Revisiting the Additionality and Durability of Carbon Uptake in Large-Scale Ocean Iron Fertilization

Revisiting the Additionality and Durability of Carbon Uptake in Large-Scale Ocean Iron Fertilization | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Revisiting the Additionality and Durability of Carbon Uptake in Large-Scale Ocean Iron Fertilization Kyung-Min Noh, Xiao Liu, Charles Stock, Dennis McGillicuddy Jr., and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7717531/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Climate stabilization pathways limiting global warming to 1.5-2 \(\:℃\) targets require emission reductions with additional carbon dioxide removal (CDR) of ~ 2–3 PgC year − 1 . Here, we assess the long-term additionality and durability of carbon uptake from large-scale ocean iron fertilization (OIF) using an Earth System Model under emissions-driven scenarios. Our simulations suggest that sustained century-scale fertilization in the southern sector of the Southern Ocean (SSO) yields a moderate contribution (~ 30PgC, or 0.3 PgC/yr). In other iron-limited regions, the additional carbon uptake induced by fertilization is largely offset (70–100%) by the non-fertilized regions. Terminating OIF after 30 years, the ocean retains 50% of the additional carbon in the SSO, while retention becomes negligible in other regions. Global CDR rates are 2 \(\:-\) 7 times lower than prior idealized estimates. OIF-induced CDR is constrained by multiple biogeochemical processes: local and remote nutrient depletion, re-entrainment of previously sequestered carbon, incomplete macronutrient drawdown, reduced phytoplankton carbon to phosphorus ratios. Incomplete macronutrient drawdown represented the largest difference from earlier studies, reflecting uptake controls by light, top-down controls, and rates of macronutrient resupply. Further insight into CDR potential requires application of our standardized design across modeling institutions, and targeted analysis of top-down and bottom-up controls on nutrient drawdown. Earth and environmental sciences/Biogeochemistry/Carbon cycle Earth and environmental sciences/Climate sciences/Ocean sciences/Marine biology Earth and environmental sciences/Climate sciences/Biogeochemistry/Carbon cycle Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Growing demands for decarbonization to mitigate climate change and achieve carbon neutrality by 2050 1 have increased focus on Carbon Dioxide Removal (CDR) technologies to offset residual emissions 2 . Given the current limitations in atmosphere and land-based CDR technologies, the ocean—the largest active carbon reservoir on Earth 3 , 4 —has been suggested as an alternative, with several promising marine CDR (mCDR) approaches under consideration 5 , 6 . However, significant uncertainties remain in the efficacy and feasibility of these approaches. Iron fertilization is one of these proposed strategies. First introduced over three decades ago 7 , this approach aims to enhance carbon sequestration in the deep ocean by stimulating biological productivity and subsequent carbon export to depth in iron-limited regions 8 , 9 , 10 . These regions are characterized by high concentrations of unused surface macronutrients but low chlorophyll levels—so-called High-Nutrient, Low-Chlorophyll (HNLC) zones. The most prominent iron-limited HNLC region is the Southern Ocean (SO) 11 , 12 , where nitrate concentrations exceed 25 mmol m⁻³. Other notable iron-limited HNLC regions include the North Pacific (NP) and the Equatorial Pacific (EP) 13 . The conceptual basis of this approach is to couple excess surface macronutrients with carbon via enhanced primary production induced through iron fertilization, facilitating the export of organic carbon to the deep ocean and enhancing CO 2 uptake from the atmosphere. This idea summarized in Martin’s 1988 remark: “Give me a half-tanker of iron, and I will give you an ice age.” ( https://earthobservatory.nasa.gov/features/Martin ) Modeling studies to test the “Martin Hypothesis” began with idealized box models and early three-dimensional carbon–climate models. These studies suggested that complete removal of Southern Ocean nutrients could draw down 106–213 PgC (around 50-107ppm) from the atmosphere 14 – 17 . Southern Ocean macronutrients, however, have since been recognized as key nutrient source for the tropical surface ocean via subduction through Subpolar Mode Waters (SPMW) and Antarctic Intermediate Waters (AAIW), supporting low-latitude primary productivity 18 – 20 . Iron fertilization can thus have unintended consequences, reducing productivity and carbon export in downstream, non-fertilized regions due to nutrient redistribution—a process known as “nutrient robbing” 21 . Such non-local effects raise critical CDR questions: Does iron fertilization lead to genuinely new carbon storage, or merely shift the location of carbon uptake that would have occurred elsewhere in its absence? Here, we revisit iron fertilization using a global coupled chemistry–carbon–climate Earth System Model (GFDL-ESM4.1 22,23 ) to quantify the large-scale and long-term additionality and durability of this mCDR strategy, and the biogeochemical mechanisms that shape them. We performed simulations under a high-emission scenario, continuously releasing iron into the surface ocean from 1990–2100 in four distinct regions (Fig. 1 a): the North Pacific, Equatorial Pacific, and the northern and southern sectors of the Southern Ocean. These regions are characterized by iron limitation and the presence of unused surface nutrients, which can fuel additional biological production when iron is added 13 , 24 . The Southern Ocean was divided to resolve the different roles of northern and southern sectors in the nutrient supply to lower-latitude 17 , 18 . A global fertilization scenario was conducted to estimate the theoretical upper limit of carbon sequestration via iron fertilization. Finally, "stop-fertilization" experiments in which fertilization was ceased after 30 years were simulated to 2100 to evaluate the durability of the fertilization-induced effects (Fig. 1 b). Together, these simulations provide insights into the mechanisms underlying the additionality and durability of iron fertilization. Limited Additionality and Durability in Ocean Carbon Uptake Fertilization-induced global carbon sequestration is small except in the Southern Ocean (Fig. 2 ). The total oceanic carbon uptake in the fertilization and stop-fertilization experiments closely overlaps with that of the control simulation across the North and Equatorial Pacific, with differences falling largely within the range of natural climate variability (Fig. 2 a-e). In contrast, the Southern Ocean exhibits a discernable fertilization signal, particularly in its southern sector (Fig. 2 d-e). Given the strong interannual variability in air–sea CO 2 fluxes, the cumulative responses in carbon uptake over time provides a clearer metric for assessing fertilization efficacy across regions (Fig. 2 f–j). Over 110 years of continuous fertilization, the Southern Ocean accounts for 31.1 PgC and 7.2 PgC of cumulative uptake in its southern and northern sectors, respectively. By contrast, the Equatorial Pacific shows minimal net gain, while the North Pacific accumulates ~ 2 PgC. Carbon uptake from global ocean fertilization is thus similar to that of the Southern Ocean. The rate of global carbon uptake decreases over time in all cases despite continuous fertilization. In the southern sector of the Southern Ocean, accumulation rates of ~ 0.45 PgC yr − 1 from continuous fertilization over the first 30 years decrease to ~ 0.2 PgC yr − 1 over the last 80 years. This results in mean century-scale accumulation rate of ~ 0.3 PgC yr − 1 . Cumulative gains from continuous fertilization in the North Pacific reach their peak after 35 years of fertilization and remain stable thereafter, while cumulative uptake in the northern Southern Ocean and equatorial Pacific declines later in the 21st century, leading to almost net zero impact of carbon sequestration by the end of the century in the latter case. In the “stop” experiments, most regions exhibit a near-complete loss of fertilization-induced carbon within a few decades after cessation of fertilization. For instance, the cumulative carbon gain in the northern Southern Ocean reaches ~ 7 PgC but declines by nearly 97% within a few decades. The notable exception is the southern Southern Ocean, which retains about 50% of its peak accumulation (~ 13 PgC) by the end of the century, corresponding to an average loss rate of ~ 0.08 PgC yr − 1 after the cessation of fertilization. Overall, while moderate carbon sequestration additionality and durability were achievable in the southern Southern Ocean, even CDR rates attained through sustained large-scale fertilization of this remote region (0.2–0.3 PgC yr − 1 ) fell short of the scale required for ambitious climate stabilization targets (2–3 PgC yr − 1 ), and those suggested by idealized nutrient-depletion experiments (Table 1 , ~ 1–2 PgC) 14 – 17 . Compared to more recent modeling studies, they are two to three times smaller than more recent studies by Aumont and Bopp (2006) 25 and Oschlies et al., (2010) 21 , yet similar in magnitude to the most recent study of Tagliabue et al., (2023) 26 . Multiple biogeochemical processes regulated the impact of OIF-induced CDR: compensating outgassing in non-fertilized regions due to stronger macronutrient limitation, re-entrainment of sequestered carbon to the surface, incomplete drawdown of surface macronutrients, reduced phytoplankton carbon to phosphorus (C:P) stoichiometry, and—in the case of the equatorial Pacific—a decline in macronutrient supply following prolonged fertilization. Table 1 Previous large-scale iron fertilization and nutrient depletion experiments. Estimates of atmospheric CO 2 removal and oceanic carbon uptake were derived using various models, ranging from simple box models to state-of-the-art 3D fully coupled Earth System Models (ESMs). Nutrient depletion experiments represent scenarios with complete depletion of upper ocean phosphate in the Southern Ocean. Aumont & Bopp (2006) fixed the surface iron concentration globally at 0.6 nM. Oschlies et al. (2010) simulated iron fertilization by doubling the phytoplankton maximum growth rate from 0.13 day⁻¹ to 0.26 day⁻¹ in the Southern Ocean. Tagliabue et al. (2023) performed two types of iron fertilization experiments: one maintaining dissolved iron concentrations at 2 nM, and another applying additional surface fluxes of dissolved iron at varying rates. CDR rates are indirectly estimated by dividing the total oceanic carbon uptake by the perturbation period. Marinov et al. (2006) simulated a new equilibrium under nutrient-depleted conditions, which does not allow for the calculation of CDR rates. Atmospheric CO 2 (ppm) Ocean Uptake (PgC) CDR Rates * (PgC/yr) Method Peng & Broecker (1991) 14 96 204 2.04 Advection-Diffusion Box-Model (Nutrient Depletion to 75m) Joos et al. (1991) 15 107 213 2.13 Advection-Diffusion Box-Model (Nutrient Depletion to 50m) Sarmiento & Orr (1991) 16 72 153 1.53 3D Ocean BGC-Model (Nutrient Depletion to 50m) Kurz & Maier-Reimer (1993) 17 50 107 1.07 3D Ocean BGC-Model (Nutrient Depletion to 50m) Marinov et