OXYGEN ISOTOPIC COMPOSITION OF THE PALEOZOIC AND PRECAMBRIAN OCEAN WATER

A fundamental parameter describing the ocean is its oxygen isotopic composition. If and by how much the O-isotopic composition of seawater may have changed over geological time has been one of the longest standing controversies in geochemistry. The comparatively low 18-O isotopic composition of most Paleozoic and older sediments suggests a major secular trend to lower 18-O oceans whereas all ophiolites and ore deposits indicate seawater 18-O has not changed by more than +/- 2 per mil. Consequences drawn from either view are profound. Some have assumed low 18-O Paleozoic oceans and concluded from sediments that atmospheric CO2 is decoupled from climate and that the volume of the oceans have radically changed. Others assume a modern value for Archean seawater and suggest a very hot ocean, from chert analyses. The 18-O of the present ocean reflects a balance between isotopic enrichments (mostly within basalts) and depletions (within dykes and gabbros) linked to seafloor alteration and plate tectonic processes. Thus, the deduction of a modern value for the Proterozoic ocean implies a similar tectonic regime at that time.

Here we report a new 18-O profile through the 1950 million-year-old Joruma ophiolite (Finland). The pillow basalts range from 8.5 to 4.0 (SMOW) (Avg. 7.2); dykes, from 4.7 to 1.4 (Avg. 3.1); gabbros, 4.7 to 4.3 (Avg. 4.5). The Jormua ophiolite records 18-O enrichments of its pillow lavas with a concomitant depletion within the sheeted dykes and gabbros as observed in the modern oceanic crust. A similar 18-O profile has already been reported for the contemporaneous Cape Smith ophiolite. The coincident 18-O enrichment by low temperature alteration of basalt and 18-O depletion of dykes and gabbros at higher temperature is only possible if Proterozoic seawater had an isotopic composition similar to the modern value. This new data, taken with published 18-O profiles from Paleozoic, Proterozoic, and even Archean ophiolites, indicates that the oxygen isotope composition of seawater has been near 0 per mil throughout Earth history. The secular trend observed in most old sediments need not reflect unusual processes but rather hotter oceans, isolated shallow seas, or sediment diagenesis