Stable Barium Isotopes : Biogeochemical Cycling and Palaeoceanographic Applications
Stable barium (Ba) isotopes have received increasing attention in the past decade as a result of improved analytical precision and their potential as a tracer for ocean circulation and for palaeoceanographic reconstructions. However, challenges exist regarding the isotope analysis and interpretation of this emerging isotopic tracer. The first major challenge is the accurate and precise determination of the natural stable Ba isotope fractionation (δ138Ba) in various samples matrices, such as seawaters, river waters, carbonates, sulphates and sediments. Multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) offers rapid sample throughput for stable Ba isotope analysis, but suffers from severe matrix effects caused by changes in matrix load and instrumental mass bias variations during the measurements. While significant efforts have been invested into improving chemical purification and refining the models of mass bias correction, the impact of plasma conditions has rarely been considered. In this thesis, a detailed investigation of non-spectral matrix effects on the accuracy and precision of stable Ba isotope analysis has been carried out under different plasma conditions quantified by the Normalised Argon Index (NAI). Based on the results of our experiments, a new approach is proposed to address how the matrix effects can be attenuated by optimising the NAI, which minimises and stabilises the instrumental mass fractionation. The improved understanding of the behaviour of the matrix-induced mass bias allows us to define a matrix tolerance state for stable Ba isotope analysis with MC-ICP-MS. The second challenge in the study of oceanic Ba isotope systematics is a lack of constraints on the dissolved Ba isotope composition of endmember water masses in the high-latitude oceans. The modern deep ocean circulation system is largely driven by the sinking of cold, saline water masses in high latitudes, where North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) acquire their respective Ba isotope compositions. In this thesis, we characterise the δ138Ba signatures of NADW and AABW by analysing a suite of depth profiles from the Nordic Seas, the Labrador Sea, and the Weddell Sea. The contrasting δ138Ba signals between these northern and southern endmembers are most likely the result of the Ba isotope fractionation signatures introduced into the return path of the thermohaline circulation by barite formation in the Southern Ocean. In combination with the previously reported low and mid-latitude δ138Ba profiles, we are able to better constrain the meridional Ba transport and mixing processes across the entire Atlantic Ocean. The systematic variation in the meridional δ138Ba profiles provides valuable insights into the biogeochemical cycling of Ba in the modern ocean and thus builds the framework for potential applications of stable Ba isotopes in palaeoceanography. Applying stable Ba isotope signatures extracted from shallow-water corals as a novel tracer for river runoff constitutes the third primary topic of this thesis. Coral-based reconstructions of riverine inputs are critical for the understanding of local and global elemental budgets in the context of climate change and anthropogenic land use. The elemental ratios of Ba/Ca in corals are the commonly used tracer for riverine runoff. However, coral Ba/Ca records at different locations show contrasting behaviour, which hinders the use of Ba/Ca as a reliable tracer for past river runoff in coastal regions. Stable Ba isotope compositions in shallow-water corals may reflect the ambient seawater Ba isotope signatures and have great potential to provide important constraints on seasonal changes in riverine inputs and to validate coral Ba/Ca records. Our results of a monthly resolved coral record from the Andaman Islands (India) show that coral δ138Ba values are generally light during the South Asian Summer Monsoon characterised by enhanced rainfall and regional river discharge suggesting that coral δ138Ba may act as a novel runoff proxy. This inference is supported by the stable Ba isotope compositions of local seawater samples, with a significantly light δ138Ba value of +0.29 ± 0.04‰ during the summer monsoon periods compared to heavy δ138Ba values of +0.44 ± 0.05‰ during other times of the year. In addition, we estimate the Ba isotope fractionation between corals and seawater, which sheds light on the effects of mineral growth kinetics and biomineralization controlling the process of Ba incorporation into coral skeletons. In summary, this thesis addresses three important objectives, which are the establishment of a matrix tolerance state for stable Ba isotope analysis with MC-ICP-MS, the characterisation of the dissolved Ba isotope distributions in water masses in the modern deep ocean, and the application of seasonal Ba isotope changes recorded by shallow-water corals to reconstruct past river runoff.
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