Sequestration by the biological carbon pump: Do we really know what we are talking about?

The biological pump—the transfer of atmospheric carbon dioxide to the ocean interior and marine sediments as organic carbon—plays a critical role in regulating the long-term carbon cycle, atmospheric composition and climate. Despite its centrality in the Earth system, the response of the biological pump to biotic innovation and climatic fluctuations through most stages of Earth’s history has been largely conjectural. Here we use a mechanistic model of the biological carbon pump to revisit the factors controlling the transfer efficiency of carbon from surface waters to the ocean interior and marine sediments. We demonstrate that a shift from bacterioplankton-dominated to more eukaryote-rich ecosystems is unlikely to have considerably impacted the efficiency of Earth’s biological pump. In contrast, the evolution of large zooplankton capable of vertical movement in the water column would have enhanced carbon transfer into the ocean interior. However, the impact of zooplankton on the biological carbon pump is still relatively minor when compared with environmental drivers. In particular, increased ocean temperatures and greater atmospheric oxygen abundance lead to notable decreases in global organic carbon transfer efficiency. Taken together, our results call into question causative links between algal diversification and planetary oxygenation and suggest that climate perturbations in Earth’s history have played an important and underappreciated role in driving both carbon sequestration in the ocean interior and Earth surface oxygenation. Ocean temperature and atmospheric oxygen concentration are key factors in the long-term efficiency of the marine biological carbon pump, according to a mechanistic model of carbon transfer from surface waters to the deep ocean interior.

The biological pumping of carbon in the ocean and its effects on ocean carbon sequestration are being studied by researchers from several disciplines. These studies address the biological carbon pump (also called organic, soft-tissue, or biogeochemical carbon pump), the carbonate pump (or counter-pump) and the microbial carbon pump, which are most often treated separately. In the present study, the three pumps are treated together for the first time, under the generic name of “biologically-driven ocean carbon pumps”, in short, “bio-pumps”. The interactions among the carbon fluxes of three interwoven bio-pumps are summarized in the bio-pump jigsaw puzzle.The bio-pump literature presents a wide range of approaches to the nature and processes of carbon pumps and the ensuing carbon sequestration in the ocean, approaches that often differ significantly. It is shown that sequestration fluxes from all three bio-pumps can occur throughout the water column, albeit in different forms, and this “continuous vertical sequestration” concept is used to propose a target framework to simplify the research on bio-pumps, and unify the studies carried out by researchers from different disciplines. The review of the wide range of approaches to ocean biologically-driven carbon pumping and sequestration in the literature includes both a look back at the initial ocean carbon pump concept and an analysis of current approaches. It also includes estimates of century-scale (≥100 years) global sequestration fluxes in the water column by the three bio-pumps, which are about 1–3, 0.7 and 0.2 Pg C y–1 for the biological, carbonate and microbial pumps, respectively. The value of 0.7 Pg C y–1 appears to be the first ever published for the carbonate pump. The review is followed by detailed analysis of the functioning of the bio-pumps and their carbon sequestration processes, which is organized around four common components, i.e. downward fluxes of biogenic carbon from the upper ocean (i.e. export), transformation fluxes of the exported biogenic carbon in the lower ocean, carbon sequestration fluxes throughout the water column, and upward flux of dissolved inorganic carbon. It is recommended that future carbon-pump publications use representations that include these four components. This leads to a synthetic description of the processes involved in the three bio-pumps and their carbon sequestration, and a proposed unification of research on the interwoven bio-pumps.

The North Pacific (>20°N) stands out as a significant carbon sink, contributing to approximately 25% of the global oceanic CO2 uptake and absorbing around 0.5 Pg C yr-1 from the atmosphere. Despite the well-established importance of the biological carbon pump in maintaining this regional carbon sink, our current understanding of the strength and efficiency of the biological pump in this vast region remains incomplete. Historical studies have primarily relied on extrapolations from a limited number of observations.In this study, we utilize data from 85 BGC-floats, covering over 160 annual cycles, to constrain essential fluxes relevant to the biological pump in the North Pacific, including net primary production, the export of distinct biogenic carbon, and air-sea CO2 flux. Furthermore, we combine the output from a well-constrained regional ecosystem model (ROMS-CoSiNE-Iron Model) to gain mechanistic insights into how the food-web dynamics drive the strength and efficiency of the biological carbon pump across different ecosystems.Overall, our study offers an integrated perspective on the North Pacific biological pump by leveraging high-resolution observations from the BGC-float array and simulation from an improved ecosystem model.

