THE STRONTIUM ISOTOPIC COMPOSITION OF BRIGANTIAN (LATE DINANTIAN) SEAWATER

Strontium isotopic determinations of different fossiliferous biominerals from Brigantian (= late Dinantian) carbonates in northern England reveal that many are affected by diagenetic overprinting of their original Sr-87/Sr-86 ratios. Only pristine non-luminescent articulate brachiopod shell material can be used to gain reliable estimates of Carboniferous and, by deduction, Palaeozoic ocean chemistries. Conodonts are found to be the only type of phosphatic biomineral capable of preserving original palaeoseawater chemistries. However, this only applies to specimens subjected to minimal burial temperatures. Other phosphatic biominerals are conclusively shown to be readily susceptible to diagenetic overprinting of original Sr isotopic ratios. A Brigantian strontium curve, compiled from pristine brachiopod calcite, shows a relatively complex evolution with time but it can be resolved into two separate components, operating on different time scales. A cyclic, sinusoidally-shaped component, with a frequency of 1 Ma appears to fluctuate about a longer-term, more gradual, linear increase in the Brigantian seawater isotopic composition.