Testing Speleothem Proxies of the Asian Monsoon: Modern Calibration Results from Heshang Cave, China

Seasonality is a key aspect of climate that may play an important role in driving abrupt climate change, yet there is a paucity of seasonal resolution paleoclimate records. Numerous speleothem δ¹⁸O records of East Asian monsoon rainfall reveal that summer insolation is the primary driver of low-frequency monsoon variability, but significant high-frequency δ¹⁸O variability also exists and is often poorly understood. A 2.5 m long annually laminated stalagmite, HS-4, collected from Heshang Cave, China formed continuously during the last 9.5 kyr and contains a high resolution record of climate. This sample contains seasonal variations in minor and trace element concentration, which are a widespread feature of calcite speleothems from a variety of climatic regimes and hold great potential as sub-annual resolution paleoclimate proxies. To test how seasonal variations in temperature and rainfall at this site are recorded in speleothem geochemistry, we have been conducting a modern calibration study in Heshang Cave since 2004. We show that seasonal cycles in cave temperature and drip rate reflect seasonal changes in regional temperature and precipitation, respectively. In addition, we compare environmental parameters and modern dripwater chemistry with the chemistry of calcite grown on glass slides every month to determine modern controls on speleothem geochemistry. Using instrumental temperature data and measured dripwater δ¹⁸O, we show that modern speleothem calcite is forming in isotopic equilibrium and that seasonal cycles are primarily controlled by cave temperature, even though low-frequency variations are thought to reflect monsoon intensity. Strong positive correlation between Mg/Ca, Sr/Ca, Ba/Ca, and δ¹³C in glass slide data and micromilled Last Glacial Maximum (~20 ka), mid-Holocene (~7 ka), and modern (1949-2001 AD) sections of Heshang Cave speleothems suggest a common environmental control, most likely related to crystal growth conditions (temperature, growth rate, degassing rate, etc.) rather than variations in drip water chemistry.