PALEOREDOX CHANGES ASSOCIATED WITH EARLY SILURIAN EXTINCTION EVENTS: NEW GEOCHEMICAL EVIDENCE FROM BALTOSCANDIA

The early Silurian was characterized by global oceanographic and biotic turnover associated with survival/recovery from one of the largest mass extinctions in the Phanerozoic. Lower Silurian (Llandovery) strata contain evidence for widespread anoxia, continued glaciation, and three positive carbon (δ¹³C) isotope excursions (CIEs): Early Aeronian, Late Aeronian, and Valgu. However, mechanisms capable of causing widespread climactic and oceanographic changes that can be linked to biotic events are poorly understood. Llandovery conodont and graptolite records document several extinction intervals, some of which are coincident with the aforementioned CIEs. Previous studies have proposed major oceanographic circulation and climatic changes as the driver for these marine extinction events but geochemical evidence available to test these hypotheses are limited.

This study presents new local redox proxy data such as iron speciation and trace metal geochemistry along with pyrite-sulfur (δ³⁴S_pyr) and organic carbon (δ¹³C_org) isotope records from Llandovery upper slope and distal shelf shale sequences within the Baltic Basin. Iron speciation and trace metal data (Mo_EF and U_EF- enrichment factors) from both drill cores indicate anoxic water column conditions, predominantly non-sulfidic, coincident with the δ¹³C records of Late Aeronian and Valgu CIEs. These new local-redox proxy data combined with lithofacies patterns in the basin suggest that a local/regional scale oxygen minimum zone may have been established along this margin of the Baltic Basin at times throughout the Llandovery. Geochemical and lithologic evidence suggest that anoxic waters spread from deeper basinal settings onto the shelf during the Aeronian-early Telychian coinciding with two positive CIE’s and associated moderate extinction events. Expansion of anoxic and intermittently euxinic waters provides a unique mechanism to tie paleontological records of extinction to perturbations of the global C cycle during the early Silurian.