Seafloor weathering and the oxygen isotope ratio in seawater: Insight from whole-rock δ18O and carbonate δ18O and Δ47 from the Troodos ophiolite

Abstract

The controls on, and history of, the oxygen isotope ratio in seawater continue to be debated after many decades of research with the lack of consensus in large part reflecting uncertainty in the role of hydrothermal exchange between seawater and the oceanic crust. We have investigated this using new carbonate Δ47 and δ18O data, and whole-rock O-isotope data, for samples from the lava section of the Troodos ophiolite. Carbonate data confirm that fluid-to-rock ratios in the upper lavas during off-axis hydrothermal circulation are generally sufficiently large that both the fluid δ18O and temperature are similar to those of bottom water. However, some samples require more complicated interpretations that could reflect changes in the rate of calcite formation. Whole-rock data indicate that O-isotope exchange in the lavas is directly linked to the major element exchange that leads to alkalinity production (i.e., CO2 consumption) and both are dependent on bottom water temperature. This means that the O-isotopic composition of seawater is linked to the long-term C-cycle. The data are used to parameterise a simple model of the evolution of the O-isotopic composition of seawater driven by changes in solid earth CO2 degassing. Alkalinity balance links the total extent of weathering of the continents and seafloor, which are sinks for high δ18O material, to CO2 degassing rate and surface temperature. The modelling suggests that if solid earth CO2 degassing and the rate of formation of oceanic crust are linked, the O-isotopic composition of the ocean (including any ice sheets) is unlikely to have varied more than ±1‰ over the Phanerozoic.