Abstract
We report the presence of clear annual cycles in trace-element (Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca) and stable-isotope (δ 18O and δ 13C) 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 δ 18O variations (− 10.8‰ to − 8.5‰) are controlled by seasonal variations in temperature and drip-water δ 18O 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 δ 13C 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 δ 13C (− 12.5‰ to − 10.7‰), may be fully explained by progressive CO 2 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, δ 13C TDIC = − 16.75‰), 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 CO 2 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 δ 13C values during drier periods. Past variations in Heshang carbonate chemistry may therefore be useful as seasonal resolution proxies for past rainfall.
Highlights
In recent years, numerous speleothem based proxy studies have significantly advanced our understanding of annual-to-decadal scale paleoclimate variability
A number of recent studies have focused in detail on the complicated mechanisms through which trace elements and stable isotopes are incorporated in speleothem calcite [2,3,4,5,6,7,8,9]
We suggest that seasonal variations in the amount of CO2 degassing and calcite precipitation in Heshang Cave occur through two primary mechanisms, both related to rainfall and, drip rate variations: (1) Prior calcite precipitation may occur within the epikarst or on the cave ceiling as stalactites
Summary
Numerous speleothem based proxy studies have significantly advanced our understanding of annual-to-decadal scale paleoclimate variability. While the majority of these studies have focused on the use of speleothem stable-isotope variations (δ18O and δ13C) to reconstruct paleorainfall, paleotemperature, and/or paleovegetation, several recent studies have investigated the paleoclimate proxy potential of annual trace-element variations preserved in speleothems [1,2,3]. Annual cycles in trace-element composition, a common feature of speleothems [4], may be useful as paleoenvironmental proxy records with seasonal resolution. In order to use speleothem trace-element variations to reconstruct past changes in climate, it is imperative that the proxies are well understood, well calibrated, and rigorously tested in any individual study area, because numerous environmental, climatic and geologic factors may affect the chemistry of speleothem calcite. Based on the comparison of the observed annual cycles with modern calcite, drip water, climatic data, and a simple model, we suggest that annual trace-element (Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca) and stable-isotope (δ13C and δ18O) cycles record past hydrologic changes which directly reflect past variations in summer monsoon rainfall
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