Session: Warm Climates (Mid-Holocene, Last interglacial, Deep-time, Pliocene)
Author: Agatha de Boer / email@example.com / Stockholm University
Co-author: Catherine Bradshaw, Met Office Hadley Centre, UK;
Petra Langebroek, Bjerknes Center for Climate Research, Norway;
Caroline Lear, Cardiff University, UK;
Daniel Lunt, University of Bristol, UK;
The geological record documents a dynamic Antarctic ice sheet during the Middle Miocene (16 – 14 Ma) against a background of relatively low CO2. Recent bottom water temperature reconstructions indicate significant bottom water temperature changes during the Middle Miocene Climate Optimum (MMCO, 17-14.7 Ma), but no significant cooling over the major ice sheet growth of the Middle Miocene Climate Transition (MMCT, 14.7-12 Ma). This implies the increase in seawater oxygen isotopic composition at the MMCT represents growth of a larger-than-modern ice sheet. Our new modelling results indicate the mechanism by which this decoupling of temperatures and ice volume can be achieved.
An ice-free Antarctic is warm and wet. Surface runoff from a very active hydrologic cycle forms a polar halocline preventing the freezing surface waters from ventilating the deep ocean. Ice sheet growth markedly reduces this precipitation and subsequent runoff, thereby making the near-freezing surface water around Antarctica saltier and able to form bottom water. Once the ice sheet has reached a continental scale, additional vertical growth does not further affect runoff significantly because precipitation has already reduced to a low level. Consequently, the polar salinity and temperatures are also little affected and hence neither is deep water production (which is in all cases produced in the south). Through orbitally paced scaling up and down of deep ocean ventilation, this mechanism is able to offer explanation for both the large amplitude variations in the MMCO benthic isotope records occurring whilst CO2 changes are no greater than 300ppm, and the lower amplitude isotopic variations following the MMCT (Holbourn et al., 2005, 2007, 2013; Kochhann et al., 2016 and references therein).
Estimates of the Antarctic ice volume increase at the prior Eocene-Oligocene Transition (34-33 Ma) are equivalent to the modern ice sheet (Lear et al., 2008; Liu et al., 2009). Taken together, our new modelling results and the existing isotope, temperature, vegetation and CO2 reconstructions suggests this large Oligocene Antarctic ice sheet had collapsed by the MMCO. This implies that the dynamism during the Middle Miocene operated on a much smaller ice sheet than previously thought.