Biogeochemistry of Mono Lake,California

Modern sediments of Mono Lake show marked variation in lipid composition with depositional environment. Constituents derived from the drainage basin, characterized by high molecular weight alkane hydrocarbons (C₂₅–C₃₁), and the steroids β-sitosterol and brassicasterol, predominate in near-shore environments. In the deepest part of the lake, sediments exhibit a combination of externally-derived constituents, and lipids derived from the lake biota; the latter characterized by low molecular-weight alkanes and alkenes (C₁₅–C₁₇), phytane, and the steroids ergost-7-en-3β-ol and 24-ethylcholest-7-en-3-β-ol. Steranes, 4-methylsteranes, and the C₁₈ and C₁₉ isoprenoids appear to be forming in the intensely reducing bottom sediments at the present time.The compositions of samples from the Pleistocene succession of Mono Basin suggest that sample-to-sample variation within the same stratum is negligible so long as unweathered samples from the same depositional environment are compared. Sediments having equivalent lithologies may or may not have similar compositions, but sediments having similar fossil contents do show similar lipid compositions. Subaerial weathering of sediments causes a marked decrease in the amount of extractable organic material, as well as distinct changes in its hydrocarbon composition. Specifically, weathered sediments exhibit a decrease in relative content of low molecular weight hydrocarbons and a relative increase in nC₂₂.Organic composition of sediments from the Pleistocene stratigraphie column cannot be correlated with depth of burial. Compositional changes with stratigraphie position are probably related to paleo-ecological factors such as population or productivity rather than depth of burial. Lithology and organic composition provide mutually-corroborating evidence regarding glacial advances in the adjacent Sierra Nevada Mountains. During glaciations, the lake sediments are rich in sandstones, and the organic composition shows a predominance of externally-derived debris, with no evidence for contributions from the lake biota.     

Arsenic distribution, speciation and solubility in shallow groundwater of Owens Dry Lake, California

Generation of dust particles from the Owens Lake playa creates a severe air pollution hazard in the western United States. Much of the dust produced from the dry lakebed is derived from salts formed by evaporation of saline groundwater that often contains high concentrations of dissolved arsenic (As). The objectives of this research were to study the spatial distribution of dissolved arsenic in the shallow groundwater, and to examine factors affecting arsenic solubility and speciation. Evapoconcentration, redox potential, pH, and mineral solubility were examined as factors regulating arsenic biogeochemistry. Dissolved arsenic concentrations ranged from 0.1 to 96 mg L⁻¹ and showed a general increase from the shoreline to the center of the lakebed. Arsenic concentrations were strongly correlated to electrical conductivity (EC) and δD suggesting that evapoconcentration is an important process regulating total As concentrations. Arsenite [As(III)] was the dominant form of inorganic arsenic at Eh values less than about −170 mV while arsenate [As(V)] was predominant at higher Eh values. Organic arsenic was negligible (<0.21%) in all shallow groundwater samples. Dissolved arsenic concentrations do not appear to be strongly regulated by solid-phase reactions. Solid-phase arsenic concentrations generally ranged between 4.0 and 42.6 mg kg⁻¹ and a maximum concentration range (20 to 40 mg kg⁻¹) was reached as solution concentration increased up to 80 mg L⁻¹, indicating minimal sorption and/or precipitation of arsenic. Chemical equilibrium modeling indicated that orpiment (As₂S₃) was the only solid phase with a positive saturation index (indicating over-saturation), but only at high arsenic and sulfide concentrations. The findings of this research are important for assessing the potential environmental impacts of elevated arsenic concentrations on dust mitigation efforts taking place at Owens Dry Lake.     

