University of the Pacific
Geological Sciences
1] Brush removal is commonly conducted to increase water availability in arid areas, such as central Texas, where water resources are stressed. The effectiveness of brush clearing to enhance recharge, however, remains uncertain as... more
1] Brush removal is commonly conducted to increase water availability in arid areas, such as central Texas, where water resources are stressed. The effectiveness of brush clearing to enhance recharge, however, remains uncertain as numerous studies have yielded contradictory results. This study assesses the effects of brush clearing on recharge to a cave at Natural Bridge Caverns, central Texas by evaluating changes in drip rate, drip water compositions, cave air CO 2 concentrations, and calcite growth in a cave underlying an area cleared of brush. Drip sites were monitored for 3 years preclearing and 2 years postclearing at five drip sites beneath and seven drip sites not beneath the surface cleared of brush. Physical and chemical drip water parameters exhibit preclearing and postclearing variability. Postclearing drip rate characteristics reflect an initial interval of anomalously heavy rainfall, then a longer dry period. Drip water 87 Sr/ 86 Sr values do not exhibit preclearing to postclearing variation at sites beneath the cleared area and indicate no change in postclearing water residence time. Significant decreases in postclearing cave air CO 2 are observed at seven of nine drip sites in the cave beneath the clearing. Decreases in cave air CO 2 influence calcite growth, which impacts postclearing drip water Mg/Ca and Sr/Ca. Decreases in cave air CO 2 immediately following brush clearing suggest that a significant portion of soil CO 2 respiration is from tree root respiration rather than from soil microbial activity. Seasonal calcite growth patterns, linked to cave air CO 2 , also exhibit variability postclearing and suggest that cave mineral deposits may record historical changes in vegetative cover.
A 4-year study in a central Texas cave quantifies multiple mechanisms that control dripwater composition and how these mechanisms vary at different drip sites. We monitored cave-air compositions, in situ calcite growth, dripwater... more
A 4-year study in a central Texas cave quantifies multiple mechanisms that control dripwater composition and how these mechanisms vary at different drip sites. We monitored cave-air compositions, in situ calcite growth, dripwater composition and drip rate every 4-6 weeks. Three groups of drip sites are delineated (Groups 1-3) based on geochemical variations in dripwater composition. Quantitative modeling of mineral-solution reactions within the host carbonate rock and cave environments is used to identify mechanisms that can account for variations in dripwater compositions. The covariation of Mg/ Ca (and Sr/Ca) and Sr isotopes is key in delineating whether Mg/Ca and Sr/Ca variations are dictated by water-rock interaction (i.e., calcite or dolomite recrystallization) or prior calcite precipitation (PCP). Group 1 dripwater compositions reflects a narrow range of the extent of water-rock interaction followed by varying amounts of prior calcite precipitation (PCP). Group 2 dripwater compositions are controlled by varying amounts of water-rock interaction with little to no PCP influence. Group 3 dripwater compositions are dictated by variable extents of both water-rock interaction and PCP. Group 1 drip sites show seasonal variations in dripwater Mg/Ca and Sr/Ca, whereas the other drip sites do not. In contrast to the findings of most previous dripwater Mg/Ca-Sr/Ca studies, these seasonal variations (at Group 1 drip sites) are independent of changes in water flux (i.e., rainfall and/or drip rate), and instead significantly correlate with changes in cave-air CO 2 concentrations. These results are consistent with lower cave-air CO 2 , related to cool season ventilation of the cave atmosphere, enhancing calcite precipitation and leading to dripwater geochemical evolution via PCP. Group 1 dripwater Mg/Ca and Sr/Ca seasonality and evidence for PCP as a mechanism that can account for that seasonality, have two implications for many other regions where seasonal ventilation of caves is likely: (1) speleothem trace-element records may provide seasonal signals, and (2) such records may be biased toward recording climate conditions during the season when calcite is depositing. Additionally, we use our results to construct a forward model that illustrates the types of speleothem Mg/Ca and Sr/Ca variations that would result from varying controls on dripwater compositions. The model provides a basis for interpreting paleodripwater controls from high frequency Mg/Ca and Sr/Ca variations for speleothems from caves at which long term monitoring studies are not feasible.
