4.1. Age of DOC to U.S. Coasts
 There are regional differences in the Δ14C-DOC in large river basins. DOC exported to the west coast of the U.S. from 4 major river basins (Columbia, San Joaquin, Sacramento and Colorado) adjusted for flow has an average Δ14C signature of 15.6‰ which corresponds to modern carbon. Although the Colorado River is shown to have consistently low ages, it has a low discharge, which results in a negligible contribution to the flow adjusted average Δ14C. Rivers flowing into the Northern Atlantic Ocean (St. Lawrence, Hudson, Connecticut) are exporting DOC that has a flow adjusted average Δ14C signature of 28.5‰, slightly more modern than rivers in the West. The Gulf of Mexico is dominated by modern DOC from the Mississippi, Atchafalaya and Mobile rivers resulting in a flow adjusted average Δ14C signature of 63.2‰ as is the Altamaha (70‰), a southeastern river draining to the South Atlantic region of the U.S. The remainder of the rivers (Susquehanna and Potomac) discharge to the Atlantic Ocean from the Mid Atlantic region of the eastern U.S., and support an average flow adjusted Δ14C-DOC of −50‰. This export corresponds to a calendar age of ∼350 years before present (1950).
4.2. Within Basin Controls on Δ14C-DOC
 Relationships across the seasonal hydrograph between the radiocarbon age of DOC and the molecular characteristics of that organic carbon remain difficult to define. There do not appear to be uniform correlations that hold within each basin (Table 4). The finding that Δ14C of DOC showed strong correlations with the concentration of DOC in the Altamaha, Atchafalaya, and Connecticut river systems suggests that these systems may be dominated by two sources of DOC with distinct 14C-DOC signatures. The Altamaha, Atchafalaya, and Connecticut river basins have a relatively high percentage of wetland and forested ecosystems (Table 1b) which could be the 14C-enriched source. However, the Mobile River also has similar land use characteristics and Δ14C-DOC shows a strong negative relationship with concentration, indicative perhaps of an alternate source derived from either mobilized organic matter from soils or non-terrestrial inputs. Regardless, the dominant source of organic matter is of modern origin.
 In contrast to the finding that SUVA254 correlates with Δ14C-DOC across basins, the within basin relationships are much weaker. Only three basins, the Mobile, Potomac and Susquehanna rivers, showed strong relationships between Δ14C-DOC and the aromaticity of that carbon. However, DOC in the Mobile River was consistently modern whereas DOC in the Potomac and Susquehanna rivers was, on average, much older. The discrepancy between modern and old does not exclude a possible uniform explanation. All of these basins show significant proportions of agriculture and forest land cover (Table 1b) which could be significant sources of less degraded organic material. If this were the case, however, we would suspect that increases in discharge would also correlated with increases in modern Δ14C-DOC [Raymond and Saiers, 2010], which does not occur within these basins. In the Colorado and Rio Grande rivers, where there were negative relationships between SUVA254 and Δ14C of DOC, SUVA254 values are very low, and FI is the highest for all of the rivers in this study (Table 2). This finding indicates that throughout the year there is a slight shift in the source of organic carbon to include even older and slightly aromatic material later in the season under lower flow conditions. Inputs from agricultural activities could be mobilizing older soil carbon [Sickman et al., 2010] but we would caution this interpretation as the variation in the measurements presented here for these river basins remains very small, and the SUVA254 and FI values themselves suggest that terrestrial organic carbon may not be the dominant source at any point throughout the year.
 The seasonal characteristics of flow differ greatly across these river systems (Figure 6). Only the San Joaquin and Connecticut (positive) and Rio Grande (negative) showed substantial correlations between Δ14C-DOC and discharge. Although the annual discharges and basin areas for the San Joaquin and Rio Grande are nearly an order of magnitude apart, these rivers represent relatively arid systems with low forest cover. We hypothesize that there are two different mechanisms that can attribute a relationship between Δ14C-DOC and discharge unique to each basin. For instance, in the San Joaquin River, modern terrestrial organic carbon is mobilized into the river system as precipitation and flow increase, while in the Rio Grande River, agricultural runoff combined with wastewater and groundwater contributions are being mobilized under higher flow conditions [Benke and Cushing, 2005]. The implication of wastewater contribution to Δ14C-DOC is discussed further below. We suggest that the coupling of discharge with Δ14C-DOC in the Connecticut is a direct result of the strong seasonality of vegetation since the basin is 89% forested (Table 1b) [Raymond and Saiers, 2010].
