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Title: Seqestration of dissolved organic carbon in the deep sea

Abstract

There are 600 GT of dissolved organic carbon (DOC) sequestered in seawater. The marine inventory of DOC is set by its concentration in the deep sea, which is nearly constant at 35+2µM C, irrespective of sample location or depth. Isotopic measurements show deep sea DOC to be depleted in radiocarbon, with an apparent radiocarbon age of between 4000ybp (Atlantic) and 6000ybp (Pacific). From the radiocarbon data, we can infer that deep sea DOC is inert and does not cycle on less than millennial time scales. However, high precision DOC measurements show deep sea concentrations are variable at the + 1-2µM DOC level, suggesting a fraction of deep sea DOC, equivalent to 15-30Gt C, is cycling on short time scales, acting as a sink for new, atmospheric carbon. This project is designed to identify and quantify the biological and physical processes that sequester DOM in the deep sea by making compound specific radiocarbon measurements on sugars and proteins extracted from deep sea DOC. Our Hawaii surface seawater sample has a DIC Δ14C value of 72 + 7 ‰ and shows the influence of bomb radiocarbon on surface water DIC values. HMWDOC Δ14C is 10 ‰, significantly depleted in radiocarbon relative tomore » DIC. Purification of HMWDOC by reverse phase HPLC yields seven neutral sugars with radiocarbon values of 47 – 67‰. Assuming the radiocarbon determinations of individual sugars in HMWDOC serve as replicates, then the average Δ14C for neutral sugars in HMWDOC is 57 + 6 ‰(1 SD, n=11), only slightly depleted in 14C relative to DIC. There has been a sharp decrease in radiocarbon values for DIC in the North Pacific Ocean over the past few decades. If neutral sugars cycle more slowly than DIC, we would expect them to have correspondingly higher radiocarbon values. Previous studies have modeled upper ocean DOC as a two component mixture of newly synthesized DOC with a radiocarbon value equal to DIC, and an old component with a radiocarbon value equal to deep sea DO14C. In order to measure the radiocarbon value of the old DOC component, we analyzed a molecularly uncharacterized carbon (MUC) fraction isolated from HMWDOC. Ten percent of HMWDOC is retained by the Biorex anion ion exchange resin, but eluted by NH4OH. This fraction has spectral characteristics nearly identical to deep sea HMWDOC (Fig. 2), and a Δ14C of–416‰. Our Δ14C value for MUC in surface water is within the range of values for HMWDOC isolated from 900-5200m at this site (-380 to –440‰), and significantly depleted relative to a sample of humic substances isolated at 10 m by adsorption onto XAD resin (-342‰; Druffel et al. (1992)). Separation of MUC from the more reactive, newly synthesized component of HMWDOC as represented by neutral sugars in surface seawater yields a MUC fraction with radiocarbon depletions similar to deep sea (> 1000-5720 m) DO14C (-501 to -536‰, Druffel et al., 1992). Our analyses therefore verify the existence of both a newly synthesized and old fraction of DOC in surface seawater with radiocarbon values equal to DIC and nearly equal to deep sea DOC. Neutral sugar concentrations decrease from 4-6 µM C or 13-21% of HMWDOC in surface samples, to 0.7µM C or 6% of HMWDOC at 600m. The carbohydrate fraction of HMWDOC can be introduced into the mesopelagic ocean through two fundamentally different mechanisms. A small fraction of the reactive carbohydrate synthesized in the euphotic zone may escape degradation and be mixed into the mesopelagic ocean by advection. These sugars will have a radiocarbon value equal to DIC at depth. Alternatively, sugars could be introduced from the dissolution of rapidly sinking large particles. Reactive DOC injected by sinking particles will have radiocarbon values similar to surface water DIC. To distinguish these two mechanisms, we compared radiocarbon values of DIC and neutral sugars in samples from 600m. DIC Δ14C and HMWDOC Δ14C values at 600m sample are –155 + 7 ‰ and –258‰ respectively, and are typical of values at this depth in the North Pacific Ocean. Neutral sugars at 600m have radiocarbon values between –108 and –133‰, and are enriched by up to 150 ‰ relative to HMWDOC. The average Δ14C value obtained by treating glucose, galactose, xylose and mannose as replicates is –123 + 10 ‰ (1SD, n=4), and is slightly enriched in radiocarbon relative to DIC. Our data suggest that some fraction of neutral sugars might be introduced by the dissolution of rapidly sinking particles. If we assume that neutral sugars at 600m are a simple mixture of new carbon with a Δ14C value equal to surface water DIΔ14C, and older carbon with a Δ14C value equal to DIΔ14C at depth, then 15% of the neutral sugars at 600m are introduced by large, rapidly sinking particles.« less