al. (2006) 19 70 147 - 3D Ocean BGC-Model (Nutrient Depletion to 50m) Aumont & Bopp (2006) 25 33 70 0.70 3D Ocean BGC-Model (Fixing Iron Concentration) Oschlies et al. (2010) 21 26 58.6 0.59 3D Fully Coupled Model (Doubling Phytoplankton Growth) Tagliabue et al. (2023) 26 10 21 0.21 3D Ocean BGC-Model (Surface Iron Flux, Concentration) This work 12 31 0.28 3D Fully Coupled Model (Surface Iron Flux) Non-Local Compensation of Carbon Uptake All four regions exhibit substantial net carbon uptake within the fertilized region in both the continuous and stop fertilization cases (Fig. S1a). However, non-fertilized regions exhibit compensatory carbon fluxes that offset a majority of the fertilization-induced uptake, complicating the detection of a clear global CDR signal (Fig. S1b). In the Equatorial Pacific, carbon loss in non-fertilized areas begins shortly after fertilization starts and ultimately reaches ~ 9.7 PgC, compensating the uptake in the fertilized zone. Similarly, over 70% of the carbon uptake in the North Pacific and northern Southern Ocean is offset by losses in non-fertilized regions. In contrast, the southern Southern Ocean maintains a relatively strong net fertilization-induced uptake (~ 31 PgC), with compensation limited to approximately 20% (Table 2 ). Table 2 Cumulative responses of fertilization-induced air-sea CO 2 flux anomalies. The anomalies are integrated over the full 110 years from 1990 to 2100 in response to SSP5-8.5 emission scenario. In the FERT experiments, iron is continuously released into the surface ocean until the end of the 21st century, whereas in the STOP experiments, iron is added only for 30 years (from 1990 to 2019), with no further addition over the remaining 80 years. The fertilized regions in the four different experiments are defined in Fig. 1 . Positive values indicate additional carbon uptake by the ocean. The compensation rate is defined as the ratio of additional carbon uptake in fertilized regions that is offset by additional carbon loss in non-fertilized areas. (PgC) 1990–2100 Type North Pacific Equatorial Pacific Southern Ocean (N) Southern Ocean (S) Global Ocean FERT 2.0 0.1 7.2 31.0 STOP 1.1 1.2 0.5 6.6 Fertilized Region FERT 6.9 9.8 24.4 39.5 STOP 2.7 6.7 8.4 9.9 Non-Fertilized Region FERT \(\:-\) 4.9 \(\:-\) 9.7 \(\:-\) 17.2 \(\:-\) 8.5 STOP \(\:-\) 1.6 \(\:-\) 5.5 \(\:-\) 8.0 \(\:-\) 3.3 Compensation (Non/Fert) FERT \(\:-\) 71% \(\:-\) 99.9% \(\:-\) 70.5% \(\:-\) 21.5% STOP \(\:-\) 59.3% \(\:-\) 82.1% \(\:-\) 95.2% \(\:-\) 33.3% Inspection of the spatio-temporal evolution of air-sea CO 2 fluxes reveals that the compensatory outgassing in non-fertilized areas emerges rapidly in adjacent waters (Equatorial Pacific and North Pacific) or as a growing response spreading from the fertilized region to more distant regions over a century (Southern Ocean, Fig. 3 left column). The growth of these compensating outgassing mirrors the declining rate of accumulation in the Southern Ocean fertilization experiments (Fig. 2 h-j). Spatial patterns of the compensatory air-sea CO 2 flux responses are largely mirrored by the particle export flux declines at 100m (Fig. 3 , middle column). This is consistent with stronger nutrient limitation of phytoplankton productivity due to “nutrient robbing” though, for reasons discussed in the next section, the relative magnitudes of the carbon export and air-sea exchange response vary. During the initial decades, fertilization effects remain confined to areas adjacent to the fertilized regions, causing immediate reductions in carbon uptake and export due to localized nutrient depletion. Over time, non-local reductions in both carbon responses emerge across the tropical oceans, particularly for the Southern Ocean fertilization. These compensatory responses become pronounced after approximately 30 years in the northern Southern Ocean and around 70 years in its southern counterpart (Fig. 3 h,k). Such delayed tropical responses to Southern Ocean fertilization constrain long-term additionality of carbon sequestered in fertilized regions. Notably, the northern part of the Southern Ocean experiences greater carbon losses in the tropical regions than the southern sector, where the propagation of fertilization signals occurs with longer timescales. The biogeochemical teleconnection pattern in Fig. 3 is well established from previous work, particularly with respect to the Southern Ocean’s role in supplying preformed nutrients to the tropical oceans 18 , especially from the subantarctic region (i.e., the northern Southern Ocean 19 ). Reductions in preformed nutrients originating in the Southern Ocean are propagated to the tropical Pacific and Atlantic Oceans in our simulations through subducted mode and intermediate waters (Fig. S2), consistent with earlier findings on nutrient depletion and redistribution 16 , 17 . Consequently, prolonged fertilization leads to declines in productivity with reduced carbon uptake across the tropical Pacific and Atlantic Oceans (Fig. 3 g, h, j, k), Local Re-Entrainment of Sequestered Carbon to Surface Waters In addition to non-local compensation, several aspects of the response within fertilized regions limited the CDR potential of large-scale fertilization. The carbon particle export flux at 100 m is about 45–55% greater than surface ocean carbon uptake from the atmosphere, except in the Equatorial Pacific (Fig. 3 , right column). In general, the biological pump transfers carbon from the surface to the subsurface ocean through various particle, mixing, and active transport processes 28 , 29 . As a result, fertilization-enhanced biological production extracts surface carbon and relocate carbon to subsurface water, thereby steepening the vertical gradient of carbon concentration. If carbon is not transported deeply enough, however, it may re-emerge at the surface via seasonal re-entrainment 21 . This re-entrainment dynamic underlies the large surplus of 100m particle export over air-sea CO 2 uptake in the high latitude regions with vigorous winter mixing (Fig. 3 ). While this response occurs primarily within the fertilized region, some also occurs in adjacent regions (e.g., the northern sector of the Southern Ocean). Consequently, export fluxes exceed net air-sea CO 2 uptake due to enhanced upwelling of carbon-rich waters (as indicated by red shading in the third column of Fig. 3 ), which contributes to the limited additionality of carbon uptake. When particle export fluxes closer to the permanent thermocline are compared to those within the euphotic zone, the net additionality of carbon export more aligns with that of air-sea CO 2 fluxes in high-latitude regions. In the equatorial Pacific, the air-sea CO 2 exchange and 100m particle export are largely in balance (Fig. 3 f), as one would expect for a system where seasonal re-entrainment is not an issue. Closer examination of the spatial pattern of differences between carbon uptake and export, however, reveals that the enhanced particle export response is restricted to the eastern equatorial Pacific while the enhanced air-sea response extends westward (Fig. S3). This reflects the fast advection timescale of surface carbon deficits away from the core upwelling region relative to the slower equilibration of the air-sea CO 2 flux. Incomplete Macronutrient Removal of Iron Fertilization A second aspect of the fertilization response that limits OIF-induced CDR is the incomplete drawdown of surface nitrate surpluses except the equatorial Pacific (Fig. 4 ). The control run captures the main contrasts in both the mean nitrate concentrations and their seasonal drawdown cycles across fertilized regions, which are commonly used to estimate net community production 29 . Continuous fertilization enhanced this nitrate drawdown, but over 50% of surface surplus remained in the North Pacific and Southern Ocean. Drawdown during the light-limited winters in high-latitude systems was particularly small. The limited nutrient drawdown contrasts with previous idealized nutrient depletion simulations that assumed complete nutrient exhaustion in the upper ocean, enabling maximum carbon drawdown from the atmosphere on the order of 70–100 ppm removal 16 , 17 , 19 . Limited nutrient depletion in our simulations results from a combination of light limitation, top-down controls 30 , 31 and continuous nutrient resupply from deeper waters 32 . Decreased phytoplankton C:P ratios in the enhanced productivity zones In fertilized regions, the phytoplankton carbon to phosphorus (C:P) ratio decreases, indicating less efficient carbon utilization of preformed nutrients—particularly in areas with enhanced carbon export (Fig. 5 b–e). This decline is driven by fertilization-induced shifts in phytoplankton community composition, with larger phytoplankton groups (e.g., diatoms and other miscellaneous taxa) becoming more abundant while smaller forms (e.g., picophytoplankton) decline (Fig. 5 b–e, circular plots). These community shifts are consistent with previous fertilization experiments and field observations 33 , 34 reflecting a shift toward larger phytoplankton that exhibit lower C:P ratios 35 . For example, large phytoplankton—especially diatoms—show marked increases in the Southern Ocean, with the strongest growth in its southern sector (Fig. 5 e). The representation of stoichiometric variations in the model is relatively simple: different characteristic ratios are specified for different plankton, with small phytoplankton having higher C:P ratios than larger phytoplankton 24 , 35 , 36 . This pattern would likely be reinforced by further unresolved elevated C:P ratios from greater investment in phytoplankton growth machinery in fertilized environments 37 , but could be counteracted by phosphate frugality as phosphate concentrations decline 38 . Degrading Macronutrient Supply in the Equatorial Pacific Although the fertilized Equatorial Pacific initially sequesters some carbon, it exhibits minimal cumulative gains (Table 2 ) and steadily declining additionality in carbon uptake (Fig. 3 d–e) over the 21st century is evident. The weak overall response can be partly attributed to strong compensatory effects outside the core upwelling regions (Fig. 3 ), but the gradual decline in carbon uptake within the Equatorial Pacific requires further explanation. In the Equatorial Pacific, macronutrient remineralization below the euphotic zone is hindered by a shallow hypoxic layer. Particle export initiated by sustained fertilization persistently depletes macronutrient in the upper water that feeds equatorial upwelling. Depletion is notable for both phosphate and nitrate, but particularly severe for nitrate (Fig. S4). Further, the pronounced deficits of nitrate are amplified by denitrification in low oxygen waters 39 . Consequently, the Equatorial Pacific exhausts its macronutrient reserves, steadily diminishing carbon-uptake efficacy productivity over time. Summary and Discussion Due to escalating atmospheric CO 2 concentration, mCDR methods have emerged in climate stabilization research aiming to address residual emissions of 2–3 PgC year − 1 sustained over the next century 5 , 6 . Among these, iron fertilization—originally proposed over 30 years ago 7 , 15 , 16 —has been revisited in this study using a fully coupled Earth system model 22 , 23 to