Please use this identifier to cite or link to this item: https://hdl.handle.net/2440/133129
Type: Thesis
Title: Calibration of alkaline earth metal isotope tracers in semi-arid coastal environments
Author: Shao, Yuexiao
Issue Date: 2021
School/Discipline: School of Physical Sciences : Earth Sciences
Abstract: 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.
Advisor: Farkaš, Juraj
Tyler, Jonathan
Gillanders, Bronwyn
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2021
Keywords: Radiogenic and stable strontium isotopes
calcium isotopes
water source mixing
carbonate precipitation/dissolution
coastal carbon cycling
mineral saturation
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals
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