4.3. Across Basin Controls on Δ14C-DOC
 The variation of the 14C-age of DOC across these river systems was not predicted by land cover. Our data suggest that DOC age measured at downstream locations of large river basins is modulated by a number of factors including: (1) sources of DOC (natural versus anthropogenic), (2) the seasonal vegetation cycle (climate) and, (3) water residence time (climate and impoundments). The radiocarbon age of DOC found within these rivers separate along a continuum where one or two of these factor appears most influential. We present a framework to identify the dominant controls on the14C-DOC inFigure 7.
Figure 7. Preliminary framework for the dominant controls on the Δ14C-DOC in large river basins across the U.S. In summary, the dominant drivers are either climate induced (precipitation influencing groundwater inputs and natural land cover), or human induced (direct inputs or manipulation of the landscape). Each set of effects from either changing climate or humans drives differences in residence time, the processing of DOC along the river continuum, and ultimately the signature of Δ14C-DOC. The rivers investigated in this study are placed along both a residence time and DOC age spectrum that changes from short (young) to long (old) respectively. Both the Rio Grande and Colorado Rivers are strongly influence by both climate and anthropogenic activities.
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 Aged organic carbon dominates the annual DOC in the Rio Grande, Colorado, Potomac, Sacramento and Susquehanna rivers. These results are similar to those reported previously for the Susquehanna, Hudson, and Delaware rivers [Raymond and Bauer, 2001b; Raymond et al., 2004]. Both the Rio Grande and the Colorado rivers, which consistently showed aged DOC, drain arid regions where groundwater and wastewater can contribute significantly to riverine discharge, water extraction activities can reduce discharge, and impoundments can change the quality of organic carbon. In these dry watershed systems, we hypothesize that three things are occurring. First, in these arid systems with large water diversions and low flow, groundwater contribution and long residence time provides a longer period for radiocarbon dead organic chemicals to build up and influence 14C-ages. That is, direct inputs from anthropogenic activities (agricultural diversions and wastewater) could represent a significant proportion of the total water flow at these arid downstream locations [Swietlik et al., 1995]. Groundwater DOC ages have been reported as low as 16,000 y.b.p [Murphy et al., 1989; Purdy et al., 1992] and could contribute a significant proportion of the total discharge in the Colorado [Warner et al., 1985] and Rio Grande systems. High concentrations of organo-chlorine pesticides have been found in water and sediments in the Colorado River Delta [Garcìa-Hernandez et al., 2001]. These pesticides are synthesized from organic compounds derived from fossil fuel and are therefore radiocarbon ‘dead’ [Reddy et al., 2002]. Second, the low terrestrial productivity of these arid systems may lead to lower contributions of aromatic, modern DOC, which is further compounded by low surface-water runoff. For instance, only 40% of the flow in the Colorado River above Cisco, Utah was accounted for by surface runoff [Michel, 1992]. Third, the potential for in situ production of DOC behind impoundments offers opportunities to incorporate inorganic carbon sources from weathering products that have very old carbon signatures.
 In addition, wastewater cannot be ignored as a potential source in all of the rivers presented here, but especially in arid, low flow systems, such as the Colorado and Rio Grande where a small percentage of surface recharge from precipitation contributes to flow. Wastewater plants have been shown to be significant contributors of aged organic carbon in coastal watersheds in Northern England and the Hudson River sampled below Newburg NY [Ahad et al., 2006; Griffith et al., 2009]. Radiocarbon based ages of wastewater DOC for measurements along the east coast ranged from modern-2650 y.b.p. but averaged 1630 y.b.p. [Griffith et al., 2009]. Griffith et al. suggest that these ages are a function of surfactants and the degradation byproducts of various detergents, cleaners, and disinfectants. Isotopic analysis of dissolved organic carbon shows that wastewater is composed of ∼25% petroleum-based DOC and ∼67% C3 vegetation, and ∼8% C4 vegetation [Griffith et al., 2009]. If riverine DOC is composed of only 1% wastewater derived compounds, the apparent reduction in age is ∼160 y.b.p. The average age of DOC for the Colorado and Rio Grande were 726 and 470 y.b.p., respectively.
 The composition of DOC in river systems continues to be used as an indication of the source and processing of carbon [Stubbins et al., 2010; Weishaar et al., 2003] and each basin in this study exhibited unique SUVA254 values that are closely tied to the HPOA% of the DOC (Figure 4). Across basins there is a positive relationship between SUVA254 and the average Δ14C (Figure 3) suggesting that systems with a larger proportion of this aromatic organic material might be more enriched in Δ14C-DOC. In addition, there is a positive relationship between the average annual MODIS Enhanced Vegetation Index (EVI) and the annual SUVA254 values for DOC (r2 = 0.4, p < 0.05). This suggests that large watershed systems that have a higher proportion of the watershed in green vegetation throughout the year export more aromatic carbon. Within basins that show strong seasonal shifts in greenness, correlations exist between SUVA254 and Δ14C-DOC that suggest vegetation is again driving organic carbon composition. There is substantial variation in the strength of the Pearson's correlation coefficients (Table 4) between SUVA254 and Δ14C-DOC within a basin. Interestingly, those systems with high EVI have strong positive correlations between Δ14C-DOC and SUVA254, while those systems that have very low EVI have strong negative correlations between SUVA254 and Δ14C (Figure 8). Removal of the Hudson River makes this comparison clearer as the river basins separate more strongly into groups.
