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[1] The evolution of subtropical (30-35°N)

High precision oxygen isotope analyses were made of phosphate extracted from 17 samples of nektonic and benthic fish debris sampled across the stratigraphic Cretaceous-Tertiary boundary in northern Morocco. A refinement of the silver phosphate method was used to isolate phosphate from biogenic materials. Measured ~180 values of 18.6-20.5%o are interpreted as reflecting high-resolution thermal variations that affected the ocean water column of the western Tethys. The warm (27°C) water masses that characterized Maastrichtian times underwent rapid cooling and stabilized at an average temperature of 19°C during the Dano-Montian and Thanetian. This period of constant and cool temperature was followed by a relatively rapid but more gradual warming to about 25°C achieved in the Middle Ypresian. Significant small shifts in 6180 values between nektonic and benthic fauna recorded only during the stages of rapid warming or cooling may correspond to averaged thermal differences within the water column that developed in response to global climatic changes. The indicated temperature distribution could have been caused by thermal changes in the atmosphere rather than some signal carried by deep ocean currents. The oxygen isotope data coupled with previous measurements of REE and eNd~TI on the same samples support the suggestion that paleo-Pacific westward currents progressed as far as the northwestern part of the African platform at the end of the Cretaceous period.

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