GEOCHRONOLOGY AND PALEOENVIRONMENT OF PLUVIAL HARPER LAKE, MOJAVE DESERT, CALIFORNIA

The Mojave River is a well-known recorder of Southern California paleoclimate with past terminations in the pluvial (upstream to downstream) Harper (Harper basin), Manix (Afton, Coyote and Troy basins), Cronese (East and West basins) and Mojave (Soda and Silver basins) lakes over the last 40 ka. Previous studies at Harper Lake yielded uncalibrated radiocarbon ages ranging from >30-24 ka for the lake highstand. The present hypothesis is that the Mojave River: 1) flowed simultaneously into Harper and Manix lakes at ~30 ka; 2) the river then flowed exclusively into Lake Manix from 28-25 ka; 3) then, resumed simultaneous flow into Harper and Manix lakes, forming the Harper Lake highstand at ~25 ka; 4) after which the river bypassed Harper basin and the lake receded. Being upstream and consisting of a single basin without internal sills, the paleohydrology of Harper Lake is comparatively uncomplicated. Here we present geologic mapping, radiocarbon and OSL ages from Harper basin. We describe a 2.1 m continuous stratigraphic section from near the highstand elevation. The section rests non-conformably on quartz monzonite, and is comprised of interbedded lacustrine sands and silts capped by 0.6 m of carbonate mud. Anodonta californiensis and ostracodes (Limnocythere, Candona, Cypridopsis, and Heterocypris) occur primarily in the upper and lower portions of the section. Cross-bedded sand and rare, reworked ostracodes characterize the middle portion of section. IRSL ages date depositional phases of the Harper Lake highstand to between 45.6 ± 3.1 ka and 27.9 ± 1.7 ka, with calibrated radiocarbon ages verifying this chronology from 40.2 ± 0.9 ka to 33.6 ± 0.3 ka. Dunes on Harper playa have OSL ages of 0.8 ± 0.7 ka and 1.3 ± 0.7 ka, which indicate Harper playa has not experienced significant flooding during historic times. The ages, facies sequence, sedimentary structures, and ostracodes, combined with a lack of pedogenesis suggest a single transgressive-regressive sequence at Harper Lake between ~45 ka and ~28 ka with no second highstand at 25 ka, as previously hypothesized. The IRSL and radiocarbon data suggest that the Mojave River flowed simultaneously into Harper and Manix lakes between ~45 ka and ~28 ka, similar to the Truckee River (Pyramid and Winnemuca Lakes) in the past and the present-day Mojave River (Soda and Cronese basins).