The Use of Deep-Sea Corals as Paleoceanographic Monitors

Abstract

<p>The study of past ocean processes is essential to the understanding of current oceanography and climate systems, and to the prediction of future global change. The study of paleoceanography requires proxies: to date many studies have used foraminifera and reef corals. Each of these tools, however, has its limitations. The time resolution of paleoceanographic reconstruction using forams can be very poor, while reef corals can only give information about shallow, tropical processes. Azooxanthellate corals, in contrast, are suitable for highly precise and accurate radiogenic dating and are found everywhere in the world oceans, at all depths. The few attempts to use this taxon for paleoceanographic work have been, for the most part, unsuccessful. This is because the techniques for decoding the environmental information stored within azooxanthellate skeleton had not yet been developed. This thesis represents the first attempt to extract paleoceanographic information from 'deep-sea' corals and is presented in five sections, each a published for publishable manuscript. Part 1 describes an investigation of corals from Orphan Knoll (Desmophyllum cristagalli from 1600 m depth), in the northwest Atlantic. They were dated by the ²³⁰Th/²³⁴U method using thermal ionization mass spectrometry (TIMS) and found to have grown around the time of the Younger Dryas cooling event (13,000 to 11,700 calendar years BP). The origin of this cooling recently been attributed to a reduction or cessation of deep-water producution in the North Atlantic and a concurrent lessening of the heat flux from low latitudes. The δ¹⁸O in the coral skeletons shift markedly, coincident with the initiation of the Younger Dryas, suggesting that profound changes in intermediate water circulation may have occurred. Part 2 describes a study of skeletogenesis in deep-sea corals in order to develop a suitable isotopic sampling scheme for 'time-series' analyses. Numerous scanning electron micrographs of D. cristagalli have shown that this organism has skeletal banding analogous to that in reef corals. A long term record will be difficult to collect from D. cristagalli since lamellae are less than 10 μm thick, band position is unpredictable and degree of band dissolution is difficult to assess. Part 3 attempted to ascertain if portions of the coral skeletons displayed minimal isotopic disequilibria. A specimen of D.cristagalli was intensively sampled on all interior and exterior coeval surfaaces. Even though the coral grew at an almost-constant temperature, δ¹⁸O varied by almost 3% and was up to 3.25% depleted with respect to aragonite-seawater oxygen isotope equilibrium. Portions of the skeleton approached equilibrium, the location of those areas were unpredictable and were not associated with any readily-identifiable characteristics. Part 4 describes the study of thirty-five azooxanthellate corals belonging to 18 species, collected at sites ranging from the Norwegian Sea to the Antarctic and of depths ranging from 10 to 5220m. All specimens showed distinct, well-defined linear correlations between carbonate oxygen and carbon isotopic composition, with slopes ranging from 0.23 to 0.67 (mean 0.45 ± 0.9) and linear correlation r² values which averaged 0.89. These pronounced isotopic disequilibria are what have rendered azooxanthellate corals unsuitable for use in paleothermometry to date. Despite the disequilibria, a reliable method for obtaining paleotemperature data was obtained. It was found that, if a δ¹³O vs δ¹³C regression line from an individual coral could be generated, the δ¹⁸Oarag value corresponding to δ¹³Carag = δ¹³Cwater and corrected for δ¹⁸Owater was a linear function of temperature: δ⁸ O= -0.25 T(℃) + 4.97. Part 5 describes a study of trace-element and stable-isotope analyses which was performed on two sets of azooxanthellate corals from the North Atlantic: one set from Orphan Knoll and the other from the Mid-Atlantic-Ridge. The Mid-Atlantic-Ridge corals had episodic pulses of Fe, Mn, Cu, Ni and Zn contained within their skeletons. We believe these metals originated in a hydrothermal discharge zone associated with the Mid-Atlantic spreading centre. With sufficient specimens of known ages, it may be possible to reconstruct the history of a particular segment of a spreading centre.</p>