Authors:
Publication Date:
Research Org.:
Woods Hole Oceanographic Institution
Sponsoring Org.:
USDOE - Office of Energy Research (ER)
OSTI Identifier:
908226
Report Number(s):
DOE/ER/62999-1
TRN: US200821%%321
DOE Contract Number:
FG02-00ER62999
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
20 FOSSIL-FUELED POWER PLANTS; CARBOHYDRATES; CARBON; DISSOLUTION; GALACTOSE; GLUCOSE; HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY; ION EXCHANGE; MANNOSE; MIXTURES; PACIFIC OCEAN; PROTEINS; PURIFICATION; RESINS; SACCHARIDES; SACCHAROSE; SEAS; SEAWATER; SURFACE WATERS; XYLOSE; ocean carbon sequestration; marine carbon cycle; dissolved organic carbon; radiocarbon

Citation Formats

Daniel J. Repeta. Seqestration of dissolved organic carbon in the deep sea. United States: N. p., 2006. Web. doi:10.2172/908226.
Daniel J. Repeta. Seqestration of dissolved organic carbon in the deep sea. United States. doi:10.2172/908226.
Daniel J. Repeta. Wed . "Seqestration of dissolved organic carbon in the deep sea". United States. doi:10.2172/908226. https://www.osti.gov/servlets/purl/908226.
@article{osti_908226,
title = {Seqestration of dissolved organic carbon in the deep sea},
author = {Daniel J. Repeta},
abstractNote = {There are 600 GT of dissolved organic carbon (DOC) sequestered in seawater. The marine inventory of DOC is set by its concentration in the deep sea, which is nearly constant at 35+2µM C, irrespective of sample location or depth. Isotopic measurements show deep sea DOC to be depleted in radiocarbon, with an apparent radiocarbon age of between 4000ybp (Atlantic) and 6000ybp (Pacific). From the radiocarbon data, we can infer that deep sea DOC is inert and does not cycle on less than millennial time scales. However, high precision DOC measurements show deep sea concentrations are variable at the + 1-2µM DOC level, suggesting a fraction of deep sea DOC, equivalent to 15-30Gt C, is cycling on short time scales, acting as a sink for new, atmospheric carbon. This project is designed to identify and quantify the biological and physical processes that sequester DOM in the deep sea by making compound specific radiocarbon measurements on sugars and proteins extracted from deep sea DOC. Our Hawaii surface seawater sample has a DIC Δ14C value of 72 + 7 ‰ and shows the influence of bomb radiocarbon on surface water DIC values. HMWDOC Δ14C is 10 ‰, significantly depleted in radiocarbon relative to DIC. Purification of HMWDOC by reverse phase HPLC yields seven neutral sugars with radiocarbon values of 47 – 67‰. Assuming the radiocarbon determinations of individual sugars in HMWDOC serve as replicates, then the average Δ14C for neutral sugars in HMWDOC is 57 + 6 ‰(1 SD, n=11), only slightly depleted in 14C relative to DIC. There has been a sharp decrease in radiocarbon values for DIC in the North Pacific Ocean over the past few decades. If neutral sugars cycle more slowly than DIC, we would expect them to have correspondingly higher radiocarbon values. Previous studies have modeled upper ocean DOC as a