Past Climate, Modern Caves, and Future Resource Management in Speleothem Paleoclimatology

Abstract

My research focuses on reconstructing past climate in southern Arizona using cave deposits called speleothems. However, this necessitates a broader perspective than simply a geochemical time series, and therefore, I also investigate modern cave systems using a combination of modeling and observational datasets. Finally, cave deposits are fundamentally non-renewable resources, and sampling for past climate reconstruction can be destructive, unlike other cave uses. My last investigation is focused on developing possible best practice recommendations for paleoclimate scientists and other cave stakeholders moving forward. We developed two new stalagmite records of past climate variability in southern Arizona over the past 7000 years. Past climate reconstruction from two caves (Cave of the Bells and Fort Huachuca Cave) highlights insolation control of southern Arizona hydroclimate from 7000-2000 years before present. Additionally, comparison between two stalagmites with different seasonal sensitivities uncovers a few eras of multi-decade long droughts in southern Arizona, which align with other regional reconstructions of past climates and elucidate forcings on Southwest paleoclimate as emergent from both external (insolation) and internal climate variability in the Pacific and Atlantic Ocean basins. Although the oxygen isotopic signal of cave calcite in speleothems is complex, agreement with these other records indicates that the speleothem records from these caves primarily record a climate signal.Modeling and monitoring of modern caves both helps us interpret paleoclimate records and enhances our understanding of cave systems in their own right. Modeling of Cave of the Bells dripwaters demonstrates the effect of storage and mixing on the dripwater oxygen isotope signal; non-climate processes can imprint on dripwater variability on multidecadal timescales. Monitoring shows that on very small spatial scales, every cave is different, and even sites within the same cave respond uniquely to surface climate. Most notably, calcite oxygen isotopic composition, used to reconstruct past climate, shows seasonal variability unrelated to dripwater and surface rainfall oxygen isotope variability. Substantial oxygen isotope disequilibrium is identified at numerous caves sites in southern Arizona, and this understanding aligns with a growing number of cave studies that demonstrate the long-held assumption of isotopic equilibrium in cave systems may not always be valid or that the way in which we define isotopic equilibrium insufficiently captures the variety of processes controlling the oxygen isotopic composition of speleothems. Overall, however, monitoring can identify stalagmites that are more sensitive to surface climate and less sensitive to these in-cave processes by identifying sites with dripwater variability responses to surface rainfall variability and sites that precipitate close to oxygen isotopic equilibrium. Finally, a major missing component in speleothem research is the fact that speleothems take thousands and sometimes hundreds of thousands of years to form. They are non-renewable resources on human timescales, and habitat for myriad microbes that have yet to be identified. Removal of speleothems for paleoclimate research is one of the only destructive uses of these deposits. With that in mind, I also analyze current methods of collecting speleothems and develop a framework based on two surveys of scientists and stakeholders to assist scientists and managers when evaluating potential methods of incorporating cave conservation into the speleothem sampling process. Thus, I approach caves from a variety of angles and timescales, from the past through the present to the future, illuminating caves as complex scientific and social systems.