Abstract
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.
Original language | English |
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Pages | 1831 |
Publication status | Published - 1 Dec 2011 |
Keywords
- [4900] PALEOCEANOGRAPHY
- [4901] PALEOCEANOGRAPHY / Abrupt/rapid climate change
- [4924] PALEOCEANOGRAPHY / Geochemical tracers
- [4954] PALEOCEANOGRAPHY / Sea surface temperature