Geochemistry of brachiopods: Oxygen and carbon isotopic records of Paleozoic oceans

Combined trace element and isotope studies of 319 brachiopods, covering the Ordovician to Permian time span, show that δ ¹³C and δ ¹⁸O in well preserved specimens varied during the Paleozoic. The overall δ ¹³C secular trend is in accord with the previously published observations, but its details are obscured by vital isotopic fractionation effects at generic level. Nonetheless, the results suggest that the negative correlation between marine δ ¹³C _carbonate and δ ³⁴S _sulphate deteriorates at time scales of ⩽ 10 ⁶ years, due to the long residence time, and thus slow response, of SO ₄^2- in the ocean. For oxygen isotopes, all Devonian and older specimens have δ ¹⁸O of ⩽ -4%, while the well preserved Permian samples have near-present day δ ¹⁸O of about -1% (PDB). This isotopic dichotomy is probably not due to post-depositional phenomena, salinity, or biogenic fractionation effects. This leaves open the perennial arguments for a change in ¹⁸O /¹⁶O of sea water versus warmer ancient oceans. The present data are difficult to explain solely by the temperature alternative. The coincidence of the proposed shift in δ ¹⁸O with the large Late Paleozoic changes in marine ⁸⁷Sr /⁸⁶Sr , ¹³C /¹²C , ³⁴S /³²S , and "sea level stands" argues for a tectonic cause and for a change in ¹⁸O /¹⁶O of sea water, although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites. Whatever the precise cause, or combination of causes, the implications for tectonism and/or paleoclimatology are of first order significance.