Biomass Storage in Anoxic Marine Basins: Initial Estimates of Geochemical Impacts and CO ₂ Sequestration Capacity

Abstract

In combination with dramatic and immediate CO ₂ emissions reductions, net‐negative atmospheric CO ₂ 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 (OM) 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 OM 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 CO ₂ 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.

Plain Language Summary

In addition to dramatically and immediately reducing CO ₂ emissions, it has become necessary to actively remove CO ₂ from the atmosphere in order to meet the goals of the Paris Agreement and avoid the worst effects of climate change. Several of the approaches to remove CO ₂ from the atmosphere that are currently being discussed involve trapping large amounts of CO ₂ as organic carbon in plants or algae and then storing that carbon in the deep ocean. Here we ask how this type of carbon storage would likely impact the ecology and chemistry of deep ocean environments, depending on the amount of material placed and its location. Within the limitations of these first simple calculations, we find that specific anoxic basins like the Black Sea may have the potential to sequester climatically relevant quantities of organic carbon for more than 1,000 years with moderate changes to deep water chemistry. With these results, it is our aim to motivate rigorous field and experimental studies that develop more nuanced models for the impacts of carbon storage in locations like the Black Sea.

Key Points

• Organic carbon sequestration may be relatively efficient in parts of the ocean without O ₂, although its durability depends on local circulation

• The Black Sea may have the potential to durably sequester climatically relevant quantities of organic carbon

• Research is urgently needed to better understand potential biomass degradation rates and the mixing and transport of degradation products