HOLOCENE WETLAND RECORDS FROM THE MOJAVE DESERT

The Desert Southwest is at risk of increased frequency of droughts due to climate change, but we know little about the possible effects on springs and seeps critical to desert ecosystems. We obtained several short (5-6 m) sediment cores from two transects along the margin of Soda Lake playa near Baker, Calif., where active springs and wetlands grade into a dry playa that is part of the floodway of the Mojave River. These cores record alternations between lacustrine, playa, wetland, phreatophyte-flat, and distal-fan conditions, suggesting significant changes through time in the extent or distribution of high water tables and wetlands. We obtained 13 ¹⁴C dates on the sediments using bulk organic samples, plant fragments, and mollusk shells. All but two samples, now considered to be young roots, are in correct stratigraphic order. One transect, located on a phreatophyte flat, showed a transition upward from lacustrine deposits at the base to organic-rich wetland muds prior to ~10,900 cal yr B.P. The wetland persisted until sometime after 9,900 cal yr B.P. and then transitioned to phreatophyte-flat and then to distal fan and playa deposits, indicating a drop in the local water table. Phreatophyte-flat conditions then returned for some time, after which aeolian sand accumulated at the surface. The second transect extended from a bio-mound adjacent to active seeps downslope to the modern playa. Lacustrine sediments at the base of these cores have ages from ~13,000 to 9,650 cal yr B.P., and are overlain by organic, possibly lagoonal sediments and then by playa deposits at the lower site and by marsh deposits at the higher site. These in turn transition upward into phreatophyte-flat deposits prior to ~1000 cal yr B.P. and then to wetland deposits ~200 cal yr B.P. Ostracode species support the depositional settings interpreted from sedimentology. The cores suggest a complicated history of groundwater table fluctuations during the Holocene. In coming months, we will acquire more continuous cores to further examine pollen, ostracodes, and other parameters to better define changes in Holocene paleohydrologic conditions in this area of the Mojave. Such information can be directly applied to develop models of wetland extent and effects on wildlife habitat under future climatic conditions, which should be beneficial to land managers.