Combining the Mg/Ca of the ostracod Cyprideis torosa with its ontogenic development for reconstructing a 28 kyr temperature record for Lake Banyoles (NE Spain)

Cyprideis torosa is a ubiquitous ostracod found in fresh to hypersaline waters, and commonly in large numbers across the Mediterranean region. Single valves of 51 adult specimens of C. torosa were separated from carapaces that were collected from Lake Banyoles in NE Spain at a depth of 5 m. The Mg/Ca of the valves was compared with known temperatures necessary for successive instar valve calcification, the latter being based on a four-year ecological study of C. torosa collected at Dievengat in northern Belgium by Carlo Heip in 1968-72. Hence, we were able to link the Mg/Ca of fossil valves of C. torosa recovered for a 28 kyr sequence at La Draga, cored on the fringe of Lake Banyoles, with the ontogenic observations of Heip, and reconstruct mean summer temperatures as well as optimal calcification temperature for C. torosa. Principal findings are: (1) the Holocene registered the highest temperatures with also very broad fluctuations; (2) three cold phases are clearly identified at 26.7-23.2, 21.6-20.3 and 16.2-14.3 ka BP; and (3) a prolonged warm phase that lasted about two millennia commenced at 19.5 ka BP and was followed by a progressive temperature decline well over three millennia. Surprisingly the Last Glacial Maximum was not the coldest phase. We finally compare our results with sea-surface temperature (SST) reconstructions from cores from the western Mediterranean Sea. Our record from La Draga clearly matches events recorded in the Alboran Sea that display SST changes obtained from the UK'37 index. The Heinrich 1 and 2 events around 16 and 28 ka BP coincide with significant low temperature excursions at Banyoles, and palynological records in the marine cores which define semi-arid conditions on land match the low temperature record in our core.