There are disagreements whether the tropical sea surface temperature during the warm early to early middle Eocene was equal to the present temperature or significantly lower. In order to determine the sea surface temperature of the eastern tropical Atlantic in the early middle Eocene we have established δ18O and δ13C profiles along the transect of growth of excellently preserved shells of Turritella gastropods and a Macoma bivalve from tropical West Africa. This method makes possible a distinction between the winter and the summer temperature. The δ18O and δ13C data indicate sea surface temperatures of 29°–31°C, which declined to 23°–25°C during seasonal upwelling. These conditions are similar to those of today and suggest that significant seasonal dynamics prevailed in the early middle Eocene tropics. The new data further contradict that the tropics cooled during the generally warm early to early middle Eocene, as inferred in studies on foraminifera δ18O.
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