USING NEODYMIUM ISOTOPES TO DECIPHER COMPLEX CIRCULATION PATTERNS IN THE SOUTH ATLANTIC AT THE END OF THE CRETACEOUS

Neodymium (Nd) isotopic values of biogenic apatite and other sedimentary phases have increasingly been used to infer Cretaceous ocean circulation patterns and rates. Nd isotopic values compared among sites and through sections have been cited as evidence that Late Cretaceous ocean circulation was largely driven by the formation of cool bottom waters at high latitudes, similar to the modern “ocean conveyor belt” with deep convection enhanced or initiated by Late Cretaceous cooling. However, evolving oceanic gateways and subsiding bathymetric highs also imposed important, but poorly constrained, tectonic controls on the flow of both intermediate (0.5 – 2.5 km) and deep (> 2.5 km) waters. For the Late Cretaceous, this uncertainty is particularly high for the bathymetrically complex South Atlantic. To better understand the influence of bathymetry on flow through the South Atlantic we measured Nd isotopes at 11 sites to create depth transects along prominent bathymetric barriers for the Campanian and Maastrichtian. For an independent assessment of circulation patterns, model simulations using the Community Climate Systems Model (CCSM4) tracked water mass age and tested sensitivity to the the depth of Vema Gap at 2 and 3.5 km. This gap is the deepest point in the main east-west trending bathymetric barrier of the Rio Grande Rise-Walvis Ridge system.

New Nd isotopic records show a shift towards lower Nd isotopic values at sites on top and to the north of the Rio Grande Rise-Walvis Ridge through the Campanian and Maastrichtian as predicted if south component waters (SCW) began flowing northward at this time, but Nd isotopic values from sites positioned between the Rio Grande Rise-Walvis Ridge barrier and the proposed source region for SCW are higher than those to both the north and the south, arguing against simple northward flow of SCW. Modeling results do not perfectly explain the Nd isotopic records at every site, but they do support the existence of multiple water masses at the end of the Cretaceous that circulated in the South Atlantic and were largely controlled by bathymetry. Thus, geochemical and model simulation results both suggest South Atlantic bathymetry was a critical variable in controlling subsurface circulation patterns, and these controls were at least as important as forcing from cooling at the end of the Cretaceous.