RESPONSES OF EVAPORITE MINERALOGY TO INFLOW WATER SOURCES AND CLIMATEDURING THE PAST 100 KY IN DEATH-VALLEY, CALIFORNIA

The mineralogy of cored sediments from Badwater Basin, Death Valley, C alifornia, provides information on water inflow sources and climate ov er the past 100 k.y. Abundant glauberite (Na2SO4 . CaSO4) and gypsum ( CaSO4 . 2H(2)O), and relatively small amounts of calcite are associate d with mudflat deposits from dry periods from 0 to 10 ka and 60 to 100 ka. In contrast, scarce CaSO4-bearing minerals but relatively abundan t calcite are associated with halite and mud layers from 10 to 60 ka, including the 25 ka wet period when Death Valley contained a perennial lake (10 to 35 ka). Different mixing ratios of inflow waters between wet and dry periods are interpreted to be responsible for the relation ship between mineral assemblages and climate. Modern Death Valley, rep resentative of dry periods, is characterized by Na-Cl-SO4 brines, prod uced by mixing two basic types of inflow waters: (1) Na-HCO3-rich and Na-Cl-SO4-HCO3-rich meteoric waters from the Amargosa River, springs, and ground waters from northern and central Death Valley; and (2) Na-C a-Cl-rich springs and ground waters from southern Death Valley, possib ly related to volcanism, hydrothermal activity, and a 15-km-deep magma body. During dry periods, relatively abundant Ca-rich spring inflow r emoves HCO3 as calcite during the early stages of brine evolution. Fur ther evaporative concentration produces gypsum and glauberite from the remaining Ca. During wetter periods, increased discharge of meteoric HCO3-rich Amargosa River water and basin-margin spring waters removes most Ca from the brine via precipitation of calcite, and, with low cal cium concentrations, CaSO4-minerals are not abundantly formed during f urther evaporative concentration. Such Ca-poor, Na-Cl-SO4-rich brines precipitate thenardite (Na2SO4) during later stages of brine evolution .