Description: The West Antarctic Ice Sheet (WAIS) is one of the largest potential sources of future sea-level rise, with glaciers draining the WAIS thinning at an accelerating rate over the past 40 years. Due to complexities in calibrating palaeoceanographic proxies for the Southern Ocean, it remains difficult to assess whether similar changes have occurred earlier during the Holocene or whether there is underlying centennial- to millennial-scale forcing in oceanic variability. Archaeal lipid-based proxies, specifically glycerol dialkyl glycerol tetraether (GDGT; e.g. TEX86 and TEXL86), are powerful tools for reconstructing ocean temperature, but these proxies have been shown previously to be difficult to apply to the Southern Ocean. A greater understanding of the parameters that control Southern Ocean GDGT distributions would improve the application of these biomarker proxies and thus help provide a longer-term perspective on ocean forcing of Antarctic ice sheet changes. In this study, we characterised intact polar lipid (IPL)-GDGTs, representing (recently) living archaeal populations in suspended particulate matter (SPM) from the Amundsen Sea and the Scotia Sea. SPM samples from the Amundsen Sea were collected from up to four water column depths representing the surface waters through to Circumpolar Deep Water (CDW), whereas the Scotia Sea samples were collected along a transect encompassing the sub-Antarctic front through to the southern boundary of the Antarctic Circumpolar Current. IPL-GDGTs with low cyclic diversity were detected throughout the water column with high relative abundances of hydroxylated IPL-GDGTs identified in both the Amundsen and Scotia seas. Results from the Scotia Sea show shifts in IPL-GDGT signatures across well-defined fronts of the Southern Ocean. Indicating that the physicochemical parameters of these water masses determine changes in IPL-GDGT distributions. The Amundsen Sea results identified GDGTs with hexose-phosphohexose head groups in the CDW, suggesting active GDGT synthesis at these depths. These results suggest that GDGTs synthesised at CDW depths may be a significant source of GDGTs exported to the sedimentary record and that temperature reconstructions based on TEX86 or TEXL86 proxies may be significantly influenced by the warmer waters of the CDW.

Description: Reconstructing the advance and retreat of past ice sheets provides important long-term context for recent change(s) and enables us to better understand ice sheet responses to forcing mechanisms and external boundary conditions that regulate grounding line retreat. This study applies various radiocarbon dating techniques, guided by a detailed sedimentological analyses, to reconstruct the glacial history of Anvers-Hugo Trough (AHT), one of the largest bathymetric troughs on the western Antarctic Peninsula (WAP) shelf. Existing records from AHT indicate that the expanded Antarctic Peninsula Ice Sheet (APIS) advanced to, or close to, the continental shelf edge during the Last Glacial Maximum (LGM; 23-19 cal kyr BP [ = calibrated kiloyears before present]), with deglaciation of the outer shelf after ∼16.3 cal kyr BP. Our new chronological data show that the APIS had retreated to the middle shelf by ∼15.7 cal kyr BP. Over this 600-year interval, two large grounding-zone wedges (GZW) were deposited across the middle (GZW2) and inner shelf (GZW3), suggesting that their formation occurred on centennial rather than millennial timescales. Expanded sequences of sub-ice shelf sediments occur seaward of the inner GZW3, which suggests that the grounding line remained stationary for a prolonged period over the middle shelf. Grounding-line retreat rates indicate faster retreat across the outer to middle shelf compared to retreat across the middle to inner shelf. We suggest that variable retreat rates relate to the broad-scale morphology of the trough, which is characterised by a relatively smooth, retrograde seabed on the outer to middle shelf and rugged morphology with a locally landward shallowing bed and deep basin on the inner shelf. A slowdown in retreat rate could also have been promoted by convergent ice flow over the inner shelf and the availability of pinning points associated with bathymetric highs around Anvers Island and Hugo Island.

