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USGS2017

Radiocarbon ages from two submerged strandline features in the western Gulf of Maine and a sea-level curve for the northeastern Massachusetts coastal region

New radiocarbon dates provide ages for two submerged strandline features on the Massachusetts inner shelf. These ages provide limited control on a relative sea-level (RSL) curve for the late Wisconsinan and Holocene. The curve indicates a late Wisconsinan high stand of RSL of +33 m about 14,000 yr ago and a very short-lived relative low stand of about -43 m at about 12,000 yr ago followed by a rise to present sea level. Rapid changes of RSL around 12,000 yr ago may be related to changes in global glacial meltwater discharge and eustatic sea-level change shown by dated corals off Barbados. Variations in the magnitude and timing of RSL change from south to north along the coast of the western Gulf of Maine are due to greater crustal depression and later deglaciation to the north.

Massachussets↗

USGS2017

Comparison and contrast of processes of soil formation in the San Timoteo Badlands with chronosequences in California

The degree of soil development associated with geomorphic surfaces in the San Timoteo Badlands area (STB), California allows correlation of the surfaces. Soil development indices, based on field descriptions and laboratory analysis, provide a basis for comparison of these soils to each other and to dated soil chronosequences at Cajon Pass, Merced, and Anza, California. The soils in this study record a complex tectonic and climatic history, include preserved surfaces intermediate in soil development to those at Cajon Pass, and do not preserve a record of the major late Pleistocene to early Holocene aggradational event observed elsewhere in the region. Pedogenesis is similar to that of regional soils in southern California. With time, the profiles develop progressively thicker argillic horizons, and they increase in redness, clay, and secondary iron oxide content, and amount and thickness of clay films. With increasing soil age the ratio of dithionite-extractable Fe to total Fe increases, while the ratios of Fe 2+ to Fe 3+ and of oxalate- to dithionite-extractable Fe decrease. These trends are more subdued in the STB than elsewhere in the region, perhaps because the slightly higher temperature and lower precipitation of the STB may slow the rate of transformation of ferrihydrite to hematite. We use weighted means of soil development indices and rubification index values to compare the soils of this study to those at Cajon Pass, Merced, and Anza. Wide-range age estimates using the maximum limits of these values result in 27,500–305,000 yr for surface Q1, 43,000–570,000 yr for surface Q2, and 300,000–700,000 yr for surfaces Q3 and Q4. Weighing most heavily the nearest dated chronosequences, Anza and Cajon Pass, yields best estimates of age. This method constrains the time of formation of the Q1 surface to between 27,500 and 67,000 yr and the Q2 surface to 43,000–67,000 yr, and does not change the age estimates of the older surfaces.

California↗

USGS2017

Cyclic aggradation and downcutting, fluvial response to volcanic activity, and calibration of soil-carbonate stages in the western Grand Canyon, Arizona

In the western Grand Canyon, fluvial terraces and pediment surfaces, both associated with a Pleistocene basalt flow, document Quaternary aggradation and downcutting by the Colorado River, illuminate the river's response to overload and the end of overload, and allow calibration of soil-carbonate stages and determination of downcutting rates. Four downcutting-aggradation cycles are present. Each begins with erosion of older deposits to form a new river channel in which a characteristic suite of deposits is laid down. The current cycle (I) started ~700 yr B.P. The oldest (IV) includes the 603,000 ± 8000 to 524,000 ± 7000 yr Black Ledge basalt flow, emplaced when the river channel was ~30 m higher than it is now. The flow is overlain by basalt-cobble gravel and basalt sand. Soils reach the stage V level of carbonate development. Calibrated ages for soil stages are Stage V, ~525,000 yr; stage IV, <525,000 yr, ≥250,000 yr; stage III, <250,000 yr, ≥100,000 yr. The monolithologic basalt sand beds represent overloading by volcanic ash produced by an eruption 30-50 km upstream. The basalt-cobble beds signal breaching and rapid destruction of lava dams and erosion of flows. These deposits show that the Colorado River responds to overload by aggrading vigorously during the overload and then downcutting equally vigorously when the overload ends. The overall downcutting rate for the interval studied is 1.6 cm/1000 yr, much lower than rates upstream. The current downcutting rate, 11-14 m/1000 yr, likely is a response both to the end of late Pleistocene and early Holocene overload and to the reduction of sediment supply caused by Glen Canyon Dam.