- by Corinne I Wong and +2
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- Cave and Karst Studies
Karst Drought Telogenetic Edwards aquifer Groundwater Texas s u m m a r y Understanding the sources and processes that control groundwater compositions and the timing and magnitude of groundwater vulnerability to potential surface-water... more
Karst Drought Telogenetic Edwards aquifer Groundwater Texas s u m m a r y Understanding the sources and processes that control groundwater compositions and the timing and magnitude of groundwater vulnerability to potential surface-water contamination under varying meteorologic conditions is critical to informing groundwater protection policies and practices. This is especially true in karst terrains, where infiltrating surface water can rapidly affect groundwater quality. We analyzed the evolution of groundwater compositions (major ions and Sr isotopes) during the transition from extreme drought to wet conditions, and used inverse geochemical modeling (PHREEQC) to constrain controls on groundwater compositions during this evolution. Spring water and groundwater from two wells dominantly receiving diffuse and conduit flow (termed diffuse site and conduit site, respectively) in the Barton Springs segment of the Edwards aquifer (central Texas, USA) and surface water from losing streams that recharge the aquifer were sampled every 3-4 weeks during November 2008-March 2010. During this period, water compositions at the spring and conduit sites changed rapidly but there was no change at the diffuse site, illustrating the dual nature (i.e., diffuse vs. conduit) of flow in this karst system. Geochemical modeling demonstrated that, within a month of the onset of wet conditions, the majority of spring water and groundwater at the conduit site was composed of surface water, providing quantitative information on the timing and magnitude of the vulnerability of groundwater to potential surface-water contamination. The temporal pattern of increasing spring discharge and changing pattern of covariation between spring discharge and surface-water (steam) recharge indicates that that there were two modes of aquifer response-one with a small amount of storage and a second that accommodates more storage.
Understanding the nature of communication between aquifers can be challenging when using traditional physical and geochemical groundwater sampling approaches. This study uses two multiport wells completed within Edwards and Trinity... more
Understanding the nature of communication between aquifers can be challenging when using traditional physical and geochemical groundwater sampling approaches. This study uses two multiport wells completed within Edwards and Trinity aquifers in central Texas to determine the degree of groundwater inter-flow between adjacent aquifers. Potentiometric surfaces, hydraulic conductivities, and groundwater major ion concentrations and Sr isotope values were measured from multiple zones within three hydrostratigraphic units (Edwards and Upper and Middle Trinity aquifers). Physical and geochemical data from the multiport wells were combined with historical measurements of groundwater levels and geochemical compositions from the region to characterize groundwater flow and identify controls on the geochemical compositions of the Edwards and Trinity aquifers. Our results suggest that vertical groundwater flow between Edwards and Middle Trinity aquifers is likely limited by low permeability, evaporite-rich units within the Upper and Middle Trinity. Potentiometric surface levels in both aquifers vary with changes in wet vs. dry conditions, indicating that recharge to both aquifers occurs through distinct recharge areas. Geochemical compositions in the Edwards, Upper, and Middle Trinity aquifers are distinct and likely reflect groundwater interaction with different lithologies (e.g., carbonates, evaporites, and siliceous sediments) as opposed to mixing of groundwater between the aquifers. These results have implications for the management of these aquifers as they indicate that, under current conditions, pumping of either aquifer will likely not induce vertical cross-formational flow between the aquifers. Inter-flow between the Trinity and the Edwards aquifers, however, should be reevaluated as pumping patterns and hydrogeologic conditions change.
