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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/hess-2020-429
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2020-429
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  08 Oct 2020

08 Oct 2020

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This preprint is currently under review for the journal HESS.

The evolution of stable silicon isotopes in a coastal carbonate aquifer, Rottnest Island, Western Australia

Ashley N. Martin1,2,3,4, Karina Meredith1,2, Andy Baker1,3, Marc D. Norman5, and Eliza Bryan1,2,3 Ashley N. Martin et al.
  • 1Connected Waters Initiative Research Centre, UNSW Sydney, Sydney, NSW2052, Australia
  • 2Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW2234, Australia
  • 3School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, NSW2052, Australia
  • 4Institut für Mineralogie, Leibniz Universität Hannover, Callinstraße 3, 30167 Hannover, Germany
  • 5Research School of Earth Sciences, Australian National University, Canberra, ACT2601, Australia

Abstract. Dissolved silicon (DSi) is a key nutrient in the oceans, but there are few data available regarding Si isotopes in coastal aquifers. Here we investigate the Si isotopic composition of 12 fresh and 17 saline groundwater samples from Rottnest Island, Western Australia, which forms part of the world’s most extensive aeolianite deposit (the Tamala Limestone Formation). Two bedrock samples were also collected from Rottnest Island for Si isotope analysis. The δ30Si values of groundwaters ranged from −0.39 to +3.60 ‰ with an (average: +1.59 ‰) and the rock samples were −0.76 and −0.13 ‰. Due to the relatively low concentrations of DSi (64 to 196 μM) and clay-forming cations in fresh groundwaters, the correlation between δ30Si values and DSi concentrations (ρ = 0.59, p = 0.02) may be explained by Si adsorption onto Fe-Al (oxy)hydroxides present in the aquifer. An increase in groundwater δ30Si in association with the occurrence of water-rock interactions may explain the spatial pattern in δ30Si across the aquifer, and is consistent with the correlation between δ30Si and tritium activities when considering all groundwaters (ρ = −0.68, p = 0.0002). In the deeper aquifer, the inverse correlation between DSi and Cl concentrations (ρ = −0.79, p = 0.04) for the more saline groundwaters is attributed to groundwater mixing with local seawater that is depleted in DSi (< 3.6 μM). Our results from this well-constrained, island aquifer system demonstrate that stable Si isotopes usefully reflect the degree of water-rock interactions, which is related to groundwater residence time and local hydrogeology. Our finding that lithogenic Si dissolution occurs in the freshwater lens and the freshwater-seawater transition zone on Rottnest Island appears to supports the recent inclusion of a marine submarine groundwater discharge term in the global DSi mass balance. Finally, geologically-young, carbonate aquifers, such as Rottnest Island, may be an important source of DSi in coastal regions with low riverine input and low oceanic DSi concentrations.

Ashley N. Martin et al.

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Short summary
The silicon isotopic composition of groundwaters from Rottnest Island, Western Australia were measured to investigate water-rock interactions in a coastal aquifer. Silicon isotopic ratios varied spatially across the island and were related to groundwater processes, such as vertical mixing and rainfall recharge frequency. We find that silicate dissolution occurs in the freshwater-seawater transition zone, supporting the recent recognition of marine groundwater discharge in oceanic silicon cycle.
The silicon isotopic composition of groundwaters from Rottnest Island, Western Australia were...
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