VERTICAL ISOTOPIC GRADIENTS AMONG FORAMINIFERA AND EVOLUTION OF THE MAASTRICHTIAN NORTH ATLANTIC THERMOCLINE

The Cretaceous represents one of the warmest intervals in Earth history, but conditions were not uniformly warm across this period.  There were intervals of warming and cooling, and the Maastrichtian is generally considered one of the latter. On Blake Nose in the North Atlantic, the regional expression of the Maastrichtian cooling was warming and increased water column stratification.

Stable isotopes from 944 single individual analyses of the late Maastrichtian planktonic foraminifera Contusotruncana contusa, Racemiguembelina fructicosa and Rugoglobigerina rugosa as well as from ~200 multispecimen analyses of six other species show a consistent negative d¹⁸O excursion of ~1.5‰.  This shift suggests an increase in temperature of approximately 6°C over the last 3 million years of the Maastrichtian.  Moreover, a d¹³C divergence among species through time is also observed which we interpret as evidence for increasing water column stratification.  Coincident with these trends C. contusa increases in size, and there is a diversification among the planktic taxa.

Increasing water stratification associated with the long term negative d¹⁸O excursion (warming) suggest a more strongly developed thermocline.  Because within sample variability of the single specimen d¹³C and d¹⁸O data does not change through the section, increased stratification is unlikely to have resulted from increased seasonality.  Rather, we suggest an intensification of the northern hemisphere polar front as a more likely mechanism.  This hypothesis has important implications for heat transport into the latest Cretaceous Arctic.