Owen Sherwood

B.Sc. (Geology), McMaster University, 2000
M.Sc. (Geology), McMaster University, 2002

Ph. D. Thesis

Deep-Sea Octocorals: Dating Methods, Stable Isotopic Composition, and Proxy Records of the Slopewaters off Nova Scotia

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For generations, fishermen in Atlantic Canada have known about corals or “trees” growing in the deep waters along the shelf break. Scientific interest dates back to the 1880s, when Gloucester Fisheries collected specimens for the Smithsonian. In recent years, growing concern about the rate and scale of climate change has prompted an examination of climate records encoded in deep-sea coral skeletons.

Focusing on the gorgonian species Primnoa resedaeformis, this thesis examines the paleoceanographic utility of deep-sea octocorals. With a lifespan of 700+ years, P. resedaeformis is the longest-living octocoral, and one of the oldest animals in the sea. Unlike their aragonitic counterparts, the scleractinians, octocorals secrete a 2-part skeleton of calcite and gorgonin, a protein. The calcite is derived from dissolved inorganic carbon at depth, while the gorgonin is derived from recently exported particulate organic matter. The gorgonin is resistant to organic diagenesis, as revealed by the amino acid composition of a nearly 2000 year old sub-fossil specimen. Clues about past environmental conditions are encoded in the chemical make-up of annual rings secreted over the lifetime of the coral. The 2-part skeleton calcite and gorgonin thus constitutes a record of surface- and deep- water properties alike.

Development of calcite- and gorgonin-based proxy records from P. resedaeformis sheds new light on the slopewater system off the Scotian Shelf/Gulf of Maine. From the skeletal calcite, a record of Δ 14C documents decadal-scale shifts in 14C reservoir age of upper intermediate waters. From the skeletal gorgonin, stable C and N isotopic composition presents a more complicated picture of the surface water environment. Following nearly 2000 years of relatively constant values, δ15N rapidly decreased during the 20th century. As a possible cause for the observed trend, a weakening of the Labrador Current is suggested, but a trophic cascade presents a plausible alternative hypothesis. Methods developed herein may be expanded to other oceanographic regions where proxy climate data are lacking.

Pages: 256
Supervisor: David Scott