HR: 13:40h
AN: PP32C-01    [PDF]
TI: A History of Water Mass Circulation in the Paleogene Southern Ocean from Nd Isotopes
AU: * Scher, H D
EM: hscher@ufl.edu
AF: University of Florida, Department of Geological Sciences, 241 Williamson Hall, PO Box 112120, Gainesville, FL 32611 United States
AU: Martin, E E
EM: emartin@geology.ufl.edu
AF: University of Florida, Department of Geological Sciences, 241 Williamson Hall, PO Box 112120, Gainesville, FL 32611 United States
AB: Using fossil fish teeth from ODP site 689 (2080m, Maud Rise) we have generated a 28 Myr record of Nd isotope ratios and concentrations for the early middle Eocene to early Miocene at an average resolution of 250-300 kyr. $\epsilon$$_{Nd}$(T) values documented in this time series range from -9.5 to -7.35, and demonstrate a pattern of secular variations that is remarkably similar to benthic foraminiferal $\delta$$^{13}$C records from this site. This correlation suggests that secular variations of $\epsilon$$_{Nd}$(T) values observed at site 689 are related to changes in ocean circulation and may provide insight into the history of water mass circulation in the Southern Ocean. Early middle Eocene $\epsilon$$_{Nd}$(T) values average -9.25 and display little variation compared to younger portions of the record, which illustrate long term oscillations beginning in the late middle Eocene. Starting at 40.8 Ma $\epsilon$$_{Nd}$(T) values increase over a 6 Myr interval from -9.4 to -7.35 in a stepwise fashion. Although $\epsilon$$_{Nd}$(T) values begin to rise during the late middle Eocene the period of most rapid change occurs in the late Eocene (after 37 Ma). $\epsilon$$_{Nd}$(T) values begin to fall in the earliest Oligocene reaching -8.75 at 30 Ma, then rise to -7.75 during the late Oligocene, but fall again to -8.75 by the end of the Oligocene. A hiatus occurs from the latest Oligocene to early Miocene. Following that interval, late early Miocene values average -8.5. Throughout the record the most radiogenic Nd isotopic compositions ($\sim$ -7.5), are associated with high $\delta$$^{13}$C values (1.2-1.4 $\permil$), while nonradiogenic Nd isotopic compositions are associated with lower $\delta$$^{13}$C values (.2-.4 $\permil$). The subsidence curve constructed for site 689 indicates a middle Eocene paleodepth of 1200-1500 m. A mean $\epsilon$$_{Nd}$(T) value of -9.25 during the middle Eocene possibly reflects upward mixing of Warm Saline Deep Water (WSDW) from the Tethys Sea, which has been documented at a deeper site on the Maud Rise (690) on the basis of an oxygen isotopic inversion between the two sites (Kennett and Stott, 1990). If this interpretation is correct, these data are the first to characterize the Nd isotopic signature of Paleogene WSDW in the Southern Ocean. The rapid shift toward radiogenic $\epsilon$$_{Nd}$(T) values at 37 Ma in this time series is coeval with a change in climate-productivity-ventilation patterns at this site (Diester-Haass and Zahn, 1996) based on stable isotopes and benthic foraminiferal accumulation rates. We suggest that the early opening of the Drake Passage at 37 Ma created or strengthened the proto Antarctic Polar Front (pAAPF) south of site 689. Rapidly increasing $\epsilon$$_{Nd}$(T) values at this time could represent the Cenozoic precursor to Antarctic Intermediate Water (AAIW) combined with the inflow of radiogenic Pacific Water through the Drake passage. This interpretation in consistent with increasing $\delta$$^{13}$C values and the apparent position of the pAAPF based on microfossil assemblages in the Southern Ocean (Cooke et al., 2002). Oligocene oscillations of $\epsilon$$_{Nd}$(T) values may be related to the appearance of cold, dense Antarctic Bottom Water (AABW) following first major Antarctic ice growth. The presence of AABW may have caused the depth of the mixing zone between WSDW and AAIW to shoal, resulting in lower $\epsilon$$_{Nd}$(T) values at site 689. By the late Oligocene there is evidence that the Drake Passage was open to deep water flow (Scher and Martin, 2003) and it is likely that Circumpolar Deep Water (CPDW) became the bottom water mass at this site.
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 4267 Paleoceanography
DE: 4283 Water masses
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting