Equilibrium H/H fractionations in organic molecules: I. Experimental calibration of ab initio calculations

Carbon-bound hydrogen in sedimentary organic matter can undergo exchange over geologic timescales, altering its isotopic composition. Studies investigating the natural abundance distribution of ¹H and ²H in such molecules must account for this exchange, which in turn requires quantitative knowledge regarding the endpoint of exchange, i.e., the equilibrium isotopic fractionation factor (α_eq). To date, relevant data have been lacking for molecules larger than methane. Here we describe an experimental method to measure α_eq for C-bound H positions adjacent to carbonyl group (H_α) in ketones. H at these positions equilibrates on a timescale of days as a result of keto-enol tautomerism, allowing equilibrium ²H/¹H distributions to be indirectly measured. Molecular vibrations for the same ketone molecules are then computed using Density Functional Theory at the B3LYP/6-311G** level and used to calculate α_eq values for H_α. Comparison of experimental and computational results for six different straight and branched ketones yields a temperature-dependent linear calibration curve with slope = 1.081−0.00376T and intercept = 8.404−0.387T, where T is temperature in degrees Celsius. Since the dominant systematic error in the calculation (omission of anharmonicity) is of the same size for ketones and C-bound H in most other linear compounds, we propose that this calibration can be applied to analogous calculations for a wide variety of organic molecules with linear carbon skeletons for temperatures below 100 °C. In a companion paper (Wang et al., 2009) we use this new calibration dataset to calculate the temperature-dependent equilibrium isotopic fractionation factors for a range of linear hydrocarbons, alcohols, ethers, ketones, esters and acids.     

Basement-controlled faulting of Paleozoic strata in southern Ontario, Canada: new evidence from geophysical lineament mapping

Basement fault reactivation is now recognized as an important control on sedimentation and fault propagation in intracratonic basins. In southern Ontario, the basement consists of complexly structured mid-Proterozoic (ca. 1.2 Ga) crystalline rocks and metasedimentary rocks that are overlain by up to 1500 m of Paleozoic sedimentary strata. Reactivation of basement structures is suspected to control the location of Paleozoic fault and fracture systems, but evaluation has been hindered by a limited understanding of the regional structural characteristics of the buried basement. New aeromagnetic- and gravimetric-lineament mapping presented in this paper better resolves the location of basement discontinuities and provides further evidence for basement controls on the distribution of Paleozoic fault and fracture systems. Lineament mapping was facilitated by reprocessing and digital image enhancement (micro-levelling, regional residual separation, derivative filtering) of existing regional gravity and aeromagnetic datasets. Reprocessed images identify new details of the structural fabric of the basement below southern Ontario and delineate several previously unrecognized aeromagnetic and gravity lineaments and linear zones. Linear zones parallel the projected trends of mid-Proterozoic terrane boundaries identified by field mapping on the exposed shield to the north of the study area, and are interpreted as zones of shearing and basement faulting. Mapped aeromagnetic and gravity lineaments show similar trends to Paleozoic faults and fracture networks and broad zones of seismicity in southern Ontario. These new data support an ‘inheritance model’ for Paleozoic faulting, involving repeated reactivation and upward propagation of basement faults and fractures into overlying cover strata.     

Crustal structure of the Ruby Mountains and southern Carlin Trend region, Nevada, from magnetotelluric data