assess its additionality and durability under large-scale and long-term scenarios. By simulating carbon and biogeochemical responses across four major iron-limited HNLC regions, we find that large-scale, sustained iron fertilization yields only limited additionality and durability of carbon uptake compared with previous large-scale fertilization studies (Table 1 ). Most HNLC regions sequester a modest fraction of the petagram-scale target of climate stabilization scenarios. This suggests that while iron fertilization could contribute to CDR strategies, it must be implemented as part of a broader portfolio of CDR solutions. Furthermore, if fertilization is not continuously sustained, much of its benefit would dissipate. These limited benefits must also be weighed against established ecosystem risks 26 , 40 . The effectiveness of carbon drawdown by OIF is regulated by diverse biogeochemical responses to fertilization. As in prior studies, non-local compensatory responses in carbon uptake emerge due to reduced preformed nutrients. This was compounded by resurfacing of sequestered carbon, shifts in plankton stoichiometry, and macronutrient exhaustion in the equatorial Pacific. The most prominent difference between prior idealized simulations, however, was incomplete macronutrient drawdown in the Southern Ocean and North Pacific. Surface macronutrients in these regions arise in the model from a complex mixture of nutrient re-supply rates, top-down controls, and light limitation. Our simulation captures the observed seasonal macronutrient drawdown patterns reasonably well (Fig. 4 ), but the drawdown expected from sustained iron fertilization is more difficult to assess against observations. We note that the nitrate drawdowns were comparable to those observed in smaller-scale fertilization experiments. (2–5 mmol/m 3 ) 10,41 . In addition, the overall sequestration from sustained fertilization was similar in magnitude to the recent sustained iron fertilization study of Tagliabue et al., (2023) 26 , which also directly simulated fertilization (Table 1 ) and used a similarly comprehensive biogeochemical model 42 . An important limitation of the current modeling approach is its inability to resolve mesoscale and smaller-scale ocean heterogeneity, which may prevent complete representation of local decoupling between phytoplankton growth and grazing, as well as bloom intensity 43 , 44 . Future work using higher resolution models resolving small-scale processes may offer deeper insights into the extent of local nutrient drawdown in the Southern Ocean. Another key factor influencing carbon sequestration efficacy is the remineralization length scale, which governs how efficiently carbon is transferred from the surface ocean to the deep ocean 45 . However, current Earth System Models (ESMs) show considerable variability in their representation of vertical carbon flux transfer efficiency 46 . This includes differences in modeling ballast effects, bacterial degradation, and the temperature- and oxygen-dependence of respiration 47 , all of which contribute to divergent remineralization responses to fertilization. Furthermore, the representation of phytoplankton functional types (PFTs) and their associated stoichiometry vary widely among models 48 . Dynamic phytoplankton stoichiometry may allow for the higher nutrient drawdown in response to fertilization 49 , resulting in the higher carbon uptake efficacies. Further insight into the CDR potential of sustained large-scale iron fertilization could be gained through the application of the standardized design herein across ESMs, and targeted analysis of top-down and bottom-up nutrient drawdown controls. Our study synthesizes large-scale, long-term iron fertilization experiments, extending earlier findings. Initial idealized nutrient depletion studies suggested iron fertilization could explain glacial–interglacial CO 2 differences (~ 70–100 ppm; Table 1 ). However, our results suggest that carbon uptake from OIF may be considerably smaller. A cross-regional analysis of four HNLC regions reveals: (1) small contributions from sustained iron fertilization in most regions, (2) moderate carbon sequestration potential in the Southern Ocean, albeit insufficient for petagram-scale climate stabilization targets, and (3) critical roles of biogeochemical processes that constrain additionality and durability. These findings provide guidance for evaluating the feasibility and scalability of mCDR strategies globally, suggesting that even large-scale fertilization would be insufficient to induce an ice-age-scale carbon drawdown. Methods Model Configuration This study utilized the Geophysical Fluid Dynamics Laboratory Earth System Model version 4.1 (GFDL-ESM4.1), a coupled carbon–chemistry–climate model 22 . The atmospheric and land components are represented by AM4 50–52 and LM4 53 , respectively, both operating at a horizontal resolution of 100 km with 49 vertical levels. The ocean model, OM4p5, integrates GFDL’s Modular Ocean Model 6 (MOM6) for ocean physics and Sea Ice Simulator 2 (SIS2) for sea ice processes 54 , with a 50 km horizontal resolution and 75 hybrid depth–density vertical layers. Marine biogeochemistry and food web dynamics are simulated using the Carbon, Ocean Biogeochemistry, and Lower Trophics version 2 (COBALTv2) model 23 . COBALTv2 uses 33 tracers to represent key elements of the ocean carbon cycle, biogeochemistry, and plankton ecosystems representing carbonate systems (dissolved organic carbon, and alkalinity), five different nutrients (nitrogen, phosphorus, iron, oxygen, silicate), with explicit three phytoplankton (Diazotroph, small and large phytoplankton group), three zooplankton (small, medium and large zooplankton group), and bacteria. Notably, it includes explicit interactions between the iron cycle and various sources, such as rivers, sediments, geothermal vents, atmospheric deposition, and icebergs 55 – 59 , and resolves energy transfer within plankton communities 60 , 61 . To compare nitrate drawdown between observations and GFDL-ESM4.1 in fertilized ocean regions, we used nitrate datasets from the World Ocean Atlas 2023 (WOA23), which are based on in situ reanalysis 62 , to calculate the seasonal cycle of nitrate concentration. The WOA23 provides present-day climatological fields interpolated onto a \(\:1^\circ\:\times\:1^\circ\:\) latitude-longitude grid in each month. Nitrate drawdown, which reflects the potential for net biological production in each region, is typically estimated as the difference between the annual maximum and minimum nitrate concentrations. Experiment Design To investigate the effectiveness of iron fertilization for marine carbon dioxide removal (mCDR), we conducted a series of targeted simulations. Large-scale fertilization experiments were designed for four high-nutrient, low-chlorophyll (HNLC) regions: the North Pacific (NP), Equatorial Pacific (EP), Northern Southern Ocean (NSO), and Southern Southern Ocean (SSO). A global fertilization experiment was also performed to estimate the upper limit of carbon uptake in the absence of iron limitation. The primary goal of these experiments was to quantify the maximum regional carbon uptake potential and to evaluate the limitations of previous small-scale patch fertilization (PSF) experiments. In all fertilization scenarios, artificial iron fluxes of approximately 0.02 mol Fe m⁻² yr⁻¹ were uniformly applied at the surface, a level sufficient to saturate iron limitation and maintain dissolved iron concentrations near 2 nM in the mixed layer. Sensitivity tests confirmed the adequacy of this flux rate. All other biogeochemical fluxes followed the configurations described in Stock et al. (2020) 23 . We conducted a control simulation without artificial iron addition, spanning the historical period (1990–2014) and the future under SSP5-8.5 (2015–2100), using emission-driven forcings (Fig. 1 b), in accordance with the esm-hist and esm-ssp585 protocols from C4MIP 63 . Two additional simulations were performed: (1) FERT : Continuous iron release through 2100 to examine long-term fertilization effects (2) STOP : Iron release ceased after 30 years to assess the persistence and durability of fertilization impacts. The effects of fertilization were quantified by comparing these simulations with the control. Specifically, the accumulated response in air-sea CO2 flux ( \(\:\eta\:\) ) was calculated as : $$\:\eta\:={\int\:}_{{t}_{0}}^{t}{\int\:}_{A}\left[{\left(C{O}_{2}\:Flux\right)}_{OIF}-{\left(C{O}_{2}\:Flux\right)}_{Control}\:\right]\:dA\:dt$$ where \(\:A\) and \(\:t\) denote area and time, respectively. Accumulated responses were computed separately over fertilized and non-fertilized areas to assess both local and global impacts. To quantify compensation effects, the compensation rate was calculated as the ratio of the CO 2 flux change in non-fertilized areas to that in fertilized areas. Uncertainty was estimated from the interannual variability of each variable. Selected Regions for the Iron-Fertilization HNLC regions—characterized by abundant macronutrients but limited dissolved iron 11 , 13 , 24 —have been the focus of several fertilization experiments 8 – 10 , 41 . These regions were selected to maximize carbon uptake responses to minimal iron additions. Efficacy may also depend on ocean circulation features such as vertical mixing and horizontal advection. The OIF forced areas were defined by first identifying regions with nitrate concentrations exceeding 1 µM 13,24 then selecting areas where phytoplankton were iron-limited in ESM4.1 23,64 . The Southern Ocean was further divided into northern and southern sectors based on the formation of intermediate and deep waters 19 , 65 , delineated using the 26.8 kg m⁻³ isopycnal surface. This partitioning remains consistent with prior studies, despite known warm and fresh biases in the Southern Ocean in ESM4.1 22 . The iron fertilization areas selected in this way include four different regions; the Northern Pacific (NP), Equatorial Pacific (EP), Southern Ocean North (NSO) associated within the main Antarctic Circumpolar Current, and Southern Ocean South (SSO) associated with the ice-influenced region around Antarctica (Fig. 1 ). Declarations Data Availability Statement The source codes for GFDL-ESM4.1 can be accessed online via Github at the following link: https://github.com/NOAA-GFDL/ESM4. All figures were generated by using software package Python with the matplotlib and basemap modules (https://matplotlib.org/, https://matplotlib.org/basemap/). The map coastlines are derived by the Global Self-consistent, Hierarchical, High-resolution Geography (GSHHG) Database (www.soest.hawaii.edu/pwessel/gshhg/), which has been distributed under the GNU Lesser General Public License and is provided with the basemap Python module. The nitrate in WOA23 is provided freely at https://www.ncei.noaa.gov/access/world-ocean-atlas-2023/ by NOAA National Centers for Environmental Information. Acknowledgments This manuscript was prepared by Kyung-Min Noh under award NA23OAR4320198 from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the author(s) and do not necessarily reflect the views of the National Oceanic and Atmospheric Administration, or the U.S. Department of Commerce. Competing Interests The authors declare no competing interest. References United Nations Framework Convention on Climate Change (2015) Paris Agreement. FCCC/CP/2015/L.9/Rev.1 National Academies of (2019) Sciences, Engineering, and Medicine. