Approaches to unravel seasonality in sea surface temperatures using paired single-specimen foraminiferal delta O-18 and Mg/Ca analyses

Seasonal changes in surface ocean temperature are increasingly recognized as an important parameter of the climate system. Here we assess the potential of analyzing single-specimen planktonic foraminifera as proxy for the seasonal temperature contrast (seasonality). Oxygen isotopes and Mg/Ca ratios were measured on single specimens of Globigerinoides ruber, extracted from surface sediment samples of the Mediterranean Sea and the adjacent Atlantic Ocean. Variability in delta O-18 and Mg/Ca was then compared to established modern seasonal changes in temperature and salinity for both regions. The results show that (1) average delta O-18-derived temperatures correlate with modern annual average temperatures for most sites, (2) the range in delta O-18- and Mg/Ca-derived temperature estimates from single-specimen analysis resembles the range in seasonal temperature values at the sea surface (0-50 m) in the Mediterranean Sea and the Atlantic Ocean, and (3) there is no strong correlation between Mg/Ca- and delta O-18-derived temperatures from the same specimens in the current data set, indicating that other parameters (salinity, carbonate ion concentration, symbiont activity, ontogenesis, and natural variability) potentially affect these proxies.