Does a strong pycnocline impact organic-matter preservation and accumulation in an anoxic setting? The case of the Orca Basin, Gulf of Mexico

The Orca Basin (an intraslope, depression located in the Gulf of Mexico) collects sedimentary particles of terrestrial origin (clastic and organic particles mainly supplied by the Mississippi River) and of marine origin (biogenic productivity). The basin is partly filled with dense brines leached from stilt diapirs cropping out on the sea floor, and is permanently stratified. A strong pycnocline induces anoxic bottom conditions, expectedly favorable to organic matter (OM) preservation. Here, we report on OM in the upper 750 cm below sea floor of Core MD02-2552 (Holocene). The organic content is dominated by marine-derived amorphous OM. The organic assemblage is unexpectedly degraded to some extent, which may be accounted for by a relatively long residence time of organic particles at the halocline-pycnocline at similar to 2240 m. Thus the organic particles are temporarily trapped and kept in contact with the dissolved oxygen-rich overlying water mass. Lastly, the land-derived organic fraction shows co-variations with the land-derived clay mineral supply. To cite this article: N. Tribovillard et at., C. R. Geoscience 341 (2009). (C) 2008 Academic des sciences. Published by Elsevier Masson SAS. All rights reserved.