ABSTRACT
Analyses of fluid inclusions in halite allow direct comparison of ancient surface temperatures and those from modern environments. These data provide a means of assessing the absolute magnitude of past temperature changes. Modern salt pans on the floor of Death Valley, California, are among the hottest, driest environments on earth. However, several ancient shoreline terraces, beach bars and tufa deposits attest to the existence of a relatively deep, perennial lake which occupied the basin in the past. A core taken from the Badwater Basin salt pan in 1993 penetrates interbedded salt and mud deposited during the last 200 ka. These sediments are deposits of saline mudflat, ephemeral lake-salt pan and perennial lake environments.
Halite deposited in the ephemeral lake-salt pan and perennial lake environments contains fluid inclusions, samples of brine from which the salt grew, trapped during the growth of the crystals. These fluid inclusions contain samples of ancient surface brines, which can be analyzed for stable isotope and major element composition and also act as tiny thermometers, preserving a record of ancient temperatures.
Temperature data from fluid inclusions in halite which formed in 1993 compare favorably with temperatures measured in the field as the salt precipitated. The temperature range for fluid inclusions in halite of the salt pan/ephemeral lake sediments (192 to 186 ka) is similar. High frequency (diurnal) variations in temperature are up to 16°C, and temperature tends to rise irregularly through beds. These data indicate that conditions during deposition of the salt were similar to modern, with the salt pan flooding in the winter or early spring and desiccating during spring to summer, but that temperatures were cooler than for the modern.
Halite of the perennial lake sequence (186 to 120 ka) also contains fluid inclusions. Hexagonal halite crystals, probable pseudomorphs after the low temperature mineral hydrohalite, are also present. High frequency (diurnal) temperature variations are about 5°C, and temperature decreases upward through beds. Data indicate spring floods, followed by evaporation and formation of salt during late summer. Some beds show an increase, followed by a decrease in temperature, probably a result of lower spring runoff, allowing earlier onset of halite precipitation during evaporation. Although there is no perennial lake in Death Valley at present, data obtained from fluid inclusions in halite are compared with air and brine temperatures from modern lakes. Comparison of these records allows us to quantify past temperature changes in Death Valley. The data point to a decrease of at least 10°C in both maximum summer and minimum winter temperature when the perennial lake existed.
