CAN FLUID INCLUSIONS IN EOLIAN GYPSUM BE USED TO INTERPRET PAST LAKE CONDITIONS, CHEMISTRY, AND MICROBIOLOGY?

Chemical sediments form by precipitation in surface saline waters, but can be physically reworked and become clastic saline grains. Eolian gypsum sand is known from Earth and Mars. Is information stored in clastic gypsum that can be used to interpret past lake environmental conditions, including water chemistry and microbiology?

Here, we present preliminary data from three modern terrestrial gypsum lakes and adjacent dunes and sand flats: (1) acid saline ephemeral Lake Aerodrome in Cowen Basin in southern Western Australia; (2) acid Salars Gorbea and Ignorado in the high Andes Mountains of northern Chile; and (3) alkaline Lake Lucero and Alkali Flat at White Sands National Monument and White Sands Missile Range in New Mexico. We first compared petrographic observations of modern eolian gypsum sands with parent lake gypsum to document provenance of the eolian gypsum. At all three locations, there are several gypsum types upwind from dunes and salt flats, including depositional and diagenetic forms. Surface-water bottom-growth gypsum is characterized by distinctive growth bands of primary fluid inclusions that are not seen in displacive gypsum or vein selenite. Therefore, presence of growth bands in eolian grains serves as an indicator that those are reworked bottom-growth lake gypsum.

Fluid inclusions in reworked bottom-growth lake gypsum have been examined with transmitted light and UV-vis petrography, microthermometry, and laser Raman microscopy. The phases in primary fluid inclusions suggest basic lake conditions. For example, all-liquid primary inclusions indicate low-temperature waters. Solids and/or gas bubbles in fluid inclusions can be remnants of salts and/or gases in past lakes. Microorganisms and organic compounds are found within fluid inclusions. Freezing-melting microthermometry is used to estimate salinity and major ions in solution. Laser Raman spectroscopy has been successfully used to identify organic compounds within fluid inclusions, despite the strong Raman signature of the host gypsum. These data can be used to reconstruct lake characteristics.

Fluid inclusions in eolian gypsum can be proxies for past saline lake waters. This work provides an impetus for a closer look at eolian gypsum grains on Mars to better evaluate past martian lake hydrology and possible martian life.