TY - JOUR T1 - Constraining the sources and cycling of dissolved organic carbon in a large oligotrophic lake using radiocarbon analyses JF - Geochimica et Cosmochimica Acta Y1 - 2017 A1 - Prosper K. Zigah A1 - Elizabeth C. Minor A1 - Ann P. McNichol A1 - Li Xu A1 - Josef P. Werne KW - atmospheric co2 KW - Carbohydrate-like substance KW - Carboxylic-rich alicyclic molecules (CRAM) KW - Contemporary DOC KW - dissolved inorganic carbon KW - dissolved organic carbon KW - Freshwater carbon cycling KW - Heteropolysaccharides (HPS) KW - High molecular weight DOC KW - Lake Superior KW - Lipid-like fraction KW - Old DOC KW - Oligotrophic lake KW - Protein-like substance KW - radiocarbon KW - Serial thermal oxidation KW - Solid phase extracted DOC KW - Stable isotope KW - Unhydrolysable material AB - Abstract We measured the concentrations and isotopic compositions of solid phase extracted (SPE) dissolved organic carbon (DOC) and high molecular weight (HMW) \{DOC\} and their constituent organic components in order to better constrain the sources and cycling of \{DOC\} in a large oligotrophic lacustrine system (Lake Superior, North America). \{SPE\} \{DOC\} constituted a significant proportion (41-71%) of the lake \{DOC\} relative to \{HMW\} \{DOC\} (10-13%). Substantial contribution of 14C-depleted components to both \{SPE\} \{DOC\} (Δ14C = 25 to 43‰) and \{HMW\} \{DOC\} (Δ14C = 22 to 32‰) was evident during spring mixing, and depressed their radiocarbon values relative to the lake dissolved inorganic carbon (DIC; Δ14C ∼ 59‰). There was preferential removal of 14C-depleted (older) and thermally recalcitrant components from \{HMW\} \{DOC\} and \{SPE\} \{DOC\} in the summer. Contemporary photoautotrophic addition to \{HMW\} \{DOC\} was observed during summer stratification in contrast to \{SPE\} DOC, which decreased in concentration during stratification. Serial thermal oxidation radiocarbon analysis revealed a diversity of sources (both contemporary and older) within the \{SPE\} DOC, and also showed distinct components within the \{HMW\} DOC. The thermally labile components of \{HMW\} \{DOC\} were 14C-enriched and are attributed to heteropolysaccharides (HPS), peptides/amide and amino sugars (AMS) relative to the thermally recalcitrant components reflecting the presence of older material, perhaps carboxylic-rich alicyclic molecules (CRAM). The solvent extractable lipid-like fraction of \{HMW\} \{DOC\} was very 14C-depleted (as old as 1270-2320 14C years) relative to the substances isolated by acid hydrolysis of \{HMW\} DOC. Our data constrain relative influences of contemporary \{DOC\} and old DOC, and \{DOC\} cycling in a modern freshwater ecosystem. VL - 208 UR - http://www.sciencedirect.com/science/article/pii/S0016703717301722 ER -