Quaternary Research↗

USGS2017

Comparison of conventional K-Ar and 40Ar/39Ar dating of young mafic volcanic rocks

K-Ar and 40Ar/39Ar ages have been measured on nine mafic volcanic rocks younger than 1 myr from the Snake River Plain (Idaho), Mount Adams (Washington), and Crater Lake (Oregon). The K-Ar ages were calculated from Ar measurements made by isotope dilution and K2O measurements by flame photometry. The 40Ar/39Ar ages are incremental-heating experiments using a low-blank resistance-heated furnace. The results indicate that high-quality ages can be measured on young, mafic volcanic rocks using either the K-Ar or the 40Ar/39Ar technique. The precision of an 40Ar/39Ar plateau age generally is better than the precision of a K-Ar age because the plateau age is calculated by pooling the ages of several gas increments. The precision of a plateau age generally is better than the precision of an isotope correlation (isochron) age for the same sample. For one sample the intercept of the isochron yielded an 40Ar/36Ar value significantly different from the atmospheric value of 295.5. Recalculation of increment ages using the isochron intercept for the composition of nonradiogenic Ar in the sample resulted in much better agreement of ages for this sample. The results of this study also indicate that, given suitable material and modern equipment, precise K-Ar and 40Ar/39Ar ages can be measured on volcanic rocks as young as the latest Pleistocene, and perhaps even the Holocene.

Quaternary Research↗

USGS2005

Integrated surface and borehole strong-motion, soil-response arrays in San Francisco, California: Empirical measurements of low-strain site coefficients at site class E and D soil sites

An integrated set of four borehole arrays and ten surface installations is installed in the city of San Francisco, California to measure the response of soft-soil deposits to strong earthquake ground motions. The borehole arrays extend through thick layers of soft water-saturated soils of Holocene age and older more consolidated soils of Pleistocene age into bedrock at depths up to 90 m. The surface installations are configured in pairs to provide simultaneous comparative surface measurements of soft soils and nearby rock. The rock locations also permit comparative measurements of rock as observed at the surface and in nearby boreholes. The arrays are designed to address a wide variety of scientific and engineering issues, and especially the issue of anelastic and nonlinear soil response at high strain levels as might be recorded during a large regional earthquake. Recordings of ground motions from the largest regional earthquakes which have occurred since the installation of the arrays show marked evidence of amplification as measured on the borehole and surface arrays. Implications of the results for low-strain site coefficients in present U.S. building codes are discussed.

Calfornia↗

USGS2021

Permafrost thaw in northern peatlands: Rapid changes in ecosystem and landscape functions

Peatlands within the northern permafrost region cover approximately 2 million km 2 and are characterized by organic soils that can be several meters thick, and a fine-scale mosaic of permafrost and non-permafrost landforms interspersed by shallow ponds and lakes. Ongoing permafrost thaw is transforming these peatlands, causing abrupt changes to their morphology, hydrology, ecology, and biogeochemistry. In this review we show how changes to individual peatlands depend on both their Holocene developmental history and their location within current permafrost zones. Permafrost thaw in peatlands often leads to land surface collapse between 0.5 and 5 m, the so-called thermokarst. Thermokarst in peatlands can lead to the development of ice-wedge troughs, waterlogged thermokarst bogs and fens, and the initiation, expansion, and drainage of thermokarst lakes. Permafrost thaw in peatlands can thus completely alter vegetation composition and shift patterns of landscape inundation and hydrological connectivity. These changes in turn have implications for magnitude and timing of runoff, downstream water quality, habitat suitability for birds and larger mammals, traditional land-use, and the exchange of greenhouse gases with the atmosphere. Ongoing permafrost thaw is largely irreversible at relevant human time-scales, and peatland thermokarst has been accelerating over the last few decades. Complete permafrost loss is expected this century for peatlands in relatively warmer permafrost zones, and all peatlands in the northern permafrost region will be profoundly transformed by permafrost thaw.

Book chapter↗

USGS2014

Paleoclimate

As host to one of the major continental-scale ice sheets, and with considerable spatial variability of climate related to its physiography and location, North America has experienced a wide range of climates over time. The aim of this chapter is to review the history of those climate variations, focusing in particular on the continental-scale climatic variations between the Last Glacial Maximum (LGM, ca. 21,000 years ago or 21 ka) and the present, which were as large in amplitude as any experienced over a similar time span during the past several million years. As background to that discussion, the climatic variations over the Cenozoic (the past 65.5 Myr, or 65.5 Ma to present) that led ultimately to the onset of Northern Hemisphere glaciation at 2.59 Ma will also be discussed. Superimposed on the large-amplitude, broad-scale variations from the LGM to present, are climatic variations on millennial-to-decadal scales, and these will be reviewed in particular for the Holocene (11.7 ka to present) and the past millennium.

