The carbonate mineralogy and distribution of habitat-forming deep-sea corals in the southwest pacific region

Abstract

Habitat-forming deep-sea scleractinian and alcyonacean corals from around the southwest Pacific were analysed for their calcium carbonate mineralogy. Scleractinian coral species Solenosmilia variabilis, Enallopsammia rostrata, Goniocorella dumosa, Madrepora oculata and Oculina virgosa were all found to be 100% aragonitic, while some members of the alcyonacean taxa Keratoisis spp., Lepidisis spp., and Paragorgia spp. were determined to be high magnesium (Mg) calcite (with 8–11mol% MgCO3) and Primnoa sp. is bimineralic with both aragonite and Mg calcite. The majority of these habitat-forming deep-sea corals are found at intermediate depths (800–1200m) in the Antarctic Intermediate Waters (AAIW) with low salinities (~34.5), temperatures of 4–8°C and high oxygen concentrations (>180μmol/kg) and currently sitting above the aragonite saturation horizon (ASH). However, habitat-forming corals have been recorded from greater depths, in cooler waters (2–4°C) that are undersaturated with respect to aragonite (Ωaragonite<1), but with oxygen levels still >160μmol/kg. To address the sampling depth bias the coral records were normalised by the number of benthic stations (sampling effort) in the same depth range. This shows that the highest number of corals per sampling effort is between 1000 and 1400m with corals present in over 5% of the stations at these depths. The normalised records and Boot Strap analyses suggests that scleractinian corals, especially S. variabilis should be present in >1% of stations down to 1800m water depth, with E. rostrata, M. oculata and G. dumosa slightly shallower. While alcyonacean corals are found in >1% down to 2600m, with Keratoisis spp. the deepest down to 2600m, while Lepidisis spp. and Paragorgia spp. found down to 1800m. This suggests that most species can probably tolerate some undersaturation of aragonite (Ωaragonite=0.8–0.9), with several species/genera (S. variabilis; Keratoisis spp.) even more tolerant of lower carbonate concentrations ([CO32−]), down to Ωaragonite of 0.7. With this tolerance for some carbonate undersaturation it is unclear how deep sea habitat-forming corals might respond to future ocean acidification. It is likely that some species/genera will cope better than others. However, future changes in oxygen concentrations and food availability, are also going to have a strong influence on the depth and spatial distribution of deep-sea corals in the southwest Pacific.