Abstract
In the southwestern United States, California (CA) is one of the most climatically sensitive regions given its low (≤250 mm/year) seasonal precipitation and its inherently variable hydroclimate, subject to large magnitude modulation. To reconstruct past climate change in CA, cave calcite deposits (stalagmites) have been utilized as an archive for environmentally sensitive proxies, such as stable isotope compositions (δ18O, δ13C) and trace element concentrations (e.g., Mg, Ba, Sr). Monitoring the cave and associated surface environments, the chemical evolution of cave drip-water, the calcite precipitated from the drip-water, and the response of these systems to seasonal variability in precipitation and temperature is imperative for interpreting stalagmite proxies. Here we present monitored drip-water and physical parameters at Lilburn Cave, Sequoia Kings Canyon National Park (Southern Sierra Nevada), CA, and measured trace element concentrations (Mg, Sr, Ba, Cu, Fe, Mn) and stable isotopic compositions (δ18O, δ2H) of drip-water and for calcite (δ18O) precipitated on glass substrates over a two-year period (November 2018 to February 2021) to better understand how chemical variability at this site is influenced by local and regional precipitation and temperature variability. Despite large variability in surface temperatures and precipitation amount and source region (North Pacific vs. subtropical Pacific), Lilburn Cave exhibits a constant cave environment year-round. At two of the three sites within the cave, drip-water δ18O and δ2H are influenced seasonally by evaporative enrichment. At a third collection site in the cave, the drip-water δ18O responds solely to precipitation δ18O variability. The Mg/Ca, Ba/Ca, and Sr/Ca ratios are seasonally responsive to prior calcite precipitation at all sites but minimally to water-rock interaction. Lastly, we examine the potential of trace metals (e.g., Mn2+ and Cu2+as a geochemical proxy of recharge and find that variability in their concentrations has high potential to denote the onset of the rainy season in the study region. The drip-water composition is recorded in the calcite, demonstrating that stalagmites from Lilburn Cave, and potentially more regionally, could record seasonal variability in weather even during periods of substantially reduced rainfall.
Highlights
The monitoring of caves and surface environments is an important approach for understanding processes that impact paleoclimate proxy records developed using cave calcite deposits (“speleothems”)
Big Yellow Hungus Thing (BYHT) drip-water δ18O compositions reach their lowest values in autumn and early winter (September–December) and rise to their highest values in the spring (March–May) (Figure 3)
We demonstrated that the trace element (TE)/Ca ratios in Lilburn Cave drip-water and calcite are sensitive to seasonal variability in precipitation amount suggesting that Lilburn Cave speleothems have the potential to accurately record seasonal precipitation changes through time
Summary
The monitoring of caves and surface environments (the karst system) is an important approach for understanding processes that impact paleoclimate proxy records developed using cave calcite deposits (“speleothems”). Monitoring the cave environment allows for a quantification of the impact of temperature, precipitation and CO2 variability, and cave processes on stable isotopic compositions (δ18O, δ2H, δ13C) and trace element concentrations (Mg, Ba, Sr, etc.), commonly applied proxies in stalagmite paleoclimate reconstruction (McDermott, 2004; Wong and Breecker, 2015). Inferring environmental conditions from stalagmite geochemical compositions is complicated by the confounding effects of multiple influences on their signatures (Lachniet, 2009; Fairchild and Baker, 2012) This reflects the fact that geologic and hydrologic conditions of the karst system (i.e., soil, epikarst, and cave) control how the climate signal is transferred to the geochemical compositions of the drip-water and mineral precipitate as well as their included fluids (e.g., Dreybrodt and Scholz, 2011; Tremaine et al, 2011; Feng et al, 2014). Spatial variability in drip-water and stalagmite geochemical compositions can potentially be unrelated to the external climate signal
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