Dennis L. Newell

Effect of dissolved CO2 on a shallow groundwater system: a controlled release field experiment.

Capturing carbon dioxide (CO(2)) emissions from industrial sources and injecting the emissions deep underground in geologic formations is one method being considered to control CO(2) concentrations in the atmosphere. Sequestering CO(2) underground has its own set of environmental risks, including the potential migration of CO(2) out of the storage reservoir and resulting acidification and release of trace constituents in shallow groundwater. A field study involving the controlled release of groundwater containing dissolved CO(2) was initiated to investigate potential groundwater impacts. Dissolution of CO(2) in the groundwater resulted in a sustained and easily detected decrease of ~3 pH units. Several trace constituents, including As and Pb, remained below their respective detections limits and/or at background levels. Other constituents (Ba, Ca, Cr, Sr, Mg, Mn, and Fe) displayed a pulse response, consisting of an initial increase in concentration followed by either a return to background levels or slightly greater than background. This suggests a fast-release mechanism (desorption, exchange, and/or fast dissolution of small finite amounts of metals) concomitant in some cases with a slower release potentially involving different solid phases or mechanisms. Inorganic constituents regulated by the U.S. Environmental Protection Agency remained below their respective maximum contaminant levels throughout the experiment.

Dissected hydrologic system at the Grand Canyon: Interaction between deeply derived fluids and plateau aquifer waters in modern springs and travertine

Geochemical study of water and gas discharging from the deeply incised aquifer system at the Grand Canyon, Arizona, provides a paradigm for understanding complex groundwater mixing phenomena, and Quaternary travertines deposited from cool springs provide a paleohydrologic record of this mixing. Geochemical data show that springs have marked compositional variability: those associated with active travertine accumulations (deeply derived endogenic waters) are more saline, richer in CO2, and elevated in 87Sr/86Sr relative to springs derived dominantly from surface recharge of plateau aquifers (epigenic waters). Endogenic waters and associated travertine are preferentially located along basement-penetrating faults. We propose a model whereby deeply derived fluids are conveyed upward via both magmatism and seismicity. Our model is supported by: (1) gas analyses from spring waters with high He/Ar and He/N2 and 3He/4He ratios indicating the presence of mantle-derived He; (2) large volumes of travertine and CO2-rich gases in springs recording high CO2 fluxes; and (3) 87Sr/86Sr in these springs that indicate circulation of waters through Precambrian basement. Geochemical trends are explained by mixing of epigenic waters of the Colorado Plateau aquifers with different endogenic end-member waters in different tectonic subprovinces. Endogenic waters are volumetrically minor but have significant effects on water chemistry. They are an important and largely unrecognized component of the hydrogeochemistry and neotectonics of the southwestern United States.

Degassing of mantle-derived CO2 and He from springs in the southern Colorado Plateau region - Neotectonic connections and implications for groundwater systems

