Sedimentary Environment and Redox Conditions of the Lower Triassic Osawa Formation in the Southern Kitakami Terrane, Japan: Insights Into Ocean Redox Stratification and Faunal Recovery

Abstract

The sedimentary environments and redox conditions of the Lower Triassic Osawa Formation in the Southern Kitakami Terrane were reconstructed based on lithofacies, trace fossils, and other paleontological content. The muddy and sandy lithofacies of the Osawa Formation lack evidence of storm waves despite the presence of storm-induced turbidites, suggesting that the oldest deposits of the Osawa Formation were deposited in the proximal part of the outer shelf. In turn, water depth increased from the lower to upper part of the formation, ultimately recording the distal part of the outer shelf. In addition to sandy lithofacies caused by turbidity and traction currents, multiple sandy layers within the muddy lithofacies would have originated via supply into the prodelta setting from a fan delta system. Collapses of the delta front or river system floods could have generated hyperpycnal flows, resulting in abundant supplies of mud and organic matter. Trace fossil analyses revealed that the degree of bioturbation (ichnofabric indices) dramatically decreased as water-depth increased, indicating a steep oxic-dysoxic gradient along the onshore-offshore transect. Diagenetic pyrite framboids indicative of dysoxic/anoxic benthic conditions are abundant at greater water depths. Abundant pyrite framboids less than 6 μm in diameter suggest intermittent euxinicity. With an increase in global weathering, abundant sediment supply including organic matter from the fan delta system could have contributed to the development of ocean redox stratification, which appears to have impacted on the adaptation of both nektic and benthic animals of this area during the Early Triassic.

Hyperthermal-driven anoxia and reduced productivity in the aftermath of the Permian-Triassic mass extinction: a case study from Western Canada

“…Consequently, marine life was in a precarious position: squeezed from above by hot sea surface temperatures and from below by a shallow redoxocline, resulting in a narrow habitable zone whose dimensions varied as global temperatures warmed or cooled, and the redoxocline rose and fell (Beatty et al, 2008;Song et al, 2014;Woods et al, 2019). Geochemical records and geochronology indicate that the Early Triassic global hothouse persisted until the end of the Early Triassic (Sun et al, 2012;Joachimski et al, 2022), while anoxia in the deep ocean may have persisted into the mid-Anisian (lower Middle Triassic) (Isozaki, 1997;Lau et al, 2016;Ishizaki and Shiino, 2023).…”

“…Palaeoenvironmental reconstructions of the Permian-Triassic interval have primarily focused on the period of time surrounding the PTME (e.g., Wignall and Twitchett, 1996;Lehrmann et al, 2003;Thomas et al, 2004;Algeo et al, 2007;Son et al, 2007;Grasby and Beauchamp, 2009;Bond and Wignall, 2010;Brookfield et al, 2010;Liao et al, 2010;Richoz et al, 2010;Varol et al, 2011). Only a handful of studies have produced continuous environmental records across the entire Lower Triassic interval of environmental instability at the regional or local scale (e.g., Shen et al, 2015;Lau et al, 2016;Grasby et al, 2021;Ishizaki and Shiino, 2023;Saito et al, 2023). Analysis of paleobiologic trends during this period suggests that recovery was often sluggish or reset by persistent environmental stresses (Pietsch et al, 2014;Song et al, 2014;Woods et al, 2019), resulting in a slow biotic rebound that frequently stretched well beyond the earliest Triassic, to perhaps as late as the early middle Triassic (Anisian) (e.g., Schubert, 1989;Hallam, 1991;Twitchett and Wignall, 1996;Boyer et al, 2004;Pruss and Bottjer, 2004;Twitchett and Barras, 2004;Nützel and Schulbert, 2005;Baucon and De Carvalho, 2016;Golding, 2021;Wang et al, 2022;Zhu et al, 2022).…”