FROM LAGERSTÄTTE TO LAG: PRELIMINARY BEDDING-SCALE TAPHONOMIC AND GEOCHEMICAL ANALYSIS OF PHOSPHATE DISTRIBUTION IN THE CINCINNATIAN

Phosphate particles in the Upper Ordovician (Cincinnatian) of the Cincinnati region are common, originate as <0.5 mm pore fillings in bioclasts, and are visibly concentrated in some grainstones. These grainstones developed as shells accumulated during periods of low sediment input, episodic winnowing, and obrution. This suggests that an organism initially buried alive would have been repeatedly reworked and mixed with freshly-buried organics as the shell bed evolved from lagerstätte to “lag”. Theoretically, the burial of organic material could have introduced phosphorus (P) into the sediment, with specific redox conditions for phosphate precipitation provided by decay-induced micro Eh gradients. This iterative burial-exhumation process may have enhanced phosphogenesis.

We tested this hypothesis by examining samples from the Maysvillian Fairview and Corryville formations. The samples represented the gamut of the shell-bed “maturity” spectrum; 1) obrution deposits, 2) slightly-winnowed shelly packstones, and 3) comminuted-shell grainstones. The distribution of P and other elements was determined by scanning XRF analysis of polished slabs and by SEM/EDS analysis of polished thin sections.

These analyses revealed interesting relationships between fossils and P. Live-buried fossils and calcareous concretions formed around such fossils are enriched in P compared to surrounding silt or mud. This P is associated with concentric rings of pyrite and/or Mn. Winnowed shell beds contain visibly recognizable grains with far higher P concentrations than those of articulated remains, yet some disarticulated columnals contain similarly high concentrations of P only in the stereom around the lumen.

These observations suggest that obrution of macrofossils contributed and/or fixed P in the sediment, and that P was mobile after deposition. The concordance between P and pyrite or Mn confirms that Eh gradients influenced precipitation. The fact that visibly-high concentrations of P are limited to disarticulated remains may signify that phosphogenesis progresses over cycles of winnowing, but the similarly high concentration of P in the lumen regions of some disarticulated columnals hints that the process was initiated in the earliest stages of decay when organic-rich lumenal tissues were still present.