In combination with dramatic and immediate CO2 emissions reductions, net-negative atmospheric CO2 removal (CDR) is necessary to maintain average global temperature increases below 2.0 °C. Many proposed CDR pathways involve the placement of vast quantities of organic carbon (biomass) on the seafloor in some form, but little is known about their potential biogeochemical impacts, especially at scales relevant for global climate. Here, we evaluate the potential impacts and durability of organic carbon storage specifically within deep anoxic basins, where organic matter is remineralized through anaerobic processes that may enhance its storage efficiency. We present simple biogeochemical and mixing models to quantify the scale of potential impacts of large-scale organic matter addition to the abyssal seafloor in the Black Sea, Cariaco Basin, and the hypersaline Orca Basin. These calculations reveal that the Black Sea in particular may have the potential to accept biomass storage at climatically relevant scales with moderate changes to the geochemical state of abyssal water and limited communication of that impact to surface water. Still, all of these systems would require extensive further evaluation prior to consideration of megatonne-scale CO2 sequestration. Many key unknowns remain, including the partitioning of breakdown among sulfate-reducing and methanogenic metabolisms and the fate of methane in the environment. Given the urgency of responsible CDR development and the potential for anoxic basins to reduce ecological risks to animal communities, efforts to address knowledge gaps related to microbial kinetics, benthic processes, and physical mixing in these systems are critically needed.

Biomass-based marine CO2 removal (mCDR) aims to harness photosynthetic organisms to remove excess CO2 from the atmosphere and sequester that fixed carbon in a long-lived marine reservoir. This strategy would contribute to a portfolio of climate mitigation efforts. To guide decision-making around testing, deploying, and regulating mCDR, we need to better understand how the deep sea and the broader Earth system would respond to increased biomass addition. The central processes driving this response are sensitive to choices about biomass type and storage site, and they stretch across spatial and temporal scales from microns to kilometers and from minutes to millenia. To organize this immense interdisciplinary challenge, we define five generalizable phases of a biomass-based mCDR project: inputs, placement, short-term response, long-term response, and functional stability. Each phase is associated with high-priority research objectives that could be achieved through thoughtful integration of direct field measurements, investigations of analog sites, experiments, and/or models. In-situ and laboratory experiments can be particularly powerful for isolating key processes; for example, in-situ “closed-system” bottle incubations can amplify small signals and reduce uncertainties created by complex physical flows. Regardless of approach, the overarching goal of biomass-based mCDR research is to develop a process-based understanding of biomass sequestration that is robust enough to project the likely outcomes of alternative choices related to mCDR. Beyond assessing carbon storage and ensuring regulatory compliance, future field experiments should prioritize generating the data required to improve models for impacts of biomass-based mCDR on the deep ocean at climatically-relevant scales.

A document by Morgan Raven. Click on the document to view its contents.

Organic matter (OM) sulfurization can enhance the preservation and sequestration of carbon in anoxic sediments, and it has been observed in sinking marine particles from marine O2-deficient zones. The magnitude of this effect on carbon burial remains unclear, however, because the transformations that occur when sinking particles encounter sulfidic conditions remain undescribed. Here, we briefly expose sinking marine particles from the eastern tropical North Pacific O2-deficient zone to environmentally relevant sulfidic conditions (20C, 0.5 mM [poly]sulfide, two days) and then characterize the resulting solid-phase organic and inorganic products in detail. During these experiments, the abundance of organic sulfur in both hydrolyzable and hydrolysis-resistant solids roughly triples, indicating extensive OM sulfurization. Lipids also sulfurize on this timescale, albeit less extensively. In all three pools, OM sulfurization produces organic monosulfides, thiols, and disulfides. Hydrolyzable sulfurization products appear within ≤ 200-m regions of relatively homogenous composition that are suggestive of sulfurized extracellular polymeric substances (EPS). Concurrently, reactions with particulate iron oxyhydroxides generate low and fairly uniform concentrations of iron sulfide (FeS) within these same EPS-like materials. Iron oxyhydroxides were not fully consumed during the experiment, which demonstrates that organic materials can be competitive with reactive iron for sulfide. These experiments support the hypothesis that sinking, OM- and EPS-rich particles in a sulfidic water mass can sulfurize within days, potentially contributing to enhanced sedimentary carbon sequestration. Additionally, sulfur-isotope and chemical records of organic S and iron sulfides in sediments have the potential to incorporate signals from water column processes.