Widespread Changes in Ocean Circulation Following the Opening of Drake Passage
Abstract
Neodymium isotope records from the Atlantic sector of the Southern Ocean illustrate a prominent shift to radiogenic ɛNd values during the late middle Eocene, reflecting the appearance of Pacific seawater following the early opening of Drake Passage (Scher and Martin, 2004; 2006). Correlations between maxima in biological productivity and episodes of increased Pacific throughflow indicate the opening was marked by a new, more vigorous mode of circulation, however the extent of these changes outside of the Atlantic sector is not known. We investigated the late middle to late Eocene section of an upper Kerguelan Plateau site in the Indian sector of the Southern Ocean (Ocean Drilling Program [ODP] Site 738, current water depth 2252 m, estimated paleo-depth 2000 m). A Nd isotope record was generated from fossil fish teeth for the age interval from 41 to 33 Ma at an average resolution of 100 kyr. Our goal was to investigate changes in ocean circulation in the Indian sector of the Southern Ocean to document regional changes in Southern Ocean circulation related to the opening of Drake Passage. This is critical for evaluating proposed mechanisms for planetary cooling during the late Eocene and across the Eocene/Oligocene boundary. The Nd isotope record from Kerguelan Plateau closely mirrors those from the Atlantic sector, showing an overall shift from nonradiogenic to radiogenic ɛNd values. ɛNd values average -9.4 from 41 to 38 Ma and increase to -9.0 by 37.5 Ma. After a 1 Myr interval of relative stability, ɛNd values increase steadily through the late Eocene, reaching a maximum of -7.8 around 34 Ma. ɛNd values decrease to -9.6 during a nonradiogenic excursion that straddles the E/O boundary, then return to -8.3 in the early Oligocene. The long-term variability observed at ODP Site 738 is remarkably similar to the Nd isotope record from ODP Site 689 (upper Maud Rise). The overall shift to radiogenic ɛNd values is interpreted as an increasing proportion of Pacific seawater in the Indian sector of the Southern Ocean, and suggests that changes in ocean circulation resulting from the opening of Drake Passage were widespread throughout the Southern Ocean. Results also demonstrate that there were distinct water masses in the Indian Ocean; ɛNd values on Ninety-east Ridge (ODP Site 757) are around -6 to -7 over the interval investigated (Martin and Scher, 2004). This observation supports the conclusion from stable isotopes that a proto polar-front had formed during the Eocene, separating water masses in the northern and southern Indian Ocean (Zachos et al., 1992). Increased wind stress on the Southern Ocean as a result of Drake Passage opening may have contributed to the formation of this front. Results from this study raise the possibility that the early and intermediate stages of this gateway opening were significant for climate through reductions in oceanic heat transport and/or stimulating biological pumping of carbon dioxide through nutrient upwelling.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMPP21B1681S
- Keywords:
-
- 1040 Radiogenic isotope geochemistry;
- 1615 Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0414;
- 0793;
- 4805;
- 4845 Nutrients and nutrient cycling (0470;
- 1050);
- 4912 Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0414;
- 0793;
- 1615;
- 4924 Geochemical tracers