For geologists and geochemists concerned about the chemical and thermal evolution of oceans, deciphering primary marine delta 13 C and delta 18 O values from ancient marine carbonates can provide useful data for the physical and chemical modelling of ancient oceans. Although abiotic aragonite and magnesian calcite marine cements have commonly been overlooked because of their susceptibility to diagenetic alteration, petrographic and chemical analysis of radiaxial fibrous calcite from two Upper Devonian (Frasnian) pinnacle reefs (Golden Spike and Nevis) of central Alberta indicates that these magnesium-rich cements preserve original marine delta 18 O and delta 13 C values. In the pursuit of primary marine isotopic compositions it is necessary to characterize the isotopic compositions of diagenetic cement phases. Subaerial exposure and meteoric diagenesis of the interior of the Golden Spike reef is recorded by vadose cements, caliche, and clear, non-ferroan, meteoric calcite spar with distinctive black and yellow cathodoluminescence (CL) zones. delta 18 O-delta 13 C crossplots of caliche and meteoric phreatic cement data form an "inverted J-curve" typical of rock-water reactions in meteoric systems. A late phase of calcite spar with variable inclusion density and Fe (super 2+) content and diffusely-zoned red-orange CL is associated with late fractures and stylolites, and it precipitated during subsequent burial of the reef. Unaltered centers of radiaxial fibrous cement crystals are nonluminescent, inclusion-free, and enriched in magnesium, while diagenetically altered crystal terminations, inter-crystalline boundaries, and crystal substrates are brightly luminescent, inclusion-rich (microdolomite and fluid inclusions), low-Mg calcite. Altered cloudy marine cements have variable delta 18 O and invariant delta 13 C values which define a trend that diverges from isotopically heavier unaltered marine cements. The isotopic compositions of inclusionrich marine cement are coincident with those of meteoric phreatic spars. Although burial spars are present in all samples, petrographic and chemical data suggest that portions of the marine cement crystals were altered by meteoric fluids. The delta 18 O values of unaltered radiaxial fibrous calcite are similar (invariant) throughout both reefs, although marine cements from older (deep) parts of both reefs are enriched in 13 C relative to younger (shallow) parts. This records temporal changes in the delta 13 C value of marine bicarbonate within the Alberta Basin. Variability of delta 18 O and delta 13 C values of unaltered marine cements is very small (+ or - 0.5 per thousand for a given suite of samples), making these cements ideal for high time resolution studies of the isotopic evolution of ancient oceans. The presence of well preserved magnesium-rich calcite from Late Devonian reefal sequences suggests that marine cements throughout the Phanerozoic should be examined more carefully, as they may preserve primary isotopic signatures. Independent confirmation of primary isotopic compositions is found in similar values for marine cements from Middle Frasnian reefs of the Canning Basin (Australia) and the Dinant Synelinorium (Belgium). The delta 18 O values for the well preserved marine cements of this study are considerably lower than modern marine carbonate values (by 3 to 4 per thousand ). Detailed chemical and petrographic analysis indicates that diagenetic alteration is not the cause of these low delta 18 O values. In addition, such a difference cannot be fully explained by warmer Late Devonian oceans. These data suggest that Late Devonian seawater had a delta 18 O value lower than modern oceans. This in turn, suggests that the balance of low temperature silicate weathering and high temperature seawater-basalt exchange reactions must have been different from today's.