ocean carbon uptake and storage :: Ocean Carbon & Biogeochemistry

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Archive for ocean carbon uptake and storage

The ocean is the largest natural carbon sink for atmospheric CO2

Posted by mmaheigan

· Friday, January 23rd, 2026

Only about half of human-made CO2 emissions remain in the atmosphere and drive global warming. The other half has so far been said to be taken up in roughly equal amounts by the biosphere on land and by physical-chemical processes in the ocean. In equal amounts?

In a new assessment, Friedlingstein et al. reassess the various components of the Global Carbon Budget. Major changes were suggested for the land and ocean sinks. For the land, the prior assumption of a preindustrial land-cover in the Dynamic Global Vegetation Models (DGVM) led to an overestimation of the natural land sink in previous studies. The land sink is further revised downwards by accounting for an anthropogenic perturbation of lateral carbon export to the ocean. For the ocean, adjustments were made for the known underestimation of the ocean sink from Global Ocean Biogeochemical Models and the cool and salty skin effect in surface fCO₂-observation-based estimates. As a result, the ocean is now estimated to have taken up 29% of anthropogenic CO₂ emissions in the last decade 2015-2024, while the land sink has taken up 21%. In this revised estimate with virtually no budget imbalance over the last decade and no significant trend in the budget imbalance since 1960, climate-driven impacts on the natural sinks are quantified: Land and ocean sinks would be 25% and 7% higher, respectively, without this carbon-climate feedback. Since 1960, the carbon-climate feedback has already contributed 8 ppm (8%) to the rise in atmospheric CO₂ concentration.

The negative imprints of earth system changes (e.g., warming, droughts, changes in wind patterns and ocean circulation, etc.) on these important carbon sinks is worrisome and is expected to intensify as warming continues. The most effective way to protect these sinks is to drastically reduce CO₂ emissions from fossil fuels and land-use changes, ultimately to net zero.

Authors
Judith Hauck (Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, University of Bremen)
Peter Landschützer (VLIZ)
Corinne Le Quéré (University of East Anglia)
Pierre Friedlingstein (University of Exeter)

Bluesky: @pfriedling @jhauck @clequere

A heat burp breaks the assumed relationship of cumulative CO2 emissions and warming

Posted by mmaheigan

· Friday, January 23rd, 2026

The ocean stores vast amounts of heat and carbon under anthropogenic CO₂ emissions, but its behavior under net-negative emission scenarios remains poorly understood. Here we use an Earth System Model of intermediate complexity and show results of an idealized future climate scenario that includes sustained net-negative emissions over centuries. After gradual global cooling, the model produces an abrupt “heat burp,” in which heat previously stored in the deep Southern Ocean resurfaces through deep convection, temporarily reversing the cooling and causing renewed warming. The release of heat is not accompanied by a comparable release of CO₂. The heat burp represents a breakdown of the assumed linear relationship between cumulative CO₂ emissions and warming, a metric that is used to calculate the remaining carbon budget. We call for assessing the robustness of how models forced with net-negative CO₂ emissions simulate durability of ocean storage of heat and CO₂, and pathways and time scales of loss to the atmosphere.

Fig caption: The temporal evolution of (a) global heat and carbon uptake and release; (b) surface air temperature (SAT) anomaly relative to preindustrial conditions; (c) Southern Ocean temperature anomaly relative to preindustrial conditions; gray shading/black bar indicate the period of comparatively abrupt ocean heat release that warms SAT, representing a climate feedback.

Authors
(all at GEOMAR)

Ivy Frenger
Svenja Frey (and Univ Copenhagen)
Andreas Oschlies
Julia Getzlaff
Torge Martin
Wolfgang Koeve

Frenger, I., Frey, S., Oschlies, A., Getzlaff, J., Martin, T., & Koeve, W. (2025). Southern Ocean heat burp in a cooling world. AGU Advances, 6, e2025AV001700. https://doi.org/10.1029/2025AV001700

A Microbial Conveyor Belt Beneath the South Pacific

Posted by mmaheigan

· Friday, October 17th, 2025

Global overturning circulation is a planetary conveyor belt: dense waters sink around Antarctica, spread through the deep ocean for centuries, and eventually rise elsewhere, redistributing heat, nutrients, and carbon. But how does this slow, pervasive movement of water impact marine microbes?

