Transient glacio-eustatic fall and its climato-environmental effects during the Frasnian-Famennian transition

The cause of the Frasnian-Famennian (F–F) boundary mass extinction, one of the largest Phanerozoic biocrises, remains uncertain. Here, we infer a short glaciation during the F–F transition based on evidence of sea-level fall and major changes in terrestrial inputs and weathering fluxes, suggesting that climatic cooling was the proximate cause of the F–F event. This study is based on an integrated elemental-isotopic-organic dataset from the West Valley core, representing prodelta facies of the Catskill Delta in western New York State. The 29-m-thick study interval records a shift from greenish-gray shales of the upper Hanover Formation to dark gray shales of the lower Dunkirk Formation that was accompanied by long-term increases in terrestrial organic and clay fluxes and more reducing watermass conditions. The 15-m interval bracketing the F–F boundary shows three shifts from near-marine (background) salinity to low-brackish conditions, implying that sea-level falls caused the study site to transiently shift into the basin’s low-salinity surface-water layer. Superimposed on the general lithologic changes in this F–F transition section are twelve sedimentary cycles of ~1–5 m thickness (avg. 2.5 m) characterized by strong variation in TOC and most geochemical proxies. These cycles record climatic variation at an orbital (possibly ~18-kyr precessional) timescale, resulting in short-term fluctuations in sea level (as shown by salinity proxies) as well as in terrestrial organic and clastic fluxes (as reflected in organic and detrital proxies). Changes in sedimentary proxies at this short timescale show that the organic-rich layers were associated with more humid climatic conditions, higher sea-level stands (consistent with less continental ice mass), and more saline and reducing environmental conditions, whereas organic-poor layers were associated with drier climatic conditions, lower sea-level stands (consistent with more continental ice mass), and less saline and reducing environmental conditions. The F–F event, which spans seven such cycles, thus represents a comparatively short glacial episode with a duration of just ~126 ± 18 kyr, although it is comparable to the more severe DevonianCarboniferous (D–C) boundary glaciation that heralded the onset of the Late Paleozoic Ice Age ~12 million years later. Both the F–F and D–C biocrises represent carbon-burial-type events and, although their ultimate cause(s) remain in debate, the existing observations favor a bioevolutionary mechanism linked to the evolution and spread of land plants.