Thermal evolution of Cretaceous Tethyan marine waters inferred from oxygen isotope composition of fish tooth enamels

摘要

The evolution of subtropical (30-35°N) upper ocean temperatures through the Cretaceous is inferred from the oxygen isotope compositions of 64 fish teeth (enamel) coming from the western Tethyan platform. Mean δ~(18)O values of 22‰ at the Berriasian-Valanginian boundary decrease, with oscillations to 18.5‰ around the Cenomanian-Turonian boundary, and progressively increase to 21.5‰ by the end of the Cretaceous. The similarity of this oxygen isotope curve for bioapatites from platform environments with those for foraminifera and bulk carbonates that were deposited in deeper waters and at other paleolatitudes indicates that they record global climatic signals. Major cooling events at the million-year scale can be distinguished: (1) at the Berriasian-Valanginian boundary and (2) during the earliest Late Valanginian. A third cooling event is detected during the earliest Aptian. These events, already proposed as icehouse interludes during the lower Cretaceous, are also recorded at subtropical latitudes. A progressive warming is identified from the Aptian to the Cenomanian-Turonian interval that corresponds to a thermal optimum, and then upper ocean temperatures decreased to the Maastrichtian. Minimum isotopic temperatures range from 15 ℃ to 28 ℃, assuming a δ~(18)O_(seawater) of -1‰, for an ice-free world. Taking more realistic δ~(18)O_(seawater) values of ～0‰ for tropical waters, during glacial periods (within the Berriasian-Valanginian interval, and earliest Aptian) or with above average salinities (possibly the Maastrichtian), temperatures are increased by 4-5 ℃. Temperature differences between climatic extremes of the Valanginian and Cenomanian-Turonian are estimated to have been 10 ℃. Latitudinal thermal gradients for the Albian-Cenomanian, Turonian, and Maastrichtian were 0.2-0.3 ℃/° latitude and thus weaker than modern oceanic values at about 0.4 ℃/° latitude.