δ18O AND δ13C ALONG THE GROWTH AXIS OF A SOREQ CAVE (ISRAEL) SPELEOTHEM: SUB-ANNUAL RESOLUTION BY ION MICROPROBE

Soreq Cave, Israel, contains a record of continuous speleothem growth from 185ka to present. The speleothems preserve geochemical signals of climate as they grow. Fluctuations in their oxygen and carbon isotope composition reflect changes in rainfall, cave-air temperature, and overlying vegetation.

Most isotopic studies of speleothems at Soreq Cave have been limited to low spatial resolution (0.5mm drill-samples), which restricts their temporal resolution to a decadal timescale. The analytical capabilities of the WiscSIMS CAMECA 1280, with a spatial resolution of 10mm while measuring d¹⁸O at 1s = ±0.15‰, allow us to analyze sub-annual growth bands in a speleothem.

Sample “2-6” is a drip-formed stalagmite (5.5cm radius) composed of low-magnesium calcite. Eight U-series ages indicate growth from ~1800 to 1000 years BP. The speleothem was cut into five 1cm portions and mounted in epoxy with UWC-3 calcite standard. The plugs were imaged using optical, laser confocal, and SEM microscopy before being analyzed for d¹⁸O and d¹³C on the ion microprobe.

Fluorescent confocal imaging reveals distinct concentric banding that occurs in light/dark couplets. Within single couplets we observe consistently smooth increases of d¹⁸O by as much as 2‰ between the light and dark bands. These bands may correspond to the annual cycle of wet and dry seasons in the region. The d¹⁸O of light portions reflects the low d¹⁸O of mid-winter rainfall. The gradual increase of d¹⁸O across the bands reflects higher d¹⁸O rainfall,

typical of the end of the winter season (similar to present-day). The highest d¹⁸O values, found in the dark portions, represent the highest d¹⁸O waters that reach the cave during the dry summer. The D¹⁸O between light and dark bands varies across the sample, with larger D¹⁸O and rhythmic oscillations being typical of prolonged wet periods, while dryer periods have lower D¹⁸O and more diffuse bands. By analyzing only the wet-season banding, it is possible to track the relative shifts in d¹⁸O_calcite (and thus, d¹⁸O_rainfall) at annual resolution.

Carbon isotope measurements track a similar pattern to d¹⁸O; low d¹³C_calcite values are found in light bands, while dark bands are enriched by up to 4‰ in ¹³C. The synchronized variation of both d¹⁸O and d¹³C highlights the influence of seasonal changes in rainfall d¹⁸O and soil-CO₂ production.