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Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier

A Corrigendum to this article was published on 14 September 2017

Abstract

The West Antarctic Ice Sheet is one of the largest potential sources of rising sea levels1. Over the past 40 years, glaciers flowing into the Amundsen Sea sector of the ice sheet have thinned at an accelerating rate2, and several numerical models suggest that unstable and irreversible retreat of the grounding line—which marks the boundary between grounded ice and floating ice shelf—is underway3. Understanding this recent retreat requires a detailed knowledge of grounding-line history4, but the locations of the grounding line before the advent of satellite monitoring in the 1990s are poorly dated. In particular, a history of grounding-line retreat is required to understand the relative roles of contemporaneous ocean-forced change and of ongoing glacier response to an earlier perturbation in driving ice-sheet loss. Here we show that the present thinning and retreat of Pine Island Glacier in West Antarctica is part of a climatically forced trend that was triggered in the 1940s. Our conclusions arise from analysis of sediment cores recovered beneath the floating Pine Island Glacier ice shelf, and constrain the date at which the grounding line retreated from a prominent seafloor ridge. We find that incursion of marine water beyond the crest of this ridge, forming an ocean cavity beneath the ice shelf, occurred in 1945 (±12 years); final ungrounding of the ice shelf from the ridge occurred in 1970 (±4 years). The initial opening of this ocean cavity followed a period of strong warming of West Antarctica, associated with El Niño activity. Thus our results suggest that, even when climate forcing weakened, ice-sheet retreat continued.

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Figure 1: Map and location of core sites on the seafloor ridge.
Figure 2: Core logs and core data for PIG sub-ice-shelf cores.
Figure 3: Processes and sedimentation beneath the PIG ice shelf.

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Acknowledgements

We thank D. Pomraning for help with designing and manning the hot-water drill equipment. Logistic and safety support was provided by K. Gibbon, D. Einerson, E. Steinarsson, F. McCarthy, S. Consalvi, S. King, the PIG support camp personnel, and the National Science Foundation (NSF) Antarctic support team. We particularly thank E. Steinarsson for his help with sediment coring. This research project was supported by NSF’s Office of Polar Programs under NSF grants including ANT-0732926 and ANT 0732730; by funding from NASA’s Cryospheric Sciences Program; by New York University Abhu Dabi grant 1204; and by the Natural Environment Research Council–British Antarctic Survey Polar Science for Planet Earth Programme. Work at the Lawrence Livermore National Laboratory (LLNL) was performed under contract DE-AC52-07NA27344; LLNL-JRNL-697878.

Author information

Authors and Affiliations

Authors

Contributions

J.A.S., R.B., D.G.V. and H.F.J.C. conceived the study, and M.S., M.T. and T.P.S. conducted the fieldwork. J.A.S. and N.F. were responsible for sediment-core analysis and J.A.S. led the writing of the paper. T.J.A. measured 210Pb and 137Cs levels and developed the age models. A.M.G. measured plutonium isotopes on the PIG B core. P.D. and A.J. provided the bathymetric compilation, multibeam imagery and knowledge of the seafloor beneath Pine Island Glacier, and C-.D.H. contributed expertise on glacial sedimentology and data interpretation. W.E. is responsible for analysis of clay minerals, organic carbon and total nitrogen, and S.C. performed the X-ray fluorescence (XRF) scanning. All authors contributed to data interpretation and writing of the manuscript.

Corresponding author

Correspondence to J. A. Smith.

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The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks M. Baskaran, M. Jakobsson and J. Wellner for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Core logs and core data for PIG sub-ice-shelf cores.

ac, For PIG C (a), PIG A (b) and PIG B (c) are shown, from left to right: simplified lithology; shear strength (closed black squares); water content (open squares); relative amounts of mud (particles of 0–63 μm; black fill), sand (particles of 63 μm to 2 mm; dark grey fill) and gravel (particles of >2 mm; light grey fill), magnetic susceptibility (MS; measured with a MS2F surface probe; red line); the percentage of smectite; Br area counts; and Corg/Ntot ratios. The classification of the facies (1, 2a or 2b) is shown at the right. Facies 1 is sedimentologically distinct from facies 2, and the measured parameters are consistent in all cores. The dashed horizontal line indicates the unit boundary.

Extended Data Figure 2 210Pb and 137Cs activity as a function of depth.

a, PIG C. b, PIG B. Error bars denote one standard deviation of 210Pb and 137Cs concentrations. Note that the concentration of 137Cs is at or below the detection limit throughout both cores. Pbxs, excess 210Pb. c, CRS modelling of the down-core profile of 210Pbxs in PIG C. The black line marks the regression analysis to calculate 210Pb concentration below 7 cm. d, CF:CS modelling of down-core 210Pbxs concentrations in PIG B. The regression line is used to calculate the CF:CS chronology for PIG B. Solid dots, data used in the regression; open dots, data not used.

Extended Data Figure 3 Age–depth models.

These models were calculated using the regression analysis in Extended Data Fig. 2. a, PIG C. b, PIG B. The horizontal dashed line represents the unit boundary between facies 1 and facies 2. Error bars denote one standard deviation and were calculated on the basis of error propagation35 (for PIG C) or the error on the regression line38 (for PIG B) (see Methods).

Extended Data Figure 4 Plutonium-isotope data.

a, Depth profile of 239 + 240Pu concentrations in PIG B (the error bars are derived from the expanded uncertainties in c). The abrupt increase in 239 + 240Pu levels at 5.25 cm to 4.5 cm depth, from levels that are below the detection limit (b.d.l.), equates to between 1951 ± 12 years and 1960 ± 6 years, according to the age model. This is consistent with the time of peak nuclear fallout recorded in Antarctica (1952–1956; refs 42, 43; Extended Data Fig. 5) and with the global peak observed in 1963. b, 239 + 240Pu levels plotted against age derived from the 210Pb-based age model (age uncertainties derived from the standard error of the linear regression). The dotted horizontal line marks the transition from facies 1 to facies 2. c, 239Pu/240Pu levels in PIG B are consistent with the Southern Hemisphere average 239Pu/240Pu fallout, 0.185 ± 0.047 (ref. 41). Expanded uncertainty is given for the 95% confidence interval; b.d.l. is below the detection limit of 0.5 fg Pu per millilitre of sample solution. Activity is calculated for sediment dry weight, using the following half-lives: 239Pu, 24,110 years; 240Pu, 6,563 years.

Extended Data Figure 5 Relative 239 + 240Pu concentrations for Antarctic ice cores.

Grey bars represent the J-9 ice core42, located on the Ross Ice Shelf; peak 239 + 240Pu concentrations are observed between 1952 and 1956; dph, disintegrations per hour. The black line represents a composite of six Antarctic ice cores, including cores from Pine Island Glacier (red line) and Thwaites Glacier (blue line)43.

Extended Data Figure 6 Core-to-core correlation between PIG C and PIG A.

Red line, PIG C; black line, PIG A. a, Ca/Ti ratios, which provide a precise measure of changes in sedimentation. b, Magnetic susceptibility (MS) values. In both panels, values have been offset to highlight correlations. The concurrent changes in physical data (matched also by sedimentological changes) and the proximity of the two cores suggest that the transition from coarse-grained to fine-grained sedimentation probably occurred at the same time in both cores.

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Smith, J., Andersen, T., Shortt, M. et al. Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier. Nature 541, 77–80 (2017). https://doi.org/10.1038/nature20136

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