Molecular Fossils

What are molecular fossils? Products of altered organic matter Mainly formed by reduction, but oxidation possible Preservation over geologic time periods Lipids Pigments Membranes Indicators of life Have unique structural and isotopic characteristics Due to lipid bilayer fluidity and membrane stability

Examples: Type I Kerogen Typically lacustrine paleoenvironment Produce liquid hydrocarbons: oil

Examples: Types II & III Kerogen Type II Mainly marine algae Produce oil and gas Type III Terrestrial land plants Produce gas and coal

What are they good for? Unique to biological processes Often can point specific organisms Species, abundance, distribution etc. Identify origins of lipid or bituminous OM Source of coal beds, oil reservoirs, etc. Used in many branches of science Paleobiology, paleoecology Geology, geochemistry Astrobiology

Uses across the sciences Paleobiology, paleoecology Earliest evolution of life Clues to paleoenvironments Can be controversial Geology, geochemistry Medicine Astrobiology Mars Titan, Europa Extrasolar planets?

Uses of Microfossils in Geology To determine the source of sedimentary organic matter To Reconstruct Paleoenvironment To correlate stratigraphic layers

What is a biomarker? A molecule that indicates the presence of past/present living organisms. Unlike physical fossils, molecular fossils represent some of the organism’s specific chemical composition. Typically these molecular fossils are dated based on the age of the rock they are found in.

How to Study Biomarkers Molecules are typically separated from one another using gas or high powered liquid chromatography, then run through a mass spectrometer. The surrounding rock is dated using radiometric dating coupled with stratigraphic age determinants. These methods are typically fairly accurate, but sometimes the sample could be contaminated by the influx of organic material/chemicals that change the perceived age of the sample. Such as the formation of Hydrocarbons later in the rock’s lifespan. In Oil- bearing Shales. Lamellar bedding can be made up of granule alga chippings. When examine under a microscope, these round or elliptical granule diameters measure 2-6 μm

Paleoenvironmental Reconstructon Methylsteranes can be controlled by the depositional environment 3-methylsteranes are concentrated in saline/near shore lacustrine sediments, moderate in shallow, and poor in deep lacustrine sediments. (Relative to 4- methylsteranes).

Dongying Depression The varying vertical abundance of the methylsteranes indicated a shift from saline to brackish lacustrine in the Dongying Depression. (Wang et al., 2010)

Molecular Fossils as Biomarkers Pilbara Craton Hopanes (used to improve plasma membrane strength – sterols in eukaryotes) were found in shales in the Pilbara Craton (W. Aus). The abundance of 2α-Methylhopanes determined that cyanobacteria were the main primary producers. indicating that oxygenic photosynthesis has evolved since 2.7 Ga. (Brocks et al. 2003)

Proterozoic Oxygen levels The discovery of many biomarkers for cyanobacteria, eukaryotes, purple, and green sulphur bacteria give forms of evidence that Protereozoic Oxygen levels were much below those of today. (Brocks et al. 2005)

Molecular Fossils in Source Rocks Ratio of Steranes/hopanes indicates the input of eukaryotic (algae) vs prokaryotic (bacteria) organisms to source rocks. High concentration of steranes and high Sterane/hopane (>1) = marine organic matter Low concentration Steranes and low Sterane/hopane = terrigenous organic matter. Typically marine organic matter produce choice source rocks. (Wang et al., 2010)