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Rapid warming and salinity changes of Cretaceous surface waters in the subtropical North Atlantic

¹ School of Civil Engineering and Geosciences, Newcastle University, Devonshire Building, Newcastle upon Tyne NE1 7RU, UK
² Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
³ Department of Marine Biogeochemistry and Toxicology, Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands
⁴ Institute for Geology and Mineralogy, University of Cologne, Zülpicher Strasse 49a, 50674 Köln, Germany

Paleomarine carbon isotopic records reveal occasional and rapid release of greenhouse gases into the atmosphere. The regional consequences on surface ocean properties and time relationships between the atmosphere and the ocean, however, are still far from understood. We observe a sudden -1.5 shift in leaf wax n-alkane ¹³C values at the onset of oceanic anoxic event (OAE) 1b from early Albian sediments off northwest Africa (Deep Sea Drilling Project Site 545) that is best explained by the rapid release of isotopically light carbon into the Cretaceous atmosphere. This -1.5 shift in the n-alkanes precedes a negative isotope excursion of similar magnitude observed in marine carbonate, organic matter, and algal steranes. A TEX₈₆-based record of sea surface temperature (SST) confirms almost instantaneous warming by 3.5 °C along with the marine isotope shifts at Site 545, paralleled by an 2 °C TEX₈₆-SST increase and a freshening of surface water salinity from 43 to 41 at Blake Nose, Ocean Drilling Program Site 1049. Multiproxy evidence indicates that, once established, these warm SSTs prevailed and were stable during OAE 1b, suggesting that the emission of ¹³C-depleted carbon started abruptly but then continued over tens of thousands of years. The SST cooled by 1–2 °C at the end of the event, not reaching pre-excursion levels. The new records provide evidence for a time lag (best estimate 1–3 k.y.) between atmospheric and oceanic processes that we interpret as a direct response to changes in the atmospheric greenhouse gas concentrations followed by propagation into the ocean and subsequent heating of surface waters.

Key Words: oceanic anoxic events • sea surface temperature • biomarkers • atmosphere-ocean interactions • biogeochemical cycling