Abstract
Oxygen isotopes from ocean sediments (δ18O) used to reconstruct past continental ice volumes additionally record deep water temperatures (DWTs). Traditionally, these are assumed to be coupled (ice-volume changes cause DWT changes). However, δ18O records during peak Middle Miocene warmth (~16–15 million years ago) document large rapid fluctuations (~1–1.5‰) difficult to explain as huge Antarctic ice sheet (AIS) volume changes. Here, using climate modelling and data comparisons, we show DWTs are coupled to AIS spatial extent, not volume, because Antarctic albedo changes modify the hydrological cycle, affecting Antarctic deep water production regions. We suggest the Middle Miocene AIS had retreated substantially from previous Oligocene maxima. The residual ice sheet varied spatially more rapidly on orbital timescales than previously thought, enabling large DWT swings (up to 4 °C). When Middle Miocene warmth terminated (~13 million years ago) and a continent-scale AIS had stabilized, further ice-volume changes were predominantly in height rather than extent, with little impact on DWT. Our findings imply a shift in ocean sensitivity to ice-sheet changes occurs when AIS retreat exposes previously ice-covered land; associated feedbacks could reduce the Earth system’s ability to maintain a large AIS. This demonstrates ice-sheet changes should be characterized not only by ice volume but also by spatial extent.
Highlights
Oxygen isotopes from ocean sediments (δ18O) used to reconstruct past continental ice volumes record deep water temperatures (DWTs)
Evidence points to a much reduced size of the dynamic wet-based Antarctic ice sheet (AIS) in the Miocene Climatic Optimum (MCO) compared with its early Oligocene counterpart[14,15,16,17], and large-amplitude δ18Oc fluctuations combined with sea-level estimates imply a highly dynamic cryosphere (Fig. 1 and Extended Data Fig. 1)
For the Miocene climatic transition (MMCT) glaciation there remains the observation of a large ice increase but little DWT cooling, raising the question of why, if there is a strong coupling between ice volume and DWT as assumed[1,3,23], did DWT vary so much less during the MMCT when ice-sheet growth was most rapid? Here we present new climate model results assessing the impact of ice-sheet size on DWT across the MCO and MMCT
Summary
Oxygen isotopes from ocean sediments (δ18O) used to reconstruct past continental ice volumes record deep water temperatures (DWTs). Over the past 40 million years, rapid expansion of the Antarctic ice sheet (AIS) during the Middle Miocene climatic transition (MMCT; ~14–13.8 million years ago (Ma))[3] stands out as one of the three periods of major ice growth in the δ18Oc record[1]. We present new climate model results assessing the impact of ice-sheet size on DWT across the MCO and MMCT.
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