The potential of near-entrance stalagmites as high-resolution terrestrial paleoclimate proxies: Application of isotope and trace-element geochemistry to seasonally-resolved chronology

Sub-annually resolved environmental proxies can be valuable archives of climate change, but they are rare in terrestrial settings, and it can be difficult to verify their annual nature. We suggest that speleothems that grow in well-ventilated zones of caves may preserve such high-resolution records. Near-entrance cave environments are characterized by year-round, near-atmospheric CO2 concentrations and are significantly influenced by surface air temperature fluctuations, particularly in temperate latitudes. Previous monitoring studies of a well-ventilated, temperate-latitude cave (Westcave Preserve, central Texas) have documented seasonal variations in the oxygen isotope composition of calcite grown on glass substrates (with winter δ18O maxima and summer δ18O minima) as well as seasonal variations in drip water trace element compositions. Extending this work to a stalagmite (WC-3) from the same drip site, we find that stalagmite δ18O variations are similar in magnitude to the seasonal δ18O variations previously observed for calcite grown on glass substrates, that stalagmite [Mg] variations have a similar seasonal period with winter minima and summer maxima, and that geochemical variations follow stalagmite growth fabric as mm-scale couplets comprising thin, slow-growing, compact sparry calcite laminae (winters) and thicker, fast-growing, porous-elongate columnar calcite laminae (summers). We interpret a high-resolution (weekly to monthly) 52-year record of δ18O, Mg, Sr, and Ba in WC-3, and report new monthly measurements of drip water and associated calcite grown on glass substrates. We find drip water δ18O and [Mg]/[Ca] are essentially invariant and that seasonal variations in WC-3 calcite δ18O and Mg concentration agree well with predicted temperature-dependent fractionation between water and calcite. WC-3 calcite Sr and Ba also vary, but with higher and more variable frequencies compared to δ18O and [Mg]. The annual nature of δ18O and [Mg] cycles is supported by monitoring and 14C bomb-peak chronology. We suggest that stalagmite δ18O and [Mg] vary primarily in response to large seasonal temperature changes in this setting, allowing for unambiguous differentiation between summer and winter calcite growth. From such δ18O- and [Mg]- derived sub-annual chronologies, the timing of enrichments in other geochemical species that are less directly coupled to external cave temperature (e.g., calcite Sr and Ba) can be considered as proxies of other important processes such as water-rock interaction in the epikarst, precipitation events, or subsurface respiration rates. The potential for this kind of multi-proxy, seasonally-resolved dating may add near-entrance stalagmites to the list (ice cores, lake varves, tree rings) of high-resolution terrestrial proxies available for paleoclimate studies.