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Title: Environmental and biological controls of Mg/Ca, Sr/Ca and Mn/Ca ratios in the shells of the bivalves Pinna nobilis, Mytilus edulis and Pecten maximus : implications for palaeo-environmental reconstructions
Author: Freitas, Pedro Seabra
Awarding Body: University of Wales, Bangor
Current Institution: Bangor University
Date of Award: 2007
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Calcium carbonate skeletons produced by organisms such as foraminifera, ostracodes, corals and molluscs have the potential to contain within their isotope and elemental composition signatures (i. e. proxies) that reflect the environment in which the organisms calcified. Bivalves offer the potential for high resolution reconstructions over a wide geographical range and in the fossil record since the Early Ordovician. However, when compared to other biogenic carbonate archives there have been relatively fewer studies that have investigated bivalve mollusc shell geochemistry, especially robust validation studies of likely geochemical proxies. This study contributes to the evaluation and validation of geochemical proxies in bivalves. More specifically, the aim was to investigate and validate the relationships between the elemental composition (Mg/Ca, Sr/Ca and Mn/Ca) of bivalve shell calcite and its primary environmental controlling factors, i. e. seawater temperature, dissolved and particulate Mn concentrations. In addition, the role of secondary control factors (i. e. "vital" effects and small-scale element heterogeneity), as a source of non-environmental variability in bivalve geochemical proxies was also investigated. Studies on the temperature dependence of Mg/Ca ratios in bivalve calcite shells have produced contradictory results. In the bivalve species studied, Pinna nobilis (Chapter 2), Mytilus edulis (Chapter 4) and Pecten maximus (Chapters 3 and 4), the temperature dependence of shell calcite Mg/Ca ratios was found to be generally weak. The occurrence of a large variability in Mg/Ca ratios at the species, inter- and intra- individual shell levels, as well as through ontogeny, together with a weak temperature control, clearly suggests a strong physiological control of calcite Mg/Ca ratios during shell biomineralization. Bivalve calcite Mg/Ca ratios do not yet appear to be a reliable and precise temperature proxy, at least in the species studied. In the three bivalve species studied, Pinna nobilis (Chapter 2), Mytilus edulis (Chapter 5) and Pecten maximus (Chapters 3 and 5), shell Sr/Ca ratios were found to be influenced by more than a single physiological control (shell growth rate, metabolic activity and even shell Mg content), which may differ from one species to another, but also vary temporally in a single species. Shell growth rate, assumed to indicate a precipitation rate control, was significantly correlated to shell Sr/Ca ratios in field- and laboratory-grown P. maximus and M. edulis. The positive relationship observed between absolute respiration rate and Sr/Ca ratios in M. edulis, grown both in laboratory and field culturing experiments, provides the first direct evidence of an influence from metabolic activity on bivalve calcite Sr/Ca ratios. The seasonal variation of Mn/Ca ratios in the shell calcite of field grown Pecten maximus specimens followed a similar intra-annual variation to dissolved Mn concentrations described previously (Chapter 5). In Mytilus edulis, shell Mn/Ca ratios were found not to be influenced by either dissolved or particulate Mn2+ concentrations (Chapter 6). Shell Mn/Ca ratios and shell growth rates showed a remarkably similar seasonal variation. However, such similarity is not indicative of a precipitation rate control since precipitation rate and Mn partition coefficient in synthetic inorganic calcite are inversely related. The influence of shell growth rate on shell Mn/Ca ratios must reflect a physiological control most likely acting at the transport of Mn into the extra-pallial fluid. Significant small-scale heterogeneity in Mg/Ca, Sr/Ca and Mn/Ca ratios in the shells of Pecten maximus and Mytilus edulis deposited at a constant temperature was observed (Chapter 7). In particular, elaborate shell features and disturbance growth marks, were associated with significant variations of the elemental content of the shell calcite and may represent an important interference in the use of geochemical proxies in bivalve shell calcite. Importantly, shell Mg/Ca ratios in the inner regions of P. maximus shells promise the potential to become a valid palaeotemperature proxy. In both bivalve species studied, elemental/Ca ratios vary significantly in shell deposited from the same extra-pallial fuild and thus strongly suggests that element incorporation in to the shell carbonate at the crystalsolution interface is a key control step in determining the element composition of shell calcite. Most of the results in this study contribute to the growing evidence that bivalve calcite element composition is controlled by physiological factors that underlie a tight control of element incorporation during shell biomineralization. Unless the secondary controls (i. e. metabolic and/or kinetic factors) on element incorporation, in particular their influence on the small-scale heterogeneity of shell elemental composition, can be understood in more detail, and subsequently compensated for, the use of the geochemical proxies Mg/Ca, Sr/Ca and Mn/Ca ratios in bivalves for reliable and accurate reconstructions of past or present environmental conditions remains unlikely, at least in the species studied to date.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available