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The Lower Triassic marine carbonate carbon isotope curve: An update and ideas for the interpretation

In recent years data and knowledge of the Lower Triassic 13C-curve has substantially increased due to continued work by several groups and thus it´s evolution is quite well known. From late Permian enriched 13C-values the curve decreases with increasing steepness in many sections. Around the Permian-Triassic-Boundary three minima have been identified. Afterwards the curve first remains at low values until the middle Griesbachian and steadily increases towards the Griesbachian-Dienerian boundary. Around that boundary a minor, short, negative excursion has been reported in several sections. In the Dienerian the 13C-values increase with several short, negative inflections and with a steepening of the slope towards the Dienerian-Smithian boundary (DSB) in many sections. Around that boundary occurs a prominent maximum beyond +8‰ in shallow water carbonate sections followed by a steep and continuous drop to low, often negative values in the Smithian. Just before the Smithian- Spathian boundary (SSB) a steep rise to a second maximum occurs that in some sections reach almost the level of the previous peak. It is followed by a saw-tooth shaped decline in the Spathian and a gentle increase to a rounded peak at the Spathian-Anisian boundary (SAB). Besides the latest peak at the Lower-Middle-Triassic boundary most maxima and minima occur rather abrupt, sudden and with significant magnitudes. These are indicators of stratification and mixing of carbon reservoirs (surface and deep water), resulting in sudden, substantial changes of 13C. Additional, independent evidence is the less developed DSB in deeper water settings with lower C-isotopes, and shallow water debris flow sediment layers show enriched values of the shallow water environment. Additional influences on the marine 13C value probably were volcanic activity and the inferred oxidation of coal, and bioproductivity. Due to the difference in shape of the curve (and the recovery of biota) at the SAB other processes than ocean stratification are assumed. However, for the SSB stratification is a likely explanation, but at that interval additional research is necessary, because no unequivocal isotope fractionation between shallow and deeper water carbonates have been documented to our knowledge.

lower Triassic; carbon isotope

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