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. National Academies Sabine CL et al (2004) The oceanic sink for anthropogenic CO 2 . 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16:33:55","extension":"html","order_by":22,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":146074,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7717531/v1/37337af2292353afe78c2999.html"},{"id":95227446,"identity":"d663e358-e8d6-485f-b81b-ae6a8885c616","added_by":"auto","created_at":"2025-11-05 16:32:30","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":388487,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Designated regions and experimental setup for iron fertilization simulations.\u003c/p\u003e\n\u003cp\u003eLarge-scale iron fertilization regions are illustrated. The broader regions suitable for large-scale iron fertilization are identified based on phytoplankton nutrient limitation and physical and biogeochemical oceanographic characteristics, including nitrate concentrations, potential density, and iron concentration (see Methods): the North Pacific (red), Equatorial Pacific (orange), northern section of the Southern Ocean (cyan), and southern section of the Southern Ocean (blue). (b) Experimental design. For each region, two simulations were conducted under high-emission scenarios extending to 2100 with a control simulation (CTRL, black), a fertilization experiment (FERT, red), and a stop experiment (STOP, blue). The control simulation follows the esm-hist and esm-ssp585 protocols from C4MIP in CMIP6, with emissions-driven forcings (Jones et al., 2016). In the FERT simulation, iron is continuously supplied to the ocean surface throughout the 21st century to stimulate phytoplankton blooms in the designated regions. To assess the persistence and durability of marine carbon dioxide removal (mCDR) through iron fertilization, the STOP scenario ceases iron addition after 30 years of fertilization in each region.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7717531/v1/ced5a474298ee094e91445eb.png"},{"id":95204908,"identity":"268fc788-a3ab-42e9-a003-a7b3a9ad1ad1","added_by":"auto","created_at":"2025-11-05 13:07:26","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1555571,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eLong-term accumulated impacts of fertilization-induced carbon uptake.\u003c/strong\u003e (a-e) Globally integrated air-sea CO\u003csub\u003e2\u003c/sub\u003e fluxes from 1990 to 2100 are shown for the control simulation (CTRL, black), the fertilization experiment (FERT, red), and the termination experiment (STOP, blue). The fertilization regions and experimental design are illustrated in Figure S1. The control simulation follows the esm-hist and esm-ssp585 protocols from C4MIP in CMIP6 (Jones et al., 2016). In the FERT scenario, iron is continuously supplied to the ocean surface throughout the 21st century to assess the large-scale application and long-term effects of climate engineering. In contrast, the STOP scenario terminates iron addition after 30 years to evaluate the persistence and durability of the induced carbon uptake. Grey vertical lines indicate the year (2020) when iron addition ceases in the STOP experiment. (f-j) Cumulative global responses of fertilization-induced carbon uptake are shown as the integrated differences in CO\u003csub\u003e2\u003c/sub\u003e flux between the FERT and STOP simulations compared to CTRL. The values represent the total additional carbon sequestered over the 110-year period. For instance, continuous iron fertilization across the global ocean results in an additional uptake of approximately 33 PgC. Decomposition of accumulated carbon uptakes in the fertilized and non-fertilized regions are provided in the Figure S1.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7717531/v1/00553e836b44df68a00e916a.png"},{"id":95227227,"identity":"e7c8e10c-1501-47bb-a6d2-665732dc0ea5","added_by":"auto","created_at":"2025-11-05 16:32:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1750319,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSpatiotemporal evolution of fertilization-induced local and non-local responses.\u003c/strong\u003e Hovmöller diagram of zonally averaged air-sea CO\u003csub\u003e2\u003c/sub\u003e fluxes, carbon export fluxes anomalies at 100 m depth, and difference between these two fluxes under iron fertilization at (a-c) North Pacific, (d-f) Equatorial Pacific, (g-i) northern Southern Ocean, and (j-l) southern Southern Ocean. Red shading indicates enhanced uptake and export production, reflecting the potential of fertilization to sequester more carbon from the surface to depth, while blue shading indicates outgassing and reduced carbon transport. Hatched areas denote regions where the fertilization-induced signal exceeds natural climate variability, defined as the interannual standard deviation during the present climate period (1990–2020) at each latitude. Dashed black lines represent the mean latitudes of north and south boundaries in fertilized regions.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7717531/v1/4567dbd245b48d325f013c8c.png"},{"id":95204906,"identity":"42416762-33a7-41db-93df-4aa5a8d05323","added_by":"auto","created_at":"2025-11-05 13:07:26","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":407532,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eComparison of nitrate drawdown in fertilized regions. \u003c/strong\u003eSeasonal variations in nitrate concentration during the historical period (1990–2014) for the fertilized regions: (a) North Pacific, (b) Equatorial Pacific, (c) Southern Ocean (N), and (d) Southern Ocean (S). Black dotted lines indicate the climatological nitrate seasonality based on the World Ocean Atlas 2023 (WOA23). Orange and red dotted lines represent the nitrate seasonality in the control simulation and large-scale iron fertilization experiments, respectively. Shaded areas denote the interannual standard deviation over the same period (1990–2014).\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7717531/v1/a3b21930b6f902056bf4795d.png"},{"id":95227389,"identity":"c5f389b4-4062-495a-8bb7-2348891e0dd5","added_by":"auto","created_at":"2025-11-05 16:32:26","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1986554,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eImplications for the marine ecosystem and carbon. \u003c/strong\u003eThe climatology and response of the carbon-to-phosphate (C:P) ratio to iron fertilization are assessed as the ratio of particulate organic carbon to phosphate at 100 m depth. Horizontal spatial distributions of the C:P ratio are shown for the present-day climate (first 30 years, 1990–2020) across the following fertilization experiments: (b) North Pacific, (c) Equatorial Pacific, (d) Southern Ocean (North), and (e) Southern Ocean (South). Hatched areas denote regions where fertilization-induced signals exceed the natural interannual variability, defined by the standard deviation during the 1990–2020 period at each grid point. Circular plots illustrate the average composition of phytoplankton functional types (PFTs) within the fertilized areas, with color codes as follows: diazotrophs (orange), small phytoplankton (red), and large phytoplankton (blue), further separated into diatoms and non-diatoms. In the Equatorial Pacific, the analysis is further restricted to regions exhibiting a positive additionality in export flux.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7717531/v1/4fb7d1b59e7c637e85b9112a.png"},{"id":95312219,"identity":"f13a24eb-5cbe-434f-9a03-0af4d147086e","added_by":"auto","created_at":"2025-11-06 15:48:16","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5405432,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7717531/v1/a014e585-db74-429d-8202-b2b3e14c405a.pdf"},{"id":95228627,"identity":"ab49d44e-717f-403e-a554-f0d5e01a9837","added_by":"auto","created_at":"2025-11-05 16:34:00","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":7936102,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-7717531/v1/a4a8ac753ad106653ecb0ab9.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Revisiting the Additionality and Durability of Carbon Uptake in Large-Scale Ocean Iron Fertilization","fulltext":[{"header":"Introduction","content":"\u003cp\u003eGrowing demands for decarbonization to mitigate climate change and achieve carbon neutrality by 2050\u003csup\u003e1\u003c/sup\u003e have increased focus on Carbon Dioxide Removal (CDR) technologies to offset residual emissions\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Given the current limitations in atmosphere and land-based CDR technologies, the ocean—the largest active carbon reservoir on Earth\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e—has been suggested as an alternative, with several promising marine CDR (mCDR) approaches under consideration\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. However, significant uncertainties remain in the efficacy and feasibility of these approaches.\u003c/p\u003e\u003cp\u003eIron fertilization is one of these proposed strategies. First introduced over three decades ago\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, this approach aims to enhance carbon sequestration in the deep ocean by stimulating biological productivity and subsequent carbon export to depth in iron-limited regions\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. These regions are characterized by high concentrations of unused surface macronutrients but low chlorophyll levels—so-called High-Nutrient, Low-Chlorophyll (HNLC) zones. The most prominent iron-limited HNLC region is the Southern Ocean (SO) \u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e, where nitrate concentrations exceed 25 mmol m⁻³. Other notable iron-limited HNLC regions include the North Pacific (NP) and the Equatorial Pacific (EP)\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. The conceptual basis of this approach is to couple excess surface macronutrients with carbon via enhanced primary production induced through iron fertilization, facilitating the export of organic carbon to the deep ocean and enhancing CO\u003csub\u003e2\u003c/sub\u003e uptake from the atmosphere. This idea summarized in Martin’s 1988 remark: “Give me a half-tanker of iron, and I will give you an ice age.” (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://earthobservatory.nasa.gov/features/Martin\u003c/span\u003e\u003cspan address=\"https://earthobservatory.nasa.gov/features/Martin\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e)\u003c/p\u003e\u003cp\u003eModeling studies to test the “Martin Hypothesis” began with idealized box models and early three-dimensional carbon–climate models. These studies suggested that complete removal of Southern Ocean nutrients could draw down 106–213 PgC (around 50-107ppm) from the atmosphere\u003csup\u003e\u003cspan additionalcitationids=\"CR15 CR16\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e–\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Southern Ocean macronutrients, however, have since been recognized as key nutrient source for the tropical surface ocean via subduction through Subpolar Mode Waters (SPMW) and Antarctic Intermediate Waters (AAIW), supporting low-latitude primary productivity\u003csup\u003e\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e–\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. Iron fertilization can thus have unintended consequences, reducing productivity and carbon export in downstream, non-fertilized regions due to nutrient redistribution—a process known as “nutrient robbing”\u003csup\u003e21\u003c/sup\u003e. Such non-local effects raise critical CDR questions: Does iron fertilization lead to genuinely new carbon storage, or merely shift the location of carbon uptake that would have occurred elsewhere in its absence?