Browsing by Author "Martin, AN"

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    The evolution of stable silicon isotopes in a coastal carbonate aquifer on Rottnest Island, Western Australia
    (Copernicus Publications, 2021-07-02) Martin, AN; Meredith, KT; Baker, AA; Norman, MD; Bryan, E
    Dissolved 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.
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    Lithium and strontium isotope dynamics in a carbonate island aquifer, Rottnest Island, Western Australia
    (Elsevier, 2020-05-01) Martin, AN; Meredith, KT; Norman, MD; Bryan, E; Baker, AA
    Water-rock interactions in aquifer systems are a key control on water quality but remain poorly understood. Lithium (Li) isotopes are useful for understanding water-rock interactions, but there are few data available for groundwater aquifers. Here we present a Li isotope dataset for rainfall and groundwater samples from a carbonate island aquifer system: Rottnest Island, Western Australia. This dataset was complemented by strontium (Sr) isotope and major and trace element data for groundwaters, and leaching experiments on bedrock samples. The δ7Li values and 87Sr/86Sr ratios of fresh groundwaters ranged from +23 to +36‰ and 0.709167 to 0.709198, respectively. Mass balance calculations indicated that silicate weathering supplied ~60 and 70% of dissolved Li and Sr in fresh groundwaters, respectively, with the remainder provided by atmospheric input, and carbonate weathering; for major cations, the majority of calcium and sodium (Na) are supplied by carbonate weathering and atmospheric input, respectively. The estimated low proportion of Sr produced by carbonate weathering was surprising in a carbonate aquifer, and the 87Sr/86Sr data indicated that the silicate Sr source had low Rb/Sr and 87Sr/86Sr ratios. There was an increase in the maximum δ7Li values in fresh groundwaters (+36‰) relative to the maximum value in rainfall and seawater (ca. +31‰). As clay minerals are undersaturated in fresh groundwaters, this increase may be explained by Li isotope fractionation associated with ion-exchange reactions on clays and iron(oxy)hydroxides. In the more saline groundwaters, the minimum δ7Li values decreased with depth to +14.5‰, suggesting increased silicate mineral dissolution in the deeper aquifer. These results reveal the importance of water-rock interactions in a coastal carbonate aquifer, and demonstrate the usefulness of Li isotopes for tracing weathering reactions in an environmental setting where traditional weathering tracers, such as sodium and Sr isotopes, are less appropriate. Crown Copyright © 2020 Published by Elsevier B.V
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    Lithium isotope dynamics of closed lake systems, Rottnest Island
    (Copernicus GmbH, 2019-04-09) Meredith, KT; Baker, AA; Martin, AN; Bischoff, K
    Stable lithium isotopes (7Li) are useful for tracing hydrochemical processes such as water-sediment interactions and delineating the source of lithium (Li). Due to developments in trace element isotope methods, 7Li has proven to be a robust tracer of catchment processes and when used with traditional isotopic methods can provide deeper insights into environmental processes. Rottnest Island contains a number of permanent hypersaline lakes (with salinities over 330 g/L) that cover approximately 4.7 km2 area within a Pleistocene to mid-Holocene eolianite bedrock (Tamala Limestone). The lakes were once connected to the ocean but over thousands of years have become closed, and are now assumed to be driven by rainfall and evaporation cycles. This field-based study uses lithium isotope ratios measured on sediments and water to investigate Li dynamics of two closed lake systems on Rottnest Island, Western Australia. This work forms part of an ongoing six-year project. Four sediment cores were collected from the deepest sections of the lakes. Sediment samples were collected from textural or sedimentological changes in the cores and were ground and ashed at 450 C for at least 5 h. Column chromatography procedures were undertaken and Li isotope ratios were analysed by multi-collector inductively coupled plasma mass spectrometer. Sediment, shell, salt crusts, soil, palaeosol, microbialite, groundwater, lake water, rainfall and seawater end-members were analysed for 7Li, Li concentration, carbon-13 and oxygen-18. The results showed that the Li isotopes of the lake sediments ranged from +17.3h to +75.6h with an average of +42.4h(n = 25). All 7Li values for sediments lie between the soil (+16.5hand palaeosol (+68.5hvalues. Other end-members such as the microbialites (+23.9h, groundwaters (range from +22.8 to +35.7h, rainfall (+24.2h, bedrock (+21.0hand seawater (+33hsamples plot within this range. The shells (+6.3hand salt crusts (+15.6h were more depleted. The reported range in isotope values is the largest observed to date, and the enriched isotope values of the sediments are related to higher Li concentrations (R2=0.7). The processes responsible for this large variation suggest evaporation trends and the change in source materials (marine vs. terrestrial) over time. The lake sediment samples provide some of the first Li isotope measurements for coastal closed lake systems in Australia. The lakes on Rottnest Island have high environmental value for the region and form a sink for 7Li. Therefore, understanding the hydrochemical processes that lead to these conditions is important for interpreting the palaeoenvironmental history of the region.