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

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dc.contributor.authorMartin, ANen_AU
dc.contributor.authorMeredith, KTen_AU
dc.contributor.authorBaker, AAen_AU
dc.contributor.authorNorman, MDen_AU
dc.contributor.authorBryan, Een_AU
dc.date.accessioned2023-02-09T23:49:44Zen_AU
dc.date.available2023-02-09T23:49:44Zen_AU
dc.date.issued2021-07-02en_AU
dc.date.statistics2022-11-21en_AU
dc.description.abstractDissolved silicon (dSi) is a key nutrient in the oceans, but data regarding Si isotopes in coastal aquifers are not widely available. Here we investigate the Si isotopic composition of 12 fresh and 16 saline groundwater samples from Rottnest Island, Western Australia, which forms part of the world's most extensive aeolianite deposit (the Tamala Limestone formation). In total, two bedrock samples were also collected from Rottnest Island for Si isotope analysis. The δ30Si values of groundwater samples ranged from −0.4 ‰ to +3.6 ‰ with an average +1.6 ‰, and the rock samples were −0.8 ‰ and −0.1 ‰. The increase in δ30Si values in fresh groundwater is attributed to the removal of the lighter Si isotopes into secondary minerals and potentially also adsorption onto Fe (oxy)hydroxides. The positive correlations between δ30Si values and dSi concentrations (ρ = 0.59; p = 0.02) and δ30Si values and Cl, but not dSi and Cl concentrations, are consistent with vertical mixing between the younger fresh groundwater and the deeper groundwater, which have undergone a greater degree of water–rock interactions. This has produced a spatial pattern in δ30Si across the aquifer due to the local hydrogeology, resulting in a correlation between δ30Si and tritium activities when considering all groundwater types (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 groundwater 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–aquifer 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. 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. © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License.en_AU
dc.description.sponsorshipThis research has been supported by the Australian Research Council (grant no. LP150100144). The publication of this article was funded by the open-access fund of Leibniz Universität Hannover.en_AU
dc.identifier.citationMartin, A. N., Meredith, K., Baker, A., Norman, M. D., & Bryan, E. (2021). The evolution of stable silicon isotopes in a coastal carbonate aquifer on Rottnest Island, Western Australia. Hydrology and Earth System Sciences, 25(7), 3837-3853. doi:10.5194/hess-25-3837-2021en_AU
dc.identifier.issn1607-7938en_AU
dc.identifier.issue7en_AU
dc.identifier.journaltitleHydrology and Earth System Sciencesen_AU
dc.identifier.pagination3837-3853en_AU
dc.identifier.urihttps://doi.org/10.5194/hess-25-3837-2021en_AU
dc.identifier.urihttps://apo.ansto.gov.au/dspace/handle/10238/14638en_AU
dc.identifier.volume25en_AU
dc.language.isoenen_AU
dc.publisherCopernicus Publicationsen_AU
dc.subjectStable isotopesen_AU
dc.subjectSiliconen_AU
dc.subjectCarbonatesen_AU
dc.subjectAquifersen_AU
dc.subjectCoastal regionsen_AU
dc.subjectGround wateren_AU
dc.subjectWestern Australiaen_AU
dc.subjectAustraliaen_AU
dc.titleThe evolution of stable silicon isotopes in a coastal carbonate aquifer on Rottnest Island, Western Australiaen_AU
dc.typeJournal Articleen_AU
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