[B-PT03] Biocalcification and the Geochemistry of Proxies -Field ecology, Laboratory culture and Paleo

[BPT03-P02] Assessment of skeletal compositions in {i}A. digitifera{/i} coral as temperature proxies

*Shoko SAKATA¹, Mayuri INOUE¹, Yasuaki TANAKA², Takashi NAKAMURA³, Kazuhiko SAKAI², Minoru IKEHARA⁴, Atsushi SUZUKI⁵ (1.Graduate School of Natural Science and Technology, Okayama University, 2.Sesoko Station, 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.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST))

Introduction
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 Porites spp. In this study, we investigate the effect of temperature on skeletal δ¹⁸O and Sr/Ca ratio in A. digitifera corals for evaluating temperature proxies for one of paleoceanographic applications.
Materials and Methods
Three colonies of A. digitiferawere collected at Sesoko Island, Okinawa, Japan. We reared coral samples in seawater with 5 different temperature settings (18, 21, 24, 27, 30℃) and set 2 tanks in each temperature treatment (3 coral nubbins× 3 colonies in a tank). Calcification rate of coral nubbins was measured by buoyant weight technique every two weeks during the period of experiments. Oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) were analyzed by a stable isotope ratio mass spectrometer, and the ratios of trace elements (Sr/Ca, Mg/Ca, U/Ca and Ba/Ca) were measured by an inductively coupled plasma mass spectrometer (ICP-MS).
Results and discussion
Skeletal δ¹⁸O of corals is often used as a seawater temperature proxy. In this study, a strong negative correlation was found between δ¹⁸O and water temperature, and the temperature dependency was comparable with that of Porites spp. Thus δ¹⁸O of A. digitifera is suggested to be useful as a temperature proxy without clear influence from growth rate. A negative correlation was also observed between Sr/Ca ratio and temperature, which is compatible with that of Porites spp., although the correlation was weaker than δ¹⁸O. But variation of Sr/Ca ratio was not controlled by skeletal growth rate, suggesting that the dominant factor controlling the skeletal Sr/Ca ratio is water temperature. Thus, skeletal δ¹⁸O and Sr/Ca ratio of Acropora spp., at least A. digitifera, can be useful as a proxies for seawater temperature as well as Porites spp., although more investigation would be required for Sr/Ca-thermometer of Acropora spp.