Molecular fossil record of environmental change in the Late Cretaceous Western Interior Seaway

Cenomanian-Turonian sediments of the Western Interior basin, North America were deposited in an epeiric sea (the Western Interior Seaway, WIS) that connected the Boreal Sea to the Tethys. Previous studies suggested anoxic bottom water conditions during parts of a second-order eustatic sea level cycle (Cenomanian-Turonian Greenhorn Cycle). The presence of isorenieratane at all sampling locations indicates that the entire WIS experienced photic zone euxinic events. The distribution of isorenieratane shows a dynamic water column structure through time. During the transgressive and regressive phases, the southern WIS was frequently euxinic. At highstand, the southern WIS was more oxygenated with rare ‘intermittent anoxic events’. In contrast, the northern WIS shows a continuous record of recurrent photic zone euxinia throughout the Greenhorn Cycle.; A new paleoceanographic model, the ‘Dynamic Fresh-Water Lid’ model, is proposed to explain the history of the water column structure of the WIS. This model describes the recurrence of a fresh water lid extending from the north. At higher latitudes, the WIS received more fresh water input from a considerably larger hinterland. This allowed a fresh water lid at higher latitudes to exist even at highstand, at least periodically. The fresh water lid only wedged far enough south during periods of lower sea levels when the surface area of the WIS was smaller.; The positive carbon isotope excursion associated with the Cenomanian-Turonian (C/T) boundary reflects an increased burial rate of 13C depleted organic carbon at the onset and during an oceanic anoxic event (OAE). A concomitant drop in atmospheric pCO₂ of 20% to 40–80% has been estimated previously. Stable carbon isotopic data are presented on long-chain n-alkanes, derived from terrestrial plants at paleolatitudes 35°N and 55°N. A shift from C₃- to a C₄-type vegetation during the C/T OAE was observed at 35°N, but not at 55°N. Based on Cenomanian pCO₂ levels and the climate regimes of C₃ and C₄ plants, we propose a transient drop in atmospheric CO ₂ concentration of 40–65%. We estimated that the amount of carbon sequestered in marine sediments during the OAE is sufficient to accommodate such a large pCO₂ drawdown.