P. Ditchfield, J. D. Marshall; Isotopic variation in rhythmically bedded chalks: Paleotemperature variation in the Upper Cretaceous. Geology ; 17 (9): 842–845. doi: https://doi.org/10.1130/0091-7613(1989)017<0842:IVIRBC>2.3.CO;2
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Detailed sampling at 2 cm intervals of a rhythmically bedded chalk-marl sequence in southeast England has revealed a cyclic variation in oxygen isotopic composition of the carbonate component. These changes are most consistent with paleotemperature fluctuations of up to 4.5 °C. Contrary to some recent models, chalk horizons are interpreted to reflect periods of warmer temperature and higher primary productivity than those responsible for marl deposition. The effects of different faunal content, diagenesis, and variations in the water chemistry can all be discounted.