Heterogeneity of natural organic matter from the Chena River, Alaska

Water samples were collected in July 2001 from the Chena River in central Alaska. The natural organic matter (NOM) was size fractionated into particulate (POM,>0.45 microm), colloidal (COM,1kDa-0.45 microm) and dissolved (DOM,<1k Da) organic matter fractions, using filtration and ultrafiltration. The size-fractionated organic matter was then analyzed for organic carbon (OC) and nitrogen (N), isotopic (delta13C and delta15N) and molecular composition, using continuous flow isotope ratio mass spectrometry and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Results of phase partitioning showed that, on average, about 6% of OC and 16% of N occurred in the form of POM while 66% of OC and 57% of N occurred in the form of COM, and 28% of the OC and 27% of the N were in the DOM form. Organic matter in the river water was found to be highly heterogeneous in terms of chemical composition and isotopic signatures. The C/N ratio was as low as 16+/-1 in the POM (n=2) to as high as 48+/-1 in the COM (n=3) and 38+/-4 in the DOM (n=3), suggesting a diagenetically younger POM. Values of delta13C increased with decreasing size, varying from -29.59+/-0.45% in the POM to -27.47+/-0.06% in the COM to -16.93+/-0.08% in the DOM. In contrast, values of delta15N decreased with decreasing size, from 2.64% in POM to 1.64% in COM to 1.33% in DOM. These results, together with radiocarbon measurements, suggest a preferential decomposition of lighter C isotope (12C) and heavier N isotopic (15N) from POM to COM to DOM. Results of py-GC/MS showed that the percentage of polysaccharides decreased with decreasing size, further supporting a degradation pathway of NOM from POM to COM and DOM in Chena River waters. More studies are needed to examine the seasonal and spatial variations of size-fractionated organic matter.