Ronald I. Dorn

Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years

Abstract During the past 35,000 years, Lake Bonneville, Lake Russell, and Lake Searles underwent a major period of lake-level change. The lakes were at moderate levels or dry at the beginning of the period and seem to have achieved highstands between about 15,000 and 13,500 yr B.P. The rise of Lake Lahontan was gradual but not continuous, in part because of topographic constraints (intrabasin spill). Lake Lahontan also had an oscillation in lake level at 15,500 yr B.P. Radiocarbon-age estimations for materials that were deposited in the lake basins indicate that Lake Bonneville rose more or less gradually from 32,000 yr B.P., and had major oscillations in level between 23,000 and 21,000 yr B.P. and between 15,250 and 14,500 yr B.P. Lake Russell and Lake Searles had several major oscillations in lake level between 35,000 and 14,000 yr B.P. The timing and exact magnitude of the oscillations are difficult to decipher but both lakes may have achieved multiple highstand states. All four lakes may have had nearly synchronous recessions between about 14,000 and 13,500 yr B.P. After the recessions, the lakes seem to have temporarily stabilized or experienced a minor increase in size between about 11,500 and 10,000 yr B.P. These data provide circumstantial evidence that the Younger Dryas Event affected climate on at least a hemispheric scale. During the Holocene, the four lakes remained at low levels, and small oscillations in lake level occurred. An important aspect of the lake-level data is the accompanying expansion of lake-surface area at the time of the last highstand. Lake Bonneville and Lake Lahontan had surface areas about 10 times larger than their mean-historical reconstructed areas whereas Lake Russell and Lake Searles had surface areas about 5 times larger than their mean-historical reconstructed areas. Differences in the records of effective wetness may have been due to the locations of the basins relative to the position of the jetstream, or they may have resulted from lake/atmosphere feedback processes.

Microbial Origin of Desert Varnish

Scanning electron microscopy and energy dispersive x-ray analyses of desert varnish reveal that microorganisms concentrate ambient manganese that becomes greatly enhanced in brown to black varnish. Specific characteristics of desert varnish and of varnish bacteria support a microbial origin for manganese-rich films. Varnish microbes can be cultured and produce laboratory manganese films. Accordingly, natural desert varnish and also manganese-rich rock varnishes in nondesert environments appear to be a product of microbial activity.

Cosmogenic chlorine-36 production rates in terrestrial rocks

Abstract Chlorine-36 is produced in rocks exposed to cosmic rays at the earth surface through thermal neutron activation of 35Cl, spallation of 39K and 40Ca, and slow negative moun capture by 40Ca. We have measured the 36Cl content of 14C-dated glacial boulders from the White Mountains in eastern California and in a 14C-dated basalt flow from Utah. Effective, time-intergrated production parameters were calculated by simultaneous solution of the 36Cl production equations. The production rates due to spallation are 4160 ± 310 and 3050 ± 210 atoms 36Cl yr−1 mol−139K and 40Ca, respectively. The thermal neutron capture rate was calculated to be (3.07 ± 0.24) × 105 neutrons (kg of rock)−1 yr−1. The reported values are normalized to sea level and high geomagnetic latitudes. Production of 36Cl at different altitudes and latitudes can be estimated by appropriate scaling of the sea level rates. Chlorine-36 dating was performed on carbonate ejecta from Meteor Crater, Arizona, and late Pleistocene morainal boulders from the Sierra Nevada, California. Calculated 36Cl ages are in good agreement with previously reported ages obtained using independent methods.

Cation-ratio dating: A new rock varnish age-determination technique

Abstract Rock varnish coats many surfaces of geomorphic and archaeologic interest in arid lands. All varnish dating techniques are limited by the time lag between the exposure of a surface to subaerial processes and the onset of varnishing. They are valid only where manganese is not remobilized after deposition, for example, in most arid environments. The premise of a new age-determination method, cation-ratio dating, is that the ratio of the more mobile cations (e.g., K and Ca) to titanium in varnish decreases with time. Although there are many inherent assumptions and potential limitations, cation-ratio dating has been verified on relative age-sequences from a Death Valley debris cone, Negev Desert talus flatirons, and prehistoric lake levels at Searles Lake in California. Varnish cation ratios have been calibrated to independently dated surfaces in the Coso volcanic field and vicinity in California. Tentative absolute dates have been assigned to geomorphic surfaces in the Coso area. Cation ratios have been used to distinguish relative ages of archaeologic artifacts in southwestern North America and to demonstrate that varnish at the South Stoddard locality, Mojave Desert, did not form in 25 yr.

