Isotopic composition of strontium in sea water throughout Phanerozoic time

Isotopic analyses of strontium in primary fossil carbonate reveal significant variations in Sr87Sr86 of sea water during the Phanerozoic. The strontium isotopic composition may have been uniform from the Ordovician through the Mississippian, with an average Sr87Sr86 of 0.7078. A subsequent decrease in this value into the Mesozoic is interrupted by two provisionally documented positive pulses in Sr87Sr86—one in the Early Pennsylvanian and one in the Early Triassic. The lowest observed value (0.7068) occurred in Late Jurassic time, and this was followed by a gradual increase to 0.7075 in the Late Cretaceous and a more rapid increase through the Tertiary to 0.7090 for modern sea water. These variations are thought to be the result of a complex interplay of periods of intense volcanism and epeirogenic movements of the continents on a worldwide scale.