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Title: The use of molluscan shell biogeochemistry in environmental reconstruction
Author: Mitchell, Lynda
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1994
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Stable carbon and oxygen isotope analysis of fast growing carbonate shells of fossil molluscs from the Plio-Pleistocene of New Zealand and Recent species from Scotland has revealed a high degree of isotopic variability and a strong correlation between δ¹³C and δ¹⁸O in each case. This phenomenon may be due to kinetic isotope effects which are inherent in fast growing shells or areas of shell. If this is the case then equilibrium isotope partitioning may not have had time to occur before the completion of calcite precipitation. Kinetic effects would favour the lighter isotopes of both carbon and oxygen; it is therefore the highest δ¹³C and δ¹⁸O values that are most likely to reflect equilibrium with the environment. Very small carbonate particles often give particularly low δ¹³C and δ¹⁸O values. These results have implications for the use of isotopic data in environmental reconstruction based on fast growing shells and may also be applicable to other carbonate precipitating organisms. Stable isotope analysis of a series of fossil and recent mollusc shells from interglacial shell beds near Wanganui, New Zealand, yielded interglacial palaeotemperatures spanning the past 3.6 million years. The results indicate a cooling of about 10°C at about 1 My from earlier warm interglacial temperatures (15-20°C) to cooler interglacial temperatures (5-10°) which have lasted to the present. This event coincides with a positive excursion in the strontium isotope ratio of seawater indicating greater erosional input to the ocean and with a significant molluscan extinction event in New Zealand. These changes may all be associated with a well documented change in the dom inant global climatic cycle from 41 ky periodicity to 100 ky periodicity which occurred at about this time. Stable oxygen and carbon isotope profiles and intracrystalline amino acid profiles (free and total) were determined for the New Zealand giant Pliocene oyster Crassostrea ingens by sampling annual growth increments along a sagittal section. These profiles reflect both ontogenetic and environmental change over the life-time of the oyster (approximately 20 years). δ¹⁸O increases gradually from the umbo towards the shell margin, levelling off about half way along the shell. The profile shows an initial sharp increase at the umbo, and then gradually decreases towards the shell margin. The amino acid profile reveals a gradual decrease in abundance from the umbo to the shell margin, indicative of a progressive increase in the relative amounts of inorganic carbonate to protein over the life of the oyster, that may also be a consequence of decreasing growth rate. Amino acid analysis was carried out on intracrystalline organic material from fossil and recent mollusc shells from South Wanganui Basin, New Zealand, ranging in age from 3.6 My to Recent. The absolute abundance of amino acids is highly variable but shows a gradual decline through time due to diagenetic effects. The proportion of peptide-bound amino acids decreases with time, and there is a corresponding increase in free amino acids as proteins are broken down by natural hydrolysis. By 0.5 My, most amino acids are free, after which there is a general decrease in most individual amino acids, presumably because they decay or become incorporated into predominantly insoluble geopolymers. Alanine is a notable exception, increasing in older samples because it is a common byproduct of the breakdown of other amino acids. Amino acid data from different species and from shell beds of different ages were compared using multivariate statistical techniques. The results indicate that, despite the effects of diagenesis, the original biochemical distinction between different groups of molluscs (i.e. different proteins within the shell) survives for at least 3.6 My, and may be detectable in older specimens provided sufficient original amino acids remain. Individual intracrystalline amino acids from Pecten maximus shells from the west coast of Scotland were separated by hplc (high pressure liquid chromatography) for carbon isotope analysis. The aim was to recover individual amino acids that had not been derivatized. However, the results do not show good separation at high sensitivity. Glycine in particular is spread across the whole range of retention times and is mixed with many of the other amino acids. Many of the peaks overlap. Some amino acids show two or more peaks. Separation of underivatized amino acids by hplc can therefore be problematical. Carbon isotope analysis of bacterially produced standard amino acid samples shows significant variation in δ¹³C between different individual amino acids. This may reflect different environments of production or may reflect biological fractionation effects.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available