CONSTRAINTS ON DEEP AND INTERMEDIATE WATER CIRCULATION THROUGH THE CENTRAL AMERICAN SEAWAY FROM THE MIDDLE MIOCENE TO PLIOCENE BASED ON NEODYMIUM ISOTOPES

The closure history of the Central American Seaway (CAS) remains controversial with estimates of closure ranging from 11 to 2.5 Ma. We have taken a paleoceanographic approach to reconstruct the history of deep and intermediate water exchange from the Atlantic and Pacific into the Caribbean. Specifically, we developed records of Neodymium (Nd) isotopes of fossil fish teeth from Ocean Drilling Program (ODP) sites in the eastern equatorial Pacific (ODP sites 845, 846, 1241) and Caribbean Basin (sites 998 and 999 ) from the middle Miocene to Pliocene (~14 to 2.5 Ma), along with a few data points in the Pleistocene postdating closure. Nd isotopes are a tracer for water mass composition and the Pacific and Atlantic have analytically distinct values.

Observed Nd isotopic values of -5.5 to 0 in the Caribbean from 18 to 10.2 Ma are similar to values recorded in the deep Pacific sites and distinct from contemporaneous values of <-10 documented in the Atlantic, indicating the Caribbean was filled with waters flowing through the CAS from the Pacific during the middle and early late Miocene. From 12 to 10.7 Ma Caribbean Nd isotopic values are largely within the range of deep Pacific values, but spike up to 0, similar to values recorded at intermediate depth Site 1241, located directly across the CAS in the Pacific. These high Nd isotopic values in the Caribbean Basin have been suggested to reflect an influx of upper-Pacific Deep Water to intermediate water through the CAS and, therefore, a change in depth of the source waters from the Pacific, consistent with estimates that the CAS shoaled to ~1000 m by this time. From 10.7 to 2.5 Ma Nd isotopic values in the Caribbean decrease from -2 to -10.5, while Pacific values remain relatively unchanged. This decrease is interpreted to represent a progressive reduction in Pacific through flow as the CAS shoaled. Unfortunately, post-closure Nd isotopic values in the Caribbean range from -5.5 to -9, reflecting a mix between North Atlantic and Antarctic sources. Therefore, the oldest possible data of closure to Pacific intermediate and deep waters is ~7 Ma, the date when the decreasing Nd isotopic trend in the Caribbean reaches the envelope of post-closure Nd isotopic values.