Relationships between 14C and the molecular quality

GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 26, GB4014, doi:10.1029/2012GB004361, 2012

Relationships between D14C and the molecular quality of dissolved organic carbon in rivers draining to the coast from the conterminous United States David Butman,1,2 Peter A. Raymond,1 Kenna Butler,2 and George Aiken2 Received 21 March 2012; revised 25 September 2012; accepted 4 October 2012; published 29 November 2012.

[1] Dissolved organic carbon (DOC) in natural waters possesses chemical and molecular qualities indicative of its source and age. The apportionment of DOC by age into millennial and decadal pools is necessary to understand the temporal connection between terrestrial and aquatic ecosystems in the global carbon cycle. We measured D14C-DOC and chemical composition indices (specific ultraviolet absorbance (SUVA254), fluorescence index (FI), hydrophobic organic acid fraction (HPOA) content) for 15 large river basins in the conterminous United States. Across all rivers the average proportion of HPOA in DOC correlated strongly with SUVA254 (r2 = 0.93 p < 0.001). Individual D14C-DOC ranged from a low of 92.9‰ (726 y.b.p.) in the Colorado River to 73.4‰ (>Modern) in the Altamaha River for the year 2009. When adjusted by total discharge, these U.S. Rivers export modern carbon at between 34 and 46‰, a signal dominated by the Mississippi River. The variation in D14C correlates to indices of the aromaticity of the DOC measured by the SUVA254 (r2 = 0.87, p < 0.001), and FI (r2 = 0.6; p < 0.001) as well as differences in annual river discharge (r2 = 0.46, p < 0.006). SUVA254 was further correlated to broad scale vegetation phenology estimated from the Enhanced Vegetation Index derived from the NASA Moderate Resolution Imaging Spectrometer (MODIS). We show that basins with high discharge, high proportions of vegetation cover, and low human population densities export DOC enriched in aromatic material that corresponds to recently fixed atmospheric CO2. Conversely old DOC is exported from low discharge watersheds draining arid regions, and watersheds more strongly impacted by humans. The potential influence from fossil carbon from human inputs to aquatic systems may be important and requires more research. Citation: Butman, D., P. A. Raymond, K. Butler, and G. Aiken (2012), Relationships between D14C and the molecular quality of dissolved organic carbon in rivers draining to the coast from the conterminous United States, Global Biogeochem. Cycles, 26, GB4014, doi:10.1029/2012GB004361.

1. Introduction [2] Rivers and streams connect the terrestrial carbon cycle to estuarine and ocean carbon cycles. In doing so, in addition to passive transport, they also provide conditions conducive to both chemical and biophysical transformations of organic matter. Humans have changed the carbon chemistry within rivers by large scale land cover conversions [Ciais et al., 2006; Raymond et al., 2004], impoundments and hydrologic diversions [Aufdenkampe et al., 2011; Downing et al., 2008; Syvitski et al., 2005] as well as direct inputs of organic pollutants and wastewater [Zeng and Masiello, 2010;

1 Yale School of Forestry and Environmental Studies, New Haven, Connecticut, USA. 2 U.S. Geological Survey, Boulder, Colorado, USA.

Corresponding author: D. Butman, Yale School of Forestry and Environmental Studies, New Haven, CT 06511, USA. ([email protected]) ©2012. American Geophysical Union. All Rights Reserved. 0886-6236/12/2012GB004361

Griffith et al., 2009; Harrison et al., 2005; Hopkinson and Vallino, 1995; Spiker and Rubin, 1975]. [3] Although a substantial fraction of terrestrial organic matter transported to rivers and streams remains confined by impoundments or is remineralized and respired along the river reach [Aufdenkampe et al., 2011], the total flux of dissolved organic carbon (DOC) from rivers to the coast is 0.2 Pg C yr-1 [Aufdenkampe et al., 2011; Hope et al., 1994; Ludwig et al., 1996; Mayorga et al., 2005; Meybeck, 1982]. This contribution of organic carbon is more than needed to balance the turnover and radiocarbon age of DOC in the world oceans [Opsahl and Benner, 1997]. Many studies have tried to evaluate the age of riverine and stream DOC. A majority of these studies demonstrate that DOC is dominated by a modern, post-bomb carbon pool [Benner et al., 2004; Evans et al., 2007; Hedges et al., 1986; Hélie and Hillaire-Marcel, 2006; Neff et al., 2006; Raymond et al., 2007; Spiker and Rubin, 1975; Striegl et al., 2007] suggesting that the majority of DOC in rivers is of recent origin. [4] Millennial aged carbon in riverine DOC was reported in a study of North American rivers feeding the North

