Carbon, oxygen and sulphur isotope variations in concretions from the Upper Lias of N.E. England

Details Carbon, oxygen and sulphur isotope variations in concretions from the Upper Lias of N.E. England Carbon, oxygen and sulphur isotope variations in concretions from the Upper Lias of N.E. England

Mar 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981gecoa..45..329c&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 45, Issue 3, pp.329-340

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Scientific paper

Carbon, oxygen and sulphur isotope data for transects across two pyrite-bearmg carbonate concretions, and their host sediments, from the Upper Lias of N.E. England show symmetrical zonation. 13 C PDB values of the calcite cement (-12.9 to -15.4%.) indicate that most of it originated from organic matter by bacterial reduction of sulphate, augmented with marine and, to a lesser extent, fermentation derived carbonate. Organic carbon ( 13 C PDB = -26.1 to -37.0%.). reflects the admixture of allochtho-nous terrestrial organic matter with marine material and the selective preservation of isotopically light organic material through microbiological degradation. Two phases of pyrite are present in each concretion. The earlier framboidal pyrite formed throughout the sediment prior to concretionary growth and has 34 S CD values of -22 to -26%. indicating formation by open system sulphate reduction. The later euhedral phase is more abundant and reaches values of - 2.5 to - 5.5%. at concretion margins. This phase of sulphate reduction provided the carbonate source for concretionary growth and occurred in a partially closed system. The 13 C and 34 S data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface. The 18 O values of the calcite in one concretion ( 18 O PDB = 2.3 to -4.8%.) indicate precipitation in pore waters whose temperature and isotopic composition was close to that of overlying seawater. The other concretion is isotopically much lighter ( 18 O PDB -8.9 to -9.9%.) and large 18 O differences between concretions in closely-spaced horizons imply that local factors control the isotopic composition of pore waters.

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