MICROORGANISMS IN THE NEOPROTEROZOIC BROWNE FORMATION HALITE OF AUSTRALIA?

Bedded halite is a documented preserver of microbial life. The Neoproterozoic (815 + 15 Ma) Browne Formation of central Australia contains undeformed chevron halite hosting primary fluid inclusions. Here, we describe petrographic and laser Raman spectroscopic evidence of microbial life in the primary fluid inclusions of the Browne Formation bedded halite.

Petrography at a magnification range up to 2000x with transmitted and UV-vis light was used to characterize suspect life forms. There appear to be a diversity of suspect microbial life that ranges in color, size, and fluorescent response. There are several distinct types of small isolated (0.25 to 2 µm) spherules. There are colorless cocci and clumps of brown cocci (0.25 to ~1 µm) that fluoresced pale blue, as well as light brown cocci (~1-2 µm) that did not fluoresce. These small spherules are likely coccoid prokaryotes. Slightly larger (~5-10 µm), round, pale-yellow spherules that fluoresce pale blue, are consistent in appearance with Dunaliella algae. A third type of microbial suspect was found trapped along growth bands in bedded halite; these are relatively large (~100-600 µm), opaque, cubic solids that fluoresce bright blue and may be similar to “hairy blob” microbial communities found in modern and ancient halites. Lack of fluorescent response of some materials may signal biodegradation due to great age.

A laser Raman microprobe equipped with a 532 nm green laser was used to examine microbial suspects. Raman spectra of some Browne Formation solids contain broad peaks for disordered graphite at ~1380 and ~1550 cm^-1. No other Raman signatures for organic compounds were detected in these samples.

It is possible that these suspect prokaryotes and Dunaleilla algae identified in the Browne Formation are in a dormant state. Future research may also further our understanding of the long-term preservation potential of halite. The Neoproterozoic Browne Formation may offer a biological window into microbial life evolution prior to the Cambrian explosion.