Benthic and planktonic foraminifers and stable isotopic analysis of mass-flow sediments in the Amazon Fan

Abstract

Amazon Fan deep mass-transport deposits (MTDs) were analyzed to investigate their dynamic nature, in particular their sediment sources. We have analyzed both the benthic foraminiferal assemblages and the stable isotopic composition of benthic and planktonic foraminifers from sediments recovered during Ocean Drilling Program Leg 155. Samples studied came from Sites 936 and 933, which were located in ~3300 m of water depth. The MTDs are referred to, respectively, as Unit R and the deep Eastern Mass-transport Deposit (EMTD). These units are situated at more than 100 m below the sediment/water interface, constituting ~50 m (Site 933) and 150 m (Site 936) of thickness. Stable isotope records and foraminiferal assemblages suggest that there are distinct blocks in the MTDs, with inclusion of interglacial material in the base of the deep EMTD. The majority of sediments in the MTDs were formed during early oxygen Stage 3 (or another early glacial period), or there has been significant mixing of interglacial and glacial material within the separate blocks. In the benthic foraminiferal assemblage, the dominance of buliminids, which are infaunal deposit feeders, suggests that the MTDs originated from bathyal environments. Sparse occurrences of characteristic species indicate that the continental shelf is another sediment source. Further details were obtained by comparing the Holocene benthic foraminiferal microfauna from samples collected on the Amazon shelf and slope, as well as with pelagic and last glacial maximum sediments, collected from the Amazon deep sea. The different assemblages found in both abyssal and MTDs demonstrate a distinct origin for the latter. Buliminids and uvigerinids, which are common on the Amazon continental slope, were found in the MTDs. Globocassidulina subglobosa and Cassidulina laevigata are common on the upper slope and were also encountered in the MTDs. Species like Quinqueloculina sp. A and Pseudononion atlanticum, which are present on the Amazon shelf, were also encountered in the MTDs. In the deep EMTD, a Bulimina dominated assemblage suggests an upper/middle bathyal environment, whereas samples dominated by Brizalina aenariensis suggest a source from shelf environments. In Unit R, the pattern is similar, with the presence of the Bulimina assemblage and dominance of Brizalina aenariensis in the top of the unit.