[BCG09-P10] Sulfur isotope transition and development of Euxinia at ~1.8 Ga:
The Nuvilic Formation, Povngunituk Group, Cape Smith belt, Canada
Keywords:Paleoproterozoic, Sulfur isotope (δ34S), Organic carbon isotope (δ13Corg), Euxinia
Our sample is the drilling core 718.3333 (50 m) which consists of an alternation of sandstone and black shale. These sediments are accumulated at northern continental margin of the Superior craton. Based on upward fining structures, the core is subdivided into 3 units (Lower, Middle and Upper units). The Lower unit is about 10 m and includes 1 m thick sandstone and greyish-black shale. The Middle unit is about 25 m and consist of thin sandstone (<10cm) and black shale layer which characterized by thin sulfide layer. The Upper unit is about 10 m and contain about 30 cm sandstone layer and black shale which is also characterized by layered sulfides.
Organic carbon isotope ratio (δ13Corg) ranges from -33 to -28 ‰ through the core. At the sand dominated portion of the Lower unit, δ13Corg value shows -28‰ and decrease to the -32 ‰ of the Middle unit sand dominant portion. In the Middle and Upper unit, δ13Corg range from -32 to -30 ‰.
Sulfur isotope (δ34S) has wide range values from -5 to +15 ‰. δ34S is +13 ‰ at the sand dominated portion of the Lower unit, and subsequently, decrease dramatically to the -5 ‰ at the sand dominant sequences of the Middle unit. Through the Middle unit, δ34S change from -5 to +15 ‰. In the Upper unit, δ34S value is steady at +10 ‰. Sulfur-Total Organic Carbon (S/C) weight ratio of the core is 0.30 on average, and it indicates the Nuvilic Formation was formed in marine environment.
The transitions of the Lithology, δ13Corg and δ34S value mentioned above are likely to reflect ocean environmental change. The change in color of the black shale from greyish-black to black indicates decreasing of the oxygen concentration in the ocean. Laminated sulfide layer implies sulfidic ocean, where hydrogen sulfide in the water column reacts with iron and syngenetic precipitates as pyrite. δ13Corg of the core is comparative with δ13Corg value fixed by modern cyanobacteria, purple sulfur bacteria and methanogenic bacteria , implying the contribution from these bacterial activity. High δ34S value suggests the activity of the sulfate reducing bacteria in the sulfate poor environment. Negative excursion of δ34S at sandstone dominated portion of the Middle unit was occurred by a large influx of sulfate followed by the activation of bacterial sulfate reduction. In this study, we concluded that increasing seawater sulfate caused active bacterial reducing and it resulted in the formation of sulfidic marine environment at 1.8 Ga.