Sedimentary process inferences from Humic substances analysis and deposition rates (Western Ross Sea, Antarctica)

The first 40 cm of sediment of three basins in the Ross Sea were sampled using a box corer. Site Y1 was located close to the coast in Terra Nova Bay; the sediment of site Y3 was collected in a more distal basin in the central sector of the Ross Sea; finally site Y5 was sampled in the deepest zone of the Joides Basin. Sediment cores were sliced and analysed with a depth resolution of 2-4.5 cm. The distribution of humic substances and their structural features along the cores were determined and related to the pattern of Total Organic Carbon (TOC) and sedimentological data. The grain size distribution and the 210Pb inventories allow the sediment of the study sites to be characterised. The humic substance content in the sediment decreases, with a change in slope between 23 and 26 cm at Y1, between 12 and 15 cm at Y3 and constant values with further depth. At Y5 the depth profile of humic substance content shows constant values in the upper 17 cm and values decrease with further depth. The pattern of humic substance yield is similar to that found for TOC. The analysis of the elemental composition of the humic acids extracted from different sediment depths shows an increasing C/N atomic ratio at sites Y1 and Y3 and constant values along the Y5 core. The depth profile of the C/N atomic ratio is confirmed by the changes observed in the structural characterisation and indicates a shift from the freshly deposited organic matter on the sediment surface to more humified material (humin). The results obtained highlight a different sedimentation rate at the three sites as deduced from sedimentological analysis.