The Middle Miocene Antarctic Climate Transition: Investigating Magnitude, Phasing, and Processes Involving Cryosphere Expansion and Global Cooling

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a project to investigate the Miocene climate transition as recorded in Ocean Drilling Program cores from the South Tasman Rise. Understanding mechanisms that force climate change is a major challenge in Earth Sciences. Middle Miocene (~13-15 Ma (million years ago)) geologic records describe significant global ocean/atmosphere cooling coincident with Antarctic cryosphere expansion and a lack of appreciable Northern Hemisphere ice sheets. This suggests that Antarctic climate and cryosphere evolution played a central role in late Cenozoic climate development. Given the complexity of traditional proxies (e.g. d18O), fundamental questions exist concerning the magnitude, phasing, and processes driving mid-Miocene ice volume and temperature change. The primary objectives of this project are to better understand: 1) magnitude and phasing of global cooling and Antarctic cryosphere expansion across this interval; 2) processes that forced Antarctic climate/cryosphere evolution at this time; and 3) effects of an expanding and increasingly stable Antarctic cryosphere on climate feedbacks at orbital periods (about 40, 100, and 400 thousand years). Recently developed geochemical paleotemperature proxies (Mg/Ca) should deconvolve mid-Miocene d18O temperature and ice volume signals. New data from ODP Site 1171 Miocene foraminifers suggest that Mg/Ca will yield paleotemperatures necessary to evaluate ice volume and temperature changes. This project will integrate high-resolution (2-6 kyr (thousand year)) planktonic and benthic foraminifer Mg/Ca, d18O, d13C, and faunal records to address these objectives. This novel multi-proxy approach will test hypotheses regarding mid-Miocene Antarctic cryosphere evolution and global cooling. Further inquiry will address: 1) processes forcing the mid-Miocene climate shift, and 2) dynamic influences of climate feedbacks across this shift. The research will: i) assess the southwest Pacific/Southern Ocean hydrographic response to this climate shift, ii) evaluate possible processes/feedbacks involved in cryospheric expansion using records with age resolution of ~2-6 kyr to distinguish 40 and 100 kyr orbital periodicity, and iii) characterize the evolution of the climate spectrum at orbital time-scales through the mid-Miocene climate transition. To achieve the proposed objectives, high-resolution (~2-6 kyr) multi-proxy geochemical and faunal investigations will be conducted on a pair (ODP Sites 1170 and 1171) of unusually high quality marine CaCO3 sequences of mid-Miocene age (~16.8 to 12 Ma) drilled on the South Tasman Rise (STR) during ODP Leg 189. STR sequences provide a rare opportunity to study the mid-Miocene global climate shift at a sensitive oceanographic interface in the high southern latitudes. In the mid Miocene, the STR was situated proximal to the confluence of subtropical Pacific circulation and the Antarctic Circumpolar Current. The STR marks the southern extent of the East Australian Current, a sensitive western boundary current supplying heat and moisture to high southern latitudes. Sites 1170 and 1171 are marked by superb-quality CaCO3 preservation, relatively high deep-sea sedimentation rates (~6 cm/kyr), and >80% recovery. The primary intellectual merit of this research is that it will provide an unprecedented opportunity to examine the magnitude, phasing, and processes influencing middle Miocene Antarctic cryosphere expansion and global cooling at orbital timescales. This research will focus the view of Cenozoic climate evolution, including processes and feedbacks influencing Earth's systems across a major climate transition. Dramatic changes occurring in Antarctica, which may be linked to observed tropical Pacific variability (ENSO), underscore the need to improve understanding of feedbacks influencing the development, extent, and stability of the Antarctic cryosphere on both long (tens of thousands to millions of years) and short (decadal to millennial) timescales. The impact of this research is broad, as it addresses major issues related to the evolution of the Antarctic cryosphere and its response to climate reorganizations that is of vital interest to society. Further broader impacts are the student training that will be accomplished during the course of the research.