COMPARATIVE ANALYSIS OF PYRITE FRAMBOIDS FROM LATE DEVONIAN BLACK SHALES

The diameter of pyrite framboids is widely used as a proxy to infer bottom water oxygen conditions. Framboid characteristics including average size, standard deviation, maximum size and abundance reflect the location of the redox boundary with respect to the sediment water interface. The goal of this research is to collect pyrite framboid size data in association with independent redox proxies in Late Devonian black shales to both compare proxies and to analyze the depositional environments. Characterizing depositional conditions through this interval has significance as this interval preserves one of the largest mass extinctions in the history of life. Pyrite framboid, trace metal (Mo, U, V/V+Ni) and trace fossil data were collected for twelve samples. Results from eleven of the samples demonstrate small average framboid size, the absence of trace fossils, and enriched values of Mo, U and V/V+Ni, which are consistent with deposition under anoxic conditions. Interestingly, one sample contains a low abundance of framboids, consistent with the oxic conditions implied by trace metal and ichnofabric data for this sample, while the small size of the framboids suggests euxinic conditions. This result could be explained by deposition under rapidly fluctuating bottom water oxygen conditions and supports the importance of using multiproxy methods to gain a more complete description of ancient depositional conditions.