Efforts to improve sea level forecasting on a warming planet have focused on determining the temperature, sea level and extent of polar ice sheets during Earth’s past interglacial warm periods1,2,3. About 400,000 years ago, during the interglacial period known as Marine Isotopic Stage 11 (MIS11), the global temperature was 1 to 2 degrees Celsius greater2 and sea level was 6 to 13 metres higher1,3. Sea level estimates in excess of about 10 metres, however, have been discounted because these require a contribution from the East Antarctic Ice Sheet3, which has been argued to have remained stable for millions of years before and includes MIS114,5. Here we show how the evolution of 234U enrichment within the subglacial waters of East Antarctica recorded the ice sheet’s response to MIS11 warming. Within the Wilkes Basin, subglacial chemical precipitates of opal and calcite record accumulation of 234U (the product of rock–water contact within an isolated subglacial reservoir) up to 20 times higher than that found in marine waters. The timescales of 234U enrichment place the inception of this reservoir at MIS11. Informed by the 234U cycling observed in the Laurentide Ice Sheet, where 234U accumulated during periods of ice stability6 and was flushed to global oceans in response to deglaciation7, we interpret our East Antarctic dataset to represent ice loss within the Wilkes Basin at MIS11. The 234U accumulation within the Wilkes Basin is also observed in the McMurdo Dry Valleys brines8,9,10, indicating11 that the brine originated beneath the adjacent East Antarctic Ice Sheet. The marine origin of brine salts10 and bacteria12 implies that MIS11 ice loss was coupled with marine flooding. Collectively, these data indicate that during one of the warmest Pleistocene interglacials, the ice sheet margin at the Wilkes Basin retreated to near the precipitate location, about 700 kilometres inland from the current position of the ice margin, which—assuming current ice volumes—would have contributed about 3 to 4 metres13 to global sea levels.
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We thank J. Schutt, G. Faure and D. Schmidt for their sample collection at the Elephant Moraine and A. Grunow and the Byrd Polar Rock Repository for providing samples with a ‘PRR’ prefix. We also thank J. Paces, S. Hemming and T. Rasbury for their input. This research was funded by NSF 1644171 to T.B. and S.T.
The authors declare no competing interests.
Peer review information Nature thanks Carli Arendt and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
a, b, Sample shown in visible light (with location of dated horizons corresponding to data reported in Extended Data Table 1) (a) and the scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDS) compositional map showing variations in manganese (green for high Mn, black for low Mn) (b).
a, b, Sample shown in visible light (with location of dated horizons corresponding to data reported in Extended Data Table 1) (a) and the SEM/EDS compositional map showing Ca and Si (b). The sample exhibits an angular unconformity, indicating that the sample physically moved beneath the ice before accumulation began again. Clct, calcite.
The inset photograph shows sample PRR39222 under visible light with the location of δ234U measurements marked. The main plot shows the measured δ234U for PRR39222 at three horizons, revealing an increasing δ234U from top to bottom (Extended Data Table 1). Because of the high thorium (Th) contents, we cannot define a formation age and thus cannot identify a reliable δ234Ui. The purple curves in Fig. 3 represent the possible δ234Ui values for the top (higher δ234U) and bottom (lower δ234U) for any formation time. What we do not know is the absolute time at which this sample formed or the duration it formed over. However, we do know that: (1) the δ234Ui for the top and bottom of the sample must lie on these purple lines; (2) the sample must be younger than 1,500 kyr given that the measured δ234U is not in secular equilibrium. In addition to these known conditions, we can assume that the calcite in PRR39222 probably formed very rapidly as indicated by: (1) morphology, specifically radiating clusters of blade-like sparite; (2) lack of unconformities; (3) shared δ18O and δ13C composition with rapidly forming calcite from PRR50489, which is constrained by geochronology. Data from the literature as well as the geochronologic constraints presented in Extended Data Table 1 provide limits on the rate of sub-ice calcite formation (0.5 mm kyr−1 is shallow and 5 mm kyr−1 is steep). Given a known sample dimension of 4.5 cm, any assumed precipitation rate translates to a time duration for sample formation of 10–90 kyr. Assuming these durations, along with the requirement that the bottom and top of the sample intersects the purple curves in Fig. 3, permits us to define possible δ234Ui ingrowth histories (black arrows in Fig. 3). The rate of modelled 234U accumulation as recorded by PRR39222 is strongly controlled by assumed formation age with only a narrow time range yielding 234U ingrowth histories consistent with the other Wilkes Basin fluid histories. For example, if the sample were to have formed at 1,000 ka, we predict a change in δ234Ui of about 300% from the top to the bottom of this sample. Such rapid ingrowth histories result in δ234U compositions that would result in δ234U compositions that far exceeds anything observed in Antarctica (>6,000‰). If, however, the sample were to have formed at about 400 ka, the projected ingrowth histories would match both model projections and measured data for the Wilkes Basin. Only scenarios that place PRR39222 formation at roughly <500 ka yield projected ingrowth histories consistent with the blue curve. In addition to the above analysis, the occurrence of low δ234U (<500‰) in subglacial fluids is apparently rare, having been identified in this region only in samples older than about 300 ka. Collectively, this suggests that the 234U ingrowth history recorded by PRR39222 is at least consistent with formation at about 400 ka.
Extended Data Fig. 4 Long-term results of measurements of NBS 4321 (5.2919 × 10−5 ± 0.013 × 10−5 (0.25%)) at UCSC using an IsotopX X62, TIMS.
All uncertainties are absolute 2σ.
Extended Data Fig. 5 Steady-state activity ratio of 234U and 238U as a function of the flushing timescale for three different values of the 234U ejection factor.
The dotted line shows the assumed level of δ234U in meltwater. The assumed weathering timescale is 100 million years.
This file contains Supplementary Methods.
This file contains source data for Extended Data Table 1.
This file contains source data for Extended Data Table 2.
This file contains source data for Extended Data Table 3.
This file contains source data for Extended Data Table 4.
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Blackburn, T., Edwards, G.H., Tulaczyk, S. et al. Ice retreat in Wilkes Basin of East Antarctica during a warm interglacial. Nature 583, 554–559 (2020). https://doi.org/10.1038/s41586-020-2484-5
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