Isotopic study of an eastern Ohio carbonate concretion

Abstract

The isotopic composition of a Pennslyvanian siderite concretion indicates that the concretion is of marine origin. The 813C of the siderite was +.7 or .6, essentially equal to the marine carbonate used as standard. The 87Sr/86Sr ratio of the siderite was slightly higher than the accepted value for Pennsylvanian seawater. This can be attributed to the insoluble silica residue in the siderite, incomplete mixing with worldwide ocean water or the particular sample area was not exactly representative of the entire depositional basin. At least some of the sulfur is secondary with the sulfate 834S value (-4.6) being too low for deposition from Pennsylvanian seawater. The sulfides present (FeS2 and ZnS) did not precipitate from the same solution. Pyrite was probably reduced from the seawater, with 32S being biologically enriched. The sulfate that is in the sample is of non-marine origin and was most likely oxidized from the sphalerite since both have the same 834S of -4.6. A source of this sulfur is either old sulfide (probably pyrite) or of magmatic origin.