Interpretation of Oxygen Isotopes in Phanerozoic Ophiolites and Sedimentary Rocks

Abstract

Abstract Two reactive transport models were developed to integrally simulate fluxes of oxygen isotopes from hydrothermal alteration of oceanic crust and weathering of continental rocks for a better understanding of the regulation of oxygen isotopes in the ocean. The hydrothermal alteration model consists of three boxes to represent pillow basalt, sheeted dike, and gabbro sections and simulates the steady‐state δ18O of solid rocks and porewaters in three individual sections along the spreading direction. The continental weathering model similarly calculates steady‐state δ18O profiles along the uplift direction. The two models were run in concert to simulate the evolution of seawater δ18O during the Phanerozoic, reflecting the relevant forcing factors to the corresponding parameter values of the models as functions of time. The seawater δ18O was calculated to have evolved from less than −5‰ in the early Paleozoic to between −2 and 0‰ for the Mesozoic and Cenozoic, as a result of declines in atmospheric CO2 and land area of igneous and high‐grade metamorphic rocks. The transition of seawater δ18O is similar to the δ18O trend that is commonly recorded in authigenic sedimentary rocks such as carbonates and cherts. The simultaneously simulated δ18O values for altered continental and oceanic rocks are decoupled from seawater δ18O because of kinetic inhibition and solid‐rock buffering at respective low and high temperatures and are similar to the shale and ophiolite δ18O records. Overall consistency between the simulation and observations supports the dynamic behavior of oceanic δ18O over the Phanerozoic eon.