Sediment concentrations in Greenlands icebergs

In Greenlands fjords, icebergs transport sediment from the continent to the coastal ocean, resulting in ice-rafted debris. Over glacial-interglacial cycles, ice-rafted debris provides a record of paleoclimate conditions in the Northern Hemisphere that can be observed through marine sediment cores. Furthermore, on annual to decadal time scales, sediment delivery from the ice sheet to fjords is thought to supply marine ecosystems with key, biolimiting nutrients. Observations of sediment concentrations and debris layer thicknesses in icebergs are limited, and estimates of the total ice-rafted debris flux rely on sparse evidence extrapolated across wide regions of the ice sheet. Over 2018 and 2019, we sampled 50 small icebergs on the East and West coasts of Greenland. We observe considerable variability in sediment concentrations and debris thickness in these icebergs, ranging over multiple orders of magnitude. To explain this variability, we present a numerical model for subglacial sediment entrainment and transport. We consider the effects of sliding velocity and effective pressure on debris layer thicknesses and sediment concentrations in basal ice. Our model results indicate that ice-rafted debris fluxes respond to the thermal regime at the base of the ice sheet, the grain-scale properties of underlying till, and the structure of the subglacial drainage system. Overall, this research provides a quantitative depiction of the controls on ice-rafted debris fluxes, and thus builds towards a more complete understanding of the interactions between the Greenland ice sheet and its subglacial sedimentary system.