Speleothem trace element signatures: A hydrologic geochemical study of modern cave dripwaters and farmed calcite

Trace element variations in ancient cave speleothems are often interpreted as indicators of changes in paleo-rainfall and hydrologic conditions. However, these records are difficult to interpret without an understanding of the physicochemical controls on stalagmite chemistry plus site-specific calibration of changes in net rainfall to variations in dripwater and speleothem chemistry. In this study we examine geochemical relationships between net rainfall (Precipitation minus Evapotranspiration; P−ET), drip rates, drip water chemistry, and contemporaneous calcite chemistry to test the hypothesis that speleothem Mg/Ca and Sr/Ca records are proxies for rainfall amount. HRC is contained within four low-magnesium limestone units capped sporadically by a remnant dolomitic limestone. Aqueous concentrations of magnesium (post evapotranspiration) decrease with increasing vertical travel distance between the soil zone and the point of in-cave drip emergence (Drip Path Length – DPL) as dissolved high-Mg solutions sourced from the dolomitic caprock are diluted with dissolved low-Mg limestone waters sourced from the host limestone. Dripwater Mg/Ca and Sr/Ca ratios covary and provide diagnostic indicators of the two dominant mechanisms controlling dripwater chemistry: (1) mixing of post-evaporative solutions derived from two geochemical endmembers (dissolution of dolomite and limestone); and (2) evolution of hydrochemistry away from dissolved bedrock compositions due to Prior Calcite Precipitation (PCP) above the drip sites. By resolving the linear mixing relationships for drip water Mg/Ca and Sr/Ca sources and the distribution coefficients for trace element transfer in the PCP dripwater-to-calcite precipitation reactions and applying these principles to our time series, we find that the extent of PCP production within the karst is directly controlled by the balance between Precipitation (P) and Evapotranspiration (ET): higher net rainfall (P−ET \u3e 1: wet conditions) reduces PCP, and lower net rainfall with increased evapotran

Speleothem calcite farmed in situ: Modern calibration of δ18O and δ13C paleoclimate proxies in a continuously-monitored natural cave system

Understanding the relationships between speleothem stable isotopes (δ13C δ18O) and in situ cave forcing mechanisms is important to interpreting ancient stalagmite paleoclimate records. Cave studies have demonstrated that the δ18O of inorganically precipitated (low temperature) speleothem calcite is systematically heavier than the δ18O of laboratory-grown calcite for a given temperature. To understand this apparent offset, rainwater, cave drip water, groundwater, and modern naturally precipitated calcite (farmed in situ) were grown at multiple locations inside Hollow Ridge Cave in Marianna, Florida. High resolution micrometeorological, air chemistry time series and ventilation regimes were also monitored continuously at two locations inside the cave, supplemented with periodic bi-monthly air gas grab sample transects throughout the cave. Cave air chemistry and isotope monitoring reveal density-driven airflow pathways through Hollow Ridge Cave at velocities of up to 1.2 m s−1 in winter and 0.4 m s−1 in summer. Hollow Ridge Cave displays a strong ventilation gradient in the front of the cave near the entrances, resulting in cave air that is a mixture of soil gas and atmospheric CO2. A clear relationship is found between calcite δ13C and cave air ventilation rates estimated by proxies pCO2 and 222Rn. Calcite δ13C decreased linearly with distance from the front entrance to the interior of the cave during all seasons, with a maximum entrance-to-interior gradient of Δδ13CCaCO3 = −7‰. A whole-cave “Hendy test” at multiple contemporaneous farming sites reveals that ventilation induces a +1.9 ± 0.96‰ δ13C offset between calcite precipitated in a ventilation flow path and calcite precipitated on the edge or out of flow paths. This interpretation of the “Hendy test” has implications for interpreting δ13C records in ancient speleothems. Calcite δ13CCaCO3 may be a proxy not for atmospheric CO2 or overlying vegetation shifts but also for changes in cave ventilation due to dissolution fissures and ceiling collapse creating an

Predictors of Variations in Residential Water Consumption in Central Texas

Background: The 100th Meridian in Texas aligns with a corridor of large and rapidly growing urban areas with a growing water demand and limited supply. Understanding the variations in residential water consumption may assist with identifying the characteristics associated with disproportionate water consumption that may be responsive to policy changes and enforcement. Methods: Data from the San Antonio Water System, the Bexar County Appraisal District, and the American Community Survey were utilized. The average daily water consumption was estimated for the seasons and a total year for more than 300,000 single-family residences between 2009 and 2015. The presence of a swimming pool, residential parcel hectares, size of the living space, and per capita income were examined as predictors of the variations in residential water consumption using hierarchal modeling. Results: The presence of swimming pools and a residential property’s value were the strongest predictors of water consumption. Parcel hectares and household income were positively associated with water consumption. A quartile analysis of select independent variables identified the disproportionate variations of water consumption of units with large yards, swimming pools, and high values. Conclusions: The findings indicate a strong association between variations in residential water consumption and both irrigation and swimming pool water used, which emphasize a need to focus conservation efforts on higher-valued housing and residences with swimming pools and the consideration of tiered pricing