Sources and flux of natural gases from Mono Lake,California

The ability to identify a formation mechanism for natural gas in a particular environment requires consideration of several geochemical factors when there are multiple sources present. Four primary sources of methane have been identified in Mono Lake. Two of these sources were associated with numerous natural gas seeps which occur at various locations in the lake and extend beyond its present boundary; the two other gas sources result from current microbiological processes. In the natural gas seeps, we observed flow rates as high as 160 moles CH₄ day⁻¹, and estimate total lakewide annual seep flux to be 2.1 × 10⁶ moles CH₄. Geochemical parameters (δ¹³CH₄,δDCH₄,CH₄/[C₂H₆+ C₃H₈]) andδ¹⁴CH₄measurements revealed that most of the seeps originate from a paleo-biogenic (δ¹³CH₄ = about −70%.). natural gas deposit of Pleistocene age which underlies the current and former lakebed. Gas seeps in the vicinity of hot springs had, in combination with the biogenic gas, a prominent thermogenic gas component resulting from hydrothermal alteration of buried organic matter.Current microbiological processes responsible for sources of natural gas in the lake included pelagic meth- anogenesis and decomposition of terrestrial grasses in the littoral zone. Methanogenesis in the pelagic sediments resulted in methane saturation (2–3 mM at 50 cm; δ₁₃CH₄ = about −85%.). Interstitial sulfate decreased from 133 mM at the surface to 35 mM by 110 cm depth, indicating that sulfate-reduction and methanogenesis operated concurrently. Methane diffused out of the sediments resulting in concentrations of about 50 μM in the anoxic bottom waters. Methane oxidation in the oxic/anoxic boundry lowered the concentration by >98%, but values in surface waters (0.1–1.3μM) were supersaturated with respect to the atmosphere. The δ¹³CH₄ (range = −21.8 to −71.8%.) of this unoxidized residual methane was enriched in ¹³C relative to methane in the bottom water and sediments. Average outward flux of this methane was 2.77 × 10⁷ moles yr⁻¹. A fourth, but minor source of methane (δ¹³CH₄ = −55.2%.) was associated with the decomposition of terrestrial grasses taking place in the lake's recently expanded littoral zone.     

Arsenic speciation in pyrite and secondary weathering phases,Mother Lode Gold District,Tuolumne County,California

Arsenian pyrite, formed during Cretaceous gold mineralization, is the primary source of As along the Melones fault zone in the southern Mother Lode Gold District of California. Mine tailings and associated weathering products from partially submerged inactive gold mines at Don Pedro Reservoir, on the Tuolumne River, contain ∼20–1300 ppm As. The highest concentrations are in weathering crusts from the Clio mine and nearby outcrops which contain goethite or jarosite. As is concentrated up to 2150 ppm in the fine-grained (<63 μm) fraction of these Fe-rich weathering products.Individual pyrite grains in albite-chlorite schists of the Clio mine tailings contain an average of 1.2 wt.% As. Pyrite grains are coarsely zoned, with local As concentrations ranging from ∼0 to 5 wt.%. Electron microprobe, transmission electron microscope, and extended X-ray absorption fine-structure spectroscopy (EXAFS) analyses indicate that As substitutes for S in pyrite and is not present as inclusions of arsenopyrite or other As-bearing phases. Comparison with simulated EXAFS spectra demonstrates that As atoms are locally clustered in the pyrite lattice and that the unit cell of arsenian pyrite is expanded by ∼2.6% relative to pure pyrite. During weathering, clustered substitution of As into pyrite may be responsible for accelerating oxidation, hydrolysis, and dissolution of arsenian pyrite relative to pure pyrite in weathered tailings. Arsenic K-edge EXAFS analysis of the fine-grained Fe-rich weathering products are consistent with corner-sharing between As(V) tetrahedra and Fe(III)-octahedra. Determinations of nearest-neighbor distances and atomic identities, generated from least-squares fitting algorithms to spectral data, indicate that arsenate tetrahedra are sorbed on goethite mineral surfaces but substitute for SO₄ in jarosite. Erosional transport of As-bearing goethite and jarosite to Don Pedro Reservoir increases the potential for As mobility and bioavailability by desorption or dissolution. Both the substrate minerals and dissolved As species are expected to respond to seasonal changes in lake chemistry caused by thermal stratification and turnover within the monomictic Don Pedro Reservoir. Arsenic is predicted to be most bioavailable and toxic in the reservoir’s summer hypolimnion.     

