A Gibraltar speleothem record of environment and regional climate for the last 200 ka

We present results of our Gibraltar project which intends to provide a speleothem based palaeoclimatic framework for the western Mediterranean realm over the last half-million years. Gibraltar climate is influenced by the Mediterranean Sea and the Atlantic Ocean in a boundary area between polar and subtropical air masses. Speleothem proxy records from Gibraltar provide opportunities to improve understanding of the long-term interplay of climatic features of this key location at the southern limit of tracks presently taken by the North Atlantic depressions delivering rainfall to Europe.

Here we focus on a composite record based on five speleothems: two from near present sea level in Ragged Staff Cave and three from an altitude of 255 masl in St. Michaels Cave. The composite chronology is constrained by over 260 U-Th dates and reveals a near continuous δ¹³C_calcite, δ¹⁸O_calcite and trace element proxy record for the last 200 ka. Extensive cave monitoring results provide a robust foundation for palaeoclimate interpretations of the proxy data. The Gibraltar δ¹⁸O record shows clear imprints of Alboran Sea SST for the last 200 ka and is strikingly similar to NGRIP but with a range of variation notably more subdued than other long speleothem records like Soreq or Hulu that appear to have insolation forcing of monsoon strength as their dominant control. We interpret the Gibraltar δ¹⁸O record with respect to changes in cave temperatures, ocean δ¹⁸O and isotopic disequilibrium during calcite deposition. The Gibraltar δ¹³C record is interpreted as signaling changes in vegetation and water balance.