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Planktonic foraminifera show 30 to 45 percent of the species disappearing during the 300,000 to 400,000 years prior to the Cretaceous/Tertiary (K/T) boundary in continental shelf (El Kef, Tunisia) and epicontinental sea (Brazos River, Texas) sections. Their disappearance appears to be linked to a sea-level regression and global cooling. At the K/T boundary a 26 percent species reduction coincides with the geochemical anomalies at El Kef; their disappearance appears to be a direct consequence of the K/T boundary event. No change is observed at Brazos River. At both El Kef and Brazos River, many species (11 and 33 percent, respectively) disappear shortly after the K/T boundary, and all but one of the Cretaceous survivors terminally decline in relative abundance beginning at the K/T boundary. This pattern of species extinctions clearly shows a significant decline in species diversity during the latest Maastrichtian, followed by a sudden decline in diversity at the K/T boundary, which drastically and permanently altered planktonic foraminiferal communities. The K/T boundary event, however, did not cause instantaneous extinctions of nearly all species, as commonly claimed. Initial recovery of the ecosystem appears to have occurred about 230,000 years after the K/T boundary event, as implied by an increase in carbonate sedimentation, carbon isotope values, and diversification of planktonic foraminifera.

Carbon and oxygen isotope records of benthic and planktonic foraminifera at Brazos River reveal remarkable new data with far-reaching implications. For instance, the δ13C shift, which characterizes the K/T boundary globally, coincides with the micropaleontologically defined boundary and not with the tsunami deposit of Bourgeois and others (1988), indicating that the latter deposit is independent of the boundary event. Moreover, the δ13C shift occurs gradually over thousands of years, and not instantaneously as recorded in deep-sea sections, implying a more gradual and long-term effect than commonly assumed. Furthermore, stable isotopic data unequivocally show the survivorship of many Cretaceous species well into the early Tertiary.

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