\u003c/p\u003e\u003cp\u003eHere, we revisit iron fertilization using a global coupled chemistry–carbon–climate Earth System Model (GFDL-ESM4.1\u003csup\u003e22,23\u003c/sup\u003e) to quantify the large-scale and long-term additionality and durability of this mCDR strategy, and the biogeochemical mechanisms that shape them. We performed simulations under a high-emission scenario, continuously releasing iron into the surface ocean from 1990–2100 in four distinct regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea): the North Pacific, Equatorial Pacific, and the northern and southern sectors of the Southern Ocean. These regions are characterized by iron limitation and the presence of unused surface nutrients, which can fuel additional biological production when iron is added\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. The Southern Ocean was divided to resolve the different roles of northern and southern sectors in the nutrient supply to lower-latitude\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. A global fertilization scenario was conducted to estimate the theoretical upper limit of carbon sequestration via iron fertilization. Finally, \"stop-fertilization\" experiments in which fertilization was ceased after 30 years were simulated to 2100 to evaluate the durability of the fertilization-induced effects (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Together, these simulations provide insights into the mechanisms underlying the additionality and durability of iron fertilization.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eLimited Additionality and Durability in Ocean Carbon Uptake\u003c/h3\u003e\n\u003cp\u003eFertilization-induced global carbon sequestration is small except in the Southern Ocean (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The total oceanic carbon uptake in the fertilization and stop-fertilization experiments closely overlaps with that of the control simulation across the North and Equatorial Pacific, with differences falling largely within the range of natural climate variability (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-e). In contrast, the Southern Ocean exhibits a discernable fertilization signal, particularly in its southern sector (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed-e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eGiven the strong interannual variability in air–sea CO\u003csub\u003e2\u003c/sub\u003e fluxes, the cumulative responses in carbon uptake over time provides a clearer metric for assessing fertilization efficacy across regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef–j). Over 110 years of continuous fertilization, the Southern Ocean accounts for 31.1 PgC and 7.2 PgC of cumulative uptake in its southern and northern sectors, respectively. By contrast, the Equatorial Pacific shows minimal net gain, while the North Pacific accumulates ~ 2 PgC. Carbon uptake from global ocean fertilization is thus similar to that of the Southern Ocean.\u003c/p\u003e\u003cp\u003eThe rate of global carbon uptake decreases over time in all cases despite continuous fertilization. In the southern sector of the Southern Ocean, accumulation rates of ~ 0.45 PgC yr\u003csup\u003e− 1\u003c/sup\u003e from continuous fertilization over the first 30 years decrease to ~ 0.2 PgC yr\u003csup\u003e− 1\u003c/sup\u003e over the last 80 years. This results in mean century-scale accumulation rate of ~ 0.3 PgC yr\u003csup\u003e− 1\u003c/sup\u003e. Cumulative gains from continuous fertilization in the North Pacific reach their peak after 35 years of fertilization and remain stable thereafter, while cumulative uptake in the northern Southern Ocean and equatorial Pacific declines later in the 21st century, leading to almost net zero impact of carbon sequestration by the end of the century in the latter case.\u003c/p\u003e\u003cp\u003eIn the “stop” experiments, most regions exhibit a near-complete loss of fertilization-induced carbon within a few decades after cessation of fertilization. For instance, the cumulative carbon gain in the northern Southern Ocean reaches ~ 7 PgC but declines by nearly 97% within a few decades. The notable exception is the southern Southern Ocean, which retains about 50% of its peak accumulation (~ 13 PgC) by the end of the century, corresponding to an average loss rate of ~ 0.08 PgC yr\u003csup\u003e− 1\u003c/sup\u003e after the cessation of fertilization.\u003c/p\u003e\u003cp\u003eOverall, while moderate carbon sequestration additionality and durability were achievable in the southern Southern Ocean, even CDR rates attained through sustained large-scale fertilization of this remote region (0.2–0.3 PgC yr\u003csup\u003e− 1\u003c/sup\u003e) fell short of the scale required for ambitious climate stabilization targets (2–3 PgC yr\u003csup\u003e− 1\u003c/sup\u003e), and those suggested by idealized nutrient-depletion experiments (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, ~ 1–2 PgC)\u003csup\u003e\u003cspan additionalcitationids=\"CR15 CR16\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e–\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Compared to more recent modeling studies, they are two to three times smaller than more recent studies by Aumont and Bopp (2006)\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e and Oschlies et al., (2010)\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e, yet similar in magnitude to the most recent study of Tagliabue et al., (2023)\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. Multiple biogeochemical processes regulated the impact of OIF-induced CDR: compensating outgassing in non-fertilized regions due to stronger macronutrient limitation, re-entrainment of sequestered carbon to the surface, incomplete drawdown of surface macronutrients, reduced phytoplankton carbon to phosphorus (C:P) stoichiometry, and—in the case of the equatorial Pacific—a decline in macronutrient supply following prolonged fertilization.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv class=\"gridtable\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003e\u003cb\u003ePrevious large-scale iron fertilization and nutrient depletion experiments.\u003c/b\u003e Estimates of atmospheric CO\u003csub\u003e2\u003c/sub\u003e removal and oceanic carbon uptake were derived using various models, ranging from simple box models to state-of-the-art 3D fully coupled Earth System Models (ESMs). Nutrient depletion experiments represent scenarios with complete depletion of upper ocean phosphate in the Southern Ocean. Aumont \u0026amp; Bopp (2006) fixed the surface iron concentration globally at 0.6 nM. Oschlies et al. (2010) simulated iron fertilization by doubling the phytoplankton maximum growth rate from 0.13 day⁻¹ to 0.26 day⁻¹ in the Southern Ocean. Tagliabue et al. (2023) performed two types of iron fertilization experiments: one maintaining dissolved iron concentrations at 2 nM, and another applying additional surface fluxes of dissolved iron at varying rates. CDR rates are indirectly estimated by dividing the total oceanic carbon uptake by the perturbation period. Marinov et al. (2006) simulated a new equilibrium under nutrient-depleted conditions, which does not allow for the calculation of CDR rates.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAtmospheric CO\u003csub\u003e2\u003c/sub\u003e (ppm)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eOcean Uptake (PgC)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCDR Rates\u003csup\u003e*\u003c/sup\u003e (PgC/yr)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eMethod\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePeng \u0026amp; Broecker (1991)\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e96\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e204\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eAdvection-Diffusion Box-Model\u003c/p\u003e\u003cp\u003e(Nutrient Depletion to 75m)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eJoos et al.\u003c/p\u003e\u003cp\u003e(1991)\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e107\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e213\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eAdvection-Diffusion Box-Model\u003c/p\u003e\u003cp\u003e(Nutrient Depletion to 50m)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSarmiento \u0026amp; Orr (1991)\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e153\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.53\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3D Ocean BGC-Model\u003c/p\u003e\u003cp\u003e(Nutrient Depletion to 50m)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eKurz \u0026amp; Maier-Reimer (1993)\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e107\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.07\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3D Ocean BGC-Model\u003c/p\u003e\u003cp\u003e(Nutrient Depletion to 50m)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMarinov et al. (2006)\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e147\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3D Ocean BGC-Model\u003c/p\u003e\u003cp\u003e(Nutrient Depletion to 50m)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAumont \u0026amp; Bopp (2006)\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e33\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3D Ocean BGC-Model\u003c/p\u003e\u003cp\u003e(Fixing Iron Concentration)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eOschlies et al. (2010)\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e58.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.59\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3D Fully Coupled Model\u003c/p\u003e\u003cp\u003e(Doubling Phytoplankton Growth)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTagliabue et al. (2023)\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3D Ocean BGC-Model\u003c/p\u003e\u003cp\u003e(Surface Iron Flux, Concentration)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eThis work\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3D Fully Coupled Model\u003c/p\u003e\u003cp\u003e(Surface Iron Flux)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eNon-Local Compensation of Carbon Uptake\u003c/h2\u003e\u003cp\u003eAll four regions exhibit substantial net carbon uptake within the fertilized region in both the continuous and stop fertilization cases (Fig. S1a). However, non-fertilized regions exhibit compensatory carbon fluxes that offset a majority of the fertilization-induced uptake, complicating the detection of a clear global CDR signal (Fig. S1b). In the Equatorial Pacific, carbon loss in non-fertilized areas begins shortly after fertilization starts and ultimately reaches ~ 9.7 PgC, compensating the uptake in the fertilized zone. Similarly, over 70% of the carbon uptake in the North Pacific and northern Southern Ocean is offset by losses in non-fertilized regions. In contrast, the southern Southern Ocean maintains a relatively strong net fertilization-induced uptake (~ 31 PgC), with compensation limited to approximately 20% (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv class=\"gridtable\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003e\u003cb\u003eCumulative responses of fertilization-induced air-sea CO\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e \u003cb\u003eflux anomalies.