Acyclic Lipids in Amazon Shelf Waters

Estuarine, Coastal and Shelf Science, Volume 50, Issue 2, February 2000, Pages 231-243
V. O. Elias, J. N. Cardoso and B. R. T. Simoneit

Summary of Points:

· Acyclic lipids were analyzed for dissolved and particulate material from surface water of the Amazon continental shelf to correlate the direct inputs with diagenetic processes of the organic matter and to assess the differences in compound distribution patterns of the samples.

· The dissolved samples contained n -alkanes with an unusual even-to-odd carbon number predominance <C₃₀and CPIs (carbon-preference index) varying from 0·35 to 1·22 (C_17¯C₂₂range) and 0·52 to 1·33 (C_23¯C₂₈range). Carbon number maxima (C_max) occurred at C₁₈and/or C₂₂Pr/Ph ratios varied from 0·28 to 2·62. The n -alkanes in particulate samples ranged from C₁₅to C₃₅(CPI_17¯22from 0·15 to 0·50 and CPI_23¯28from 0·52 to 1·12) with C_maxat C₁₈and C₂₀, while pristane and phytane on the other hand were present in particulate matter only at trace amounts and hence Pr/Ph ratios could not be calculated.

· Both dissolved and particulate phases showed saturated C₁₄, C₁₆and C₁₈alkanoic acids and an unsaturated C_16:1acid as major components. The chemical composition of these acyclic lipids indicates an origin from micro-organisms, probably diatoms.

· Although minor, a contribution of components derived from plant waxes are more abundant in the particulate phase with profiles indicating a remineralized residue. Plant waxes are effectively biodegraded in the shelf waters during downstream transport in the Amazon.

· Furthermore, the Pr/Ph ratios calculated for the dissolved samples indicate that the organic matter in the surface of the highly turbid waters passed through an anaerobic stage. The differences in the acyclic hydrocarbon profiles of the dissolved and particulate phases show that organic inputs are preserved differently in smaller and larger particles.

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