UC Santa Cruz
Insight Into Climate Variations Of The Early Middle Eocene: High-Resolution Benthic Stable Isotope Data From Site 1408, Newfoundland Ridge
- Author(s): Wu, Fei
- Advisor(s): Zachos, James
- et al.
The middle Eocene is a transitional period from the greenhouse early Eocene to the ice-house Eocene-Oligocene Transition. Due to lack of high-resolution records, the details of this long-term cooling trend are not clear. Low-resolution atmospheric CO2 records suggest declining, but highly variable levels during the middle Eocene which might have driven transient climate variations. However, high-resolution stable isotope records are lacking to document the true climate patterns over this period. Moreover, orbital scale climate reconstructions are essential for reconciling how the sensitivity to orbital forcing changed, if at all, as the greenhouse boundary conditions changes from the early to late Eocene. High-resolution isotope and trace metal records are also essential for constraining the evolution of northern hemispheric sea-ice and continental glaciation as well as North Atlantic Deep Water in the middle Eocene.
To address these shortcomings, I have generated high-resolution stable isotope and trace element data for a site in the northern Atlantic on Newfoundland Ridge (IODP, Site 1408). The records span Chron20r (46.4~43.8 Ma), and are the first orbital-scale stable isotope records for the early middle Eocene. With the exception of the uppermost portion of the section, the δ18O data indicate gradual cooling of 0.8~1.2 ⁰C/Myr over much of the section from 46.4Ma to 44.3 Ma which is consistent with the long-term global cooling trend. This is accompanied by a decrease in Mg/Ca records, while the paired δ18O-Mg/Ca analysis shows no pronounced shift in δ18Osw implying no variation in ice volume. In the upper part of the section (44.3~43.8 Ma), the δ18O shows a slight reversal consistent with warming, while Mg/Ca proxy indicates a continuous cooling. One hypothesis for the apparent between the two records is that the water mass reflects changes in the salinity of local bottom water and a possible shift toward a higher-latitude, cooler and less salty water source. In addition, clear astronomically paced cycles are observed in the stable isotopic and lithological records, and spectral analysis demonstrate that the cycles are dominated by 41-kyr obliquity cycles, a unique feature of Paleocene-Eocene records which are typically dominated by precession and eccentricity. This could reflect either shift in orbital configuration (low eccentricity signals or change in Earth’s tilt), or gradual climatic variation toward the ice-house world in the middle Eocene. Interbasin δ13C gradient reveals relatively low δ13C values at this northern Atlantic site compared with records from the Southern Ocean, indicating the meridional circulation was northward and/or North Atlantic Deep Water formation was absent or weak during this time.