Browsing by Subject "Dissolved organic carbon"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Biological and photochemical degradation of dissolved organic carbon in peatland ecosystems.
    (2012-08) Jacobson, Meghan McCarthy Funke
    Approximately one half of terrestrial carbon runoff is processed by inland waters and released into the atmosphere as carbon dioxide (CO2) prior to reaching the oceans, and bacterial consumption of dissolved organic carbon (DOC) comprises a dominant proportion of this carbon loss. Though peatlands export more DOC per area than most other ecosystems, the sources and biodegradability of peatland DOC and their effects on downstream DOC loads and fluxes are poorly understood. Moreover, photochemical degradation plays an important role in the loss of carbon from aquatic ecosystems, especially in peatlands with high DOC concentrations rich in photochemically reactive humic and phenolic compounds, but its contribution to global CO2 evasion from inland waters has not been quantified. This dissertation focuses on predictors of biodegradable DOC (BDOC) in aquatic ecosystems, the sources and biodegradability of DOC in peatland watersheds, and the contribution of photochemical degradation of DOC to global CO2 evasion from peatlands and fluvial ecosystems. Key findings from these studies were that SUVA, a measurement broadly used in ecology and environmental engineering and fairly simple to obtain, is an excellent predictor of the amount of long-term BDOC concentrations in Minnesota lake ecosystems. The peatland bog may be the most important source of BDOC exported from peatlands annually, rather than the upland. And, photochemical enhancement of bacterial respiration in peatland and fluvial ecosystems contributes approximately 0.11-0.22 Pg C yr-1 to the total global CO2 evasion from inland waters (~1.4 Pg C yr-1), or 9-18% of all inland water CO2 evasion. The results from this dissertation will lend insight into how future changes in hydrology and surface water DOC concentrations will alter the sources, biodegradability, and photochemical enhancement of DOC in aquatic ecosystems in the northern hemisphere, especially peatlands.
  • Thumbnail Image
    Item
    Modeling marine dissolved organic carbon response to climate change
    (2022-06) Gilchrist, Maya
    At 662 Pg C, the marine reservoir of dissolved organic carbon (DOC) represents the ocean’s largest pool of reduced carbon, holding over 200 times the carbon contained in marine biomass and rivaling the atmospheric carbon inventory. Recent work has suggested that the size of the DOC reservoir may respond to future changes in sea temperatures and global overturning circulation strength. Moreover, mobilization of marine DOC has been implicated in several paleoclimate change events. Despite these suggestions, however, the temporal dynamics of the marine DOC reservoir are poorly understood, and previous carbon cycle modeling work has generally assumed this reservoir to be static. In this study, we utilized an Earth system model of intermediate complexity calibrated with respect to DOC observations to assess the response of the marine DOC reservoir to climate changes representative of the last glacial maximum climate state, reduced ocean overturning circulation strength, and future warming scenarios. Our results indicate that the marine DOC reservoir is mobile in response to climate forcings and may shrink or expand depending on changes in its production rate. Moreover, variability in the ocean’s DOC reservoir was directly linked to changes in atmospheric CO2 concentrations, explaining a significant portion of CO2 drawdown or ventilation by the ocean across three sets of climate change experiments. These findings point to an integral role of marine DOC in the global carbon cycle and indicate that consideration of this reservoir is critical in improving our understanding of the connection between ocean processes and global climate of the past, present, and future.
  • Thumbnail Image
    Item
    Sources, biogeochemical cycling, and fate of organic matter in Lake Superior: an investigation using natural abundance radiocarbon and stable isotopes
    (2012-01) Zigah, Prosper Kojo
    The natural abundance radiocarbon and stable isotopic distributions of bulk dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved inorganic carbon (DIC), zooplankton, size-fractionated organic matter, and biochemical compound classes were used to investigate the sources, biogeochemical cycling, and fate of organic matter in the water column of Lake Superior. DIC pool appears to reset rapidly, showing radiocarbon values similar to atmospheric values from approximately 3 years previous to sampling. DIC concentrations and isotopic compositions were mostly homogeneous across the entire lake. POC was generally more depleted in stable carbon isotopic values than concurrent DOC. POC was also consistently depleted in radiocarbon (thus, older) relative to DOC and DIC. Radiocarbon ages of POC was spatially heterogeneous (range, modern to 2,840 year BP), and appear to be related to total water depth, exhibiting more older and more variable ages in the deepest basins of the lake. The ages and reactivity of bulk DOC did not change radically across the lake. DOC pool appears to be semi-reactive, recycling over up to 60 years in the entire water column. The radiocarbon signatures of the various DOC size fractions show that they recycle on similar time scales, with consistently modern (post 1950) radiocarbon values. Radiocarbon and Nuclear Magnetic Resonance (NMR) data show most of the high molecular weight dissolved organic matter (HMW DOM) originates from contemporary origin and was dominated by carbohydrates, aliphatic compounds, and acetate, with little aromatic compounds. Total hydrolyzable free carbohydrates and amino acids within HMW DOM exhibited modern radiocarbon signatures and recycled rapidly in the lake. In contrast, extractable lipid was pre-aged (20 to 2,320 years BP) due to older sources and/or general long term persistence in the lake. Coupled radiocarbon and stable carbon isotopic values indicate multiple sources, and variable formation pathways for the acid insoluble organic fraction within HMW DOM in the lake. Radiocarbon and stable isotopic values show zooplankton in Lake Superior selectively feed on within-lake produced organic matter even though other organic carbon sources represented a considerable portion of the available food resource.