OCR Text |
Show associated with dry periods ( mudflat deposits 0 - 10 ka and 60 - 120 ka and saline pan sediments 186 - 192 ka). In contrast, scarce CaS04- bearing minerals but relatively abundant calcite ( 20- 50% of the > 1 micron size insoluble fraction) are associated with halite and mud layers from wetter periods ( 10 - 60 ka and 120 - 186 ka). Different mixing ratios of inflow waters between wet and dry periods are interpreted to be responsible for the relationship between mineral assemblages and climate. Modern Death Valley, representative of dry periods, is characterized by Na- Cl- S04 brines, produced by mixing two basic types of inflow waters: 1) Na- HC03- rich and Na- Cl- S04- HC03- rich meteoric waters from the Amargosa River, springs, and groundwaters from northern and central Death Valley; and 2) Na- Ca- Cl- rich springs and groundwaters from southern Death Valley, possibly related to volcanism, hydrothermal activity, and a 15 km- deep magma body. During dry periods, relatively abundant Ca- rich spring inflow removes HC03 as calcite during the early stages of brine evolution. Further evaporative concentration produces gypsum and glauberite from the remaining Ca. During wetter periods, increased discharge of meteoric HC03- rich Amargosa River water and basin- margin spring waters removes most Ca from the brine via precipitation of calcite, and, with low calcium concentrations, CaS04- minerals are not abundantly formed during further evaporative concentration. Such Ca- poor, Na- Cl- S04- rich brines precipitate thenardite ( Na2S04) during later stages of brine evolution. The paleoclimate record of Death Valley from core DV- 93 is dominated by 2 cold/ wet and warm/ dry cycles that occur on a 100 ka time scale. Of particular note are the major lacustrine phases 10 - 35 ka and 120 - 186 ka ( oxygen isotope stages 2 and 6); the penultimate glacial lake was deeper and far longer- lasting than the last glacial lake. The Death Valley climate record can be divided into 6 periods: ( 1) 0- 10 ka: dry arid conditions featuring mudflat/ saline pan environments, minor inflow of meteoric waters, and a water table normally below the surface. ( 2) 10 - 35 ka: cold wet climate with plentiful inflow that fed a perennial lake, at times up to 90 m deep. Overall, for this 25 ka period, fluid inclusion T^^ x data suggest brine temperatures depressed 4° to 15° C below modern values. Of note is that during the latest lake phase, 11- 15 ka, ThMAx values of halites are only 4° C below modem values, whereas T^^ j^ values decrease significantly, to 7°- 8° C below the modern, in halites from approximately 10- 11 ka. ( 3) 35 - 60 ka: cold climate, with moderate aridity, dominated by saline pans, but with enough inflow to accumulate thick salts relatively rapidly, and maintain shallow saline lakes approximately 50 - 54 ka. Homogenization temperatures ( Ty^ x between 23° and 28° C) are 6° to 11° C below the modern. Of note is the relatively low temperature interval, 10° C below the modern, at approximately 47 to 50 ka. ( 4) 60 - 120 ka: dry period dominated by mudflat environments. Limited data from fluid inclusions in two halite intervals suggest generally cool conditions, 6° to 7° C below the modem, approximately 80 and 92 ka. However, T^^ values from 10 halite intervals around 100 ka are close to the modern, which suggests a warm, arid climate not unlike the modern at 100 ka. ( 5) 120 - 186 ka: cold climate, with abundant inflow to continually maintain a perennial lake, at times > 300 m deep. Of note is the period, 146 - 186 ka, which was particularly cold ( TVMAX 7° to 20° C below the modern), and, based on high shoreline tufa ages, commonly had lakes hundreds of meters deep. The latest lake phase, 120- 128 ka, resulted from reduced inflow, which ultimately led to the desiccation of the lake at 120 ka. Warmer temperatures, comparable the modern T^^ x, mark the terminal lake stage at 120 ka. ( 6) 186 - 192 ka: warm climate, dominated by saline pans and shallow ephemeral lakes. Inflow was sufficient to allow the rapid accumulation of 25 m of salt in 6 ka. T^^ x values are 30° C or greater for 8 of 12 stratigraphic intervals, indicating temperatures similar to or slightly below the modern. |