FOSSILIZED FILAMENTOUS BACTERIA FROM THE EOCENE GREEN RIVER FORMATION OF WYOMING: EVIDENCE SUGGESTING A BACTERIAL GENESIS OF SIDERITE

The discovery of exquisitely preserved filamentous bacteria from the Green River Formation, Wyoming suggests a relationship between bacteria and the formation of the iron carbonate, siderite (FeCO₃). Comparative mineralogical studies of the Green River Formation have suggested previously undefined controlling mechanisms for siderite precipitation/distribution. Most abundant at the of the Tipton/Wilkins Peak boundary, siderite has been characterized geochemically as part of a drying and concentrating episode of Lake Gosiute. This period in the lake’s history would have had a high potential for increased alkalinity and available ionic iron leached from volcaniclastic sources. Siderite is known to precipitate from saturated solutions which are in equilibrium with CO₂ gas or contain a fixed amount of carbonate ions. Another possibility for siderite formation that has been suggested is a bacterial origin involving the reduction of iron in anaerobic sediments. Certain modern filamentous bacteria for example, Liptothrux and Crenothrix, are characterized by flocculent masses of hydrated ferric hydroxide collected on a bacterial sheath. As the bacteria shed their old sheath and grow a new one, they leave behind an iron-rich bacterial framework which might serve as a center for siderite nucleation. Eocene Age fossilized bacteriamorphs from the Tipton/Wilkins Peak boundary, which bear an uncanny resemblance to their modern counterparts, have been identified by Scanning Electron Microscopic (SEM) suggesting a possible validation for a bacterial genesis of siderite.