Sedimentary pyrite: A window into the microbial past

Abstract

Microbes are ubiquitous in modern sediments, and must have been a similarly common constituent in the past. After death, however, they degrade readily and usually do not become part of the rock record. Especially for our understanding of early earth history and the evolution of life, however, finding preserved cellular remains has been critical. Verified microbial remains from early earth history (Awramik et al., 1983) have all been reported from sediments that experienced very early cementation by fine crystalline quartz (Schopf and Walter, 1983). The quartz cement renders these rocks (cherts) transparent and allows examination of microbial fossils in the context of the rock matrix (3). Because such cherts are rare, the microbial record of early as well as later earth history is still poorly known. Pyrite, another early diagenetic mineral that forms as a result of microbial processes in sediments, has so far received little attention as a source of microbial fossils. Pyrite grains ranging in age from Archean to Jurassic were examined by scanning and transmission electron microscope, and most of them show coccoid, rod-shaped, and even filamentous features that are interpreted as microbial. Although pyrite represents a much more common sedimentary mineralization than chert, its opaque nature has in the past rendered the search for contained microbial remains very difficult. The identification of microbial remains in sedimentary pyrite opens up the prospect to greatly expand our knowledge of microbial life in very old sediments, as well as allowing us much more detailed analysis of microbial life throughout Earth history. Sedimentary pyrite grains may also represent a good prospect to find traces of past life on Mars.

What Do We Really Know about the Role of Microorganisms in Iron Sulfide Mineral Formation?

“…Our understanding of the evolution of the Earth's surface environment is mainly based on analyses of iron sulfides preserved in ancient or modern sediments. Additionally the presence of pyritized organisms in the sedimentary record of Archaean to Jurassic age implies a potential role for Fe sulfides in the preservation of micro-and macrofossils (Sagemann et al, 1999;Grimes et al, 2001;Schieber, 2002;Cosmidis et al, 2013;Vietti et al, 2015). Deciphering the role of microorganisms in the formation of sulfide minerals is therefore crucial to better constrain the evolution of the biogeochemical cycles of Fe, S, and C.…”

Microfacies Analysis and Resrvoir Potential of Sakesar Limestone, Nammal Gorge (Western Salt Range), Upper Indus Basin, PakistanZain

“…The sedimentary diagenetic pyrite is formed by microbial processes under reducing conditions. Pyrite grains of age ranging from Archean to Jurassic have been examined by scanning and transmission electron microscopes, mostly showed the coccoid, rod shaped and filamentous features which are interpreted as microbial (Schieber, 2002). The pyrite formation takes place close enough to the sediment surface, when the pore spaces are being teemed with microbes (Canfield and Raiswell, 1991;Bird et al, 2001).…”

Fossil associations from the middle and upper Eocene strata of the Pamplona Basin and surrounding areas (Navarre, western Pyrenees)

“…Microbial mats can greatly enhance the preservation of surface traces (i.e., Marty et al, 2009;Carmona et al, 2012). The reducing chemical conditions beneath the microbial mats may have contributed to the formation of "anoxic" minerals, such as pyrite (Schieber, 1999(Schieber, , 2002, which has also been observed in the surface of some slabs of the Liédena Sandstone. These anoxic conditions could explain the scarcity and low diversity of invertebrate traces in the Liédena Sandstone.…”