Organic matter provenance, palaeoproductivity and bottom water anoxia during the Cenomanian/Turonian oceanic anoxic event in the Newfoundland Basin (northern proto North Atlantic Ocean)

Abstract

Free and sulfur-bound biomarkers in sediments deposited in the northern proto North Atlantic (Newfoundland Basin, ODP Site 1276) during the Cenomanian-Turonian oceanic anoxic event 2 (OAE-2) were studied. The δ 13C records of phytane and lycopane confirmed the stratigraphic position of the positive carbon isotope excursion associated with OAE-2, previously reported for total organic carbon (TOC) and β,β-homohopane. Sediments before and after the OAE-2 interval were poor in organic matter (OM) and comprised numerous gravity flow deposits. The interval itself was composed of pelagic sediments with occasionally a much higher TOC content of up to 12.7%. The OAE-2 sediments were characterized by a low amount of terrestrial OM since the dominant biological sources of the biomarkers were aquatic in origin. High hopane, pentamethylicosane (PMI), and squalane abundances in the OM-rich sediments pointed to a relatively high input of prokaryotes, partly derived from cyanobacteria, as suggested by the occasional occurrence of 2-methylhopanes. PMI comprised both the regular and irregular isomer and changes in the δ 13C of PMI are thought to reflect contributions from methanogenic and methanotrophic archea. The high relative concentration of lycopane indicated that bottom water conditions were anoxic during large parts of the OAE-2 interval. In one horizon, trace amounts of isorenieratane provided evidence for the occasional occurrence of photic zone anoxia. Taken together, the data imply that oceanic anoxia, and probably also high productivity, reached the northernmost part of the proto-North Atlantic during OAE-2, albeit that photic zone anoxia was much less common than in the southern proto-North Atlantic.