Contrasting Recent Sedimentation Rates and Sediment Characteristics Between Fluvially Connected and Disconnected Distal Coastal Marshes in Southeastern Louisiana, USA.
Abstract
The lower delta plain of the Mississippi River Delta contains approximately five coastal sedimentary basins that are topographically defined, and one shelf-crossing depocenter (the Birds Foot Delta). These depositional systems receive varying quantities of sediment from fluvial and marine sources, and have rates of coastal land loss that are roughly inversely proportional to fluvial sediment supply. As part of a larger Coastal SEES project, we are studying vertical wetland accretion rates and sediment composition in two basins with the highest contrasts in sediment supply and land loss: Atchafalaya Bay (high sediment supply from the Atchafalaya River) and Terrebonne Bay (cut off from direct fluvial supply since 1904). We have analyzed five push cores from the eastern marsh of Fourleague Bay, a bay adjacent to the Atchafalaya outlet, and five push cores from western Terrebonne Bay, using grain size profiles, loss-on-ignition data, and density profiles. Additionally, vertical accretion rates (VAR; cm yr-1) and mass accumulation rates (MAR; g cm-2 yr-1) are calculated using 137Cs, a radioisotope generated from hydrogen bomb testing during the mid-20th century, and 210Pb, a naturally occurring radioisotope generated from the decay of 238U. Together, these isotopes can provide sedimentation rates for approximately the past century. Terrebonne data analysis is ongoing. Initial results from Fourleague bay display an average VAR of 1.18 ± 0.41cm yr-1, and an average mineral sediment only MAR of 0.72 ± 0.33 g cm-2 yr-1. Individual VAR rates for sites range from 0.60 to 1.84 cm yr-1, while mineral sediment only MAR rates range from 0.23 - 2.51 g cm-2 yr-1, both with decreasing average trends as one moves further south, away from the Atchafalaya outlet (R2 = 0.59 & 0.71, respectively). These results place the sphere of influence of Atchafalaya borne sediment at 25 km from river mouth.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMGC53B..07R
- Keywords:
-
- 1620 Climate dynamics;
- GLOBAL CHANGEDE: 1630 Impacts of global change;
- GLOBAL CHANGEDE: 4323 Human impact;
- NATURAL HAZARDSDE: 4217 Coastal processes;
- OCEANOGRAPHY: GENERAL