The estimated path of seawater 87Sr/86Sr during the Silurian and Devonian is based on 86 analyses (84 whole rocks) presented here and in other published results. The seawater 87Sr/86Sr rose steadily throughout the Silurian from a poorly constrained value near 0.70793 (Δsw −114) at the Ordovician–Silurian boundary. The 87Sr/86Sr value at the Silurian–Devonian boundary is near 0.70871 (Δsw −36) based on an earliest Devonian limestone suite from New York. The 87Sr/86Sr falls through the Early Devonian to reach a low near 0.70772 (Δsw −135) in the Eifelian and Givetian and rises to near 0.70803 (Δsw −104) in Famennian time. The seawater 87Sr/86Sr at the Devonian–Mississippian boundary is estimated to be near 0.70805 (Δsw −102). The middle Paleozoic sample suite is judged less suitable for defining seawater 87Sr/86Sr than those from other Paleozoic periods. The middle Paleozoic seawater 87Sr/86Sr 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 87Sr/86Sr during the middle Paleozoic.
