| OCR Text |
Show and Candona Caudata, which, although not abundant, indicate that the salinity of the Death Valley lakes in which the ostracodes lived was typically below 10,000 ppm, and at times, less than 3000 ppm for the salinity sensitive species Candona Caudata. Less saline lake phases are inferred from ostracode occurrences at 10 m, 14 m, and 18 m ( approximately 16 ka, 25 ka, and 34 ka) for the last glacial lake and 113- 116 m, 125- 126 m, 138- 143 m, 157 m, and 162 m ( approximately 121- 123 ka, 127- 129 ka, 146- 154 ka, 179 ka, and 186 ka, for the long- lived penultimate glacial lake. Shoreline carbonate tufas provide information on the depths of Pleistocene lakes in Death Valley. Shoreline tufas, most prominant approximately 90 m above sea level, form horizontal terraces encrusting bedrock on the eastern side of Death Valley ( Black Mountains, between Badwater and Mormon Point). 13 samples of the high tufa have been dated by uranium- series methods. 9 of the 13 samples are 150 - 185 ka, which correlates with deep lake sediments observed in core DV- 93 from 128 to 186 ka, and which therefore serves as an independent check confirming the reliability of the dating methods. These tufas indicate lake depths of up to 330 m, ignoring faulting. 4 high shoreline tufas, dated between 194 and 216 ka, are older than any sediments from DV- 93; these oldest tufas suggest the existence of yet a third perennial lake stage in Death Valley approximately 200 ka. Petrographic and geochemical studies indicate that the tufas have not recrystallized nor have they undergone major later cementation, both of which could complicate interpretation of age dates. Samples consist of porous fine grained carbonate as ( 1) downward radiating " bushes" composed of micrite, ( 2) concentrically- banded " ball" structures composed of micrite, and less commonly radial sparry calcite or radial fibrous calcite fans. Most samples are quite porous with relatively minor calcite cement Elemental mapping of samples for Mg and Sr using the electron microprobe indicates that chemical zonation of these elements coincides with textural and mineralogical ( calcite and aragonite) zones in the tufas, which confirms their unrecrystallized primary origin. The paleotemperature component of the climate record is provided by maximum homogenization temperatures ( TUMAX) of fluid inclusions in subaqueous halites from core DV- 93, which give maximum brine temperatures during salt crystallization. TiMAX may be used for paleoclimate interpretations based on the close relationship between saline lake temperatures and mean air temperatures from modern settings. Halite samples from 64 stratigraphic intervals, 0- 184 m ( 0- 192 ka) commonly have fluid inclusion homogenization temperatures below the modern ThMAx ° f 34° ( halite precipitation, late April 1993). Lacustrine halites deposited during the last glacial period, 10- 35 ka, have very low fluid inclusion maximum homogenization temperatures ( ThMAx= 19- 30° C), which suggests brine temperatures approximately 4° C to 15° C below modern values. Ephemeral saline lake halites precipitated 35 to 60 ka have T^^^ between 23° C and 28° C, 6- 11° C below modern values. Relatively high T^^ x values, up to 34° C and 35° C, occur in halite formed approximately 100 ka and 120 ka, in a climate regime similar to the modern. Generally colder conditions are recorded in fluid inclusions in halite for the 120- 186 ka perennial lake sequence. T^^^ values are particularly low ( 14° C to 27° C, 7° to 20° C below the modern) in halite from five stratigraphic intervals between 139 m and 161 m, covering the glacial period between 146 and 186 ka. A return to higher Ty^ x values, similar to the modern, occurs in the bottom 25 meters of core DV- 93 ( 164 to 184 m, 186 to 192 ka): 8 of 12 stratigraphic intervals have Ty^ x values greater than or equal to 30° C. Mineralogy of the Death Valley core provides information on water inflow sources and climate over the past 200 ka. Abundant glauberite ( CaS04Na2S04) and gypsum ( CaS04.2H20) and relatively small amounts of calcite ( 13- 35% of the > 1 micron size insoluble fraction) are |
