[B-PT04] Biomineralization and the Geochemistry of Proxies

[BPT04-P04] Variation of geochemical tracers in coral skeletons (Acropora digitifera vs Porites australiensis) based on the temperature controlled culture experiment

Shoko Sakata¹, *Mayuri Inoue¹, Yasuaki Tanaka⁵, Takashi Nakamura³, Kazuhiko Sakai², Minoru Ikehara⁴, Atsushi Suzuki⁶ (1.Department of Earth Sciences, Okayama University, 2.Tropical Biosphere Research Center, University of the Ryukyus, 3.Faculty of Science, University of the Ryukyus, 4.Center for Advanced Marine Core Research, Kochi University, 5.Faculty of Science, Universiti Brunei Darussalam, 6.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST))

Keywords:geochemical proxies, coral skeleton, temperature

While biogenic carbonates such as foraminifera and coccolithophorid are attractive tools to reconstruct the past environments, scleractinian corals also provide environmental data around tropical to subtropical region with much higher time resolution. For example, oxygen isotope ratio (δ¹⁸O) and strontium-calcium (Sr/Ca) ratio have been used for reconstructing sea surface temperature and salinity by generally using massive Porites sp. However, reconstructions of paleoenvironments using only Porites are sometime limited to Indo-Pacific region and specific time windows. Therefore in this study, we focus on Acropora digitifera, in addition to Porites, which dwell even in the Caribbean Sea in addition to Indo-Pacific ocean and are often found as fossil corals. We compare chemical components contained in A. digitifera and P. australiensis based on temperature controlled culture experiments in which three colonies of both corals were used. As a result, A. digitifera showed a strong negative correlation between the mean δ¹⁸O and water temperature (R²= 1.0), and the temperature dependency was comparable with that of Porites sp. Thus δ¹⁸O of A. digitifera was suggested to be useful as a temperature proxy although they were also slightly influenced by skeletal growth rate. A negative strong correlation was also observed between mean Sr/Ca ratio and temperature in A. digitifera and P. australiensis (R²= 1.0 and 0.93, respectively) without clear effects from skeletal growth rate. Therefore, it was suggested that skeletal Sr/Ca ratio in coral was predominantly controlled by water temperature although large deviations of Sr/Ca in A. digitifera even at same temperature settings were found. This deviation would be improved by subsampling along an appropriate skeletal structure composed of single polyp.