Constraining the sources and cycling of dissolved organic carbon in a large oligotrophic lake using radiocarbon analyses

TitleConstraining the sources and cycling of dissolved organic carbon in a large oligotrophic lake using radiocarbon analyses
Publication TypeJournal Article
Year of Publication2017
AuthorsZigah, PK, Minor, EC, McNichol, AP, Xu, L, Werne, JP
JournalGeochimica et Cosmochimica Acta
Keywordsatmospheric co2, Carbohydrate-like substance, Carboxylic-rich alicyclic molecules (CRAM), Contemporary DOC, dissolved inorganic carbon, dissolved organic carbon, Freshwater carbon cycling, Heteropolysaccharides (HPS), High molecular weight DOC, Lake Superior, Lipid-like fraction, Old DOC, Oligotrophic lake, Protein-like substance, radiocarbon, Serial thermal oxidation, Solid phase extracted DOC, Stable isotope, Unhydrolysable material

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.