\u003c/b\u003e The anomalies are integrated over the full 110 years from 1990 to 2100 in response to SSP5-8.5 emission scenario. In the FERT experiments, iron is continuously released into the surface ocean until the end of the 21st century, whereas in the STOP experiments, iron is added only for 30 years (from 1990 to 2019), with no further addition over the remaining 80 years. The fertilized regions in the four different experiments are defined in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Positive values indicate additional carbon uptake by the ocean. The compensation rate is defined as the ratio of additional carbon uptake in fertilized regions that is offset by additional carbon loss in non-fertilized areas.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003e(PgC)\u003c/p\u003e\u003cp\u003e1990–2100\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eType\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eNorth\u003c/p\u003e\u003cp\u003ePacific\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEquatorial\u003c/p\u003e\u003cp\u003ePacific\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eSouthern\u003c/p\u003e\u003cp\u003eOcean (N)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eSouthern\u003c/p\u003e\u003cp\u003eOcean (S)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eGlobal\u003c/p\u003e\u003cp\u003eOcean\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFERT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e31.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSTOP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e1.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eFertilized\u003c/p\u003e\u003cp\u003eRegion\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFERT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e6.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e9.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e24.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e39.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSTOP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e6.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e9.9\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eNon-Fertilized Region\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFERT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e4.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e9.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e17.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e8.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSTOP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e1.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e5.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e8.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e3.3\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eCompensation (Non/Fert)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFERT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e71%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e99.9%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e70.5%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e21.5%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSTOP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e59.3%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e82.1%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e95.2%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e33.3%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eInspection of the spatio-temporal evolution of air-sea CO\u003csub\u003e2\u003c/sub\u003e fluxes reveals that the compensatory outgassing in non-fertilized areas emerges rapidly in adjacent waters (Equatorial Pacific and North Pacific) or as a growing response spreading from the fertilized region to more distant regions over a century (Southern Ocean, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e left column). The growth of these compensating outgassing mirrors the declining rate of accumulation in the Southern Ocean fertilization experiments (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh-j). Spatial patterns of the compensatory air-sea CO\u003csub\u003e2\u003c/sub\u003e flux responses are largely mirrored by the particle export flux declines at 100m (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e, middle column). This is consistent with stronger nutrient limitation of phytoplankton productivity due to “nutrient robbing” though, for reasons discussed in the next section, the relative magnitudes of the carbon export and air-sea exchange response vary.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eDuring the initial decades, fertilization effects remain confined to areas adjacent to the fertilized regions, causing immediate reductions in carbon uptake and export due to localized nutrient depletion. Over time, non-local reductions in both carbon responses emerge across the tropical oceans, particularly for the Southern Ocean fertilization. These compensatory responses become pronounced after approximately 30 years in the northern Southern Ocean and around 70 years in its southern counterpart (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003eh,k). Such delayed tropical responses to Southern Ocean fertilization constrain long-term additionality of carbon sequestered in fertilized regions. Notably, the northern part of the Southern Ocean experiences greater carbon losses in the tropical regions than the southern sector, where the propagation of fertilization signals occurs with longer timescales.\u003c/p\u003e\u003cp\u003eThe biogeochemical teleconnection pattern in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e is well established from previous work, particularly with respect to the Southern Ocean’s role in supplying preformed nutrients to the tropical oceans\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, especially from the subantarctic region (i.e., the northern Southern Ocean\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e). Reductions in preformed nutrients originating in the Southern Ocean are propagated to the tropical Pacific and Atlantic Oceans in our simulations through subducted mode and intermediate waters (Fig. S2), consistent with earlier findings on nutrient depletion and redistribution\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Consequently, prolonged fertilization leads to declines in productivity with reduced carbon uptake across the tropical Pacific and Atlantic Oceans (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003eg, h, j, k),\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eLocal Re-Entrainment of Sequestered Carbon to Surface Waters\u003c/h3\u003e\n\u003cp\u003eIn addition to non-local compensation, several aspects of the response within fertilized regions limited the CDR potential of large-scale fertilization. The carbon particle export flux at 100 m is about 45–55% greater than surface ocean carbon uptake from the atmosphere, except in the Equatorial Pacific (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e, right column). In general, the biological pump transfers carbon from the surface to the subsurface ocean through various particle, mixing, and active transport processes\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. As a result, fertilization-enhanced biological production extracts surface carbon and relocate carbon to subsurface water, thereby steepening the vertical gradient of carbon concentration. If carbon is not transported deeply enough, however, it may re-emerge at the surface via seasonal re-entrainment\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThis re-entrainment dynamic underlies the large surplus of 100m particle export over air-sea CO\u003csub\u003e2\u003c/sub\u003e uptake in the high latitude regions with vigorous winter mixing (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e). While this response occurs primarily within the fertilized region, some also occurs in adjacent regions (e.g., the northern sector of the Southern Ocean). Consequently, export fluxes exceed net air-sea CO\u003csub\u003e2\u003c/sub\u003e uptake due to enhanced upwelling of carbon-rich waters (as indicated by red shading in the third column of Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e), which contributes to the limited additionality of carbon uptake.\u003c/p\u003e\u003cp\u003eWhen particle export fluxes closer to the permanent thermocline are compared to those within the euphotic zone, the net additionality of carbon export more aligns with that of air-sea CO\u003csub\u003e2\u003c/sub\u003e fluxes in high-latitude regions. In the equatorial Pacific, the air-sea CO\u003csub\u003e2\u003c/sub\u003e exchange and 100m particle export are largely in balance (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003ef), as one would expect for a system where seasonal re-entrainment is not an issue. Closer examination of the spatial pattern of differences between carbon uptake and export, however, reveals that the enhanced particle export response is restricted to the eastern equatorial Pacific while the enhanced air-sea response extends westward (Fig. S3). This reflects the fast advection timescale of surface carbon deficits away from the core upwelling region relative to the slower equilibration of the air-sea CO\u003csub\u003e2\u003c/sub\u003e flux.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eIncomplete Macronutrient Removal of Iron Fertilization\u003c/h3\u003e\n\u003cp\u003eA second aspect of the fertilization response that limits OIF-induced CDR is the incomplete drawdown of surface nitrate surpluses except the equatorial Pacific (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The control run captures the main contrasts in both the mean nitrate concentrations and their seasonal drawdown cycles across fertilized regions, which are commonly used to estimate net community production\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Continuous fertilization enhanced this nitrate drawdown, but over 50% of surface surplus remained in the North Pacific and Southern Ocean. Drawdown during the light-limited winters in high-latitude systems was particularly small.