Figure 8. Linear regression of the Pearson's correlation coefficient (r) between Δ14C-DOC and specific ultraviolet absorbance (SUVA254) within a basin and the annual average MODIS Enhanced Vegetation Index (E.V.I.) for the watersheds of 15 large rivers in the conterminous United States. Red points represent those sites with either positive or negative significant correlations between SUVA and Δ14C-DOC.
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 In systems like the Rio Grande and Colorado rivers there is a negative correlation between SUVA254 and Δ14C-DOC suggesting that, in less productive watersheds, other factors are controlling the relationship. Both the Colorado and Rio Grande rivers are heavily impounded. Reservoirs increase residence time and have been shown to alter the composition of downstream organic matter through carbon inputs from primary production [Parks and Baker, 1997; Westerhoff and Anning, 2000] and removal of terrestrial DOC. The replacement of allochthonous DOC with algal sources, which have low SUVA254, could explain the strong negative relationship between aromaticity of DOC and its radiocarbon age within the Colorado and Rio Grande rivers. High fluorescence index values also support the importance of in situ production of DOC in these systems (Table 2). However, algal DOC can incorporate the isotopic signature from sources that range in ages from modern (atmospheric CO2 pool) to aged (terrestrial weathering) [Mayorga et al., 2005]. There are insufficient data to discern the exact sources of inorganic carbon in the Colorado and Rio Grande rivers but alkalinity within the Colorado were on average 167 ± 9.1 mg L−1 as CaCO3 for 2009 suggesting a strong influence from carbonate rich weathering materials imported from groundwater sources (USGS National Water Information System – NWIS; http://waterdata.usgs.gov/nwis). In addition, large sources of oil and grease exist along the Rio Conchos, a major tributary above the sampling location associated with the NASQAN station at Brownsville, Texas [Benke and Cushing, 2005]. Therefore, in the case of these arid basins, small inputs of radiocarbon dead material may disproportionately affect the age of DOC while measurements of carbon quality reflect an autochthonous source.
 While our data show aged Δ14C-DOC values for the Sacramento River DOC (Table 2), our results are enriched compared to those reported in a previous study [Sickman et al., 2010]. Sickman et al. argued old organic matter found within the Sacramento and San Joaquin rivers is the result of contributions of aged organic material derived from upland soils recovering from agricultural disturbance [Sickman et al., 2010]. However, Michel  demonstrated that only 35% of the outflow of the Sacramento River was from runoff held in the basin for less than one year. The remainder was from older sources, such as groundwater. In addition, the San Joaquin River has also been shown to be impacted by agricultural chemicals [Bergamaschi et al., 2001] that may influence the radiocarbon signatures. Our data Δ14C-DOC for the main stems of these rivers are consistent with agricultural and groundwater influences and suggest either less contribution from older soil organic carbon, a larger in situ source of modern carbon mixing with older carbon, or large inter-annual variation.
 In watersheds like the Mobile, Potomac and Susquehanna rivers, SUVA254 correlates strongly with its radiocarbon age. These basins also show dynamic vegetation cycles throughout the year indicated by the EVI (Figure 8). In the case of the Mobile River, the lower stretch flows through coastal plain marshes and the strong relationship may be due to contributions of young, aromatic DOC from the marshes during high flow. The situation for both the Potomac and Susquehanna Rivers is likely due to the relative contributions of surface runoff and groundwater in these rivers. Previous research using tritium to trace water residence times determined that groundwater was an important fraction of flow in both the Susquehanna (20%) and Potomac River basins (54%) [Michel, 1992]. In particular, the diminished contribution of modern surface runoff in the Potomac River (46% of flow) could explain why 14C-DOC correlates strongly with SUVA254. Under conditions when groundwater dominates flow, the river contains older, less aromatic DOC. When surface runoff dominates, the river contains more terrestrially derived organic matter that is younger and more aromatic. The Susquehanna River has a similar pattern, but has a greater contribution of surface runoff (80%). Our data suggest that organic carbon quality and its bulk radiocarbon age responds quickly to both changes in the vegetation on the land surface and the subsequent effect that has on the hydrologic cycle, however determining the portion of the watershed that has the most influence on organic carbon requires more research.
