ANALYSIS OF THREE COSMOGENIC ISOTOPES IN SUBGLACIAL COBBLES HELPS UNRAVEL GREENLAND’S EXPOSURE AND EROSION HISTORY

The size and erosion history of the Greenland Ice Sheet over the Pleistocene remain poorly constrained despite extensive research employing a variety of different chemical, physical, and isotopic techniques. Here, we report results of a new approach, cosmogenic nuclide analysis of subglacial and englacial cobbles, which allows us to infer the behavior of the ice sheet over time and space.

We collected 86 cobbles from glacial ice at the current margin and from proximal outwash channels at three locations in western Greenland: Kangerlussuaq (67°N), Ilulissat (69°N), and Upernavik (72°N). We measured the concentration of in situ ¹⁰Be in all cobbles (n = 86) and ²⁶Al and ¹⁴C in a subset (n= 14, those with the highest ¹⁰Be). Most cobbles contain very low concentrations of ¹⁰Be (median 1.0 x 10³ atoms g^-1), although the population is highly skewed; several cobbles have ~10⁴ and one cobble has ~10⁵ atoms g^-1 of ¹⁰Be. Ratios of ²⁶Al/¹⁰Be, which reflect exposure/burial histories, range from 5.0 ± 1.1 to 8.4 ± 1.2. Nine cobbles have ¹⁴C above detection limits, and the ¹⁴C concentrations form a significant linear relationship with the ¹⁰Be and ²⁶Al concentrations.

Overall, the low concentration of ¹⁰Be in most subglacial cobbles suggests that their source areas under the Greenland Ice Sheet have been deeply eroded since the ice sheet was established ~2.5 Ma. The source of the low but measurable ¹⁰Be is likely from muon-induced production in deep bedrock prior to ice sheet growth; the rocks we analyzed were then exhumed, plucked, and transported to the margin with little or no surface exposure. The few cobbles containing higher concentrations of ¹⁰Be were re-exposed during at least one warm period when the ice sheet was smaller than at present. The range of ²⁶Al/¹⁰Be ratios, with most showing no extended burial, but several indicating burial of at least hundreds of ky, indicates a variety of exposure/burial histories. The presence of measurable ¹⁴C (10 cobbles) and the positive ¹⁴C/¹⁰Be covariance (8 of 10 cobbles) suggest near-surface exposure after the Last Glacial Maximum, presumably during the Holocene Climate Optimum. The ¹⁴C/¹⁰Be ratio of ~6 that we measure is more consistent with muon-induced production under ice or soil cover than surface exposure (for which ¹⁴C/¹⁰Be is ~3).