The world’s oceans are a major sink for atmospheric carbon dioxide (CO2). The biological carbon pump plays a vital role in the net transfer of CO2 from the atmosphere to the oceans and then to the sediments, subsequently maintaining atmospheric CO2 at significantly lower levels than would be the case if it did not exist. The efficiency of the biological pump is a function of phytoplankton physiology and community structure, which are in turn governed by the physical and chemical conditions of the ocean. However, only a few studies have focused on the importance of phytoplankton community structure to the biological pump. Because global change is expected to influence carbon and nutrient availability, temperature and light (via stratification), an improved understanding of how phytoplankton community size structure will respond in the future is required to gain insight into the biological pump and the ability of the ocean to act as a long-term sink for atmospheric CO2. This review article aims to explore the potential impacts of predicted changes in global temperature and the carbonate system on phytoplankton cell size, species and elemental composition, so as to shed light on the ability of the biological pump to sequester carbon in the future ocean.

The marine biological carbon pump substantially contributes to the glacial-interglacial CO2 change. Compared to the late Holocene, proxy data for the Last Glacial Maximum (LGM) generally agree on an increased export production, associated with an enhanced marine biological carbon pump, in the subantarctic region of the Southern Ocean (SO). By contrast, global export production during the LGM is poorly constrained due to the sparseness and uncertainty of proxy data. The efficiency of the biological pump is mainly controlled by phytoplankton growth, ocean circulation and the sinking and remineralisation of organic matter. Previous modelling studies primarily focused on the sensitivity regarding the former two factors. By far, few studies have discussed the impact of marine particle sinking on glacial ocean biogeochemistry.In this study, we examine the impact of two different sinking schemes for biogenic particles on the LGM ocean biogeochemistry in the Max Planck Institute Earth System Model (MPI-ESM). In the default sinking scheme, sinking velocities of particulate organic matter (POM), biogenic minerals (CaCO3 and opal) and dust are prescribed and kept the same between LGM and pre-industrial (PI) state. Such a scheme is also widely applied in other ocean biogeochemical models. In a new Microstructure, Multiscale, Mechanistic, Marine Aggregates in the Global Ocean (M4AGO) sinking scheme, the size, microstructure, heterogeneous composition, density and porosity of marine aggregates, consisting of POM, CaCO3, opal and dust, are explicitly represented, and the sinking speed is prognostically computed. We discuss the effect of the two particle sinking schemes under two LGM circulation states: “deep LGM AMOC” with a similar NADW/AABW boundary compared to PI, which is produced in many existing models, and “shallow LGM AMOC” with a shallower NADW/AABW boundary, which agrees better with proxy data. Furthermore, we conducted sensitivity studies regarding LGM dust deposition as the latter is subject to considerable uncertainties.We find that for the deep LGM AMOC, the difference between the impact of the two particle sinking schemes on the ocean biogeochemical tracers is small. On the contrary, for shallow LGM AMOC, the M4AGO scheme yields more remerineralised carbon in the deep ocean and, therefore, better agreement with δ13C data, suggesting the quantitative impact of particle sinking schemes strongly depends on the background LGM circulation state. For the default sinking scheme, increased glacial dust deposition increases iron fertilisation and thus leads to a rise in both primary production and export production. For the M4AGO scheme, however, the iron fertilisation effect is surpassed by the ballasting effect that reduces the surface nutrient concentration, and LGM primary production decreases with dust deposition. This preliminary result shows that the new marine aggregate sinking scheme adds further complexities to the marine biological carbon pump response to the climate states. Our further analysis will encompass the other nutrients and dissolved oxygen, as well as the comparison to corresponding proxy data. 

