Letter pubs.acs.org/journal/estlcu
Fossil Carbon in Particulate Organic Matter in the Gulf of Mexico following the Deepwater Horizon Event J. Cherrier,† J. Sarkodee-Adoo,† T. P. Guilderson,‡ and J. P. Chanton*,§ †
School of the Environment, Florida A&M University, Tallahassee, Florida 32307, United States Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, and Department of Ocean Sciences, University of California, Santa Cruz, California 95064, United States § Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida 32306, United States ‡
ABSTRACT: In 2010, the Deepwater Horizon event released large quantities of natural gas into the Gulf of Mexico in addition to oil. Several studies have reported evidence of the introduction of petro-based carbon into the Gulf planktonic foodweb. This study reports results consistent with the hypothesis that methane-derived carbon entered the food web through small particles indicative of methanotrophy. Suspended particulate organic carbon (POCsusp), collected from the Gulf water column in 2011 and 2012, was depleted of δ13C and Δ14C relative to surface planktonic production. The suspended particulate organic fraction was as depleted as −37‰ in δ13C and −618‰ in Δ14C. The 13C and 14C values were strongly correlated and indicated the admixture of modern surface carbon with a depleted radiocarbon source that had a δ13C value indicative of methane input. Mixing models indicated that 28−43% of the POCsusp may have been derived from fossil CH4.
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during the oil spill.13 Methanotrophic biomass retains its distinctive depleted stable isotopic signature with subsequent transfer up the food web. If the methane source is ancient, that biomass will bear a distinctive radiocarbon-depleted signature, as well.12 In addition to seep communities, methanotrophs have been shown to be important to the food web of other environments, including that of protists in soils and grazing zooplankton in lakes.14−16 The goal of this study was to investigate the manner in which methane or petroleum carbon may have entered into the Gulf food web. Specifically, we used natural 13C and 14C to investigate the carbon sources of the suspended particulate organic carbon (i.e., POCsusp, operationally, the particulate material that is retained on a 0.7 μm glass fiber filter). POCsusp is distinguished from larger sinking particles that are collected in sediment traps.17−20 The water column was sampled in the DeSoto Canyon (Figure 1), a location known to be affected by hydrocarbon plumes during the spill (D. Hollander, personal communication, 2013).
INTRODUCTION The 2010 hydrocarbon blowout in the central Gulf of Mexico released from 0.9 to 1.2 × 1010 mol of natural gas, mostly in the form of methane.1−4 Measurements at the Gulf surface indicated that little methane was released to the atmosphere,5 so most was presumably dissolved in the water column where it was subsequently oxidized over the next months by methanotrophic bacteria, to produce CO2, energy, and biomass.1,2 Hydrocarbons released in the summer of 2010 appear to have entered the planktonic food web of the Gulf as indicated by 13C measurements.6 These results were subsequently confirmed with natural abundance 14C.7 On the basis of the covariance of 14C and 13C in plankton samples, Chanton et al.7 hypothesized that methane may have been an important contributor to the food web. The microbial consumption of methane lends itself to relatively high percentages of assimilation into biomass, relative to CO2 production.8 Gommers et al.8 calculated that the theoretical ratio of CO2 production during methane consumption is 0.12 assuming methane assimilation by the ribulose monophosphate pathway of formaldehyde fixation. Borjesson et al.9,10 measured (CO2 produced)/(methane consumed) ratios that varied from 0.16 to 0.4, indicating a large portion of consumed methane was transferred into biomass. This high rate of transfer of methane into biomass is significant and allows the highly successful symbiotic relationship of methanotrophic bacteria with seep fauna, particularly mussels.11,12 Du and Kessler suggest a biomass conversion ratio of 0.36 ± 0.11 © 2013 American Chemical Society
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METHODS Water was recovered from various depths, surface to bottom, using a CTD rosette water sampler on the Florida Institute of Received: Revised: Accepted: Published: 108
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Figure 1. Sampling sites in the DeSoto Canyon. Water depth varied from 26 to 1300 m, and sites were from 24 to 156 km offshore. Locations of sites were as follows: DS1, 29.2050 N, 87.0617 W, 780 m depth, 122 km from shore; DS2, 30.1670 N, 86.6630 W, 26 m depth, 24 km from shore; DS3, 28.8259 N, 88.2678 W, 1300 m depth, 156 km from shore; DS4, 29.1832 N, 87.7487 W, 1000 m depth, 115 km from shore. DWH marks the location of the Deepwater Horizon well.
