A review of carbon isotope excursions, redox changes and marine red beds of the Early Triassic with insights from the Qinling Sea, northwest China

The Early Triassic was an interval characterized by frequent, large global carbon isotope excursions (CIEs), multiple and widespread anoxic phases, and generally high but fluctuating temperatures. In order to comprehensively understand their inter-relationship, we have investigated the Yiwagou section from the little-known Qinling shelf sea, at the eastern corner of Paleo-Tethys, and evaluated the global marine red bed (MRB) occurrences, redox conditions and CIEs. The anoxic episodes show great variations between different regions, but only during the earliest Griesbachian were they of almost global extent, whilst during other Early Triassic intervals there is great regional variation of redox trends. Even during the early Griesbachian anoxia was absent in South Tibet and Qinling shelf seas. Smithian MRBs in the latter region are dominated by intraclasts or ooids and are likely to be caused by ferruginous ocean waters advected from the adjacent Paleo-Tethys Ocean. The more widespread Spathian MRBs, mostly developed in the middle Spathian, were also likely a product of advection of anoxic ferruginous waters into shelf areas. Vigorous upwelling at this time is unlikely because this would stimulate high productivity and diagenetic conditions that would have reduced the iron oxyhydroxides responsible for the MRBs. Compilation of 54 delta 13Ccarb records reveals that the negative excursion in the Smithian, from P2 to N3, ranges in magnitude from-2.9%o to-9.7%o, the positive excursion from N3 to P3 across the Smithian-Spathian boundary ranges from 2.6%o to 11.9%o and its amplitude is greatest in the Northern Yangtze Platform. The similarity of N3 values at Yiwagou with those from the seamount carbonates of the Panthalassa Ocean indicates a good oceanic connectivity at the time of Smithian MRBs formation. The global average of delta 13C values during the P2 and N3 CIEs shows that values are 2.0%o and 1.1%o heavier in shallow settings compared to deep settings respectively. In contrast, there is no consistent variation with water depth of the subsequent P3 CIE. Analysis of Delta delta 13Cvert values shows that there are large differences between regions that likely reflects the different stratification histories of epicontinental basins, but there is no global signal at this time. Previous studies have suggested a collapse of the water column carbon isotope gradient during P3 associated with vigorous upwelling, but this pattern is not widespread and is likely a regional signal. The origins of the extreme light carbon isotope values during the mid-Smithian N3 excursion, and the heavy early Spathian P3 excursion, remain unclear and are not easily reconciled with global redox changes. Changes in the proportions of carbonate carbon and organic carbon burial may be important.