Evidence for anoxia in deep environments of Northeast Asia at the Permian-Triassic transition

Abstract

It is well known that the Permian‐Triassic transition was marked by significant changes in the biota and sedimentation settings. Evidence for the development of anoxic environments in various world areas during events at the Permian‐Triassic transition in both shallow- and deep-water settings are reported in [1‐8 and others]. Such data have been unavailable thus far for the Russian territory. The study of Permian‐Triassic boundary deepwater sediments in Northeast Asia provided the first evidence for H 2 S contamination of bottom waters in this region. Both shallow- and deep-sea basins existed in Northeast Asia during the Permian Period [9]: Ayan-Yuryakh, Balygychan, Sugoi, and Gizhiga. They were characterized by avalanche sedimentation, which was responsible for accumulation of thick (up to 7 km) sandy‐clayey and volcanogenic (frequently, turbidite) sediments with impoverished fossil assemblages. The fossils are represented by Inoceramus-like bivalves and subordinate ctenodont bivalves and gastropods ( Straparolus ). In the terminal Permian, the system of sea basins in Northeast Asia experienced some shoaling. In deep marginal sea basins, bathyal settings recorded by the accumulation of turbidite sequences gave way to shallower conditions comparable with deep shelf settings, which differed, however, from typical shelf environments. They reflected substantial shoaling of marginal basins in response to the global regression in the terminal Permian [10]. The rocks are characterized by abundant structural distortions and traces of intense bioturbation (distinctly or vaguely patchy structures). Benthic fossils are more common as compared with the underlying strata. The sea basins became deeper during the Permian‐ Triassic transition. This is evident from the change in the sedimentation pattern in the section: the complete disappearance of distorted bedding, spotty structures, and any other signs of vital activity of benthic organisms, as well as their remains. Instead, thin horizontal and, less commonly, oblique lamination appears. The rocks become fine-grained. All these features imply intense activity of contour currents and lack of benthic life. Such a lithological change in deep-water settings is characteristic of spacious areas of Northeast Asia extending from the Verkhoyansk region to the Taigonos Peninsula. Lower Triassic sediments (up to the Tompophiceras pascoei Zone) lack benthic fossils and bioturbation. The Orotukan segment of the Balygychan basin is one of the most promising areas for the study of the Permian‐Triassic boundary sediments (Fig. 1). The upper part of the Permian section (200 m) is represented here by foliated silty argillites of the Pautovaya Formation, while the lower part of the Triassic sequence is composed of silty argillites with thin siltstone laminae of the Gerba Formation [10]. The Late Permian (late Changhsingian) age of the upper Pautovaya Formation is substantiated by a finding of an impression of the bivalve Claraioides aff. primitivus (Yin) [11]. The Early Triassic age of the Gerba Formation is confirmed by ammonoid Tompophiceras pascoei (Spath) remains found several kilometers northwest of the study section 70‐80 m above the Permian‐Triassic boundary. The rocks are characterized by an elevated C org content (0.91‐0.95% on average, up to 4.92% in some samples), which makes them similar to carbonaceous black shales [12]. Organic matter is finely dispersed. Its origination is most likely related to zoo- and phytoplankton [13]. The above-mentioned lithological features of rocks and lack of benthic fossils indicate that the sediments accumulated in anoxic settings. Kholodov and Nedumov, who studied Quaternary and Tertiary sediments of the Black Sea region [14] and Cretaceous organic-rich clayey rocks of the Atlantic [15], suggested using the Mo/Mn ratio as an indicator of anoxic environments. They established that Mo/Mn values ranging from 0.0 n to 0. n correspond to H 2 S-contaminated environments in