ABSTRACT FINAL ID: PP23A-1831
We here present (isotope)geochemical and sedimentological data from marine sediment cores from the northern Gulf of Mexico to approximate the temporally and spatially varying terrigenous sediment contribution via the Mississippi River and the related spread of freshwater over the last glacial-interglacial cycles, with specific focus on the last ca. 42.000 years. Our study is based on cores from the DeSoto Canyon (MD02-2576 and 2575), from ~90 km southeast off the Mississippi River delta (M78-181), and from southwest of the delta (IODP 1319A). The geochemical signature of the eastern cores closely matches that of the Mississippi catchment area rather than those of the Alabama and Mobile River catchments. In particular, the siliciclastic major element potassium (K), estimated from calibrated XRF core scanning, serves as a suitable proxy for Mississippi River sediment discharge, becoming less concentrated with distance from the delta.
The K variability suggests enhanced glacial phase terrigenous influx triggered by strengthened fluvial runoff and changing fluvial and ice sheet dynamics. Mississippi River influx was at a maximum during glacial MIS 2/3, late MIS 8 and MIS 10, reflected by sedimentation rates being 4 to 5 times higher than in the Holocene. Late glacial to deglacial fluvial sediment supply, however, decreased abruptly at ca. 20 ka at our easternmost core location (MD02-2576), and ca. 2 kyr later at our core location closest to the Mississippi Delta, implying a gradual westward shift of the Mississippi outflow. Due to synchronous changes in sea-surface temperatures, we hypothesize an increasing impact of the northward extending Loop Current on the Mississippi outflow pattern.
Combined stable oxygen isotope and element ratios from shallow and deep-dwelling as well as benthic foraminifers allow to approximate paleosalinity, and hence to follow the dispersal of freshwater across the Gulf of Mexico. According to our data, Mississippi freshwater discharge events appeared during the last glacial and during Termination II, but were mostly confound to the southwest of the Mississippi River delta. The prominent discharge event during Heinrich 1 and the Boelling/Alleroed warm period is also observed at core location M78-181, but not further to the east. Notably, sediment supply during this megadischarge event is insignificant compared to full glacial conditions, suggesting that signals of freshwater and sediment supply became decoupled.
Holocene changes of Mississippi discharge are closely related to the sea-surface temperature and salinity development in the northern Gulf, most likely amplified by the migration of the Innertropical Convergence Zone, related dislocations of the Hadley Cell, and changes in climatic zones.
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