Book chapter↗

USGS2011

Late quaternary climate variations reflected in Baltic Sea sediments

Late Pleistocene to Holocene climate change of the Atlantic and the northern European realm is reflected by the facies of sediments in the Baltic Sea. The sedimentary sequence have been subdivided into zones reflecting the main postglacial stages of the Baltic Sea basin development according to sediment echosounder profiling and investigating sediment cores from the central Baltic. The changes in the environment of Baltic Sea bottom water is displayed by sediment physical, geochemical, and microfossil proxies. These proxies mark the main shift in the sedimentary facies of the Baltic Basin at 8.14 cal. years BP, from a freshwater to a brackish/marine environment due to the Littorina transgression of marine water masses from the North Sea. The downhole physical facies variation from the Eastern Gotland can be correlated basinwide. Thickness maps of the freshwater and the brackish sediments ascribe the general change in the hydrographic circulation from a coast-to-basin to a basin-to-basin system along with the Littorina transgression. Variations in the salinity of the brackish Littorina Baltic Basin are attributed to changes in the North Atlantic Oscillation (NAO) ascribing the wind forces driving the inflow of marine water into the Baltic Basin. Time series analysis of facies variation reveals distinct periodicities of 900 and 1,500 years. These periods can be compared with data from North Atlantic marine sediments and Greenland ice cores identifying global climate change effects in Baltic Basin sediments.

Book chapter↗

USGS2011

Porosity variability in limestone sequences

Porosity is the state of being porous, as measured by the percentage of bulk volume of a rock or soil that is occupied by space, whether isolated or connected. In hydrocarbon-bearing limestone settings, subsurface porous strata containing the oil or gas usually underlie non-porous caprock through which hydrocarbons cannot pass. In settings, subsurface freshwater aquifers beneath caprock can become contaminated by saltwater intrusion during periods of drought. Islands of the Florida Keys consist of two types of emergent 125-ka limestone, a highly porous fossil coral reef with large voids and a less porous oolite with small grains and interstices. Both limestones are capped by impervious laminated Holocene calcrete whose dimensions differ greatly (Figure 1a and b). Porosity variability in the limestones is thought to be the cause. The less permeable oolite retained rainfall moisture longer, allowing longer periods of calcrete buildup. Reddish and brownish layers in both illustrated calcrete samples represent periods of influx of non-carbonate minerals on African dust. The hiatus or gap in these rock records represents an interval of >115 kyr during which no marine or terrestrial deposition is recorded.

Book chapter↗

USGS2010

Estimating the empirical probability of submarine landslide occurrence

The empirical probability for the occurrence of submarine landslides at a given location can be estimated from age dates of past landslides. In this study, tools developed to estimate earthquake probability from paleoseismic horizons are adapted to estimate submarine landslide probability. In both types of estimates, one has to account for the uncertainty associated with age-dating individual events as well as the open time intervals before and after the observed sequence of landslides. For observed sequences of submarine landslides, we typically only have the age date of the youngest event and possibly of a seismic horizon that lies below the oldest event in a landslide sequence. We use an empirical Bayes analysis based on the Poisson-Gamma conjugate prior model specifically applied to the landslide probability problem. This model assumes that landslide events as imaged in geophysical data are independent and occur in time according to a Poisson distribution characterized by a rate parameter λ. With this method, we are able to estimate the most likely value of λ and, importantly, the range of uncertainty in this estimate. Examples considered include landslide sequences observed in the Santa Barbara Channel, California, and in Port Valdez, Alaska. We confirm that given the uncertainties of age dating that landslide complexes can be treated as single events by performing statistical test of age dates representing the main failure episode of the Holocene Storegga landslide complex.

Book chapter↗

USGS2001

Integrated surface and borehole strong-motion, soil-response arrays in San Francisco, California

An integrated set of four borehole arrays and ten surface installations is installed in the city of San Francisco, California to measure the response of soft-soil deposits to strong earthquake ground motions. The borehole arrays extend through thick layers of softwater-saturated soils of Holocene age and older more consolidated soils of Pleistocene age into bedrock at depths up to 100 m. The surface installations are configured in pairs to provide simultaneous comparative surface measurements of soft soils and nearby rock. The rock locations also permit comparative measurements of rock as observed at the surface and in nearby boreholes. Complementary structural response arrays alsoare installed near each array. The arrays are designed to address a wide variety of scientific and engineering issues, and especially the issue of anelastic and nonlinear soil response at high strain levels. Exact anelastic models have been developed to account for contrasts in anelastic properties at boundaries and the resultant inhomogeneity of propagating wave fields. These models predict that significant amounts of energy may be trapped in soil basins with resultant larger amplifications than can be predicted using conventional homogeneous wave-field models with damping. Results of these models are discussed.

