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.