STORIES FROM UNDER THE ICE: INVESTIGATING GLACIAL HISTORY AND PROCESS WITH COSMOGENIC NUCLIDES IN ICEBOUND COBBLES (Invited Presentation)

Surfaces below ice sheets have the potential to preserve long-term records of glacial history and process. Here, we study the isotopic composition of cobble-sized rocks sourced subglacially and transported to the ice margin by ice flow and/or the subglacial hydrologic system. These cobbles were at some time eroded from up-ice bedrock surfaces and contain cosmogenic nuclides indicative of the exposure, burial, and erosion history of the now subglacial landscape. Measured cosmogenic nuclides may have been produced during previous periods of surface or near-surface exposure (including interglacial periods as well as shorter-lived episodes of ice retreat such as the Holocene Climatic Optimum) and/or by deep muogenic production before initial glaciation of the landscape in the Pliocene.

We analyzed in situ cosmogenic ¹⁰Be in 86 cobbles collected directly from ice and outwash channels at three locations in western Greenland: Kangerlussuaq, Ilulissat, and Upernavik. Measured ¹⁰Be/⁹Be ratios were above blank level for all but several samples and form a right-skewed distribution. Calculated ¹⁰Be concentrations are generally low (median = 1.0 x10³ atoms g^-1) but 6 of the 86 clasts have concenetrations exceeding 10⁴ atoms g^-1. One sample contains 1.1 x10⁵ atoms g^-1, two orders of magnitude above the median and equivalent to >20 ky of surface exposure at sea level. Concentrations of ¹⁰Be are not separable by sample site or type. Analyses of cosmogenic ¹⁴C and ²⁶Al in the samples with the highest ¹⁰Be concentrations are in progress and will provide further insight about cobble exposure history.

The isotopic concentrations of these cobbles indicate a wide range of histories. Cobbles with little ¹⁰Be may record pre-glacial, deep muogenic production with no surface exposure until they were plucked and transported by erosive ice. Cobbles with more ¹⁰Be have been sourced from areas that experienced one or more past periods of surface exposure or areas with less erosive ice cover that did not strip pre-existing nuclides. The presence of ¹⁴C in these samples will indicate Holocene Optimum exposure while discordant ²⁶Al/¹⁰Be data will indicate older periods of exposure and long burial durations.