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BUTMAN ET AL.: D14C OF DOC IN US RIVERS

Western Atlantic Ocean [Raymond and Bauer, 2001b]. In this work it was suggested that bacterial utilization of DOC preferentially removes modern material reducing the D14C signature of the remaining carbon. Aged organic carbon has recently been determined in a number of river systems [Blair et al., 2003; Dickens et al., 2004; Goñi et al., 1997; Hood et al., 2009; Masiello and Druffel, 2001; Raymond and Bauer, 2001b; Sickman et al., 2010; Striegl et al., 2007]. Thus an older more recalcitrant pool may be present in some rivers and hidden due to the heterogenic nature of DOC that is often dominated by a young 14C-bomb enriched pool. A small number of studies attempt to estimate the contribution of both modern and aged organic carbon to the oceans across the water year [Neff et al., 2006; Raymond et al., 2007; Sickman et al., 2010]. Research demonstrates that basins can export older DOC under both base flow and high flow scenarios [Masiello and Druffel, 2001; Raymond and Bauer, 2001b; Raymond et al., 2004; Sickman et al., 2010]. Few studies provide additional information on the nature or chemistry of the DOC in these rivers. The composition of DOC together with its radiocarbon age may help determine the apportionment between fast (annual to decadal turnover) and slow (>decadal turnover) carbon pools. [5] D14C measurements of natural organic matter in rivers to determine its age must be used with caution. Studies have found that petroleum-based radiocarbon dead (devoid of 14C due to decay) DOC can be introduced via rainwater [Avery et al., 2006; Raymond, 2005; Hood et al., 2009; Stubbins et al., 2012] as well as from wastewater treatment plants [Ahad et al., 2006; Griffith et al., 2009]. Although old, a significant proportion of this anthropogenic petroleum fraction can potentially biodegrade [Petrovic and Barceló, 2004; Hood et al., 2009]. Thus, it is possible that the export of modern DOC is modulated by both the bacterial processing of DOC in rivers and estuaries, and by the introduction of petroleum-based DOC that might leave an aged DOC pool for oceanic export. The export of aged organic carbon, therefore, may be less a function of old soil organic matter turnover and more related to the anthropogenic influence of wastewater, atmospheric inputs and other anthropogenic activities. [6] Regardless, broad continental-scale systematic evaluations of the radiocarbon age of DOC in temperate rivers to determine the relative importance of modern versus aged organic carbon have not yet been completed. Studies to date where the D14C of DOC have been measured suggest we do not have a complete understanding of the potential sources of aged material moving through our rivers. To address this we provide an analysis of the quality and radiocarbon age of DOC leaving 15 large rivers basins in the U.S.

2. Site Description [7] Fifteen major U.S. river basins that drain to the coast were sampled across the hydrograph in this study (Figure 1). In total, these basins drain 6.4 million km2 of the conterminous U.S., Mexico and Canada and represent 79% of the land area of the contiguous 48 states and 90% of all the freshwater discharge in 2009. Basin sizes range from 2.9 million km2 for the Mississippi River to 28,183 km2 for the Connecticut River. Average annual precipitation is lowest in the Colorado River basin at 328 mm yr 1 and highest in the Mobile River basin at 1437 mm yr 1. Water management