Delineating the climate processes governing precipitation variability in drought-prone Texas is critical for predicting and mitigating climate change effects, and requires the reconstruction of past climate beyond the instrumental record.... more
Delineating the climate processes governing precipitation variability in drought-prone Texas is critical for predicting and mitigating climate change effects, and requires the reconstruction of past climate beyond the instrumental record. We synthesize existing paleoclimate proxy data and climate simulations to provide an overview of climate variability in Texas during the Holocene. Conditions became progressively warmer and drier transitioning from the early to mid Holocene, culminating between 7 and 3 ka (thousand years ago), and were more variable during the late Holocene. The timing and relative magnitude of Holocene climate variability, however, is poorly constrained owing to considerable variability among the different records. To help address this, we present a new speleothem (NBJ) reconstruction from a central Texas cave that comprises the highest resolution proxy record to date, spanning the mid to late Holocene. NBJ trace-element concentrations indicate variable moisture conditions with no clear temporal trend. There is a decoupling between NBJ growth rate, trace-element concentrations, and d18O values, which indicate that (i) the often direct relation between speleothem growth rate and moisture availability is likely complicated by changes in the overlying ecosystem that affect subsurface CO2 production, and (ii) speleothem d18O variations likely reflect changes in moisture source (i.e., proportion of Pacific-vs. Gulf of Mexico-derived moisture) that appear not to be linked to moisture amount.
- by Corinne I Wong
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Speleothems have become a cornerstone of the approach to better understanding Earth's climatic tel- econnections due to their precise absolute chronologies, their continuous or semicontinuous deposition and their global terrestrial... more
Speleothems have become a cornerstone of the approach to better understanding Earth's climatic tel- econnections due to their precise absolute chronologies, their continuous or semicontinuous deposition and their global terrestrial distribution. We review the last decade of speleothem-related research, building off a similar review by McDermott (2004), in three themes e i) investigation of global tele- connections using speleothem-based climate reconstructions, ii) refinement of climate interpretations from speleothem proxies through cave monitoring, and iii) novel, technical methods of speleothem- based climate reconstructions. Speleothem records have enabled critical insight into the response of global hydroclimate to large climate changes. This includes the relevant forcings and sequence of climatic responses involved in glacial terminations and recognition of a global monsoon response to climate changes on orbital and millennial time scales. We review advancements in understanding of the pro-
cesses that control speleothem d13C values and introduce the idea of a direct atmospheric pCO2 influence. We discuss progress in understanding kinetic isotope fractionation, which, with further advances, may help quantify paleoclimate changes despite non-equilibrium formation of speleothems. This feeds into the potential of proxy system modeling to consider climatic, hydrological and biogeochemical processes with the objective of quantitatively interpreting speleothem proxies. Finally, we provide an overview of emerging speleothem proxies and novel approaches using existing proxies. Most recently, technical advancements made in the measurement of fluid inclusions are now yielding reliable determinations of paleotemperatures.
cesses that control speleothem d13C values and introduce the idea of a direct atmospheric pCO2 influence. We discuss progress in understanding kinetic isotope fractionation, which, with further advances, may help quantify paleoclimate changes despite non-equilibrium formation of speleothems. This feeds into the potential of proxy system modeling to consider climatic, hydrological and biogeochemical processes with the objective of quantitatively interpreting speleothem proxies. Finally, we provide an overview of emerging speleothem proxies and novel approaches using existing proxies. Most recently, technical advancements made in the measurement of fluid inclusions are now yielding reliable determinations of paleotemperatures.
We investigate climate dynamics and teleconnections governing moisture transport to the western U.S. during past warm and cool intervals of the last deglaciation using paleoclimate simulations of the Bølling warm (~14 ka) and Younger... more
We investigate climate dynamics and teleconnections governing moisture transport to the western U.S. during past warm and cool intervals of the last deglaciation using paleoclimate simulations of the Bølling warm (~14 ka) and Younger Dryas cool (~12 ka) events. Results suggest that the waning continental ice sheet weakened atmospheric pressure centers in the region leading to a progression from a more sinuous to more zonal Pacific winter storm track throughout the deglaciation. Furthermore, variations in meltwater flux to the Atlantic influenced the meridional temperature gradient over the Pacific and thereby modulated storm track intensity. Changing sinuosity of the storm track may be reflected in broad increases in modeled δ 18 O precip and observed δ 18 O speleothem values from the western U.S. over the last deglaciation, whereas abrupt δ 18 O speleothem shifts are dynamically consistent with the response of storm track intensity to variations in meltwater flux to the Atlantic.