CARBOHYDRATE DIAGENESIS IN HYPERSALINE ENVIRONMENTS - APPLICATION OF GC-IRMS TO THE STABLE-ISOTOPE ANALYSIS OF DERIVATIZED SACCHARIDES FROMSURFICIAL AND BURIED SEDIMENTS

The stable carbon isotope ratios of individual neutral monosaccharides isolated from microbial mats, mangrove tissue and paleosoils from the Abu Dhabi lagoon-sabkha system were determined by gas chromatography- isotope ratio mass spectrometry (GC-IRMS). Neutral monosaccharides in hydrolysates from surficial and buried ''flat'' mats dominated by the filamentous cyanobacteria Microcoleus chtonoplastes and Lyngbya aestua rii, have deltaC-13 (PDB) averaging - 5.6 +/- 1.3 and - 4.6 +/- 1.5 pa rts per thousand respectively. This is the first time such heavy delta C-13 values have been reported for individual biosynthetic components, to our knowledge. So-called ''blister'' mats, formed by decomposition of flat mats by aerobic bacteria, yield slightly lower deltaC-13 valu es for individual monosaccharides, around - 10.4 +/- 1.4 parts per tho usand. The stable carbon isotope ratios of the monosaccharides isolate d from the fresh mangrove leaf (Avicennia marina) are low with deltaC- 13 around - 22.5 +/- 2.6 parts per thousand, typical for low latitude C3 plants. Neutral monosaccharides from the mangrove paleosoils from 1 250 and 6000 yr BP, however, have deltaC-13 of - 16.0 +/- 1.2 and - 13 .8 +/- 2.9 parts per thousand respectively. This is attributed to the presence of anaerobic bacterial contributions in the paleosoils. The d eltaC-13 values of saccharides from the paleosoils indicate that galac tose, mannose and fucose are mainly derived from bacteria, whereas glu cose and arabinose still seem to be largely derived from the mangrove tissues.