Toarcian Ocean Anoxic Event from an open-ocean setting in the eastern Tethys

The Early Toarcian "Oceanic Anoxic Event" (T-OAE) is a very significant paleoenvironmental perturbation of the Mesozoic that coincided with marked disruptions to both the climate system and marine ecosystems. This event is associated with widespread burial of marine carbon and a large negative carbon-isotope excursion (CIE) in carbonates, organic matter, and fossil wood. These features have been attributed to a large-scale injection of isotopically light carbon into the ocean-atmosphere system, possibly from the release of volcanogenic CO2 from the emplacement of the Karoo-Ferrar Large Igneous Province (LIP) in southern Gondwana and/or from dissociation of methane hydrates.

The T-OAE has been well documented in the western Tethyan and Boreal realms. Only a few studies on the T-OAE have been reported from the northwest, central, and northeast Panthalassic Ocean. Therefore, the global expression of the T-OAE needs to be established through analysis of Early Toarcian rocks in other paleogeographic locations.

In the eastern Tethys, previous researchers proposed a correlation of the organic-rich shales in the Bilong Co area of northern Tibet to the Lower Toarcian anoxic black shales of Europe. The Bilong Co section deposited in a lagoonal environment displays a trend and distinct negative carbon-isotope excursion that are similar to those in the T-OAE strata of the western Tethyan and Boreal realms of NW Europe. However, this restricted setting for the T-OAE was not representative of the main Panthalassic Ocean.

Here, we present high-resolution carbon-isotope data, bulk geochemistry, mineral characterization from an open-ocean setting in the eastern Tethys. The studied section (Sewa section) documents a high-resolution record of the T-OAE from an open-ocean setting in the eastern Tethys (Figure 1). The succession displays a ca. –8.4‰ negative excursion in δ¹³C_carb (CIE) with trends that are characteristic of T-OAE sections in the western Tethys and NW Europe. Compare with other T-OAE records from the nearby lagoonal Bilong Co section and the open-ocean setting of southern margin of the eastern Tethys, the δ¹³C variation patterns of these sites appear to be similar (Figure 1). These data suggest that the T-OAE was probably extensive in the eastern Tethyan realm.

A key feature of the Toarcian CIE interval from an open-ocean setting in the Qiangtang Basin is the common occurrence coarser-grained deposits (silt-sized sediments) associated with high-energy conditions within the otherwise low-energy claystone deposits (Figure 2). Three possible scenarios have been proposed to elucidate fine-grained siliciclastic accumulation: (1) a relative sea-level fall extends to this area; (2) climatic variations enhance clastic input from the source regions; and (3) a combination of both. However, a global sea-level rise that occurred during the Lower Toarcian was extensively accepted, and the transgression was also recorded in the Qiangtang Basin. The occurrence of coarser grained deposit during the most negative interval of the T-OAE negative excursion in the Sewa section coincides stratigraphically with similar lithologies in other marine basins. The temporally restricted occurrence of coarser-grained sediment supply, in geographically widespread marine basins, suggests a climatic control rather than only local tectonic activity. The observation suggests an increased supply of clastic sediments into marginal and deeper marine basin in response to increased runoff and/or storm-driven currents. Therefore, a fluctuating, silt-sized sediment to the Qiangtang Basin during the T-OAE is possibly indicative of global changing climatic and environmental conditions on nearby landmasses and in shallow marine environments.

The low C_org:P_total ratios in combination with bioturbated structure and depleted or slightly enriched in redox-sensitive trace elements V, Mo, and U (Figures 2 and 3) indicate a long-term oxygenation event during the T-OAE interval at the Sewa succession, and hence, anoxia may not play a fundamental role during the Toarcian negative CIE in this setting. The long-term oxygenation in the study area results a generally low accumulation of organic-matter sediments.

Many episodes of kaolinite enrichment were identified in the Sewa section (Figure 3), which is attributed to a change in sediment provenance. Our sedimentological (coarser-grained deposits) and geochemical analyses (CIA values; Figure 3) reveal that a rapid climatic variation occurred at the base of the T-OAE. The early Toarcian climate exhibits a warming and more humid condition began at the start of the T-OAE. A warmer and more humid climate during the T-OAE in the Qiangtang Basin is a regional response to global warming occurring in this interval.

TOC abundance shows two main fluctuations (prominent increase) within the Sewa section. The first prominent increase in TOC content coincident with the onset of the T-OAE (Figure 3); here, an increase in silt-sized sediment supply was also observed. These lines of evidence suggest that accumulation of organic-matter sediments during the T-OAE is generally controlled by global climatic change. The second prominent increase in TOC content coincident with a rapid change in sediment provenance (Figure 3). These data indicate that regional sedimentary conditions might influence the preservation of organic matter.

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See the article:

A Toarcian Ocean Anoxic Event record from an open-ocean setting in the eastern Tethys: Implications for global climatic change and regional environmental perturbation

https://doi.org/10.1007/s11430-020-9753-1

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