Year

2012

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

The organic matter (OM) content of surface sediments was characterised for two contrasting sedimentary environments: the tropical Arafura Sea continental shelf sediments between north Australia and East Timor, and the temperate Shoalhaven River and associated continental shelf sediments on the southeast Australian margin. The main goal of both investigations was to establish the sources and transformation of terrestrial organic matter (OM) and its delivery to continental shelf sediments. The study on the Arafura Sea focuses on the identification of OM of terrestrial origin in off-shore marine sediments. Accordingly, the OM was characterised by coupling compound-specific δ13C data of n-alkanes, n-alcohols and sterols to δ13C and Δ14C measurements of total organic carbon (TOC). As a result, it is demonstrated that the δ13C and Δ14C values of TOC resembling partial input of terrestrial OM (about 50%) in the most inshore sample are actually due to other biogeochemical processes that recycle carbon derived from the biodegradation of OM buried within the Holocene sequence. The study stresses the need for a multi-proxy method to characterise the origin of OM in marine sediments which, in this case, would have been otherwise misinterpreted as of partial terrestrial origin on the basis of δ13C and Δ14C values of TOC alone.

The study on the southeast Australian margin focuses on the factors controlling the preservation and composition of terrestrial OM during its transport. Accordingly, the origin of OM within surface sediments from the Shoalhaven River and associated shelf sediments was determined by coupling lipid biomarker analysis of n-alkanes, polycyclic aromatic hydrocarbons, n-alcohols, sterols, hopanols, n-fatty acids, iso and anteiso fatty acids and pentacyclic triterpenoid acids to the δ13C and Δ14C values of TOC. Furthermore, particle size analysis of bulk sediments and XRD and XRF measurements of the < 63 μm particle size fraction were utilised. The main outcome of the study was that the preservation efficiency of terrestrial OM was similar to that reported for similar types of continental margins, that is, about 70% of the terrestrial input is degraded in the oceanic environment. However, while on passive margins, remineralisation of OM seems to occur mostly in the lowland plain environment and little marine OM is added to the river-dominated delta deposits, in the Shoalhaven River and adjacent continental shelf, remineralization of terrestrial OM seems to occur mostly in the oceanic environment, and such loss is replaced by accumulation of OM from in situ marine production. At the same time, probably because of more limited residence times of OM in catchment areas, an amount of fossil OM escapes remineralisation in soils and is transported to the continental shelf sediments (contributing up to 25% of the marine sedimentary OM) where it is ultimately reburied and removed from oxidation and participation in the current carbon cycle.

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