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