Geology topics

L. Benson

Publications and source records attributed to L. Benson.

14 recordsLinked to original sources

USGS2017

Factors Affecting 14C Ages of Lacustrine Carbonates: Timing and Duration of the Last Highstand Lake in the Lahontan Basin

Two processes contribute to inaccurate 14C age estimates of carbonates precipitated within the Lahontan basin, NevadaCalifornia: low initial 14C/C ratios in lake water (reservoir effect) and addition of modern carbon to calcium carbonate after its precipitation. The mast reliable set of 14C ages on carbonates from elevations > 1310 m in the Pyramid and Walker Lake subbasins indicate that lakes in all seven Lahontan subbasins coalesced ???14,200 14C yr B.P. forming Lake Lahontan. Lake Lahontan achieved its 1330-m highstand elevation by ???13,800 14 C yr B.P. and receded to 1310 m by ???13,700 14C yr B.P. Calculations, based on measured carbonate-accumulation rates, of the amount of time Lake Lahontan exceeded 1310 and 1330 m (500 and 50 yr) are consistent with this chronology. The timing of the Lake Lahontan highstand is of interest because of the linkage of highstand climates with proximity to the polar jet stream. The brevity of the Lahontan highstand is interpreted to indicate that the core of the southern branch of the polar jet stream remained only briefly over the Lahontan basin.

Quaternary Research↗

USGS2017

A revised chronology for the last Pleistocene Lake cycle in the central Lahontan Basin

Radiocarbon dates of plant materials from packrat middens in caves below the elevation of the last high stand of Pleistocene Lake Lahontan, in conjunction with radiocarbon dates of ancient archaeological materials, provide evidence that the last high stand terminated before 12,070 yr B.P. This new information suggests that the last major fluctuation in the level of Lake Lahontan was approximately synchronous with those of Lakes Bonneville and Russell, and that these three lakes probably responded contemporaneously to the same regional paleoclimatic changes. ?? 1986.

Quaternary Research↗

USGS2017

Uranium-series age estimates and paleoclimatic significance of Pleistocene tufas from the Lahontan basin, California and Nevada

An extended chronology of Lahontan basin lake levels based on uranium-series age estimates correlates with the global ice-volume record. Lake highstands occur at or shortly after times of maximum ice-sheet size. Moderate size lakes occur when the global ice volume is about 80% of its maximum. The data indicate that lake levels rise and fall relative to the proximity of the mean position of the jetstream. When the continental ice sheet is above some threshold size or shape, it appears that the large-scale circulation and climatic conditions are right for producing lakes of moderate to large size within the Lahontan basin. ?? 1988.

Quaternary Research↗

USGS2007

Simulation and observations of annual density banding in skeletons of Montastraea (Cnidaria: Scleractinia) growing under thermal stress associated with ocean warming

We present a model of annual density banding in skeletons of Montastraea coral species growing under thermal stress associated with an ocean-warming scenario. The model predicts that at sea-surface temperatures (SSTs) <29??C, high-density bands (HDBs) are formed during the warmest months of the year. As temperature rises and oscillates around the optimal calcification temperature, an annual doublet in the HDB (dHDB) occurs that consists of two narrow HDBs. The presence of such dHDBs in skeletons of Montastraea species is a clear indication of thermal stress. When all monthly SSTs exceed the optimal calcification temperature, HDBs form during the coldest, not the warmest, months of the year. In addition, a decline in mean-annual calcification rate also occurs during this period of elevated SST. A comparison of our model results with annual density patterns observed in skeletons of M. faveolata and M. franksi, collected from several localities in the Mexican Caribbean, indicates that elevated SSTs are already resulting in the presence of dHDBs as a first sign of thermal stress, which occurs even without coral bleaching. ?? 2007, by the American Society of Limnology and Oceanography, Inc.

Limnology and Oceanography↗

USGS2005

New data for Late Pleistocene Pinedale alpine glaciation from southwestern Colorado