Groundwaters of the southern Colorado Plateau–Arizona Transition Zone region are a heterogeneous mixture of chemically diverse waters including meteoric (epigenic) fluids, karst-aquifer waters, and deeply sourced (endogenic) fluids. We investigate the composition of travertine-depositing CO 2 -rich springs to determine the origin, transport, and mixing of these various components. The San Francisco Mountain recharge area has little surface flow. Instead, waters discharge through major springs hundreds of kilometers away. About 70% (9340 L/s) of the total recharge (13,500 L/s) discharges 100 km to the north in the incised aquifer system at Grand Canyon. Most of this water (85%; 8070 L/s) emerges through two travertine-depositing karst spring systems: Blue Springs (6230 L/s) and Havasu Springs (1840 L/s). About 30% of recharge (4150 L/s) flows to the south and discharges along NW-striking faults in the Arizona Transition Zone, forming the base flow for the Verde River. Geochemical data define regional mixing trends between meteoric recharge and different endogenic end members that range from bicarbonate waters to sulfate waters. Water quality in the region is dictated by the percentage and character of the endogenic inputs that cause a measurable degradation of groundwater quality for water supply. Sources for the high CO 2 include dissolution of limestone and dolostone (C carb ) and “external carbon” (C external ). C external is computed as the bicarbonate alkalinity (dissolved inorganic carbon [DIC]) minus the C carb (C external = DIC - C carb ). C external is deconvolved using carbon isotopes into biogenically derived sedimentary carbon (C organic ) and deep CO 2 inputs (C endogenic ). Measured δ 13 C values are −17‰ to +3‰ versus Pee Dee Belemnite (PDB). Assuming δ 13 C carb = +2‰, δ 13 C organic = −28‰, and δ 13 C endogenic = −5‰, water chemistry mixing models indicate that an average of 42% of the total DIC comes from dissolution of carbonate rocks, 25% from organic carbon, including soil-respired CO 2 ,and 33% from deep (endogenic) sources. Helium isotope values ( 3 He/ 4 He) in gases dissolved in spring waters in the southern Colorado Plateau region range from 0.10 to 1.16 R A (relative to air) indicating that a significant component of the deeply derived fluid is from the mantle (mean of 5% asthenospheric or 10% subcontinental lithospheric mantle source). Measured CO 2 / 3 He ratios of 2 × 10 9 to 1.4 × 10 13 are adjusted by removing the proportion of CO 2 from C carb and C organic to give values 10 for all but four samples. Various mixing models using CO 2 / 3 He suggest that the mantle-derived components of the CO 2 load are highly variable from spring to spring and may make up an average of ~10% of the total CO 2 load of the regional springs. Fluid-rock interactions involving endogenic fluids are suggested by 87 Sr/ 86 Sr, δ 18 O, and other tracers. The endogenic CO 2 component, multiplied by discharge for each spring, yields an integrated annual flux of deeply derived CO 2 to the groundwater system of ~1.4 × 10 9 mol/yr. This CO 2 emission from the Colorado Plateau region reflects a complex tectonic evolution involving Laramide hydration of the lithosphere above the Farallon slab, addition of fluids from mid-Tertiary mantle tectonism during slab removal, and ongoing fluid movement induced by neotectonic small-scale asthenospheric convection.

Orogen-parallel extension and exhumation enhanced by denudation in the trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal

Focused denudation and mid-crustal flow are coupled in many active tectonic settings, including the Himalaya, where exhumation of mid-crustal rocks accommodated by thrust faults and low-angle detachment systems during crustal shortening is well documented. New structural and (U-Th)/He apatite data from the Mount Everest region demonstrate that the trans-Himalayan Ama Drime Massif has been exhumed at a minimum rate of ~1 mm/yr between 1.5 and 3.0 Ma during orogen-parallel extension. The Ama Drime Massif offsets the South Tibetan detachment system, and therefore the South Tibetan detachment system is no longer capable of accommodating south-directed mid-crustal flow or coupling it with focused denudation. Previous investigations interpreted the NNE-SSW–striking shear zone on the west side of the Ama Drime Massif as the Main Central thrust zone; however, our data show that the Ama Drime Massif is bounded on either side by 100–300-m-thick normal-sense shear zone and detachment systems that are kinematically linked to young brittle faults that offset Quaternary deposits and record active orogen-parallel extension. When combined with existing data, these results suggest that the Ama Drime Massif was exhumed during orogen-parallel extension that was enhanced by, or potentially coupled with, denudation in the trans-Himalayan Arun River gorge. This model provides important insights into the mechanisms that exhumed trans-Himalayan antiformal structures during orogen-parallel extension along the southern margin of the Tibetan Plateau.

