Going beyond blue carbon: Organic carbon storage rates in clastic strata outpace rates in organic-rich strata in freshwater deltaic environments
Abstract
It is widely believed that coastal ecosystems (salt marshes, mangroves, and seagrass meadows) play an important role in the global carbon cycle and account for a significant portion of the terrestrial carbon sink. However, large alluvial systems contain potentially significant carbon sinks located immediately landward of the blue carbon zone. In particular, environments that accumulate organic-rich deposits could be expected to sequester organic carbon (OC) that can be stored over long (centennial to millennial) timescales. Here we present new data on OC storage rates from freshwater environments in the Mississippi Delta, derived from sediment cores with 14C and OSL-based geochronologies for the past ~6000 years, along with paleoecological information. This enables us to develop precise age-depth models for peats that accumulated in freshwater swamps, overlain by mud-dominated overbank deposits formed by a precursor of the Mississippi River. Surprisingly, we find that the clastic deposits with OC contents of only a few percent store OC at rates that generally exceed 100 g/m2/yr, equal to or higher than rates in subjacent peat beds. This is mainly due to the large difference in accretion rates; proximal overbank deposits form at rates that average a few centimeters per year, an order of magnitude higher than the accretion rates seen in swamp peat. The inferred OC storage rates are also roughly comparable with present-day rates as seen near the shoreline in the Mississippi Delta. Our findings suggest that freshwater settings (i.e., floodplains and delta plains) immediately inland of the traditional blue carbon zone can be at least as important in storing OC. Finally, we highlight the counterintuitive result that organic-matter content is not always a good predictor of OC storage efficiency.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP41A..08T
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 1030 Geochemical cycles;
- GEOCHEMISTRY;
- 1815 Erosion;
- HYDROLOGY;
- 1862 Sediment transport;
- HYDROLOGY