THE SEARCH FOR MICROORGANISMS IN BRINE INCLUSIONS IN HALITE: AN UPDATE

Studies of ancient microorganisms inside crystals done in conjunction with sedimentological and geochemical characterization of the host rocks provides information on the timing (surface versus burial diagenetic) and physical setting (temperature and brine chemistry from fluid inclusions) of crystal growth and microorganism entrapment. An integrated geobiological approach has been used to study halites from the Permian Salado Formation, New Mexico, and the Silurian Salina Formation (F-Salt), Michigan.

The recent report on the cultivation of a 250-million-year-old bacterium from the Salado salts has been challenged because Bacillus species 2-9-3 was extracted from a coarse, clear halite cement of indeterminate age and ambiguous origin. New work on these Salado halite cements from the same location (depths of 569 m) shows that they formed syndepositionally by evaporation of seawater-derived groundwater brines at temperatures of 17º to 30º C. Although the timing of cement precipitation is ambiguous petrographically, the chemistry of the fluid inclusions, when plotted on ion-ion diagrams, shows that they follow the evaporation curve of Permian seawater, which confirms a near surface origin in which concentration of brines was driven by evaporation.

The search for bacteria in the Salina Formation salts has so far been negative. The major ion chemistry of fluid inclusions in halite from the F-Salt shows that Silurian seawater was different from modern seawater, relatively depleted in Mg, SO4, and Na, and enriched in Ca. Fluid inclusions from these halites were cultured in seawater with Silurian chemistry. However, incubation of samples at 30º and 40º C may have killed any viable bacterial populations. Fluid inclusion homogenization temperatures done by microthermometry shows that the halite crystallized at surprisingly low temperatures, between 5º and 25º C, and an average of 14º C. New samples are currently being cultured using seawater of Silurian chemistry at 15º C.