Strontium isotope variation in the dissolved load and suspended sediments of Northern Hemisphere land terminating glaciers

Abstract:

Bedrock, hydrology and microbial communities are primary contributors to sub-glacial chemical weathering and therefore mediate the chemical composition of bulk glacial outflow. Chemical weathering associated with glaciers has attracted attention due to the possible link between increased chemical weathering during glacial retreat and control of the marine radiogenic strontium (⁸⁷Sr/⁸⁶Sr) ratios [1-3]. Here we contrast the differences in strontium isotope (⁸⁷Sr/⁸⁶Sr) compositions of the dissolved load (DL) and suspended sediments (SS) from bulk subglacial outflows from three, northern hemisphere ice masses. We sampled from sub-glacial outflows draining geographically and geologically distinct glacial termini from the (1) the South, East and West of the Greenland Ice sheet (GIS), (2) the Juneau Icefield and (3) the Columbia Icefield. The diversity in regional outlet facilitates the comparison of glaciers with differing climate, size, hydrology and bedrock. The magnitude of offset in ⁸⁷Sr/⁸⁶Sr ratios between the SS and DL (Δ^87/86Sr, ‰, = (⁸⁷Sr/⁸⁶Sr_SS - ⁸⁷Sr/⁸⁶Sr_DL)*1000) varies between -62 to +7 ‰ and shows a positive correlation with pH, regardless of differences in glacier size and bedrock lithology. Here we believe the magnitude of the offset between the DL and SS is due to variations in subglacial weathering environments driven primarily by residence times of both water and sediment within the glacial system, and secondarily by bedrock lithology. The most radiogenic Sr compositions (both DL and SS) are found draining the GIS (up to ⁸⁷Sr/⁸⁶Sr = 0.80716) indicating the GIS may have provided a significant source of radiogenic Sr to the oceans during times of deglaciation.

[1] Armstrong, R.L., (1971) Nature, v. 230, p. 132-133

[2] Capo, R.C., De Paolo, D.J., (1990) Science, v. 249, no. 4964, p. 51-55.

[3] Vance, D., Teagle, D.A.H., Foster, G.L. (2009), Nature, v. 458 p. 493-496.