Table 1. POCsusp Isotopic Values depth (m)
δ13C (PDB)
Δ14C
±
CAMS #
size (μmol of C)
μmol of C/L 10.0 17.9 12.1 12.5 8.9 2.8 1.5 3.4 3.3 7.8 6.1 9.2 8.9 4.3 2.7 2.7 2.8 3.8 2.3
2011 DS1 DS2 DS3 DS4 DS2 DS1 DS3 DS1 DS4 DS3
5 5 5 5 20 350 700 780 980 1200
−25.7 −27.0 −26.6 −28.9 −29.0 −35.3 −35.6 −35.0 −32.0 −34.6
−28 −74 −39 −46 −143 −410
6 4 5 4 5 4
157586 157596 157587 157589 157599 158412
−542 −228 −520
5 9 5
157597 157590 157588
11.3 20.1 13.6 14.1 10.0 3.2 1.7 3.8 3.7 8.8
DS1 DS2 DS3 DS4 DS4 DS3 DS1 DS4 DS3
5 5 5 5 500 600 816 980 1248
−32.0 −30.2 −29.8 −28.4 −36.7 −37.0 −37.0 −37.2 −35.3
−116 −181 −274 −172 −618 −594 −454 −547 −404
5 4 3 7 9 9 8 6 11
158338 158340 158341 158346 158345 158342 158339 158344 158343
6.9 10.4 10.0 4.8 3.0 3.0 3.2 4.3 2.6
2012
Oceanography’s RV Weatherbird. At selected depths, 3 L water samples were collected from duplicate rosette bottles into 14C clean, acid-washed polycarbonate carboys from which replicate 1.5 L aliquots were filtered through precombusted (4 h at 525 °C) 0.7 μm, 47 mm glass fiber filters. Each filter was sealed in precombusted aluminum foil packets and then frozen for subsequent analysis. All filters were fumed with HCl for 24 h and then dried. Filters for δ13C were analyzed on a Carlo Erba
elemental analyzer coupled to a ThermoFisher isotope ratio mass spectrometer. One-quarter of a filter was run for 13C. The 13 C values were corrected for blanks (one-quarter combusted unused acidified filter and cups), which represented ∼10% of the signal of samples on the elemental analyzer and had a value of −27.1‰. Results are presented relative to VPDB [δ13C = (Rsam/Rstd − 1) × 1000, where R = 13C/12C]. Filters were then taken to Lawrence Livermore National Laboratory, where they 109
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were combusted, converted to graphite targets, and analyzed for 14 C by accelerator mass spectrometry.21 Values are reported according to the Δ notation put forth by Stuiver and Polach.22 The Δ notation corrects the radiocarbon content of samples for kinetic fractionation to the same δ13C value (−25‰) and time point and is a linear scale starting at −1000‰ when a sample has 0% modern carbon.23 The 14C values include a background subtraction for combustion and graphitization and a filter blank of 0.5 μmol of C with a Δ14C value of −50.0 ± 6‰. The sizes of the deeper samples were close to but within the lower range of the Lawrence Livermore National Laboratory’s ability to run them with confidence.