Description: Following the Last Glacial Maximum (LGM; ca. 23-19 calibrated [cal.] kyr before present [BP]), atmospheric and oceanic warming, together with global sea-level rise, drove widespread deglaciation of the Antarctic Ice Sheet, increasing the flux of freshwater to the ocean and leading to substantial changes in marine biological productivity. On the Antarctic continental shelf, periods of elevated biological productivity, often preserved in the sediment record as laminated (and sometimes varved) diatomaceous oozes (LDO), have been reported from several locations and are typically associated with the formation of calving bay re-entrants during ice sheet retreat. Understanding what drives the formation and deposition of LDOs, and the impact of deglacial processes on biogenic productivity more generally, can help inform how Antarctic coastal environments will respond to current and future ice sheet melting. In this study we utilise a suite of sediment cores recovered from Anvers-Hugo Trough (AHT), western Antarctic Peninsula shelf, which documents the transition from subglacial to glacimarine conditions following retreat of an expanded ice stream after the LGM. We present quantitative absolute diatom abundance (ADA) and species assemblage data, to investigate changes in biological productivity during the Last Glacial Transition (19-11 cal kyr BP). In combination with radiocarbon dating, we show that seasonally open marine conditions were established on the mid-shelf by 13.6 cal kyr BP, but LDOs did not start to accumulate until ∼11.5 cal kyr BP. The ∼1.4 kyr delay between the onset of seasonally open marine conditions and LDO deposition indicates that physiographic changes, and specifically the establishment of a calving bay in AHT, is insufficient to explain LDO deposition alone. LDO deposition in AHT coincides with the early Holocene climatic optimum (∼11.5 – 9.0 kyr) and is therefore explained in terms of increased atmospheric/ocean temperatures, high rates of sea and glacial ice melt and the formation of a well-stratified water column in the austral spring. An implication of our study is that extensive bathymetric mapping in conjunction with detailed core analyses is required to reliably infer environmental controls on LDO deposition.

Description: The Eocene-Oligocene Transition (EOT; ~34.4–33.7 Ma) was a major shift in Earth’s long-term climatic evolution, marking the cooling from the early Paleogene greenhouse to the icehouse regime that has prevailed from the Oligocene until today. However, it remains uncertain which landmasses were already covered by ice sheets during the Early Oligocene Glacial Maximum (EOGM; ~33.7–33.2 Ma), an interval of peak glaciation immediately following the EOT. The scarcity of earliest Oligocene climate records in both Arctic and Antarctic regions hitherto prevented the reconstruction of environmental conditions and ice-sheet extent during the EOGM. Such constraints, however, are critical for assessing ice–ocean–atmosphere interactions during the early stages of the Cenozoic icehouse. Here, we present the first shallow-marine drill-core record of earliest Oligocene environmental conditions in West Antarctica’s Pacific sector. It comprises marine mudstones documenting the presence of a cool-temperate Nothofagus-dominated forest situated within a marine archipelago at 73.5°S palaeolatitude. Any evidence for marine-terminating glaciers is lacking, thus no land-based ice or only small ice caps existed in West Antarctica during the EOGM. Our new EOGM temperature and topographical constraints allow for more reliable verification of a fully-coupled Earth System Model. It simulates a large East Antarctic ice sheet already during the EOGM. However, West Antarctica does not glaciate until ~26 Ma, thereby illustrating the significance of asymmetric Antarctic ice sheet response during initial Cenozoic glaciation and highlighting the importance of differential regional response for future cryospheric change.

Description: The Eocene-Oligocene Transition (EOT; ~34.4–33.7 Ma) was a major shift in Earth’s long-term climatic evolution, marking the cooling from the early Paleogene greenhouse to the icehouse regime that has prevailed from the Oligocene until today. However, it remains uncertain which landmasses were already covered by ice sheets during the Early Oligocene Glacial Maximum (EOGM; ~33.7–33.2 Ma), an interval of peak glaciation immediately following the EOT. The scarcity of earliest Oligocene climate records in both Arctic and Antarctic regions hitherto prevented the reconstruction of environmental conditions and ice-sheet extent during the EOGM. Such constraints, however, are critical for assessing ice–ocean–atmosphere interactions during the early stages of the Cenozoic icehouse. Here, we present the first shallow-marine drill-core record of earliest Oligocene environmental conditions in West Antarctica’s Pacific sector. It comprises marine mudstones documenting the presence of a cool-temperate Nothofagus-dominated forest situated within a marine archipelago at 73.5°S palaeolatitude. Any evidence for marine-terminating glaciers is lacking, thus no land-based ice or only small ice caps existed in West Antarctica during the EOGM. Our new EOGM temperature and topographical constraints allow for more reliable verification of a fully-coupled Earth System Model. It simulates a large East Antarctic ice sheet already during the EOGM. However, West Antarctica does not glaciate until ~26 Ma, thereby illustrating the significance of asymmetric Antarctic ice sheet response during initial Cenozoic glaciation and highlighting the importance of differential regional response for future cryospheric change.