Relationships between development of organic-rich shallow shelf facies and variation in isotopic composition of pyrite (Middle Triassic, Spitsbergen)

• Published in: Polish Polar Research
• Main Author: Karcz, Przemysław
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Walter de Gruyter GmbH 2010
• Subjects: Ecology; Ecology, Evolution, Behavior and Systematics; Bravaisberget; Polar Research; Spitsbergen
• Online Access: http://dx.doi.org/10.2478/v10183-010-0005-1; https://content.sciendo.com/view/journals/popore/31/3/article-p239.xml; https://www.degruyter.com/view/j/popore.2010.31.issue-3/v10183-010-0005-1/v10183-010-0005-1.pdf

Relationships between development of organic-rich shallow shelf facies and variation in isotopic composition of pyrite (Middle Triassic, Spitsbergen) 51 samples from the Middle Triassic black shales (organic carbon-rich silt-stones; up to 4.9% TOC - Total Organic Carbon) from the stratotype section of the Bravaisberget Formation (western Spitsbergen) were analyzed with respect to isotopic composition of pyritic sulphur (Δ 34 S) and TOC. Isotopic composition of syngenetic pyrite-bound sulphur shows wide (Δ 34 S from -26‰ to +8‰ VCDT) and narrow (Δ 34 S from -4‰ to +17‰ VCDT) variation of the Δ 34 S in upper and lower part of the section, respectively. Range of the variation is associated with abrupt changes in dominant lithology. Wide Δ 34 S variation is found in lithological intervals characterized by alternation of black shales and phosphorite-bearing sandstones. The narrow Δ 34 S variation is associated with the lithological interval dominated by black shales only. Wide and narrow variation of the Δ 34 S values suggests interplay of various factors in sedimentary environment. These factors include oxygen concentration, clastic sedimentation rate, bottom currents and burrowing activity. Biological productivity and rate of dissimilatory sulphate reduction had important impact on the Δ 34 S variation as well. Wide variation of the Δ 34 S values in the studied section resulted from high biological productivity and high rate of dissimilatory sulphate reduction. Variable degree of clastic sedimentation rate and burrowing activity as well as the activity of poorly oxygenated bottom currents could also cause a co-occurrence of isotopically light and heavy pyrite in differentiated diagenetic micro-environments. Occurrence of organic matter depleted in hydrogen could also result in a wide variation of the Δ 34 S values. Narrow variation of the Δ 34 S values was due to a decrease of biological productivity and low rate of dissimilatory sulphate reduction. Low organic matter supply, low oxygen concentration and bottom currents and burrowing activity were also responsible for narrow variation of the Δ 34 S. The narrow range of the Δ 34 S variation was also due to occurrence of hydrogen-rich organic matter. In the studied section the major change in range of the Δ 34 S variation from wide to narrow appears to be abrupt and clearly associated with change in lithology. The change of lithology and isotopic values may suggest evolution of the sedimentary environment from high- to low-energy and also facies succession from shallow to deeper shelf. The evolution should be linked with the Late Anisian regional transgressive pulse in the Boreal Ocean.