two component mixture of newly synthesized DOC with a radiocarbon value equal to DIC, and an old component with a radiocarbon value equal to deep sea DO14C. In order to measure the radiocarbon value of the old DOC component, we analyzed a molecularly uncharacterized carbon (MUC) fraction isolated from HMWDOC. Ten percent of HMWDOC is retained by the Biorex anion ion exchange resin, but eluted by NH4OH. This fraction has spectral characteristics nearly identical to deep sea HMWDOC (Fig. 2), and a Δ14C of–416‰. Our Δ14C value for MUC in surface water is within the range of values for HMWDOC isolated from 900-5200m at this site (-380 to –440‰), and significantly depleted relative to a sample of humic substances isolated at 10 m by adsorption onto XAD resin (-342‰; Druffel et al. (1992)). Separation of MUC from the more reactive, newly synthesized component of HMWDOC as represented by neutral sugars in surface seawater yields a MUC fraction with radiocarbon depletions similar to deep sea (> 1000-5720 m) DO14C (-501 to -536‰, Druffel et al., 1992). Our analyses therefore verify the existence of both a newly synthesized and old fraction of DOC in surface seawater with radiocarbon values equal to DIC and nearly equal to deep sea DOC. Neutral sugar concentrations decrease from 4-6 µM C or 13-21% of HMWDOC in surface samples, to 0.7µM C or 6% of HMWDOC at 600m. The carbohydrate fraction of HMWDOC can be introduced into the mesopelagic ocean through two fundamentally different mechanisms. A small fraction of the reactive carbohydrate synthesized in the euphotic zone may escape degradation and be mixed into the mesopelagic ocean by advection. These sugars will have a radiocarbon value equal to DIC at depth. Alternatively, sugars could be introduced from the dissolution of rapidly sinking large particles. Reactive DOC injected by sinking particles will have radiocarbon values similar to surface water DIC. To distinguish these two mechanisms, we compared radiocarbon values of DIC and neutral sugars in samples from 600m. DIC Δ14C and HMWDOC Δ14C values at 600m sample are –155 + 7 ‰ and –258‰ respectively, and are typical of values at this depth in the North Pacific Ocean. Neutral sugars at 600m have radiocarbon values between –108 and –133‰, and are enriched by up to 150 ‰ relative to HMWDOC. The average Δ14C value obtained by treating glucose, galactose, xylose and mannose as replicates is –123 + 10 ‰ (1SD, n=4), and is slightly enriched in radiocarbon relative to DIC. Our data suggest that some fraction of neutral sugars might be introduced by the dissolution of rapidly sinking particles. If we assume that neutral sugars at 600m are a simple mixture of new carbon with a Δ14C value equal to surface water DIΔ14C, and older carbon with a Δ14C value equal to DIΔ14C at depth, then 15% of the neutral sugars at 600m are introduced by large, rapidly sinking particles.},
doi = {10.2172/908226},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Mar 01 00:00:00 EST 2006},
month = {Wed Mar 01 00:00:00 EST 2006}
}

