1. Geochemical signals in Antarctic corals as paleoceanographic proxies of Southern Ocean water mass properties
1Paolo Montagna, 2Malcolm McCulloch, 1Alessandro Remia, 3Stefano Schiaparelli, 1Marco Taviani, 2Julie Trotter
1 Institute of Marine Sciences (ISMAR)-CNR, Bologna, Italy
2 Oceans Institute and School of Earth and Environment, The University of Western Australia, Perth, Australia
3 Dipartimento di Scienze della Terra, dell’Ambiente e della Vita, Università di Genova, Genova, Italy
INTRODUCTION
Waters south of the Polar Front in the high latitude Southern Ocean become undersaturated with respect to aragonite and calcite, strongly limiting carbonate accumulation and preservation. Cold-water corals are among the few calcifying organisms that can cope with this corrosive environment and can, therefore, represent good candidates for the reconstruction of specific marine parameters at various temporal scales. The physical and chemical properties of the deep-water masses may be encoded as specific geochemical signals within their banded calcareous skeleton, potentially providing century long records at sub-decadal resolution. Among various potentially suitable taxa inhabiting Antarctic waters, where scleractinians are occurring at present as solitary cup corals, we selected as a prime candidate for our pilot-study Flabellum spp. because previous studies (e.g. Shirai et al., 2005) suggest that this genus may be a reliable paleoclimate archive. Further candidates for geochemically-oriented studies will include in the future also the two extant commonest cup corals in Antarctica i.e. Caryophyllia antarctica and Gardineria antarctica, occurring also in late Pleistocene and Holocene uplifted deposits.
Photo credit: Greg Rouse (Levin et al., 2015)
MATERIAL AND METHODS
Our material consists of freshly-collected Flabellum spp. from the Balleny Islands (66°50’S – 162°25’E, 760m) and the Ross Sea (73°14’S – 175°38’E, 389m). Multiple trace elements profiles of 7Li, 11B, 25Mg, 43Ca, 84Sr, 138Ba, 238U were obtained using a 193nm ArF excimer laser ablation coupled to a ICP-MS Varian 820, following previously described methods (Montagna et al., 2014). In addition, fragments of the coral skeleton collected along the growth axis have been recently analysed for boron isotopes (d11B) using the method of McCulloch et al. (2015) and seawater pH has been reconstructed following the approach of Trotter et al. (2011) and McCulloch et al. (2012).
RESULTS
The geochemical high-resolution study of the different microstructures in Flabellum reveals that most elemental variations along the marginal thecal wall could result from the mixture of different microstructures (such as early mineralization zones and fibrous aragonite), with different minor and trace element compositions. Compared to the fibrous aragonite, the early mineralization zones show depleted concentration of U/Ca, B/Ca and Ba/Ca and to a lesser degree Sr/Ca and an enrichment of Mg/Ca and Li/Ca, which is likely due to the higher calcification rate within the early mineralization zones. Most of the elements show a high degree of reproducibility between adjacent tracks in almost all the coral specimens, mainly at low-frequency fluctuations. These common features represent a promising evidence for evaluating this coral species as a paleoclimate archive since they are likely due to environmental control that partly influences the uptake of minor and trace element into the aragonite skeleton.
1 cm
We found that the outer septal face, which is composed only by fibrous aragonite, displays a more homogeneous geochemical variation with the amplitude of the element variations being significantly smaller. These results suggest that this coral portion of Flabellum spp is certainly the more suitable portion for deriving paleoclimate information and part of the “vital effect” processes might be avoided using a careful sampling technique.
REFERENCES: McCulloch et al., Rapid Communications in Mass Spectrometry; McCulloch et al Nature Climate Change; Montagna et al Geochimica et Cosmochimica Acta; Shirai et al Chemical Geology; Trotter et al Earth and Planetary Science Letters.
We are grateful to members of the XXIX Antarctic expedition (austral summer ) and in particular to Navy Marshals Francesco Reale, Daniele Risina, and Gianluca Giannotti who supported all on ship operations. The GEOSMART project (GEOchemical Signatures in the Antarctic MARine carbonate sysTem: present, past and future implications; PI: Paolo Montagna ISMAR-CNR) is funded by the Italian Antarctic Research Program (PNRA)