Isotopic composition of apatite-bound sulphur in the Triassic phosphogenic facies in Svalbard

• Autorzy: Krajewski K. P.
• Języki publikacji: EN

Abstrakty

EN

Isotopic composition of apatite-bound sulphate sulphur in the Middle Triassic phospho- genic facies in Svalbard (Bravaisberget and Botneheia formations) shows wide lateral variation, with d34S values ranging from +24.9‰ to -2.0‰. After correction for likely value of coeval seawater sulphate (d34S = +15.1‰ š 1.5‰), the D34S (= d34SCFA - d34Scoeval evaporite) values of the Middle Triassic apatites fall in a range between approx. +10‰ and -17‰, suggesting complex and variable isotopic modifications of seawater sulphate in bottom phosphogenic environments. This variation was primarily forced by changing bottom depositional conditions along the Svalbard shelf which affected the nature, allocation, and intensity of phosphogenic processes in surficial sediments. Four bottom diagenetic systems leading to different isotopic ranges of apatite-bound sulphur are suggested: (1) rapid burial semi-closed system (D34S = +10‰ to +5‰); (2) shielded interface semi-closed system (D34S = 0‰ to -17‰); (3) open oxic/anoxic system (D34S = +3‰ to -1‰); and (4) semi-closed anoxic system (D34S = +6‰ to +4‰). The rapid burial semi-closed system and the open oxic/anoxic system dominated the formation of apatite in the shallow and deep parts of the Svalbard shelf, respectively. Seawater sulphur values of apatites formed in organic-rich deep shelf facies of the Botneheia Formation in central and eastern Svalbard reflect high original porosity of the sediment and nearsurface location of the phosphogenic zone in a thin suboxic and in the upper part of anoxic sulphidic pore environments. Heavier sulphur values of apatites formed in silty to sandy shallow shelf facies of the Bravaisberget Formation in western and southwestern Spitsbergen also reflect phospho- genesis in the suboxic/upper anoxic sulphidic pore environment, but located deeper in sediment column and characterized by limited sulphate exchange with shelf water. The semi-closed anoxic system and the shielded interface semi-closed system developed locally in bottom shelf environ- ments, reflecting peculiar, spatially restricted depositional conditions. Isotopically heavy apatitic sulphur in organic-rich facies of the Botneheia Formation deposited to the east of the Billefjorden Fault Zone in Spitsbergen reflects phosphogenesis in semi-closed anoxic system developed in surficial sediments overlain by anoxic bottom water enriched in residual dissolved sulphate. Isotopically light apatitic sulphur in microbial mat-generated phosphorite horizons occurring locally in both the shallow (Bravaisberget Formation) and deep shelf (Botneheia Formation) facies in Spitsbergen reflects phosphogenesis in nearsurface environment shielded by gradient-type microbial mats. The light apatitic sulphur originated from intense oxidation of hydrogen sulphide at a narrow oxic/anoxic interface stabilized by the growth and metabolic activity of the mat-forming communities.

• Wydawca: Instytut Nauk Geologicznych PAN
• Czasopismo: Studia Geologica Polonica
• Rocznik: 2000
• Tom: Vol. 116
• Strony: 85--109

Twórcy

autor

Krajewski K. P.

• Polish Academy of Sciences, Institute of Geological Sciences, ul. Twarda 51/55, 00-818 Warszawa, Poland,