Integrated record of Ludlow (Upper Silurian) oceanic geobioevents – Coordination of changes in conodont, and brachiopod faunas, and stable isotopes

The Ludlow Epoch (Silurian) was marked by several globally recognized but mechanistically poorly understood biotic events. The most pronounced of them was the Lau Event, which strongly decimated conodont, graptolite, and brachiopod faunas. Additionally, this event coincides with the largest positive stable carbon isotopic anomaly in the whole Phanerozoic, as well as the resurgence of the so-called “anachronistic” microbial facies that were frequently encountered during survival episodes of the major mass extinction events. In this contribution, based on the analysis of the outer shelf facies succession (Milaičiai-103 core), from the Lithuanian part of the Silurian Baltic Basin, as integrated quantitative record of conodont, brachiopod and δ13C changes across most of the Ludlow is presented. The succession was subdivided into four conodont zones that served as a stratigraphic framework for analyzing the δ13C and palaeoecological trends. The depth constrained cluster analysis revealed successions of three statistically distinct conodont and six statistically distinct brachiopod assemblages that replace each other near the change points in the stable carbon isotopic curve. The application of a newly developed mathematical technique based on the analysis of recurrence patterns of the fossil assemblages revealed that both conodonts and brachiopods are represented by highly time specific assemblages in the aftermath of the Lau event (O. snajdri Interval Zone). The anomalous interval is confined to the transgressive and highstand phases of the 3rd order post-Lau transgression. The discussed interval is coeval with the extensive development of stromatolitic communities in the nearshore environments around the world. The results allow for the first time to quantify the profound ecosystem-wide geobiological impact of the mid-Ludfordian event that lasted up to the latest Ludfordian.