High rates of organic carbon burial in submarine deltas maintained on geological timescales

Burial of terrestrial organic carbon in marine sediments can draw down atmospheric CO2 levels on Earth over geologic timescales (>= 10(5) yr). The largest sinks of organic carbon burial in present-day oceans lie in deltas, which are composed of three-dimensional sigmoidal sedimentary packages called clinothems, dipping from land to sea. Analysis of modern delta clinothems, however, provides only a snapshot of the temporal and spatial characteristics of these complex systems, making long-term organic carbon burial efficiency difficult to constrain. Here we determine the stratigraphy of an exhumed delta clinothem preserved in Upper Cretaceous (similar to 75 million years ago) deposits in the Magallanes Basin, Chile, using field measurements and aerial photos, which was then combined with measurement of total organic carbon to create a comprehensive organic carbon budget. We show that the clinothem buried 93 +/- 19 Mt terrestrial-rich organic carbon over a duration of 0.1-0.9 Myr. When normalized to the clinothem surface area, this represents an annual burial of 2.3-15.7 t km(-2) yr(-1) organic carbon, which is on the same order of magnitude as modern-day burial rates in clinothems such as the Amazon delta. This study demonstrates that deltas have been and will probably be substantial terrestrial organic carbon sinks over geologic timescales, a long-standing idea that had yet to be quantified.

Automated summary

This study investigates an exhumed delta clinothem in the Magallanes Basin, Chile, to determine its stratigraphy and create a comprehensive organic carbon budget. The results show that over a period of 0.1-0.9 Myr, the clinothem buried a significant amount of terrestrial organic carbon, indicating that deltas have been and will continue to be important sinks for organic carbon over geologic timescales.