Abstract
Abstract. The volume of the Antarctic continental ice sheet(s) varied substantially during the Oligocene and Miocene (∼34–5 Ma) from smaller to substantially larger than today, both on million-year and on orbital timescales. However, reproduction through physical modeling of a dynamic response of the ice sheets to climate forcing remains problematic, suggesting the existence of complex feedback mechanisms between the cryosphere, ocean, and atmosphere systems. There is therefore an urgent need to improve the models for better predictions of these systems, including resulting potential future sea level change. To assess the interactions between the cryosphere, ocean, and atmosphere, knowledge of ancient sea surface conditions close to the Antarctic margin is essential. Here, we present a new TEX86-based sea surface water paleotemperature record measured on Oligocene sediments from Integrated Ocean Drilling Program (IODP) Site U1356, offshore Wilkes Land, East Antarctica. The new data are presented along with previously published Miocene temperatures from the same site. Together the data cover the interval between ∼34 and ∼11 Ma and encompasses two hiatuses. This record allows us to accurately reconstruct the magnitude of sea surface temperature (SST) variability and trends on both million-year and glacial–interglacial timescales. On average, TEX86 values indicate SSTs ranging between 10 and 21 ∘C during the Oligocene and Miocene, which is on the upper end of the few existing reconstructions from other high-latitude Southern Ocean sites. SST maxima occur around 30.5, 25, and 17 Ma. Our record suggests generally warm to temperate ocean offshore Wilkes Land. Based on lithological alternations detected in the sedimentary record, which are assigned to glacial–interglacial deposits, a SST variability of 1.5–3.1 ∘C at glacial–interglacial timescales can be established. This variability is slightly larger than that of deep-sea temperatures recorded in Mg ∕ Ca data. Our reconstructed Oligocene temperature variability has implications for Oligocene ice volume estimates based on benthic δ18O records. If the long-term and orbital-scale SST variability at Site U1356 mirrors that of the nearby region of deep-water formation, we argue that a substantial portion of the variability and trends contained in long-term δ18O records can be explained by variability in Southern high-latitude temperature and that the Antarctic ice volume may have been less dynamic than previously thought. Importantly, our temperature record suggests that Oligocene–Miocene Antarctic ice sheets were generally of smaller size compared to today.
Highlights
Numerical paleoclimate models predict that with the current rate of ice volume loss several sectors of the West Antarctic marinebased ice sheet will disappear within the coming few centuries (e.g., Joughin et al, 2014; The IMBRIE Team, 2018), favored by ocean warming-induced collapse
If the longterm and orbital-scale sea surface temperature (SST) variability at Site U1356 mirrors that of the nearby region of deep-water formation, we argue that a substantial portion of the variability and trends contained in long-term δ18O records can be explained by variability in Southern high-latitude temperature and that the Antarctic ice volume may have been less dynamic than previously thought
Observations show that glaciers on East Antarctica are vulnerable to basal melt through warming of the ocean waters when they are grounded below sea level (Greenbaum et al, 2015; Miles et al, 2016; Shen et al, 2018; The IMBRIE Team, 2018), making the East Antarctic Ice Sheet (EAIS) not as stable as previously thought (Mcmillan et al, 2014)
Summary
Numerical paleoclimate models predict that with the current rate of ice volume loss (up to 109 ± 56 Gt yr−1; The IMBRIE Team, 2018) several sectors of the West Antarctic marinebased ice sheet will disappear within the coming few centuries (e.g., Joughin et al, 2014; The IMBRIE Team, 2018), favored by ocean warming-induced collapse. These models show that sensitivity to global warming is high where the ice sheet is grounded below sea level (Fretwell et al, 2013), such as in the Wilkes Land Basin (Golledge et al, 2017; Shen et al, 2018) On both glacial–interglacial (Parrenin et al, 2013) and longer-term Cenozoic timescales (Pagani et al, 2011; Zachos et al, 2008), Antarctic ice volume changes have been mostly linked to changes in atmospheric CO2 concentrations (pCO2; see e.g., Foster and Rohling, 2013; Crampton et al 2016), modulated by astronomically forced changes in insolation (e.g., Holbourn et al, 2013; Liebrand et al, 2017; Miller et al, 2017; Pälike et al, 2006b; Westerhold et al, 2005). Given that observations clearly link the recent instability of marine-based ice sheets to ocean warming, it becomes important to better constrain near-field sea surface temperatures (SSTs) from the Antarctic margin during the Oligocene and Miocene to improve our understanding of past ice sheet dynamics and the projections for the future
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