Marine and terrigenous lipids in coastal sediments from the Peru upwelling region at 15°S: Sterols and triterpene alcohols

Abstract

The distributions of free 4-desmethyl sterols in sediments from the Peru coastal zone at 15°S have been determined. Major free sterols in the surface sediments include cholesterol, which is mainly derived from zooplankton, and two C 28 sterols: 24-methylcholesta-5,24(28)-dien-3β-ol and 24-methylcholesta-5,22E-dien-3β-ol both of which are derived fro diatoms. Their concentrations decrease by almost an order of magnitude in the top 20 cm of sediment depth, indicating that free sterols are rapidly degraded in this sedimentary environment. Lipids from higher plants were also detected: long chain fatty acids and alcohols and various triterpenoid alcohols, including taraxerol, lupeol and α- and β-amyrin. The concentrations of most terrigenous lipids varied by less than a factor of 3 over the same depth, and these changes were not correlated with changes in the concentrations of total organic carbon. Below 3 cm, lipids from higher plants predominated in the extractable lipid distributions due to the more rapid degradation of marine lipids. We postulate that there are significant marine sources of the higher plant sterols 24-ethylcholesterol, 24-ethylcholesta-5,22E-dien-3β-ol and 24-methylcholesterol in these sediments. A high proportion of many of the terrigenous lipids in these sediments are probably transported into the coastal zone by rivers, rather than from the atmosphere, and then redistributed by bottom currents.