87Sr/ 86Sr and Sr/Ca in speleothems for paleoclimate reconstruction in Central China between 70 and 280 kyr ago

Abstract

Limestone cave deposits (speleothems) provide archives for past changes in regional climates over a range of timescales. While δ 18O and δ 13C in speleothem calcite have been commonly used for reconstruction of paleoclimates, we report here further efforts in the use of 87Sr/ 86Sr and Sr/Ca signals in speleothem calcite to deduce paleomonsoon variability near the Loess Plateau of central China. A two end-member mass-balance model of concentration and isotopic composition of strontium in a cave system is used to estimate variation of the 87Sr/ 86Sr ratio in sediments overlying a limestone cave. We show that this ratio reflects climate-driven variations in the provenance and the extent of chemical weathering of the epikarstic sediments. The measurements of 87Sr/ 86Sr made on a well-dated stalagmite, SFL, from Buddha Cave (33 o40N′ 109 o05′E) show ratios of 0.71092 to 0.71133 (±0.00001 as 2σ) during relatively cold periods (e.g., Marine Isotope Stage (MIS) 5b, 5d, and 8), lower than ratios of 0.71133 to 0.71194 during relatively warm periods (e.g., MIS 5a, 5c, 5e, and 7). As changes in the Sr/Ca ratio may affect speleothem 87Sr/ 86Sr, we show that the direct use of speleothem 87Sr/ 86Sr is less ideal than our modeled 87Sr/ 86Sr for the exogenic Sr source above the cave as a paleomonsoon proxy. Using the δ 18O, δ 13C, Sr/Ca, and 87Sr/ 86Sr records of the stalagmite, we reconstruct the variability of the East Asian monsoon for the time period between 70 and 280 kyr ago. The results show that summer monsoons were more intense during interglacial periods than during glacial periods.