title: Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier.
abstract: The West Antarctic Ice Sheet is one of the largest potential sources of rising sea levels. Over the past 40 years, glaciers flowing into the Amundsen Sea sector of the ice sheet have thinned at an accelerating rate, 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 underway. Understanding this recent retreat requires a detailed knowledge of grounding-line history, 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.
authors.affiliation: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.
authors.forename: J A
authors.initials: JA
authors.lastname: Smith
authors.affiliation: Center for Permafrost (CENPERM), Department of Geosciences and Natural Resource Management, University of Copenhagen, 1350 Copenhagen K, Denmark.
authors.forename: T J
authors.initials: TJ
authors.lastname: Andersen
authors.affiliation: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.
authors.forename: M
authors.initials: M
authors.lastname: Shortt
authors.affiliation: Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA.
authors.forename: A M
authors.initials: AM
authors.lastname: Gaffney
authors.affiliation: Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775-7320, USA.
authors.forename: M
authors.initials: M
authors.lastname: Truffer
authors.affiliation: Department of Oceanography, Naval Postgraduate School, Monterey, California 93943, USA.
authors.forename: T P
authors.initials: TP
authors.lastname: Stanton
authors.affiliation: NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
authors.forename: R
authors.initials: R
authors.lastname: Bindschadler
authors.affiliation: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA.
authors.forename: P
authors.initials: P
authors.lastname: Dutrieux
authors.affiliation: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.
authors.forename: A
authors.initials: A
authors.lastname: Jenkins
authors.affiliation: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.
authors.forename: C-D
authors.initials: CD
authors.lastname: Hillenbrand
authors.affiliation: Institute for Geophysics and Geology, University of Leipzig, Talstrasse 35, D-04103 Leipzig, Germany.
authors.forename: W
authors.initials: W
authors.lastname: Ehrmann
authors.affiliation: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.
authors.forename: H F J
authors.initials: HF
authors.lastname: Corr
authors.affiliation: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.|Department of Earth Sciences, University of Geneva, 13 Rue des Maraîchers, CH-1205 Geneva, Switzerland.
authors.forename: N
authors.initials: N
authors.lastname: Farley
authors.affiliation: Department of Earth Sciences, University of Geneva, 13 Rue des Maraîchers, CH-1205 Geneva, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Crowhurst
authors.affiliation: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.
authors.forename: D G
authors.initials: DG
authors.lastname: Vaughan