The regional climate and sea-level fingerprint of the ACR across the mid- to high-latitude Southern Hemisphere is difficult to reconcile. Recent modelling studies have demonstrated that it is possible to reconstruct the spatial pattern without substantial fresh water forcing in the Southern Ocean11,14. However, our modelling, together with the results of previous studies2,13,18, suggests that a significant fresh water input into the Southern Ocean provides a potential trigger for the ACR signal, a hypothesis supported by our field data. Our inferred decoupling of ice-sheet elevation from air temperature across the LGT implies ocean forcing was a primary driver of Antarctic-wide ice-sheet dynamics through this period. Independent ice-sheet modelling experiments, driven by transient Earth System Model (LOVECLIM) outputs that include fresh water hosing in the Ross and Weddell seas17 (see Supplementary Information), predict similar changes in ice-sheet geometry and ice-flow dynamics (Figs 3 and 4 and Supplementary Information). The modelled increase in freshwater flux strongly suggests the drawdown of the AIS during the ACR was sustained by a positive feedback operating within the Southern Ocean. Crucially, we find a freshening of surface waters leads to a weakening of Southern Ocean overturning, resulting in reduced Antarctic Bottom Water (AABW) formation, enhanced stratification and sea-ice expansion17 (Fig. 4). The increased delivery of relatively warm Circumpolar Deep Water (CDW)26 onto the continental shelf close to the grounding line of the AIS thermally erodes marine-based ice, maintaining a positive ice-ocean feedback (Fig. 4)17. High resolution ice sheet modelling suggests that this mechanism predicts increases in ice mass loss across the AIS during the ACR in excess of 800 Gt/a, with an average of ~400 Gt/a, making a GMSL contribution of ~0.3 to 0.1 m per century17, importantly this rate almost doubles during the period defined by MWP-1A (Fig. 3E). Modelling of the following millennium implies a marked reduction in mass loss from all sectors of the AIS including the Weddell Sea (Fig. 3E), reflecting reduced Southern Ocean stratification and resumption of AABW formation post ACR, in agreement with our observations from the Patriot Hills BIA.

Figure 4 LOVECLIM transient model simulations of Southern Ocean fresh water forcing showing temperature anomalies (°C) for the ACR (14 ka minus 15 ka; left-hand panels)17,18, and subsequent surface warming during the Younger Dryas chronozone (12 ka minus 14 ka right-hand panels), with sea surface temperatures and 0.1 m sea ice contour (A,B), ocean temperature anomalies at depth (C,D, averaged over 484–694 m), and ocean temperature anomalies across the Weddell Sea (60°W to 15°W) (E,F) (constructed using ferret http://ferret.pmel.noaa.gov/Ferret/). Full size image

The coincidence between changes in AIS elevation from the Patriot Hills, enhanced iceberg flux2, atmospheric temperature trends22,23, and Southern Hemisphere mid-latitude westerly airflow9 through the LGT (Fig. 3B,C) implies a tight coupling between the ice-ocean-atmosphere system. Recent work using absolutely-dated tree ring chronologies has identified an abrupt increase in the inter-hemispheric radiocarbon gradient as a result of increased upwelling of 14CO 2 –depleted abyssal waters from 12.7 ka12, coincident with the maximum southerly extent of the Intertropical Convergence Zone (ITCZ) and strengthening Southern Hemisphere Westerlies (SHW)9. Our results are consistent with these findings, suggesting that weaker SHW during the ACR enhanced Southern Ocean stratification and maintained a positive ice-sheet-ocean feedback that drove substantial drawdown of the AIS (Fig. 4). This positive feedback appears to have only been disrupted by the re-expansion of the tropical belt and Hadley circulation during subsequent Northern Hemisphere cooling, and anti-phase southern warming after 12.7 ka (Fig. 3), suggesting AIS dynamics are highly sensitive to global atmospheric circulation.

The Patriot Hills preserves a record of significant AIS ice-sheet drawdown, mass loss and meltwater discharge during the ACR and across the LGT, contrasting markedly with previous interpretations of the configuration in the Weddell Sea sector of the AIS, which predict limited ice sheet drawdown since the local Last Glacial Maximum (LGM)15. Previous terrestrial reconstructions, based upon cosmogenic isotope analysis, predict a maximum thinning of ~480 m since the LGM, that occurred predominately during the mid-Holocene, suggesting that the WSE only made a minor contribution to GMSL rise since the LGM25. These estimates contrast with model-based reconstructions from far-field sites16, recent ice-sheet modelling studies8, reconstructions of IRD in the Scotia Sea2, and, crucially, our estimate of ~600 m of ice sheet surface elevation change across the ACR and MWP-1A (Fig. 3). Whilst we cannot define an upper altitudinal limit of the pre-ACR ice sheet across the WSE, the results reported here are inconsistent with estimates based upon terrestrial cosmogenic reconstructions25.

We suggest these contrasts may reflect two factors: firstly, there is a question over the effectiveness of terrestrial cosmogenic isotope studies to truly reflect the former elevation of the LGM ice-sheet surface in areas of cold based non-erosive polythermalice sheet settings27,28. Secondly, it is possible under a scenario of dynamic deglaciation during the LGT that rapid regional bedrock isostatic variations may have effectively masked rapid ice-sheet elevation changes that have occurred during deglaciation and subsequently during the Holocene (Supplementary Information). Therefore, terrestrial cosmogenic isotope reconstructions from mountains and nunataks across the WSE are only likely to robustly reconstruct ice-sheet surface elevation changes during Holocene deglaciation25,29,30. This is an issue which requires future detailed analysis, with multiple lines of evidence pointing towards a dynamic history of ice-sheet change across the WSE during the Holocene29,30,31, with significant implications for defining the pre-Holocene history of this sector of the AIS. Innovative reconstructions, such as that provided by the Patriot Hills BIA, are urgently required to define in detail dynamic Antarctic ice-climate feedbacks and better constrain the ice sheets contribution to global sea level rise during periods of rapid climate transition such as the LGT.

Supported by marine geological evidence of enhanced iceberg calving2, and independent ice-sheet and Earth system modelling experiments17, the Patriot Hills BIA provides the first direct terrestrial evidence that the Antarctic ice sheet was highly responsive to global ice-ocean-atmosphere feedbacks during the LGT2,17. Modelling suggests this pattern could be Antarctic wide, sustained by ice-ocean feedbacks amplified by the delivery of CDW onto the Antarctic Continental Shelf. The counterintuitive finding of sustained ice-sheet mass loss across this sector of the AIS during a period of atmospheric cooling suggests that Southern Ocean AIS feedbacks were likely modulated by global atmospheric teleconnections during a period of asynchronous hemispheric climate change. Defining the details of these atmosphere-ocean-ice feedbacks is crucial to reducing uncertainty in sea level projections4,32, and understanding the implications of observed high-latitude Southern Hemisphere environmental changes today6,7, which may conspire to amplify future Antarctic ice mass loss.