OSTRACODES AND FOSSIL GROUNDWATER DISCHARGE DEPOSITS IN THE AMERICAN SOUTHWEST

Scientists are investigating the utility of fossil groundwater deposits in the American Southwest as archives of climate change in desert environments. Approximately 130 fossil groundwater deposits have been recently mapped in the Mojave Desert alone, in addition to deposits in southern Nevada and southern Arizona, yet few of these deposits have been intensively studied. Current detailed studies are focusing on deposits in the Mojave Desert, CA, and at Murray Springs, AZ.

Groundwater deposits form where water tables intersect the earth's surface. The resulting wetlands create diverse habitats that are exploited by a variety of ostracode species. Ostracode species distributions are dictated by the physical and chemical characteristics of the host water. Key ostracode species inhabit discrete water chemistries, and ostracode assemblages can readily differentiate between lacustrine, fluvial, and spring-discharge environments. Preliminary investigations at Valley Wells, 40 km northeast of Baker, CA, revealed the presence of Cavernocypris wardi, an ostracode with strong affinities for cold, dilute spring discharge. Investigations at Lake Dumont (the most distal basin along the Mojave River drainage, approximately 60 km northwest of Baker, CA) revealed that the late Pleistocene (18-28 ka) sediments preserved in that basin are groundwater deposits rather than lacustrine sediments as originally interpreted. Lake Dumont sediments contain ostracodes indicative of a high Ca:Alk groundwater-derived chemistry (e.g., Cyprideis beaconensis) in contrast to all other pluvial lake sediments in the Mojave River drainage which contain ostracodes indicative of Mojave River-derived, low Ca:Alk water chemistries (e.g., Limnocythere ceriotuberosa). Oxygen isotope ratios (del 18O) in ostracode valve calcite are being studied at a well dated, Late Pleistocene- to early Holocene-aged groundwater deposit at Murray Springs, 8 km east of Sierra Vista, AZ, in an effort to detect changes in the del 18O of spring discharge across the Pleistocene-Holocene boundary in southern Arizona.

Fossil groundwater deposits are potential sources of paleoclimate information in the deserts of the American Southwest, augmenting existing pluvial lake, pack-rat midden, and speleothem records.