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RESULTS AND DISCUSSION The δ13C values for POCsusp were depleted relative to surface production and varied from −26 to −37‰ (Table 1). The error in 13C analysis was 0.15‰ based on replicate samples. The δ13C values were more depleted at depth relative to that at the surface. The most 13C-enriched values of POCsusp were similar to the most depleted postspill 13C values for surface plankton,6,7 which ranged from −21 to −25‰ and were apparently affected by petro carbon inputs. Following the trend of the δ13C values, the POCsusp 14C values became more depleted at depth (Table 1). The POCsusp Δ14C ranged from −28 to −618‰. The amount of POC on the filters decreased with depth (Table 1) as observed by others.17−20 The surface seawater dissolved inorganic carbon (DIC) collected along the same transect in 2011 was modern with a Δ14C value of 41.0 ± 3.0‰. 7 Had the POC been produced from surface phytoplankton production, it would have been similar to the 14 C of DIC with a 13C signature similar to that of uncontaminated phytoplankton, −20 to −22‰.17,18 However, the POC in the Gulf of Mexico at the time we sampled was significantly depleted of 14C relative to surface DIC, and the 13 C was depleted relative to phytoplankton, indicating the admixture of a source of organic carbon that was depleted of both 13C and 14C. The 14C sources of POCsusp can be envisioned as two endmembers. For 14C, one endmember was radiocarbon-dead petro-based carbon (oil or methane) with a value of −1000‰. The second endmember was surface organic matter with a 14C value of 41‰.7,24 The contributors that control the value of 13 C for these vary from a value of −21‰ for surface production17,18 to −27‰ for petroleum6 to −61.1 ± 2.2‰ (n = 18)2 to −57.4 ± 0.4‰ (n = 27) (S. Joye and J. P. Chanton, unpublished observations) for methane released from the Deepwater Horizon oil spill. POCsusp 13C was plotted versus POCsusp 14C, and the two isotopes were significantly correlated [p < 0.001 (Figure 2)]. Extrapolating the linear regression to the δ13C value at which 14 C reached complete depletion yielded a value of −51‰. This value is similar to those measured for Gulf of Mexico seep methane,25 although it was slightly 13C-enriched relative to the blowout-derived methane values reported by Valentine et al.2 and S. Joye and J. P. Chanton (unpublished observations), which ranged from −61 to −57‰. One might expect to observe isotopic fractionation with methanotrophy, but with quantitative consumption, that would not be expressed. At the time the 2011 POCsusp samples were collected, plankton Δ14C ranged from 58 to −15‰,7 indicating the possible input of fossil carbon.6 Like the POC data, the 13C and 14 C plankton values showed a significant correlation, and 14C
Figure 2. Correlation of 13C and 14C POC values. Diamonds are the data from the DeSoto Canyon (Table 1 and Figure 1), and the line is y = 40.7x + 1047. Triangles are data from ref 20 and represent POCsusp from the Mid-Atlantic Bight Margin, and the line is y = 43x + 976. Squares are data from ref 18 and are from the mid-Atlantic Sargasso Sea. These deep water open ocean samples show less radiocarbon depletion and little variation of δ13C from values typical of surface planktonic production.
