INVITED: CONSTRAINTS FROM GLOBAL CYCLES ON THE OXYGEN ISOTOPIC COMPOSITION OF SEAWATER: IMPLICATIONS FOR EPICONTINENTAL SEAWAYS

The history of the oxygen isotope composition of seawater is inferred from two differing lines of evidence: the first from interpretations of phases precipitated directly from seawater and the second from the patterns of hydrothermal exchange between surface-derived fluids and crustal rocks. The fluxes deduced from the latter studies suggest that for elements such as oxygen and strontium, the competition between fluxes resulting from fluid-rock interaction at mid-ocean ridges and fluxes from chemical weathering controls the isotopic composition of seawater. When exchange rates are cast using rates amenable to tectonic analysis, e.g. spreading rates for the ridge processes and volumetric weathering rates for chemical weathering, both isotopic systems are described by the same tectonic parameters. The Sr isotope ratio of seawater should change at the 2-3 per mil level on short (Myr) timescales whereas the oxygen isotope ratio of seawater is constrained by the ~100 Myr characteristic time for oxygen isotope exchange to vary within the 1 per mil level. For the former studies, there is a trade-off between temperature of formation and changes in the inferred composition of seawater. For recent paleoclimatic analysis, the oxygen isotopic composition of the seawater is known to vary within narrow limits relating to the ice volume, driven by the meteoric water cycle. For longer time intervals, the ¹⁸O-depleted character of ancient (e.g. the Paleozoic) marine samples presents a conundrum for literal interpretations of the measurements that requires that formation temperatures be biologically unreasonable or that the oxygen isotopic composition of the vary in ways in conflict with data from ancient oceanic crust or from inferences on the isotopic composition of ancient meteoric waters. There are enough data from Cretaceous epicontinental seaways to suggest a resolution of the paradox posed by marine carbonate data sets. The volume of a maximum epicontinental seaway can be easily shifted downwards from the global average of seawater by as much as 5 per mil while only affecting the bulk ¹⁸O/¹⁶O ratio of the ocean at the 0.1 per mil level. This interpretation also has consequences for the magnitude of inferred temperature changes for interior tropical seaways.