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regimes and overall hydrology differ greatly among these basins, which are reflected in the internal variation in discharge. Mean annual discharge varied during the 2009 sampling period from 20,190 m3 s 1 for the Mississippi River to 6 m3 s 1 for the Rio Grande River (USGS National Water Information System (NWIS); http://waterdata.usgs.gov/nwis). Additional site information is available in Tables 1a–1b. 2.1. Sample Collection [8] Freshwater samples were collected through the United States Geological Survey National Stream Quality Accounting Network (NASQAN) at 13 coastal locations, along with the Hudson and Connecticut rivers (Figure 1). At each location, 4–10 samples were collected during January–December 2009, and discharge measurements were made at the time of sample collection (Table 1a). For the NASQAN effort, depthintegrated samples were collected at multiple locations across the main channel and subsequently churned to create a mixed sample [U.S. Geological Survey, 1997–1999, chapters A1–A9] (updates and revisions are ongoing and can be viewed at http://water.usgs.gov/owq/FieldManual/mastererrata.html). Unfiltered water was shipped on ice to the USGS in Boulder CO within 24 h of collection and immediately filtered using pre-combusted Whatman GF/F filters at 0.7-mm nominal pore size. Subsamples for DOC concentration and composition were stored in precombusted amber glass bottles fitted with Teflon lined caps and refrigerated until analysis. Subsamples for radiocarbon analyses were placed in acid washed 125 ml polycarbonate bottles, acidified to pH 2.5 using high purity 60% H3PO4, immediately frozen, and shipped overnight to Yale University. Grab samples from the Connecticut and Hudson rivers were collected at the surface only at an approximate depth of 0.5 m. For the Connecticut River, all samples were collected from the main channel near Deep River, CT except those collected in winter when the presence of river ice necessitated collecting from a dock extending offshore by 10 m. Hudson River samples were collected from the town landing at Newburgh, NY. Both the Connecticut River and Hudson River samples were collected unfiltered into pre-combusted amber glass bottles, placed on ice, and shipped overnight to Boulder CO. All further handling of samples from these two rivers is similar to that for the NASQAN samples. 2.2. Concentration and Optical Characterization of Organic Matter [9] DOC concentration was measured utilizing the platinum catalyzed persulfate wet oxidation method on an O.I. Analytical Model 700 TOC Analyzer™ using established methods [Aiken et al., 1992]. Reported values are the averages of duplicate analyses. Standard deviation for the DOC measurement was determined to be 0.2 mg carbon L 1. Fluorescence and absorbance measurements were made within 48 h of sample filtration on a JY-Horiba Fluoromax-3 fluorometer and an Agilent Model 8453 UV-Visible spectrophotometer, respectively. Fluorescence measurements were obtained and corrected according to Murphy et al. [2010], and the fluorescence index (FI) calculated as the ratio of the emission at 470 nm to that measured at 520 nm when excited at 370 nm [McKnight et al., 2001]. Decadal absorption coefficients were measured at 1 nm wavelength intervals with a path length of 1 cm. Specific ultraviolet

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Figure 1. Map showing the watersheds and sampling site of North American Rivers reported on in this paper. absorbance at 254 nm (SUVA254) was calculated according to Weishaar et al. [2003], and reported in units of [L/(mg carbon * m)]. Absorbance at 254 nm was chosen because of its common association with aromatic moieties in a sample [Weishaar et al., 2003].

2.3. Organic Acid Fractionation [10] Organic acid fractions were chromatographically separated using columns packed with Amberlite™ XAD-8 and XAD-4 resin in a modified version of the method described by

Table 1a. NASQAN Coastal Sampling Sites Used for This Analysis With the Hudson and Connecticut Addeda Location of Sampling Station Basin

Basin Area (km2)

Latitude (dd)

Altamaha Atchafalaya Colorado Columbia Connecticut Hudson Lower Atchafalaya Mississippi Mobile Potomac Rio Grande Sacramento San Joaquin St. Lawrence Susquehanna

36,532 241,865 662,195 715,270 28,183 31,471 252,417 2,968,242 110,929 29,973 564,117 75,501 72,093 775,673 70,162

31.427167 30.690743 32.718659 46.181221 41.390660 41.504476 29.702708 29.857151 31.086566 38.929555 25.898969 38.456020 37.676041 45.006160 39.657913