To find out, researchers collected over 300 water samples spanning the full depth of the ocean along the GO-SHIP P18 line in the South Pacific. They found that microbial genomes cluster into six spatial cohorts that are not only delineated by depth, but also circulatory features, like Antarctic Bottom Water formation, and ventilation age. Distinct functional signatures also emerged across these circulation-driven zones. For example, genes for light harvesting and iron uptake dominate in surface waters, while adaptations for cold, high pressure, or anaerobic metabolism characterize deep and ancient waters. Antarctic Bottom Water communities also carry hallmarks of rapid genetic exchange, suggesting horizontal gene transfer may help microbes adapt as they sink into the deep ocean. Even in waters isolated from the atmosphere for over a thousand years, many microbial genomes have coverage patterns that imply active replication, demonstrating that long-isolated water masses still support active microbial populations. In considering patterns of microbial diversity, researchers also identified a pervasive “prokaryotic phylocline” in which richness spikes just below the surface mixed layer and remains high to full ocean depth, only dipping slightly in very old water.

These results demonstrate that physical circulation, not just temperature or nutrients, partitions the ocean into microbial biomes. Understanding this linkage is critical because microbes determine the amount of carbon that is recycled or stored long-term in the deep ocean. As climate change alters overturning circulation, the functioning of these hidden microbial ecosystems and their role in regulating atmospheric CO₂ may shift in unexpected ways.

Authors
Bethany C. Kolody (University of California San Diego; UC Berkeley; J. Craig Venter Institute)
Rohan Sachdeva (UC Berkeley)
Hong Zheng (J. Craig Venter Institute)
Zoltán Füssy (UC San Diego; J. Craig Venter Institute)
Eunice Tsang (UC Berkeley)
Rolf E. Sonnerup (University of Washington)
Sarah G. Purkey (UC San Diego)
Eric E. Allen (UC San Diego)
Jillian F. Banfield (UC Berkeley; Lawrence Berkeley National Laboratory; Monash University)
Andrew E. Allen (UC San Diego; JCVI)

Social media
Twitter/X: @science_doodles, @Scripps_Ocean, @JCVenterInst
Bluesky: @banfieldlab.bsky.social, @bethanykolody.bsky.social, @scrippsocean.bsky.social, @jcvi.org

https://www.science.org/doi/10.1126/science.adv6903
Overturning circulation structures the microbial functional seascape of the South Pacific
Science

Marine plant metabolites give marine microbes gas

Posted by mmaheigan

· Friday, October 17th, 2025

A recent study in Nature Geosciences observed high concentrations of methane overlying permeable (sand) sand sediments in bays in Denmark and Australia. These environments are not one would expect to see methane because they are highly oxygenated and the high concentrations of sulfate in seawater typically inhibit methanogenesis. The authors showed that the methane was not being imported from local groundwater using geochemical methods. Using a combination of biogeochemical, microbial isolation, culturing and genomic approaches, revealed that methane was being produced by fast growing microbes resistant to oxygen exposure using plant produced substrates such as dimethylsulfide and amines. This work shows that where marine plants such as seaweed and seagrass grow and accumulate there may be high and sporadic production of methane. This has implications for how we account for the carbon sequestering capacity of coastal environments and the climate impact of increasing algal blooms such as coastal Ulva and the great sargassum bloom.

Authors
Perran Cook (Monash University)
Ning Hall (University of free spirit)

How does a persistent eddy impact the biological carbon pump?

