The giant oyster Hyotissa hyotis from the northern Red Sea as a decadal-scale archive for seasonal environmental fluctuations in coral reef habitats

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 delta O-18 and delta C-13 records show sinusoidal fluctuations, which are independent of shell microstructure. The delta C-13 fluctuations exhibit the same wavelength as the delta O-18 fluctuations but are phase shifted. The delta O-18 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 delta O-18(seawater) value is estimated at 2.17aEuro degrees, i.e., 0.3-0.8aEuro degrees higher than published open surface water delta O-18 values (1.36-1.85aEuro degrees) from the region. The mean water temperature deviates by only 0.4A degrees C from the expected value, and the minimum and maximum values are 0.5A degrees C lower and 2.9A degrees C higher, respectively. When comparing the mean monthly values, however, the sea surface temperature discrepancy between reconstructed and global grid datasets is always < 1.0A degrees C. The delta C-13 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.