Longitude (dd) 81.605386 91.736226 114.718844 123.183454 72.413762 74.004516 91.202048 89.977847 87.977220 77.116923 97.497484 121.501344 121.266329 74.794910 76.174175

Dischargeb (m3 s 1)

Sample Date Range Start

End

Min

Max

Mean

18-Feb-09 20-Jan-09 29-Jan-09 02-Mar-09 22-Jan-09 18-Feb-09 22-Jan-09 21-Jan-09 15-Jan-09 12-Jan-09 25-Feb-09 26-Jan-09 07-Jan-09 21-Jan-09 08-Jan-09

14-Dec-09 05-Oct-09 29-Aug-09 21-Oct-09 20-Oct-09 29-Jul-09 07-Oct-09 06-Oct-09 08-Dec-09 06-Oct-09 31-Aug-09 26-Aug-09 27-Oct-09 21-Oct-09 02-Dec-09

100 5,465 48 3,200 221 267 2,308 12,261 490 46 2 252 17 6,230 447

3,030 15,348 92 9,288 898 560 8,467 30,016 3,596 496 10 1,209 78 8,212 2,220

680 9,226 65 6,785 438 457 4,828 20,190 1,574 245 6 451 39 7,622 1,228

a Sample date range for D14C-DOC analysis. Basin areas were delineated using NHDPlus and ArcHydro and are reported in square kilometers. Sampling locations are those provided by the USGS National Water Information System (http://waterdata.usgs.gov/nwis). Discharge measurements from the 2009 sampling period have been converted from cubic feet per second derived from the NWIS database. b Discharge measurements have been compiled for the sampling period only.

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Table 1b. Physical and Climatic Attributes for the 15 Large River Basinsa E.V.I.c

L.S.T.d (deg C)

% Land Covere

River Basin

Avg Precipb (mm yr 1)

Min

Max

Mean

Min

Max

Mean

Forest

Agriculture

Urban

Wetland

Water


1209 1016 328 602 1141 1113 1036 742 1437 1014 381 790 463 849 1005

0.23 0.05 0.10 0.16 0.22 0.19 0.05 0.09 0.23 0.19 0.13 0.21 0.19 0.16 0.16

0.52 0.53 0.18 0.31 0.65 0.64 0.54 0.44 0.56 0.63 0.25 0.30 0.35 0.31 0.64

0.37 0.31 0.15 0.22 0.38 0.38 0.31 0.27 0.38 0.39 0.17 0.25 0.25 0.22 0.38

7.7 16.4 0.8 8.6 14.3 13.6 16.3 5.5 5.9 2.4 10.7 1.1 7.2 11.8 9.5

31.7 33 43.6 34.7 23.8 24.7 33 34.3 31.6 29.3 44.5 37.6 39.9 23.1 26.3

22.1 23.9 24.1 12.9 10.0 10.9 23.8 17.7 20.9 16.1 29.7 20.2 24.5 8.9 13.1

62.3 29 16 33.5 88.7 77.8 28 20.9 67.3 61.8 9.6 35.1 15.8 37.9 67.6

18.8 31 1.7 9.1 1.7 13.7 31.3 40.8 18.5 33.9 4.5 10.6 30.1 18 27.5

10 1.1 0.3 0.6 6.2 4.8 1.1 1.8 3.9 4.1 0.4 1.4 1.9 3 4

7.9 8.1 0.1 0.1 2 1.6 9.9 1 8.5 0 0 0.5 0.2 4 0.1

0.7 1.3 0.2 1.1 1.5 2.1 1.3 1.2 1.2 0.2 0.2 1.4 0.5 34.5 0.8

a Average precipitation does not cover the sampling dates since a seamless high resolution precipitation data set that covers watershed basins that are beyond the conterminous U.S. does not exist yet. b Average precipitation is from a global model data set averaged through 2005 [Hijmans et al., 2005]. c Enhanced Vegetation Index (E.V.I.) derived from monthly MODIS EVI at 0.05 deg resolution is averaged for the sampling period. d Land Surface Temperature (L.S.T) is derived from the MODIS LST product at 0.05 deg resolution and averaged for the sampling period. Both E.V.I. and L.S.T. cover the dates of the sampling period. e Land Cover data is derived from the North American Land Change Monitoring System at 250-m resolution [Homer et al., 2004].

