TRACE METAL RECORDS OF RAPID ANOXIA DEVELOPMENT IN A MEDITERRANEAN SATELLITE BASIN DURING THE PRE-EVAPORITIC MESSINIAN STAGE

Black shales alternating with marl/limestone/dolomite layers occur in a 10.3 m section below the first evaporite cycle of the Gessoso-solfifera Formation in the Vena-del-Gesso basin in the Northern Apennines, Italy. In this study we utilize a suite of redox-sensitive trace metals (Mo, V, U, Cr, Mn) as the primary investigative tool in order to assess the paleo- redox conditions in the basin leading up to the Messinian Salinity Crisis (MSC). Our assays indicate that Mo (~9 ppm) and U (~120 ppm) are significantly enriched (EF of 3 and 25, respectively) compared to reported average shale values. In contrast, Mn (~34ppm), V (~1.0ppm) and Cr (~ 3.2ppm) levels are at or below average shale values. Although no significant relationships between the trace metals are observed, a systematic enrichment sequence (V→Cr→Mn→Mo→U) can be discerned. The trace metal data suggest that the basin evolved rapidly (approx. 100,000 yrs or less) from euxinic to anoxic to suboxic conditions that culminated with the deposition of primary evaporite cycles of the MSC. The evolving redox conditions in the Vena-del-Gesso Basin were likely caused by a coupling of upward shoaling related to tectonic uplift with contemporaneous water body stratification related to regional aridification. The redox evolution stages of the Mediterranean satellite basin, imprinted in the trace metal records, can serve as a useful analog for anoxia development in much larger Precambrian basins that underwent similar redox transitions.