Sedimentation rates control trace element composition of sedimentary phosphorites: Anomalously low uranium and cadmium levels in Paleozoic shelly phosphorites from the Baltica Paleobasin

Abstract

Phosphorites – made up of chemically precipitated authigenic apatite or composed of bioapatitic fossil remains – become enriched in a variety of trace elements such as Rare Earth Elements (REE), Cd, and U during diagenesis. The Cambrian-Ordovician phosphorites of the Baltica Paleobasin that are almost exclusively made up of fossilized remains of phosphatic brachiopod shells are, however, exceptional in this regard, showing abnormally low levels of trace elements. In this study, we examined the depositional environment and the trace elemental composition of the Baltic Paleobasin shelly phosphorites. We show that hydrodynamic conditions during deposition exert control on the trace elemental composition of shelly phosphorites. A combination of high sedimentation rates, redox conditions, and the biogenic origin of the shells led to the depletion of bio-toxic Cd and U relative to other similar phosphorite deposits. Similarly, the concentrations of REE are below the global phosphorites average; however, the shelly phosphorites show a higher degree of Middle Rare Earth Element (MREE) enrichment. These findings have further implications for exploration — phosphorites that contain abundant bioapatite and/or were deposited in areas with high sedimentation rates and oxic conditions are likely less enriched in trace elements.