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Date of Award

1-1997

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

Supervisor

M.J. Risk

Abstract

Coral tissue and skeleton contain physical and chemical proxy signatures of the surrounding environment. The purpose of this thesis is to broaden or refine the use of corals as environmental recorders/tracers by examining the fidelity of both new and existing techniques. Corals from the Caribbean and Indo-Pacific are utilised, with primary reference to samples from Jamaica and Indonesia.

Coral skeletons from Banda Api, Indonesia, contain evidence of a short term pulse of volcanic ash and hydrothermal fluids which affected the surrounding reef during a May, 1988 eruption. Partial burial of corals by volcanic ash led to the formation of highly bioeroded death surfaces and the incorporation of ash into underlying skeletal pores. Subsequent recovery of the coral resulted in preservation of these features as death/regrowth surfaces. Hydrothermal activity is preserved in the skeleton as a distinct orange layer of iron hydroxide which has been termed the "Banda Band". X-radiography confirms that the location of these features within the coral skeletons coincides with the timing of the eruption. Preservation of an environmental pulse lasting only a few days demonstrates the fine scale resolution that can be obtained from coral skeletal records.

Coral tissues from Jamaica and Zanzibar, collected along depth/light gradients, were analysed for δ¹⁵N. Coral tissue δ¹⁵N was found to decrease with decreasing light availability. A conceptual steady state model has been developed to explain this relationship. In higher light conditions, corals and/or their symbiotic algae significantly deplete the internal pool of dissolved inorganic nitrogen, resulting in minimal fractionation during uptake. Under lower light conditions, however, less dissolved nitrogen is assimilated and fractionation is more fully expressed, with the light isotope of nitrogen being taken up preferentially. These results suggest corals are not conservative tracers of nitrogen. It is proposed, however, that shallow water (<5 m), autotrophic species of corals may exhibit minimal fractionation and prove to be excellent tracers of dissolved inorganic nitrogen on reefs.

δ¹⁵N and δ¹³C signals of shallow water corals collected from seven different reefs were compiled. Significant inter-reef variability in δ¹⁵N was found. Both light availability and the isotopic composition of source nitrogen are thought to contribute to the variation between reefs. Other factors such as nutrient concentration may also be important. δ¹³C variability was much less significant in the data set, with most reefs having enriched coral tissue signals, consistent with a primarily autotrophic diet.

The tissue and underlying skeleton of Jamaica and Zanzibar corals were also analysed for δ¹³C. Tissue δ¹³C was found to decrease with decreasing light availability. Skeletal δ¹³C, however, shows no relationship with depth/light availability. It is proposed that strong kinetic isotope effects associated with the rate of calcification mask any such relationship. A simple data transformation is applied to account for the presence of kinetic isotope effects, the result of which is a much clearer relationship between skeletal δ¹³C and light availability. These results suggest that it is possible to correct for the presence of kinetic isotope effects associated with the rate of skeletogenesis and to resolve meaningful environmental/metabolic information from skeletal carbon isotopic records.

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