A COMPARISON OF DEPOSITIONAL THICKNESSES, ORGANIC MATTER CONTENT, CARBON, AND NITROGEN IN DIFFERENT FLOODPLAIN SETTINGS ALONG THE EMBANKED LOWER MISSISSIPPI RIVER NEAR NATCHEZ, MISSISSIPPI, DURING THE 2018, 2019, AND 2020 FLOODS

The Lower Mississippi River (LMR) experienced major floods in 2018, 2019, and 2020. Sediment deposition in the embanked floodplains during floods represent important storage and sequestration opportunities for carbon and nutrients from ~40% of the continental USA. This research aims to compare depositional thicknesses, organic matter (OM), and carbon (C) & nitrogen (N) concentrations in floodplain sediments deposited by the combined 2018-19 floods to the 2020 flood along the LMR near Natchez, Mississippi. For 2018-19, depositional thicknesses ranged from 12 mm to 1.4 meters with averages of 181 mm for natural levees, 30 mm for meander scrolls, and 108 mm for backswamps. For 2020, thicknesses ranged from 3.3–162 mm with averages of 101 mm for natural levees, 10 mm for meander scrolls, and 28 mm for backswamps. For 2018-19, OM content ranged from 0.11–4.38% with averages of 0.68% for natural levees, 2.82% for meander scrolls, and 2.11% for backswamps. For 2020, OM content ranged from 0.15–3.32% with averages of 0.32% for natural levees, 2.26% for meander scrolls, and 2.09% for backswamps. For 2018-19, total C and N concentrations (by weight) averaged 2.78% and 0.23%, respectively, with averages of 0.51% C and 0.02% N for natural levees, 3.36% C and 0.28% N for meander scrolls, and 2.41% C and 0.20% N for backswamps. For 2020, total C and N concentrations averaged 2.86% and 0.23%, respectively, with averages of 0.40% C and 0.02% N for natural levees, 3.35% C and 0.27% N for meander scrolls, and 2.62% C and 0.22% N for backswamps. Greater depositional thicknesses in 2018-19 are best explained by their combined flood durations; the 2019 flood was the longest in recorded history. Slightly higher levels of OM in 2018-19 sediments could indicate preservation favored by the extended duration of saturated conditions and/or increased organic productivity in the warm overbank water column during the later summer interval of the 2019 flood. Comparable concentrations of total C and N indicate that post-flood organic productivity rapidly converts available C and N to biological forms while diverse soil compositions and redox conditions equilibrate the remaining soil C & N to predictable levels commensurate with the floodplain setting (i.e., natural levees, backswamps).