Stable carbon isotope ratios of tandem fractionated humic substances from different water bodies

Abstract

The isolation and fractionation of dissolved organic matter from aquatic environments using a two column array of XAD-8 and XAD-4 resins in series was carried out prior to isotopic characterization. Waters from marine, estuarine and fluvial environments have been used in this study. The hydrophobic organic acids composed of humic and fulvic acids are isolated from the sample on XAD-8, followed by the isolation of the more hydrophilic organic acids on XAD-4. The XAD-4 fraction holds a higher content of hydrophilic structures with low molecular size that would be lost for analysis if the tandem XAD-8/XAD-4 were not used. For the samples studied, the hydrophilic acids have lower carbon and higher oxygen contents than both the humic and fulvic acids. Fulvic and humic acids isolated with XAD-8 resin have similar d13C values. Nevertheless, d13C data obtained from XAD-4 shows that this fraction is composed of organic compounds which have a higher net content in 13C, like polysaccharides and amino acids. Two hypotheses that may explain the 13C enrichment of XAD-4 fractions relative to respective fulvic and humic acids have been considered. Selective preservation of the more resistant organic matter or condensation of dead biomass can explain this enrichment. Each of these hypotheses lead to a different pathway to the formation of XAD-4 fractions so further work on the 14C would help to evaluate the ages of the three fractions of humic substances and correlate with their structural characterization.