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

Surficial sediments collected along the Amazon shelf during the November 1991 cruise of the AmasSeds program were analyzed for n-alkanols and 4-desmethyl sterols. Despite the high water column productivity and Amazon River inputs, total concentrations are low (usually <1 μg/g). Principal component analysis (PCA) indicates that the n-C16 alkanol and cholesterol/cholestanol are the best biomarkers for autochthonous inputs while the n-C28 alkanol and most C29 sterols are the best for trace river-borne material. 24-Ethylcholesterol is the major sterol throughout the shelf, but PCA modeling shows that this C29 sterol has mixed marine and terrigenous sources. The concomitant evaluation of alkanol and sterol biomarkers allows the identification of areas where river-borne organic matter (OM) accumulates, mainly in the outer topset northwest of the river mouth and to a smaller degree in the near shore region along the Amapá shelf. The contribution of compounds produced 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.