All pelagic JOIDES cores penetrating the Cretaceous-Tertiary boundary exhibit a major unconformity between the Cretaceous and Tertiary. The gaps in these cores are greater than most marine shelf sections, although all are paleontologically unconformable. Furthermore, calcareous nannofossil biostratigraphy indicates that the Cretaceous-Tertiary hiatus is nearly identical in magnitude for these shelf sections. In a nearly continuous Cretaceous-Tertiary sequence in Alabama, almost all Cretaceous species disappear within 3 m of the boundary, or in about 10⁵ years, assuming a pelagic sedimentation rate of 20 m/m.y. Observed nondeposition and/or dissolution at the top of the Cretaceous could have resulted from low carbonate production when widespread tectonic quiesce ce would yield low phytoplankton nutrient supplies. Low carbonate production began to raise compensation depth because O₂ production curtailment in the oceans caused by phytoplankton depletion led to CO₂ buildup in atmosphere and ocean.

Quantitative nannofossil biostratigraphy shows cyclic fluctuations within a continuous uppermost Maestrichtian section in Alabama. With the 20 m/m.y. sedimentation rate, these are similar in duration to climatic variations predicted by astronomical insolation variations or "Milankovitch curves."

It is postulated that an astronomical cycle of low insolation coinciding with widespread nutrient and carbonate depletion led to carbonate undersaturation in the photic zone, producing mass extinctions of nannoplankton and dependent groups at the close of the Cretaceous. The observed magnitude of the Cretaceous-Tertiary unconformity, as indicated by calcareous fossils, is a function of paleobathymetry, with pelagic sections having been below compensation depth longer than shelf sections and consequently exhibiting greater hiatuses.

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