Oxygen isotope evolution of Phanerozoic seawater

A compilation of over 2000 measurements of 18O and 13C on Phanerozoic low-Mg calcite shells, such as brachiopods, belemnites and oysters, delineates secular 18O/16O and 13C/12C variations that are similar to those previously described for whole rocks. The trend for the δ18O suggests about ∼5±2‰ enrichment from the Cambrian to today. In contrast, the δ13C rise during the Paleozoic is followed by its decline in the Mesozoic and Cenozoic. Optical (textural) and chemical criteria suggest that the interior “secondary” layer of the brachiopod shells, the material that carries these signals, is well preserved in many samples and the extracted secular isotopic trends are therefore a primary feature of the geologic record. The similarity of the δ18O/δ13C isotope patterns in ancient and modern brachiopods also supports such an interpretation. In our view, the 18O enrichment in progressively younger samples is principally, although not exclusively, a reflection of the evolving 18O/16O composition of seawater. If so, a delineation of this trend may ultimately result in development of a valuable paleoclimatic and paleoceanographic tracer for the Phanerozoic.