West Antarctic Ice Sheet retreat driven by Holocene warm water incursions

Abstract

Glaciological and oceanographic observations coupled with numerical models show that warm Circumpolar Deep Water (CDW) incursions onto the West Antarctic continental shelf cause melting of the undersides of floating ice shelves. Because these ice shelves buttress glaciers feeding into them, their ocean-induced thinning is driving Antarctic ice-sheet retreat today. Here we present a multi-proxy data based reconstruction of variability in CDW inflow to the Amundsen Sea sector, the most vulnerable part of the West Antarctic Ice Sheet, during the Holocene epoch (from 11.7 thousand years ago to the present). The chemical compositions of foraminifer shells and benthic foraminifer assemblages in marine sediments indicate that enhanced CDW upwelling, controlled by the latitudinal position of the Southern Hemisphere westerly winds, forced deglaciation of this sector from at least 10,400 years ago until 7,500 years ago—when an ice-shelf collapse may have caused rapid ice-sheet thinning further upstream—and since the 1940s. These results increase confidence in the predictive capability of current ice-sheet models.

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Figure 1: Map of modern bottom-water temperatures on the ASE shelf.
Figure 2: Proxy data from Holocene marine sediments constraining environmental changes in PIB.
Figure 3: Variability of CDW advection onto the ASE shelf in comparison to potential forcing mechanisms of ice-sheet change in West Antarctica since 12 kyr bp.
Figure 4: Variability of CDW advection onto the Amundsen Sea shelf and SHWW during the past 150 yr.

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Acknowledgements

This study is part of the Polar Science for Planet Earth Programme of the British Antarctic Survey and the PACES II (Polar Regions and Coasts in the changing Earth System) programme of the Alfred-Wegener-Institut. It was funded by the Natural Environment Research Council (NERC), NERC grant NE/M013081/1 and the Helmholtz Association. This work was also funded (in part) by The European Research Council (ERC grant 2010-NEWLOG ADG-267931 HE). We thank the captain, crew, shipboard scientists and support staff participating in RV Polarstern expeditions ANT-XXIII/4 and ANT-XXVI/3, and are grateful to R. Downey, T. Williams, V. Peck, H. Blagbrough, R. Fröhlking and S. Wiebe for their assistance. Furthermore, we thank D. Hodgson, D. Vaughan and P. Dutrieux for discussions, and S. Schmidtko for providing data.

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Contributions

C.-D.H. conceived the idea for the study and together with J.A.S., G.K. and R.D.L. wrote the manuscript. G.K., C.-D.H. and K.G. collected the PS69 sediment core and together with J.A.S., P.E.J. and J.P.K. the PS75 cores. C.-D.H., J.A.S. and S.J.R. developed the 14C age models for the PS75 cores. R.D.L. designed Fig. 1. T.J.A. conducted the 210Pb measurements on the PS69 core and provided its age model. G.K., C.-D.H., J.A.S., J.P.K. and P.E.J. undertook the sedimentological analyses. D.A.H. measured stable isotopes on the foraminifer shells, while M.G. and H.E. analysed the trace metals. C.R.P., S.K. and M.W. analysed the foraminifer assemblages. All co-authors commented on the manuscript and provided input to its final version.

Corresponding author

Correspondence to Claus-Dieter Hillenbrand.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks R. McKay 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

Extended Data Figure 1 Radiocarbon chronologies for Holocene sediments from PIB.

The age–depth plots for cores PS75/160 (a) and PS75/167 (b) are based on AMS 14C dates obtained from calcareous microfossils (Extended Data Table 1). The red diamonds mark the calibrated median AMS 14C ages, with the error bars indicating the maximum and minimum ages. The vertical dashed line marks the overlapping age of 8.2 kyr bp, corresponding to 380 cm depth in core PS75/160 and 230 cm depth in core PS75/167, where the benthic and planktic δ13C records of the two records were spliced (Fig. 3a, b).

