Seasonal-Resolution δ18O in Speleothems by Ion Microprobe: Revealing Asian Monsoon Dynamics

Abstract:

Over the last decade, ion microprobe analysis of speleothems (cave carbonates) has increased the temporal resolution of their oxygen isotope (δ¹⁸O) paleoclimate proxy records. Recent improvements in methodology, standardization, and imaging at the WiscSIMS lab make it possible to examine sub-annual patterns of δ¹⁸O variability at 10-µm-scale, revealing new seasonal paleoenvironmental information. We applied this technique to an important suite of Chinese stalagmites with conventional drill-sampled δ¹⁸O records that reflect changes in Asian Monsoon dynamics across the last deglaciation.

Seasonal-resolution δ¹⁸O analyses in the Chinese stalagmites reveal regular patterns of annual δ¹⁸O variability. Quantitative assessment of the patterns identifies two important components in the δ¹⁸O records. First, the source and rainout histories of water vapors that ultimately yield rainfall over China play a primary role in determining the δ¹⁸O value of speleothem calcite year-round. Second, intra-annual patterns of calcite δ¹⁸O variability indicate that the annual proportion of monsoon precipitation changes systematically during the last deglaciation; the annual proportion of monsoon rainfall is greater during the Holocene and Bølling-Allerød than during the Younger Dryas.

This is the first time these components have been characterized in any speleothem δ¹⁸O record of monsoon dynamics because seasonal δ¹⁸O variability is lost by conventional drill-sampling. Ion microprobe analysis of speleothems can also produce year-by-year records of δ¹⁸O across abrupt climate change events. At the Younger Dryas-Holocene transition in a Kulishu Cave stalagmite, which spanned 16 years at 11.53 ky BP, there is a relatively smooth decrease in year-round δ¹⁸O(calcite). In contrast, the intra-annual δ¹⁸O patterns indicate that the increase in the annual proportion of monsoon rainfall across this transition is stochastic, implying that this record can distinguish the regional monsoon response from the hemisphere-wide atmospheric reorganization.