Posted by mmaheigan

· Friday, September 26th, 2025

The Lofoten Basin Eddy (LBE) is a unique and persistent anticyclonic feature of the Norwegian Sea that stirs the water column year-round. However, its impact on biogeochemical processes that influence region carbon storage, including carbon fixation, particle aggregation and fragmentation, and remineralization, has remained largely unknown.

Figure caption: (a) Map of the Lofoten Basin Eddy study region including locations of 1886 profiles from 22 Biogeochemical-Argo floats (2010–2022) and a heatmap showing the relative extent of the LBE influence zone over the timeseries. (b–d) Mean monthly profiles and the difference (Δ) determined as inside minus outside the LBE influence zone of the mass concentration of particulate organic carbon in small particles (POC_s). Arrows indicate key mechanisms regulating the regional biological carbon.

Using 12 years of data from Biogeochemical-Argo floats and satellite altimetry to track eddy movements, Koestner et al. (2025) examined how the LBE influences the seasonal transport of organic carbon from surface waters to the deep ocean. While the LBE can enhance carbon export during certain months, like during spring shoaling and late autumn subduction, it generally reduces the efficiency of the biological carbon pump. Inside the eddy, warmer subsurface waters and slower-sinking particles often lead to more respiration and remineralization, meaning less carbon reached the deep sea.

The LBE’s persistent influence on organic carbon cycling could affect regional climate feedbacks and marine ecosystems, including key fisheries in Norway. Understanding how features like the LBE modulate carbon sequestration is vital for improving climate models and managing ocean resources in a warming Arctic.

Authors
Daniel Koestner (University of Bergen)
Sophie Clayton (National Oceanography Centre)
Paul Lerner (Columbia University)
Alexandra E. Jones-Kellett (MIT & WHOI)
Stevie L. Walker (University of Washington)

Microbial Iron limitation in the ocean’s twilight zone

Posted by mmaheigan

· Monday, March 31st, 2025

How deep in the ocean do microbes feel the effects of nutrient limitation? Microbial production in one third of the surface ocean is limited by the essential micronutrient iron (Fe). This limitation extends to at least the bottom of the euphotic zone, but what happens below that?

In a study that recently published in Nature we investigated the abundance and distribution of siderophores, small metabolites synthesized by bacteria to promote Fe uptake. When environmental Fe concentrations become limiting and microbes become Fe deficient, some bacteria release siderophores into the environment to bind iron and facilitate its uptake. Siderophores are therefore a window into how microbes “see” environmental Fe. We found that siderophore concentrations were high in low Fe surface waters, but surprisingly we also found siderophores to be abundant in the twilight zone (200-500 m) underlying the North and South Pacific subtropical gyres, two key ecosystems for the marine carbon cycle. In shipboard experiments with siderophores labeled with the rare ⁵⁷Fe isotope, we found rapid uptake of the label in twilight zone samples. After removing ⁵⁷Fe from the ⁵⁷Fe-siderophores complex, bacteria released the now unlabeled siderophores back into seawater to complex additional Fe (Figure. 1).

Figure 1: Iron-siderophore cycling in the twilight zone. When the seawater becomes Fe-deficient, some bacteria are able to synthesize siderophores and release them into the environment (middle left). These metabolites bind Fe (middle right) and the Fe-siderophore complex is taken up by bacteria using specialized TonB dependent transporters (TBDT; bottom right). Inside the cell, Fe is recovered from the Fe-siderophore complex (bottom left) and the siderophore excreted back into the environment to start the cycle anew.

Our results show that in large parts of the ocean microbes feel the effects of nutrient limitation deep in the water column, to at least 500 m. This greatly expands the region of the ocean where nutrients limit microbial metabolism. The effects of limitation this deep in the water column are unexplored, but twilight zone Fe deficiency could have unanticipated consequences for the efficiency of the ocean’s biological carbon pump.

