ISOTOPIC EVIDENCE FOR GEOCHEMICAL DECOUPLING BETWEEN ANCIENT EPEIRIC SEAS AND BORDERING OCEANS - IMPLICATIONS FOR SECULAR CURVES

Isotopic analysis of conodonts and their host limestones sampled betwe en two regionally extensive, altered volcanic ash layers in eastern La urentia shows that a 454 Ma epeiric sea maintained large lateral diffe rences in Nd and C isotope compositions. This is consistent with infer red temperature-salinity-defined epicontinental water masses and restr icted circulation between epicontinental and oceanic environments. Bec ause the majority of old marine fossils and sedimentary rocks are know n from epeiric seas, some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses, rather than global-scale changes in the state of the earth-ocean system.