PACIFIC OCEAN CIRCULATION DURING THE LATE CRETACEOUS

Considerable progress in understanding Late Cretaceous ocean circulation and its potential relationship to climate change has been made by determining neodymium isotope (ε_Nd) patterns within and among globally distributed sites. The ε_Nd dataset has now grown to a point that details of geographic and bathymetric patterns can be examined. In this study, we compare ε_Nd patterns with those that would be expected given either 1) a progressive increase in the rate of export of intermediate- and deep-waters generated in the South Atlantic or proto-Indian sectors of the Southern Ocean, or 2) a dominance of intermediate- to deep-water production in North and/or South Pacific Ocean. Data include new ε_Nd results from 4 sites in the equatorial Pacific (Mid-Pacific Mountains; DSDP Site 463) and South Pacific Ocean (two on Ontong Java Plateau; DSDP Site 289, and ODP Site 1186 and one on Manihiki Plateau; DSDP Site 317) that expand the Campanian and Maastrichtian coverage in the Pacific and will be compared to previously published ε_Nd and stable oxygen and carbon isotopic ratios.

Preliminary results show a consistent 0.5 to 1 unit excursion towards lower ε_Nd values in the early Maastrichtian that correlates with a brief episode of cooling, but no clear long term trends in ε_Nd seem to be present through the Maastrichtian despite widespread cooling during this interval. These results suggest that relationships between circulation and climate exist in the Late Cretaceous Pacific but gradual cooling during the last 10 million years of the Cretaceous does not seem to correlate with a reorganization of Pacific circulation patterns. Geographically sites in the North Pacific show generally higher (up to 1 ε_Nd unit) values than sites in the equatorial and southern Pacific. There do not appear to be consistent bathymetric offsets among ε_Nd values. However, differences do exist among sites that were positioned at similar paleodepths as well as at similar paleolatitudes. This variability may suggest that complicated combinations of mixing, multiple source regions, reversible scavenging and/or boundary exchange played a significant role in the Nd budget of Pacific Campanian and Maastrichtian intermediate- and deep-waters.