Detecting the onset and effects of major northern hemisphere glaciation in the abyssal tropical Atlantic Ocean

Abstract. The cooling trend of the Neogene resulted in the diachronous development of glacial conditions, southern high latitudes glaciating before northern. This cooling culminated in the early Pleistocene (2.54 Ma) onset of major glaciation, during which large areas of high latitude land and sea in both hemispheres were periodically blanketed with ice. However, this onset and its impact at low latitude abyssal depths are elusive. This paper examines the abyssal benthic foraminiferal commun ity in the Atlantic Ocean near the Northern Equatorial Countercurrent, in ODP Hole 926A (Ceara Rise), off the River Amazon. Using an assemblage turnover index (ATI), a related conditioned-on-boundary index (CoBI) and SHE analysis, we show how a change in mean assemblage turnover coincides with the early Pleistocene onset of northern hemisphere major glaciation. The community comprises primarily phytodetritivores (Alabaminenella weddellensis, Epistominella exigua and Globocassidulina subglobosa), the proportional abundances of which differ before and after 2.54 Ma. Coupled with changes in the abundances of Nuttallides umbonifera, Bulimina spp. and Uvigerina spp., alterations in the abundances of the phytodetritivores after 2.54 Ma show that the adjustment in biotic turnover at depth resulted from enhanced organic carbon flux from sea surface water. This was probably related to forced southward repositioning of the inter-tropical convergence zone (ITCZ) by the growth of the northern hemisphere ice caps. This was coupled with an increase in bottom current velocity, as shown by a change in the abundance of Cibicidoides wuellerstorfi.