Dissolved organic carbon (DOC) is important in the acid‐base chemistry of acid‐sensitive freshwater systems; in the complexation, mobility, persistence, and toxicity of metals and other pollutants; and in lake carbon metabolism. Carbon isotopes (13C and 14C) are used to study the origin, transport, and fate of DOC in a softwater catchment in central Ontario. Precipitation, soil percolates, groundwaters, stream, beaver pond, and lake waters, and lake sediment pore water were characterized chemically and isotopically. In addition to total DOC, isotopic measurements were made on the humic and fulvic DOC fractions. The lake is a net sink for DOC. Δ14C results indicate that the turnover time of most of the DOC in streams, lakes, and wetlands is fast, less than 40 years, and on the same time scale as changes in acidic deposition. DOC in groundwaters is composed of older carbon than surface waters, indicating extensive cycling of DOC in the upper soil zone or aquifer. Copyright 1990 by the American Geophysical Union.

Export of dissolved organic carbon (DOC) from forested catchments is governed by competing processes of production, decomposition, sorption and flushing. To examine the sources of DOC, carbon isotopes (14C and13C) were analyzed in DOC from surface waters, groundwaters and soils in a small forested catchment on the Canadian Shield in central Ontario. A significant fraction (greater than 50%) of DOC in major inflows to the lake is composed of carbon incorporated into organic matter, solubilized and flushed into the stream within the last 40 years. In contrast,14C in groundwater DOC was old indicating extensive recycling of forest floor derived organic carbon in the soil column before elution to groundwater in the lower B and C soil horizons. A small upland basin had a wide range in14C from old groundwater values at baseflow under dry basin conditions to relatively modern values during high flow or wetter antecedent conditions. Wetlands export mainly recently fixed carbon with little seasonal range. DOC in streams entering the small lake may be composed of two pools; an older recalcitrant pool delivered by groundwater and a young labile pool derived from recent organic matter. The relative proportion of these two pools changes seasonally due the changes in the water flowpaths and organic carbon dynamics. Although changes in local climate (temperature and/or precipitation) may alter the relative proportions of the old and young pools, the older pool is likely to be more refractory to sedimentation and decomposition in the lake setting. Delivery of older pool DOC from the catchment and susceptibility of this older pool to photochemical decomposition may consequently be important in governing the minimum DOC concentration limit in lakes.

Dissolved inorganic carbon (DIC) is the main acid buffer in forested lake watersheds in Canada. We usedcarbon isotopes (13C, 14C) to evaluate the production and cycling of DIC in an acid-sensitive lake watershed of thePrecambrian Shield. Soil C02, groundwater and stream DIC were characterized chemically and isotopically. Soil CO2 concentration profiles reflect both changes in production and in losses due to diffusion. S13C soil CO2 profiles (b13C values of -23%o in summer, slightly enriched during the fall and -25%o during the winter) are a reflection of the isotopic composition of the sources and changes in isotopic fractionation due to diffusion. Carbon isotopic composition (13C, 14C) of the groundwater and stream DIC clearly indicate that weathering of silicates by soil CO2 is the main source of DIC in these watersheds. 14C data show that, in addition to recent groundwater, an older groundwater component with depleted 14C activity is also present in the bedrock. The carbon isotope pattern in the groundwater also implies that, besides the main springtime recharge events, contributions to the groundwater may also occur during late winter/early spring.

Paleoecological and geomorphological studies indicate that, during the middle Holocene, there was a predominance of drier conditions with grassy savannahs replacing forests across the South American continent. Modern savannahs are composed mainly of C4 plants and soils developed under this type of vegetation show enrichment in ¹³C compared to soils under C3 vegetation cover. If soils contain stabilized organic matter formed in the middle Holocene, we hypothesize that former C4 vegetation would be evidenced by a large enrichment of ¹³C in soil organic matter (SOM). We investigate this possibility examining the depth variation of carbon isotopic composition in 21 soil profiles collected by different researchers at 14 different sites in Brazil. Of these, profiles from only three sites showed a marked increase of ¹³C with depth (9-10‰ enrichment in δ¹³C difference between the surface soil and deepest depth); two sites showed intermediate enrichment (4-5‰), and nine sites showed a small enrichment of approximatelly 2.5‰. The majority of sites showing all-C3 derived SOM were in the Amazon region. Possible causes for the absence of a large ¹³C enrichment with depth are: (1) dominance of C3 rather than C4 grasses in mid-Holocene savannahas, (2) soil profiles did not preserve organic matter derived from mid-Holocene plants, (3) the retreat of forest areas did not occur on a regional scale, but was a much more localized phenomenon.