Aiken et al. [1992]. The method separates DOM in a water sample into 5 operationally defined fractions: hydrophobic organic acids (HPOA), hydrophobic organic neutrals (HPON, calculated by difference), low molecular weight hydrophilic acids (HPI), transphilic organic acids (TPIA), and transphilic neutrals (TPIN, not reported). The distribution of DOM in these fractions can provide information about carbon quality for bacterial mineralization, photodegradation, and reactivity. In brief, 1 L of acidified filtered water was passed through a 20-mL glass column packed with XAD-8 resin at a flow rate of 4 mL/minute. The XAD-8 effluent was then passed through a 20-mL glass column packed with XAD-4 resin at the same flow rate. The effluent from the XAD-4 column was collected as HPI. Both columns were back-eluted with 0.1 N NaOH at a flow rate of 2 mL/minute. The eluates, HPOA from the XAD-8 and TPIA from the XAD-4, were collected and acidified for analyses. Each fraction was weighed to determine sample volume, and analyzed for both UV absorbance and DOC concentration. HPON was then calculated by the mass balance difference between the XAD-8 effluent and the HPOA fraction. Data are reported as averaged percentages from duplicate fractionations, based on mass of organic matter in each fraction with a standard deviation of 2 percent. 2.4. Isotopic Analysis [11] Samples were prepared for isotopic analysis using established methods [Druffel and Williams, 1992; Raymond and Bauer, 2001a]. Frozen samples were allowed to thaw in a refrigerator at 4 C for 12 h. For each sample, 120 mL were placed into a 160-mL quartz reaction vessel connected to a vacuum extraction line. The previously acidified samples were sparged with ultra-high purity N2 for 10 min to remove inorganic CO2. Ultra-high purity O2 was then bubbled through the samples for 5 min to add additional O2 as an oxidant. The sample was irradiated (5 h) using a high energy UV light to convert DOC to CO2 [Bauer et al., 1992; Eadie et al., 1978; Williams and Gordon, 1970], which was then trapped and cryogenically purified within the vacuum system using liquid

nitrogen. The purified CO2 was sealed in a 6-mm combusted Pyrex© tube and sent for accelerator mass spectrometry (AMS). [12] All isotopic measurements were conducted at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility at the Woods Hole Oceanographic Institution (WHOI). For D14C analysis for DOC, the CO2 gas samples were converted to graphite targets by reducing CO2 with an iron catalyst under 1 atm H2 at 550 C. The graphite targets were analyzed for D14C and corrected for potential fractionation using d13C results, following the conventions of Stuiver and Polach [1977]; d13C values were obtained with an Optima mass spectrometer at the NOSAMS facility.

3. Results 3.1. Across Basin Trends [13] Average DOC concentrations ranged from 10.0 2.6 mg C L 1 in the Altamaha River to 2.0 0.2 mg C L 1 in the Columbia River (Table 2) in 2009. The average DOC concentration for all 15 rivers presented here was 4.2 2.0 mg C L 1 (Table 2). Average D14C values ranged from a low of 92.9‰ in the Colorado River to 73.4‰ in the Altamaha. These D14C values correspond to DOC calendar ages of 726 30 y.b.p. to modern (Figure 2). Modern carbon is any organic carbon that has been taken up from the atmosphere since 1950 when bomb testing was prevalent and has a positive D14C value. Flow weighted values, based on all samples collected during 2009, produce a range from 120.8‰ in the Sacramento to 70.0‰ in both the Altamaha and Mississippi rivers corresponding to 970 Yrs BP to modern ages (Table 2). The flow weighted average D14CDOC exported from the conterminous U.S. is 46‰ driven primarily by the modern carbon signal from the Mississippi River. The Colorado, Potomac, Rio Grande, Sacramento and Susquehanna rivers showed aged DOC across the annual hydrograph while all other rivers exported modern carbon