became more negative with decreasing values of δ13C indicating the admixture of a depleted source. A two-endmember mass balance indicated that the plankton contained 5−13% petro carbon and that the best agreement resulted when methane was used as an endmember in the 13C calculations.7 In the study presented here, we applied a dual-isotope three-endmember mixing model20,26,27 to evaluate the contribution of methane to deep sea POCsusp (>350 m). The three-endmember model is based on a three-equation matrix system: f1 + f2 + f3 = 1.0
(1)
N1f1 + N2f2 + N3f3 = N
(2)
E1f1 + E2f2 + E3f3 = E
(3)
where f is the relative fractional contribution of three potential sources, N is the Δ14C POC, and E is the δ13C POC. Equation 1 establishes the condition that all fractional contributions must total 1.0, and eqs 2 and 3 simultaneously determine the contribution of each source based on N, observed Δ14C values, and E, observed δ13C values, respectively. The endmembers used were methane (δ13C = −57 to −61‰, and Δ14C = −1000‰), petroleum (δ13C = −27‰, and Δ14C = −1000‰), and modern plankton (δ13C = −21 and Δ14C = 41‰). The model indicates that 28−43% of the observed deep POCsusp signatures can be explained by fossil methane, 4−24% can be explained by petroleum, and plankton explains 37−74% of the variability. It would appear from the results that methane was successful in entering the plankton and POCsusp. The results are consistent with the unique 13C-depleted signature of many of the data. This result may be in part due to the high efficiency of conversion of methane into biomass relative to respiration to form CO2 as discussed above.8−10,13 110
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In conclusion, it appears that the POCsusp fraction in the Gulf of Mexico shelf and slope waters has been affected with inputs consistent with fossil methane, possibly associated with the Macondo oil spill. At this point, because of a lack of prior data, it is difficult to say what the POCsusp in the Gulf of Mexico looked like prior to the spill and to what extent it may be affected by natural seepage, as indicated for the MAB samples (Figure 2). Druffel et al.18 have estimated a decadal residence (8.5 years) time for POCsusp in the Central Pacific Ocean, similar to the residence time results of 5−10 years obtained by Bacon and Anderson,34 who used a thorium isotope approach. If the residence time of POCsusp of the Gulf of Mexico is on the order of a decade, then it may be several years before we are able to answer this question. Nonetheless, the data presented here are consistent with the hypothesis of Chanton et al.,7 namely that a small size fraction of 13C- and 14C-depleted carbon affected the planktonic food web and this fraction was likely affected by methanotrophy.
Relative to suspended particulate organic carbon in the central Atlantic, Pacific, or Arctic Ocean, the Gulf POCsusp is depleted of 13C and 14C.17−20,28 Druffel et al.18 report midocean POCsusp profiles that exhibit δ13C values ranging from −20 to −26‰, and the most 13C-depleted values were found near the surface. In the Pacific Ocean, Δ14C ranged from 150‰ at the surface to −20‰ at depths of 6000 m. In the Atlantic Ocean, the Δ14C of particles was 0‰ from the surface to 3800 m and then decreased to −160‰.18 A similar pattern was recently observed in the Arctic Ocean.28 The enriched 14C endmember will partly depend upon the year of collection due to weapons testing that occurred in the mid-20th Century.23 In ocean margin environments more similar to the Gulf margin we sampled, suspended POC more depleted of both 13 C and 14C relative to the open ocean sites has been reported. Bauer and Druffel19 observed that both POCsusp and DOC (dissolved organic carbon) collected along the margins of the North Atlantic Ocean and North Pacific Ocean were depleted of 14C relative to midocean material. The POC was reported to be as low as −200‰,19 while δ13C was as depleted as −24.9‰. Druffel et al.17 observed depleted 14C in the Pacific Ocean confined to near bottom samples. These investigators speculated that the negative 14C values observed in POCsusp