Technical Report:

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  • Samples of sterilized sea water were bottled and sent to a total of fifteen laboratories for analysis for total dissolved inorganic carbon. These were located in Canada (2), Germany (1), The Netherlands (1), Sweden (1), the United States (9), and the United Kingdom (1). All except one laboratory used their current implementation of an extraction/coulometric procedure to analyze the samples; the other laboratory, Dr. Keeling's laboratory at the Scripps Institution of Oceanography, analyzed the samples using their extraction/manometric procedure which is considered a definitive'' method. The within-laboratory standard deviation of the results reported by the various laboratories (omitting the twomore » that had substantial difficulties) ranged from 0.56--2.75 {mu}mol{center dot}kg{sup {minus}1} with a pooled standard deviation of 1.55 {mu}mol{center dot}kg{sup {minus}1}. The weighted mean calculated from the results of these twelve laboratories was 1977.8 {mu}mol{center dot}kg{sup {minus}1}; almost identical to the certified value obtained by extraction/manometry (1978.8 {mu}mol{center dot}kg{sup {minus}1}). This indicates that the extraction/coulometric procedure is capable of making unbiased measurements. Unfortunately, the spread of the mean values from these twelve laboratories was substantial (17.2 {mu}mol{center dot}kg{sup {minus}1}) and only five sets of results had a mean that was equivalent to the certified value (with 95% confidence).« less
  • Samples of sterilized sea water were bottled and sent to a total of fifteen laboratories for analysis for total dissolved inorganic carbon. These were located in Canada (2), Germany (1), The Netherlands (1), Sweden (1), the United States (9), and the United Kingdom (1). All except one laboratory used their current implementation of an extraction/coulometric procedure to analyze the samples; the other laboratory, Dr. Keeling`s laboratory at the Scripps Institution of Oceanography, analyzed the samples using their extraction/manometric procedure which is considered a ``definitive`` method. The within-laboratory standard deviation of the results reported by the various laboratories (omitting the twomore » that had substantial difficulties) ranged from 0.56--2.75 {mu}mol{center_dot}kg{sup {minus}1} with a pooled standard deviation of 1.55 {mu}mol{center_dot}kg{sup {minus}1}. The weighted mean calculated from the results of these twelve laboratories was 1977.8 {mu}mol{center_dot}kg{sup {minus}1}; almost identical to the certified value obtained by extraction/manometry (1978.8 {mu}mol{center_dot}kg{sup {minus}1}). This indicates that the extraction/coulometric procedure is capable of making unbiased measurements. Unfortunately, the spread of the mean values from these twelve laboratories was substantial (17.2 {mu}mol{center_dot}kg{sup {minus}1}) and only five sets of results had a mean that was equivalent to the certified value (with 95% confidence).« less
  • The concentration, major fractions, and contribution of dissolved organic carbon (DOC) to stream chemistry were examined in two paired streams draining upland catchments in eastern Maine. Although SO(-2) was the dominant stream anion, followed by Cl(-), organic anions were also determined to be an important component of these stream waters, especially during storm events. This illustrates that even in streams with low DOC, such as these studied here, organic anions can contribute significantly to stream acidity.
  • The photochemical formation of carbon monoxide (CO) in water samples obtained from wetlands, lakes, and near-coastal/shelf areas and in aqueous solutions of soil organic matter was investigated. All of these samples contained dissolved organic matter that was largely derived from terrestrial sources. The studies show that, although the water samples had widely varying optical properties and CO photoproduction rates, the efficiencies for photochemical CO formation were remarkably similar in all waters examined. Model calculations further indicated that photodegradation of terrestrial dissolved organic matter (e.g., in wetland and near-coastal environments) may be an important global source of carbon monoxide and amore » key process in cycling of dissolved organic matter in these environments.« less
  • Dissolved inorganic carbon (DIC) carbon-14 ( 14C) is used to estimate groundwater ages by comparing the DIC 14C content in groundwater in the recharge area to the DIC 14C content in the downgradient sampling point. However, because of chemical reactions and physical processes between groundwater and aquifer rocks, the amount of DIC 14C in groundwater can change and result in 14C loss that is not because of radioactive decay. This loss of DIC 14C results in groundwater ages that are older than the actual groundwater ages. Alternatively, dissolved organic carbon (DOC) 14C in groundwater does not react chemically with aquifermore » rocks, so DOC 14C ages are generally younger than DIC 14C ages. In addition to chemical reactions, 14C ages may also be altered by the physical process of matrix diffusion. The net effect of a continuous loss of 14C to the aquifer matrix by matrix diffusion and then radioactive decay is that groundwater appears to be older than it actually is. Laboratory experiments were conducted to measure matrix diffusion coefficients for DOC 14C in volcanic and carbonate aquifer rocks from southern Nevada. Experiments were conducted using bromide (Br-) as a conservative tracer and 14C-labeled trimesic acid (TMA) as a surrogate for groundwater DOC. Outcrop samples from six volcanic aquifers and five carbonate aquifers in southern Nevada were used. The average DOC 14C matrix diffusion coefficient for volcanic rocks was 2.9 x 10 -7 cm 2/s, whereas the average for carbonate rocks was approximately the same at 1.7 x 10 -7 cm 2/s. The average Br- matrix diffusion coefficient for volcanic rocks was 10.4 x 10 -7 cm 2/s, whereas the average for carbonate rocks was less at 6.5 x 10 -7 cm 2/s. Carbonate rocks exhibited greater variability in DOC 14C and Br- matrix diffusion coefficients than volcanic rocks. These results confirmed, at the laboratory scale, that the diffusion of DOC 14C into southern Nevada volcanic and carbonate aquifers is slower than DIC 14C. Because of the apparent sorption of 14C-labeled TMA in the experiments, matrix diffusion coefficients are likely even lower. The reasons for the higher than expected Br-/ 14C-labeled TMA are unknown. Because the molecular size of TMA is on the low end of the range in molecular size for typical humic substances, the matrix diffusion coefficients for the 14C-labeled TMA likely represent close to the maximum diffusion rates for DOC 14C in the volcanic and carbonate aquifers in southern Nevada.« less