H34A-01 INVITED
Quantitative Modeling of Processes
The same modeling approaches that work for igneous petrogenesis work for sewage.
H34A-02
Validation of Isotope Data in the World of Interdisciplinary Science: A Cautionary Tale
Isotope geoscientists, as well as their colleagues in the geologic, hydrologic and biologic science communities, generally have great confidence in both the precision and accuracy of their isotopic data. Moreover, isotope geoscientists are increasingly realizing the potential of their isotope tracing techniques to be used in applied studies in other research disciplines, such as clinical medicine and forensic science. However, researchers in these other disciplines may not have the same confidence in the quality of isotope data as applied to their field of study, due to either lack of familiarity with the techniques or cultural differences in research approach. For example, in a recent attempt to publish a manuscript in which we described the use of Ca stable isotopes in urine to assess the effects of bedrest on bone loss in human subjects, we were confronted by a reviewer who challenged the validity of our isotope data as measured in the urine matrix. In response to this reviewer's concerns, we developed a supplemental study to demonstrate that we were indeed obtaining `the right answer` on the urine samples with the double-spiking, thermal ionization mass spectrometry technique used for the study. First, using the two urine samples from the dataset that had the lightest and heaviest Ca isotope compositions observed in the study (i.e., δ44Ca of -1.72‰ and +0.36‰ relative to seawater), we generated a series of mixtures of the lighter sample with seawater (δ44Ca = 0‰) and the heavier sample with NIST SRM 915a (δ44Ca = -2.01‰). In each case, the amount of standard added to each sample was minute, thus maintaining the urine matrix. Measured Ca isotope compositions agreed with those calculated assuming a simple mixing relationship, confirming the lack of a matrix effect on the accuracy of the data. Second, in order to test the robustness of the double spiking approach, we analyzed the two urine samples under optimally-spiked, under-spiked and over-spiked conditions and found no significant difference in Ca isotope ratios reported by the spike subtraction algorithm. Third, during processing of the two urine samples that had been optimally spiked, we collected Ca off the cation exchange resin purification columns in two fractions, the first 50% eluted being heavy and the second 50% eluted being light due to isotopic fractionation along the pathway through the resin matrix. In each case, the measured Ca isotope compositions of each fraction were identical, implying that the urine matrix did not impact the separation procedure. In the end, these results assuaged the reviewer`s concerns, and this experience demonstrates the challenges isotope geoscientists face as they move further into the realm of applied interdisciplinary science.
H34A-03
Major Ion Chemistry and Mixing Proportions of Nitrate Sources in Urban Groundwater
Working with Dr. Gilbert Hanson has allowed me to apply general mixing equations to identification of nonpoint sources of groundwater contamination. These methods have not commonly been used in hydrologic studies, as they involve a more classical petrologic approach, one which Dr. Hanson has pioneered. Our drinking water supplies are becoming more susceptible to contamination and knowing the chemistry of contaminate sources will yield precise determination of potential sources to groundwater and allow government agencies to adopt policies to reduce or prevent contamination. The geochemistry of soil water from below fertilized turfgrass sites and of sewage from septic tank/cesspools was used to place constraints on the sources of nitrate in groundwater of an unconsolidated aquifer in Suffolk County, Long Island, New York, USA. Twenty four sewage samples were acquired from Suffolk County Public Works. Soil water samples, from suction lysimeters, were acquired monthly during 2003, totaling 70 samples. We found that soil water concentrations were elevated in Ca, Mg and SO4 relative to sewage and sewage had higher concentrations of Cl, N-NO3, PO4, Na and K. This difference in the major ion chemistry allows identification of the source signatures in groundwater. We then compared the source signatures to 28 groundwater wells on binary ion diagrams of SO4, Cl and N- NO3 and created a cation sorption model for Na, Ca, Mg and K, in order to model cation concentrations on binary ion diagrams. These diagrams allow estimates of the relative contributions of each source to each well. Groundwater wells plotted according to their major land use and show that wells of similar land use have similar geochemistry and similar source contributions. The estimates of source contributions show that the proportions of soil water and sewage increase as residential land use increases. Although volumetric source proportions to groundwater wells are similar for soil water and sewage within a given land use, sewage contributes a greater proportion to the nitrate concentration in groundwater wells. For example, sewage contributes between 86-100% of the nitrate in wells sourced in medium density residential land use, even when accounting for a 50% reduction in nitrate concentrations from the septic tank/cesspool system. Our results indicate that to decrease the nitrate concentrations in groundwater one must reduce the load from septic tank/cesspool systems.
