PLIOCENE GREENLAND ICE SHEET GROWTH RECORDED BY IN SITU 10BE DECREASE IN MULTIPLE MARINE SEDIMENT CORES (Invited Presentation)

Scant evidence constrains the multi-million year history of the Greenland Ice Sheet, and two recently published cosmogenic nuclide studies offer potentially divergent views. ¹⁰Be concentrations and ²⁶Al/¹⁰Be ratios in rock at the bottom of the GISP2 ice core imply a few longer or many shorter ice-free periods (Schaefer et al., 2016). Low ¹⁰Be concentrations in two marine sediment cores off east Greenland are more consistent with persistent ice cover, although ²⁶Al/¹⁰Be ratios suggest inclusion of material that had near-surface exposure (Bierman et al., 2016). It is unclear, however, how well these records represent the behavior of the entire ice sheet.

We increase spatial coverage of marine core records with 51 in situ ¹⁰Be measurements of sediment from four Ocean Drilling Program sites (645, 1307, 918, 987) off the southern half of Greenland that span the last 3.5 to 8 Myr. ¹⁰Be concentrations on the Greenlandic landscape should have evolved through time in response to glaciation, with nuclide production during pre-glacial exposure and subsequent deglaciation events, and nuclide removal via radio-decay and erosion when the landscape was ice covered. Erosion strips sediments from the landscape with ¹⁰Be concentrations reflecting a convolution of these processes and delivers them to core sites.

Decay-corrected ¹⁰Be concentrations in Miocene-age sediments from the two oldest cores (sites 645 and 918) are >100,000 atoms g^-1, and concentrations in all four records exhibit a several-fold decline over the Plio-Pleistocene, reaching 5,000-30,000 atoms g^-1 by the late Quaternary. This common long-term decrease likely reflects a continent-wide expansion of glaciation that halted ¹⁰Be production and began the process of eroding deeper-sourced, ¹⁰Be-poorer material. Some regional differences are apparent, however, with consistently higher ¹⁰Be concentrations off the southern tip of Greenland than further north, suggesting that the southern part of the ice sheet has retreated more frequently and/or for longer durations during past warm periods. Shorter-term drops in ¹⁰Be are unrelated between the records but they line up with increases in sand content within each core, perhaps because they reflect more localized episodes of intensified glacial erosion that excavated deeper-sourced material containing less ¹⁰Be.