Stable isotope ratios of a oyster Hyotissa hyotis from the northern Red Sea

This study explores the giant oyster Hyotissa hyotis as a novel environmental archive in tropical reef environments of the Indo-Pacific. The species is a typical accessory component in coral reefs, can reach sizes of tens of centimetres, and dates back to the Late Pleistocene. Here, a 70.2-mm-long oxygen and carbon isotope transect through the shell of a specimen collected at Safaga Bay, northern Red Sea, in May 1996, is presented. The transect runs perpendicularly to the foliate and vesicular layers of the inner ostracum near the ligament area of the oyster. The measured d18O and d13C records show sinusoidal fluctuations, which are independent of shell microstructure. The d13C fluctuations exhibit the same wavelength as the d18O fluctuations but are phase shifted. The d18O record reflects the sea surface temperature variations from 1957 until 1996, possibly additionally influenced by the local evaporation. Due to locally enhanced evaporation in the semi-enclosed Safaga Bay, the d18Oseawater value is estimated at 2.17 per mil, i.e., 0.3-0.8 per mil higher than published open surface water d18O values (1.36-1.85 per mil) from the region. The mean water temperature deviates by only 0.4°C from the expected value, and the minimum and maximum values are 0.5°C lower and 2.9°C higher, respectively. When comparing the mean monthly values, however, the sea surface temperature discrepancy between reconstructed and global grid datasets is always <1.0°C. The d13C signal is weakly negatively correlated with regional chlorophyll a concentration and with the sunshine duration, which may reflect changes in the bivalve's respiration. The study emphasises the palaeogeographic context in isotope studies based on fossils, because coastal embayments might not reflect open-water oceanographic conditions.

本研究以巨牡蛎（Hyotissa hyotis）为研究对象，将其作为印太热带珊瑚礁海域中的新型环境记录载体。该物种是珊瑚礁中的典型伴生类群，成体体长可达数十厘米，其演化历史可追溯至晚更新世。本文报道了1996年5月于红海北部萨法加湾（Safaga Bay）采集的一枚牡蛎壳体的70.2毫米长氧、碳同位素剖面。该剖面垂直于牡蛎韧带区附近的内壳层叶状层与泡状层延伸。实测得到的d¹⁸O与d¹³C记录呈现正弦式波动，且该波动与壳体微结构无关。d¹³C波动与d¹⁸O波动的波长一致，但存在相位偏移。d¹⁸O记录反映了1957年至1996年的海表温度变化，同时可能受到局地蒸发作用的额外影响。由于半封闭的萨法加湾局地蒸发作用增强，海水d¹⁸O值估算为2.17‰，较该区域已发表的开阔海域表层水d¹⁸O值（1.36~1.85‰）高出0.3~0.8‰。重建得到的平均水温与预期值仅相差0.4℃，最低温与最高温分别较预期值偏低0.5℃、偏高2.9℃。但对比月平均数据时，重建海表温度与全球格点数据集之间的偏差始终小于1.0℃。d¹³C信号与区域叶绿素a（chlorophyll a）浓度及日照时长呈弱负相关，这一相关性可能反映了双壳类的呼吸作用变化。本研究强调了基于化石的同位素研究中古地理背景的重要性，因为近岸海湾或许无法准确反映开阔海域的海洋学条件。