OXYGEN-ISOTOPE STUDY OF MIDOCEAN RIDGE HYDROTHERMAL FLUIDS - IMPLICATION FOR THE O-18 BUDGET OF THE OCEANS

New data are reported for the O-18 composition of hot hydrothermal flu ids from various sites with different tectonic settings. The measured delta(18)O, in the range 0.53-2.3 parts per thousand confirm earlier d ata suggesting that mid-ocean ridge hydrothermal vents are a major sou rce of O-18 to the ocean. In light of this new data, we attempt to re- evaluate the marine O-18 budget from direct quantification of the main sources and sinks of oceanic oxygen-18. Within the uncertainty of the flux estimates, O-18 inputs and outputs are found to balance each oth er, showing that oceanic O-18 is presently at a steady state. Using, t hese present fluxes, we examine the past evolution of the O-18 composi tion of the ocean and speculate that the initial O-18 oceanic content may have evolved rapidly towards its present value. This suggests, as already pointed out in previous studies, that ocean water may have had a rather constant O-18 composition over most of the Earth's history a nd that delta(18)O measurements in ancient cherts and carbonates may p rimarily record the evolution of the ocean temperature. Copyright (C) 1997 Elsevier Science Ltd.