As ocean Carbon Dioxide Removal techniques are being considered, it is critical that they be evaluated against our scientific understanding of the global biological carbon pump. In a recent paper Nowicki et al. (2022, https://doi.org/10.1029/2021GB007083) provide an innovative and comprehensive breakdown of the different mechanistic pathways of carbon sequestration through the present‐day biological pump but then speculate that “These results suggest that ocean carbon storage will weaken as the oceans stratify and the subtropical gyres expand due to anthropogenic climate change.” Essentially, the authors combine their steady state result that oligotrophic subtropical gyres have lower residence times than other areas with the climate change result of these areas increasing under climate warming and extrapolate—assuming “all else is equal”—that the overall ocean will suffer a reduction in carbon sequestration efficiency. Expressing global changes in carbon sequestered by the ocean's biological pump as the summation of local changes in the sequestered carbon, timescale of return to the surface, and biogeographical area, I discuss how all three terms are tightly coupled, and summarize decades of climate change modeling consistently indicating that the global scale physical sequestration response is an increase ‐ in opposition of what one would infer from changes in subtropical area alone.

Every year, large numbers of zooplankton migrate from the surface ocean to depths of 500–2000 m to hibernate. Through this migration, they actively transport organic carbon to the deep ocean, where it is used to fuel metabolic needs. This active transport of carbon is thought to be highly efficient, as carbon metabolized by copepods is directly injected deep into the ocean's interior. The significance of this process in view of global carbon cycling remains an open question. Here, we focus on five representative, diapausing copepod species (Calanus finmarchicus, Calanus hyperboreus, Calanoides acutus, Calanoides natalis, and Neocalanus tonsus) distributed in the Arctic, Atlantic, Indian, and Southern Oceans. For each species, we compute both carbon injection (how much carbon is transported below the euphotic zone during zooplankton migration and left there as dissolved inorganic carbon) and carbon sequestration (the amount of carbon stored in the ocean's interior following diapausing zooplankton‐mediated injection). In total, the five species considered here contribute 0.4–0.8% of total biological carbon export, and 0.8–3.3% of total carbon sequestration mediated by the biological pump (assuming a total carbon export of ~ 10 PgC yr−1 and sequestration of ~ 1300 PgC). Including other species in this inventory would increase the contribution of diapausing copepods to the biological carbon pump, but requires more precise estimates of copepods' distribution, abundance, and metabolic requirements.

Once fixed by photosynthesis carbon becomes part of the marine food web. The fate of this carbon has two possible outcomes, it may be respired and released back to the ocean and potentially to the atmosphere as CO2 or retained in the ocean interior and/or marine sediments for extended time scales. The most important biologically mediated processes responsible for long-term carbon storage in the ocean are the biological carbon pump (BCP) and the microbial carbon pump (MCP). While acting simultaneously in the ocean, the balance between these two mechanisms is thought to vary depending on the trophic state of the environment. Using previously published formulations, we propose a modelling framework to simulate variability in the MCP:BCP ratio as a function of external nutrients. Our results suggest that the role of the MCP might become more significant under future climate change conditions where increased stratification enhances the oligotrophic nature of the surface ocean. Based on these model results, we propose a conceptual framework in which the internal stoichiometry of phytoplankton, modulating both grazing pressure and dissolved organic matter production (via phytoplankton exudation), plays a crucial role in regulating the MCP:BCP ratio.