near the margins were due to resuspension of 14C-depleted sediments. However, in the DeSoto Canyon, the sediments vary in δ13C from −20.6 to −22.7‰ (J. P. Chanton, unpublished observations). POC associated with the outflow from the Mississippi River has been reported to include a Δ14C from −86 to −223‰ and a δ13C from −23.3 to −26.0‰ (ref 29 and personal communication in 2013 with L. Guo). Neither of these possible sources is consistent with the Gulf of Mexico POCsusp observations (Table 1). Bauer and Druffel19 suggested that older 14C-depleted DOC could be absorbed to POC and thus affect its radiocarbon content. The idea that hydrocarbon seepage could impart depleted values to the DOC pool was suggested,19 but it was mentioned that this effect would be less likely to affect the POCsusp pool. Hydrocarbon seeps have since been shown to contribute 14C- and 13C-depleted DOC to overlying waters in the Gulf of Mexico and the Pacific Ocean.30,31 It is also possible that DOC absorbed to the filters during the filtration process.28,32 Therefore, our measurements could reflect some contamination by DOC, but open water DOC is generally not as depleted of 13C as our POC values.19,28,33 The idea that hydrocarbon seeps might influence POCsusp was revisited.20 δ13C POCsusp values for deep Mid-Atlantic Bight slope water were among the most depleted 13C values observed in the deep ocean (−31.6 to −23.0‰). POCsusp Δ14C were as depleted as −660‰. Bauer et al.20 labeled a subset of the POCsusp values that were depleted of δ13C (less than −30‰) and Δ14C (−300 to −660‰) as “unconstrained” but suggested that they might have a petroleum seep origin. We have replotted this MAB data from July and August 1996, Central, Southern, and Northern transects in Figure 2 (triangles). The data are similar to the data from the Gulf of Mexico, in that 13C and 14C both decrease and are correlated. Solving for the δ13C value of the line fit to the data from ref 20 when Δ14C is −1000‰ yields a δ13C value of −46‰, similar to that of thermogenic methane. The open ocean nonmargin values of Druffel et al.18 from the Sargasso Sea are also shown in Figure 2. These deep water open ocean samples show less radiocarbon depletion and little variation in δ13C from values typical of surface phytoplanktonic production.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This research was supported by grants from the BP/The Gulf of Mexico Research Initiative through the Florida Institute of Oceanography, the Deep C Consortium administered by Florida State University (#SA 12-12,GoMRI-008), and the Ecosystem Impacts of Oil and Gas Inputs to the Gulf (ECOGIG) Consortium administered by the University of Mississippi, and the National Oceanic and Atmospheric Administration, Office of Education Educational Partnership Program award (NA11SEC4810001) to the Environmental Cooperative Science Center (ECSC) administered by Florida A&M University. The contents of this publication are solely the responsibility of the award recipient and do not necessarily represent the official views of the U.S. Department of Commerce, National Oceanic and Atmospheric Administration. We thank Alex Harper, Kelsey Rogers, Samantha Bosman, and Ale Mickle of the Department of Earth, Ocean and Atmospheric Science (Florida State University) and Yingfeng Xu at the National High Magnetic Field Laboratory for assistance with 13C and 14C work. We also thank P. Zermeño at Lawrence Livermore National Laboratory for excellent AMS work. We thank editor Bruce Logan and three reviewers for their helpful comments. The data registry numbers for GRIIDC are R1.x.132.134:0031 and R1.x138.078:0019. A portion of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
(1) Kessler, J. D.; Vaentine, D. L.; Redmond, M. C.; Du, M.; Chan, E. W.; Mendes, S. D.; Quiroz, E. W.; Villanueva, C. J.; Shusta, S. S.; Werra, L. M.; Yvon-Lewis, S. A.; Weber, T. C. A persistent oxygen anomaly reveals the fate of spilled methane in the deep Gulf of Mexico. Science 2011, 331, 312−315. (2) Valentine, D. L.; et al. Propane respiration jump-starts microbial response to a deep oil spill. Science 2010, 330, 208−211.
