Seqestration of dissolved organic carbon in the deep sea

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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 ... continued below

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1045-1053

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Repeta, Daniel J. March 1, 2006.

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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.

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  • Report No.: DOE/ER/62999-1
  • Grant Number: FG02-00ER62999
  • DOI: 10.2172/908226 | External Link
  • Office of Scientific & Technical Information Report Number: 908226
  • Archival Resource Key: ark:/67531/metadc889547

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  • March 1, 2006

Added to The UNT Digital Library

  • Sept. 22, 2016, 2:13 a.m.

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  • Nov. 7, 2016, 2:33 p.m.

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Repeta, Daniel J. Seqestration of dissolved organic carbon in the deep sea, report, March 1, 2006; United States. (digital.library.unt.edu/ark:/67531/metadc889547/: accessed October 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.