Antarctic ice sheet sensitivity to atmospheric CO ₂ variations in the early to mid-Miocene

Abstract

Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23–14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3–4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

Southern Ocean phytoplankton turnover in response to stepwise Antarctic cooling over the past 15 million years

“…At the same time, sea level dropped by between 30 and 60 m (23), consistent with expansion of a terrestrial Antarctic Ice Sheet across the continental shelf and into marine basins in East and West Antarctica. The presence of grounded ice on the continental shelf is indicated in the ANDRILL-1B core and by regional seismic records (9) and supported by ice-sheet-climate models (8). Model simulations also indicate that sea-ice expansion is likely to have occurred in concert with advance of grounded ice sheets onto the shelf (24).…”

“…(We note that certain key diatom taxa used to infer the presence of sea ice in ref. 9, notably Fragilariopsis truncata and Synedropsis cheethamii, have not been consistently identified in older floral lists from the Southern Ocean and failed to meet the threshold for inclusion in the present CONOP analyses.) In addition, probable sea-ice associated Miocene diatoms have been recorded from the Ross Sea region (26,27).…”

“…Highly variable ice sheets that characterized the mid-Miocene Climate Optimum (8,9) subsequently expanded during the cooling associated with the Miocene Climate Transition ∼14 Ma (10,11), at which time a large terrestrial ice sheet became a relatively permanent feature on the East Antarctic continent. Marine-based ice, grounded on bedrock below sea level in West and East Antarctica, continued to expand and contract through variable climates of the late Miocene and into the Pliocene (14-3 Ma) (12, 13), driving global sea-level fluctuations of between 20-to 30-m amplitude and up to 20 m above the present-day level (14).…”

Evolution of the early Antarctic ice ages

“…(ii) Why was hysteresis (i.e., glacial− interglacial asymmetry) apparently stronger for both the large OMT and the smaller Early Miocene ice sheets than for the large ice sheets of the Oligocene? One explanation for the long-term change in ice volume is that the large glacial ice volumes of the MOGI were possible because of higher topography in West Antarctica (34) that permitted formation of a large terrestrial ice sheet that also buttressed growth of ice sheets on East Antarctica (25,35). In this interpretation, tectonic subsidence and glacial erosion during the Late Oligocene caused a shift to a smaller marine-based ice sheet in West Antarctica (25,35), which limited the maximum size of the Early Miocene Antarctic ice sheets during peak glacial intervals.…”

“…The Early Miocene ice sheets may have been less responsive to astronomically paced changes in radiative forcing because of colder polar temperatures under lower CO 2 conditions from ∼24 My ago onward (7) or restriction of ice sheets to regions of East Antarctica above sea level following the Late Oligocene subsidence of West Antarctica (25,35). Another possibility is that the large ice sheets that characterized the peak glacials of the MOGI underwent rapid major growth and decay because of higher-amplitude glacial−interglacial CO 2 changes than during the Early Miocene.…”