California↗

USGS2017

Application of molluscan analyses to the reconstruction of past environmental conditions in estuaries

Molluscs possess a number of attributes that make them an excellent source of past environmental conditions in estuaries: they are common in estuarine environments; they typically have hard shells and are usually well preserved in sediments; they are relatively easy to detect in the environment; they have limited mobility as adults; they grow by incremental addition of layers to their shells; and they are found in all the major environments surrounding estuaries—terrestrial, freshwater, brackish, and marine waters. Analysis of molluscan assemblages can contribute information about past changes in sea level, climate, land use patterns, anthropogenic alterations, salinity, and other parameters of the benthic habitat and water chemistry within the estuary. High-resolution (from less than a day to annual) records of changes in environmental parameters can be obtained by analyzing the incremental growth layers in mollusc shells (sclerochronology). The shell layers retain information on changes in water temperature, salinity, seasonality, climate, river discharge, productivity, pollution and human activity. Isotopic analyses of mollusc shell growth layers can be problematic in estuaries where water temperatures and isotopic ratios can vary simultaneously; however, methods are being developed to overcome these problems. In addition to sclerochronology, molluscs are important to Holocene and Pleistocene estuarine palaeoenvironmental studies because of their use in the development of age models through radiocarbon dating, amino acid racemization, uranium-thorium series dating, and electron spin resonance (ESR) dating.

Book chapter↗

USGS1992

Late Cenozoic lacustrine and climatic environments at Tule Lake, northern Great Basin, USA

Cores of lake sediment to a depth of 334 m in the town of Tulelake, Siskiyou County, northern California, document the late Cenozoic paleolimnologic and paleoclimatic history of the northwestern edge of the Great Basin. The cores have been dated by radiometric, tephrochronologic and paleomagnetic analyses. Lacustrine diatoms are abundant throughout the record and document a nearly continuous paleolimnologic history of the Tule Lake basin for the last 3 Myr. During most of this time, this basin (Tule Lake) was a relatively deep, extensive lake. Except for a drier (and cooler?) interval recorded by Fragilaria species about 2.4 Ma, the Pliocene is characterized by a dominance of planktonic Aulacoseira solida implying a warm monomictic lake under a climatic regime of low seasonality. Much of the Pleistocene is dominated by Stephanodiscus and Fragilaria species suggesting a cooler, often drier, and highly variable climate. Benthic diatoms typical of alkaline-enriched saline waters commonly appear after 1.0 Ma, and tephrochronology indicates slow deposition and possible hiatuses between about 0.6 and 0.2 Ma. The chronology of even-numbered oxygen isotope stages approximately matches fluctuations in the abundance of Fragilaria since 800 ka indicating that glacial periods were expressed as drier environments at Tule Lake. Glacial and interglacial environments since 150 ka were distinct from, and more variable than, those occurring earlier. The last full glacial period was very dry, but shortly thereafter Tule Lake became a deep, cool lacustrine system indicating a substantial increase in precipitation. Aulacoseira ambigua characterized the latest glacial and Holocene record of Tule Lake. Its distribution indicates that warmer and wetter climates began about 15 ka in this part of the Great Basin. Diatom concentration fluctuates at 41 000 year intervals between 3.0 and 2.5 Ma and at approximately 100 000 year intervals after 1.0 Ma. In the late Pliocene and early Pleistocene, Aulacoseira solida percentages wax and wane in an approximately 400 000 year cycle. The possible response of Tule Lake diatom communities to orbitally-induced insolation cycles underscores the importance of this record for the study of late Cenozoic paleoclimate change. ?? 1992 Springer-Verlag.

