Unveiling a new oceanic anoxic event at the Norian/Rhaetian boundary (Late Triassic)

Abstract

The latest Triassic was characterised by protracted biotic extinctions concluding in the End-Triassic Extinction (~ 200 Ma) and a global carbon cycle perturbation. The onset of declining diversity is closely related to reducing conditions that spread globally from upper Sevatian (uppermost Norian) to across the Norian-Rhaetian boundary, likely triggered by unusually high volcanic activity. We correlate significant organic carbon cycle perturbations to an increase of CO₂ in the ocean-atmosphere system, likely outgassed by the Angayucham igneous province, the onset of which is indicated by the initiation of a rapid decline in ⁸⁷Sr/⁸⁶Sr and ¹⁸⁸Os/¹⁸⁷Os seawater values. A possible causal mechanism involves elevated CO₂ levels causing global warming and accelerating chemical weathering, which increased nutrient discharge to the oceans and greatly increased biological productivity. Higher export production and oxidation of organic matter led to a global O₂ decrease in marine water across the Norian/Rhaetian boundary (NRB). Biotic consequences of dysoxia/anoxia include worldwide extinctions in some fossil groups, such as bivalves, ammonoids, conodonts, radiolarians.

Figure 1

δ¹³C_org, Ba, and redox-sensitive elements (Mn, Cr, V, U, and Mo) show enrichments in the study sections. The stratigraphic position of the NRB is discussed in Supplementary Information. Blue dots represent raw data, and red lines show moving average on three samples.

Figure 2

U_EF vs. Mo_EF diagram for samples in the study sections. Base figure modified from Algeo and Tribovillard.

Figure 3

Geochemical profile of Pignola-Abriola section. Enrichment factors (EF) were calculated relative to UCC^,. δ⁹⁸Mo_auth for carbonate-absent is of Fe–Mn hydroxides and for carbonate-bearing is Fe–Mn hydroxides plus carbonate. Strontium curve modified after. A description of the taxonomic range of conodonts is as follows: 1. Mockina bidentata; 2. Parvigondolella andrusovi; 3. Misikella hernsteini; 4. Mi. hernsteini/posthernsteini; 5. Mi. posthernsteini; 6. Mi. ultima.

Figure 4

Stratigraphic profiles of K_EF, Rb_EF, and CIA in the Pignola-Abriola section. Enrichment factors (EF) were calculated relative to UCC. K_EF and Rb_EF decrease with increasing CIA values across the NRB.