9627908 McLennan Siliciclastic sediments preserved within the Amazon subaqueous delta and deep-sea fan were derived from the Amazon drainage basin under contrasting climatic conditions. The delta represents relatively recent deposition largely under tropical conditions whereas the fan represents deposition during various Pleistocene glacial stages when the shelf was by-passed during low sea level stands. Stratigraphically constrained samples have only recently been made available for the Amazon Fan from ODP Leg 155. This proposal is concerned with quantitatively evaluating the relative importance of Andean versus cratonic provenance for these sediments during these contrasting climatic conditions. Preliminary petrographic, geochemical and isotopic data suggest a dominance of Andean sources but also suggest that in detail the provenance history is complex. The study will evaluate the sources of both muds and sands using neodymium and lead isotopes and a variety of geochemical and petrographic techniques. For the sands, the lithology most often used in provenance studies of ancient sequences, we will further evaluate the provenance of the major individual components, quartz, feldspar and heavy minerals (using zircon as the proxy) using a combination of Pb and Nd measurements. The purpose of this aspect of the study is twofold: (1) to determine if the provenance inferred from whole rock Pb and Nd isotope measurements can be applied to the major minerals that contain relatively little Nd (quartz and feldspar) or Pb (quartz); (2) to determine if the provenance inferred from detrital zircon studies is similar to that inferred for major minerals (quartz and feldspar), bulk sands and associated muds. From these data, we will be able to reconstruct the provenance of both sands and muds with some precision and evaluate if the craton plays a greater role for sands that muds, as suggested by others, and whether the craton plays a greater or lesser role during contrasting climatic conditions. Th is study will provide fundamental information regarding limitations on how various approaches to provenance (e.g., petrography, geochemistry, radiogenic isotopes) can be interpreted. The Amazon drainage system is the largest in the world and understanding the origin and history of sediment derived from this system is of the great interest in its own right. Any contrasts between sediment preserved on the fan and delta would have significance for paleoclimatological studies of this drainage system.
