CHANGES IN OCEANIC PRIMARY PRODUCTIVITY TRIGGERED BY THE CRETACEOUS/TERTIARY BOUNDARY EVENT AT BLAKE NOSE (NW ATLANTIC): INFERENCES FROM BENTHIC FORAMINIFERA

During Ocean Drilling Program Leg 171B the K/T boundary interval was recovered in Hole 1049C at Blake Nose, NW Atlantic, ~1600 km from the Chicxulub impact site. In this hole the K/T boundary is located at the base of a 9 cm-thick layer with abundant spherules, topped by a limonitic layer. Uppermost Maastrichtian oozes below, and lowermost Danian pelagic oozes above the spherule-bed contain well-preserved benthic foraminifera. We performed a detailed, quantitative analysis of benthic foraminifera in sections 1049C-8X-5 through -8X-1). Benthic foraminifera in the uppermost Maastrichtian and lowermost Danian oozes indicate deposition in a lower bathyal environment, but those in the spherule-bed vary strongly in their preservation, and consist of a mixture of neritic and bathyal species. These assemblages probably resulted from downslope transport related to the K/T boundary impact. Upper Maastrichtian benthic foraminiferal assemblages indicate a stable environment characterized by an abundant organic flux to the sea floor, probably resulting from upwelling in the region of Blake Nose. At Blake Nose as at other locations worldwide benthic foraminiferal extinction at the K/T boundary was not significant, but nutrient supply to the sea-bottom floor decreased during the earliest Danian, as indicated by a decrease in the relative abundance of infaunal morphogroups. The decrease in primary productivity was most likely caused by impact-induced mass extinction of primary producers and persisted over a time period less than Biozone P alpha. Increased relative abundances of infaunal taxa indicate a recovery of the food flux, but low-diversity assemblages suggest that normal marine conditions did not return until ~ 400 kyr after the K/T boundary. We suggest that primary productivity during the period of low diversity could have been dominated by blooms of non-calcareous primary producers, which created a stressful environment for the benthos.