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(3) Joye, S. B.; MacDonald, I. R.; Leifer, I.; Asper, V. Magnitude and oxidation potential of hydrocarbon gases released from the BP oil well blowout. Nat. Geosci. 2011, 4, 160−165. (4) McNutt, M. K.; Camilli, R.; Crone, T. J.; Guthrie, G. D.; Hsieh, P. A.; Ryerson, T. B.; Savas, O.; Shaffer, F. Review of flow rate estimates of the Deepwater Horizon oil spill. Proc. Natl. Acad. Sci. U.S.A. 2012, 109 (50), 20260−20267. (5) Yvon-Lewis, S. A.; Hu, L.; Kessler, J. Methane flux to the atmosphere from the Deepwater Horizon oil disaster. Geophys. Res. Lett. 2011, 38, DOI: 10.1029/2010GL045928. (6) Graham, W. M.; Condon, R. H.; Carmichael, R. H.; D’Ambra, I.; Patterson, H. K.; Linn, L. J.; Hernandez, F. J., Jr. Oil carbon entered the coastal planktonic food web during the Deepwater Horizon oil spill. Environ. Res. Lett. 2010, 5, 045301. (7) Chanton, J. P.; Cherrier, R. M.; Wilson, J.; Sarkodee-Adoo, S.; Boseman, A.; Mickle; Graham, W. M. Radiocarbon indicates that carbon from the Deepwater Horizon Spill entered the planktonic food web of the Gulf of Mexico. Environ. Res. Lett. 2012, 7, 045393. (8) Gommers, P. J. F.; van Schie, B. J.; van Dijken, J. P.; Kuenen, J. G. Biochemical limits to microbial growth yields: An analysis of mixed substrate unilization. Biotechnol. Bioeng. 1988, 32, 86−94. (9) Borjesson, G.; Chanton, J. P.; Svensson, B. H. Methane oxidation in two Swedish Landfill Cover Soils Determined with the Use of 13 C/12C Isotope Ratios. J. Environ. Qual. 2001, 30, 369−376. (10) Börjesson, G.; Sundh, I.; Tunlid, A.; Frostegard, A.; Svensson, B. H. Microbial oxidation of CH4 at high partial pressures in an organic landfill cover soil under different moisture regimes. FEMS Microbiol. Ecol. 1998, 26, 207−217. (11) Childress, J. M.; Fisher, C. R.; Brooks, J. M.; Kennicutt, M. C., II; Bidigare, R.; Anderson, A. E. A methanotrophic marine molluscan (Bivalvia, Mytilidae) symbiosis: Mussels fueled by gas. Science 1986, 233, 1306−1308. (12) Paull, C. K.; Martens, C. S.; Chanton, J. P.; Neumann, A. C.; Coston, J.; Jull, A. T.; Toolin, L. J. Fossil methane carbon in living chemosynthetic organisms and young carbonate cements at the Florida Escarpment. Nature 1989, 342, 166−168. (13) Du, M.; Kessler, J. D. Assessment of the spatial and temporal variability of bulk hydrocarbon respiraton following the Deepwater Horizon Oil Spill. Environ. Sci. Technol. 2012, 46, 10499−10507. (14) Murase, J.; Frenzel, P. Selective grazing of methanotrophs by protozoa in a rice field soil. FEMS Microbiol. Ecol. 2008, 65, 408−414. (15) Jones, R. I.; Grey, J.; Sleep, D.; Arvola, L. Stable isotope analysis of zooplankton carbon nutrition in humic lakes. Oikos 1988, 86, 97− 204. (16) Bastviken, D.; Ejlertsson, J.; Sundh, I.; Tranvik, L. Methane as a source of carbon and energy for lake pelagic food webs. Ecology 2003, 84, 969−981. (17) Druffel, E. R. M.; Griffin, S.; Bauer, J. E.; Wolgast, D. M.; Wang, X. C. Distribution of particulate organic carbon and radiocarbon in the water column from the upper slope to the abyssal NE Pacific Ocean. Deep Sea Res., Part II 1998, 45, 667−687. (18) Druffel, E. R. M.; Bauer, J. E.; Griffin, S.; Hwang, J. Penetration of anthropogenic carbon into organic particles of the deep ocean. Geophys. Res. Lett. 2003, 30, 1744. (19) Bauer, J. E.; Druffel, E. R. M. Ocean margins as a significant source of organic matter to the deep ocean. Nature 