Extended Data Figure 2 Down-core concentrations of radiogenic lead and caesium isotopes in sediments from the outer Amundsen Sea shelf.

The concentration of unsupported 210Pb (210Pbxs) for sediments from giant box core PS69/251 is displayed on a logarithmic scale (a), while the concentration of 137Cs is displayed on a linear scale (b). Error bars denote ±1 s.d. of the 210Pb and 137Cs concentrations. The 210Pbxs concentration is about 115 Bq kg−1 at the sediment surface and declines exponentially with core depth in the upper 6 cm. The 210Pb activity is at the detection limit or lower below 6 cm depth. The calculated 210Pbxs flux is 100 Bq m−2 yr−1, which is in reasonable agreement with the expected flux from atmospheric deposition56. The 137Cs activity is at or below the detection limit throughout the core.

Extended Data Figure 3 Constant rate of supply (CRS) modelling of the down-core 210Pbxs profile on the outer Amundsen Sea shelf.

The CRS modelling (Methods) was conducted on the sediments from core PS69/251 using a modified method56. The unsupported 210Pb concentrations (210Pbunsupp.) are plotted versus mass depth; the dashed line marks the regression used to calculate the 210Pb concentration below 6 cm core depth. Open circles highlight samples with 210Pbxs concentrations at or below the detection limit. Error bars denote ±1 s.d. of the 210Pbxs concentrations.

Extended Data Figure 4 Age–depth relationship for sediments from the outer Amundsen Sea shelf.

The age model for the sediments from core PS69/251 is based on CRS modelling of the down-core 210Pbxs profile in the uppermost part of the core. Error bars denote ±1 s.d. of the calculated ages.

Extended Data Figure 5 Geochemical and chronological data of Holocene sediments from PIB.

Shown are down-core profiles of planktic (N. pachyderma sin.) and benthic (A. angulosa) δ18O and δ13C ratios, log-normalized Ba/Ti (LN(Ba/Ti)) and Ba/Zr (LN(Ba/Zr)) peak area ratios61 and radiocarbon dates for cores PS75/160 (a) and PS75/167 (b). AMS 14C dates were obtained from various calcareous microfossils19,21,53 (Extended Data Table 1) and are displayed as mean calibrated ages with ±2 s.d. error. The asterisk at the lowermost date from core PS75/160 indicates average of replicate dates from the same sample horizon. The ranges of δ13C values typical for AASW and CDW are indicated by the blue-green and yellow shaded areas, respectively. Seawater δ18O composition reported for the ASE is about −0.7‰ to −0.5‰ with reference to the international Vienna Standard Mean Ocean Water (VSMOW) standard in AASW and −0.3‰ to −0.1‰ VSMOW in CDW91, yielding an offset of about −0.4‰ VSMOW between surface and deep waters. The average offsets between the planktic and benthic δ18O records of cores PS75/160 and PS75/167 are −0.45‰ VPDB and −0.38‰ VPDB, respectively, which is consistent with the reported surface to deep water δ18O gradient (we note that the relation between the SMOW scale and the VPDB scale is linear92). While prominent peaks in LN(Ba/Ti) below about 220 cm depth in core PS75/167 are considerably higher than the background LN(Ba/Ti) values, the LN(Ba/Zr) peaks are not much higher in this core section. This relation suggests that the LN(Ba/Ti) and Ba/Ti peaks are caused by barium input through increased supply of terrigenous heavy minerals (including barite and zircon), which is confirmed by the down-core increase of terrigenous sand layers below approximately 220 cm core depth (see figure 2 in ref. 19).

Extended Data Figure 6 Magnesium/calcium ratios of benthic foraminifer shells in PIB.