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe limited nutrient drawdown contrasts with previous idealized nutrient depletion simulations that assumed complete nutrient exhaustion in the upper ocean, enabling maximum carbon drawdown from the atmosphere on the order of 70–100 ppm removal\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Limited nutrient depletion in our simulations results from a combination of light limitation, top-down controls\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e and continuous nutrient resupply from deeper waters\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eDecreased phytoplankton C:P ratios in the enhanced productivity zones\u003c/h3\u003e\n\u003cp\u003eIn fertilized regions, the phytoplankton carbon to phosphorus (C:P) ratio decreases, indicating less efficient carbon utilization of preformed nutrients—particularly in areas with enhanced carbon export (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e5\u003c/span\u003eb–e). This decline is driven by fertilization-induced shifts in phytoplankton community composition, with larger phytoplankton groups (e.g., diatoms and other miscellaneous taxa) becoming more abundant while smaller forms (e.g., picophytoplankton) decline (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e5\u003c/span\u003eb–e, circular plots). These community shifts are consistent with previous fertilization experiments and field observations\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e reflecting a shift toward larger phytoplankton that exhibit lower C:P ratios\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. For example, large phytoplankton—especially diatoms—show marked increases in the Southern Ocean, with the strongest growth in its southern sector (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e5\u003c/span\u003ee).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe representation of stoichiometric variations in the model is relatively simple: different characteristic ratios are specified for different plankton, with small phytoplankton having higher C:P ratios than larger phytoplankton\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. This pattern would likely be reinforced by further unresolved elevated C:P ratios from greater investment in phytoplankton growth machinery in fertilized environments\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e, but could be counteracted by phosphate frugality as phosphate concentrations decline\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eDegrading Macronutrient Supply in the Equatorial Pacific\u003c/h3\u003e\n\u003cp\u003eAlthough the fertilized Equatorial Pacific initially sequesters some carbon, it exhibits minimal cumulative gains (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and steadily declining additionality in carbon uptake (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003ed–e) over the 21st century is evident. The weak overall response can be partly attributed to strong compensatory effects outside the core upwelling regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e), but the gradual decline in carbon uptake within the Equatorial Pacific requires further explanation. In the Equatorial Pacific, macronutrient remineralization below the euphotic zone is hindered by a shallow hypoxic layer. Particle export initiated by sustained fertilization persistently depletes macronutrient in the upper water that feeds equatorial upwelling. Depletion is notable for both phosphate and nitrate, but particularly severe for nitrate (Fig. S4). Further, the pronounced deficits of nitrate are amplified by denitrification in low oxygen waters\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. Consequently, the Equatorial Pacific exhausts its macronutrient reserves, steadily diminishing carbon-uptake efficacy productivity over time.\u003c/p\u003e"},{"header":"Summary and Discussion","content":"\u003cp\u003eDue to escalating atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration, mCDR methods have emerged in climate stabilization research aiming to address residual emissions of 2–3 PgC year\u003csup\u003e− 1\u003c/sup\u003e sustained over the next century\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Among these, iron fertilization—originally proposed over 30 years ago\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e—has been revisited in this study using a fully coupled Earth system model\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e to assess its additionality and durability under large-scale and long-term scenarios.\u003c/p\u003e\u003cp\u003eBy simulating carbon and biogeochemical responses across four major iron-limited HNLC regions, we find that large-scale, sustained iron fertilization yields only limited additionality and durability of carbon uptake compared with previous large-scale fertilization studies (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Most HNLC regions sequester a modest fraction of the petagram-scale target of climate stabilization scenarios. This suggests that while iron fertilization could contribute to CDR strategies, it must be implemented as part of a broader portfolio of CDR solutions. Furthermore, if fertilization is not continuously sustained, much of its benefit would dissipate. These limited benefits must also be weighed against established ecosystem risks\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe effectiveness of carbon drawdown by OIF is regulated by diverse biogeochemical responses to fertilization. As in prior studies, non-local compensatory responses in carbon uptake emerge due to reduced preformed nutrients. This was compounded by resurfacing of sequestered carbon, shifts in plankton stoichiometry, and macronutrient exhaustion in the equatorial Pacific. The most prominent difference between prior idealized simulations, however, was incomplete macronutrient drawdown in the Southern Ocean and North Pacific. Surface macronutrients in these regions arise in the model from a complex mixture of nutrient re-supply rates, top-down controls, and light limitation. Our simulation captures the observed seasonal macronutrient drawdown patterns reasonably well (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e4\u003c/span\u003e), but the drawdown expected from sustained iron fertilization is more difficult to assess against observations. We note that the nitrate drawdowns were comparable to those observed in smaller-scale fertilization experiments. (2–5 mmol/m\u003csup\u003e3\u003c/sup\u003e)\u003csup\u003e10,41\u003c/sup\u003e. In addition, the overall sequestration from sustained fertilization was similar in magnitude to the recent sustained iron fertilization study of Tagliabue et al., (2023)\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, which also directly simulated fertilization (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) and used a similarly comprehensive biogeochemical model\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eAn important limitation of the current modeling approach is its inability to resolve mesoscale and smaller-scale ocean heterogeneity, which may prevent complete representation of local decoupling between phytoplankton growth and grazing, as well as bloom intensity\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Future work using higher resolution models resolving small-scale processes may offer deeper insights into the extent of local nutrient drawdown in the Southern Ocean.\u003c/p\u003e\u003cp\u003eAnother key factor influencing carbon sequestration efficacy is the remineralization length scale, which governs how efficiently carbon is transferred from the surface ocean to the deep ocean\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e. However, current Earth System Models (ESMs) show considerable variability in their representation of vertical carbon flux transfer efficiency\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. This includes differences in modeling ballast effects, bacterial degradation, and the temperature- and oxygen-dependence of respiration\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e, all of which contribute to divergent remineralization responses to fertilization.\u003c/p\u003e\u003cp\u003eFurthermore, the representation of phytoplankton functional types (PFTs) and their associated stoichiometry vary widely among models\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. Dynamic phytoplankton stoichiometry may allow for the higher nutrient drawdown in response to fertilization\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e, resulting in the higher carbon uptake efficacies. Further insight into the CDR potential of sustained large-scale iron fertilization could be gained through the application of the standardized design herein across ESMs, and targeted analysis of top-down and bottom-up nutrient drawdown controls.\u003c/p\u003e\u003cp\u003eOur study synthesizes large-scale, long-term iron fertilization experiments, extending earlier findings. Initial idealized nutrient depletion studies suggested iron fertilization could explain glacial–interglacial CO\u003csub\u003e2\u003c/sub\u003e differences (~ 70–100 ppm; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). However, our results suggest that carbon uptake from OIF may be considerably smaller. A cross-regional analysis of four HNLC regions reveals: (1) small contributions from sustained iron fertilization in most regions, (2) moderate carbon sequestration potential in the Southern Ocean, albeit insufficient for petagram-scale climate stabilization targets, and (3) critical roles of biogeochemical processes that constrain additionality and durability. These findings provide guidance for evaluating the feasibility and scalability of mCDR strategies globally, suggesting that even large-scale fertilization would be insufficient to induce an ice-age-scale carbon drawdown.\u003c/p\u003e"},{"header":"Methods","content":"\u003ch3\u003eModel Configuration\u003c/h3\u003e\u003cp\u003eThis study utilized the Geophysical Fluid Dynamics Laboratory Earth System Model version 4.1 (GFDL-ESM4.1), a coupled carbon–chemistry–climate model\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. The atmospheric and land components are represented by AM4\u003csup\u003e50–52\u003c/sup\u003e and LM4\u003csup\u003e53\u003c/sup\u003e, respectively, both operating at a horizontal resolution of 100 km with 49 vertical levels. The ocean model, OM4p5, integrates GFDL’s Modular Ocean Model 6 (MOM6) for ocean physics and Sea Ice Simulator 2 (SIS2) for sea ice processes\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e, with a 50 km horizontal resolution and 75 hybrid depth–density vertical layers.