Hydroclimate interpretations of stalagmite δ18O records from tropical regions requires an understanding of the temporal integration of rainfall amount and its isotopic composition by drip waters that form stalagmite deposits. This study presents oxygen (δ18O) and hydrogen (δD) isotopic results from over 1200 groundwater, rainfall and drip water samples, collected at ~weekly time intervals, over three hydrological years at Río Secreto Cave, in the Yucatán Peninsula, Mexico. Cave environmental conditions and the isotopic composition of drip water were monitored in three chambers with different degrees of air ventilation, along with temperature and relative humidity conditions at the surface. We examined 16 drips and observed that annual δD and δ18O variability reflects the isotopic variability of rainfall to varying degrees. The observed annual amplitude of drip water isotopic variability represents between 5% and 95% of that of rainfall, reflecting epikarst water reservoir size and the complexity of flow paths. Drips that closely reflect the isotopic variability of rainfall and best preserve the isotopic signal of individual rainfall events are observed, but they are uncommon. Only two drips out of 16 were found to have potential to record rainfall isotopic shifts associated with tropical cyclones if sampled at weekly resolution. The relationship between δD and δ18O in drip water suggests that recharge is biased toward the rainy season (June to November), which represents up to 80% of total annual precipitation. We find that over the course of a year most drips reflect the annual δ18O composition of rainfall, in support of quantitative precipitation estimates from stalagmite δ18O records. We find evidence that the effective recharge in this cave system is controlled by precipitation amount and that recharge is not limited to the months when precipitation exceeds evaporation.
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Determining the source of speleothem δ18O variability from in situ measurements of seasonal and inter-annual isotope trends in precipitation, cave drip water and modern calcite from sites in the Central Peruvian Andes
In the past decade, Huagapo and Pacupahuain Caves in the Central Peruvian Andes have become sources of speleothem oxygen isotope (δ18O) paleoclimate records. These studies identify the South American Summer Monsoon (SASM) as the main climate system controlling δ18O variability. While this interpretation is verified through inter-proxy record comparisons on millennial scales, interpretation of the high-resolution variability within these records is limited by a lack of modern proxy calibration studies at these sites. Here we present results from a modern cave monitoring study undertaken to address the controls on the δ18O values of precipitation at these sites and how surface and in-cave processes affect the δ18O value of speleothem calcite. Speleothem calcite δ18O values reflect an integrated signal of atmospheric processes (e.g., rainout, Raleigh distillation, upstream moisture recycling, changes in moisture source), evaporation and mixing during infiltration in the soil and epikarst, and in-cave processes such as degassing and evaporation. In consideration of these factors, we compare isotope trends in precipitation, cave drip water and modern farmed calcite from the two cave sites. We find that precipitationδ18O values during peak monsoon activity (January -February) shows considerable inter-annual variation with averages of -16.7‰ for 2020, -18.5‰ for 2021 and -13.8‰ in 2022. We investigate the source of this variability in regional atmospheric circulation patterns using weather station data and back trajectories. The mean annual precipitation (MAP) from outside Huagapo Cave is δ18O = -15.5+/- 6‰, while seasonal samples of drip water δ18O = -14.5+/- 1‰, are offset from MAP possibly due to evaporation during infiltration. Cave drip waterδ18O has low variability over inter-annual and seasonal timescales indicating homogenization in the epikarst. Using geochemical and sensor data (e.g. cave relative humidity, temperature, and drip rate) as inputs for a karst based forward model, we simulate modern speleothem δ18O to quantitatively assess the combined effects of hydroclimate processes integration to the isotope record.
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- Award ID(s):
- 2102996
- NSF-PAR ID:
- 10426084
- Date Published:
- Journal Name:
- American Geophysical Union Conference. Chicago, IL
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.
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