THE POINT GRATIOT BED GEOCHEMICAL PARADOX: EVIDENCE FOR A GLOBAL OCEAN METAL SINK?

The Upper Devonian Point Gratiot Bed (PGB), which occurs near the top of the Hanover Shale in the Appalachian Basin, which represents the Upper Kellwasser extinction event, exhibits a geochemical paradox where geochemical characteristics suggest opposing bottom water redox conditions during deposition. Continuous samples through this decimeter-thick black shale bed were collected from numerous exposures along a 100 km outcrop belt in western New York, and were analyzed for major and trace element concentration, total organic carbon (TOC) content and pyrite framboid diameter analysis. Enrichment in Molybdenum (Mo) and uranium (U) indicates anoxic to euxinic conditions. These values are highest in the base of the PGB and decrease upsection suggesting the most euxinic portion of the PGB occurred in the lower portion of the bed. Conversely, TOC, and total pyrite concentrations increase, while pyrite framboid diameters decrease upsection. TOC preservation and pyrite production may increase with increasing anoxia. Moreover, small framboids are associated with accumulation under an euxinic water column. This indicates that bottom water anoxia/euxinia increased from the base to the top of the PGB, which directly conflicts with the interpretation of Mo and U enrichment profiles. A plausible explanation for the decreasing Mo and U values is a sink for these metals in the global ocean. Widespread anoxia/euxinia in the global ocean would have led to the uptake of these metals by abyssal muds leaving a water mass depleted in metals to circulate into epeiric seas. As water in the epeiric seas became increasingly anoxic/sulfidic, the preservation of organic carbon and the production of framboidal pyrite would increase without an attendant rise in Mo and U.