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

• Published in: Chemical geology Vol. 145; no. 3; pp. 263 - 273
• Main Author: Muehlenbachs, Karlis
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.04.1998
• Subjects: Ophiolites; Oxygen isotope; Seafloor; Seawater; Sediments; Seafloor; Ophiolites; Oxygen isotope; Sediments; Seawater
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/S0009-2541(97)00147-2

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 δ 18O 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 δ 18O of seawater is mainly buffered by hydrothermal and weathering processes at mid-ocean ridges to a δ 18O value of about 0‰ (SMOW). At values other than 0±2‰, a large countervailing flux of 18O would return seawater to near 0‰. The δ 18O of Paleozoic, Proterozoic and Archean ophiolites support the above model. The discrepancy in δ 18O 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 δ 18O in sediments and fossils; (2) warmer paleoenvironments that lead to the precipitation of lower δ 18O 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.