Why are we researching ocean-based CO2 removal (CDR)?
Humans continue to emit vast quantities of CO2 to the atmosphere, largely from fossil fuels. This rapid and uncontrolled experiment on the Earth system is driving extreme weather, food insecurity, and sea level rise that is likely to displace entire cities and nations (IPCC AR6). These effects will fall hardest on the under-resourced communities that have contributed least to the problem (UNDP).
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The international scientific consensus is that CO2 removal from the atmosphere is now required to avoid the most severe consequences of climate change and stay below global average temperature increases of 2 C (IPCC factsheet). We have already reached the critical threshold of 1.5 C (BBC), and time is short.
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In the NOISE Lab, our expertise is in understanding how Earth naturally preserves organic carbon in parts of the deep ocean and sediments that lack oxygen (O2). Generally speaking, these anoxic environments can be hotspots for efficient organic carbon storage. At several points in Earth history, enhanced organic carbon burial in anoxic basins appears to have driven global cooling (1).
We aim to better understand whether this climate-stabilizing feedback could be enhanced to remove anthropogenic CO2 over decades rather than thousands of years. How much carbon could be stored safely? How would enhanced carbon storage affect marine ecosystems, and how could negative impacts be minimized?
Our goal is to provide actionable information about the potential scale and impacts of ocean-based CDR to empower regulators, governments, investors, and the public to make informed decisions about how to best navigate the challenges ahead.
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unpublished data on this page: Martinez, Evans, Raven et al. (in prep)
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What is Marine Anoxic Carbon Storage (MACS)?
Marine anoxic carbon storage (MACS) is a potential CDR approach that takes advantage of the CO2 uptake that plants do every year. In the fall and winter, most of this carbon returns to the atmosphere through respiration and burning. MACS approaches hope to prevent some fraction of that biomass from oxidizing back to CO2 by sequestering it in a long-term marine reservoir – essentially shortening the orange arrow at right.
Raven et al. (2024), AGU Advances
​​​Most of the ocean today is rich in dissolved O2. Biomass in these oxic waters can be efficiently respired, producing dissolved CO2, nutrients, organics, and an O2 deficit that, at large scales, could pose significant risks to marine life (2,3). Breakdown in physically restricted, anoxic waters avoids impacts to animals and oxygen but could produce sulfide or methane. ​
Organic carbon cycling in a deep, anoxic brine pool
Our research has focused on deep anoxic brine pools, which can be found in U.S. waters in the Gulf of Mexico. Orca Basin is a relatively large and well-known example, with a brine volume of ~10.24 km3 (4). Sediments from Orca Basin are known to show exceptional organic matter preservation (5).
In a series of four cruises to the Orca Basin in 2023 and 2024, we collected geochemical profiles of seawater, brine, and sediments.
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Orca Basin has been isolated for ~8,000 yrs by a strong density interface, 2,200 m below the sea surface (6).
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Over that time, the brine has accumulated high concentrations of dissolved iron, sulfate, organic matter, and hydrocarbons. Dynamic redox cycles linking iron, manganese, sulfur, and organic matter span the density interface (4,7).​
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coming soon: DOM + friends profiles
What happens to plant materials when they are placed in anoxic brine?
We conducted biomass incubations in the lab and in Orca Basin to quantify carbon loss and identify the microbial processes involved.
Some dissolves:
Approximately 1.3% of the carbon in bagasse dissolves into brine over hours to weeks, releasing DOC with a woody or humic composition. This abiotic process is highly reproducible within each particular biomass preparation type.
Bottle incubations in lab ran for 180 days, harvested in triplicate at 7 time points.
4 treatments: sugarcane bagasse, γ-sterilized bagasse, dried Sargassum, control
2 conditions: anoxic brine (Orca Basin) vs. anoxic seawater (Santa Barbara Basin)
No detectable microbial breakdown in brine:
Microbial sulfate reduction occurs in Orca Basin and, with abundant sulfate in the brine, is likely a principal pathway for anoxic biomass breakdown (8). We use radiolabeled sulfate (35S) to sensitively measure MSR rates in lab. Sulfate reduction rates remain below detection over 120 days – equivalent to less than 0.002% of added carbon per year. In field incubations with bagasse, we also detected no dissolved sulfide after 200 days.
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Methanogenesis is also active in Orca Basin (9) and could contribute to biomass breakdown. However, we found no evidence for methane accumulation after 256 days (so far).
