We present the first published uranium-series measurements from modern Greenland Ice Sheet (GrIS) runoff and proximal seawater, and investigate the influence of glacial melt on global seawater δ234U over glacial-interglacial (g-ig) timescales. Climate reconstructions based on closed-system uranium-thorium (U/Th) dating of fossil corals assume U chemistry of seawater has remained stable over time despite notable fluctuations in major elemental compositions, concentrations, and isotopic compositions of global seawater on g-ig timescales. Deglacial processes increase weathering, significantly increasing U-series concentrations and changing the δ234U of glacial meltwater. Analyses of glacial discharge from GrIS outlet glaciers indicate that meltwater runoff has elevated U concentrations and differing 222Rn concentrations and δ234U compositions, likely due to variations in subglacial residence time. Locations with high δ234U have the potential to increase proximal seawater δ234U. To better understand the impact of bulk glacial melt on global seawater δ234U over time, we use a simple box model to scale these processes to periods of extreme deglaciation. We account for U fluxes from the GrIS, Antarctica, and large Northern Hemisphere Continental Ice Sheets, and assess sensitivity by varying melt volumes, duration and U flux input rates based on modern subglacial water U concentrations and compositions. All scenarios support the hypothesis that global seawater δ234U has varied by more than 1‰ through time as a function of predictable perturbations in continental U fluxes during g-ig periods.
Bibliographical noteFunding Information:
Funding for this project was provided by the University of Michigan’s Rackham Graduate School and Department of Earth and Environmental Science through research grants awarded to C.A.A. and through Packard funding awarded to S.M.A.; and by the Woods Hole Oceanographic Institution’s Ocean and Climate Change Institute Arctic Research Initiative research grant to S.B.D. We thank Air Greenland for assistance with field logistics and the WHOI field team for aiding in our sample collection in Ilulissat. We thank Dr. Gideon Henderson for his generosity in allowing us access to the original box-model used in Henderson, 2002 . We would like to acknowledge Editor Timothy J. Shaw and our anonymous reviewers for their insightful comments and assistance in improving the presentation and accessibility of the methods utilized and outcomes derived in this study.
© 2018 Elsevier Ltd
- Greenland ice sheet
- Radiogenic isotopes
- Seawater chemistry
- Subglacial processes