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

The Mojave River is a well-known recorder of Southern California paleoclimate with a complex paleohydrology and past terminations in pluvial (upstream to downstream) Harper (Harper basin), Manix (Afton, Coyote & Troy basins), and Mojave (Soda & Silver basins) lakes over the last 30,000 years. Previous studies yielded uncalibrated radiocarbon ages ranging from 24 to >30 ka yrs BP for highstand lake deposits near 656 m elevation. Based on several studies, the present hypothesis is that the Mojave River: 1) flowed simultaneously into Harper and Manix lakes ~30 ka; 2) the river then flowed exclusively into Manix Lake 28-25 ka; 3) then, resumed simultaneous flow into Harper and Manix lakes, forming the Harper Lake highstand ~25 ka; 4) the Mojave River ceased flowing into Harper basin and the lake receded. Being upstream and consisting of a single basin without internal sills, pluvial Harper Lake is relatively uncomplicated compared to the other terminal basins. Here we present geologic mapping (1:12,000), a measured stratigraphic section and radiocarbon ages from the Red Hill area. The 2.1-m-thick continuous stratigraphic section is near the highstand elevation and comprised of interbedded sand, silt and silty sand capped by a 0.6-m-thick sequence of carbonate mud resting nonconformably on quartz monzonite. Lacustrine sediments contain four shell horizons (Anodonta californiensis) and ostracodes (genera Limnocythere, Candona, and Heterocypris). Each shell horizon was sampled yielding seven calibrated radiocarbon ages ranging from 33,645 ± 343 to 40,155 ± 924 cal yrs B.P. Our radiocarbon ages and the continuity of the section support a single Harper Lake highstand between 40 and 33 ka with no subsequent hiatus or second highstand at 25 ka as previously hypothesized. Preliminary ostracode analysis yields a tentative interpretation that Harper Lake water may have been periodically less saline than other Mojave River-fed pluvial lakes. These tentative observations suggest that Harper Lake overlaps phases of Lake Manix.