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Pyrite geochemistry (isotope and trace element composition, degree of pyritization, S/C org ratio) was used in context of selected lithogeochemical parameters (major and trace elements, including sulphur, organic carbon, and δ 13 C of carbonate carbon) to constrain fluctuations in depositional conditions during the Early to Middle Frasnian carbon isotopic perturbation (punctata Event) in the Kostomloty Basin, Poland. Based on the ratio between the sum of oxyanionic elements and transition metals in pyrite, a new proxy parameter (index of syngenetic pyrite, ISYP) is proposed for assessing the relative amount of syngenetic pyrite in a sample. The distribution of the ISYP along the Kostomloty – Male Gorki section (upper Szydlowek to the basal Kostomloty beds) is in concert with conclusions inferred from paleoecologic data and other geochemical parameters (degree of pyritization, S/C org , δ 34 S pyrite ). According to these, the lower segment of the Szydlowek Beds was deposited in a normally oxygenated environment, but undergoing increasing primary productivity in surface water, as indicated by an increase in δ 13 C carb and in Cu/Zr ratio in bulk rock, which triggered the periodic deposition of sediments slightly enriched in organic matter, notably within the pyrite-ammonoid level (= Goniatite Level). Fluctuating, but in general high S/C org ratios, DOP R values and ISYP values suggest that during this time – against the background of a generally dysoxic environment – shorter or longer lasting episodes of more restricted (anoxic and possibly even euxinic) bottom water conditions developed. Low sedimentation rates enabled a continuous and practically unlimited supply of sulphate during bacterial sulphate reduction (BSR), which in turn led to a strong depletion of pyrite sulphur in 34 S in this interval (constantly around –29‰). In contrast, below and above the Goniatite Level, higher δ 34 S values (up to + 3‰), are compatible with closed system conditions and higher sedimentation rates. The Styliolina Horizon is distinguished by the overall highest portion of syngenetic pyrite, which suggests that in this marker layer most of the pyrite precipitated from euxinic bottom waters. |
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