Freshwater control of ice-rafted debris in the last glacial period at Mono Lake, California, USA

The type section silts of the late Pleistocene Wilson Creek Formation at Mono Lake contain outsized clasts, dominantly well-rounded pebbles and cobbles of Sierran lithologies. Lithic grains > 425 μm show a similar pattern of variability as the > 10 mm clasts visible in the type section, with decreasing absolute abundance in southern and eastern outcrops. The largest concentrations of ice-rafted debris (IRD) occur at 67–57 ka and 46–32 ka, with strong millennial-scale variability, while little IRD is found during the last glacial maximum and deglaciation.Stratigraphic evidence for high lake level during high IRD intervals, and a lack of geomorphic evidence for coincidence of lake and glaciers, strongly suggests that rafting was by shore ice rather than icebergs. Correspondence of carbonate flux and IRD implies that both were mainly controlled by freshwater input, rather than disparate non-climatic controls. Conversely, the lack of IRD during the last glacial maximum and deglacial highstands may relate to secondary controls such as perennial ice cover or sediment supply. High IRD at Mono Lake corresponds to low glacial flour flux in Owens Lake, both correlative to high warm-season insolation. High-resolution, extra-basinal correlation of the millennial peaks awaits greatly improved age models for both records.     

Analysis and simulated diagenesis of kerogen in a Recent bottom mud from Mono Lake,California: a comparison with selected ancient kerogens

The insoluble organic matter, or kerogen, in a Recent bottom mud (<1000yr old) from Mono Lake, California, has been analyzed by vacuum pyrolysis-GC-MS and compared with the kerogen from several Precambrian rocks, including the Belingwe and Transvaal stromatolites. The Mono Lake kerogen consists mainly of cyclic and acyclic aliphatic components with lesser amounts of aromatics present. It is less aromatic and more susceptible to thermal degradation than the Precambrian kerogens, and its products show a much greater diversity especially among the unsaturated aliphatics and the heteroatomic components. The presence of these compounds, most notably 2,5-dimethylfuran, in the Mono Lake kerogen indicates a relatively rapid formation and incorporation into a young kerogen and suggests that their presence in the Precambrian kerogens, such as the Belingwe stromatolite, may be consistent with an ancient biological origin.In simulated diagenesis experiments the Recent mud was heated at 150°C for 3 months or 225°C for 8 months. The former was insufficient to affect the kerogen pyrolysis products. The latter, however, caused a large decrease in the heteroatomic components and a slight increase in the abundance of n-alkanes relative to that of the cyclic and branched alkanes. This suggests that the presence of some of these components in Precambrian rocks should be consistent with a high degree of preservation of these rocks, as appears to be the case for the Belingwe and Transvaal stromatolites.     

Arsenic in freshwater systems: Influence of eutrophication on occurrence,distribution, speciation,and bioaccumulation

Arsenic exists in a variety of chemical forms, and microbial metabolism results in the occurrence of thermodynamically unstable arsenite (As^III) and methylarsenic compounds in freshwaters (rivers and lakes). The inorganic forms (As^V and As^III) and the methylated forms (methylarsonic acid; MMAA^V and dimethylarsinic acid; DMAA^V) are the main species of As in freshwaters while the bulk of the total dissolved As is inorganic species. Although the predominant forms of methylarsenic compounds are consistently DMAA^V followed by MMAA^V, the DMAA^III and MMAA^III species have also been found in freshwaters. Several observations have revealed that phytoplankton activities are responsible for the seasonal variations of methylarsenic compounds in freshwaters. Although it was unclear if the occurrences of methylarsenic compounds were from the breakdown of larger molecules or the end-products of phytoplankton biosynthesis, recent studies have revealed that less toxic As–glutathione complexes are intermediates in the biosynthesis of organoarsenic compounds by phytoplankton. Recent studies have also revealed that eutrophication plays an important role in the production, distribution, and cycling of methylarsenic compounds in freshwaters. In this review, the recent reports on the influence of eutrophication on distribution, speciation, and bioaccumulation in freshwaters are discussed.     

