Seasonal trace-element and stable-isotope variations in a Chinese speleothem: The potential for high-resolution paleomonsoon reconstruction

We report the presence of clear annual cycles in trace-element (Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca) and stable-isotope (delta O-18 and delta C-13) composition in an annually banded stalagmite from Heshang Cave, Hubei Province, China (30.44 degrees N, 110.42 degrees 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 modem climatic and environmental data we created composite annual cycles for each proxy by stacking 6 well-defined years. Speleothem 8180 variations (-10.8 parts per thousand to -8.5 parts per thousand) are controlled by seasonal variations in temperature and drip-water delta O-18 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 the other geochemical cycles. The composite cycles reveal a strong positive correlation between Mg/Ca, Sr/Ca, Ba/Ca, and delta C-13 values in the micromilled section (R-2=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 delta C-13 (-12.5 parts per thousand to -10.7 parts per thousand), 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 degrees C, Mg/Ca-solution = 0.84 mol/mol, Sr/Ca-solution = 0.69 mmol/mol, delta C-13(TDIC) = -16.75 parts per thousand), we find that the observed relationships can be produced by using D-Mg=0.016 and D-Sr=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 produce 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 delta C-13 values during drier periods. Past variations in Heshang carbonate chemistry may therefore be useful as seasonal resolution proxies for past rainfall. (c) 2006 Elsevier B.V. All rights reserved.