Distribution and Drivers of Organic Carbon Sedimentation Along the Continental Margins

• Published in: AGU advances Vol. 5; no. 4
• Main Authors: Tegler, Logan A., Horner, Tristan J., Galy, Valier, Bent, Shavonna M., Wang, Yi, Kim, Heather H., Mete, Öykü Z., Nielsen, Sune G.
• Format: Journal Article
• Language: English
• Published: Hoboken John Wiley & Sons, Inc 01.08.2024; Wiley
• Subjects: Carbon; carbon cycle; Carbon dioxide; Continental margins; Efficiency; Estimates; Euphotic zone; Global climate; marine organic carbon; Marine sediments; Ocean floor; Oceans; organic C sink; Organic carbon; Oxygen; Productivity; Sedimentation; Sedimentation & deposition; Sediments; Statistical analysis; terrestrial organic carbon; Water; Water column; Water depth
• ISSN: 2576-604X; 2576-604X
• DOI: 10.1029/2023AV001000

Organic carbon (OC) sedimentation in marine sediments is the largest long‐term sink of atmospheric CO2 after silicate weathering. Understanding the mechanistic and quantitative aspects of OC delivery and preservation in marine sediments is critical for predicting the role of the oceans in modulating global climate. Yet, estimates of the global OC sedimentation in marginal settings span an order of magnitude, and the primary controls of OC preservation remain highly debated. Here, we provide the first global bottom‐up estimate of OC sedimentation along the margins using a synthesis of literature data. We quantify both terrestrial‐ and marine‐sourced OC fluxes and perform a statistical analysis to discern the key factors influencing their magnitude. We find that the margins host 23.2 ± 3.5 Tmol of OC sedimentation annually, with approximately 84% of marine origin. Accordingly, we calculate that only 2%–3% of OC exported from the euphotic zone escapes remineralization before sedimentation. Surprisingly, over half of all global OC sedimentation occurs below bottom waters with oxygen concentrations greater than 180 μM, while less than 4% occurs in settings with <50 μM oxygen. This challenges the prevailing paradigm that bottom‐water oxygen (BWO) is the primary control on OC preservation. Instead, our statistical analysis reveals that water depth is the most significant predictor of OC sedimentation, surpassing all other factors investigated, including BWO levels and sea‐surface chlorophyll concentrations. This finding suggests that the primary control on OC sedimentation is not production, but the ability of OC to resist remineralization during transit through the water column and while settling on the seafloor. Plain Language Summary Particulate organic carbon (OC) sedimentation is a major sink of atmospheric carbon dioxide. Most OC is thought to be sedimented along the margin, the marine area near land with water depths shallower than 1,500 m. However, estimating how much OC is sedimented in these environments has proven challenging with current methods. Here, we compiled and studied information on hundreds of seafloor sediment samples that were collected from around the world. We find that the margins account for approximately 92% of total marine OC sedimentation. Surprisingly, less than 4% of this sedimentation occurs in low‐oxygen regions of the ocean, contradicting a long‐standing assumption about the importance of low‐oxygen regions to global carbon sedimentation. Our results show that the breakdown of particulate OC during its transit through the water column and while settling at the seafloor is the main control on how much OC is sedimented. This information helps to constrain the flux of several other biologically cycled elements and will be important for predicting how global climate change will affect marine OC sedimentation. Key Points Between 17.4 and 24.5 Tmol (0.2–0.3 Pg) of organic carbon (OC) are sedimented on the seafloor annually, of which 90% is along continental margins More than 95% of marine OC sedimentation occurs in regions with bottom‐water oxygen concentrations >50 μmol kg−1 The ability to escape remineralization, rather than production at the sea surface, sets the global pattern of OC sedimentation