Temperature and productivity influences on U37K′ and their possible relation to solar forcing of the Mediterranean winter

To assess the extent and nature of human-induced climate change, we need to understand natural climate variability. This heavily relies on proxy-based climate reconstructions, calling for excellent understanding of the conditions represented by the proxies. Here we report on an alkenone-based (U37 K′) sea surface temperature (SST) proxy record originating from south Italian marine sediments, and covering 1305 A.D. to 1979 A.D. with a 3.87 year resolution. The shallow-water cores, extracted from the Gallipoli terrace in the Gulf of Taranto, were dated by the Torino group with high accuracy over the last two millennia, using radiometric and tephroanalysis methods. On the basis of comparison with historical and satellite-derived climate and productivity data, we propose that U37 K′ reflects mainly SST of the cooler part of the year and is modified by variation in magnitude and timing of peak alkenone production on the seasonal temperature cycle. Comparison of the U37 K′ record with the record of atmospheric Δ14C, a proxy for solar energy variability, shows a high correlation (r = −0.73) for the period between 1420 A.D. and before human interference with the Δ14C record by combustion of fossil carbon and the release of radioactive isotopes by nuclear tests. This suggests a centennial-scale solar forcing. We propose that wind-induced mixing resulting from Mediterranean climate dynamics plays a crucial role in translating solar activity to alkenone productivity and SST during the cooler part of the year.