Continental-scale links between the mantle and groundwater systems of the western United States: Evidence from travertine springs and regional He isotope data

to understand regional mantle degassing, we compiled new and existing helium isotope data measured in hot springs, gas fields, and travertine-depositing cool springs and compared these geochemical data with mantle velocity structure determined from tomographic studies. these data suggest heterogeneous mantle degassing, with regions of highest He/He in groundwaters (hence, highest mantle helium contribution) corresponding to regions of lowest mantle velocity, a reflection of tectonically active and partially molten mantle. new He isotope and water chemistry data from travertinedepositing cool springs of the western United States show marked variability consistent with mixing between surface water recharge and inputs from deep crustal and mantle sources. the deeply sourced end-member fluids of these mixing trends have high He/He, high dissolved cO 2 , and high salinity compared to shallow recharge waters, and commonly have elevated trace element concentrations. consequently, these fluids cause degradation of water quality in western U.S. aquifers. Our conclusions highlight a connection between neotectonics (e.g., mantle degassing) and water quality in the western United States. INTRODUCTION Distributed deformation associated with the western north American plate margin extends >1000 km inboard from the San Andreas fault zone to the rocky Mountain and western Great Plains regions. this region forms an orogenic plateau with high average heat flow and is characterized by relatively low upper mantle P-wave velocities with marked heterogeneity (Godey et al., 2003; Humphreys et al., 2003). Progressive geochemical depletion of the upper mantle during generation of basaltic GSA Today: v. 15, no. 12, doi: 10.1130/1052-5173(2005)015 2.0.cO;2 melt likely occurred in several episodes since the Proterozoic (Karlstrom et al., 2005). the mantle was hydrated by flat-slab subduction during the laramide orogeny (Humphreys et al., 2003) and now is partially molten, leading to small-scale convective exchange between an upwelling asthenosphere (Gao et al., 2004) and compositionally variable lithosphere (Dueker et al., 2001; Karlstrom et al., 2005). the mantle underlying western north America is marked by one of the largest known shear wave velocity contrasts on earth (van der lee and nolet, 1997). At the continental scale, this transition reflects the heterogeneous thinning and warming of north America’s lithospheric keel as the plate moved southwest in absolute plate motion in the cenozoic into a wide zone of warm asthenosphere (cDrOM Working Group, 2002). We hypothesize that cO 2 -rich mineral springs and related travertine deposits in the western United States are a manifestation of this mantle tectonism, and hence the geochemistry of spring waters and gases can be used in conjunction with geophysical data sets to understand mantle heterogeneity and the processes of lithosphere-asthenosphere interaction. We report new water and gas chemistry with associated carbon and helium isotope data in the context of a synthesis of the existing noble gas isotope chemistry database for western north America. Our literature synthesis (table Dr1) builds on previous work in the area, with the regional helium isotope data presented in the context of a tomographic image of today’s mantle. We also show that travertine-depositing cool springs contain mantle-derived volatiles in a variety of locations and tectonic settings throughout the western United States, such that many aquifer systems are influenced by mixing of deeply sourced and circulated waters. HE ISOTOPES—BACKGROUND the isotope geochemistry of noble gases is a sensitive tracer of mantle-derived volatiles even with a large input of volatiles derived from earth’s crust. this is because the mantle has retained a significant fraction of the terrestrial inventory of the primordial isotope He acquired during earth formation (clarke et al., 1969), and it is still leaking to earth’s surface. in contrast, the crust has been extensively reworked over geological time and has retained very little He: its helium inventory is dominated by radiogenic He produced from the decay of Uand th-series nuclides. consequently, helium presently emanating from regions of mantle melting, such as mid-oceanic ridges or helium trapped in glass and phenocrysts in mid-oceanic-ridge basalts (MOrB), is characterized by a relatively high He/He ratio (r) of 8 ± 1 times that of air (r A ), which has a He/He ratio of 1.4 × 10 (Graham, 2002). indeed, values as high as 37 × r A have been observed in some ocean island basalts (Hilton et al., 1999) and are thought to be related to deep plumes tapping less degassed mantle reservoirs. When mantle-derived fluids are injected into the crust, mantle helium becomes progressively diluted by crustal helium characterized by low He/He ratios of ~0.02 r A . therefore, any value higher than 0.1 GSA Data repository item 2005199, a description of sampling and analytical methods and geochemical data tables Dr1–Dr3, is available online at www. geosociety.org/pubs/ft2005.htm or on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, cO 80301-9140, USA, or editing@geosociety.org.

