The oxygen isotopic composition of the oceans, sediments and the seafloor

Abstract

The oxygen isotope history of seawater remains controversial despite decades of study. Isotopic data from fossils and chemical sediments generally indicate significantly ¹⁸O-depleted seas during the Paleozoic and earlier, whereas the isotopic composition of contemporaneous ophiolites imply an ocean indistinguishable from today's. Uncertainty in δ¹⁸O of seawater limits the usefulness of oxygen isotope geothermometry as well as challenges current paradigms of plate tectonics. This paper reviews the controversy and revises earlier estimates on the ¹⁸O-balance within the hydrosphere. The conclusion is that the δ¹⁸O of seawater is mainly buffered by hydrothermal and weathering processes at mid-ocean ridges to a δ¹⁸O value of about 0‰ (SMOW). At values other than 0±2‰, a large countervailing flux of ¹⁸O would return seawater to near 0‰. The δ¹⁸O of Paleozoic, Proterozoic and Archean ophiolites support the above model. The discrepancy in δ¹⁸O between most (but not all) post-Mississippian fossils and chemical sediments and ophiolites must be explained as a combination of: (1) loss of integrity of δ¹⁸O in sediments and fossils; (2) warmer paleoenvironments that lead to the precipitation of lower δ¹⁸O exogenic products; (3) isotopically distinct water masses in inland seas that host the fossils but which are not well mixed with the open ocean that alters the seafloor.