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Abstract Application of novel proxies, such as the stable isotope compositions and noble gas concentrations of fossil drip water trapped as inclusions in stalagmites, have the potential to provide unique constraints on past hydroclimate states and surface temperatures. Geochemical analysis of inclusion waters, however, requires an understanding of the three‐dimensional spatial distribution of dominantly liquid‐ versus air‐filled inclusions in a given stalagmite. Here we couple neutron computed tomography and medium‐ to high‐resolution X‐ray computed tomography to map out the three‐dimensional calcite density and distribution of liquid‐ versus air‐filled inclusions within a Sierra Nevada stalagmite (ML‐1), which formed during the last deglaciation (18.5 to 11.7 ka). Comparison of coupled neutron computed tomography‐X‐ray computed tomography results with a time series of stalagmite calcite fabrics indicates that although highest density calcite contains abundant liquid (fluid)‐filled inclusions, calcite density and fabric overall were secondary controls on the liquid inclusion distribution (LID). Furthermore, a multistatistical evaluation of the stalagmite time series indicates a significant relationship at the multicentury‐ to millennial‐scale between LID and calcite δ¹⁸O and δ¹³C that suggests a potential link between LID and water availability to the cave. 

Cave carbonate minerals are an important terrestrial paleoclimate archive. A few studies have explored the potential for applying carbonate clumped isotope thermometry to speleothems as a tool for constraining past temperatures. To date, most papers utilizing this method have focused on mass-47 clumped isotope values (Δ47) at a single location and reported that cave carbonate minerals rarely achieve isotopic equilibrium, with kinetic isotope effects (KIEs) attributed to CO2 degassing. More recently, studies have shown that mass-47 and mass-48 CO2 from acid digested carbonate minerals (Δ47 and Δ48) can be used together to assess equilibrium and probe KIEs. Here, we examined 44 natural and synthetic modern cave carbonate mineral samples from 13 localities with varying environmental conditions (ventilation, water level, pCO2, temperature) for (dis)equilibrium using Δ47-Δ48 values, in concert with traditional stable carbon (δ13C) and oxygen (δ18O) isotope ratios. Data showed that 19 of 44 samples exhibited Δ47-Δ48 values indistinguishable from isotopic equilibrium, and 18 (95 %) of these samples yield Δ47-predicted temperatures within error of measured modern temperatures. Conversely, 25 samples exhibited isotopic disequilibria, 13 of which yield erroneous temperature estimates. Within some speleothemsamples, we find Δ47-Δ48 values consistent with CO2 degassing effects, however, the majority of sampleswith KIEs are consistent with other processes being dominant. We hypothesize that these values reflect isotopicbuffering effects on clumped isotopes that can be considerable and cannot be overlooked. Using a Raleigh Distillation Model, we examined carbon and oxygen isotope exchange trajectories and their relationships with dual clumped isotope disequilibria. Carbon isotope exchange is associated with depletion of both Δ47 and Δ48 relative to equilibrium, while oxygen isotope exchange is associated with enrichment of both Δ47 and Δ48 relative to equilibrium. Cave rafts collected from proximate locations in Mexico exhibit the largest averagedepartures from equilibrium (ΔΔ47 = − 0.032 ± 0.007, ΔΔ48 = − 0.104 ± 0.035, where ΔΔi is the measured value – the equilibrium value). This study shows how the Δ47-Δ48 dual carbonate clumped isotope framework can be applied to a variety of tcave carbonate mineral samples, enabling identification of isotopic equilibria and therefore quantitative application of clumped isotope thermometry for paleoclimate reconstruction, or alternatively, constraining the mechanisms of kinetic effects. 

Atmospheric rivers (ARs) bring concentrated rainfall and flooding to the western United States (US) and are hypothesized to have supported sustained hydroclimatic changes in the past. However, their ephemeral nature makes it challenging to document ARs in climate models and estimate their contribution to hydroclimate changes recorded by time-averaged paleoclimate archives. We present new climate model simulations of Heinrich Stadial 1 (HS1; 16,000 years before the present), an interval characterized by widespread wetness in the western US, that demonstrate increased AR frequency and winter precipitation sourced from the southeastern North Pacific. These changes are amplified with freshwater fluxes into the North Atlantic, indicating that North Atlantic cooling associated with weakened Atlantic Meridional Overturning Circulation (AMOC) is a key driver of HS1 climate in this region. As recent observations suggest potential weakening of AMOC, our identified connection between North Atlantic climate and northeast Pacific AR activity has implications for future western US hydroclimate. 

Stalagmites are an important archive of terrestrial climate information. However, there remains questions about the ability of stalagmites to form in oxygen isotopic equilibrium and thus record, in a simple manner, the oxygen isotopic composition and temperature of formational fluids. Recent studies have suggested that the combined application of 48 and 47 carbonate clumped isotope measurements can quantify the extent of kinetic isotope fractionation in stalagmites and thus used to correct for these kinetic isotope effects and solve for the original formation temperatures. Here we measure the 47 and 48 values from 16 different samples of the same stalagmite from central California that spans the deglaciation (11 to 20 kya). Each sample is replicated three to five times. We find that based on these measurements the extent of kinetic fractionation present in the carbonate from this stalagmite is minimal. The temperature calculated from 47 in this stalagmite ranges from 11.6 to 19.9 °C, in agreement with regional reconstructions of temperatures from 47 values of lake carbonates. In contrast, previously published the 18O and 2H values of inclusion fluids (Wortham et al., 2022) from this stalagmite suggest periods of increasing kinetic fractionation of the water isotopes at 13 and 15 ka. These periods have been previously interpreted to be times of a reduction in effective moisture regionally. We suggest by this comparison that the use of both water isotopes and the dual clumped isotope system in stalagmites can aide the interpretation of where kinetic fractionation occurs in the hydrologic and carbonate precipitation system in caves. We will discuss the work’s implications for paleoclimate records from stalagmites and other terrestrial systems in seasonally dry and Mediterranean regions. 

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.