The lundgreni event in the Silurian of the East European Platform, Poland

Abstract

The lundgreni event (late Wenlock, Silurian) is here interpreted in terms of palaeontologic (graptolites, microphytoplankton, benthos), biostratigraphic and geochemical evidence derived from Homerian deposits drilled in the Bartoszyce well (East European Platform in Poland). The lundgreni event in the graptolite record of Bartoszyce comprises extinction, survival and recovery phases. The graptolite extinction proceeded in three steps: lundgreni, testis and flemingii-dubius. The testis extinction eliminated the largest number of species. Only the long-lasting Pristiograptus dubius survived the combined extinctions and was joined during the survival phase by the opportunistic Gothograptus nassa and Pristiograptus parvus. The graptolite recovery is expressed by the diachronous arrival of new species in association with P. dubius. All documented phytoplankton taxa survived the lundgreni event. Phytoplankton frequency and diversity patterns, together with total organic carbon (TOC) fluctuations, indicate that oligotrophy/productivity regime was unsteady during the testis and flemingii-dubius extinctions and stable during the early graptolite survival. There is no evidence for extinction within benthic fauna assemblages. Diversified benthos appeared in a great abundance during the early survival phase. This records a redox-related event of substrate colonization that took place simultaneously with local decline in primary productivity. The last two phases of graptolite extinction and the onset of the survival correlate with strong, positive anomalies of δ 18O, δ 13C org and δ 13C carb. The anomalies coincide with the fall in local primary productivity and the enhanced storage of organic carbon in peri-Gondwanan upwelling zones (Sudeten, Thuringia). The stepped graptolite extinction in Bartoszyce occurred in an environment that was situated within the low-latitude Baltica shelf and was markedly stable in terms of bathymetry, distance from land and oxic regime. This local extinction was most likely caused by catastrophic fluctuations in nutrient supply, breaking primary productivity. The trophic perturbations on the Baltica shelf were coeval with and related to eutrophication of the high-latitude upwelling zones of the Rheic Ocean. The postulated spatial fluctuations in the trophic resource continuum may reflect palaeooceanographic changes that accompanied the global, late sea-level highstand of that time.