REDOX SITUATION OF THE EARLY TRIASSIC OCEAN: CLUES FROM MOESSBAUER SPECTROSCOPY

The Permian/Triassic boundary was the severest mass extinction in Earth history. The exact mechanisms that culminated to this devastating result are still not entirely unravelled, also the causes that led to a strongly extended survival interval after the extinction are still dubious. Investigations of the δ13C isotope values of Lower Triassic shallow marine carbonates gave a curve with profound and short timed oscillations. One hypothesis to explain the observed features in the Lower Triassic is a strong variation in oxygen availability in sea water caused by periods of well circulated ocean water masses followed by periods of intense stratification.

We applied ⁵⁷Fe Mössbauer spectroscopy, which enables us to identify Fe²⁺ und Fe³⁺ phases, giving evidence for reducing and oxidizing sedimentation environments, respectively. As a result we only discriminated 4 phases in combinations of maximally three: pyrite, a Fe²⁺-phase, paramagnetic Fe²⁺ and paramagnetic Fe³⁺. The samples have been analyzed from the Chaohu section in south China. Due to the low iron content of the samples, they had to be measured for several weeks. This explains the small number of samples. The sample from the uppermost Permian contains exclusively pyrite as Fe bearing phase. In the Lower Triassic a sample directly from the δ13C maximum at the Induan/Olenekian boundary contains mainly pyrite, less paramagnetic Fe²⁺ and a minor amount of a Fe²⁺- phase. Two samples from the Smithian, that gives very light carbon isotope values have a Fe³⁺- (probably paramagnetic Fe³⁺) and the Fe²⁺-phase as their major Fe phases and minor amounts of paramagnetic Fe²⁺. At the base of the Spathian pyrite is the major Fe phase together with smaller, almost equal amounts of paramagnetic Fe²⁺ and the Fe²⁺-phase. We interpret these results by a sedimentation environment at Chaohu that remained for the most time of the investigated interval under reducing conditions. Only in the Smithian there existed a period of increased availability of free oxygen. This can be explained with a “stratified ocean” situation, only interrupted by a short period of circulation, bringing oxygen rich water to the seafloor. The interpretation is in good agreement with the δ13C isotope curve of the Chaohu section, which generally shows depleted isotope values compared to shallow water sections.