We have collected about 150 magnetotelluric (MT) soundings in northeastern Nevada in the region of the Ruby Mountains metamorphic core complex uplift and southern Carlin mineral trend, in an effort to illuminate controls on core complex evolution and deposition of world-class gold deposits. The region has experienced a broad range of tectonic events including several periods of compressional and extensional deformation, which have contributed to the total expression of electrical resistivity. Most of the soundings reside in three east–west profiles across increasing degrees of core uplift to the north (Bald Mountain, Harrison Pass, and Secret Pass latitudes). One short cross-line was also taken to assess an east–west structure to the north of the northern profile. Model resistivity cross-sections were derived from the MT data using a 2-D inversion algorithm, which damps departures of model parameters from an a priori structure. Geological interpretation of the resistivity combines previous seismic, potential field and isotope models, structural and petrological models for regional compression and extension, and detailed structural/stratigraphic interpretations incorporating drilling for petroleum and mineral exploration. To first order, the resistivity structure is one of a moderately conductive, Phanerozoic sedimentary section fundamentally disrupted by intrusion and uplift of resistive crystalline rocks. Late Devonian and early Mississippian shales of the Pilot and Chainman Formations together form an important conductive marker sequence in the stratigraphy and show pronounced increases in conductance (conductivity–thickness product) from east to west. These increases are attributed to graphitization caused by Elko–Sevier era compressional shear deformation and possibly by intrusive heating. The resistive crystalline central massifs adjoin the host stratigraphy across crustal-scale, steeply dipping fault zones. The zones provide pathways to the lower crust for heterogeneous, upper crustal induced, electric current flow. Resistive core complex crust appears steeply bounded under the middle of the neighboring grabens and not to deepen at a shallow angle to arbitrary distances to the west. The numerous crustal breaks imaged with MT may contribute to the low effective elastic thickness (T_e) estimated regionally for the Great Basin and exemplify the mid-crustal, steeply dipping slip zones in which major earthquakes nucleate. An east–west oriented conductor in the crystalline upper crust spans the East Humboldt Range and northern Ruby Mountains. The conductor may be related to nearby graphitic metasediments, with possible alteration by middle Tertiary magmatism. Lower crustal resistivity everywhere under the profiles is low and appears quasi one-dimensional. It is consistent with a low rock porosity (<1 vol.%) containing hypersaline brines and possible water-undersaturated crustal melts, residual to the mostly Miocene regional extension. The resistivity expression of the southern Carlin Trend (CT) in the Pinon Range is not a simple lineament but rather a family of structures attributed to Eocene intrusion, stratal deformation, and alteration/graphitization. Substantial reactivation or overprinting by core complex uplift or Basin–Range extensional events seems likely. We concur with others that the Carlin Trend may result in part from overlap of the large Eocene Northeast Nevada Volcanic Field with Precambrian–Paleozoic deep-water clastic source rocks thickening abruptly to the west of the Pinon Range, and projecting to the north–northwest.     

A scientifically based nationwide assessment of groundwater quality in the United States

Beginning in 1986, the U.S. Geological Survey began an effort to develop a National Water-Quality Assessment Program. The basic premise underlying this initiative is that a better understanding of the quality of water resources across the country, both surface- and groundwater, is needed to develop effective programs and policies to meet the nation's water-quality concerns. The program will focus on water-quality conditions that are prevalent or large in scale, such as occur from nonpoint sources of pollution or from a high density of point sources.The design of the program is substantially different from the traditional approach of a diffuse national monitoring network.The major activities of the assessment program will be clustered within a set of hydrologic systems (river basins and aquifer systems), referred to as study units. In aggregate, the study units will account for a large part of the nation's water use and represent a wide range of settings across the country.Unique attributes of the program include: (1) the use of consistent study approaches, field and laboratory methods, water-quality measurements, and ancillary data measurements for all study units; (2) the development of a progressive understanding of water-quality conditions and trends in each study unit through long-term studies that rotate periods of intensive data collection and analysis with periods during which the assessment activities are less intensive; and (3) the focus of considerable effort on synthesizing results from among the study units to provide information on regional and national water-quality issues.Paper presented at 28th International Geological Congress, Washington, D.C., July 10, 1989.     