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10.0 (2.6) 5.9 (0.58) 2.9(0.10) 2.0 (0.20) 3.4 (0.59) 3.7 (0.37) 5.6(0.54) 4.4(0.39) 5.9(1.2) 3.3(0.25) 4.8(0.37) 2.6(1.12) 3.8(1.50) 2.7(0.15) 2.7(0.14)

DOC (mg L 1) 4.5 3.5 1.6 2.7 3.1 3.1 3.3 3.1 3.3 2.3 1.9 2.5 2.4 1.3 2.4

(0.33) (0.34) (0.10) (0.52) (0.13) (0.17) (0.84) (0.28) (0.84) (0.38) (0.11) (0.58) (0.24) (0.21) (0.24)

SUVA254 (L (mg C * m)–1) 1.32 (0.02) 1.41 (0.01) 1.49 (0.05) 1.39 (0.02) 1.41 (0.02) 1.43 (0.04) 1.42 (0.01) 1.44 (0.01) 1.45 (0.06) 1.49 (0.02) 1.53 (0.01) 1.42 (0.03) 1.47 (0.03) 1.41 (0.02) 1.47 (0.03)

FI 56 49 38 43 45 48 48 44 50 39 34 41 40 29 41

(3) (3) (2) (3) (3) (1) (3) (2) (2) (5) (3) (6) (2) (3) (4)

HPOA (%) 27.79 (1.09) 26.82 (0.72) 25.7 (1.09) 25.21 (2.26) 27.15 (0.46) 27.33 (0.66) 26.41 (1.86) 25.82 (1.01) 20.19 (13.13) 26.55 (0.39) 27.24 (1.97) 21.57 (18.33) 25.16 (2.6) 25.38 (1.93) 25.45 (1.63)

d 13C-DOC‰ 73.46 (12.57) 51 (23.11) 92.94 (20.02) 18.01 (16.61) 22.14 (22.06) 30.05 (27.01) 39.25 (29.36) 45.46 (33.83) 23.53 (27.88) 40.6 (107.19) 62.53 (46.85) 34.7 (82.41) 3.91 (26.03) 21.75 (19.09) 22.64 (38.39)

D14C-DOC‰ >Mod >Mod 726 (540–1020) >Mod (10 >Mod) >Mod >Mod >Mod (115 >Mod) >Mod >Mod (155 >Mod) >Mod (2470 >Mod) 470 (170–1260) 222(1940 >Mod) >Mod (275 >Mod) >Mod (60 >Mod) 126 (545 >Mod)

Calendar Ages (y.b.p) 70.0 (>Mod) 59.7 (>Mod) 95.5 (748) 26.6 (>Mod) 46.7 (>Mod) 51.2 (>Mod) 52.6 (>Mod) 70.0 (>Mod) 29.8 (>Mod) 111.5 (892) 108.2 (862) 120.8 (976) 22.2 (>Mod) 26.1 (>Mod) 38.6 (258)

Flow Weighted D14C‰ (y.b.p)

680 (905) 9226 (3359) 65 (17) 6250 (2496) 438 (278) 457 (133) 4828 (2072) 20190 (6771) 1574 (1249) 231 (172) 6 (3) 451 (311) 39 (20) 7622 (767) 1228 (559)

Discharge (m3 s 1)

a SUVA254 units are [L (mg C * m) 1] and all isotopic data are in ‰. Calendar ages are calculated as [age = 8033*ln(Fm)] where Fm is the fraction modern [Stuiver and Polach, 1977]. Calendar ages are presented as >Mod when the fraction modern exceeded 1 and represent fixed bomb carbon since 1950.


