THE STRONTIUM ISOTOPIC COMPOSITION OF BRIGANTIAN (LATE DINANTIAN) SEAWATER

Strontium isotopic determinations of different fossiliferous biominera ls from Brigantian (= late Dinantian) carbonates in northern England r eveal that many are affected by diagenetic overprinting of their origi nal Sr-87/Sr-86 ratios. Only pristine non-luminescent articulate brach iopod shell material can be used to gain reliable estimates of Carboni ferous and, by deduction, Palaeozoic ocean chemistries. Conodonts are found to be the only type of phosphatic biomineral capable of preservi ng original palaeoseawater chemistries. However, this only applies to specimens subjected to minimal burial temperatures. Other phosphatic b iominerals are conclusively shown to be readily susceptible to diagene tic overprinting of original Sr isotopic ratios. A Brigantian strontiu m 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 seawat er isotopic composition.