Abstract

Two records of seawater sulfate isotope composition from the Early Jurassic demonstrate that large isotopic gradients existed between the European epicontinental sea and the open Tethys Ocean. These differences can be explained by the modification of open-ocean sulfate isotopic compositions by water-mass isolation, sea-level rise, and the effects of changing regional weathering and pyrite burial fluxes, during a time of rapid environmental change. Both records contain large positive isotopic excursions. In the section from Europe (Yorkshire, UK), a 6‰ excursion begins in the early exaratum subzone of the Toarcian in the middle of the organic-rich shale representing a well-known oceanic anoxic event. An open Tethyan margin record from Tibet records a much larger 19‰ excursion, but the section is less well dated. Two age interpretations are possible: sparse biostratigraphic evidence places this excursion in the Aalenian, but we suggest that it may correlate with the positive excursion in Yorkshire. Hence these records may document both a Toarcian event and an Aalenian sulfur isotope event, or the early Toarcian anoxic event alone. Conservative estimates of the rate of isotopic change with time based on the Tibetan section suggest that Early Jurassic seawater sulfate concentrations were between 1 and 5 mM, much lower than previously thought.

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