H34A-04 INVITED
Overview and Brief History of the Boron Isotope Proxy for Past Seawater pH
In 1992 Hemming and Hanson (GCA, vol. 56, p. 537-543) showed that a variety of modern marine carbonates revealed a boron isotopic composition close to the isotopic composition of dissolved borate at modern seawater pH, suggesting this was the boron species preferentially adsorbed and incorporated into marine carbonates. With a constant offset between the trigonal and tetrahedrally coordinated boron species and a pH-dependent variation in their fractions, it appeared that this system would be sensitive to pH changes in the natural range of seawater. Accordingly, it was suggested that the boron isotope composition of marine carbonates is a proxy for past seawater pH. Subsequent culture studies with living planktic foraminifers and corals, as well as synthetic precipitation experiments confirmed that the boron isotopic composition follows the isotopic composition of borate across a wide range of seawater pH. In order to use the proxy with confidence, however, all other controls apart from pH need to be thoroughly understood. Recent laboratory and sediment experiments have demonstrated that vital effects and partial shell dissolution have the potential to modify the primary seawater pH signal recorded in the boron isotopic composition of planktic foraminifers. However it has also been shown that careful sample selection allows for avoiding these potential complications. A record of reconstructed surface seawater pH and estimated aqueous PCO2 shows a remarkable match between boron isotope based atmospheric pCO2 estimates and the Vostok ice core CO2 record. This convincingly demonstrates that boron isotopes in planktic foraminifers allow quantitative estimates of atmospheric pCO2 in the past, and confirms that glacial surface ocean pH was ~0.2 units higher compared to interglacial periods. We are going to review and discuss the achievements generated in Gil Hanson's lab over the past 15 years in the light of recent empirical measurements of the boron isotope fractionation between boric acid and borate in seawater.
H34A-05 INVITED
Where is the U in Calcite?
In order to better understand the potential for U-Pb dating of calcite, we have been working to characterize the distribution of U in calcite. We use phosphor imaging to compare macroscopic calcite structures with U distribution in hand specimens, fission track maps to compare microscopic structures of calcite to U distribution, and synchrotron micro-XRF analyses to map the distribution of U with respect to other elements, and X-ray Adsorption Near Edge Structure (XANES) analyses to determine the oxidation state(s) of U. Based on our work and published work of others on U in calcite we recognize (at least) three independent avenues for U incorporation in calcite, all of which have yielded reliable U-Pb ages based on comparison to more traditional dating tools: 1) reduced U that substitutes for Ca in the crystal lattice; 2) oxidized U associated with organic matter that substitutes for Ca in the crystal lattice; 3) oxidized U in calcite that has been neomorphosed from an aragonite precursor and substitutes into the crystal lattice. While it may be suggested that the presence of U in the crystal lattice ‘proves the U-Pb dating technique,' we suggest that instead by understanding U behavior in various depositional contexts, we can infer something about the fluids involved in calcite precipitation. Importantly, we do not know of an example of calcite with mixed U oxidation states; all published examples are either fully reduced or oxidized. Thus, the kinetics of mineral formation as an explanation of U oxidation state or alteration after formation do not appear to be viable explanations for the U oxidation state in calcite. Instead it appears that a reduced U species may be soluble in some hydrothermal fluids (perhaps CO2 rich brines), and that oxidized U can be removed from solution by coprecipitation in calcite (and possibly by complexation to coprecipitated organic functional groups in the calcite) in the oxidized state rather than being removed from solution by reduction. Although these tenets are partly contrary to paradigms of U behavior they are the most direct explanations for our observations.
H34A-06
40Ar/39Ar Dating of the Middle Miocene Skyline Tuff, Barstow Formation, CA
The Miocene Barstow Formation, located in the Mojave region of southern California, is famous for its abundant mammalian fossil record. It also contains a series of volcanic ash horizons intercalated within the sedimentary sequence. MacFadden et al. (1990, GSAB, v.102) and Woodburne et al. (1990, GSAB, v.102) used K-Ar and 40Ar/39Ar dating of these ashes to constrain the age of the stratigraphy, magnetostratigraphy and biostratigraphy of the type Barstow Fm. and to calibrate the early Barstovian North American Land Mammal Age (NALMA). One of the most recognizable layers of the Barstow sequence is the Skyline Tuff. The resistant Skyline Tuff stands in relief and forms the top of many of the cliffs in the Barstow Syncline, particularly in Rainbow Basin. Stratigraphically, it forms the base of the upper member of the Barstow Fm., and is situated between the "Dated" and Hemicyon Tuffs. Published geochronological work on this important marker bed is limited to K-Ar ages of (diagenetic?) K-feldspar, illite/smectite, and clinoptilolite cuts. Unlike many of the ashes in the Barstow Fm., including the "Dated" and Hemicyon tuffs, the Skyline Tuff lacks biotite. We have sampled the Skyline Tuff from three locations, two in the Rainbow Basin, and one in the Owl Canyon. Single sanidine crystals were co-irradiated along with an inter-lab monitor standard cross calibration. We anticipate that by using the ages of these samples, we can directly compare the Skyline Tuff age with the biotite derived ages on the "Dated" and Hemicyon Tuffs. Better age control on the Skyline Tuff may therefore have important implications for calibration of the Barstow magnetostratigraphy, biochronology of the early to middle part of the Barstovian NALMA, and will improve correlation with endemic Great Plains faunas of middle to late Barstovian age.