Acyclic lipids in Amazon shelf waters

Acyclic lipids were analysed for dissolved (<1.2 mu m) and particulate (>1. 2 mu m) material from surface water of the Amazon continental shelf to corr elate 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 car bon number predominance <C-30 and CPIs (carbon-preference index) varying fr om 0.35 to 1.22 (C-17-C-22 range) and 0.52 to 1.33 (C-23-C-28 range). Carbo n number maxima (C-max) occurred at C-18 and/or C-22 Pr/Ph ratios varied fr om 0.28 to 2.62. The n-alkanes in particulate samples ranged from C-15 to C -35 (CPI17-22 from 0.15 to 0.50 and CPI23-28 from 0.52 to 1.12) with C-max at C-18 and C-20, while pristane and phytane on the other hand were present in particulate matter only at trace amounts and hence Pr/Ph ratios could n ot be calculated. Both dissolved and particulate phases showed saturated C- 14, C-16 and C-18 alkanoic acids and an unsaturated C-16:1 acid as major co mponents. The chemical composition of these acyclic lipids indicates an ori gin 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 ef fectively biodegraded in the shelf waters during downstream transport in th e Amazon River. Furthermore, the Pr/Ph ratios calculated for the dissolved samples indicate that the organic matter in the surface of the highly turbi d waters passed through an anaerobic stage. The differences in the acyclic hydrocarbon profiles of the dissolved and particulate phases show that orga nic inputs are preserved differently in smaller and larger particles. (C) 2 000 Academic Press.