Michał Rakociński

Mercury enrichments and the Frasnian-Famennian biotic crisis: A volcanic trigger proved?

The Frasnian-Famennian (F-F) global event, one of the five largest biotic crises of the Phanerozoic, has been inconclusively linked to rapid climatic perturbations promoted in turn by volcanic cataclysm, especially in the Viluy large igneous province (LIP) of Siberia. Conversely, the triggers of four other Phanerozoic mass extinction intervals have decisively been linked to LIPs, owing to documented mercury anomalies, shown as the diagnostic proxy. Here, we report multiple Hg enrichments in the two-step late Frasnian (Kellwasser) crisis interval from paleogeographically distant successions in Morocco, Germany, and northern Russia. The distinguishing signal, > 1 ppm Hg in the domain of closing Rheic Ocean, was identified in different lithologies immediately below the F-F boundary and approximately correlated with the onset of the main extinction pulse. This key Hg anomaly, comparable only with an extreme spike known from the end-Ordovician extinction, was not controlled by increased bioproductivity in an anoxic setting. We suggest, therefore, that the global chemostratigraphic pattern near the F-F boundary records a greatly increased worldwide Hg input, controlled by the Center Hill eruptive pulse of the Eovariscan volcanic acme, but likely not manifested exclusively by LIP(s). Consequently, all five major biotic crises of the Phanerozoic have now been more reliably linked to volcanic cataclysms.

Temporal dynamics of encrusting communities during the Late Devonian: a case study from the Central Devonian Field, Russia

Abstract. In this study we focused on the dynamics of encrusting assemblages preserved on brachiopod hosts collected from upper Frasnian and lower Famennian deposits of the Central Devonian Field, Russia. Because the encrusted brachiopods come from deposits bracketing the Frasnian/Famennian (F/F) boundary, the results also shed some light on ecological differences in encrusting communities before and after the Frasnian—Famennian (F-F) event. To explore the diversity dynamics of encrusting assemblages, we analyzed more than 1300 brachiopod valves (substrates) from two localities. Taxon accumulation plots and shareholder quorumsubsampling (SQS) routines indicated that a reasonably small sample of brachiopod host valves (n=50) is sufficient to capture themajority of the encrusting genera recorded at a given site. The richness of encrusters per substrate declined simultaneously with the number of encrusting taxa in the lower Famennian, accompanied by a decrease in epibiont abundance, with a comparable decrease in mean encrustation intensity (percentage of bioclasts encrusted by one or more epibionts). Epibiont abundance and occupancy roughlymirror each other. Strikingly, few ecological characteristics are correlated with substrate size, possibly reflecting random settlement of larvae. Evenness, which is negatively correlated with substrate size, shows greater within-stage variability among samples than between Frasnian and Famennian intervals and may indicate the instability of early Famennian biocenoses following the faunal turnover. The occurrence distribution of encrusters points to nonrandomassociations and exclusions among several encrusting taxa. However, abundance and occupancy of microconchids remained relatively stable throughout the sampled time interval. The notable decline in abundance (∼60%) and relatively minor decline in diversity (∼30%) suggest jointly that encrusting communities experienced ecological collapse rather than a major mass extinction event. The differences between the upper Frasnian and lower Famennian encrusting assemblages may thus record a turnover associated with the F-F event.