Geochronology and paleoenvironment of pluvial Harper Lake, Mojave Desert, California

The Mojave River has provided a record of Southern California paleoclimate changes over the last 500 ka in the paleolacustrine deposits of pluvial lakes Harper, Manix, Cronese, and Mojave. Previous studies at Harper Lake yielded uncalibrated radiocarbon ages ranging from >30--24 ka for the 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. In this study, geologic mapping, radiocarbon, and optically stimulated luminescence data indicate the Harper Lake highstand deposits are between 45.6 +/- 3.1 ka and 27.9 +/- 1.7 ka. Ostracode assemblages are consistent with inflow of Mojave River water and indicative of wetland and beach environments associated with a filling Harper Lake. Based on the estimated surface areas of Harper Lake and Lake Manix, the Mojave River could fill and maintain both lakes simultaneously by doubling the present day average annual discharge of 9.5 x 106 m3 and decreasing evaporation to 0.4 times present day conditions. 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 and that the Mojave River could have flowed simultaneously into Harper and Manix lakes at that time.