LEG 155

Amazon Deep-Sea Fan

The Amazon Fan is a typical passive margin deep-sea fan. These deep-sea fans are characterized by the thickest accumulations of sediment on modern continental margins and are particularly sensitive to land climate change, sea-level change, and tectonic activity in the source area. ODP Leg 155 drilled 17 sites on this fan (Sites 930-946), recovering material which, through foraminiferal, isotopic, and paleomagnetic analyses will permit an unprecedented level of time resolution for paleoclimate studies and for correlation between different fan environments. This 250,000-year record of continental and oceanic paleoclimate is potentially of the same high resolution as that of the Greenland ice-core record, but in a critical equatorial area.

The Amazon deep-sea fan grew as a result of rapid deposition of channel-levee systems that have prograded across medium- to coarse-grained sands deposited at the downstream ends of channels. Bioturbated mud on the levee crests of abandoned channels had "low" sedimentation rates of 1-3 m/k.y., whereas the levees built by active channels accumulated at rates of 5-10 m/k.y. In contrast, calcareous clays were deposited across the entire fan during interglacial high-stands at sedimentation rates of only 0.1 m/k.y., showing that the vast supply of terrigenous sediment to the fan is temporarily diverted onto the continental shelf in response to sea-level rise. The rapid glacial- age sediment accumulation in this area is illustrated by the recording of the Lake Mungo paleomagnetic excursion at sub-bottom depths of 24 to 150 meters.

During Leg 155, ODP achieved the first-ever systematic coring of the entire thickness of a debris flow on the mid-fan and of the sandy fill of an active aggrading channel in a modern deep-sea fan, where transport distance and morphology of channel and levee are known; this documentation will provide a reference for the interpretation of channel processes and a link to studies of modern and ancient sedimentary processes and sequences. Each major levee complex of the Amazon Fan corresponds to a glacial stage and is capped by an interglacial calcareous clay. Within the latest Pleistocene levee complex, major shifts in the position of the active channel have occurred by avulsion on the upper fan every 5 to 10 k.y. Many individual channel-levee systems show fluctuations in the abundance of overbank silt on a scale of 0.5 to 1.5 k.y., requiring controls other than sea level on sediment supplied to the fan.

Shore-based studies of organic geochemistry, pollen, mineralogy, and geochemistry of terrigenous detritus will provide important information on the climatic, vegetation, and weathering history of the Amazon basin and surrounding highlands and isotopic and paleontological analysis of the organic fraction will enable us to establish very high-resolution records of oceanic circulation, productivity, and temperature in an area sensitive to tropical climate change.