Determining the climatic drivers of speleothem proxy variability: coupling modern cave monitoring with a multicomponent reactive transport model

Abstract:

Speleothem isotopic and geochemical proxy records can illuminate changes in climate patterns, soils, vegetation, and the amount of seepage water flow into a cave. However, the number of potential chemical and transport mechanisms that influence drip water composition can complicate speleothem records. A thorough understanding of processes affecting isotopic and geochemical compositions of modern cave waters is essential for interpreting proxies in a climate context. We couple a reactive transport model, CrunchTope, with modern measurements from Blue Spring Cave, Tennessee to understand the factors controlling proxy variability. For 2 years we have monitored surface and soil temperature, precipitation and soil moisture, cave temperature and pCO₂, drip rate, and drip water chemistry, δ¹⁸O, and δ¹³C of dissolved inorganic carbon (DIC). The range of variability in drip water δ¹⁸O indicates some drips are fed by fracture flow from the surface, while others are fed by more diffuse flow paths. For both drip types, δ¹³C_DIC is inversely correlated with monthly rainfall. Cave air pCO₂ suggests seasonal ventilation driven by surface air temperature change. Drip water Sr/Ca and Mg/Ca indicate prior carbonate precipitation (PCP) occurs in the epikarst, but do not appear to reflect cave ventilation.

To improve interpretations of drip water geochemical variation, we parameterize CrunchTope with horizons representing soil, epikarst and karst. We use this model to simulate water chemistry changes due to coupled fluid transport and water-mineral interactions in the soils and bedrock. Initial model runs reproduce mean drip water [Sr] and [Mg]. Accurate simulation of drip water [Ca] requires inclusion of a low pCO₂ layer that drives PCP. With the inclusion of isotope systematics, a baseline model calibrated with modern data will be available to simulate the effects of long-term climate change on cave waters, thus enhancing the quantitative interpretation of speleothem proxy records.