PALEOENVIRONMENTAL CHANGES ACROSS THE LATE DEVONIAN-EARLY MISSISSIPPIAN TRANSITION

The Late Devonian witnessed two mass extinctions including the Kellwasser Event near the Frasnian-Famennian (F-F) boundary and the Hangenberg Event near the Devonian-Carboniferous (D-C) boundary. Each of these events is accompanied by a positive carbon isotope (δ¹³C) excursion and evidence of significant ocean anoxia. The 13-million-year interval between the Kellwasser and Hangenberg events also contains multiple smaller-scale δ¹³C excursions and ocean redox variations, but the nature of these events is much less well understood. Here we report δ¹³C and rare earth element (REE+Y) records of three sections from western Utah and Nevada (DG-Devil’s Gate; BM-Bactrian Mountain; MH-Mountain Home), covering the F-F to D-C boundary interval in the late Devonian Antler foreland basin. The positive δ¹³C excursion across the F-F boundary is present in all three sections, but with smaller magnitudes (≤ 2‰) in the BM and MH sections compared to the 4‰ excursion in the DG section. Above the F-F boundary, three positive δ¹³C shifts with magnitudes up to 3‰ are identified at the P. rhomboidea, P. gracilis expansa, and S. praesulcata condont biozones, possibly corresponding to the global Condroz, Dasberg, and Hangenberg events, respectively. The Ce/Ce* [Ce/Ce* = Ce_SN/(Pr_SN²/Nd_SN)] in petrographically and geochemically screened samples (e.g., Y/Ho > 36; Al < 1000 ppm; Fe < 800 ppm; Th < 0.5 ppm; total REE < 20 pm), which has been used as an indicator for shallow-water redox conditions, is relatively constant, ranging from 0.8 to 1.1, across the F-F to D-C interval. The high Ce/Ce* (≥ 0.8) during the Famennian are in contrast with the much lower values (≤ 0.6) of the Early Mississippian strata in the same sections, suggesting that the latest Devonian depositional environments in the Great Basin area were mostly dysoxic or anoxic. Whether such redox conditions have global implications requires tests in other sections globally.