Abstract
Abstract. To quantify the intensity of oceanic oxygen isotope buffering through hydrothermal alteration of the oceanic crust, a 2D hydrothermal circulation model was coupled with a 2D reactive transport model of oxygen isotopes. The coupled model calculates steady-state distributions of temperature, water flow and oxygen isotopes of solid rock and porewater given the physicochemical conditions of oceanic crust alteration and seawater δ18O. Using the present-day seawater δ18O under plausible modern alteration conditions, the model yields δ18O profiles for solid rock and porewater and fluxes of heat, water and 18O that are consistent with modern observations, confirming the model's validity. The model was then run with different assumed seawater δ18O values to evaluate oxygen isotopic buffering at the midocean ridges. The buffering intensity shown by the model is significantly weaker than previously assumed, and calculated δ18O profiles of oceanic crust are consistently relatively insensitive to seawater δ18O. These results are attributed to the fact that isotope exchange at shallow depths does not reach equilibrium due to the relatively low temperatures, and 18O supply via spreading solid rocks overwhelms that through water flow at deeper depths. Further model simulations under plausible alteration conditions during the Precambrian showed essentially the same results. Therefore, δ18O records of ophiolites that are invariant at different Earth ages can be explained by the relative insensitivity of oceanic rocks to seawater δ18O and do not require constant seawater δ18O through time.
Highlights
Hydrothermal alteration of oceanic crust at midocean ridges works as the dominant source and/or sink of several elements and/or isotopes in the ocean (e.g., Wolery and Sleep, 1976; Elderfield and Schultz, 1996)
After confirming the model’s validity by comparing the model results that assume the present-day seawater δ18O with modern observations, we examine the intensity of oceanic δ18O buffering by hydrothermal alteration of oceanic crust at midocean ridges by changing seawater δ18O
The 2D reactive transport model of oxygen isotopes combined with 2D hydrothermal circulation simulations enables us to predict distributions of temperature, water flow and oxygen isotopes of solid rocks and porewaters within oceanic crust based on mass, momentum and energy conservations
Summary
Hydrothermal alteration of oceanic crust at midocean ridges works as the dominant source and/or sink of several elements and/or isotopes in the ocean (e.g., Wolery and Sleep, 1976; Elderfield and Schultz, 1996). The close balance between the addition and removal of the heavy isotope, together with the huge oxygen supply from the mantle, has led to a hypothesis that the water–rock interactions at midocean ridges have buffered oceanic δ18O at the present-day value (0 ‰ relative to standard mean ocean water – SMOW) throughout the Earth’s history (e.g., Muehlenbachs and Clayton, 1976; Gregory and Taylor, 1981; Holland, 1984; Muehlenbachs, 1998). Age-invariant δ18O records of ophiolites (ancient oceanic crust) have been argued to support the hypothesis (e.g., Holmden and Muehlenbachs, 1993). If constituting minerals of sedimentary rocks were formed in equilibrium with seawater, and if the later diagenetic or metamorphic modification of δ18O was negligible, δ18O records of sedimentary rocks can be utilized to infer surface temperatures of the past, using temperature-
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