IMPLICATIONS OF LONG-TERM SURVIVAL OF ALLUVIAL GOLD IN GLACIATED LANDSCAPE AS REVEALED BY COSMOGENIC 3He

The high density of native metal grains tends to result in their transport and storage on bedrock surfaces in river systems. The ability to measure cosmogenic ³He in individual alluvial metal grains [1] provides the potential to determine their long-term transport-storage history beyond the residence of bedload sediment. While extremely long exposure histories have been measured in alluvial metal grains from unglaciated regions [1], the efficiency with which repeated glacial cycles have removed alluvial metal grains stored on bedrock surfaces is unknown. The ³He content of 2-50 mg native gold grains from nine streams in upland Scotland are significantly in excess of nucleogenic production based on measured Li concentration (<1 ppb) in the gold. Using cosmogenic ³He production rate of 25 atoms/g/year and shielding equivalent to typical stream water depths, over 95% of the grains have cosmogenic He concentrations that require surface residence times beyond the Last Glacial Maximum, with 20% of grains appearing to have survived from before the onset of northern hemisphere glaciation. This implies that bedrock glacial erosion and sub-glacial fluvial erosion over the last 2.7 Myr have not efficiently stripped land surfaces of pre-existing sediment. We will explore the extent to which this can be used to constrain glacial erosion of mountain landscapes in mid-latitudes.

[1] O. Yakubovich et al. (2019). Chemical Geology 517, 22-33.