Direct measurement of the combined effects of lichen, rainfall, and temperature onsilicate weathering ☆

Abstract A key uncertainty in models of the global carbonate–silicate cycle and long-term climate is the way that silicates weather under different climatologic conditions, and in the presence or absence of organic activity. Digital imaging of basalts in Hawaii resolves the coupling between temperature, rainfall, and weathering in the presence and absence of lichens. Activation energies for abiotic dissolution of plagioclase (23.1 ± 2.5 kcal/mol) and olivine (21.3 ± 2.7 kcal/mol) are similar to those measured in the laboratory, and are roughly double those measured from samples taken underneath lichen. Abiotic weathering rates appear to be proportional to rainfall. Dissolution of plagioclase and olivine underneath lichen is far more sensitive to rainfall.

Age and geomorphic history of Meteor Crater, Arizona, from cosmogenic 36Cl and 14C in rock varnish

Abstract Using cosmogenic 36 Cl buildup and rock varnish radiocarbon, we have measured the exposure age of rock surfaces at Meteor Crater, Arizona. Our 36 Cl measurements on four dolomite boulders ejected from the crater by the impact yield a mean age of 49.7 ± 0.85 ka, which is in excellent agreement with an average age of 49 ± 3 ka obtained from thermoluminescence studies on shock-metamorphosed dolomite and quartz. These ages are supported by undetectably low 14 C in the oldest rock varnish sample.

Accelerator mass spectrometry radiocarbon dating of rock varnish

Accelerator mass spectrometry (AMS) 14C dating off rock varnish provides minimum-limiting ages for landforms and archaeological artifacts in arid and semiarid lands that are undatable by conventional radiocarbon methods. Experiments on sample collection, the effect of different individuals preparing samples, the influence of different chemical-extraction procedures, the incorporation of carbonate detritus, possible contamination from rock underlying varnish, the role of bio-geochemical erosion of varnish, and other influences reveal procedures that yield reproducible results. Conventional radiocarbon dates from charcoal beneath lava flows of Hualalai Volcano, Hawaii, and from arid sites in western North America provide controls to test varnish radiocarbon dating. AMS radio-carbon ages on the very bottom layer of varnishes are typically ≤10% younger than the 14C ages of these controls. Applications are illustrated here for fluvial, aeolian, peri-glacial, hillslope, lacustrine, and glacial geo-morphology, as well as rock-art research in archaeology.

Geomorphic and paleoclimatic implications of latest Pleistocene radiocarbon dates from colluvium-mantled hollows, California

Radiocarbon analyses of charcoal from basal colluvium in 11 California hollows show a clustering of dates between 9000 and 15,000 B.P., an indication that changes in the storage and discharge of colluvium from hillslopes accompanied the Pleistocene-Holocene transition. Hollows are sites of topographically induced convergence and deposition of colluvial debris, and evacuation of this debris was apparently more thorough and possibly more frequent in the latest Pleistocene, perhaps due to a combination of changes in vegetation and rainfall characteristics. One hypothesis is that greater storm intensities occurred in the latest Pleistocene and induced a higher frequency of landslides in hollows and a regional extension of channel heads upslope relative to the Holocene. During the last Pleistocene, California hollows apparently contained smaller volumes of colluvium in storage. The increased storage of debris during the Holocene may have resulted in a diminished supply of sediment to stream channels.

Radiocarbon and cation-radio ages for rock varnish on Tioga and Tahoe marainal boulders of Pine Creek, eastern Sierra Nevada, California, and their paleoclimatic implications

Abstract Accelerator mass spectrometry 14 C analyses of organic matter extracted from rock varnishes on morainal boulders yield limiting minimum ages for three crests of the Tioga glaciation. At Pine Creek in the eastern Sierra Nevada, varnish started to form on boulders of the outermost Tioga moraine before 19,000 yr B.P., and varnish originated on the innermost Tioga moraine before 13,200 yr B.P. Comparisons with lake-level, paleohydrological, paleoecological, colluvial, and rock varnish micromorphological data indicate that central-eastern California and western Nevada experienced a moisture-effective period during the late Pleistocene but after the Tioga maximum, and perhaps as Tioga glaciers receded from the mouth of Pine Creek canyon. Varnishes on Tahoeage morainal boulders at Pine Creek have cation-ratio ages of about 143,000–156,000 yr B.P., suggesting that the Tahoe glaciation should not be correlated with oxygen-isotope stage 4 in the early Wisconsin, but rather with stage 6. Varnishes on morainal boulders of an older glaciation at Pine Creek are dated by cation ratio at about 182,000–187,000 yr B.P.

Cation-ratio and accelerator radiocarbon dating of rock varnish on Mojave artifacts and landforms

The first accelerator radiocarbon dates of rock varnishes are reported along with potassium/argon ages of lava flows and conventional radiocarbon dates of pluvial lake shorelines, in an empirical calibration of rock varnish K+ + Ca2+/Ti4+ ratios with age in the Mojave Desert, eastern California. This calibration was used to determine the cation-ratio dates of 167 artifacts. Although cation-ratio dating is an experimental method, some dates suggest human occupation of the Mojave Desert in the late Pleistocene.