Climate Dynamics↗

USGS1990

Paleoclimatic implications of the high stand of Lake Lahontan derived from models of evaporation and lake level

Based on previous climate model simulations of a split of the polar jet stream during the late Pleistocene, we hypothesize that (1) 20-13.5 ka BP, season-to-season variation in the latitudinal maximum of the jet stream core led to enhanced wetness in the Great Basin, and (2) after 13.5 ka BP, northward movement of the jet stream resulted in increased aridity similar to today. We suggest that the enhanced effective wetness was due to increased precipitation combined with an energy-limited reduction in evaporation rates that was caused by increased summer cloud cover. A physically based thermal evaporation model was used to simulate evaporation for Lake Lahontan under various hypothesized paleoclimates. The simulated evaporation rates, together with hypothetical rates of precipitation and discharge, were input to a water balance model of Lake Lahontan. A 42% reduction in evaporation rate, combined with maximum historical rates of precipitation (1.8 times the mean annual rate) and discharge (2.4 times the mean annual rate), were sufficient to maintain Lake Lahontan at its 20-15 ka BP level. When discharge was increased to 3.8 times the present-day, mean annual rate, the ??? 13.5 ka BP maximum level of Lake Lahontan was attained within 1400 years. A 135-m drop from the maximum level to Holocene levels was simulated within 300 years under the imposition of the present-day hydrologic balance. ?? 1990 Springer-Verlag.

Climate Dynamics↗

USGS1992

Volcán Quizapu, Chilean Andes

Quizapu is a flank vent of the basalt-to-rhyodacite Holocene stratocone, Cerro Azul , and lies at the focus of a complex Quaternary volcanic field on the Andean volcanic front. The Quizapu vent originated in 1846 when 5 km 3 of hornblende-dacite magma erupted effusively with little accompanying tephra. Between &sim; 1907 and 1932, phreatic and strombolian activity reamed out a deep crater, from which 4 km 3 of dacite magma identical to that of 1846 fed the great plinian event of 10–11 April 1932. Although a total of >9 km 3 of magma was thus released in 86 years, there is no discernible subsidence. As the pre-plinian crater was lined by massive lavas, 1932 enlargement was limited and the total plinian deposit contains only &sim; 0.4 wt % lithics. Areas of 5-cm and 1-cm isopachs for compacted 1932 fallout are about half of those estimated in the 1930's, yielding a revised ejecta volume of &sim;9.5 km 3 . A strong inflection near the 10-cm isopach (downwind &sim;110 km) on a plot of log Thickness vs Area 1/2 reflects slow settling of fine plinian ash — not of coignimbrite ash, as the volume of pyroclastic flows was trivial (<0.01 km 3 ). About 17 vol.% of the fallout lies beyond the 1-cm isopach, and &sim; 82 wt% of the ejecta are finer than 1 mm. A least 18 hours of steady plinian activity produced an exceptionally uniform fall deposit. Observed column height (27–30 km) and average mass eruption rate (1.5x10 8 kg/s) compare well with values for height and peak intensity calculated from published eruption models. The progressive “aeolian fractionation” of downwind ash (for which Quizapu is widely cited) is complicated by the large compositional range of 1932 juvenile pumice (52–70% SiO 2 ). The eruption began with andesitic scoria and ended with basaltic scoria, but >95% of the ejecta are dacitic pumice (67–68% SiO 2 ); minor andesitic scoria and frothier rhyodacite pumice (70% SiO 2 ) accompanied the dominant dacite. Phenocrysts (pl>hb&sim;opx>mt>ilm&sim;cpx) are similar in both abundance and composition in the 1846 (effusive) and 1932 (plinian) dacites. Despite the contrast in mode of eruption, bulk compositions are also indistinguishable. The only difference so far identified is a lower range of δ D values for 1846 hornblende, consistent with pre-eruptive degassing of the effusive batch.

Bulletin of Volcanology↗

USGS1986

The evolution of young silicic lavas at Medicine Lake Volcano, California: Implications for the origin of compositional gaps in calc-alkaline series lavas

