58. Li/Mg and Boron Isotopes in Antarctic Corals as Temperature and pH Proxies of Southern Ocean Water Masses

Paolo Montagna (1)*, Malcolm McCulloch (2), Julie Trotter (3), Vincenzo Ricca (1), Marco Taviani (1)

1 Institute of Marine Sciences, National Research Council, 40129 Bologna, Italy
2 School of Earth and Environment, UWA Oceans Institute and ARC Centre of Excellence for Coral Reef Studies, The University of Western Australia, Crawley, WA, 6009, Australia
3 School of Earth and Environment and UWA Oceans Institute, The University of Western Australia, Crawley, WA, 6009, Australia

Background
Attempts to decipher the role of the Southern Ocean in modulating past-climate has been particularly difficult due to the paucity of calcium carbonate-precipitating organisms, such as foraminifera, which are commonly used as paleoclimate archives in other oceans. Waters south of the Polar Front become undersaturated with respect to aragonite and calcite, strongly limiting carbonate accumulation and preservation. Cold-water corals are one of the few calcifying organisms that can cope with this corrosive environment, so are potential candidates for reconstructing temperature and pH records at high resolution (annual) over centennial timescales.

Methods
Trace elements and boron isotopes were measured in four specimens of the deep-sea coral Flabellum, retrieved live from the Ross Sea and offshore Bouvet Island at depths ranging between 390 and 760m. The specimens were sub-sampled along the growth axis and analysed using both quadrupole and MC-ICPMS methods.

Findings
The temperature-sensitive elements (Li/Mg and Sr/Ca) and the boron isotope (11B) pH proxy show a consistent pattern between different transects within each specimen, with excellent reproducibility, suggesting minimal influence from ‘vital effects’. Both the Li/Mg-derived temperatures and the 11B-derived pH of the younger portion of the corals are consistent with in-situ instrumental values. Importantly, these corals record a general trend of decreasing pH over the past few decades, as well as both warming and cooling trends which depend on the ambient water masses (High Salinity Shelf Water vs. Circumpolar Deep Water).

Conclusions
Specific geochemical signals encoded in the aragonite skeleton of the Antarctic corals are shown to be robust proxies for the physical and chemical properties of the water masses in which the corals grew. In particular, the Li/Mg and boron isotopic composition of the coral Flabellum vary with temperature and pH respectively, providing a new tool to reconstruct the variability of these key parameters in deep water environments.