Rapid mineral surface weathering beneath the Greenland Ice Sheet shown by radium and uranium isotopes

Abstract

Glaciated watersheds are regions of intense physical and chemical weathering. In order to gain new insight on subglacial weathering processes, we measured uranium and radium isotopes from a proglacial river draining the Greenland Ice Sheet (GrIS). Time series samples were collected from the spring to mid-summer, a time period during which subglacial drainage pathways are thought to transition from slow-inefficient (distributed) to fast-efficient (channelized) systems. The 228Ra/226Ra activity ratio of the dissolved load varied from 5.2 ± 0.9–16.9 ± 3.6, which was significantly higher than the 228Ra/226Ra ratio of a suspended sediment load sample of 2.1 ± 0.07 and crustal values of ~1. The high 228Ra/226Ra in the dissolved load relative to the source rock material is indicative of mineral surface weathering induced by rapid and continuous flushing of the subglacial drainage network during the course of the melt season and those prior. The 234U/238U ratio (δ234U) varied between 33 and 106‰ with a discharge-weighted mean of 67‰; the seasonal evolution of δ234U did not correlate to geochemical indicators of subglacial meltwater storage time. An experiment designed to measure changes in δ234U with increasing meltwater storage times found that δ234U in the dissolved phase decreased rapidly with increasing storage time. Similarly, samples collected along a transect moving downstream from the ice sheet terminus had decreasing δ234U values from 63 to 15‰ further indicating that with increased weathering, the δ234U of meltwater decreases. Coupled with the relatively low δ234U and high 228Ra/226Ra, U appears to be impacted by rapid chemical weathering of subglacial and suspended sediments. The Leverett River discharge weighted U concentration was 0.13 nM; if this system is considered representative of the broader GrIS, then the total dissolved U flux from the GrIS would be on the order of 6.4 × 104 mol/y. Using a similar set of assumptions, the dissolved 228Ra and 226Ra flux from the GrIS was ~1.1 × 1014 dpm/y and ~ 5.5 × 1013 dpm/y, respectively. These estimates suggest that the 226Ra flux to the ocean from the GrIS is globally significant and that the 228Ra flux in particular is larger than most river inputs.