Diane K. McDaniel,2 Scott M. McLennan,2 and Gilbert N. Hanson2


Pleistocene clastic sediments of the Amazon Fan provide a record of their continental provenance and of the climatic conditions under which they were weathered and transported. We report on Nd and Pb isotopic compositions of muds from the Amazon Fan and use these data to evaluate the relative importance of the Andean highlands and cratonic lowlands as sources of sediment. Changing environmental conditions have the potential to change the relative contribution of Andean and cratonic sources to fan sediment. During the time of Amazon Fan deposition, sea-level was at a low stand, and the Andes were glaciated. It has also been postulated that the Amazon drainage basin was cooler and more arid than today. The major-element composition of Amazon Fan muds, analyzed on Leg 155, indicates that they experienced only moderate degrees of weathering, unlike many of the soils developed on the South American craton today. Nd model ages (TDM) range from 1620 to 1770 Ma. These ages are older than reported ages for Andean igneous rocks but similar to those for sediment currently being shed from the Andes highlands. Amazon Fan muds have 206Pb/204Pb and 207Pb/204Pb somewhat higher than Andean igneous rocks, indicating some component of Precambrian Pb in the provenance. This Precambrian component may be derived in large part from recycled sediments that exist within the Andes highlands. We conclude from geochemical compositions and Nd and Pb isotopic compositions that muds of the Amazon Fan are derived dominantly from the Andean highlands. Furthermore, during their journey to the Atlantic Ocean, they were not affected by the extreme weathering conditions such as that existing in the Amazon drainage basin today.

1Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Eds.), 1997. Proc. ODP, Sci. Results, 155: College Station, TX (Ocean Drilling Program).
2Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100, U.S.A. Mcdaniel@PBisotopes.ess.sunysb.edu