A Gibraltar speleothem record of environment and regional climate for the last glacial period

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 caves 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.

Deployment of logging instruments from 2004 to 2012 plus water and air sampling in two Gibraltar caves - St. Michaels and Ragged Staff - revealed that seasonality in speleothem growth rates is most strongly influenced by seasonally reversing convective ventilation that permeates the entire Rock. Average rates over longer timescales depend on drip rates (reflecting hydroclimate at the surface), combined with conversion into dissolved inorganic carbon of organic matter washed down by deep percolation into the epikarst and bedrock. The geochemistry of calcite precipitation is thus regulated by environmental conditions including net organic production by vegetation, temperatures and water balance. The monitoring results provide a robust foundation for palaeoclimate research. Data from twenty-four dated speleothems provide a palaeoclimatic framework with insights into regional climate as recorded by chemical proxies over seasonal to multimillennial timescales, over the last half-million years.

Here, we concentrate on the last glacial period and present a composite record based on four speleothems: two from near present sea level in Ragged Staff (Gib 10d and Gib 10e) and two (Gib 06a and Gib 06b) from an altitude of 255 masl in St. Michaels Cave. The composite chronology is constrained by over 170 U-Th dates and provides a continuous δ¹³O_calcite, δ¹⁸O_calcite and trace element proxy record of palaeoclimate from 100 ka to 10 ka, at a resolution that varies between ~5 and ~100 years. The Gibraltar δ¹⁸O record is strikingly similar to NGRIP but with a range of variation notably more subdued than other long speleothem records such as Soreq and Hulu that appear to have insolation forcing of monsoon strength as their dominant control. Our δ¹⁸O_calcite data show clear imprints of Alboran Sea SST associated with D-O cycles during MIS 2-5, superimposed on a general trend of higher δ¹⁸O during stadials and lower values in interstadials and interglacials. The latter trend may have multiple causes including changes in cave temperatures, ocean δ¹⁸O and isotopic disequilibrium during calcite deposition. Changes in atmospheric circulation are another possibility but currently difficult to evaluate as neither δ¹⁸O of individual rainfall events nor the GNIP monthly record show any relationship to modern synoptic circulation patterns. The δ¹³C record is interpreted as signaling changes in vegetation and water balance. Higher δ¹³C values are caused by greater degassing due to lower drip rates and/or weakened net organic matter production and downwash during cooler periods with less recharge. The trace element records across DO events provide supporting observations.