AUTHOR=Sakata Shoko , Inoue Mayuri , Tanaka Yasuaki , Nakamura Takashi , Sakai Kazuhiko , Ikehara Minoru , Suzuki Atsushi 

TITLE=Assessment of chemical compositions in coral skeletons (Acropora digitifera and Porites australiensis) as temperature proxies

JOURNAL=Frontiers in Marine Science

VOLUME=11

YEAR=2024

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2024.1329924

DOI=10.3389/fmars.2024.1329924

ISSN=2296-7745

ABSTRACT=<p>Although biogenic carbonates, such as foraminifera and coccolithophorids, are valuable tools for reconstructing past environments, scleractinian corals also offer environmental data from tropical to subtropical regions with a higher time resolution. For example, oxygen isotopes (δ<sup>18</sup>O) and strontium-calcium (Sr/Ca) ratios have been utilized to reconstruct sea surface temperatures and salinity, primarily through the use of massive-type <italic>Porites</italic> sp. from the Pacific, as well as corals like <italic>Diploria</italic> and <italic>Montastrea</italic> from the Atlantic. While a few types of corals other than <italic>Porites</italic> have been utilized in paleoclimate studies, comprehensive evaluations of their geochemical tracers as temperature proxies have not been thoroughly conducted. Therefore, in this study, we focused on branching-type <italic>Acropora</italic>, which are found worldwide and are often present in fossil corals. We conducted a comparison of the chemical compositions (δ<sup>18</sup>O, δ<sup>13</sup>C, Sr/Ca, U/Ca, Mg/Ca, and Ba/Ca) of <italic>Acropora digitifera</italic> and <italic>Porites australiensis</italic> through temperature-controlled culture experiments. The validity of using the chemical components of <italic>A. digitifera</italic> as temperature proxies was then evaluated. Three colonies of <italic>A. digitifera</italic> and <italic>P. australiensis</italic> were collected for culture experiments on Sesoko Island, Okinawa, Japan. We reared coral samples in seawater with five different temperature settings (18, 21, 24, 27, 30°). The calcification rate and photosynthesis efficiency (<italic>Fv/Fm</italic>) of each nubbin were measured during the experimental period. After the culture experiment for 77 days, chemical components in skeletal parts grown during the experiment were then measured. Consequently, the mean growth rates and <italic>Fv/Fm</italic> throughout the experiment were higher for <italic>A. digitifera</italic> (0.22%/d and 0.63 for growth rate and <italic>Fv/Fm</italic>) compared to those for <italic>P. australiensis</italic> (0.11%/d and 0.38 for growth rate and <italic>Fv/Fm</italic>). This suggests that the higher efficiency of photosynthesis in <italic>A. digitifera</italic> would promote greater calcification compared to <italic>P. australiensis</italic>. Regarding the potential use as temperature proxies, <italic>A. digitifera</italic> exhibited a strong negative correlation, on average, between δ<sup>18</sup>O and the water temperature (r = 0.95, p&lt; 0.001). The temperature dependency was found to be comparable to that reported in <italic>Porites</italic> corals (-0.11 and -0.17 ‰/°C for <italic>P. australiensis</italic> and <italic>A. digitifera</italic>, respectively). Thus, the δ<sup>18</sup>O of <italic>A. digitifera</italic> appeared to be a useful temperature proxy, although it was also slightly influenced by skeletal growth rate at the same temperature. A strong negative correlation was also observed between the mean Sr/Ca ratio and temperature in <italic>A. digitifera</italic> (r = 0.61, p&lt; 0.001) as well as <italic>P. australiensis</italic> (r = 0.56, p&lt; 0.001), without a clear influence from the skeletal growth rate. Therefore, the skeletal Sr/Ca ratio in corals may have been primarily influenced by water temperature, although large deviations in Sr/Ca were observed in <italic>A. digitifera</italic>, even at the same temperature settings. This deviation can be reduced by subsampling an apical part of a polyp including the axis of skeletal growth. The U/Ca ratio of <italic>A. digitifera</italic> appeared to be affected by internal pH variation within the corals, especially at 30°C. Similar to U/Ca ratios, metabolic and kinetic effects on corals were observed in δ<sup>13</sup>C of <italic>A. digitifera</italic> at 18 and 30°C. In addition, considering the variation pattern of both U/Ca and δ<sup>13</sup>C of <italic>A. digitifera</italic> at 30°C, it has been suggested that respirations may overwhelm photosynthesis for coral samples at 30°C. Therefore, the U/Ca and δ<sup>13</sup>C of <italic>A. digitifera</italic> could potentially be used as proxies of biomineralization processes, whereas the δ<sup>18</sup>O and Sr/Ca displayed a high possibility of acting as temperature proxies.</p>