Image of deep-sea coral taken from a submersible. Image courtesy of S. Fallon.

Corals grow in a range of different morphologies: as solitary forms, as isolated colonies, in small patch reefs several metres across, or as large reefs  up to 300m high (think Great Barrier Reef). Most reef-building (hermatypic) corals grow in the photic zone where there is sufficient light to support the photosynthetic activity of their symbiotic algae. As such, the majority of reef-forming corals are found in shallow tropical waters. However there is a vast, world-wide occurrence of deep/cold-water coral systems: in fjords, continental shelves, canyons and seamounts ranging from 50 m to 4,000 m. These corals may be reef-building or solitary, and thrive in the abyss of the deep ocean. While they lack the symbiotic algae of surface dwelling corals, it is important to remember that deep-sea corals have a wide trophic niche. They feed on a mixed diet of surface derived phytoplankton and zooplankton, acting as obligate filter feeders and even supplementing their diet with resuspended aged particulates during seasonal shortages. Deep sea corals have been shown to harvest bacterial symbionts by ingesting their own mucopolysacharide layer that houses the bacteria. Eeeeew.

Bamboo coral skeleton with alternating organic nodes and inorganic internodes.

At the Research School of Earth Sciences we are concerned with two species of coral in particular: bamboo corals and black corals than inhabit South East Australian waters. Tasmanian deep-sea corals anchor to rocky substrates and form deep reefs on the region’s seamounts and continental margins. Bamboo corals produce unique archives due to their skeletal composition, consisting of alternating inorganic Mg- calcite internodes and organic protein nodes. The former provides strength to the organism whereas the latter confers flexibility along the skeletal axis. Previous srudies have used radiocarbon to demonstrate that the carbon in the organic nodes had a distinct surface signature, and is therefore largely derived from surface particulates, whereas the calcite is derived from the surrounding deepwater dissolved inorganic carbon. As such, surface and deepwater archives are paired in the alternating skeletal components.