Climate transitions across the Cenozoic: Insight from elemental ratios in benthic foraminifera and marine gastropods.

摘要

The Cenozoic climate record derived from the oxygen isotope ratios of benthic foraminifera (delta18Ob) displays several, short term steps signifying the glaciation of Antarctica (∼33.7 Ma) and the development (∼2.7 Ma) and intensification (∼0.9 Ma) of large-scale northern hemisphere glaciation (NHG), termed hereafter the late Pliocene transition (LPT) and mid-Pleistocene transition (MPT), respectively. The interpretation of the delta18Ob record, which is controlled by both temperature and the oxygen isotopic composition of seawater (delta o), is, however, not straightforward.;In the work presented here, I:(1) used Mg/Ca benthic foraminiferal paleothermometry paired with delta18Ob to construct high resolution records of deep ocean temperature and global ice volume to understand the underlying mechanisms of Pliocene-Pleistocene climate transitions and (2) developed a new salinity independent paleothermometer based on Sr/Ca ratios in marine gastropods and examined its potential for Cenozoic low-latitude sea surface temperature reconstructions and I constructed and applied a new regional Mg/Ca-temperature calibration based on downcore Mg/Ca variations to encompass changes in temperature and carbonate saturation in deep Atlantic Ocean.;The new, high resolution bottom water temperature record from North Atlantic Deep Sea Drilling Project site 607 indicates that the deep ocean cooled across both the LPT and MPT. The cooling across the MPT preconditioned the high latitudes for ice-sheet growth by modifying heat transport through changes in meridional temperature gradients. Across the LPT, the mean trend in sea level decreased by 20+/-25m whereas the MPT is associated with an increase in glacial-interglacial amplitude of sea level. I propose that the MPT is related to a change in ice sheet dynamics, specifically growth of thicker ice sheets, and that a threshold response to high-latitude cooling is not sufficient to explain the MPT.;The Sr/Ca- temperature calibration study, based on the marine gastropod Conus ermineus, shows that strontium incorporation into the aragonitic gastropod shell is most likely driven by a temperature influence on growth rate. To minimize the ontogenetic effect, I separated the Sr/Ca-temperature calibration into juvenile and adult growth stages. The application of the new low-latitude paleothermometer to fossil specimens shows that the low latitudes cooled along with high latitudes throughout the Cenozoic.