New cosmogenic surface-exposure ages of moraine-crest boulders from southwestern Colorado are compared with published surface-exposure ages of boulders from moraine complexes in north-central Colorado and in west-central (Fremont Lake basin) Wyoming. 10Be data sets from the three areas were scaled to a single 10Be production rate of 5.4 at/g/yr at sea level and high latitude (SLHL), which represents the average 10Be production rate for two high-altitude, mid-latitude sites in the western United States (US) and Austria. Multiple nuclide ages on single boulders indicate that this 10Be production rate yields ages comparable to those calculated with a commonly used 36Cl production scheme. The average age and age range of moraine-crest boulders on terminal moraines at the southwestern Colorado and Wyoming sites are similar, indicating a retreat from their positions ???16.8 36Cl ka (Cosmogenic ages in this paper are labeled 10Be or 36Cl ka or just ka when both 10Be or 36Cl ages are being discussed; radiocarbon ages are labeled 14C ka, calibrated radiocarbon are labeled cal ka, and calendar ages are labeled calendar ka. Errors (??1??) associated with ages are shown in tables. Radiocarbon ages were calibrated using the data of Hughen et al. (Science 303 (2004) 202). This suggests a near-synchronous retreat of Pinedale glaciers across a 470-km latitudinal range in the Middle and Southern Rocky Mountains. Hypothetical corrections for snow shielding and rock-surface erosion shifts the time of retreat to between 17.2 and 17.5 10Be ka at Pinedale, Wyoming, and between 16.3 and 17.3 36Cl ka at Hogback Mountain, Colorado. ?? 2004 Elsevier Ltd. All rights reserved.

Quaternary Science Reviews↗

USGS2004

The probable importance of snow and sediment shielding on cosmogenic ages of north-central Colorado Pinedale and pre-Pinedale moraines

Eight uncorrected 36Cl ages for Pinedale boulders in north-central Colorado fall in the range 16.5 to 20.9 kyr. 10Be age determinations on four of five boulders are in close agreement (???6% difference) with 36Cl determinations. Hypothetical corrections for snow shielding increased the 36Cl ages of Pinedale boulder surfaces by an average of ???12%. Most ages for pre-Pinedale (Bull Lake) boulders fall within marine-isotope stage (MIS) 5, a time when continental and Sierran ice accumulations were small or nonexistent. Under the assumption that these boulders were deposited on moraines that formed before the end of MIS 6 (???140 kyr BP), calculations indicated that rock-surface erosion rates would have had to range from 5.9 to 10.7 mm kyr-1 to produce the observed 36Cl values. When compared to rates that have been documented for the past 20 kyr, these erosion rates are extremely high. Snow shielding accounts for 0-48% of the additional years needed to shift pre-Pinedale dates to MIS 6. This suggests that some combination of snow shielding, sediment shielding, or 36Cl leakage has greatly decreased the apparent ages of most pre-Pinedale boulders. Inability to account for the effects of these processes seriously hinders the use of cosmogenic ages of pre-Pinedale boulders as estimators of the timing of alpine glaciation.

Quaternary Science Reviews↗

USGS2003

Ancient maize from Chacoan great houses: Where was it grown?

In this article, we compare chemical ( 87 Sr/ 86 Sr and elemental) analyses of archaeological maize from dated contexts within Pueblo Bonito, Chaco Canyon, New Mexico, to potential agricultural sites on the periphery of the San Juan Basin. The oldest maize analyzed from Pueblo Bonito probably was grown in an area located 80 km to the west at the base of the Chuska Mountains. The youngest maize came from the San Juan or Animas river floodplains 90 km to the north. This article demonstrates that maize, a dietary staple of southwestern Native Americans, was transported over considerable distances in pre-Columbian times, a finding fundamental to understanding the organization of pre-Columbian southwestern societies. In addition, this article provides support for the hypothesis that major construction events in Chaco Canyon were made possible because maize was brought in to support extra-local labor forces.

Proceedings of the National Academy of Sciences of↗

USGS2003

Age of the Mono Lake excursion and associated tephra

The Mono Lake excursion (MLE) is an important time marker that has been found in lake and marine sediments across much of the Northern Hemisphere. Dating of this event at its type locality, the Mono Basin of California, has yielded controversial results with the most recent effort concluding that the MLE may actually be the Laschamp excursion (Earth Planet. Sci. Lett. 197 (2002) 151). We show that a volcanic tephra (Ash #15) that occurs near the midpoint of the MLE has a date (not corrected for reservoir effect) of 28,620 ?? 300 14C yr BP (??? 32,400 GISP2 yr BP) in the Pyramid Lake Basin of Nevada. Given the location of Ash #15 and the duration of the MLE in the Mono Basin, the event occurred between 31,500 and 33,300 GISP2 yr BP, an age range consistent with the position and age of the uppermost of two paleointensity minima in the NAPIS-75 stack that has been associated with the MLE (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009). The lower paleointensity minimum in the NAPIS-75 stack is considered to be the Laschamp excursion (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009).

