title: Heterogeneity of natural organic matter from the Chena River, Alaska.
abstract: 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.
authors.affiliation: International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK 99775, USA. guol@iarc.uaf.edu
authors.forename: Laodong
authors.initials: L
authors.lastname: Guo
authors.forename: Jaime K
authors.initials: JK
authors.lastname: Lehner
authors.forename: Daniel M
authors.initials: DM
authors.lastname: White
authors.forename: D Sarah
authors.initials: DS
authors.lastname: Garland
mesh_terms: D000413:Alaska; D002247:Carbon Isotopes / Q000032:analysis; D003102:Colloids; D004784:Environmental Monitoring; D008401:Gas Chromatography-Mass Spectrometry; D009587:Nitrogen Isotopes / Q000032:analysis; D009930:Organic Chemicals / Q000032:analysis*; D010316:Particle Size; D014873:Water Pollutants / Q000032:analysis*
chemical_list: D002247:Carbon Isotopes; D003102:Colloids; D009587:Nitrogen Isotopes; D009930:Organic Chemicals; D014873:Water Pollutants