Construction of the Silurian and Devonian seawater Sr-87/Sr-86 curve

The estimated path of seawater Sr-87/Sr-86 during the Silurian and Devonian is based on 86 analyses (84 whole rocks) presented here and in other published results. The seawater Sr-87/Sr-86 rose steadily throughout the Silurian from a poorly constrained Value near 0.70793 (Delta sw - 114) at the Ordovician-Silurian boundary. The Sr-87/Sr-86 value at the Silurian-Devonian boundary is near 0.70871 (Delta sw - 36) based on an earliest Devonian limestone suite from New York. The Sr-87/Sr-86 falls through the Early Devonian to reach a low near 0.70772 (Delta sw -135) in the Eifelian and Givetian and rises to near 0.70803 (Delta sw - 104) in Famennian time. The seawater Sr-87/Sr-86 at the Devonian-Mississippian boundary is estimated to be near 0.70805 (Delta sw - 102). The middle Paleozoic sample suite is judged less suitable for defining seawater Sr-87/Sr-86 than those from other Paleozoic periods. The middle Paleozoic seawater Sr-87/Sr-86 variation indicates domination of continental Sr contributions during the Silurian and later Devonian. The high ratio (continental) Sr contribution is controlled by the climate and the area, age and composition of exposed crystalline rocks. The dominance of the oceanic (low ratio) Sr during the Early and part of Middle Devonian is coincident with a reported increase in continental volcanism. The estimated rise and fall of sealevel, leading to flooding or exposure of the continents, does not appear to control seawater Sr-87/Sr-86 during the middle Paleozoic.