Quaternary Science Reviews↗

USGS2002

Holocene multidecadal and multicentennial droughts affecting Northern California and Nevada

Continuous, high-resolution δ 18 O records from cored sediments of Pyramid Lake, Nevada, indicate that oscillations in the hydrologic balance occurred, on average, about every 150 years (yr) during the past 7630 calendar years (cal yr). The records are not stationary; during the past 2740 yr, drought durations ranged from 20 to 100 yr and intervals between droughts ranged from 80 to 230 yr. Comparison of tree-ring-based reconstructions of climate change for the past 1200 yr from the Sierra Nevada and the El Malpais region of northwest New Mexico indicates that severe droughts associated with Anasazi withdrawal from Chaco Canyon at 820 cal yr BP (calendar years before present) and final abandonment of Chaco Canyon, Mesa Verde, and the Kayenta area at 650 cal yr BP may have impacted much of the western United States.During the middle Holocene (informally defined in this paper as extending from 8000 to 3000 cal yr BP), magnetic susceptibility values of sediments deposited in Pyramid Lake's deep basin were much larger than late–Holocene (3000–0 cal yr BP) values, indicating the presence of a shallow lake. In addition, the mean δ 18 O value of CaCO 3 precipitated between 6500 and 3430 cal yr BP was 1.6‰ less than the mean value of CaCO 3 precipitated after 2740 cal yr BP. Numerical calculations indicate that the shift in the δ 18 O baseline probably resulted from a transition to a wetter (>30%) and cooler (3–5°C) climate. The existence of a relatively dry and warm middle-Holocene climate in the Truckee River–Pyramid Lake system is generally consistent with archeological, sedimentological, chemical, physical, and biological records from various sites within the Great Basin of the western United States. Two high-resolution Holocene-climate records are now available from the Pyramid and Owens lake basins which suggest that the Holocene was characterized by five climatic intervals. TIC and δ 18 O records from Owens Lake indicate that the first interval in the early Holocene (11,600–10,000 cal yr BP) was characterized by a drying trend that was interrupted by a brief (200 yr) wet oscillation centered at 10,300 cal yr BP. This was followed by a second early-Holocene interval (10,000–8000 cal yr BP) during which relatively wet conditions prevailed. During the early part of the middle Holocene (8000–6500 cal yr BP), high-amplitude oscillations in TIC in Owens Lake and δ 18 O in Pyramid Lake indicate the presence of shallow lakes in both basins. During the latter part of the middle Holocene (6500–3800 cal yr BP), drought conditions dominated, Owens Lake desiccated, and Lake Tahoe ceased spilling to the Truckee River, causing Pyramid Lake to decline. At the beginning of the late Holocene (∼3000 cal yr BP), Lake Tahoe rose to its sill level and Pyramid Lake increased in volume.

Quaternary Science Reviews↗

USGS2002

HIBAL: A hydrologic-isotopic-balance model for application to paleolake systems

A simple hydrologic-isotopic-balance (HIBAL) model for application to paleolake ??18O records is presented. Inputs to the model include discharge, on-lake precipitation, evaporation, and the ??18O values of these fluid fluxes. Monthly values of climatic parameters that govern the fractionation of 18O and 16O during evaporation have been extracted from historical data sets and held constant in the model. The ability of the model to simulate changes in the hydrologic balance and the ??18O evolution of the mixed layer has been demonstrated using measured data from Pyramid Lake, Nevada. Simulations of the response in ??18O to step- and periodic-function changes in fluid inputs indicate that the hydrologic balance and ??18O values lag climate change. Input of reconstructed river discharges and their ??18O values to Pyramid and Walker lakes indicates that minima and maxima in simulated ??18O records correspond to minima and maxima in the reconstructed volume records and that the overall shape of the volume and ??18O records is similar. The model was also used in a simulation of abrupt oscillations in the ??18O values of paleo-Owens Lake, California.

Quaternary Science Reviews↗

USGS1996

Carbonate deposition, Pyramid Lake subbasin, Nevada: 3. The use of 87Sr values in carbonate deposits (tufas) to determine the hydrologic state of paleolake systems

Sierran rivers that discharge to the Lahontan basin have much lower (∼4.5‰) δ 87 Sr values than the Humboldt River which drains northeastern Nevada. The δ 87 Sr values of tufas deposited during the last lake cycle were used to determine when Humboldt derived Sr entered the Pyramid Lake subbasin. Prior to ∼15,000 yr B.P., the Humboldt River flowed to the Smoke Creek-Black Rock Desert subbasin. During the recession of Lake Lahontan, the Humboldt River diverted to the Carson Desert subbasin. This study has demonstrated that 87 Sr can be used to determine drainage histories of multi-basin lake systems if the δ 87 Sr values of rivers that discharge to the basins are sufficiently different.

