U016-09
Floodplain architecture and organic carbon storage in discontinuous permafrost

Monday, 14 December 2020: 11:59
Madison Douglas1, Michael P. Lamb2, Gen Li2, Joel C Rowland3, A. Joshua West4, Preston Cosslett Kemeny2, Jon Schwenk5, Anastasia Piliouras3, Austin John Chadwick6 and Woodward W Fischer7, (1)Massachusetts Institute of Technology, Cambridge, MA, United States, (2)California Institute of Technology, Pasadena, CA, United States, (3)Los Alamos National Laboratory, Los Alamos, NM, United States, (4)University of Southern California, Department of Earth Sciences, Los Angeles, CA, United States, (5)Los Alamos National Laboratory, Earth and Environmental Sciences, Los Alamos, NM, United States, (6)California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States, (7)Caltech, Pasadena, CA, United States
Abstract:
The Arctic contains major river systems that deliver organic carbon to the Arctic Ocean and are responding rapidly to climate change since their watersheds are underlain by permafrost. In particular, rivers migrate laterally and may locally erode up to tens of meters of riverbank per year, allowing rivers in permafrost to entrain and transport organic carbon (OC) from deposits far below the depth of annual thaw. However, the fate of particulate OC during fluvial erosion and mobilization is uncertain, as it may be oxidized and released to the atmosphere as greenhouse gases that amplify climatic warming, or be deposited downstream in sedimentary bars for temporary sequestration in a river floodplain. To quantify particulate OC cycling due to cutbank erosion into carbon-rich permafrost and deposition of point bars during river lateral migration, we collected riverbank and floodplain sediment samples along the Koyukuk River in central Alaska. The Koyukuk contributes 12% of the water and sediment discharge of the Yukon River and meanders through discontinuous permafrost. To characterize OC stocks in floodplain deposits, we measured sediment total OC (TOC), total nitrogen (TN), OC stable isotopes, radiocarbon fraction modern (Fm), water content, bulk density, grain size, and floodplain stratigraphy. TOC, TN, and Fm were higher in finer-grained samples, and we used this relation between grain size and OC content to map OC content onto measured floodplain stratigraphic sections. We calculated that cutbanks and point bars had similar OC stocks, regardless of whether the banks contained permafrost. Sediment samples from both cutbanks and unfrozen point bars contained radiocarbon-depleted (Fm < 1) biospheric OC, indicating that aged organic material was eroded by the river and re-deposited in point bars, rather than being respired to the atmosphere. The process of aging, erosion and re-deposition of floodplain organic material may be intrinsic to river-floodplain dynamics for meandering rivers, generating an aged radiocarbon signal in floodplain sediments regardless of permafrost content.
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