PRESERVED FOSSIL BACTERIA IN THE EARLIEST MARINE PHOSPHORITE DEPOSITS: 1.85 BILLION YEAR-OLD BARAGA GROUP, MICHIGAN

Bacteria are found in modern sedimentary phosphate mineral grains and their fossils mark analogs throughout the Phanerozic. Bacteria are important in the phosphorus cycle; they break down organic compounds, release phosphorus, and lead to precipitation of sedimentary apatite that can encapsulate them. It is unclear how this relationship developed in the early oceans and what role oxygen played in these processes. To understand this relationship, we examined phosphorites of the 1.85 billion year-old Baraga Group, Upper Peninsula, Michigan. This shallow marine succession contains one of the earliest marine phosphorite deposits that formed approximately coincident with the end of a major period of iron formation deposition and a hypothesized oceanic transition to sulfidic deep water.

Rod, spherical, and filamentous fossil bacteria forms were identified using SEM within phosphatic crusts lining fractures and within stromatolites. The fossil bacteria may be internal casts, and are 500 nm in length--consistent with the size of modern bacteria (340 nm to 200 µm). Many rod-shaped forms observed here have “bent” and/or “beaded-chain” morphologies, as well as occurring in clusters or swarms. Inorganic subhedral to euhedral francolite crystals surround the bacteria casts, indicating that the bacteria predate the formation of these crystals and may have acted as nucleation sites. These are the oldest reported fossil bacteria preserved within sedimentary phosphate minerals; their presence in supratidal to shallow subtidal environments suggests that bacteria may have played a fundamental role in fixing phosphate in marine sediments as atmospheric oxygen levels reached a point where iron oxyhydroxides could transport phosphorus to the seafloor. Before this relationship developed phosphorus may have remained bound to organic compounds and remained dissolved in seawater.