O-18-O-16 AND C-13-C-12 IN LOWER PALEOZOIC ARTICULATE BRACHIOPODS - IMPLICATIONS FOR THE ISOTOPIC COMPOSITION OF SEAWATER

Three hundred and eleven articulate brachiopods, with extensive geographic coverage, spanning the Cambrian to Silurian stratigraphic interval, were analyzed for oxygen and carbon isotopic composition. Cambrian samples have delta-O-18 less-than-or-equal-to 7, parts per thousand, Ordovician samples less-than-or-equal-to -2.4 parts per thousand, and Silurian samples less-than-or-equal-to -1.9 parts per thousand, confirming the previously established trend towards lighter isotopic compositions with increasing age throughout the Lower Paleozoic. Forty-nine "best preserved" Ordovician and Silurian samples were identified based on scanning electron microscopy and trace element analysis. They were found to bracket the isotopic compositions of over 85% of specimens from these stratigraphic intervals supporting widespread preservation of the isotopic signal. Some latest Ordovician and Lower Ludlovian samples associated with shale sequences are apparently "enriched" in O-18. These are interpreted as an environmental phenomenon, perhaps related to water temperature and glaciation. A number of Silurian samples of varying genera and stratigraphic levels are highly enriched in C-13, up to +6 parts per thousand. Some are shale related, but some are associated with carbonate-bearing basins. These are also thought to represent "near-original" compositions, but a single environmental cause is unknown. The present data show that luminescence is not decisive criterion for evaluating the degree of brachiopod preservation. Whole-shell values were isotopically similar to their nonluminescent portions for both oxygen and carbon.