BLACK MATS, SPRING-FED STREAMS, AND LATE-GLACIAL-AGE RECHARGE IN THE SOUTHERN GREAT-BASIN

Black mats are prominent features of the late Pleistocene and Holocene stratigraphic record in the southern Great Basin. Faunal, geochemical , and sedimentological evidence shows that the black mats formed in se veral microenvironments related to spring discharge, ranging from wet meadows to shallow ponds. Small land snails such as Gastrocopta tappan iana and Vertigo berryi are the most common mollusk tars present. Semi aquatic and aquatic taxa are less abundant and include Catinellids, Fo ssaria parva, Gyraulus parvus, and others living today in and around p erennial seeps and ponds. The ostracodes Cypridopsis okeechobi and Sco ttia tumida, typical of seeps and low-discharge springs today, as well as other taxa typical of springs and wetlands, are common in the blac k mats. Several new species that lived in the saturated subsurface als o are present, but lacustrine ostracodes are absent. The delta(13)C va lues of organic matter in the black mats range from -12 to -26 parts p er thousand, reflecting contributions of tissue from both C-3 (sedges, most shrubs and trees) and C-4 (saltbush, saltgrass) plants. Carbon-1 4 dates on the humate fraction of 55 black mats fall between 11,800 to 6300 and 2300 C-14 yr B.P. to modern. The total absence of mats in ou r sample between 6300 and 2300 C-14 yr B.P. likely reflects increased aridity associated with the mid-Holocene Altithermal. The oldest black mats date to 11,800-11,600 C-14 yr B.P., and the peak in the C-14 bla ck mat distribution falls at similar to 10,000 C-14 yr B.P. As the for mation of black mats is spring related, their abundance reflects refil ling of valley aquifers starting no later than 11,800 and peaking afte r 11,000 C-14 yr B.P. Reactivation of spring-fed channels shortly befo re 11,200 C-14 yr B.P. is also apparent in the stratigraphic records f rom the Las Vegas and Pahrump Valleys. This age distribution suggests that black mats and related spring-fed channels in part may have forme d in response to Younger Dryas (YD)-age recharge in the region. Howeve r, the inception of black mat formation precedes that of the YD by at least 400 C-14 yr, and hydrological change is gradual, not rapid. (C) 1998 University of Washington.