\u003c/p\u003e\u003cp\u003eMarine biogeochemistry and food web dynamics are simulated using the Carbon, Ocean Biogeochemistry, and Lower Trophics version 2 (COBALTv2) model\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. COBALTv2 uses 33 tracers to represent key elements of the ocean carbon cycle, biogeochemistry, and plankton ecosystems representing carbonate systems (dissolved organic carbon, and alkalinity), five different nutrients (nitrogen, phosphorus, iron, oxygen, silicate), with explicit three phytoplankton (Diazotroph, small and large phytoplankton group), three zooplankton (small, medium and large zooplankton group), and bacteria. Notably, it includes explicit interactions between the iron cycle and various sources, such as rivers, sediments, geothermal vents, atmospheric deposition, and icebergs\u003csup\u003e\u003cspan additionalcitationids=\"CR56 CR57 CR58\" citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e–\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u003c/sup\u003e, and resolves energy transfer within plankton communities\u003csup\u003e\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e,\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eTo compare nitrate drawdown between observations and GFDL-ESM4.1 in fertilized ocean regions, we used nitrate datasets from the World Ocean Atlas 2023 (WOA23), which are based on in situ reanalysis\u003csup\u003e\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u003c/sup\u003e, to calculate the seasonal cycle of nitrate concentration. The WOA23 provides present-day climatological fields interpolated onto a \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:1^\\circ\\:\\times\\:1^\\circ\\:\\)\u003c/span\u003e\u003c/span\u003e latitude-longitude grid in each month. Nitrate drawdown, which reflects the potential for net biological production in each region, is typically estimated as the difference between the annual maximum and minimum nitrate concentrations.\u003c/p\u003e"},{"header":"Experiment Design","content":"\u003cp\u003eTo investigate the effectiveness of iron fertilization for marine carbon dioxide removal (mCDR), we conducted a series of targeted simulations. Large-scale fertilization experiments were designed for four high-nutrient, low-chlorophyll (HNLC) regions: the North Pacific (NP), Equatorial Pacific (EP), Northern Southern Ocean (NSO), and Southern Southern Ocean (SSO). A global fertilization experiment was also performed to estimate the upper limit of carbon uptake in the absence of iron limitation.\u003c/p\u003e\u003cp\u003eThe primary goal of these experiments was to quantify the maximum regional carbon uptake potential and to evaluate the limitations of previous small-scale patch fertilization (PSF) experiments. In all fertilization scenarios, artificial iron fluxes of approximately 0.02 mol Fe m⁻\u0026sup2; yr⁻\u0026sup1; were uniformly applied at the surface, a level sufficient to saturate iron limitation and maintain dissolved iron concentrations near 2 nM in the mixed layer. Sensitivity tests confirmed the adequacy of this flux rate. All other biogeochemical fluxes followed the configurations described in Stock et al. (2020)\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eWe conducted a control simulation without artificial iron addition, spanning the historical period (1990\u0026ndash;2014) and the future under SSP5-8.5 (2015\u0026ndash;2100), using emission-driven forcings (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), in accordance with the esm-hist and esm-ssp585 protocols from C4MIP\u003csup\u003e\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e. Two additional simulations were performed: (1) \u003cb\u003eFERT\u003c/b\u003e : Continuous iron release through 2100 to examine long-term fertilization effects (2) \u003cb\u003eSTOP\u003c/b\u003e : Iron release ceased after 30 years to assess the persistence and durability of fertilization impacts. The effects of fertilization were quantified by comparing these simulations with the control. Specifically, the accumulated response in air-sea CO2 flux (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\eta\\:\\)\u003c/span\u003e\u003c/span\u003e) was calculated as :\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\eta\\:={\\int\\:}_{{t}_{0}}^{t}{\\int\\:}_{A}\\left[{\\left(C{O}_{2}\\:Flux\\right)}_{OIF}-{\\left(C{O}_{2}\\:Flux\\right)}_{Control}\\:\\right]\\:dA\\:dt$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:A\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:t\\)\u003c/span\u003e\u003c/span\u003e denote area and time, respectively. Accumulated responses were computed separately over fertilized and non-fertilized areas to assess both local and global impacts. To quantify compensation effects, the compensation rate was calculated as the ratio of the CO\u003csub\u003e2\u003c/sub\u003e flux change in non-fertilized areas to that in fertilized areas. Uncertainty was estimated from the interannual variability of each variable.\u003c/p\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003eSelected Regions for the Iron-Fertilization\u003c/h2\u003e\u003cp\u003eHNLC regions\u0026mdash;characterized by abundant macronutrients but limited dissolved iron\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e\u0026mdash;have been the focus of several fertilization experiments\u003csup\u003e\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. These regions were selected to maximize carbon uptake responses to minimal iron additions. Efficacy may also depend on ocean circulation features such as vertical mixing and horizontal advection.\u003c/p\u003e\u003cp\u003eThe OIF forced areas were defined by first identifying regions with nitrate concentrations exceeding 1 \u0026micro;M\u003csup\u003e13,24\u003c/sup\u003e then selecting areas where phytoplankton were iron-limited in ESM4.1\u003csup\u003e23,64\u003c/sup\u003e. The Southern Ocean was further divided into northern and southern sectors based on the formation of intermediate and deep waters\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e\u003c/sup\u003e, delineated using the 26.8 kg m⁻\u0026sup3; isopycnal surface. This partitioning remains consistent with prior studies, despite known warm and fresh biases in the Southern Ocean in ESM4.1\u003csup\u003e22\u003c/sup\u003e. The iron fertilization areas selected in this way include four different regions; the Northern Pacific (NP), Equatorial Pacific (EP), Southern Ocean North (NSO) associated within the main Antarctic Circumpolar Current, and Southern Ocean South (SSO) associated with the ice-influenced region around Antarctica (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe source codes for GFDL-ESM4.1 can be accessed online via Github at the following link: https://github.com/NOAA-GFDL/ESM4. All figures were generated by using software package Python with the matplotlib and basemap modules (https://matplotlib.org/, https://matplotlib.org/basemap/). The map coastlines are derived by the Global Self-consistent, Hierarchical, High-resolution Geography (GSHHG) Database (www.soest.hawaii.edu/pwessel/gshhg/), which has been distributed under the GNU Lesser General Public License and is provided with the basemap Python module. The nitrate in WOA23 is provided freely at https://www.ncei.noaa.gov/access/world-ocean-atlas-2023/ by NOAA National Centers for Environmental Information.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis manuscript was prepared by Kyung-Min Noh under award NA23OAR4320198 from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the author(s) and do not necessarily reflect the views of the National Oceanic and Atmospheric Administration, or the U.S. Department of Commerce.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eUnited Nations Framework Convention on Climate Change (2015) Paris Agreement. FCCC/CP/2015/L.9/Rev.1\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eNational Academies of (2019) Sciences, Engineering, and Medicine. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. National Academies\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSabine CL et al (2004) The oceanic sink for anthropogenic CO\u003csub\u003e2\u003c/sub\u003e. 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NOAA Atlas NESDIS 92\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eJones CD et al (2016) C4MIP \u0026ndash; The Coupled Climate\u0026ndash;Carbon Cycle Model Intercomparison Project: Experimental protocol for CMIP6. Geosci Model Dev 9:2853\u0026ndash;2880\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDrenkard EJ et al (2023) The importance of dynamic iron deposition in projecting climate change impacts on Pacific Ocean biogeochemistry. \u003cem\u003eGeophys. Res. Lett.\u003c/em\u003e 50, e2022GL102058\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMorrison AK et al (2022) Ventilation of the Southern Ocean pycnocline. Annu Rev Mar Sci 14:405\u0026ndash;430\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAnderson LA, Sarmiento JL (1994) Redfield ratios of remineralization determined by nutrient data analysis. Global Biogeochem Cycles 8:65\u0026ndash;80\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7717531/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7717531/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eClimate stabilization pathways limiting global warming to 1.5-2\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:℃\\)\u003c/span\u003e\u003c/span\u003e targets require emission reductions with additional carbon dioxide removal (CDR) of ~\u0026thinsp;2\u0026ndash;3 PgC year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Here, we assess the long-term additionality and durability of carbon uptake from large-scale ocean iron fertilization (OIF) using an Earth System Model under emissions-driven scenarios. Our simulations suggest that sustained century-scale fertilization in the southern sector of the Southern Ocean (SSO) yields a moderate contribution (~\u0026thinsp;30PgC, or 0.3 PgC/yr). In other iron-limited regions, the additional carbon uptake induced by fertilization is largely offset (70\u0026ndash;100%) by the non-fertilized regions. Terminating OIF after 30 years, the ocean retains 50% of the additional carbon in the SSO, while retention becomes negligible in other regions. Global CDR rates are 2\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:-\\)\u003c/span\u003e\u003c/span\u003e7 times lower than prior idealized estimates. OIF-induced CDR is constrained by multiple biogeochemical processes: local and remote nutrient depletion, re-entrainment of previously sequestered carbon, incomplete macronutrient drawdown, reduced phytoplankton carbon to phosphorus ratios. Incomplete macronutrient drawdown represented the largest difference from earlier studies, reflecting uptake controls by light, top-down controls, and rates of macronutrient resupply. Further insight into CDR potential requires application of our standardized design across modeling institutions, and targeted analysis of top-down and bottom-up controls on nutrient drawdown.\u003c/p\u003e","manuscriptTitle":"Revisiting the Additionality and Durability of Carbon Uptake in Large-Scale Ocean Iron Fertilization","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-05 13:07:21","doi":"10.21203/rs.3.rs-7717531/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"81a1e9ed-de98-4e74-ad9f-913d03605f7e","owner":[],"postedDate":"November 5th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":56295800,"name":"Earth and environmental sciences/Biogeochemistry/Carbon cycle"},{"id":56295801,"name":"Earth and environmental sciences/Climate sciences/Ocean sciences/Marine biology"},{"id":56295802,"name":"Earth and environmental sciences/Climate sciences/Biogeochemistry/Carbon cycle"}],"tags":[],"updatedAt":"2025-11-05T13:07:21+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-05 13:07:21","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7717531","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7717531","identity":"rs-7717531","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}