Geomorphic,geographic, and hydrographic basis for resolving the Mono Lake controversy

One of the most highly publicized and protracted land-use conflicts currently being waged in the western United States involves Mono Lake, a large, hypersaline water body that lies in the lee of California's Sierra Nevada. Since 1940, when the Los Angeles Department of Water and Power began to divert the tributary streams that feed Mono Lake, the lake surface has dropped 45 ft (14 m), lake volume has been halved, and lake salinity has doubled. Dust storms resulting from deflation of newly exposed playa surfaces have increased in intensity, and the spatial dimensions of lacustrine habitats have been reduced. Islands used by nesting gulls have become peninsulas, permitting coyotes to invade and disrupt the rookeries.An essential first step in resolving the Mono Lake predicament involves quantifying the relationship between lake behavior and the environmental variables that lie at the center of the controversy. This article documents these relationships, and explores some of the problems inherent in developing a management plan for Mono Lake. The approach taken here may be useful in resolving environmental problems at other of the world's closed lakes which, in increasing numbers and to an increasing degree, are being drawn down by the diversion of water.     

Accumulation and speciation of selenium in evaporation basins in California,USA

The accumulation of selenium in evaporation basins (or ponds) in the San Joaquin Valley, California is of a great concern due to its potential hazards to environments. In this study, the accumulation, speciation and concentrations of Se were examined in waters as well as sediments in a system of the evaporation ponds. A significant decrease in the total dissolved Se concentration in Cell 1 in which drainage water with higher Se concentration was pumped from Inlet Channels indicated that the immobilization of Se was active in the Cell 1 and resulted in the higher Se concentration in sediments compared to the terminal cell such as Cell 9. The percentage of reduced Se species such as selenite [Se(IV)] and org-Se of total Se in drainage waters was also found increased in Cell 1 compared to Inlet Channels. The total dissolved Se concentrations in water along flow paths from Cell 1 were relatively constant except for terminal cells such as Cells 9 and 10, which showed higher total dissolved Se concentrations due to evapoconcentration. The percentage of reduced Se forms of total Se was inversely proportional to the percentage of Se(VI) depending on the redox condition of evaporation ponds along the flow paths. Sequential extractions of Se species in sediments indicated that organic associated Se and elemental Se were prevalent forms in sediments in the ponds system. The higher concentrations of elemental Se and organic associated Se in sediments in Cell 1 indicated that the immobilization of Se was active in the sediments compared to Cell 9, while the percentage of both fractions of total Se in sediments in Cells 1 and 9 was relatively constant. The organic materials from algae might provide carbon sources for Se reduction and Se sink in sediments in its elemental and organic associated forms.     

Arsenic accumulation and speciation in plants from different habitats

Understanding As accumulation in plants is necessary in order to alleviate problems with As in the environment and to improve sustainable As phytotechnologies. To find suitable candidates for phytoremediation purposes and to investigate specific accumulation patterns due to growth habitat and plant groups, As accumulation in 124 plant species collected from different habitats and speciation in 6 of these plant species, was determined. The data show that submerged plants have a higher accumulation than emergent and terrestrial plants. The As concentration in terrestrial and emergent plants were correlated with the [As]_soil, while the accumulation factor correlated negatively with [As]_soil. Gymnosperms had a high [As]_shoot:[As]_root ratio. The inorganic As species, arsenate and arsenite were found in plants from all habitats and methylarsonic acid (MMA) in all but one plant species. Arsenate predominated in submerged plants. The results suggest that the habitat and the [As]_soil have a strong influence on the As accumulation in plants and that submerged plants and/or gymnosperms might be suitable for phytoremediation of As.     

