DataSheet_1_Assessment of chemical compositions in coral skeletons (Acropora digitifera and Porites australiensis) as temperature proxies.docx

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 (δ18O) and strontium-calcium (Sr/Ca) ratios have been utilized to reconstruct sea surface temperatures and salinity, primarily through the use of massive-type Porites sp. from the Pacific, as well as corals like Diploria and Montastrea from the Atlantic. While a few types of corals other than Porites 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 Acropora, which are found worldwide and are often present in fossil corals. We conducted a comparison of the chemical compositions (δ18O, δ13C, Sr/Ca, U/Ca, Mg/Ca, and Ba/Ca) of Acropora digitifera and Porites australiensis through temperature-controlled culture experiments. The validity of using the chemical components of A. digitifera as temperature proxies was then evaluated. Three colonies of A. digitifera and P. australiensis 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 (Fv/Fm) 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 Fv/Fm throughout the experiment were higher for A. digitifera (0.22%/d and 0.63 for growth rate and Fv/Fm) compared to those for P. australiensis (0.11%/d and 0.38 for growth rate and Fv/Fm). This suggests that the higher efficiency of photosynthesis in A. digitifera would promote greater calcification compared to P. australiensis. Regarding the potential use as temperature proxies, A. digitifera exhibited a strong negative correlation, on average, between δ18O and the water temperature (r = 0.95, p< 0.001). The temperature dependency was found to be comparable to that reported in Porites corals (-0.11 and -0.17 ‰/°C for P. australiensis and A. digitifera, respectively). Thus, the δ18O of A. digitifera 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 A. digitifera (r = 0.61, p< 0.001) as well as P. australiensis (r = 0.56, p< 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 A. digitifera, 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 A. digitifera 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 δ13C of A. digitifera at 18 and 30°C. In addition, considering the variation pattern of both U/Ca and δ13C of A. digitifera at 30°C, it has been suggested that respirations may overwhelm photosynthesis for coral samples at 30°C. Therefore, the U/Ca and δ13C of A. digitifera could potentially be used as proxies of biomineralization processes, whereas the δ18O and Sr/Ca displayed a high possibility of acting as temperature proxies.

尽管有孔虫（foraminifera）与颗石藻（coccolithophorids）等生物成因碳酸盐（biogenic carbonates）是重建古环境的宝贵工具，但石珊瑚（scleractinian corals）也可提供热带至亚热带区域的环境数据，且具备更高的时间分辨率。例如，氧同位素（δ¹⁸O）与锶钙比（Sr/Ca）已被用于重建海表温度与盐度，相关研究主要依托太平洋的块状滨珊瑚属（Porites sp.），以及大西洋的双珊瑚属（Diploria）和单星珊瑚属（Montastrea）等类群。尽管古气候研究中也曾利用过少量非滨珊瑚属的珊瑚类群，但针对其地球化学示踪剂作为温度代用指标的全面评估尚未充分开展。因此，本研究聚焦于全球分布广泛且常出现于化石珊瑚中的分枝型鹿角珊瑚属（Acropora）。我们通过控温培养实验，对比了指形鹿角珊瑚（Acropora digitifera）与澳大利亚滨珊瑚（Porites australiensis）的化学成分（δ¹⁸O、δ¹³C、Sr/Ca、U/Ca、Mg/Ca及Ba/Ca），并评估了指形鹿角珊瑚的化学成分作为温度代用指标的有效性。我们在日本冲绳县濑底岛采集了3株指形鹿角珊瑚与澳大利亚滨珊瑚样本用于培养实验，将珊瑚断枝饲养于5个不同温度梯度（18、21、24、27、30℃）的海水中。实验期间测定了每个断枝的钙化速率与光合效率（Fv/Fm）。经过77天的培养实验后，对实验期间生长的骨骼部分进行化学成分测定。结果显示，实验全程的平均钙化速率与光合效率，指形鹿角珊瑚（钙化速率0.22%/d，Fv/Fm为0.63）均高于澳大利亚滨珊瑚（钙化速率0.11%/d，Fv/Fm为0.38），这表明指形鹿角珊瑚更高的光合效率可促进其相较于澳大利亚滨珊瑚更强的钙化作用。就作为温度代用指标的潜力而言，指形鹿角珊瑚的δ¹⁸O与水温整体呈显著负相关（r=0.95，p<0.001），其温度依赖性与已报道的滨珊瑚类群相当（澳大利亚滨珊瑚与指形鹿角珊瑚的温度系数分别为-0.11‰/℃与-0.17‰/℃）。因此，指形鹿角珊瑚的δ¹⁸O似乎是一种有效的温度代用指标，尽管在相同温度下其也会略微受到骨骼生长速率的影响。指形鹿角珊瑚（r=0.61，p<0.001）与澳大利亚滨珊瑚（r=0.56，p<0.001）的平均Sr/Ca比均与温度呈较强负相关，且未受骨骼生长速率的明显影响。因此，珊瑚骨骼的Sr/Ca比可能主要受水温调控，尽管即使在相同温度设置下，指形鹿角珊瑚的Sr/Ca仍存在较大偏差，该偏差可通过取包含骨骼生长轴的珊瑚虫顶端部分进行分样来降低。指形鹿角珊瑚的U/Ca比似乎受珊瑚体内pH变化的影响，尤其在30℃条件下。与U/Ca比类似，18℃与30℃下的指形鹿角珊瑚δ¹³C也表现出代谢与动力学效应。此外，结合指形鹿角珊瑚在30℃下的U/Ca与δ¹³C变化模式，有研究表明此时珊瑚样本的呼吸作用可能超过了光合固碳作用。因此，指形鹿角珊瑚的U/Ca与δ¹³C或可作为生物矿化过程的代用指标，而δ¹⁸O与Sr/Ca则具备作为温度代用指标的较高潜力。