River

Table 2. Average Dissolved Organic Carbon Concentration, Specific Ultraviolet Absorbance (SUVA254), Fluorescence Index (FI), d 13C, D14C, and Mean Discharge Data During the 2009 Sampling Period for North American Rivers (Table 1a) With Standard Deviation in Parenthesesa

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(Table 2). The flow weighted average D14C-DOC for systems that exported aged DOM suggest a further depletion of radiocarbon under higher discharge, while those rivers that exported more modern DOM show increases in radiocarbon ratios under higher flow (Table 2). [14] Annual average SUVA254 values ranged from 4.5 0.33 L mg-C 1 m 1 in the Altamaha River down to 1.3 0.21 L mg-C 1 m 1 in the St. Lawrence River with the average across all systems being 2.7 0.80 L mg-C 1 m 1. Annual average D14C signatures showed a statistically significant linear increase across a range of SUVA254 values from 1 to 5 L mg-C 1 m 1 (r2 = 0.6, p < 0.001 Figure 3). The St. Lawrence River appears as an outlier to the linear trend with very low SUVA254 values and an average modern carbon isotopic signature of 21.8 19.09 ‰. The FI ranged from a high of 1.53 0.01 in the Rio Grande to 1.32 0.02 in the Altamaha River. There was a strong negative relationship between D14C-DOC and FI (r2 = 0.6, p < 0.001, Figure 3). [15] The percentage of DOC as the hydrophobic organic acid fraction (HPOA), which is comprised of aquatic humic substances, ranged from 56 3% in the Altamaha to a low of 34 3% in the Rio Grande (Table 2). There is a strong relationship between SUVA254 and the percentage of HPOA in DOM across the basins (r2 of 0.93) (Figure 4). This result illustrates the applicability of a simple measure like SUVA254 toward understanding the compositional structure of DOM in freshwaters. [16] River discharge is highly variable across basins with the Mississippi River having an average of 20,190 6771 m3 s 1 down to 6 3 m3 s 1 for the Rio Grande for 2009. Differences in hydrology correlated with the D14C-

Figure 2. Box plot of D14C-DOC across 15 large rivers in the U.S. Samples are taken from 2009 across the annual hydrograph. In all cases, peak flow is included. Numbers in parentheses represent the number of D14C-DOC measurements made. Dark lines are the median values and the red dashed lines are the means. Error bars represent the 5th and 95th percentiles with outliers represented by the black points.

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3.2. Within Basin Trends [17] All measured constituents and their variability are presented in Table 2. In general there were mixed trends within river basins. D14C showed positive correlations with DOC concentration in the Altamaha River (r = 0.79), Atchafalaya River (r = 0.78), Connecticut River (r = 0.84) and Lower Atchafalaya River (r = 0.82) while the St. Lawrence River showed a significant negative correlation (r = 0.79) (Table 4). The across system trends between D14C and SUVA254 (Figure 6) were not always evident within basins. D14C and SUVA254 showed strong positive relationships in the Columbia, Mobile, Potomac, and Susquehanna rivers (Tables 3 and 4). In rivers where aged carbon dominated the D14C signature, such as the Rio Grande and the Colorado (Tables 3 and 4), negative relationships were shown between SUVA254 and radiocarbon isotopic ratios. Those river basins that had a large range in SUVA254, across the water year, such as the Mobile River, had the strongest correlations between SUVA254 and D14C. [18] The within basin correlation between FI, percent HPOA (HPOA%), discharge and D14C-DOC showed significant variation (Table 4). FI showed a negative relationship (p < 0.05) with D14C in the Altamaha (r = 0.86), Atchafalaya (r = 0.80), Connecticut (r = 0.93), and the Susquehanna (r = 0.86). Other basins exhibited no significant relationships between FI and D14C. As HPOA% increased so did the D14C signature (p < 0.05) in the Altamaha (r = 0.80), the Potomac (r = 0.78) the Susquehanna (r = 0.73) and the Hudson River (r = 0.98). In all basins except the Colorado, the relationship between the HPOA% and SUVA254 was positive, however, only the Atchafalaya,

Figure 3. (a) Average specific ultraviolet absorbance (SUVA254) and (b) fluorescence index (FI) data versus D14C-DOC across 15 large rivers in the conterminous U.S. Error bars represent 1 standard deviation in the estimate. Removal of the St. Lawrence improves the relationship significantly to r2 of 0.87 with SUVA254. DOC. The basins with the highest water yield export modern carbon while systems in arid environments (Rio Grande, Colorado), and those potentially impacted by urban systems (Sacramento, Susquehanna and Potomac), showed aged DOC (Figures 2 and 4). Figure 5 shows the log transformed average discharge for each basin along with the average annual precipitation. There is a significant relationship between the D14C signature in a river basin and large-scale hydrologic processes such as precipitation and river discharge (r2 = 0.46, p < 0.006) (Figure 5) where wetter basins have younger DOC.

