Sources and fate of n-alkanols and sterols in sediments of the Amazon shelf

Surficial sediments collected along the Amazon shelf during the November 19 91 cruise of the AmasSeds program were analyzed for n-alkanols and 4-desmet hyl sterols. Despite the high water column productivity and Amazon River in puts, total concentrations are low (usually <1 mu g/g). Principal component analysis (PCA) indicates that the n-C-16 alkanol and cholesterol/cholestan ol are the best biomarkers for autochthonous inputs while the n-C-28 alkano l and most C-29 sterols are the best for trace river-borne material. 24-Eth ylcholesterol is the major sterol throughout the shelf, but PCA modeling sh ows that this C-29 sterol has mixed marine and terrigenous sources. The con comitant evaluation of alkanol and sterol biomarkers allows the identificat ion of areas where river-borne organic matter (OM) accumulates, mainly in t he outer topset northwest of the river mouth and to a smaller degree in the near shore region along the Amapa shelf The contribution of compounds prod uced locally is less and associated with low polar lipid levels in the mid- topset northwest of the Amazon mouth and in the deepest water column in the foreset/bottomset area. (C) 1999 Elsevier Science Ltd. All rights reserved .