THE FRASNIAN-FAMENNIAN MASS EXTINCTION: UNDER THE VOLCANO BUT WITHOUT THE BURP

Two sections are introduced for the Frasnian-Famennian boundary. The first section is from the terrestrial Old Red Sandstone in East Greenland and is hugely expanded with a very high sedimentation rate. A δ¹³C_TOC isotope profile from terrestrial plant material shows significant injections of light carbon normally interpreted as resulting from a volcanic source. Coincidentally the sedimentary system responds with two episodes of a very active fluvial system that was quite atypical for the otherwise rather arid basin. These active fluvial systems then transition to two episodes of sustained aridity. Palynological correlations show that these are the terrestrial equivalents of the lower and upper Kellwasser Events. The response of the sedimentary system is entirely consistent with modelled responses of fluvial systems/weathering in a volcanically stimulated high CO₂ world. The sustained aridity was the result of CO₂ reduction as the system over-compensates.

The other section is a marginal marine upper Kellwasser equivalent drilled in a scientific borehole at Sosnogorsk, near Ukhta, Komi Republic, Russia. This revealed a remarkably well preserved 8 m thick interval of unconsolidated grey mudstone which contains a pristine record of both organic and carbonate stable isotope data integrated with pyrite framboid diameters, %S, TOC, calcite % and palynology. The TOC and calcite content reveals a pattern of orbital forcing and hence a time framework. High resolution isotope results from the key interval shows 2 negative excursions in δ¹³C_org and a single excursion in δ¹³C_CaCO3. This data, integrated with quantitative palynological counts and silt content enables us to identify a peak in fresh water runoff. Interpretation of the %S, pyrite framboid sizes and δ¹³C_AOM enables us to identify incursions of euxinic water with a component of remineralised carbon. Metabolized by green sulphur bacteria then produces an excursion that could otherwise be interpreted as methane release from clathrates.