Arsenic distribution, concentration and speciation in groundwater of the Osijek area, eastern Croatia

Groundwater is the main source of drinking water for the population of nearly 200,000 people in eastern Croatia. The largest town in the region is Osijek whose citizens are supplied with drinking water obtained from groundwater from the “Vinogradi” well field. This study investigated and determined As occurrence in groundwater of the Osijek area. Groundwater samples were taken from 18 water wells and 12 piezometers with a depth ranging between 21 and 200 m. Over the 10-a period to 2007, a mean As concentration of 240 μg L⁻¹ was found. There was no statistically significant secular change in concentration over that period, however small but significant seasonal variations were noted, with the highest seasonal As concentrations over the period May 2006-February 2007 being observed in summer. The predominant As species observed was As(III), constituting 85% and 93% of total As in piezometers and water wells, respectively. Higher concentrations of As tended to be found in deeper wells with the mean As concentration in shallow groundwater (<50 m) and deep groundwater (>50 m) being 27 μg L⁻¹, and 205 μg L⁻¹, respectively. Geochemically, the groundwaters show similarities to those in other parts of the Pannonian Basin. Arsenic(tot) is weakly correlated with pH and Fe, negatively correlated with Mn and has no significant correlation with any of EC, COD-Mn or alkalinity.     

The prediction of borate mineral equilibria in natural waters: Application to Searles Lake, California

The chemical equilibrium model of et al. (1984) has been extended to include borate species. The model is based upon the semi-empirical equations of (1973) and coworkers and is valid to high ionic strength (≈14 m) and high borate concentration. Excellent agreement with the existing emf, isopiestic and solubility data in the system (Na-K-Ca-Mg-H-Cl-SO₄-CO₂-B(OH)₄-H₂O) is obtained. Calculated mineral solubilities are in general within 10% of their experimental values, even at high ionic strengths.

The model was applied to the multicomponent, high ionic strength (I ˜ 10) and high borate concentration (B_T ˜ 0.5 m) Searles Lake evaporite deposit. Utilizing the chemical composition of the interstitial brine, the model predicts equilibrium between the brine and only those minerals which are known to be in contact with the brine. These calculations clearly demonstrate the applicability of the model to high ionic strength, high borate concentration natural waters.

The model was also utilized to calculate the mineral sequences which should result from evaporation of the major source of water for Searles Lake, the Owens River. The geochemical conditions necessary for the formation of the most recent mud and saline units are examined. The final results indicate that the mineral sequences found in the most recent saline unit in Searles Lake can be produced by evaporation of a water close in composition to present Owens River water, provided primary dolomite formation is delayed and back reaction between the Parting Mud and the Upper Salt is inhibited.     

Mineral Equilibria in the Searles Lake Evaporites, California

The Searles Lake evaporites (late Quaternary) consist of salineand mud layers permeated by brines. Two mineral pairs (gaylussite-pirssonite,mirabilite-thenardite) are used as indicators of relative aH₂othroughout the stratigraphic column. Variations are attributedmostly to changes in brine salinity and partly to temperature. These aH₂o-sensitive minerals locally coexist with trona, nahcolite,burkeite, northupite, tychite, hanksite, and aphthitalite, whichare sensitive to both aH₂o and a_co. Such assemblages permitconstruction of schematic isothermal aH₂o-a_co, diagrams. Fieldboundary sequences are derived from both theoretical considerationsand observed assemblages; their slopes are determined by thestoichiometry of possible reactions. Predicted assemblages invariablyagree with observed assemblages. By means of these diagrams, present-day lateral and stratigraphicvariations in relative aH₂o and aco₂ in the deposit are reconstructed.They show that aH₂o and a_co2 vary independently. Many of thepresent activities reflect depositional conditions; some indicatepost-depositional events.     

