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

Abstract

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 (Δ34S) and TOC. Isotopic composition of syngenetic pyrite-bound sulphur shows wide (Δ34S from -26‰ to +8‰ VCDT) and narrow (Δ34S from -4‰ to +17‰ VCDT) variation of the Δ34S in upper and lower part of the section, respectively. Range of the variation is associated with abrupt changes in dominant lithology. Wide Δ34S variation is found in lithological intervals characterized by alternation of black shales and phosphorite-bearing sandstones. The narrow Δ34S variation is associated with the lithological interval dominated by black shales only. Wide and narrow variation of the Δ34S 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 Δ34S variation as well. Wide variation of the Δ34S 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 Δ34S values. Narrow variation of the Δ34S 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 Δ34S. The narrow range of the Δ34S variation was also due to occurrence of hydrogen-rich organic matter. In the studied section the major change in range of the Δ34S 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.