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University of Adelaide(2021)

Calibration of alkaline earth metal isotope tracers in semi-arid coastal environments

Shao, Yuexiao

Titre : Calibration of alkaline earth metal isotope tracers in semi-arid coastal environments

Auteur : Shao, Yuexiao

Université de soutenance : University of Adelaide

Grade : Doctor of Philosophy (PhD) 2021

Coastal systems in semi-arid areas are characterised by complex physico-chemical processes involving mixing of marine and continental water sources as well as precipitation of evaporitic and carbonate minerals. The latter processes involving carbonate cycling also represent an important but currently poorly constrained component of the coastal carbon budget. This thesis fills important knowledge gaps in our understanding of water source mixing and local carbonate cycling in a semi-arid coastal system in South Australia – the Coorong, Lower Lakes and Murray Mouth (CLLMM) estuary, using selected alkaline earth metals (Ca and Sr) and their isotopes with the following research components : 1. Application of radiogenic Sr isotopes ((87)Sr/(86)Sr), stable Ca isotopes (δ(44/40)Ca) and elemental ratios, complemented by mineralogical analysis of top-sediment samples and geochemical (PHREEQC) modelling of carbonate saturations in the CLLMM waters to constrain the water source apportionment and local carbonate output in the Coorong lagoon. 2. Development and validation of high-precision stable Sr isotope analysis (δ(88/86)Sr) using thermal ionisation mass spectrometry (TIMS) and follow up calibration of δ(88/86)Sr in the CLLMM waters with respect to changing salinity and carbonate saturation states. 3. Application of (87)Sr/(86)Sr and δ(88/86)Sr tracers, along with elemental concentration data, to monitor seasonal variations (i.e., every 3 months) in water source mixing and carbonate dynamics (i.e., dissolution vs precipitation) in the CLLMM. 4. Reconstruction of palaeo-hydrology and salinity in the Coorong South Lagoon throughout the past 2400 years, based on (87)Sr/(86)Sr, δ(88/86)Sr and Mg/Sr analysed in fossil bivalve shell species (Arthritica helmsi) collected from sediment cores. The above data were complemented by radiocarbon (14C) and pollen-based geochronological models. Overall, the results from the thesis showed that the modern North Lagoon waters are mainly sourced from the Southern Ocean, with transient freshwater inputs sourced from the River Murray and Lower Lakes and/or local groundwater discharge. In contrast, the hypersaline South Lagoon waters are basically highly evaporated ‘brackish waters’ with significant contribution of Sr from continental water sources. Importantly, stable Ca and Sr isotope tracers and water chemistry data indicate that the South Lagoon acts as a net sink for dissolved inorganic carbon (DIC) in the form of precipitated carbonate minerals (mostly aragonite). Both δ(44/40)Ca and δ(88/86)Sr in the CLLMM waters seem to be controlled by mass-dependent isotope fractionation, most likely related to carbonate dissolution and precipitation. Despite the current uncertainty regarding the role of local groundwater discharge on the chemistry of Coorong waters, the results indicate that an increased alkalinity supply (mainly from the Salt Creek) may locally promote CaCO3 precipitation and increase in δ(88/86)Sr of waters in the South Lagoon. Finally, the multi-proxy analysis ((87)Sr/(86)Sr, δ(88/86)Sr and Mg/Sr) of fossil shells revealed that over the past two millennia the South Lagoon waters were never purely marine but originally rather comprised brackish waters (estimated minimum salinities of 6-23 PSU) with at least 60% contribution from continental water. Overall, the findings of this thesis improved our understanding of modern and past water source mixing and carbonate cycling in the CLLMM system and can hopefully benefit future water management strategies and plans


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Page publiée le 19 mars 2023