Diatom Response to Global Warming in Douhu Lake,Southeast China

A large number of lacustrine sedimentary records indicate that global warming is the main factor leading to significant changes in diatom communities in lakes of the northern hemisphere.However,due to the intensification of human activities since 1850,some scholars have emphasized that the increasing lake trophic level may be the main reason for the changes in diatom communities.The debate is ongoing.In order to avoid falling into the complex relationship between diatom changes and the seasonal cycle that characterizes lakes in mid and high latitudes,we chose a lake located at a low latitude,where the relationship between diatoms and temperature is not indirect but direct.The diatom record spans the past ca.100 years and reveals that the abundance of Aulacoseira granulata increased from 1900 until 1985, replacing the previously dominant Aulacoseira ambigua.These changes are in agreement with the increasing trend in global temperature.Since 1985,the percentages of the small-celled Discostella stelligera and the benthic diatom Navicula heimansioides have increased,while Aulacoseira granulata has decreased.This latest shift is caused by further global warming.We conclude that warming is the main factor leading to changing diatom communities in Douhu Lake.     

Distribution and speciation of phosphorus in sediments of Dongping Lake,North China

Dongping Lake is an important regulator for the Eastern Route of South-to-North Water Diversion Project in China, and the water quality assurance of it is of great importance. To investigate the distribution characteristics and burial process of phosphorus (P) in Dongping Lake as well as their relationship with regional economic development and human activities, 33 spatially distributed surface sediment samples and one lake sediment core were extracted from Dongping Lake. A harmonized procedure for the P forms in freshwater sediments developed by the European Programme, Standards, Measurements, and Testing was used in this paper. The results show that NaOH–P (P bound to Al, Fe, Mn oxides or hydroxides), HCl–P (Ca-bound P), inorganic phosphorus (IP), organic phosphorus (OP), and total phosphorus (TP) in the surface sediments show a strong spatial variability. The highest NaOH–P and OP concentrations were observed in the eastern and south-eastern region of the lake and decreased gradually away from the mouth area of Dawen River. We deduced that anthropogenic input via the Dawen River is the main contributor. The higher concentration of HCl–P occurred in the north-west region of the lake and increased in a gradient away from the mouth area of Dawen River, and this pattern of distribution could be related to grain-size effects and higher transport energies close to riverine inlets. Pollution reflected by phosphorus in the core sediments of Dongping Lake is well consistent with the economic development history of Dongping County. From the beginning of the 20th century to the middle 1970s, the concentrations of each P speciation did not vary largely and runoff of the Yellow River mainly contributed to the P accumulation in the sediment, which might reflect the background values of them in the lake in pre-industrial period. The stable Mz (mean grain size), low loss-on-ignition (LOI), and high sedimentary flux also support this. The rapid increase of NaOH–P and OP at 10–0 cm (especially 5–0 cm) indicates the enhancing anthropogenic eutrophication since the middle 1970s. The LOI peak period and a coarsening of particle size also support this interpretation. However, the decreasing of TP and HCl–P was attributed to the change in sediment supply. As due to construction of dams around the lake, runoff of the Dawen River mainly contributed to the P accumulation in the sediment during this period.     

Arsenic and iron speciation in uranium mine tailings using X-ray absorption spectroscopy