1998, 392, 482− 485. (20) Bauer, J. E.; Druffel, E. R. M.; Wolgast, D. M.; Griffin, S. Temporal and regional variability in sources and cycling of DOC and POC in the northwest Atlantic continental shelf and slope. Deep Sea Res., Part II 2002, 49, 4387−4419. (21) Vogel, J. S.; Southon, J. R.; Nelson, D. E.; Brown, T. A. Performance of catalytically condensed carbon for use in accelerator mass spectrometry. Nucl. Instrum. Methods Phys. Res., Sect. B 1984, 5, 289−293. (22) Stuiver, M.; Polach, H. A. Reporting of 14C Data. Radiocarbon 1977, 19, 355−363. (23) McNichol, A. P.; Aluwihare, L. I. The power of radiocarbon in biogeochemical studies of the marine carbon cycle: Insights from
studies of dissolved and particulate organic carbon (DOC and POC). Chem. Rev. 2007, 107, 443−466. (24) Graven, H. D.; Guilderson, T. P.; Keeling, R. F. Observations of radiocarbon in CO2 at LaJolla, California USA 1992−2007, Analysis of a long term trend. J. Geophys. Res. 2012, 117 (D2), DOI: 10.1029/ 2011JD016533. (25) Sassen, R.; Milkov, A. V.; Ozgul, E.; Roberts, H. H.; Hunt, J. L.; Beeunas, M. A.; Chanton, J. P.; DeFreitasa, D. A.; Sweet, S. T. Gas venting and subsurface charge in the Green Canyon area, Gulf of Mexico continental slope: Evidence of a deep bacterial methane source? Org. Geochem. 2003, 34, 1455−1465. (26) McCallister, S. L.; Bauer, J. E.; Cherrier, J.; Ducklow, H. Assessing sources and ages of organic matter supporting river and estuarine production: A multiple isotope (Δ14C, δ13C and δ15N) approach. Limnol. Oceanogr. 2004, 49 (5), 1687−1702. (27) Dorsett, A.; Cherrier, J.; Martin, J. B.; Cable, J. E. Assessing hydrologic and biogeochemical controls on pore-water dissolved inorganic carbon cycling in a subterranean estuary: A 14C and 13C mass balance approach. Mar. Chem. 2011, 127 (1−4), 77−89. (28) Griffith, D. R.; McNichol, A. P.; Xu, L.; McLaughlin, F. A.; Macdonald, R. W.; Brown, K. A.; Eglinton, T. I. Carbon dynamics in the western Atctic Ocean: Insights from full-depth carbon isotope profiles of DIC, DOC, and POC. Biogeosciences 2012, 9, 1217−1224. (29) Rosenheim, B. E.; Roe, K. M.; Roberts, B. J.; Kolker, A. S.; Allison, M. A.; Johannesson, K. H. River discharge influences on particulate organic carbon age structure in the Mississippi/Atchafalaya River System. Global Biogeochemial Cycles 2013, 27, 154−166. (30) Wang, X.-C.; Chen, R. F.; Whelan, J.; Eglinton, L. Contribution of ‘old’ carbon from natural marine hydrocarbon seeps to sedimentary and dissolved organic carbon pools in the Gulf of Mexico. Geophys. Res. Lett. 2001, 28, 3313−3316. (31) Pohlman, J. W.; Bauer, J. E.; Waite, W. F.; Osburn, C. L.; Chapman, N. R. Methane hydrate-bearing seeps as a source of aged dissolved organic carbon to the oceans. Nat. Geosci. 2011, 4, 37−41. (32) Gardner, W. D.; Richardson, M. J.; Carlson, C. A.; Hansell, D.; Mishonov, A. V. Determining true particulate organic carbon: Bottles, pumps and methodologies. Deep Sea Res., Part II 2003, 50, 655−674. (33) Benner, R.; Biddanda, B.; Black, B.; McCarthy, M. Abundance, size distribution, and stable carbon and nitrogen isotopic compositions of marine organic matter isolated by tangential-flow ultrafiltration. Mar. Chem. 1997, 57, 243−263. (34) Bacon, M. P.; Anderson, R. F. Distribution of thorium isotopes between dissolved and particulate forms in the deep sea. J. Geophys. Res. 1982, 87, 2045−2056.
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