Spondylus gaederopus: A new Mediterranean climate archive — Based on high-resolution oxygen and carbon isotope analyses

Abstract

The presented study evaluates the potential of the Mediterranean bivalve Spondylus gaederopus Linné, 1758 as a palaeoclimatic archive. Its fixosessil life in water depths < 50 m, its thick shell with heights of more than 10 cm, a lifespan of potentially several decades, regular annual growth bands in its inner aragonitic shell, its dense shell providing a good preservation potential, and its common occurrence in Pleistocene shallow-water deposits in the Mediterranean region makes this species an ideal candidate. Stable oxygen ( δ 18O) and carbon ( δ 13C) isotope data obtained from high-resolution sampling of a modern S. gaederopus from Rhodes, Greece, collected alive in 4 m water depth in spring 2002, suggest that the outer calcitic shell layer, as well as the inner aragonitic shell layer is precipitated in equilibrium with seawater. Due to the complexity of the outer calcitic shell layer with its spines and prickles, a chronological sampling of this part of the shell is not possible. In contrast, the inner aragonitic shell layer with its well-developed growth bands allows a fast chronologic sampling. 18 aragonite growth bands were sampled from the inner shell layer of the studied specimen, corresponding to 9 years of growth with 9–10 samples per year. Carbon stable isotopes ( δ 13C Ar) yield mean values of 0.70 ± 0.6‰ and show a remarkable negative shift of about 1.8‰ (from about 1.6‰ to about − 0.2‰) through time. This ontogenetic depletion in 13C might be related to physiological developments like sexual maturity, as it is the case in other bivalves. Seasonal variations with an average amplitude of 0.85‰ are superimposed on the observed secular trend. These cycles show an increase in amplitude that has not been described before and cannot be explained so far. Oxygen isotopes ( δ 18O Ar) exhibit mean values of 1.11 ± 0.5‰. Clearly pronounced seasonal cycles with an average amplitude of 1.45‰ show the same wavelength as the carbon cycles, but run out of phase. The δ 18O Ar correlates well with sea surface temperature and correlation fits best during summer periods. Deviations between measured and predicted δ 18O Ar (input data: sea surface temperature, seawater δ 18O) show that maximal discrepancies occur during winter periods. These discrepancies can be best explained by a decrease in local seawater δ 18O due to increased freshwater influx. On Rhodes maxima in precipitation, hence river runoff, occur during winter months.