In northern Saskatchewan, Canada, high-grade U ores and the resulting tailings can contain high levels of As. An environmental concern in the U mining industry is the long-term stability of As within tailings management facilities (TMFs) and its potential transfer to the surrounding groundwater. To mitigate this problem, U mill effluents are neutralized with lime to reduce the aqueous concentration of As. This results in the formation of predominantly Fe³⁺–As⁵⁺ secondary mineral phases, which act as solubility controls on the As in the tailings discharged to the TMF. Because the speciation of As in natural systems is critical for determining its long-term environmental fate, characterization of As-bearing mineral phases and complexes within the deposited tailings is required to evaluate its potential transformation, solubility, and long-term stability within the tailings mass. In this study, synchrotron-based bulk X-ray absorption spectroscopy (XAS) was used to study the speciation of As and Fe in mine tailings samples obtained from the Deilmann TMF at Key Lake, Saskatchewan. Comparisons of K-edge X-ray absorption spectra of tailings samples and reference compounds indicate the dominant oxidation states of As and Fe in the mine tailings samples are +5 and +3, respectively, largely reflecting their generation in a highly oxic mill process, deposition in an oxidized environment, and complexation within stable oxic phases. Linear combination fit analyses of the K-edges for the Fe X-ray absorption near edge spectra (XANES) to reference compounds suggest Fe is predominantly present as ferrihydrite with some amount of the primary minerals pyrite (8–15% in some samples) and chalcopyrite (5–15% in some samples). Extended X-ray absorption fine structure (EXAFS) analysis of As K-edge spectra indicates that As⁵⁺ (arsenate) present in tailings samples is adsorbed to the ferrihydrite though an inner-sphere bidentate linkage.     

Transtensional deformation in the Lake Tahoe region, California and Nevada, USA

Dextral transtensional deformation is occurring along the Sierra Nevada–Great Basin boundary zone (SNGBBZ) at the eastern edge of the Sierra Nevada microplate. In the Lake Tahoe region of the SNGBBZ, transtension is partitioned spatially and temporally into domains of north–south striking normal faults and transitional domains with conjugate strike-slip faults. The normal fault domains, which have had large Holocene earthquakes but account only for background seismicity in the historic period, primarily accommodate east–west extension, while the transitional domains, which have had moderate Holocene and historic earthquakes and are currently seismically active, primarily record north–south shortening. Through partitioned slip, the upper crust in this region undergoes overall constrictional strain.Major fault zones within the Lake Tahoe basin include two normal fault zones: the northwest-trending Tahoe–Sierra frontal fault zone (TSFFZ) and the north-trending West Tahoe–Dollar Point fault zone. Most faults in these zones show eastside down displacements. Both of these fault zones show evidence of Holocene earthquakes but are relatively quiet seismically through the historic record. The northeast-trending North Tahoe–Incline Village fault zone is a major normal to sinistral-oblique fault zone. This fault zone shows evidence for large Holocene earthquakes and based on the historic record is seismically active at the microearthquake level. The zone forms the boundary between the Lake Tahoe normal fault domain to the south and the Truckee transition zone to the north.Several lines of evidence, including both geology and historic seismicity, indicate that the seismically active Truckee and Gardnerville transition zones, north and southeast of Lake Tahoe basin, respectively, are undergoing north–south shortening. In addition, the central Carson Range, a major north-trending range block between two large normal fault zones, shows internal fault patterns that suggest the range is undergoing north–south shortening in addition to east–west extension.A model capable of explaining the spatial and temporal partitioning of slip suggests that seismic behavior in the region alternates between two modes, one mode characterized by an east–west minimum principal stress and a north–south maximum principal stress as at present. In this mode, seismicity and small-scale faulting reflecting north–south shortening concentrate in mechanically weak transition zones with primarily strike-slip faulting in relatively small-magnitude events, and domains with major normal faults are relatively quiet. A second mode occurs after sufficient north–south shortening reduces the north–south S_hmax in magnitude until it is less than S_v, at which point S_v becomes the maximum principal stress. This second mode is then characterized by large earthquakes on major normal faults in the large normal fault domains, which dominate the overall moment release in the region, producing significant east–west extension.     

A Late Pleistocene Tephra Layer in the Southern Great Basin and Colorado Plateau Derived from Mono Craters, California

A newly identified tephra in stratified deposits in southwestern Utah, dated 14,000 ¹⁴C yr B.P., may aid in correlating late Pleistocene deposits across parts of the southern Great Basin and west-central Colorado Plateau. Geochemical analyses of the ash suggest the tephra originated from Mono Craters, California, and most probably correlates with Wilson Creek ash #3. Because the ash is 2 mm thick 550 km from its source, the event may have been larger than others correlated to Mono Craters eruptions.