Cenozoic Deep-Sea Circulation: Evidence from Deep-Sea Benthic Foraminifera

Abstract

Deep-sea benthic foraminiferal faunas reflect the deep oceanic environment, the character of which is determined by interaction of deepwater circulation patterns, physicochemical parameters of the surface waters in the deepwater source areas, and nutrient influx from primary productivity in overlying surface waters. Three periods of turnover in deep-sea benthic foraminiferal assemblages can be recognized in Cenozoic sequences: (1) rapid (<10 4 yr), global extinction in the latest Paleocene, followed by migration and diversification; (2) gradual turnover in the late middle Eocene through early Oligocene, characterized by a decrease in diversity, a decrease in relative abundance of Nuttallides truempyi followed by its extinction, and a decreasing relative abundance or disappearance of Bulimina species in the lower bathyal to upper abyssal zones; and (3) gradual turnover in the late early through middle Miocene, characterized by the decrease in relative abundance or disappearance of uniserial species from the lower bathyal to abyssal reaches, the migration of miliolid species into these regions, and the evolution of Cibicidoides wuellerstorfi. The rapid mass extinction (35-50% of species) of deep-sea benthic foraminifera in the latest Paleocene was coeval with a transient 1-2%o decrease in oxygen and carbon isotope ratios in benthic as well as planktonic foraminifera, superimposed on longer-term changes. The extinction could have resulted from a shift in dominant deepwater formation from high to low latitudes. Such a shift would change temperature and oxygen content of the intermediate to deep waters, but it would also change local nutrient input by changing global patterns of upwelling of nutrient-rich waters to the surface and thus of high-productivity areas. Faunal evidence suggests that this reversed pattern of oceanic circulation persisted no longer than the early Eocene, and possibly not more than about half a million years. The two periods of gradual benthic faunal changes overlap in time with two relatively rapid (of the order of 105 years) shifts toward heavier oxygen isotopic values of benthic foraminifera, in the earliest Oligocene and middle Miocene. Faunal changes started before the isotopic changes and were more gradual. The faunal changes might reflect periods of gradual change in the physicochemical character of surface waters in the source areas of deepwater formation (e.g., decrease in temperature), as well as changes in oceanic productivity. The more rapid changes in oxygen isotopic values are not directly reflected in benthic foraminiferal assemblage changes and might represent, at least in part, a rapid buildup of ice volume on land, a process that cannot be reflected in the benthic foraminifera faunas.