Establishing records of Antarctic and Northern Hemisphere Glaciations
The publication of the Cenozoic benthic δ18O record (Miller et al., 1987) established a new baseline for understanding climate change during the Cenozoic Era. The development of benthic foraminifera Mg/Ca thermometry has provided, however, new means for evaluating the evolution of Earth’s cryosphere in the context of Cenozoic climate change. Our main interests have been in the transitions associated with the Antarctic and Northern Hemisphere glaciations. The benthic δ18O record showed that the Antarctic glaciation at the Eocene/Oligocene transition occurred in two major steps. Using Mg/Ca we showed that the early inception was associated primarily with global cooling of 3-4°C, with little ice growth whereas most of the sea level fall (~60 m) occurred rapidly during the final step (Katz et al., 2008; Lear et al. 2008; Wade et al., 2012). We also showed that the East Antarctic Ice sheet (EAIS) experienced substantial fluctuations through the Oligocene (Lear et al., 2004) and suggested, based on paired δ18O, Mg/Ca and Li/Ca measurements that following the peak deglaciation at the mid-Miocene Climate Optimum, the Antarctic ice sheet increased to near modern size, likely in response to 1-2°C global cooling driven by declining atmospheric pCO2 levels as reflected in increased seawater carbonate saturation state (Lear and Rosenthal, 2006; Lear et al., 2010). These records provide the framework for understanding the climate and eustatic response to Neogene climate change and the establishment of deep ocean circulation (e.g., Lear et al., 2003; Katz et al., 2011; Miller et al., 2010).
More recently, I have been focused on the transition from the warm Pliocene period to the Pleistocene climate dominated by strong glacial-interglacial variability. Our studies suggested that on average SST in the western Pacific warm pool (WPWP) remained stable during the Pleistocene, which led us to question the role of atmospheric pCO2 in driving the transition from the 41 to 100 kyr worlds (de Garidel-Thoron et al., 2005). While the stability of WPWP SST is debated, due to the uncertainty about changing seawater Mg/Ca at that time, subsequent studies support the idea that changes in pCO2 were small, and therefore likely not the direct cause of the Plio-Pleistocene and mid-Pleistocene transitions. Using benthic foraminifera Mg/Ca we documented cooling of ~3-4°C of North Atlantic deep water since the mid-Pliocene warm period (~3.2 Myr ago) to the present, consistent with marine and terrestrial temperature records from northern hemisphere sites (Sosdian and Rosenthal, 2009, 2010; Lawrence et al., 2010, McClymont et al., 2013). Combined with a new North Pacific record, which shows no long-term cooling at the same time, we now argue that ~21±10 m sea level equivalent of ice growth occurred from 3.15 to 2.75 Ma implying that significant portion of the sea level fall prior to the intensification of NHG occurred due to ice growth on Antarctica. We posit that the expanded Antarctic glaciation imparted changes to inter-hemispheric oceanic budgets of heat and salt, which fundamentally altered deep ocean circulation and contributed to the NHG (Woodard et al., in revision for Science as of July 2014). These new observations underscore our arguments for significant role of changes in deepwater circulation and interhemispheric heat exchange in the Plio-Pleistocene and mid-Pleistocene transitions (Lawrence et al., 2010; Woodard et al., in revision). These studies also highlight the sensitivity of the Antarctic ice sheets and its implications to eustatic sealevel changes to climate perturbations both past and present (Sosdian and Rosenthal, 2009; Miller et al., 2012, Woodard et al., in revision).
Changes in seawater chemistry introduces the largest uncertainties in our long term Mg/Ca reconstructions. To overcome the limitation of Mg/Ca paleothermometry, we are currently developing novel approach for reconstructing secular variations in seawater chemistry using both foraminifera and corals. The work combines both culture experiments and down core analysis.