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

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. δ¹³C_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 (δ¹³C_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 δ³⁴S_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 δ¹³C and δ³⁴S data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface. The δ₁₈O values of the calcite in one concretion (δ¹⁸O_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 (δ¹⁸O_PDB?8.9 to ?9.9%.) and large δ¹⁸O differences between concretions in closely-spaced horizons imply that local factors control the isotopic composition of pore waters.