The magnesium/calcium (Mg/Ca) ratios, which were measured on shells of the benthic foraminifer species A. angulosa from core PS75/160, are shown with an adjustment for a potential diagenetic Mg contribution by assuming a magnesium/manganese (Mg/Mn) ratio of 0.15 ± 0.05 mol mol−1 in the diagenetic coating (a), and with an adjustment for a potential diagenetic Mg contribution by assuming Mg/Mn ratios ranging from 0.00 to 0.25 mol mol−1 in the diagenetic coating (b). The Mg/Ca ratios in b are displayed as averages of samples taken from the time intervals 10.0–7.5 kyr bp (4 samples), 7.5–4.0 kyr bp (2 samples) and 4.0–0 kyr bp (4 samples), both without adjustment and with adjustment for potential diagenetic Mg contributions assuming Mg/Mn ratios in diagenetic coatings of 0.10, 0.15, 0.20 and 0.25 mol mol−1 (refs 68, 69, 70, 71, 72). In a, vertical error bars highlight the uncertainty of ±0.05 mol mol−1 for the Mg/Mn composition of the coating, and horizontal error bars show the ±2 s.d. range of the ages of the Mg/Ca samples, which were calculated from the calibrated AMS 14C dates obtained from neighbouring sample horizons (Extended Data Table 1).

Extended Data Figure 7 Holocene changes in foraminifer assemblages from PIB.

Benthic foraminifer assemblages (only the most abundant and selected benthic taxa are displayed), abundance of N. pachyderma sin. morphotype 2 (mt. 2) shells (in relation to all planktic foraminifera), abundance of agglutinated benthic foraminifera (in relation to all benthic foraminifera) and total foraminifer concentration (individuals per gram dry sediment) were analysed on core PS75/160. The δ13C composition of planktic and benthic foraminifer shells is also shown. Note major shift in foraminifer abundances and assemblages centred at 7.5 kyr bp. Benthic foraminifer taxa (Methods): Alabaminella weddellensis, Angulogerina angulosa, Angulogerina pauperata, Angulogerina spp. (=sum of all Angulogerina species), Globocassidulina biora, Globocassidulina subglobosa, Globocassidulina spp. (=sum of all Globocassidulina species), Nonionella bradii, Nonionella iridea, Nonionella spp. (=sum of all Nonionella species).

Extended Data Figure 8 SEM images of planktic foraminifer shells in Holocene sediments from PIB.

Whole shells and detailed shell surfaces of N. pachyderma sinistral morphotypes 1 and 2 from core PS75/160 are shown. Morphotype 1 (a–d) is encrusted with gametogenic calcite and dominates the upper section of the core, while thin-walled and non-encrusted morphotype 2 (e–g) dominates the lower section of the core. A scale bar (white dots) is shown in the lower right corner of each photo, and its length in μm is given under, at right. Note the indistinct chambers and gametogenic calcite secreted around the whole shell in morphotype 1, while the individual chambers and porous shells of morphotype 2 are clearly visible, thereby showing the beginning of encrustation (white calcite around pores; compare refs 41, 93, 94, 95). Morphotype 1 usually dominates the lower part of the water column in Antarctic and Arctic waters and is preserved in marine sediments, whereas morphotype 2 is abundant in the upper part of the water column and not preserved in the sediments41,93,94,95. Encrusted morphotype 1 is typical for the terminal life stage of N. pachyderma sin., while non-encrusted morphotype 2 is typical for its neanic (that is, adolescent) to adult stage41,93,94,95,96. In analogy with the SEM-defined progressive dissolution steps distinguished in ref. 97, the shells in a and b are affected by intermediate dissolution, the shell in image c is affected by initial to intermediate dissolution, the shell in image d is affected by initial dissolution and the shells in images e–g are well preserved.

Extended Data Table 1 Core locations and conventional and calibrated AMS 14C dates
Extended Data Table 2 Chronology of the upper 6 cm of core PS69/251

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Hillenbrand, CD., Smith, J., Hodell, D. et al. West Antarctic Ice Sheet retreat driven by Holocene warm water incursions. Nature 547, 43–48 (2017). https://doi.org/10.1038/nature22995

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