Incorporating solid solutions in reactive transport equations using a kinetic discrete-composition approach

A new approach is proposed for incorporating solid solution reactions into mass conservation equations describing reaction paths in both closed and open systems. The method is applicable to problems involving advective, dispersive, and diffusive transport in a porous medium. By representing the continuously variable solid solution composition with a discrete set of stoichiometric solids that span composition space, combined with a kinetic formulation of their rates of reaction, a self-determining spatial and temporal evolution of the solid solution concentration and composition is obtained. It is demonstrated that equilibrium of an aqueous solution with a stoichiometric solid derived from a solid solution corresponds to equilibrium of the solid solution itself if and only if equilibrium of the stoichiometric solid is stable. One advantage of this approach is that it is unnecessary to introduce any additional compositional variables to represent the solid solution. Discretization may be over the entire range of composition space, or over some subset depending on the system. A major consequence of the kinetic discrete-composition solid solution representation is that modeling solid solutions is similar to modeling pure mineral phases with the exception of a weighting factor applied to reaction rates of stoichiometric solids corresponding to a common solid solution. With this approach, precipitation leads to a discrete zonation of the solid solution that approximates the continuous variation in composition expected for the actual solid solution. The approach is demonstrated for a hypothetical ideal and non-ideal binary solid solution A_xB_1−xC for a reaction path formulation and reactive transport involving advection and diffusion.     

Smoothing and Change Point Detection for Gamma Ray Count Data

Gamma ray detectors are used to measure the natural radioactivity of rocks. For a number of boreholes drilled at a site the gamma ray detector is lowered into each borehole and counts of gamma ray emissions at different depths are recorded as the instrument is gradually raised to ground level. The profile of gamma counts can be informative about the geology at each location. The raw count data are highly variable, and in this paper we describe the use of adaptive smoothing techniques and change point models in order to identify changes in the geology based on the gamma logs. We formulate all our models for the data in the framework of the class of generalized linear models, and describe computational methods for Bayesian inference and model selection for generalized linear models that improve on existing techniques. Application is made to gamma ray data from the Castelreagh Waste Management Centre which served as a hazardous waste disposal facility for the Sydney region between March 1974 and August 1998. Understanding the geological structure of this site is important for further modelling the transport of pollutants beneath the waste disposal area.     

Retrograde fluids in the Archean Shawmere anorthosite, Kapuskasing Structural Zone, Ontario, Canada

The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite ± plagioclase show that the vein minerals were stable at T < 600 °C, XCO₂ < 0.4 at P ≈ 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T < 600 °C and XCO₂ < 0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T < 500 °C with XCO₂ > 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H₂O-rich to CO₂-rich. The calcite in the retrograde veins has δ¹⁸O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ¹³C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO₂-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO₂ ≈ 1.13 to 1.05 g/cm³). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO₂-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO₂-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO₂-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path. Received: 2 April 1996 / Accepted: 15 November 1996     

Sources and history of heavy metal contamination and sediment deposition in Tivoli South Bay, Hudson River, New York

Persistent inorganic constitutents preserved in sediments of aquatic ecosystems record temporal variability of biogeochemical functioning and anthropogenic impacts.²¹⁰Pb and¹³⁷Cs dating techniques were used to study the past variations of heavy metals (Pb, Cu, and Zn) and accumulation rates of sediments for Tivoli South Bay, in the Hudson River National Estuarine Research Reserve ecosystem. South Bay, a tidal freshwater embayment of the Hudson, may play an important role in the sediment dynamics of this important river. The measured sedimentation rate range of 0.59 to 2.92 cm yr⁻¹ suggests that rapid accumulation occurred during the time period represented by the length of the cores (approximately the past 50 yr). Direct measurements of sediment exchange with the Hudson River reveal high variability in the sediment flux from one tidal cycle to the next. Net exchange does not seem to be adequate to explain sediment accumulation rates in the bay as measured by²¹⁰Pb and¹³⁷Cs. The difference may be supplied from upland streams or the Hudson River during storm events. Concentrations of the metals Pb, Cu and Zn were found to be well correlated with each other within individual cores at five of six sites tested. This suggests a common proximate source for the three metals at a specific site. The evidence is consistent with mixing in some environmental compartment before delivery to the bay. While metals self-correlate within individual cores, absolute concentrations, depth distribution patterns, and ratios of the metals to each other vary among the cores collected at different locations within the bay. Organic matter, Fe content, and particle size distribution of sediments do not account for the intercore variations in metal concentration. It is likely that cores collected from different sites may have derived metals from different sources, such as watershed streams and tidal exchange with the Hudson River.     

