Million-year lag times in a post-orogenic sediment conveyor

Abstract

Understanding how sediment transport and storage will delay, attenuate, and even erase the erosional signal of tectonic and climatic forcings has bearing on our ability to read and interpret the geologic record effectively. Here, we estimate sediment transit times in Australia’s largest river system, the Murray-Darling basin, by measuring downstream changes in cosmogenic 26Al/10Be/14C ratios in modern river sediment. Results show that the sediments have experienced multiple episodes of burial and reexposure, with cumulative lag times exceeding 1 Ma in the downstream reaches of the Murray and Darling rivers. Combined with low sediment supply rates and old sediment blanketing the landscape, we posit that sediment recycling in the Murray-Darling is an important and ongoing process that will substantially delay and alter signals of external environmental forcing transmitted from the sediment’s hinterland.

Controls on denudation along the East Australian continental margin

“…[2018a], quantifying sediment residence times in the hillslope domain is possible via detailed sampling of colluvium along longitudinal transects. Introducing a third cosmogenic radionuclide -namely, in-situ produced 14 C with a substantially shorter half-life (5730 years) -also reduces the minimum sediment residence times that can be resolved, as demonstrated by Hippe et al [2012] in the Bolivian Altiplano and by Fülöp et al [2020] in Australia's Murray-Darling Basin.…”

“…In total, we report 10 Be results in 160 samples, and 26 Al results in 67 samples. Of the reported data, 40 10 Be and 47 26 Al are previously unpublished data, and the rest have been compiled from the OCTOPUS database [Codilean et al, 2018] and more recent studies, namely, Portenga et al [2016], Godard et al [2019], Eccleshall [2019], Fülöp et al [2020], and Mariotti et al, [2021]. Basin areas for the mainstem samples range between 35 and 139,000 km 2 for Macquarie Rivulet (Illawarra) and Fitzroy River, respectively, draining east, and between 3,300 and 150,000 km 2 for Lachlan River and Thomson River, respectively, draining west.…”

“…26 Al/ 10 Be ratios range between 4.99 ± 0.31 (± 1s) (Mitchell) and 7.86 ± 0.77 (Don), east of the continental divide, and between 3.96 ± 0.32 (Barcoo) and 6.39 ± 0.40 (Maranoa), west of the continental divide (Table S5). On both sides of the divide, all but a few of the samples plot below the 26 Al/ 10 Be steady-state erosion -continuous exposure island ( Figure S3; see also Figure 2 in Fülöp et al [2020] and Figure 6.12 in Eccleshall [2019]), indicating that those samples have experienced complex exposure histories. As noted above, burial ages are calculated assuming a two-step history: continuous exposure in the source zone followed by deep burial (during which production totally ceases) within the colluvial/fluvial system resulting in a decrease of the 26 Al/ 10 Be ratio from its initial input value, and finally recent reexposure.…”

“…Rivers may be tapping older sediment -for example previously buried floodplain material or adjacent older Pleistocene terraces [e.g., Wittmann et al, 2011] or dune sands [e.g., Vermeesch et al, 2010;Eccleshall, 2019] -with depressed 26 Al/ 10 Be ratios being the result of deep burial and decay of both 26 Al and 10 Be. Depressed Al/ 10 Be ratios have also been observed in settings where instead of deep burial, overall sediment fluxes are low and thus rivers continuously rework shallow sediment deposits that are widespread [e.g., Struck et al 2018a,b, Fülöp et al, 2020. Here burial is not deep enough to completely stop the production of 26 Al and 10 Be, but both hillslope and fluvial transit times are long enough that even with (or sometimes even in the absence of) shallow burial there is sufficient decay of 26 Al and 10 Be to detectably alter the Al/Be signal.…”

“…In this study we provide a comprehensive inventory of 10 Be-based denudation rates (n=160) and 26 Al/ 10 Be ratios (n=67) from rivers draining Australia's Great Dividing Range (Figure 1) between the Mitchell River in southeast Victoria (≈38° S latitude) and the Barron River in north Queensland (≈16° S latitude). We compiled published data from the OCTOPUS database [Codilean et al, 2018] and more recent studies [Portenga et al, 2016;Godard et al, 2019;Eccleshall, 2019;Fülöp et al, 2020;Mariotti et al, 2021], and present new 10 Be and 26 Al data from a further 40 and 47 basins, respectively. Our aim is to identify major first order controls on 10 Be-derived denudation rates along the length of the margin, explore east-west differences in rates of denudation, and place these rates in a global context by comparing to other high-elevation passive continental margins.…”