Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation

Abstract

Our understanding of the deglacial evolution of the Antarctic Ice Sheet (AIS) following the Last Glacial Maximum (26,000–19,000 years ago)1 is based largely on a few well-dated but temporally and geographically restricted terrestrial and shallow-marine sequences2,3,4. This sparseness limits our understanding of the dominant feedbacks between the AIS, Southern Hemisphere climate and global sea level. Marine records of iceberg-rafted debris (IBRD) provide a nearly continuous signal of ice-sheet dynamics and variability. IBRD records from the North Atlantic Ocean have been widely used to reconstruct variability in Northern Hemisphere ice sheets5, but comparable records from the Southern Ocean of the AIS are lacking because of the low resolution and large dating uncertainties in existing sediment cores. Here we present two well-dated, high-resolution IBRD records that capture a spatially integrated signal of AIS variability during the last deglaciation. We document eight events of increased iceberg flux from various parts of the AIS between 20,000 and 9,000 years ago, in marked contrast to previous scenarios which identified the main AIS retreat as occurring after meltwater pulse 1A3,6,7,8 and continuing into the late Holocene epoch. The highest IBRD flux occurred 14,600 years ago, providing the first direct evidence for an Antarctic contribution to meltwater pulse 1A. Climate model simulations with AIS freshwater forcing identify a positive feedback between poleward transport of Circumpolar Deep Water, subsurface warming and AIS melt, suggesting that small perturbations to the ice sheet can be substantially enhanced, providing a possible mechanism for rapid sea-level rise.

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Figure 1: Location map.
Figure 2: Climate development from the Last Glacial Maximum to the Holocene (25–7 kyr ago).
Figure 3: IBRD flux in the Scotia Sea compared to climate changes during the last deglaciation.
Figure 4: Three-dimensional pattern of temperature anomalies at 14.8–14 kyr ago (AID6).

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Acknowledgements

We acknowledge support from the Deutsche Forschungsgemeinschaft (DFG grant numbers We2039/7-1, Ri525/17-1 and Ku683/9-1 to M.E.W. and G.K.), the University of Cologne (to M.E.W.), the US NSF Antarctic Glaciology Program (grant numbers ANT-1043517 to P.U.C. and ANT-1341311 to A.T.), the US NSF Paleoclimatology Program and the Japan Agency for Marine-Earth Science and Technology (to A.T.), and Helmholtz funding through the Polar Regions and Coasts in the changing Earth System (PACES) programme (to X.Z., G.L. and G.K.). Our study was also part of the Southern Ocean Initiative of the International Marine Past Global Change Study (IMAGES) program. We thank W. F. Budd for comments on Antarctic ice-sheet dynamics, and M. Winstrup and S. Rasmussen for advice on comparing ice-core chronologies. Experiments with the Bern3D were performed in the Department of Climate and Environmental Physics, University of Bern, and with funding through the Oeschger Center for Climate Change.

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Contributions

M.E.W. conceived the idea for the study and, with P.U.C., wrote most of the manuscript. G.K. selected the core sites and provided geochemical data. A.T. oversaw the modelling contributions and helped write the manuscript. R.G. provided insight into iceberg routing and associated ice-sheet modelling. D.S. helped develop the age model and provided biogenic opal data. G.L. and X.Z. contributed results from the COSMOS model. L.M., M.O.C. and T.F. contributed results from Bern3D and LOVECLIM models. C.O. contributed uncertainty estimates on the different age models. All authors commented on the manuscript.

Corresponding author

Correspondence to M. E. Weber.

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The authors declare no competing financial interests.

Additional information

Further data are available at http://dx.doi.org/10.1594/PANGAEA.819646.

Extended data figures and tables

Extended Data Figure 1 Deglacial dust chronology.

Five common tie points (TP1 to TP5, indicated by green vertical bands) depict consistent changes in slope and reproducible lows and highs between magnetic susceptibility (a, b), Fe (c) and Ca (d) records of deep-sea sites MD07-31333 and MD07-313412, and the non-sea-salt Ca record (e) of the EDML ice core27.

Extended Data Figure 2 Uncertainty estimates for AIDs.

Conservative error estimates (2σ) rely on bootstrapping of different age models and projecting them on to AIDs. a, Errors of the MD age model12 based on tie point correlation only. Black dots depict the centre of the AID and its absolute uncertainty range. Black error bars at the boxes mark the relative uncertainty with respect to the centre. Grey error bars show the absolute uncertainty of the beginning and end of each AID. b, Errors including the EDML1 (ref. 47), and EDC3 (ref. 46) uncertainties. c, Relative duration of AIDs and related uncertainties. d, Error propagation of the three different age scales through the last deglaciation. IU is interpolation uncertainty. Note that uncertainties are highly correlated for nearby ages. Accounting for this correlation, the duration of each AID as well as the time between two AIDs is significantly more accurate than its absolute age uncertainty.

Extended Data Figure 3 X-radiograph images from Scotia Sea Site MD07-3134.

IBRD (bright dropstones) are embedded in a matrix-supported diatomaceous mud. Low IBRD contents are documented for the Last Glacial Maximum (LGM, 24.7 kyr ago) and the Holocene (8.8 kyr ago), whereas higher numbers indicate enhanced iceberg routeing through Iceberg Alley during three distinct deglaciation phases (centre panels): AID8 (MWP-19KA), AID7 and AID6 (MWP-1A).

Extended Data Table 1 Uncertainty estimates

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Weber, M., Clark, P., Kuhn, G. et al. Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation. Nature 510, 134–138 (2014). https://doi.org/10.1038/nature13397

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