Abstract. Increased accumulation of respired carbon in the deep ocean associated with enhanced efficiency of the biological carbon pump is thought to be a key mechanism of glacial CO2 drawdown. Despite greater oxygen solubility due to seawater cooling, recent quantitative and qualitative proxy data show glacial deep-water deoxygenation, reflecting increased respired carbon accumulation. However, the mechanisms of deep-water deoxygenation and contribution from the biological pump to glacial CO2 drawdown have remained unclear. In this study, we report the significance of iron fertilization from glaciogenic dust in glacial CO2 decrease and deep-water deoxygenation using our numerical simulation, which successfully reproduces the magnitude and large-scale pattern of the observed oxygen changes from the present to the Last Glacial Maximum. Sensitivity experiments show that physical changes contribute to only one-half of all glacial deep deoxygenation, whereas the other one-half is driven by iron fertilization and an increase in the whole ocean nutrient inventory. We find that iron input from glaciogenic dust with higher iron solubility is the most significant factor in enhancing the biological pump and deep-water deoxygenation. Glacial deep-water deoxygenation expands the hypoxic waters in the deep Pacific and Indian oceans. The simulated global volume of hypoxic waters is nearly double the present value, suggesting that glacial deep water was a more severe environment for benthic animals than that of the modern oceans. Our model underestimates the deoxygenation in the deep Southern Ocean because of enhanced ventilation. The model–proxy comparison of oxygen change suggests that a stratified Southern Ocean is required for reproducing the oxygen decrease in the deep Southern Ocean. Iron fertilization and a global nutrient increase contribute to a decrease in glacial CO2 of more than 30 ppm, which is supported by the model–proxy agreement of oxygen change. Our findings confirm the significance of the biological pump in glacial CO2 drawdown and deoxygenation.

Abstract. The daily vertical migrations of fish and other metazoans actively transport organic carbon from the ocean surface to depth, contributing to the biological carbon pump. We use an oxygen-constrained, game-theoretic food-web model to simulate diel vertical migrations and estimate near-global (global ocean minus coastal areas and high latitudes) carbon fluxes and sequestration by fish and zooplankton due to respiration, fecal pellets, and deadfalls. Our model provides estimates of the carbon export and sequestration potential for a range of pelagic functional groups, despite uncertain biomass estimates of some functional groups. While the export production of metazoans and fish is modest (∼20 % of global total), we estimate that their contribution to carbon sequestered by the biological pump (∼800 PgC) is conservatively more than 50 % of the estimated global total (∼1300 PgC) and that they have a significantly longer sequestration timescale (∼250 years) than previously reported for other components of the biological pump. Fish and multicellular zooplankton contribute about equally to this sequestered carbon pool. This essential ecosystem service could be at risk from both unregulated fishing on the high seas and ocean deoxygenation due to climate change.

We develop novel locally defined diagnostics for the efficiency of the ocean's biological pump by tracing carbon throughout its lifetime in the ocean from gas injection to outgassing and counting the number of passages through the soft‐tissue and carbonate pumps. These diagnostics reveal that the biological pump's key controls on atmospheric pCO2 are the mean number of lifetime pump passages per dissolved inorganic carbon (DIC) molecule at the surface and the mean aphotic sequestration time of regenerated DIC. We apply our diagnostics to an observationally constrained carbon‐cycle model that features spatially varying stoichiometric ratios and is embedded in a data‐assimilated global ocean circulation. We find that for the present‐day ocean an average of 44 ± 4% of DIC in a given water parcel makes at least one lifetime passage through the soft tissue pump, and about 4% makes at least one passage through the carbonate pump. The global mean number of lifetime pump passages per molecule, including the fraction with zero passages, is and for the soft‐tissue and carbonate pumps. Using idealized perturbations to sweep out a sequence of states ranging from zero biological activity ( ppmv) to complete surface nutrient depletion ( ppmv), we find that fractional changes in are dominated by fractional changes in the number of soft‐tissue pump passages. At complete surface nutrient depletion, the mean fraction of DIC that has at least one lifetime passage through the soft‐tissue pump increases to 69 ± 5% with .

A central component of the ocean's biological carbon pump is the export of sinking, photosynthetically derived, particulate organic carbon (POC). Bacteria colonize these particles and produce enzymes that hydrolyse sinking POC thereby acting as one of the major controls on the biological pump. Here we provide evidence that a bacterial cell-cell communication mechanism, quorum sensing (QS), may influence the activity of hydrolytic enzymes on sinking particles. We collected sinking POC from a site off Vancouver Island, Canada and found that it contained acylated homoserine lactones (AHLs), a suite of well-known bacterial communication molecules. Furthermore, we observed that the addition of exogenous AHLs to incubations containing sinking POC affected the activity of key hydrolytic enzymes involved in POC degradation in some cases. Our results suggest that AHL-based QS could play an important role in regulating the degradation of sinking POC and that variability in AHL-triggered POC hydrolysis is a heretofore unrecognized process that impacts the marine biological carbon pump.

