41. Amazon Fan Sedimentation: the Relationship to Equatorial Climate Change, Continental Denudation, and Sea-level Fluctuations

Seventeen sites on the Amazon Fan were drilled during Leg 155. Although the oldest recovered sediment dates from isotopic Stage 8 (~0.25 Ma), our sampling concentrated on the last glacial cycle. Sites were correlated by seismic reflection profiles, paleomagnetic orientation and intensity, stable isotopes of planktonic foraminifers, and biostratigraphic methods. The Amazon Fan aggraded during lowstands of sea level and switched depocenters following major highstands, when pelagic and/or hemipelagic calcareous clay was deposited across the fan. Only one distributary channel was active at any one time, although spillover of turbidity currents led to channel-related deposition on abandoned levees. Depositional rates on the levees of active channels reached 25 m/k.y. Although we have identified the position of the last interglacial (Stage 5) on the fan, we have not yet identified a deposit formed at the initial lowering of sea level at the Stage 4/5 boundary in cores or on seismic profiles. Recovered and logged sediments show that prominent, coarse-grained deposits are present in flat-lying layers that underlie middlefan channels as well as in channels. These coarse layers form units up to 25 m thick, and were formed following avulsions. These units extend downfan to form the bulk of the lower fan. The existence of coarse, sheet sediment deposits in this overall muddy fan is at odds with the view that significant coarse-grained sediments do not exist in muddy fans. Since leveed channels can transport coarse-grained sediments to the middle and lower fan, separate sedimentary processes are not required to create a sandy lower fan and channel-levee systems on the upper and middle fan: these deposits are contemporaneous and form whenever the fan is active. The rapidly accumulated sediments contain a high-resolution record of both oceanic and continental paleoclimate that will require much more study. Surface gradients in the ocean are steep, resulting from the complex interaction of saline South Atlantic water and freshwater discharge from the Amazon River. Detrital palynomorphs and phytoliths show that there was only modest cooling and an extension of savannah in the Amazon Basin during the last glacial maximum. The organic matter deposited on the Amazon Fan is broadly similar in composition to the modern Amazon River. The enhanced transport of organic matter to the Amazon Fan when the fan is active reduces the amount of carbon remineralization. Leg 155 has demonstrated both the feasibility and value of high-resolution drilling on continental margins, for solving paleoenvironmental, sedimentological, and geochemical questions. In particular, we have demonstrated that the sampling of mudrich deep-sea fans can be a practical tool for the sampling of integrated terrestrial and oceanic paleoclimate records; a technique that can be followed in other regions.