2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 102-4
Presentation Time: 8:45 AM

IMPACTS OF MELT RUNOFF FROM THE GREENLAND ICE SHEET ON SEAWATER 234U/238U COMPOSITION


ARENDT, Carli A., ACIEGO, Sarah M. and STEVENSON, Emily I., Glaciochemistry and Isotope Geochemistry Lab, University of Michigan, 1100 N. University Ave, Ann Arbor, MI 48109-1005, carliana@umich.edu

Certain fluxes within the oceanic geochemical budget, including cyclic changes in the Sr, Pb, and Os isotopic composition of seawater, have been studied in relation to glacial/inter-glacial events. In contrast, the oceans’ U budget is thought to remain relatively stable on glacial/inter-glacial timescales because the residence time of U in seawater is high. However, recent studies have indicated substantial variations in the oceans’ U budget over time. The cause of these variations has been linked to the contribution of significant amounts of glacial melt (that accumulate excess 234U) to seawater. At locations where subglacial water have long residence times the U concentration in the subglacial water will increase and potentially contribute to a larger change in the oceans’ U budget and in the oceanic 234U /238U isotopic composition compared to locations where subglacial water have short residence times.

In this study we have examine the Greenland Ice Sheet (GrIS) by sampling both subglacial drainage at the toe of outlet glaciers directly connected to the Southern GrIS, and proximal seawater at three depths ~20 km off of the GrIS coast at four unique locations: Narsarsuaq, Ilulissat, Nuuk and Kulusuk. All four outlet glaciers in this study are located on unique bedrock of varying ages which provide distinctive chemical compositions of subglacial water draining from these locations. Glaciers with the highest residence times (as inferred from high pH, cation concentrations) have the highest U concentration and 234U/238U activity ratios and have the largest impact on seawater chemistry (see Table below).

Narsarsuaq Ilulissat Nuuk Kulusuk
Subglacial [U] (ppb) 1.010 0.046 0.006 0.005
Subglacial 234U/238U 1.263 1.008 1.082 1.089
Seawater 234U/238U 1.152 1.145 1.146 1.149

Chemical analysis of both subglacial drainage from GrIS outlet glaciers and adjacent seawater show that melt from Southern GrIS (Narsarsuaq) has a strong influence on the U composition of seawater. Our findings of notable variations of U in seawater related to changing subglacial melt contributions have substantial implications for paleoclimate studies that have relied on the assumption that seawater U compositions remain stable through time.