Influence of the Amazon River development and constriction of the Central American Seaway on Middle/Late Miocene oceanic conditions at the Ceara Rise

Sediment samples from ODP 154 Site 926A (Ceara Rise, western equatorial Atlantic Ocean) spanning the Neogene from 12.8 to 9.2 Ma were investigated on their calcareous dinoflagellate contents to better understand the oceanographic changes in relation to the closure of the Central American Seaway and the development of the Amazon River. Intervals with increased cyst accumulation rates and dissolution sensitive species occur from 12.4 Ma onward. They correspond to periods of enhanced North Atlantic Deep Water production indicating the presence of this water mass at the research site during these intervals. This suggests that pulses of North Atlantic Deep waters sporadically flew into the South Atlantic related to uplift phases of the Panama Sill.

At about 11.2 Ma the first appearance of Leonella granifera indicates river influence at the sample site. This first indication of river influence in the western equatorial Atlantic can be linked to the developing Amazon River. The cyst association changes at about 11.2 Ma from almost monospecific to highly diverse and the permanent presence and increased abundance of L. granifera suggest that river waters were able to reach the study site by now, probably as a result of the southward flowing North Brazil Current. After 10.5 Ma the cyst association indicates a decrease in Amazon influence at Site 926A. This change can be correlated to a reducing inflow of Pacific waters through the Central American Seaway leading to a reverse of the North Brazil Current to its modern northwards flow pattern.