Figure 4. The relationship between specific ultraviolet absorbance (SUVA254) and the amount of hydrophobic organic acids as a percentage of whole water dissolved organic carbon concentration. Error bars represent 1 standard deviation in the estimate.

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depict how flow, SUVA254, DOC, FI and HPOA % correlate for 2009. The modified Kendal’s tau parameter [Buonaccorsi et al., 2001] is a measure of synchrony between two entities where a value of 1 corresponds to a consistent negative correspondence while a value of 1 corresponds to consistent synchrony. Table 4 shows each chemical constituent as it moves with D14C-DOC. The degree of synchrony represented by the tau values illustrates the limited correlation between D14C-DOC and each measured parameter for DOM and discharge.

4. Discussion

Figure 5. Relationship between the annual average D14C signal to (a) discharge and (b) precipitation across 15 large rivers in North America.

Columbia, Connecticut, Potomac, Sacramento and Susquehanna rivers were statistically significant at the p < 0.05 level. [19] Within basins, discharge was positively correlated to D14C- DOC in the San Joaquin (r = 0.76) and negatively correlated in the Rio Grande (r = 0.89) only. In general, within system variation in discharge was not related to the age of the DOC. Figure 6 shows D14C, SUVA254, and flow for each of the 15 basins. Across each of these basins the annual flow characteristics are very different. The Colorado and St. Lawrence rivers showed limited variability due to episodic events and very small standard deviations for 2009, while others, like the Sacramento and Altamaha, are dominated by significant seasonal variability. Figure 6 and Table 4

4.1. Age of DOC to U.S. Coasts [20] In terms of the total contribution to coastal waters, the U.S. exports modern carbon (34–46‰) when adjusted for flow. This number is controlled by the river basins with the largest freshwater flows, the Mississippi and Atchafalaya Rivers. Modern organic carbon signatures have been shown in the Amazon Basin [Hedges et al., 1986; Mayorga et al., 2005] and Arctic river basins [Benner et al., 2004; Neff et al., 2006; Raymond et al., 2007] as well as forested and peat dominated systems [Palmer et al., 2001; Schiff et al., 1997] across the globe. [21] There are regional differences in the D14C-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 D14C 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 D14C. Rivers flowing into the Northern Atlantic Ocean (St. Lawrence, Hudson, Connecticut) are exporting DOC that has a flow adjusted average D14C 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 D14C 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 D14C-DOC of 50‰. This export corresponds to a calendar age of 350 years before present (1950). 4.2. Within Basin Controls on D14C-DOC [22] 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 D14C 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 D14C-DOC shows a strong negative relationship with concentration, indicative perhaps of an alternate source derived

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Figure 6. Specific ultraviolet absorbance (SUVA254), D14C-DOC, and discharge for 15 large river basins in the U.S. for 2009. Black lines and points represent SUVA254, red lines and points represent D14C-DOC. 8 of 15

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Table 3. Linear Relationship Between Specific Ultraviolet Absorbance (SUVA254nm) and D14C -DOC River

Slope

Intercept

r2

# of Sample Days

p


15.89 44.04 155 36.78 104.28 73.048 53.5 55.5 159.2 269.3 243.9 26.3 41.3 40 135.1

0.6 105.3 152.4 85.6 297 254.7 138 128.2 565 650.8 400.9 100.5 95.6 31.6 336.7

0.16 0.34 0.58 0.97 0.4 0.2 0.3 0.22 0.77 0.85 0.33 0.03 0.15 0.18 0.65

9 10 6 5 5 4 9 8 7 6 6 8 7 6 8

0.2 0.06 0.08