CONSTRUCTION OF THE TRIASSIC AND JURASSIC PORTION OF THE PHANEROZOIC CURVE OF SEAWATER SR-87/SR-86

The Triassic and Jurassic variation of 87Sr/86Sr in seawater is based on 128 87Sr/86Sr values obtained from samples distributed among 18 states and countries in North America, Europe, Africa and Asia. Approximately 90% of the data points are enclosed by upper and lower lines that define a band. For any given time, the correct seawater ratio probably lies within this band. A line drawn within the band represents our estimate of the actual seawater ratio as a function of time. The Triassic and Jurassic curve of seawater 87Sr/86Sr appears to be influenced by a combination of tectonic, eustatic and climatic effects. Decline in the 87Sr/86Sr of seawater for this interval corresponds with systematic rifting of Pangea and gradual submergence of continental areas. Plate interactions and seafloor spreading associated with the breakup of Pangea created new loci for hydrothermal circulation of seawater through oceanic crust, thereby increasing the exchange of low-ratio oceanic-crust Sr with higher-ratio seawater Sr. Progressive flooding of continental areas during this period and increased aridity in the Late Jurassic restricted the input of high-ratio continental Sr to the oceans. These events combined to change the relative contribution of high-ratio and low-ratio Sr entering the ocean such that the seawater curve exhibits a decline from high values in the Early Triassic to low values in the Late Jurassic. Increase in the rate of decline of the seawater curve probably corresponds with the development of the Atlantic spreading center in the Mid-Jurassic. © 1990.