Age estimates for the opening of Drake Passage range from 49 to 17 million years ago (Ma), complicating interpretations of the relationship between ocean circulation and global cooling. Secular variations of neodymium isotope ratios at Agulhas Ridge (Southern Ocean, Atlantic sector) suggest an influx of shallow Pacific seawater approximately 41 Ma. The timing of this connection and the subsequent deepening of the passage coincide with increased biological productivity and abrupt climate reversals. Circulation/productivity linkages are proposed as a mechanism for declining atmospheric carbon dioxide. These results also indicate that Drake Passage opened before the Tasmanian Gateway, implying the late Eocene establishment of a complete circum-Antarctic pathway.

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The Central American Seaway played a pivotal role in shaping global climate throughout the late Cenozoic. Recent geological surveys have provided new constraints on timing of the seaway shoaling, while neodymium isotopic (ε Nd ) data measured on fossil teeth, debris, and ferromanganese crusts have helped define the history of water masses in the region. Here we provide the first 3-D simulations of ε Nd responses to the shoaling seaway. Our model suggests that a narrow and shallow seaway is sufficient to affect interoceanic circulation, that inflow/ outflow balance between the Caribbean and the Antilles responds nonlinearly to sill depth, and that a seaway narrower than 400 km is consistent with an active Atlantic meridional overturning circulation during the late Miocene. Simulated ε Nd values in the Caribbean confirm that inputs from radiogenic Pacific waters in the Caribbean decrease as the seaway shoals. Despite model limitations, a comparison between our results and ε Nd values recorded in the Caribbean helps constrain the depth of the Central American Seaway through time, and we infer that a depth between 50 and 200 m could have been reached 10 Ma ago.

Late Cretaceous fish debris from Demerara Rise exhibits a dramatic positive excursion of 8 Ej^j units during ocean anoxic event 2 (OAE2) that is superimposed on extremely low Ej^jj,) values (-14 to -16.5) observed throughout the rest of the studied interval. The OAE2 E[,j excursion is the largest yet documented in marine sediments, and the majority of the shift is estimated to have occurred over <20 k.y. Low background E^j values on Demerara Rise are explained as the Nd isotopic signature of the South American craton, whereas eruptions of the Caribbean large igneous province or enhanced mixing of intermediate waters in the North Atlantic could have caused the excursion.

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