Antarctic ice sheet and climate evolution during the mid-Miocene
Authors:   , Chorley Hannah 
Published: March 2021

<p>The mid-Miocene provides an important example relevant to the response of the East Antarctic Ice Sheet (EAIS) to future anthropogenic climate change. Geological observations and earth system modelling show a broad link between declining carbon dioxide (CO2) concentrations and increasing size and sensitivity of ice sheets in the past. Future projections show CO2 concentrations could reach up to 1000 ppm before the end of the century, with global temperatures 4-5°C warmer - a climate not seen since the mid-Miocene. This time period is therefore becoming increasingly important to understanding future Antarctic Ice Sheet (AIS) response, as CO2 concentrations are already at Pliocene levels (∼400 ppm). An improved, more detailed understanding of the response of the AIS to past climatic variability provides important context for interpreting how the AIS will respond to future climate change under high CO2 scenarios.
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<p>A dynamic EAIS characterised the mid-Miocene, with major variations in both volume and extent of terrestrial and marine ice sheets. While global climate remained warmer than present-day throughout, this interval was punctuated by an episode of unusual warmth within the Miocene Climatic Optimum (MCO, ∼17-15 Ma). The MCO is one of the warmest intervals since the onset of Antarctic glaciation, with CO2 concentrations of up to 840 ppm during peak warmth and coastal regions characterised by temperate vegetation and mean summer temperatures (MST) of up to ∼10°C. This warmth terminated with major cooling and ice expansion across the mid-Miocene Climate Transition (MMCT, ∼14.8-13.8 Ma).
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<p>A ∼50 m thick ice-cemented terrestrial glacial sequence was recovered in drill cores from the Friis Hills, McMurdo Dry Valleys in 2016. A chronostratigraphic framework for the cores based on magnetostratigraphy, 40Ar/39Ar isotopic ages, and limited biostratigraphic constraints, revealed 15 sedimentary cycles of the advance and retreat of a temperate alpine glacier system between ∼15.1-13.8 Ma. Each cycle consists of traction tills and moraines deposited during ice advance and intervening glacio-fluvial to glacio-lacustrine lithofacies deposited during ice retreat. This record highlights the influence of increasing glacial-interglacial variability across the MMCT, with till facies becoming progressively thicker, drier and of wider provenance post 14.4 Ma, while interglacial sediments remained similar to those that characterised the late-MCO, sustaining tundra vegetation and MSTs of 6-7°C.
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<p>An ensemble of model simulations were produced for a recently published mid-Miocene topography and a range of CO2 concentrations, Transantarctic Mountain (TAM) uplift scenarios, and glacial-interglacial orbits in order to better understand the mechanisms driving EAIS variability during the early to mid-Miocene. Sedimentological and palynological data for glacial-interglacial periods of the early to mid-Miocene provide the primary constraint on ice extent and temperature variability. Results of this model-data comparison were used to assess the likely boundary conditions for the MCO and MMCT, and inferred TAM elevations of 300-500 m lower than present-day, modelled CO2 concentrations up to 780 ppm during periods of peak warmth, and a transition to lower CO2 across the MMCT. The onset of marine-based ice advance across the continental shelf was inferred between 280-460 ppm modelled CO2, however, the persistence of a significantly retreated, thick EAIS under even the highest modelled CO2 concentrations is not consistent with proxy data constraints and implies a strong hysteresis effect in the model. The presence of localised tundra vegetation under low CO2 concentrations in the model supports the persistence of higher plants in coastal lowlands post-MMCT, following their extinction at higher elevations after ∼13.8 Ma.
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<p>Terrestrial, marine, and far-field records were reconciled to assess glacial-interglacial variability and evolution of the EAIS across the mid-Miocene. While 15 cycles were identified within the Friis Hills record, only 7.5 of these are well enough constrained by the age model to be correlated to climate cycles in the δ 18O record, spanning ∼160 ka of the late-MCO and inferring a terrestrial-terminating AIS responding to local insolation controlled by precession. This is consistent with eccentricity modulated precession control implied in other coastal Antarctic and far-field records during the MCO, but results presented here also support a two stepped climatic shift at ∼14.6 and ∼13.8 Ma during the MMCT. This stepwise shift in climatic cooling is attributed to declining CO2, with two boundaries in long-term atmospheric CO2 concentrations crossed during this time: (1) A shift to CO2 concentrations below 460 ppm in the model supported the growth of annual sea-ice and advance of small-scale marine-based ice into the Ross Sea. (2) At 13.8 Ma, a further decline in CO2 concentrations to below 280 ppm supported perennial sea-ice development, limiting the influence of warm, deep-water upwelling, resulting in large-scale marine-based ice advance, ultimately stabilising the AIS. This stepwise mid-Miocene cooling implies threshold behaviour of the AIS during a long-term 200-300 ppm general decline in CO2 proxy records.
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