CO2/Brine transport into shallow aquifers along fault zones.

Unintended release of CO(2) from carbon sequestration reservoirs poses a well-recognized risk to groundwater quality. Research has largely focused on in situ CO(2)-induced pH depression and subsequent trace metal mobilization. In this paper we focus on a second mechanism: upward intrusion of displaced brine or brackish-water into a shallow aquifer as a result of CO(2) injection. Studies of two natural analog sites provide insights into physical and chemical mechanisms controlling both brackish water and CO(2) intrusion into shallow aquifers along fault zones. At the Chimayó, New Mexico site, shallow groundwater near the fault is enriched in CO(2) and, in some places, salinity is significantly elevated. In contrast, at the Springerville, Arizona site CO(2) is leaking upward through brine aquifers but does not appear to be increasing salinity in the shallow aquifer. Using multiphase transport simulations we show conditions under which significant CO(2) can be transported through deep brine aquifers into shallow layers. Only a subset of these conditions favor entrainment of salinity into the shallow aquifer: high aspect-ratio leakage pathways and viscous coupling between the fluid phases. Recognition of the conditions under which salinity is favored to be cotransported with CO(2) into shallow aquifers will be important in environmental risk assessments.

Travertine deposits from along the South Tibetan Fault System near Nyalam, Tibet

A newly investigated travertine deposit in southern Tibet provides a window into Holocene hydrological, geomorphic and climatic processes near the boundary of the Tibetan Plateau and High Himalaya. Travertine, deposited as a result of the degassing of CO 2 -rich groundwater as it emerges on the Earth’s surface, is in many cases formed along the trace of major crustal-scale faults in primarily extensional tectonic regimes. A travertine platform measuring roughly 1 km by 0.5 km exists near the town of Nyalam in southern Tibet along a major Himalayan down-to-the-N normal fault, the South Tibetan Fault System. A wide variety of travertine depositional textures and features are recorded in the platform on a series of terraces. Active travertine deposition was observed from spring mounds and seeps along the base of the platform at the modern river level. Palaeotemperatures of spring water, calculated from δ 18 O of the travertine, range from 9 to 25 °C, which closely matches the temperatures recorded from modern springs in the area. A complex geomorphological landscape records interaction between growing alluvial fans, travertine accumulation, and a rapidly down-cutting river with associated fluvial terraces. River incision was contemporaneous with travertine deposition, as indicated by cemented fluvial river gravel layers interbedded with travertine. High 87 Sr/ 86 Sr ratios in the travertine (mean of 0.7168) indicate subsurface fluid interaction with radiogenic crystalline rocks of the underlying Greater Himalaya. Uranium-series ages of the travertine platform range from 5400 a (± 950 a) to 11 600 a (± 1000 a), and indicate a younging progression from higher terraces near the valley wall to lower terraces at present-day river level. Travertine that overlies a river gravel terrace 18 m above river level formed at 11 600 a. This date yields a local incision rate of 1.6 mm a −1 , consistent with estimated fluvial incision rates in the High Himalaya. The range of our U-series ages coincides with an interval of higher precipitation associated with greater intensity of the Indian monsoon, which led to elevated spring discharge and carbonate precipitation in this part of the High Himalayas.

