| Summary: | We report the presence of clear annual cycles in trace-element (Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca) and stable-isotope (δ¹⁸O and δ¹³C) composition in an annually banded stalagmite from Heshang Cave, Hubei Province, China (30.44°N, 110.42°E). Through a combination of micromilling and in situ analysis (LA-MC-ICPMS), we measured geochemical variations across 16 annual growth bands, to assess their potential as seasonal resolution paleomonsoon proxies. To facilitate comparison with modern climatic and environmental data we created composite annual cycles for each proxy by stacking 6 well-defined years. Speleothem δ¹⁸O variations (-10.8‰ to -8.5‰) are controlled by seasonal variations in temperature and drip-water δ¹⁸O which lead to maximum values during May, around the time of summer monsoon onset. This provides a chronological marker which can be used to constrain the timing of other geochemical cycles. The composite cycles reveal a strong positive correlation between Mg/Ca, Sr/Ca, Ba/Ca, and δ¹³C values in the micromilled section (R² = 0.65-0.98), with minimum values occurring around May. Maximum U/Ca values occur at the same time. We present simple models which show that these correlations, as well as the observed ranges of Mg/Ca (14.1 to 22.4 mmol/mol), Sr/Ca (0.2 to 0.4 mmol/mol), and δ¹³C (-12.5‰ to -10.7‰), may be fully explained by progressive CO₂ degassing and calcite precipitation from an initially saturated solution. Using realistic initial conditions for Heshang Cave (T = 18°C, Mg/Ca solution = 0.84 mol/mol, Sr/Ca solution = 0.69 mmol/mol, δ¹³C TDIC = -16.75‰), we find that the observed relationships can be produced by using DMg = -.016 and DSr = 0.30, within the range of expected values. The model suggests that the fraction of Ca removed from the solution ranges from 0 to 30% to produced the observed seasonal cycles. This variation may be due to two related processes which occur during drier periods: (1) increased prior precipitation of calcite in the epikarst or on the cave ceiling, and/or (2) a greater degree of CO₂ degassing and calcite precipitation on stalagmite surfaces when drip-rates are lower. Both mechanisms would have the effect of enriching speleothem Mg/Ca, Sr/Ca, Ba/Ca, and δ¹³C values during drier periods. Past variations in Heshang carbonate chemistry may therefore be useful as seasonal resolution proxies for past rainfall.
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