Abstract:
Maastrichtian foraminiferal assemblages at two ODP sites on Blake Nose (western North Atlantic) separated by approximately 30 km show similar trends through time as well as consistent subtle differences between correlative samples. At both sites the relative abundance and diversity of large, ornamented taxa increases upsection consistent with warming and greater surface stratification through the Maastrichtian. In addition, the more offshore, deeper site (1050) contains distinct meter-scale cyclic variations in the relative abundance of many taxa that varies in parallel with cyclic variations in sediment color, stable isotopic values, and mineralogy. Cycles are effectively absent at the more shoreward, shallower site (1052), and the assemblages at Site 1052 are close to one end of the range of cyclic variability seen in similar-aged samples from Site 1050. Patterns of abundance among Heterohelix spp. (relatively abundant onshore and in the higher productivity portions of the offshore cycles) and Globotruncana spp. (relatively abundant in the low productivity portions of the offshore cycles) are particularly notable. A number of other taxa show consistently higher relative abundances in either the nearshore (e.g., Archaeoglobigerina) or offshore (e.g., Rugoglobigerina) site. Combined with previously documented onshore-offshore trends among nannofossil taxa, these observations are interpreted as indicating that Site 1052 was shoreward of a western boundary current while this current flowed near or over Site 1050 through most the Maastrichtian. The apparent proximity of the current to Site 1050 through time despite the considerable paleoceanographic changes that occurred during the Maastrichtian argues that some permanent feature (e.g., local bathymetry) acted to anchor the position of the current.