Controls on the formation of microbially induced sedimentary structures and biotic recovery in the Lower Triassic of Arctic Canada

Abstract

Microbially induced sedimentary structures (MISS) are reportedly widespread in the Early Triassic and their occurrence is attributed to either the extinction of marine grazers (allowing mat preservation) during the Permo-Triassic mass extinction or the suppression of grazing due to harsh, oxygenpoor conditions in its aftermath. Here we report on the abundant occurrence of MISS in the Lower Triassic Blind Fiord Formation of the Sverdrup Basin, Arctic Canada. Sedimentological analysis shows that mid-shelf settings were dominated by deposition from cohesive sand-mud flows that produced heterolithic, rippled sandstone facies that pass down dip into laminated siltstones and ultimately basinal mudrocks. The absence of storm beds and any other "event beds" points to an unusual climatic regime of humid, quiet conditions characterized by near continuous run off. Geochemical proxies for oxygenation (Mo/Al, Th/U, and pyrite framboid analysis) indicate that lower dysoxic conditions prevailed in the basin for much of the Early Triassic. The resultant lack of bioturbation allowed the development and preservation of MISS, including wrinkle structures and bubble textures. The microbial mats responsible for these structures are envisaged to have thrived, on sandy substrates, within the photic zone, in oxygen-poor conditions. The dysoxic history was punctuated by betteroxygenated phases, which coincide with the loss of MISS. Thus, Permo-Triassic boundary and Griesbachian mudrocks from the deepest-water settings have common benthos and a well-developed, tiered burrow profile dominated by Phycosiphon. The presence of the intense burrowing in the earliest Triassic contradicts the notion that bioturbation was severely suppressed at this time due to extinction losses at the end of the Permian. The notion that Early Triassic MISS preservation was caused by the extinction of mat grazers is not tenable.

Phosphorus Cycle and Primary Productivity Changes in the Tethys Ocean During the Permian-Triassic Transition: Starving Marine Ecosystems

“…Intense ocean anoxia during the crisis interval was recognized early based on the widespread occurrence of sedimentary facies typical for deposition under hypoxic conditions (Wignall and Hallam, 1992;Wignall and Twitchett, 1996;Isozaki, 1997). This was confirmed by various proxies such as framboidal pyrite analysis, trace metal concentrations, stable sulfur and uranium isotopes and iron speciation further refining the record of P-T ocean anoxia (e.g., Riccardi et al, 2006;Grasby et al, 2013;Clarkson et al, 2016;Lau et al, 2016;Wignall et al, 2020). In contrast to the previously proposed P-T superanoxia, the current understanding is that of a spatially and temporally heterogenous anoxic event that initiated ~70 ka prior to the main marine extinction level (Zhang et al, 2018).…”

A diverse gastropod fauna from the shallow marine carbonate platform of the Yangou section (south China) in the immediate aftermath of the Permian–Triassic mass extinction

“…In addition, in the immediate aftermath of the PTME, PTBM proliferation occurred rapidly all over the world (e.g., Kershaw et al, 2009). The anoxia was extensively recorded across the extinction horizon, whereas oxygenation has only been recorded in south China, Arctic Canada, and Oman (Bond & Wignall, 2010; Brosse et al, 2019; Wignall et al, 2020; Xiao et al, 2018). However, although these studies discovered this oxygenation event in the microbialites‐bearing sections, the redox conditions of PTBMs were still dysoxic rather than fully oxic, which were insufficient for the inhabitation of many metazoans (e.g., Song et al, 2014).…”

Resilience of infaunal ecosystems during the Early Triassic greenhouse Earth

“…Whether the behavioral and ecologic diversifications of trace-making animals facilitated the initial recovery of skeletonized animals during the Early Triassic has been debated (15,16,22). When and where infauna flourished in this Early Triassic greenhouse world (23) and potential environmental controls remain poorly understood (7,(24)(25)(26)(27)(28).…”