Controlling variables of denudation across catchments draining to the Great Barrier Reef

Fluvial sediments transported to the Great Barrier Reef (GBR) can have a negative impact on the health of the reef, stemming from associated pollutants and the smothering corals. Policies have been attempting to to manage and limit sediment export from the inland catchments to the ocean for the past few decades. However, in order to plan for an efficient and sustainable land management, spatial and temporal variables controlling the denudation rates and thus sediments flux must be examined.

Here we use preliminary results from both ¹⁰Be concentrations in river sediments and gauge measurements to infer time integrated sediment fluxes integrating over the last 60 ka and the last few years, respectively. This study includes 7 major catchments situated along the east coast of Australia, spanning most of the GBR region from the Wet Tropics (-16°S) to the Fitzroy catchment (-25°S), presenting variables climatic and geomorphic settings. Samples collected for ¹⁰Be measurements from 44 sub-catchments were used to determine ¹⁰Be-derived denudation rates ranging from 4.2 ± 0.2 mm.ka^-1 to 101.7 ± 10.3 mm.ka^-1 with an overall mean of 18.3 ± 1.1 mm.ka^-1. 11 gauge stations have been selected near ¹⁰Be sampling points to act as spatial replicates with a shorter integrated measurement interval. The gauged derived denudation rates range from 0.9 ± 0.4 mm.ka^-1 to 66.7 ± 49.1 mm.ka^-1 with an overall mean of 23.4 ± 18.9 mm.ka^-1.

We explore the differences between these two datasets in order to constrain the variables controlling denudation rates at a short time scale (gauge data) and at a longer time scale (¹⁰Be data). We find that ¹⁰Be denudation rates are mostly controlled by the average slope (R²: 0.65) of the catchment and the yearly rainfall (R²: 0.47). Conversely, the gauged derived denudation rates don't appear to be controlled by simple climatic or geomorphic parameters. The ratio of ¹⁰Be/gauged denudation rates is highly variable, from 0.3 to 4.7 and does not correlate with expected variables, such as land use or gully density. These results highlight that the impact of post-European land use on sediment yield has been variable across the GBR. This suggests that management policies will need to be carefully targeted. Focusing on areas with high ¹⁰Be/gauged denudation ratios, may assist with reaching local water quality targets.