Organic matter sources, transport, degradation and preservation on a narrow rifted continental margin: Shoalhaven, southeast Australia
The continental delivery of fine grained siliciclastic sediment, together with the composition of exported terrigenous organic matter (OM), has a strong impact on the content and composition of sedimentary OM accumulating on contemporary oxic continental shelf sediments. The composition of terrigenous OM appears to be, in turn, largely related to the geodynamic setting of the drained margin. This study provides results from a comprehensive source-to-sink investigation along the Shoalhaven River (New South Wales) and its associated non-deltaic deposits on the SE Australian continental margin, a margin whose dimensions and morphology differ from those of standard passive margins. The OM content of 23 surface sediment samples collected along the river course, estuary and adjacent continental shelf was investigated by integrating bulk OM measurements, such as total organic carbon content (TOC), TOC/total N (C/N), carbon stable isotope (δ 13 C TOC ) and radiocarbon content of TOC (Δ 14 C TOC ), with lipid biomarker analysis. Geochemical data were coupled with sediment particle size analysis. The TOC content correlated strongly with the clay + silt content in all the environments, confirming the importance of fine grained particles - OM association as an influential preservation factor. A three-component mixing model was used to show that OM in continental shelf sediments consisted of 15-20% terrigenous biospheric OM (mostly modern plants), 15-25% fossil C, and marine OM. While such bimodal terrigenous input (modern plants and fossil C) is typical of active margins, this study reinforces the paradigm that OM residence times within major bioactive reservoirs during transfer (in turn related to geomorphic characteristics) strongly influence the composition of exported OM.
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