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Paper in press: Constraining the margin organic C flux

By Tristan Horner | 2024-08-17

We’re excited to share the publication of ‘Distribution and Drivers of Organic Carbon Sedimentation Along the Continental Margins’ in AGU Advances, led by Logan Tegler (Ph.D., 2023). This study explores how organic carbon settles into marine sediments—a major long-term sink for atmospheric carbon dioxide, second only to silicate weathering.

Estimating organic carbon sedimentation, especially along continental margins (depths shallower than 1,500 m), has traditionally relied on top-down approaches. These use satellite observations of variables like sea-surface chlorophyll-a to model net primary production, export, and water-column transfer. While useful, these models depend on parameterizations of complex transfer processes that aren’t fully understood, resulting in wide variability in estimates.

In Tegler et al., we took a different approach—one that is bottom-up and data-driven. By compiling and analyzing nearly 1,000 measurements of organic carbon fluxes, we estimated global sedimentation patterns based directly on observational data. The top-line results:

  • Total marine organic carbon sedimentation is approximately 25 Tmol C per year
  • About 90 % (23 Tmol C per year) occurs on continental margins (shallower than 1,500 m)
  • Of this margin flux, roughly 85 % is marine-derived and 15 % terrestrial origin

Our analysis of spatial patterns and controls produced some surprising insights. Notably, bottom-water oxygen levels—long considered a key control on organic carbon preservation—do not appear to play a major role. Over half of organic carbon sedimentation occurs beneath well-oxygenated waters (>180 μM), while less than 5 % occurs under low-oxygen conditions (<50 μM). This challenges the prevailing view that low oxygen strongly enhances organic carbon burial. Similarly, we found no significant relationship between sedimentation and sea-surface chlorophyll-a, a common proxy for productivity. This suggests that surface productivity may not directly govern how much organic matter ultimately reaches the seafloor.

So, what does matter for organic matter? Our statistical analysis points to water depth as the strongest predictor of organic carbon sedimentation—not production, and not oxygen-mediated degradation, but simply the distance traveled. We interpret this to mean that what really matters is the ability of organic matter to survive transit through the water column and during settling on the seafloor.

This was a fascinating study to work on, and we’re grateful to the reviewers and editor for helping sharpen our conclusions. The paper is featured in EOS, and you can access it here.

Citation: Tegler, L. A., Horner, T. J., Galy, V., Bent, S. M., Wang, Y., Kim, H. H., Mete, Ö. Z., & Nielsen, S. G. (2024). Distribution and drivers of organic carbon sedimentation along the continental margins. AGU Advances, 5(4), e2023AV001000, doi:10.1029/2023AV001000.

Posted in Papers