Evaluation of triterpenols and sterols as organic matter biomarkers in a mangrove ecosystem in northern Brazil

Mangrove leaves, sediment, and excrement from the mangrove crab Ucides cordatus from the coastal areas of the Bragana peninsula in North Brazil were analysed to determine suitable biomarkers for mangrove-derived organic matter. Leaves of Rhizophora mangle (red mangrove), the dominant species in the area, were characterised by high amounts of beta-amyrin, germanicol, taraxerol, and lupeol. Avicennia germinans (black mangrove) mainly contained betulin, lupeol, and beta-sitosterol, whereas significant quantities of beta-sitosterol and lupeol were typical of Laguncularia racemosa (white mangrove), the locally least abundant species. Except for betulin, the excrement of U. cordatus contained all of the above substances, but most strongly reflected the triterpenol signature of R. mangle leaves, the predominant diet of this crab. Surface sediments from various mangrove locations had relatively uniform compositions that possibly reflect tidal mixing. Sediment extracts were dominated by taraxerol and contained smaller amounts of beta-amyrin, germanicol, and lupeol. Only sediments in a marsh area, dominated by Sporobolus virginicus (seashore dropseed) and Eleocharis sp. (spikerush), revealed a different biomarker distribution. Core samples of subrecent sediment (up to 4000 C-14 yr BP), for which previous pollen analysis indicated vegetation dominated by mangroves, had compositions similar to that of the surface sediment. Taraxerol was the main component in the examined mangrove sediments and may be a marker for mangrove matter in this region, although analysis of plant material did not unequivocally support this. Germanicol is suggested to be a biomarker for organic matter from R. mangle in North Brazil. It was detected in older sediments, and was not significantly affected by ingestion by land crabs.