A multi-trace element coral record of land-use changes in the Burdekin River catchment, NE Australia

Previous studies have provided evidence for greater sediment input into the inner Great Barrier Reef (GBR) since the mid-nineteenth century, presumably caused by an increase in erosion due to intensive animal grazing after European settlement. Here we report a new trace element (Ba, Y, Mn) study of a Porites coral from Magnetic Island, northeastern Australia that grew continuously for 175 yr from 1813–1986. Increasing Ba and Y concentrations in the 1850–1890 coral growth bands are attributed to increased soil erosion caused by the introduction of livestock, mainly cattle, into the adjacent Burdekin River catchment, the largest contributor of suspended sediment to the GBR lagoon. In our study Y levels continued to rise with agricultural intensification; however, Ba concentrations reach a plateau after 1890, indicating that Ba/Ca ratios in near-shore corals may reflect additional factors than sediment input.Mn concentrations in the coral follow a more complicated pattern, which is clearly related to changes in catchment land-use and related erosion. Before the initiation of European settlement and the introduction of sheep into the Burdekin River catchment in 1854, coral Mn concentrations were uniformly low (< 1 ppm), yielding typical values for coral aragonite. However, after 1854, coral Mn levels rose dramatically, fluctuating in parallel with livestock numbers but with ever smaller amplitudes before reaching more normal values again by 1900. The coincidence in timing between the initial rise in coral Mn and the introduction of grazing animals in the Suttor and Belyando sub-catchments pinpoints the trace element source as the Burdekin River. A decrease in Mn levels since 1865 against a background of generally increasing land clearing, livestock numbers and sediment flux points to the existence of a transient, surficial Mn reservoir in the southern Burdekin River catchment, which is tentatively identified here as the pre-European topsoil on the Peak Range basaltic province. A brief return to moderate Mn levels between 1945–1960 is attributed to the expansion of the cattle industry into the upper Burdekin, Bowen and Suttor sub-catchments after World War II.This geochemical record confirms that trace elements in massive corals can be used to trace sediment input from neighbouring river catchments. In our study, Y concentrations are closely correlated with predicted increases in sediment flux due to land-use changes and prolonged droughts and so may be more useful than Ba for this purpose. Our Mn results attest to the rapidity of the ecosystem imbalance caused by the introduction of grazing animals, in particular, sheep into NE Australia. Within 30 yr, sheep had efficiently reworked and entirely removed the Mn-rich topsoil, which had conceivably accumulated during thousands of years of regular burning by Australia's indigenous population.