 The St. Lawrence River is an outlier in Figure 3 with a modern radiocarbon age and very low SUVA254. Samples reported on here were collected at Cornwall, Ontario where 99% of the flow is from Lake Ontario and the contribution of runoff is relatively low [Benke and Cushing, 2005]. In this system, therefore, more aromatic, terrestrially derived organic matter is less of a factor and the SUVA254 signal is dominated by the production of autochthonous DOC in the Great Lakes. In addition, water residence times for this portion of the St Lawrence are substantially longer due to the Laurentian Great Lakes, and photochemical oxidation of terrestrially derived DOC would result in lower SUVA254 values [Osburn et al., 2001].
 In the context of the large temperate river basin chemistry explored here, we suggest that there are two major influences on the radiocarbon age of DOC, climate and humans (Figure 7). Climate determines both the terrestrial productivity of organic material as well as transport of organic matter from terrestrial systems under different precipitation regimes, and, to some degree, the balance between surface water runoff and groundwater contributions to flow. Systems with high productivity and precipitation such as the Connecticut, Altamaha, and the Atchafalaya have a 14C-signature that is dominated by young, aromatic DOM. In an arid system, aquatic residence times are longer, allowing for the removal of young terrestrial DOM and the potential for older sources (e.g., groundwater, agricultural and wastewater byproducts) to express themselves as witnessed in both the Rio Grande and Colorado rivers. This effect is seen in the Potomac as well even with higher rates of precipitation. Although preliminary, we present a framework to think about the broad scale controls on Δ14C-DOC sources to temperate watersheds (Figure 7). Until we are able to definitively identify sources to aged organic carbon within a basin, caution must be used when applying the bulk Δ14C-DOC as a tool to understand the turnover of terrestrial carbon in watersheds.
4.4. Caveats With the Use of Δ14C-DOC
 The fact that there is a strong separation of river basins with regard to SUVA254, FI and the Δ14C- DOC suggests that broad scale processes and characteristics influence DOC composition in large river basins. Rivers are considered integrators of the entire watershed, and, in general, the concentrations and fluxes of organic carbon from rivers have been correlated with hydrology, land use, and underlying soils and lithology [Likens and Bormann, 1995]. With respect to DOC age, however, there could be the direct influence of anthropogenic inputs of petroleum-based compounds through surface runoff and wastewater that would dramatically alter the isotopic signature of these rivers [Ahad et al., 2006; Griffith et al., 2009]. In addition, relatively small, point source additions of 14C dead petrochemicals could alter the age of the DOC [Spiker and Rubin, 1975].
 We tried to evaluate this effect by exploring the distribution of Δ14C related to the population within a basin using the LandScan (2009) high-resolution global population data set © UT-Battelle, LLC (Oak Ridge National Labs). Population density within 10 km of the sampling station and the flow weighted Δ14C signatures for each river basin are presented in Table 5. There is a trend that suggests local population centers in close-upstream proximity to a sampling location may contribute to an aged DOC signal (Figure 9, r2 = 0.55, p < 0.05). Without the Colorado River the relationship improves (r20.7, p < 0.01). These data raise important questions on the impacts of anthropogenic, petroleum-based carbon sources on the overall14C isotopic signatures of whole water samples. Misinterpretation of the bulk Δ14C-DOC as representative of the turnover of terrestrial carbon as it moves to coastal systems in cases where the introduction of radiocarbon dead material contributes to those ages, would have significant effects on the calculated cycling of carbon within estuarine and marine systems [Opsahl and Benner, 1997]. At the scale of the conterminous U.S. over annual time periods, the dominant source of organic carbon to the coastal ocean is the Mississippi River and, at this scale, the use of radiocarbon as a tracer for terrestrial processing is a powerful tool. However, the research community would greatly benefit from the expansion of compound specific applications of radiocarbon dating to tease apart exactly what components of the organic carbon pool are contributing to its age.
Table 5. Population Information Based on the LANDSCAN Gridded Population Product for the Year 2009 High Resolution Global Population Data Seta
|River||Population Density (km2–1) (Basin)||Population Density (km−2) (10 km Upstream)||Ratio 10 km: Basin|
Figure 9. Relationship between population near the location of Δ14C-DOC samples and the average flow weighted Δ14C across 15 large river basins in the U.S. Population density is calculated from the LandScan (2009) data set. A 10 km buffer was applied to each sampling point in ArcGIS and used to summarize population density upstream of the sampling location only (Table 5).
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