Dispersion and recruitment of crab larvae in the Chesapeake Bay plume: Physical and biological controls

As part of the Microbial Exchanges and Coupling in Coastal Atlantic Systems (MECCAS) Project, crab larvae were collected in the shelf waters off Chesapeake Bay in June and August 1985 and April 1986. We conducted hydrographic (temperature, salinity, nutrients) and biological (chlorophyll, copepods) mapping in conjunction with Eulerian and Lagrangian time studies of the vertical distribution of crab larvae in the Chesapeake Bay plume. These abundance estimates are used with current meter records and drifter trajectories to infer mechanisms of larval crab dispersion to the shelf waters and recruitment back into Chesapeake Bay. The highest numbers of crab larvae were usually associated with the Chesapeake Bay plume, suggesting that it was the dominant source of crab larvae to shelf waters. Patches of crab larvae also were found in the higher salinity shelf waters, and possibly were remnants of previous plume discharge events. The distribution of crab larvae in the shelf waters changed on 1–2 d time scales as a consequence of both variations in the discharge rate of the Chesapeake Bay plume and local wind-driven currents. Downwelling-favorable winds (NW) intensified the coastal jet and confined the plume and crab larvae along the coast. In April during a downwelling event (when northwesterly winds predominated), crab zoeae were transported southward along the coast at speeds that at times exceeded 168 km d⁻¹. During June and August the upwelling-favorable winds (S, SW) opposed the anticyclonic turn of the plume and, via Ekman circulation, forced the plume and crab larvae to spread seaward. Plume velocities during these conditions generally were less than 48 km d⁻¹. The recruitment of crab larvae to Chesapeake Bay is facilitated in late summer by the dominance of southerly winds, which can reverse the southward flow of shelf waters. Periodic downwelling-favorable winds can result in surface waters and crab larvae moving toward the entrance of Chesapeake Bay. Approximately 27% of the larval crabs spend at least part of the day in bottom waters, which have a residual drift toward the bay mouth. There appears to be a variety of physical transport mechanisms that can enhance the recruitment of crab larvae into Chesapeake Bay.     

The geoarchaeology of Iron Age “moated” sites of the Upper Mae Nam Mun Valley,N.E. Thailand. I: Palaeodrainage,site–landscape relationships and the origins of the “moats”

The study focuses on the “moated” Iron Age sites of N.E. Thailand, first identified as significant prehistoric settlement sites in the 1940s from aerial photography. Two more recent photograph sets are used to map the surficial geology and prehistoric site distribution for a study area west of Phimai, N.E. Thailand, with a focus on site–landscape relationships and, in particular, relationships between site location and form and patterns of palaeodrainage. The derived record of the surficial geology reflects several phases of palaeodrainage, characterized by differing locations and types of former river channels. Of note is the differentiation between a recent period in which river channels, including those presently active, are single-string meandering channels, and an older period of broad belts of meandering multistring channels. The prehistoric site distribution correlates closely with the older drainage, and for many, the encircling channels (the “moats”) are closely associated with former river channels. These relationships provide a critical and novel model for site distribution; several implications arise, supported by emerging field evidence, and introducing issues for archaeological debate: (i) there is no need, as has been done in the past, to invoke prehistoric artificial forms of drainage associated with the sites; (ii) the definition of the encircling channels as “moats” is seriously called into question; and (iii) the inferred geomorphological evolution of the floodplain implies past changes in environmental parameters such as run-off, climate or biophysical environments. Since the sites are all located in or beside ancient meander belts, these parameters should now be introduced into archaeological discussions regarding the establishment, history, evolution, and abandonment of the Iron Age sites. Methodologically, this article illustrates the need to be aware of the complexity of aerial photograph interpretation in archaeological survey, showing that careful analysis of aerial photograph information may have a significant impact upon the modeling of prehistoric interpretations. Further stratigraphical studies will be reported subsequently, and will refine the models presented here. © 1999 John Wiley & Sons, Inc.