Authors
Jingxuan Li, Lydia Babcock-Adams and Daniel Repeta
(all at Woods Hole Oceanographic Institution)

Persistent bottom trawling impairs seafloor carbon sequestration

Posted by mmaheigan

· Friday, February 28th, 2025

Bottom trawling, a fishing method that uses heavy nets to catch animals that live on and in the seafloor, could release a large amount of organic carbon from seafloor into the water, that metabolizes to CO₂ then outgasses to the atmosphere. The magnitude of this indirect emission has been heavily debated, with estimates spanning from negligibly small to global climate relevant. Thus, a lack of reliable data and insufficient understanding of the process hinders management of bottom trawling for climate protection.

We set out to solve this problem in two steps. First, we analyzed a large field dataset containing more than 2000 sediment samples from one of the most intensely trawled regions globally, the North Sea. We identified a trawling-induced carbon reduction trend in the data, but only in samples taken in persistently intensively trawled areas with multi-year averaged swept area ratio larger than 1 yr^-1. In less intensely trawled areas, there was no clear effect. In a second step, we applied numerical modelling to understand the processes behind the observed change (Fig. 1). Our model results suggest that bottom trawling annually releases one million tonnes of CO₂ in the North Sea and 30 million tonnes globally. Along with sediment resuspension in the wake of the trawls, the main cause for altered sedimentary carbon storage is the depletion of macrofauna, whose locomotion and burrowing effectively buries freshly deposited carbon into deeper sediment layers. By contrast, macrofauna respiration is reduced owing to trawling-caused mortality, partly offsetting the organic carbon loss. Following a cessation of trawling, the simulated benthic biomass can recover in a few years, but the sediment carbon stock would take several decades to be restored to its natural state.

Figure 1. (a) Benthic–pelagic coupling in a natural system. (b) Processes involved in bottom trawling. (c) Model-estimated source and sink terms of organic carbon in surface sediments in the No-trawling (solid fill, n = 67 annual values for 1950–2016) and trawling (pattern fill, n = 67 ensemble-averaged values for 1950–2016) scenarios of the North Sea. © 2024, Zhang, W. et al., CC BY 4.0.

Marine conservation strategies traditionally favor hard bottoms, such as reefs, that are biologically diverse but accumulate limited amounts of organic carbon. Our results indicate that carbon in muddy sediments is more susceptible to trawling impacts than carbon in sand and point out a need to safeguard muddy habitats for climate protection. Our methods and results might be used in the context of marine spatial planning policies to gauge the potential benefits of limiting or ending bottom trawling within protected areas.

Zhang, W., Porz, L., Yilmaz, R. et al. Long-term carbon storage in shelf sea sediments reduced by intensive bottom trawling. Nat. Geosci. 17, 1268–1276 (2024). https://doi.org/10.1038/s41561-024-01581-4

Authors
Wenyan Zhang (Hereon)
Lucas Porz (Hereon)
Rümeysa Yilmaz (Hereon)
Klaus Wallmann (GEOMAR)
Timo Spiegel (GEOMAR)
Andreas Neumann (Hereon)
Moritz Holtappels (AWI)
Sabine Kasten (AWI)
Jannis Kuhlmann (BUND)
Nadja Ziebarth (BUND)
Bettina Taylor (BUND)
Ha Thi Minh Ho-Hagemann (Hereon)
Frank-Detlef Bockelmann (Hereon)
Ute Daewel (Hereon)
Lea Bernhardt (HWWI)
Corinna Schrum (Hereon)

OA could boost carbon export by appendicularia

Posted by mmaheigan

· Wednesday, December 4th, 2024

Gelatinous zooplankton comprise a widespread group of animals that are increasingly recognized as important components of pelagic ecosystems. Historically understudied, we have little knowledge of how much key taxa contribute to carbon fluxes. Likewise, there’s a critical knowledge gap of the impact of ocean change on these taxa.

