Controls on denudation along the East Australian continental margin

Abstract

We report a comprehensive inventory of 10 Be-based basin-wide denudation rates (n=160) and 26 Al/ 10 Be ratios (n=67) from 48 drainage basins along a 3,000 km stretch of the East Australian passive continental margin. We provide data from both basins draining east of the continental divide (n=37) and discharging into the Tasman and Coral Seas, and from basins draining to the west as part of the larger Murray-Darling and Lake Eyre river systems (n=11). 10 Be-derived denudation rates in mainstem samples from east-draining basins range between 7.7 ± 1.9 (± 1s; Mary) and 54.6 ± 13.7 mm kyr -1 (North Johnstone). Denudation rates in tributary samples range between 3.0 ± 0.7 (Burdekin) and 70.2 ± 18.9 mm kyr -1 (Liverpool). For west-draining basins, denudation rates are overall lower and with a more restricted range of 4.8 ± 1.2 (Barcoo) to 15.4 ± 3.6 mm kyr -1 (Maranoa) in mainstem samples, and between 4.4 ± 1.0 (Murrumbidgee) and 38.5 ± 7.8 mm kyr -1 (Murray) in tributary samples. East Australian denudation rates (median = 14.5 mm kyr -1 ) are similar to those found in other postorogenic landscapes (global median = 12.4 mm kyr -1 ) and the medians of the top 10% denudation rates recorded here (46.5 mm kyr -1 ) and in other passive margin settings are also similar, despite differences in topography and precipitation.These median denudation rate values are close to the 95 th percentile denudation rate for all tectonically passive basins (≈53 mm kyr -1 ) and are very similar to the global silicate weathering speed limit (≈58 mm kyr -1 ) calculated as the 95 th percentile of global soil weathering rates. The above suggests that in post-orogenic terrain, the overall rates of topographic decay have a 'speed limit' that is imposed by the rate at which rock is converted to soil by chemical weathering. Denudation rates along the East Australian margin correlate with topographic metrics at both the mainstem basin scale and at the smaller tributary basin scale suggesting that topography exerts the main control on rates of landscape lowering in this setting. An important link between denudation rate and rainfall is also inferred: the highest mainstem 10 Be denudation rates all occur in basins which also have the highest rainfall amounts and there is a strong correlation between the distance knickpoints have travelled upstream from the river mouth and basin area -a proxy for discharge and so to some extent also rainfall. On both sides of the divide, in all but a few of the samples, 26 Al/ 10 Be ratios are consistently lower than what is expected in a setting where sediments experience a simple and continuous exposure history. East of the continental divide, the lowest 26 Al/ 10 Be ratios are found in basins that also experience increased flood variability. We posit that there is a causal link between the hydrological variability that characterises the coastal rivers of eastern Australia and the observed non-steady state 26 Al/ 10 Be ratios: the periodic stripping of vertically accreted floodplains by large floods means that deepe...

Comparison of basin-scale in situ and meteoric <sup>10</sup>Be erosion and denudation rates across a rainfall, slope, and elevation gradient at George River, northeast Tasmania, Australia

“…This result differs from most studies, which show strong correlations between ε and mean basin slope at a global scale (Portenga and Bierman, 2011), at regional scales, across the Great Dividing Range on Australia's mainland (Fig. 9; Codilean et al, 2021;Nichols et al, 2014), and despite prior assessments of George River that suggest slope imparts a large control over erosion and sediment generation in the catchment (Jerie et al, 2003;Kragt and Newham, 2009). Instead, our finding is consistent with Mishra et al's (2018) suggestion that in low-slope, low-elevation, post-tectonic settings, the relationship between slope and erosion becomes secondary to precipitation, and this study presents one of the clearest examples of erosion along a topographically induced precipitation gradient in 10 Bei erosion literature.…”

“…Compared to measurements of ε on the Australian mainland, the mean value of ε for George River (10.5 ± 0.8 mm kyr -1 ) is of similar magnitude as the median erosion rate for all catchments draining the eastern flanks of the Great Dividing Range passive margin of mainland Australia (15.9 mm kyr -1 ; Fig. 9; Codilean et al, 2021). Average ε from George River is most consistent, however, with the erosion rates of mainland basins at the southernmost extent of the Great Dividing Range, across the Bass Strait, which are those that share similar topographic characteristics and geological histories as George River (Codilean et al, 2021).…”

“…Tasmania separated from mainland Australia during Cretaceous rifting of Antarctica and Australia and sits at the southern end of the Great Australian Escarpmenta steep arch-type escarpment that formed during the separation of Zealandia from mainland Australia in the Mid-to Late-Cretaceous (Fig. 1; Codilean et al, 2021;Crowder et al, 2019;Etheridge et al, 1987;Gaina et al, 1998;Griffiths, 1971;Gunn, 1975;Hayes and Ringis, 1973;Lanyon et al, 1993;Matmon et al, 2002;McDougall and van der Lingen, 1974;Mortimer et al, 2017;Persano et al, 2002;Sutherland et al, 2001;Weissel and Hayes, 1977).…”

“…CC BY 4.0 License. primary and secondary controls on erosion across thousands to tens of thousands of years (Aguilar et al, 2014;Carretier et al, 2018;Codilean et al, 2021;Delunel et al, 2020;Starke et al, 2020). For example, Delunel et al (2020) find that 10 Bei erosion rates across the European Alps are strongly linked to mean basin slope and influenced by uplift and glaciations.…”

“…A new compilation and analysis of 10 Bei erosion rates across the Great Dividing Range on eastern Australia, however, is the first to analyze landscape dynamics across a continent-spanning, passive, post-orogenic rift margin and finds that basin slope is most closely related to erosion at all spatial scales, more so than any other potential driver of erosion (Codilean et al, 2021).…”