EARLY SILURIAN (LLANDOVERY) BIOTIC AND OCEANOGRAPHIC EVENTS: INSIGHTS FROM NEW STABLE ISOTOPE GEOCHEMISTRY WITHIN THE BALTIC BASIN

The early Silurian was a period of widespread environmental, oceanographic, and biotic change in the aftermath of one of the largest mass extinctions in the Phanerozoic. Lower Silurian (Llandovery) strata worldwide generally record a period of episodic biotic extinction/origination possibly linked to widespread anoxia, continued glaciation, and dynamic nutrient and C-cycling. Three notable graptolite (C. vesiculosus, S. sedgwickii, S. turriculatus zones) and two conodont (Sandvika, Valgu) extinction events have been previously documented within Rhuddanian through early Telychian strata. Some of these extinction events coincide with documented perturbations of the C-cycle and sea-surface temperature proxies. However causal mechanisms linking these biotic intervals to paleoceanographic change, climate, and marine redox conditions remain poorly constrained.

Here we present new pyrite-sulfur δ³⁴S_pyr and organic matter δ¹³C_org data from a shale-dominated Hirnantian–Llandovery sequence from a drill core in southern Sweden within the Baltic Basin. We document three globally recognized carbon isotope excursions within the Lindegård Mudstone and Kallholn Formation: HICE, Late Aeronian, and Valgu. Positive shifts in δ¹³C_org range from +2 to +4‰ in magnitude, while corresponding δ³⁴S_pyr data record positive shifts up to +20‰ in places. These preliminary data indicate that some early Silurian extinction events are linked to large amounts of organic matter and pyrite being buried under reduced oceanic oxygen conditions. Additionally these results present a complex and dynamic redox landscape within this basin with potential implications for global changes to the marine environments. Future and ongoing work is now focused on Fe-speciation and trace metal geochemistry to integrate with stable isotope and biotic data across this basin to provide the most complete redox picture possible during this period of episodic biotic extinction and recovery associated with climatic change in the early Silurian. Furthermore we hope to evaluate potential mechanisms, feedbacks, and controls of these understudied Paleozoic biotic and δ¹³C events, using an integrated geochemical, sedimentological, and paleobiological approach.