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.