Appendicularia are the most abundant gelatinous zooplankton in the world oceans. Their population dynamics display typical boom-and-bust characteristics, i.e. high grazing rates in combination with a short generation time and life cycle, results in intense blooms. The most prominent feature of appendicularians is their mucous feeding-structure (“house”), which is produced and discarded several times per day. These sinking houses can contribute substantially to carbon export.

Figure 1: Influence of ocean acidification on the Appendicularia Oikopleura dioica and carbon export. Appendicularian populations display typical boom-and-bust characteristics, resulting in intense blooms. The sinking of appendicularians’ discarded mucous feeding-structure several times per day can contribute substantially to carbon export. Low pH conditions (as expected for future ocean acidification extreme events) enhanced its population growth and contribution to carbon fluxes shown above (red lines/diamonds) vs ambient (blue lines/diamonds).
(Figure sources: Picture by Jean-Marie Bouquet, data plots from Taucher et al. (2024): The appendicularian Oikopleura dioica can enhance carbon export in a high CO2 ocean. Global Change Biology, doi:10.1111/gcb.17020)

A recent study in Global Change Biology quantified how much appendicularia can contribute to carbon export via the biological pump, and how this carbon flux could markedly increase under future ocean acidification and associated extreme pH events.

The findings are based on a large-volume in situ experimental approach that allowed observing natural plankton populations and carbon export under close-to-natural conditions for almost two months. Thereby, O. dioica population dynamics could be directly linked to sediment trap data to quantify the influence of this key species on carbon fluxes at unprecedented detail. During the appendicularia bloom up to 39% of total carbon export was attributed to them.

The most striking finding was that high CO₂ conditions elevated carbon export by appendicularia increased by roughly 50%. Appendicularians physiologically benefit from low pH conditions, giving them a competitive advantage over other zooplankton, allowing them to contribute to a disproportionally large role in carbon export from the ecosystem.

Authors
Jan Taucher (GEOMAR)
Anna Katharina Lechtenbörger (GEOMAR)
Jean-Marie Bouquet (University of Bergen)
Carsten Spisla (GEOMAR)
Tim Boxhammer (GEOMAR)
Fabrizio Minutolo (GEOMAR)
Lennart Thomas Bach (University of Tasmania)
Kai T. Lohbeck (University of Konstanz)
Michael Sswat (GEOMAR)
Isabel Dörner (GEOMAR)
Stefanie M. H. Ismar-Rebitz (GEOMAR)
Eric M. Thompson (University of Bergen)
Ulf Riebesell (GEOMAR)

The fate of the 21st century marine carbon cycle could hinge on zooplankton’s appetite

Posted by mmaheigan

· Wednesday, September 11th, 2024

Both climate change and the efforts to abate have the potential to reshape phytoplankton community composition, globally. Shallower mixed layers in a warming ocean and many marine CO₂ removal (CDR) technologies will shift the balance of light, nutrients, and carbonate chemistry, benefiting certain species over others. We must understand how such shifts could ripple through the marine carbon cycle and modify the ocean carbon reservoir. Two new publications in Geophysical Research Letters and Global Biogeochemical Cycles highlight an often over looked pathway in this response: The appetite of zooplankton.

We have long known that the appetite of zooplankton—i.e. the half-saturation concertation for grazing—varies dramatically. This variability is largely based on laboratory incubations of specific species. An open-ocean perspective has been much more elusive. Using two independent inverse modelling approaches, both studies reached the same conclusion: Even at the community level, the appetite of zooplankton in the open-ocean is incredibly diverse.

Moreover, variability in zooplankton appetites maps well onto the biogeography of phytoplankton species. As these phytoplankton niches evolve, the composition of the zooplankton will likely follow. To help understand the impact of this response on the biological pump, we compared two models, one with only two types of zooplankton, and another with an unlimited amount, each with different appetites, all individually tuned to their unique environment. Including more realistic diversity reduced the strength of the biological pump by 1 PgC yr^-1.