In the event of insufficient mitigation efforts, net-negative CO2 emissions may be required to return climate warming to acceptable limits as defined by the Paris Agreement. The ocean acts as an important carbon sink under increasing atmospheric CO2 levels when the physico-chemical uptake of carbon dominates. However, the processes that govern the marine carbon sink under net-negative CO2 emission regimes are unclear. Here we assessed changes in marine CO2 uptake and storage mechanisms under a range of idealized temperature-overshoot scenarios using an Earth system model of intermediate complexity over centennial timescales. We show that while the fate of CO2 from physico-chemical uptake is very sensitive to future atmospheric boundary conditions and CO2 is partly lost from the ocean at times of net-negative CO2 emissions, storage associated with the biological carbon pump continues to increase and may even dominate marine excess CO2 storage on multi-centennial timescales. Our findings imply that excess carbon that is attributable to the biological carbon pump needs to be considered carefully when quantifying and projecting changes in the marine carbon sink.

Marine organisms, from plankton to fish, provide a wealth of ecosystem services, including carbon sequestration in a process known as the ocean’s biological carbon pump (BCP). The BCP brings carbon from the atmosphere to the ocean depths where it is stored for decades to centuries. Although parts of the ocean’s BCP are under threat from human activities,  BCP carbon sequestration rarely features as an objective in efforts to protect ocean spaces. Moreover, although BCP carbon sequestration services could support discussions of conservation and climate finance,  its economic value has yet to be estimated in space and time.Biogeochemical modeling and mapping efforts have grown in recent years, and emerging results could potentially help to fill in important spatially explicit and economic knowledge gaps that could inform the protection of the BCP. We developed a new metric to map and quantify the global ocean’s BCP long-term carbon sequestration and computed its value on a potential carbon market. We show the  global spatial patterns and valuation in relation to geopolitical and management boundaries, and highlight options for governance and management. Our results highlight potential opportunities for preserving the climate services of the BCP both nationally and in areas beyond national jurisdiction , and can be used to inform discussions about marine protected areas, environmental impact assessment, and conservation finance.

Marine organisms, from plankton to fish, provide a wealth of ecosystem services, including carbon sequestration in a process known as the ocean’s biological carbon pump (BCP). The BCP brings carbon from the atmosphere to the ocean depths where it is stored for decades to centuries. Although parts of the ocean’s BCP are under threat from human activities, BCP carbon sequestration rarely features as an objective in efforts to protect ocean spaces. Moreover, although BCP carbon sequestration services could support discussions of conservation and climate finance, its economic value has yet to be estimated in space and time and thus the BCP has not been discussed in relation to the blue economy.We performed a spatial analysis and financial valuation of the carbon pump service in relation to geo-political and management boundaries. We developed a new metric to map and quantify the global ocean’s BCP long-term carbon sequestration accounting for the carbon that remains stored in the ocean’s interior for more than 50 years, what we call 50-year carbon sequestration rate. We show the global spatial patterns and valuation in relation to geopolitical and management boundaries, and highlight options for governance and management. We estimate that, annually, the biological carbon pump adds in the ocean 2.81 Gt of carbon (range 2.44 - 3.53) with a storage time of at least 50 years (±25 years). This ecosystem service is worth US$383 billion/year (range 336 - 471) within all Exclusive Economic Zones, US$545 billion/year (range 471 - 694) in areas beyond national jurisdiction, and US$2.2 trillion (range 1.9 - 2.7; sum of discounted values for 2023-2030). These results quantify the climate and economic importance of the biological carbon pump and the important role of Small Island Developing Nations in carbon sequestration. These findings can support discussions in climate finance and in the COP global stocktake for climate action for developing a more equitable and sustainable ocean economy through opportunities for preserving the climate services of the BCP both nationally and in Areas Beyond National Jurisdiction.