The middle Cretaceous (125-88 Ma) greenhouse world was characterized by high atmospheric CO (sub 2) levels, the general absence of polar ice caps, and much higher global temperatures than at present. Both delta (super 18) O-based and model-based temperature reconstructions indicate extremely high sea-surface temperatures (SSTs) at high latitudes. However, there are a number of uncertainties with SST reconstructions based on delta (super 18) O isotope data of foraminifera due to diagenetic overprinting effects and tenuous assumptions with respect to the delta (super 18) O value of Cretaceous seawater, the paleoecology of middle Cretaceous marine organisms and seawater pH. Here we applied a novel SST proxy (i.e., TEX (sub 86) [tetraether index of 86 carbon atoms], based on the membrane lipids of marine crenarchaeota) derived from middle Cretaceous sedimentary rocks deposited at low latitudes. The TEX (sub 86) proxy indicates that tropical SSTs in the proto-North Atlantic were at 32-36 degrees C during the early Albian and late Cenomanian-early Turonian. This finding agrees with SST estimates based on delta (super 18) O paleothermometry of well-preserved foraminifera as well as global circulation model calculations. The TEX (sub 86) proxy indicates cooler SSTs (27-32 degrees C) for the equatorial Pacific during the early Aptian, which is in agreement with SST estimates based on delta (super 18) O paleothermometry.