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Vertebrate isotopes and the environment

Final Report Summary - VISE (Vertebrate isotopes and the environment)

Modelling of future climate relies heavily on the accuracy of reconstructions of previous climatic regimes to validate and corroborate the predictions being made. Of particular importance is the production of well-founded reconstructions of past surface seawater temperatures (SSTs) and oceanic thermal structures as these are a critical variable in the Earth's climate system. The most widely cited proxies used here are the oxygen isotopic composition of biogenic minerals produced by fossil organisms that inhabited the marine realm. A potentially robust temperature proxy for ancient seawater is the δ18O of biogenic apatites such as that of fish and shark teeth. The major advantage of apatite phosphate oxygen is that isotopic exchange with water is very rapid in early biochemical enzyme reactions but extremely slow in inorganic systems over geological timescales. Fossil shark teeth are increasingly used as a proxy within these studies due to high fossilization potential, abundance (partially a consequence of regular shedding of teeth in chondrichthyans) and assumed geochemical robustness of their tissues, but the fundamental basis for their utility in palaeoenvironmental reconstructions requires validation from studies conducted on extant taxa in well-constrained environments.

Although extant aquatic vertebrates have been demonstrated to be in isotopic continuity in the freshwater realm, similar studies of marine vertebrates have all been conducted upon wild-caught taxa where the relationship between the organism and the body of water is less clearly defined. Many of the modern taxa that have been examined have extensive ecological ranges vertically and laterally within the ocean basins, encountering water masses of different isotopic compositions and of different temperatures. Host water masses also vary in their isotopic composition as a function of freshwater input and evaporation, adding a significant unknown variable in palaeotemperature reconstruction.

Additionally, variability in the response of different tooth hard tissue layers on a gross scale has been well documented in the forensic geochemistry and archaeological literature. Furthermore, the influence of different processing methods on isotopic composition in fossil vertebrates has also been recently been highlighted where the hard tissue of choice is enamel or enameloid.
As a consequence of the increasing utility of vertebrate bioapatite in palaeoclimate reconstructions, together with an appreciation of the variables that will feed into the acquired isotopic values, the aim of the project was to analyse vertebrate tissues derived from within the well constrained (in terms of temperature and water composition) environments provided by the University of Birmingham’s (UoB) research relationship with the Sea Life Centre network across Europe. Access to the marine aquaria provided by the Sea Life Centre network facilitated regular sampling of water composition (to account for seasonal variability) across a range of stable tank temperatures (ranging from 11°C to 24.5°C), as well as supplying a range of materials from the fish hosted within these tanks.
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