Slow microbial breakdown in anoxic seawater:
Unlike in Orca brine, in "normal" anoxic seawater, biomass breakdown proceeds through the expected sequence of microbial metabolisms: nitrate reduction, iron reduction, sulfate reduction, and methanogenesis.
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Sulfate reduction rates for anoxic seawater increased over the experiment for all biomass types. For a rough scale, if sulfate reduction were constant at the rates observed after 120 days, this metabolism could oxidize ~0.9%/yr of biomass carbon.
log scale! :)
Exceptional preservation in field incubations:
Multiple biomass types were recovered after 200 days in Orca Basin, including freeze-dried Macrocystis (kelp) and softwood in 30-micron nylon mesh bags. Even under anoxic conditions, a substantial fraction of kelp biomass will typically degrade over days to weeks. In Orca brine, however, we did not detect the loss of insoluble carbon in any of our samples or standards (with ~0.6 wt% uncertainties).
Biomass after 200 days in Orca Basin
References and Resources
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Martinez, Evans, Raven (in prep). “Defining an environmental baseline for marine anoxic carbon storage in a deep anoxic brine.”
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Evans, Martinez, Fishburn, Raven (in prep) “Anaerobic degradation of terrestrial and marine biomass in seawater and brine.”
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Martinez, Evans, Druschel, Dawson, Doyle-Jacobson, Raven (in prep) “Metabolic constraints and biogeochemical cycling in an isolated brine.”
(1) Bodin S., Meissner P., Janssen N., Steuber T., Mutterlose J., (2015). “Large Igneous Provinces and Organic Carbon Burial: Controls on Global Temperature and Continental Weathering during the Early Cretaceous,” Global and Planetary Change 133 p. 238–53, https://doi.org/10.1016/j.gloplacha.2015.09.001.
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(2) Roberts K. E. et al. (preprint 2024). "Potential impacts of climate interventions on marine ecosystems." Under review at Reviews of Geophysics, https://doi.org/10.22541/essoar.173482381.11257447/v1.
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(3) Levin L. A. et al. (2023). “Deep-Sea Impacts of Climate Interventions,” Science 379, no. 6636: p. 978–81, https://doi.org/10.1126/science.ade7521.
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(4) Diercks A. et al. (2019). “Vertical Marine Snow Distribution in the Stratified, Hypersaline, and Anoxic Orca Basin (Gulf of Mexico).” Elementa: Science of the Anthropocene 7: 10, https://doi.org/10.1525/elementa.348.
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(5) Kennett J. and Penrose N., (1978). “Fossil Holocene Seaweed and Attached Calcareous Polychaetes in an Anoxic Basin, Gulf of Mexico.” Nature 276, no. 5684 p. 172–73, https://doi.org/10.1038/276172a0.
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(6) Addy S. and Behrens E. W., (1980). “Time of Accumulation of Hypersaline Anoxic Brine in Orca Basin (Gulf of Mexico).” Marine Geology 37, no. 3–4 p. 241–52, https://doi.org/10.1016/0025-3227(80)90104-8.
(7) Van Cappellen P. et al. (1998). “Biogeochemical Cycles of Manganese and Iron at the Oxic−Anoxic Transition of a Stratified Marine Basin (Orca Basin, Gulf of Mexico).” Environmental Science & Technology 32, no. 19: p. 2931–39, https://doi.org/10.1021/es980307m.
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(8) Hurtgen M. T. et al. (1999). “Anomalous Enrichments of Iron Monosulfide in Euxinic Marine Sediments and the Role of H 2 S in Iron Sulfide Transformations; Examples from Effingham Inlet, Orca Basin, and the Black Sea.” American Journal of Science 299, no. 7–9: p. 556–88, https://doi.org/10.2475/ajs.299.7-9.556.
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(9) Zhuang G-C et al. (2018). “Effects of Pressure, Methane Concentration, Sulfate Reduction Activity, and Temperature on Methane Production in Surface Sediments of the Gulf of Mexico.” Limnology and Oceanography 63, no. 5: p. 2080–92, https://doi.org/10.1002/lno.10925.
Funding: Grantham Foundation for the Protection of the Environment, California NanoSystems Institute (CNSI), the University of California, Santa Barbara Office of Research, and the state of California
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Disclosures: Outside of her faculty position, PI Raven serves and the Chief Science Officer for Carboniferous. No other team members have real or perceived COIs to disclose. An unconflicted PI is involved in all CDR-related funding and mentorship plans in the group.