The origin and turnover of dissolved organic carbon in forested watersheds determined by carbon isotopic ( (super 14) C and (super 13) C) measurements.
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This article presents a case study of Lower Lough Erne, a humic, alkaline lake in northwest Ireland, and uses the radiocarbon method to determine the source and age of carbon to establish whether terrestrial carbon is utilized by heterotrophic organisms or buried in sediment. Stepped combustion was used to estimate the degree of the burial of terrestrial carbon in surface sediment. Δ 14 C, δ 13 C, and δ 15 N values were measured for phytoplankton, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). Δ 14 C values were used to indicate the presence of different sources of carbon, including bedrock-derived inorganic carbon, “modern,” “recent,” “subsurface,” and “subfossil” terrestrial carbon in the lake. The use of 14 C in conjunction with novel methods (e.g. stepped combustion) allows the determination of the pathway of terrestrial carbon in the system, which has implications for regional and global carbon cycling.
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The effects of nonconservative sources (inputs) and sinks (outputs) of carbon are indicated by the behavior of Δ 14 C and δ 13 C of the total dissolved inorganic carbon (ΣCO 2 ) in San Francisco Bay and Chesapeake Bay. Isotopic distributions and model calculations indicate that in North San Francisco Bay the net CO 2 flux to the atmosphere and carbon utilization in the water column are balanced by benthic production. Municipal waste appears to be a dominant source in South San Francisco Bav. In Chesapeake Bay, atmospheric exchange has increased the Δ 14 C and δ 13 C in the surface water. Decomposition of organic matter in the water column is indicated to be the dominant source of excess ΣCO 2 in the deep water.
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Carbon isotope measurements in soil CO 2 are presented and discussed. Soil CO 2 concentration and 13 C profiles were measured using a new technique. A simple model describing the CO 2 transport from the soil to the atmosphere is derived. The finding that CO 2 in the soil is richer in 13 C than the CO 2 leaving the soil is attributed to isotopic fractionation in molecular diffusion.
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Organic matter in sediments and suspended matter is a complex mixture of constituents with different histories, sources and stabilities. To study these components in a suspended matter sample from the Ems-Dollard Estuary, we used combined molecular analysis with pyrolysis/gas chromatography/mass spectrometry and stable and radioactive carbon isotope analyses of the bulk and separated chemical fractions. Carbohydrates and proteins, ca. 50% of the total organic carbon (TOC), are much younger than the bulk sample and have a somewhat higher δ 13 C value. Lipids and the final residue are considerably older and have lower δ 13 C values. The final residue, ca. 17% of the total carbon, consists mainly of aliphatic macromolecules that could be derived from algae or terrestrial plants. The δ 13 C value points to a marine origin.
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Abstract Dissolved organic carbon (DOC) originating in peatlands can be mineralized to carbon dioxide (CO 2 ) and methane (CH 4 ), two potent greenhouse gases. Knowledge of the dynamics of DOC export via run‐off is needed for a more robust quantification of C cycling in peatland ecosystems, a prerequisite for realistic predictions of future climate change. We studied dispersion pathways of DOC in a mountain‐top peat bog in the Czech Republic (Central Europe), using a dual isotope approach. Although δ 13 C DOC values made it possible to link exported DOC with its within‐bog source, δ 18 O H2O values of precipitation and run‐off helped to understand run‐off generation. Our 2‐year DOC–H 2 O isotope monitoring was complemented by a laboratory peat incubation study generating an experimental time series of δ 13 C DOC values. DOC concentrations in run‐off during high‐flow periods were 20–30 mg L −1 . The top 2 cm of the peat profile, composed of decaying green moss, contained isotopically lighter C than deeper peat, and this isotopically light C was present in run‐off in high‐flow periods. In contrast, baseflow contained only 2–10 mg DOC L −1 , and its more variable C isotope composition intermittently fingerprinted deeper peat. DOC in run‐off occasionally contained isotopically extremely light C whose source in solid peat substrate was not identified. Pre‐event water made up on average 60% of the water run‐off flux, whereas direct precipitation contributed 40%. Run‐off response to precipitation was relatively fast. A highly leached horizon was identified in shallow catotelm. This peat layer was likely affected by a lateral influx of precipitation. Within 36 days of laboratory incubation, isotopically heavy DOC that had been initially released from the peat was replaced by isotopically lighter DOC, whose δ 13 C values converged to the solid substrate and natural run‐off. We suggest that δ 13 C systematics can be useful in identification of vertically stratified within‐bog DOC sources for peatland run‐off.
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