The estimated path of seawater 87 Sr / 86 Sr during the Cambrian and Ordovician is based on 172 analyses. The sample suite is largely from locations in Oklahoma, New York, Tennessee and Texas where rocks of this age are widely exposed and well studied. The oldest reliable samples examined in this study are of Middle Cambrian age and yield an 87 Sr / 86 Sr near 0.70888 (Δsw−19). The estimated ratio rises to near 0.70915 (Δsw+8) in the lower Upper Cambrian, the highest value during the Phanerozoic and probably for all geologic time. The estimated path drifts lower to near 0.70903 (Δsw−4) at the Cambrian–Ordovician boundary. This further declines to near 0.70875 (Δsw−32) at the end of the Ibexian and reaches a value near 0.70821 (Δsw−86) at the Whiterockian–Mohawkian boundary. The decline in the seawater 87 Sr / 86 Sr continues into later Mohawkian time to flatten abruptly near 0.70782 (Δsw−125). This ratio is essentially constant through the latest Ordovician. The curve rises near to a Ordovician–Silurian boundary value, poorly constrained from our data, near 0.70784 (Δsw−123). The early Paleozoic seawater curve shows the alternating dominance of continental and oceanic sources of Sr. Continental Sr dominated seawater from the beginning of the Cambrian into Late Cambrian. From that time, as the Cambrian transgression proceeded and radiogenic crystalline rocks were covered, oceanic Sr began to dominate. Sr from low ratio (oceanic) sources dominated through early Mohawkian time. An abrupt balance is struck between oceanic and continental sources to end the decline in the Late Ordovician and maintained until near the end of the Ordovician. The general curve shape follows the pattern suggested by proposed sealevel changes. The rise and fall of seawater 87 Sr / 86 Sr during the Cambrian and Ordovician characterized seawater variation during all the Paleozoic.
