Marine bivalve molluscs are useful archives of past environmental conditions. These archives contain structural, chemical and/or isotopic proxies that provide a record of how environmental conditions have changed naturally, or due to anthropogenic change. The utility of these proxies lies in their ability to help compare the impacts of
natural climate changes with recent climate warming caused by human activities, to reconstruct environmental conditions in inaccessible environments and to record environmental change over the same time scale as shell growth. Proxies in bivalve
shells provide high resolution seasonal records of shell growth and environmental conditions through analysis of their geochemical composition. However, a reliable utilization of geochemical proxies, such as 8180, 813C, Mg/Ca, Sr/Ca, Ba/Ca and Mn/Ca, in bivalve shells is dependent on confirmation of robust and consistent relationships between the supposed proxies and the environmental parameters of interest ( e.g. seawater temperature and chemistry). Bivalves are particularly useful because they are geographically widely distributed, from estuaries to coastal and deep
waters and from mid to high latitudes.
The potential of proxies in bivalve shells to examine the organism's environmental settings during growth in more novel environments such as deep sea hydrothermal vents is the main aim of the present PhD thesis. The results should help understand how variations in the organism's physico-chemical environment (seawater temperature and chemistry) can affect their nutrition and growth. To examine these issues a wide ranging study of the vent mussels, Bathymodiolus azoricus and B. puteoserpentis, from five physically and chemically contrasting hydrothermal vent fields on the Mid-Atlantic Ridge (MAR) was undertaken. Laboratory experiments were undertaken to study the growth of hydrothermal vent mussels under wellconstrained laboratory conditions and to provide calibration for geochemical proxies for temperature reconstruction (Chapter 2). Records of shell-banding were used to reconstruct growth characteristics of hydrothennal vent mussels and relate any changes in microgrowth patterns to environmental conditions via elemental and isotopic proxies (Chapters 3, 4, 5, 6 and 7).
The results presented in this PhD thesis do not support the use of B. azoricus and B. puteoserpentis shell elemental and carbon isotopic composition, apart from oxygen isotopic composition, as geochemical proxies of ambient environmental conditions.
Vent mussel shell chemistry is mainly influenced by the combination of physico- chemical properties of the ambient seawater. However, environmental records were compromised to different degrees by physiological factors when using the carbon stable isotope composition (813C) and elemental composition (e.g. Mg/Ca, Sr/Ca, Ba/Ca and Mn/Ca). In contrast, the oxygen stable isotope composition (8180) of shells of B. azoricus and B. puteoserpentis seems to provide a reliable proxy of ambient seawater temperature, although direct calibration and validation of this proxy is still needed.
Unless the growth characteristics of bivalve shells and the mechanisms that control element incorporation into bivalve shells are better understood, only restricted use of archive materials such as bivalve shells for palaeoceanographic reconstructions will be possible.