Climate deterioration in the Eastern Mediterranean as revealed by ion microprobe analysis of a speleothem that grew from 2.2 to 0.9 ka in Soreq Cave, Israel

Abstract

Analysis of oxygen isotope ratios (δ 18O) by ion microprobe resolves a sub-annual climate record for the Eastern Mediterranean from a Soreq Cave stalagmite that grew between 2.2 and 0.9 ka. In contrast to conventional drill-sampling methods that yield a total variation of 1.0‰ in δ 18O calcite values across our sample, the methods described here reveal up to 2.15‰ variation within single annual growth bands. Values of δ 18O measured by ion microprobe vary in a regular saw-tooth pattern that correlates with annual, fluorescent growth banding where calcite grades from light to dark fluorescence. Modern records of precipitation and of cave dripwater indicate that variable δ 18O calcite values record regular seasonal differences in δ 18O rainfall modified by mixing in the vadose zone. Large differences in δ 18O values measured across a single band (i.e., between the dark and light fluorescent calcite, or Δ 18O dark-light) are interpreted to indicate wetter years, while smaller differences represent drier years. Oxygen isotopes record: 1) month-scale growth increments, 2) changes in Δ 18O dark-light that represent seasonality, 3) a systematic, long-term decrease in maximum Δ 18O dark-light values, and 4) an overall increase in average δ 18O calcite values through time. These results suggest a drying of regional climate that coincides with the decline of the Roman and Byzantine Empires in the Levant region.