CHANGES IN INTERMEDIATE WATER SOURCES DURING THE EARLY MAASTRICHTIAN COOLING EVENT: EVIDENCE FROM ND-ISOTOPES AND FORAMINIFERA ASSEMBLAGES

The latest Cretaceous (Campanian–Maastrichtian) was characterized by gradual, global cooling following the extreme warmth of the mid Cretaceous greenhouse, but the cooling was not uniform. At multiple sites a short-lived, early Maastrichtian cooling event (~71–70 Ma) has been recognized based on positive oxygen isotope excursions in benthic foraminferal tests. Changes in the production of intermediate and deep water masses have been proposed as a mechanism to explain this climate event, but the exact source regions are poorly defined. We evaluated benthic foraminifera assemblages and Nd isotopes from fish teeth before (84–71 Ma), during (71–70 Ma), and after (70–67 Ma) the early Maastrichtian cooling interval at ODP site 1050 (paleodepth ~1500 m) on Blake Nose in the western North Atlantic. The ratio of epifaunal to infaunal foraminifera, the Benthic Foraminifera Oxygenation Index (BFOI), and marker species were used to interpret bottom water oxygen content and organic carbon fluxes; Nd isotopes were used to track water mass sources. These data highlight conditions across the early Maastrichtian cooling. Before and after the cooling event, conditions are characterized by relatively low epifauna/infauna ratios, low BFOI, and low Nuttalides truempyi abundances, indicating high organic matter fluxes and low bottom water oxygen concentrations. During the cooling event, these same proxies and the relatively high abundance of Reussella szajnochae indicate low organic fluxes and high bottom water oxygenation. Values for ε_Nd range from -6 to -8 before and after the excursion, but decrease to -9 to -10 during the cooling event. This Nd isotopic shift suggests observed differences in water mass characteristics are related to changes in circulation rather than surface productivity. Our initial micropaleontological and Nd isotopic data, thus, suggest a short-term change in the production of intermediate water during the early Maastrichtian cooling event. Additional data on faunal assemblages and north-to-south water mass aging and isotopic trends will be required to pinpoint the source regions of North Atlantic intermediate waters from the late Campanian through the Maastrichtian.