Suspended sediment beneath permanent and seasonal ice, Ross Ice Shelf, Antarctica

Abstract

Time-series measurements of suspended particulate matter (SPM) concentrations over a tidal cycle at 7 sites in McMurdo Sound revealed little correlation with depth, current velocity, or salinity. Much of the homogeneity is attributed to vertical mixing that is possibly caused by the descent of wind-driven surface water near the edge of the Ross Ice Shelf. Current speeds preclude settling of all but the coarsest SPM particles on the sandy bottom of the Sound. Most particles remain in suspension to be swept by the mean flow towards the ice shelf. The main SPM components are terrigenous clay (44–74%), indeterminate biogenic particles (12–35%), diatoms (5–30%), and aggregates of mainly fecal origin (0–18%). Fecal pellets are abundant in plankton-rich waters near the seasonal ice edge, but diminish markedly to the south because of rapid settling and reduced plankton numbers. Terrigenous SPM also decreases proportionally southwards because of increasing biogenic SPM (including diatoms) accompanying intrusion of phytoplanktonbearing water from the Ross Sea and/or liberation of biogenic material from the melting seasonal ice. SPM at Ross Ice Shelf Project hole J9, 600 km to the south, has markedly lower concentrations (x = 0.68 mg/L) and a higher proportion of very fine grained, terrigenous components compared to McMurdo SPM (x = 3.00 mg/L). These differences reflect, respectively, settling during transport from the Ross Sea and a much reduced biomass in J9 waters. The diatom component at J9 is often highly worn and corroded which is consistent with a resuspension origin from diatom-bearing bottom sediments.