Pyrite framboids associated with the Mesozoic Jehol Biota in northeastern China:Implications for microenvironment during early fossilization

Despite of the recent influx of reports describing important fossil specimens from the Mesozoic Jehol Biota, detailed in-formation on the mechanism of fossilization and paleoenvironment in which these fossils were deposited is scanty. We present an analysis ofmicroenvironment based upon scanning electron microscope observations of in situ pyrite framboids and microcrystallines of plant and ver-tebrate feather fossils in the Jehol Biota. Pyrite microcrystallines and framboids occur extensively inside and on surface of plant fossils.Framboids found on feathers and in sedimentary matrix were in a lower abundance. These framboids have diameters ranging from 6 μm to31μm with an average of 20μm, indicating a dysoxic aqueous condition with free oxygen level less than 30μmol/L for the microenviron-ment where these framboids were formed. The outgrowth of framboids inside plant tissues suggests the presence of water molecules andfree oxygen at the cellular level during pyritization; the relative timing betwee