Accretionary aragonite from a specimen of the bivalve Venericardia hatcheplata collected from the early Eocene Hatchetigbee Formation in southwestern Alabama was milled out along a transect across the growth axis of the valve and the samples analyzed for their stable isotopic compositions. A total of 140 samples define the range of seasonal variation in d¹⁸O for 8 years of growth. Mean composition falls about -3.5‰, and ranges from about -5‰ to -2‰. The amplitude of variation is very regular, with peaks and troughs falling at about the same values except for some attenuation the last two years. This pattern of isotopic variation is similar (in both mean and amplitude of seasonal variation) to that reported for Venericardia planicosta from the early middle Eocene near Hampshire, England, by Purton and Brasier (1999). Unlike the Alabama specimen, however, the English record is cuspate, indicating growth cessations during the cold month(s).

The isotopic values reported here are more negative than expected, even for early Eocene greenhouse conditions. Assuming an ice-free world and an ocean composition of -1‰, paleotemperatures for the Alabama bivalve calculated using Grossman and Ku (1986) range from 25° to 38°C, with a mean of about 31.5°C. Possible explanations for these warm temperatures include freshwater mixing, vital effects resulting in nonequilibrium precipitation, and/or unusually light Eocene shelfal waters. The regular and consistent pattern of variation in this record suggests that reduced salinity due to seasonal influx of meteoric water is not responsible because negative seasonal peaks would be expected to be more variable. In addition, it is unlikely that the same fluctuating salinity conditions would exist within these two localities separated by considerable distance and several million years of time.

As the last explanation is difficult to substantiate, it is most likely that Venericardia did not precipitate shells in isotopic equilibrium with seawater. Importantly, the range of seasonal temperature variation recorded in these two shells, regardless of mean values, is about 13°-14°C. This range is surprisingly high for early Eocene presumably equable climates, and may necessitate a reevaluation of climate conditions in the low to mid latitudes during this time.