At Medicine Lake Volcano, California, the compositional gap between andesite (57-62 wt.% SiO2) and rhyolite (73-74 wt.% SiO2) has been generated by fractional crystallization. Assimilation of silicic crust has also occurred along with fractionation. Two varieties of inclusions found in Holocene rhyolite flows, hornblende gabbros and aphyric andesites, provide information on the crystallization path followed by lavas parental to the rhyolite. The hornblende gabbros are magmatic cumulate residues and their mineral assemblages are preserved evidence of the phases that crystallized from an andesitic precursor lava to generate the rhyolite lavas. The andesitic inclusions represent samples of a parental andesite and record the early part of the differentiation history. Olivine, plagioclase and augite crystallization begins the differentiation history, followed by the disappearance of olivine and augite through reaction with the liquid to form orthopyroxene and amphibole. Further crystallization of the assemblage plagioclase, amphibole, orthopyroxene, magnetite, and apatite from a high-SiO2 andesite leads to rhyolite. This final crystallization process occurs on a cotectic that is nearly horizontal in temperature-composition space. Since a large amount of crystallization occurs over a limited temperature interval, a compositional gap develops between rhyolite and high SiO2 andesite. Liquidus surfaces with shallow slopes in temperature-composition space are characteristic of several late-stage crystallization assemblages in the andesite to rhyolite compositional range. Experimentally produced plagioclase+ amphibole+orthopyroxene+magnetite and plagioclase+ augite+low-Ca pyroxene+magnetite cotectics have liquidus slopes that are nearly flat. At other calc-alkaline volcanic centers crystallization processes involving large compositional changes over small temperature intervals may also be important in the development of bimodal volcanism (i.e. the existence of a composition gap). At Mt. Mazama and Mt. St. Helens, USA and Aso Caldera and Shikotsu, Japan the amphibole-bearing assemblage was important. At Krakatau, Indonesia and Katmai, USA, an augite+orthopyroxene-bearing assemblage was important. In addition to its role in the production of a compositional gap between intermediate and rhyolitic lavas, the crystallization process increases the H2O content of the residual liquid. This rapid increase in residual liquid volatile content which results from the precipitation of a large proportion of crystalline solids may be an important factor among several that lead to explosive silicic eruptions. ?? 1986 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS1994

Paleohydrology and paleochemistry of Lake Manitoba, Canada: the isotope and ostracode records

Lake Manitoba, the largest lake in the Prairie region of North America, contains a fine-grained sequence of late Pleistocene and Holocene sediment that documents a complex postglacial history. This record indicates that differential isostatic rebound and changing climate have interacted with varying drainage basin size and hydrologic budget to create significant variations in lake level and limnological conditions. During the initial depositional period in the basin, the Lake Agassiz phase (???12-9 ka), ??18O of ostracodes ranged from -16??? to -5??? (PDB), implying the lake was variously dominated by cold, dilute glacial meltwater and warm to cold, slightly saline water. Candona subtriangulata, which prefers cold, dilute water, dominates the most negative ??18O intervals, when the basin was part of proglacial Lake Agassiz. At times during this early phase, the ??18O of the lake abruptly shifted to higher values; euryhaline taxa such as C. rawsoni or Limnocythere ceriotuberosa, and halobiont taxa such as L. staplini or L. sappaensis are dominant in these intervals. This positive covariance of isotope and ostracode records implies that the lake level episodically fell, isolating the Lake Manitoba basin from the main glacial lake. ??18O values from inorganic endogenic Mg-calcite in the post-Agassiz phase of Lake Manitoba trend from -4??? at 8 ka to -11??? at 4.5 ka. We interpret that this trend indicates a gradually increasing influence of isotopically low (-20??? SMOW) Paleozoic groundwater inflow, although periods of increased evaporation during this time may account for zones of less negative isotopic values. The ??18O of this inorganic calcite abruptly shifts to higher values (-6???) after ???4.5 ka due to the combined effects of increased evaporative enrichment in a closed basin lake and the increased contribution of isotopically high surface water inflow on the hydrologic budget. After ???2 ka, the ??18O of the Mg-calcite fluctuates between -13??? and -7???, implying short-term variability in the lake's hydrologic budget, with values indicating the lake varied from outflow-dominated to evaporation-dominated. The ??13C values of Mg-calcite remain nearly constant from 8 to 4.5 ka and then trend to higher values upward in the section. This pattern suggests primary productivity in the lake was initially constant but gradually increased after 4.5 ka. ?? 1994 Kluwer Academic Publishers.

Journal of Paleolimnology↗

USGS1978

Earthquake swarms and local crustal spreading along major strike-slip faults in California

Earthquake swarms in California are often localized to areas within dextral offsets in the linear trend in active fault strands, suggesting a relation between earthquake swarms and local crustal spreading. Local crustal spereading is required by the geometry of dextral offsets when, as in the San Andreas system, faults have dominantly strike-slip motion with right-lateral displacement. Three clear examples of this relation occur in the Imperial Valley, Coso Hot Springs, and the Danville region, all in California. The first two of these areas are known for their Holocene volcanism and geothermal potential, which is consistent with crustal spreading and magmatic intrusion. The third example, however, shows no evidence for volcanism or geothermal activity at the surface. ?? 1978 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