Aqueous and isotope geochemistry of mineral springs along the southern margin of the Tibetan plateau: Implications for fluid sources and regional degassing of CO2

Springs issuing from different faults and shear zones along the crest of the Himalayas tap three different levels of crust beneath the Tibetan Plateau. From structurally highest to lowest these are the Tingri Graben, the South Tibetan Detachment System (STDS), and the Ama Drime massif (ADM). The aqueous chemistry reflects water-rock interactions along faults and is consistent with mapped rock types. Major ion chemistry and calculated temperatures indicate that spring waters have circulated to greater depths along the N-S trending faults that bound the Tingri Graben and Ama Drime detachment (ADD) compared to the STDS, suggesting that these structures penetrate to greater depths. Springs have excess CO2, N2, He, and CH4 compared to meteoric water values, implying addition from crustal sources. The 3He/4He ratios range from 0.018 to 0.063 RA and are consistent with a crustal source for He. The δ13C values of dissolved inorganic carbon (DIC) and CO2 gas range from −5.5 to +3.8‰ and −13.1 to −0.3‰ versus Peedee belemnite, respectively. Sources of carbon are evaluated by calculating isotopic trajectories associated with near-surface effervescence of CO2. Positive δ13C values of the Tingri graben and STDS springs are consistent with decarbonation of marine carbonates as the source of CO2. Negative values for the ADD springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon. The δ15N values of N2 range from −2.2 to +2.1‰ (versus AIR) and are explained by mixtures of air-derived nitrogen, metamorphic devolatilization of sedimentary nitrogen, and nitrogen from near-surface biogenic processes. CO2 flux is estimated by scaling from individual springs (∼105 mol a−1 per spring) to extensional structures across the southern limit of the Tibetan Plateau and likely contributes between 108 and 1011 mol a−1 (up to 10%) to the global carbon budget.

Hot faults: Iridescent slip surfaces with metallic luster document high- temperature ancient seismicity in the Wasatch fault zone, Utah, USA

and localized iridescence in the footwall dam- age zone of the Wasatch fault (Utah, USA) and document elevated temperatures on these faults. We propose that the formation of these iridescent slip surfaces requires frictional heat generated at geometric asperities at seismic slip rates. WASATCH FAULT ZONE ABSTRACT We document new geological indicators of ancient seismicity in the form of highly reflec- tive, iridescent, hematite-coated fault surfaces. Small faults that cut the Paleoproterozoic Farmington Canyon Complex in the footwall damage zone of the Brigham City segment of the Wasatch fault (Utah, USA) are smooth to striated surfaces, tens of square centimeters to 30 m 2 in area. The dull-rusty to high-metallic luster and moderate- to high-gloss surfaces exhibit multicolored elliptical iridescent patches ~0.5-3 cm across. Preexisting hematite crystals were deformed during slip on 1-200-mm-thick slip surfaces. Textural observations, X-ray diffraction, X-ray photoelectron spectroscopy, electron backscattered diffraction analysis, surface metrology, and similarity to experimentally formed iridescent spots in rocks and metals indicate that iridescence is associated with high-temperature (>300 °C) reduction of iron (Fe 3+ to Fe 2+ ) and associated conversion of hematite to magnetite. We pro- pose that the iridescent slip surfaces in the Wasatch fault damage zone are the result of seismic slip and flash heating at asperities along the small faults. The thousands of these sur - faces represent coseismic or aftershock deformation down to magnitude -3 in the exhumed footwall damage zone of the Wasatch fault.

Quantitative ichnology of triassic crayfish burrows (Camborygma eumekenomos): Ichnofossils as linkages to population paleoecology

Analysis of 201 crayfish burrows (Camborygma eumekenomos) from three fluvial beds of the Chinle Formation (Upper Triassic, Utah, U.S.A.) provides results consistent with knowledge of Holocene crayfish ecology. Thus, many aspects of their population ecology may have remained unchanged since the early Mesozoic. A significant increase in crayfish size away from the fluvial channel reflects size (age) segregation along an environmental gradient. The high lateral (within‐bed) variation in burrow density may have been caused by spatial heterogeneity in water table and soil moisture levels. In each of the three analyzed beds, the burrows record a single ecologic generation of a monospecific crayfish population. The three beds differ in terms of the mean burrow diameter (this may reflect differences either in the average size/age of the resident crayfish or in the lateral extent of sampling). However, the overall shape of the burrow‐size distribution is similar for all three beds (unimodal and close‐to‐normal). T...