Positive unintended consequences of urbanization for climate-resilience of stream ecosystems

Abstract Developing sustainable urban systems is a fundamental societal challenge for the 21st century, and central Texas faces particularly synergistic challenges of a rapidly growing urban population and a projected increasingly drought-prone climate. To assess the history of urbanization impacts on watersheds here, we analyzed 51 cores from bald cypress trees in paired urban and rural watersheds in Austin, Texas. We find a significant contrast between rural and urbanized watersheds. In the rural watershed, tree-ring-width growth histories (“chronologies”) from 1844–2018 significantly and positively correlate (p < 0.01) with (1) one another, and (2) regional instrumental and proxy records of drought. In the urbanized watershed, by contrast, chronologies weakly correlate with one another, with instrumental records of drought, and with the rural chronologies and regional records. Relatively weak drought limitations to urban tree growth are consistent with the significant present-day transfer of municipal water from urban infrastructure by leakage and irrigation to the natural hydrologic system. We infer a significant, long-term contribution from infrastructure to baseflow in urbanized watersheds. In contrast to the common negative impacts of ‘urban stream syndrome’, such sustained baseflow in watersheds with impaired or failing infrastructure may be an unintended positive consequence for stream ecosystems, as a mitigation against projected extended 21st-century droughts. Additionally, riparian trees may serve as a proxy for past impacts of urbanization on natural streams, which may inform sustainable urban development

Assessment of Texas water resources in the context of changing climate: toward alignment of research agendas and capabilities with stakeholder needs.

Long-range water planning is complicated by changes in climate, population, and water use. In Texas, the current approach is to maintain water supplies sufficient to provide adequate water through a repeat of the driest episode in instrumented history. However, the top-level state water plan does not take into consideration potential declines in surface water supply as a function of a drying climate and associated extreme weather events. In that context we review some of the climate factors that may have a large impact on water management. To illustrate how modeled predictions might serve to increase Texas water resiliency, we align these parameters the needs of a prototypical large surface water supplier. We examine the stakeholder perspective on different kinds of climate data, including actionable, incompatible, and unavailable information. Finally, we provide an example of a recent study that attempts to translate climate projections into actionable management information. While it is clear that stakeholders value the predictive capability contained in climate model outputs, we find that currently available data are generally insufficient for supporting true resilience across numerous economic sectors. Indeed, this requires a new suite of tools that provide both short and long-term, stakeholder-specific adaptive planning capacity.Office of the VP for Researc

Unprecedented Drought Challenges for Texas Water Resources in a Changing Climate: What Do Researchers and Stakeholders Need to Know?

Long‐range water planning is complicated by factors that are rapidly changing in the 21st century, including climate, population, and water use. Here, we analyze climate factors and drought projections for Texas as an example of a diverse society straddling an aridity gradient to examine how the projections can best serve water stakeholder needs. We find that climate models are robust in projecting drying of summer‐season soil moisture and decreasing reservoir supplies for both the eastern and western portions of Texas during the 21st century. Further, projections indicate drier conditions during the latter half of the 21st century than even the most arid centuries of the last 1,000 years that included megadroughts. To illustrate how accounting for drought nonstationarity may increase water resiliency, we consider generalized case studies involving four key stakeholder groups: agricultural producers, large surface water suppliers, small groundwater management districts, and regional water planning districts. We also examine an example of customized climate information being used as input to long‐range water planning. We find that while stakeholders value the quantitative capability of climate model outputs, more specific climate‐related information better supports resilience planning across multiple stakeholder groups. New suites of tools could provide necessary capacity for both short‐ and long‐term, stakeholder‐specific adaptive planning.Nielsen‐Gammon, Banner, Wong, and Tremaine were supported in part by the National Science Foundation Coupled Natural and Human Systems program, Grant AGS‐1518541, the Cynthia and George Mitchell Foundation Grant G‐1809‐55892, and by The University of Texas at Austin's Planet Texas 2050 Bridging Barriers research initiative. We thank Katharine Hayhoe, Ron Anderson, and Tim Finley for their insights and contributions to the manuscript. We thank the participants of the CNH project and the Texas Water Research Network for helping to define the problem and identify issues.Office of the VP for Researc