Figure Caption. A) Variability in the abundance and characteristic composition of phytoplankton drives B) large differences in the associated appetite and characteristic composition of zooplankton in two independent inverse modelling studies. C) When more realistic diversity in the appetite of zooplankton is simulated in models, the strength of biological pump is dramatically reduced.

That is the same order as the most optimistic scenarios for ocean iron fertilization. This means that when simulating the efficacy of many CDR scenarios, the bias introduced by insufficiently resolved zooplankton diversity could be just as large as the signal. Moving forward, it is imperative to improve the representation of zooplankton in Earth System Models to understand how the marine carbon sink will respond to inadvertent and deliberate perturbations.

Authors (GRL):
Tyler Rohr (The University of Tasmania; Australian Antarctic Program Partnership)
Anthony Richardson (The University of Queensland; CSIRO)
Andrew Lenton (CSIRO)
Matthew Chamberlain (CSIRO)
Elizabeth Shadwick (Australian Antarctic Program Partnership; CSIRO)

Authors (GBC):
Sophie Meyjes (Cambridge)
Colleen Petrick (Scripps Institute of Oceanography)
Tyler Rohr (The University of Tasmania; Australian Antarctic Program Partnership)
B.B. Cael (NOC)
Ali Mashayek (Cambridge)

Inorganic carbon outwelling as important blue carbon sink

Posted by mmaheigan

· Wednesday, May 29th, 2024

Blue carbon ecosystems—mangroves, saltmarshes, and seagrass meadows—carbon sequestration powerhouses that can help us mitigate climate change. For many years, our community has focused on studying and quantifying organic carbon storage in the soils of these ecosystems and crediting it as Blue Carbon in carbon markets.

A new paper in Nature Communications reveals that much of that carbon sequestered by mangroves and saltmarshes is actually exported as inorganic carbon to the ocean. Inorganic carbon export dominates blue carbon budgets and rivals or even surpasses carbon stored in soils. Inorganic carbon exports had an alkalinity: dissolved inorganic carbon ratio of 0.8 ± 0.2, impacting the carbonate system and carbon cycling along the coast. Most of the inorganic carbon is exported as bicarbonate which stays permanently dissolved in the ocean and is therefore a permanent atmospheric carbon sink. When we ignore inorganic carbon export, we highly underestimate the potential of mangroves and saltmarshes to mitigate climate change. Consequently, inorganic carbon export should be integrated into blue carbon frameworks to adequately inform carbon markets, which encourage landowners to restore and preserve mangrove and saltmarsh ecosystems.

Authors
Gloria M. S. Reithmaier (University of Gothenburg) Twitter: @GReithmaier@Barefoot_Lab
Alex Cabral (University of Gothenburg)
Anirban Akhand (Hong Kong University of Science and Technology)
Matthew J. Bogard (University of Lethbridge)
Alberto V. Borges (University of Liège)
Steven Bouillon (KU Leuven)
David J. Burdige (Old Dominion University)
Mitchel Call (Southern Cross University)
Nengwang Chen (Xiamen University)
Xiaogang Chen (Westlake University)
Luiz C. Cotovicz Jr (Leibniz Institute for Baltic Sea Research)
Meagan J. Eagle (U.S. Geological Survey)
Erik Kristensen (University of Southern Denmark)
Kevin D. Kroeger (U.S. Geological Survey)
Zeyang Lu (Xiamen University)
Damien T. Maher (Southern Cross University)
Lucas J. Pérez-Lloréns (University of Cádiz)
Raghab Ray (University of Tokyo)
Pierre Taillardat (National University of Singapore)
Joseph J. Tamborski (Old Dominion University)
Rob C. Upstill-Goddard (Newcastle University)
Faming Wang (Chinese Academy of Sciences)
Zhaohui Aleck Wang (Woods Hole Oceanographic Institution)
Kai Xiao (Southern University of Science and Technology)
Yvonne Y. Y. Yau (University of Gothenburg)
Isaac R. Santos (University of Gothenburg)

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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