Nevada↗

USGS1995

Carbonate deposition, Pyramid Lake subbasin, Nevada: 2. Lake levels and polar jet stream positions reconstructed from radiocarbon ages and elevations of carbonates (tufas) deposited in the Lahontan basin

Most of the tufas in the Pyramid Lake subbasin were deposited within the last 35,000 yr, including most of the mound tufas that border the existing lake. Many of the older tufas (> 21,000 yr B.P.) contained in the mounds were formed in association with ground-water discharge. The radiocarbon ( 14 C) ages of the older tufas represent maximum estimates of the time of their formation. Lake Lahontan experienced large and abrupt rises in level at ∼22,000, 15,000, and 11,000 yr B.P. and three abrupt recessions in level at ∼16,000, 13,600, and 10,000 yr B.P. The lake-level rises that were initiated at ∼23,500 and 15,500 yr B.P. are believed to indicate the passage of the polar jet stream over the Lahontan basin. During expansion of the Laurentide Ice Sheet, the jet stream moved south across the basin, and during the contraction of the Ice Sheet, the jet stream moved north across the basin. The bulk of the carbonate contained in the mound tufas was deposited during the last major lake cycle (∼23,500–12,000 yr B.P.), indicating that ground- and surface-water discharges increased at ∼23,500 and decreased at ∼12,000 yr B.P. A lake-level oscillation that occurred between 11,000 and 10,000 yr B.P. is represented by a 2-cm thick layer of dense laminated tufa that occurs at and below 1180 m in the low-elevation tufa mounds and at 1205 m in the Winnemucca Lake subbasin.

Nevada↗

USGS1994

Carbonate deposition, Pyramid Lake subbasin, Nevada: 1. Sequence of formation and elevational distribution of carbonate deposits (Tufas)

During the late Quaternary, the elevation of terrace cutting and carbonate deposition in the Pyramid Lake subbasin were controlled by constancy of lake level imposed by spill to adjoining subbasins. Sill elevations are 1177-1183 m (Mud Lake Slough Sill), 1207 m (Emerson Pass Sill), and 1265 m (Darwin Pass Sill). Carbonate deposition was favored by: (1) hydrologic closure, (2) proximity to a source of calcium, (3) elevated water temperature, and (4) a solid substrate. The thickness and aspect of tufa are a function of lake-level dynamics. Relatively thin sheets and pendant sheets were deposited during a rising or falling lake. The upper parts of thick reef-form tufas have a horizontal aspect and were deposited in a lake which was stabilized by spill to the Carson Desert subbasin. The lower parts of the reef-form tufas are thinner and their outer surface has a vertical aspect, indicating that the lower part formed in a receding lake. The thickest and most complete sequences of tufa are mounds that border the Pyramid Lake shore. The tops of the tallest mounds reach the elevation of the Darwin Pass Sill and many mounds have been eroded to the elevations of the Mud Lake Slough Sill of the Emerson Pass Sill. The sequence of tufa formation (from oldest to youngest) displayed in these mounds is: (1) a beachrock containing carbonate-cemented volcanic cobbles, (2) broken and eroded old spheroids that contain thinolitic tufa and an outer rind of dense laminated tufa, (3) large cylindrical (tubular) tufas capped by (4) coatings of old dense tufas, and (5) several generations of old branching tufa commonly associated with thin, platy tufas and coatings of thinolitic tufa, (6) young spheroids that contain poorly oriented young thinolitic tufa in the center and several generations of radially oriented young thinolitic tufas near the outer edge, (7) a transitional thinolite-to-branching tufa, (8) two or more layers of young branching tufa, (9) a 0.5-cm-thick layer of fine-grained dolomite, (10) a 2-cm-thick layer of young dense laminated tufa, (11) a 0.1-cm-thick layer of encrusting tufa that was covered by a beach deposit and (12) a 1.0-cm-thick layer of porous encrusting tufa that coated the beach deposit and the sides of tufa mounds. The elevational ranges of the principal varieties of tufa are not related to terrace or spill elevations. It seems likely that the distribution of tufa varieties is related to variation in the thermal structure or chemistry of Lake Lahontan. It is hypothesized that the thinolitic tufa (ikaite) formed in the near freezing hypolimnion and the braching tufa (calcite) formed in the seasonally warmer epiliminion of the lake.

Nevada↗

USGS1992

Lake-size variations in the Lahontan and Bonneville basins between 13,000 and 9000 14C yr B.P

Recessions of Lakes Lahontan and Bonneville that commenced ∼13,500 14 C yr B.P. were interrupted at ⪖11,500 14 C yr B.P. in the Lahontan basin and ∼12,200 14 C yr B.P. in the Bonneville basin by relatively large perturbations in lake level that persisted for ∼ 2000 years. Minor glacial readvances in the Sierra Nevada and White Mountains of California-Nevada occurred during the latter half of this interval (between 11,000 and 9700 14 C yr B.P.). The hydrologic response of Lakes Lahontan and Bonneville and the mountain glacial advances were concurrent with the Allerød/Younger Dryas climatic intervals recorded in vegetational and glacial records of western and central Europe.

Nevada, Utah↗