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Seawater Microbial Loop and DOC Cycling: A Radiocarbon Study

This study explores the dynamics of dissolved organic carbon (DOC), nitrogen, and phosphorus in seawater, impacting the microbial loop and carbon sequestration. It investigates the reactivity, cycling, and radiocarbon dating of DOC, highlighting deep-sea gradients and the global carbon reservoirs. Through a mixing model, it determines the flux needed to maintain the aged marine DOC reservoir and compares it with other carbon fluxes. This research sheds light on the origins of DOC from terrestrial plants, marine and river sources, soil, and carbonates, emphasizing the ocean as a crucial carbon repository. Radiocarbon measurements and models provide insights into the age and cycling of DOC in surface and deep waters of the Atlantic and Pacific Oceans, with implications for understanding global carbon dynamics and the fate of organic matter in marine ecosystems.

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Seawater Microbial Loop and DOC Cycling: A Radiocarbon Study

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  1. Dissolved Organic Carbon, Nitrogen, and Phosphorous in seawater Fuels the microbial loop Sequesters a large amount of carbon, as well as nitrogen and phosphorus Complexes trace metals and affects their concentration and bioavailability Absorbs UV and PA radiation

  2. Semi-reactive DOC NonreactiveDOC Very reactive DOC Reactivity and the cycling of DOC in seawater Heterotrophic microbial production ?? 0o and 140 oW Carlson and Ducklow, DSR II v 42; 639-656

  3. Reactivity and the cycling of DOC in seawater High production in the surface (high microbial activity & production) It’s all Semi-reactive DOC Low production in the deep (low microbial activity & production) Carlson and Ducklow, DSR II v 42; 639-656

  4. Deep sea gradients in [DOC] D. Hansell and C. Carlson, Nature 1998

  5. Deep sea gradients in [DOC] D. Hansell and C. Carlson, Nature 1998 Unpublished data removed AAIW NADW AABW

  6. History of radiocarbon in the Atmosphere and ocean Frigate shoals Figi 14C per mil Galapagos Prebomb value of -80 per mil

  7. DOC cycling via DO14C Williams, Oeschger, and Kinney; Nature v224 (1969) UV photooxidation 1000L HCO3- + H+ H2O + CO2

  8. Depth 14C(‰) Age 1880m -351 ‰ -3470+330 ybp 1920m -341 ‰ -3350+300 ybp DOC cycling via DO14C Williams, Oeschger, and Kinney; Nature v224 (1969) UV photooxidation 1000L

  9. How do we measure radiocarbon today ? LN2 trap For CO2 Reduce to graphite press into a target 100-500 mls Mass spectrometer Analysis (>25 µg C needed)

  10. Accelerator Mass Spectrometry (AMS) NOSAMS at Woods Hole C-14 PP measurements use 106x more C-14 than natural abundance!!!!!

  11. Radiocarbon in the Atlantic and Pacific Oceans Peter M. Williams and Ellen Druffel; Nature 1987, JGR 1992

  12. Radiocarbon in the Atlantic and Pacific Oceans DIC 14C in surface waters of the Atlantic and Pacific has the same isotopic value.

  13. Radiocarbon in the Atlantic and Pacific Oceans DIC 14C in surface waters of the Atlantic and Pacific has the same isotopic value. The deep Pacific DIC is older than the deep Atlantic DIC

  14. Radiocarbon in the Atlantic and Pacific Oceans DIC 14C in surface waters of the Atlantic and Pacific has the same isotopic value. DOC is always older than DIC (by 4 kyrs in surface water) DIC-> POC -> DOC

  15. Radiocarbon in the Atlantic and Pacific Oceans DIC 14C in surface waters of the Atlantic and Pacific has the same isotopic value. DOC is always older than DIC by 4 kyrs in surface water 14C of DIC and DOC is about the same in the deep Atlantic and Pacific

  16. Radiocarbon in the Atlantic and Pacific Oceans DIC 14C in surface waters of the Atlantic and Pacific has the same isotopic value. DOC is older than DIC by 4 kyrs in surface water 14C of DIC and DOC is about the same in the deep Atlantic and Pacific Deep ocean values of DOC are equal to a radiocarbon age of 4000-5000 yrs Either there is a source of “old” DOC, or DOC persists for several ocean mixing cycles

  17. Why is DOC so old (2000 ybp) in surface water? DIC DOC

  18. Why is DOC so old (2000 ybp) in surface water? “DOC Background” Pete Williams and Ellen Druffel

  19. Why is DOC old in surface water? reactive DOC 14C=DIC “nonreactive” DOC 14C = DOC(deep) 14C= 14C“reactive” DOC + “nonreactive” DOC

  20. Using a simple 2 component mixing model of old “non-reactive” DOC with deep sea 14C and [DOC] values, and a new “reactive” component with DIC14C, and [DOC] = [DOC]total-[DOC]deep, [DOC]total14C total = [DOC]deep14C deep + [DOC]new14C new DOC 14C DOC 14C Depth

  21. Using a simple 2 component mixing model of old “non-reactive” DOC with deep sea 14C and [DOC] values, and a new “reactive” component with DIC14C, and [DOC] = [DOC]total-[DOC]deep, [DOC]total14C total = [DOC]deep14C deep + [DOC]new14C new DOC 14C DOC 14C Modeled and measured DOC 14C values agree pretty well… Depth

  22. 14C= “reactive” DOC + “nonreactive” DOC Atlantic surface water 14Ccalc = -120 ‰ 14Cobs = -127 ‰ Pacific surface water 14Ccalc = -147 ‰ 14Cobs = -148 ‰ reactive DOC 14C=DIC nonreactive DOC 14C = DOC(deep) Nonreactive DOC = 650 GT C Reactive DOC = 30-50 GT C

  23. What flux of carbon is needed to maintain the “old” marine DOC reservoir? Global inventory/residence time = annual flux 680 GT C/ 5000-6000 yr = 0.11-0.14 GT C/yr !!! How does this compare with other C fluxes?

  24. Where does doc come from? DOC and major carbon reservoirs and fluxes Terrestrial Plants 900 GT C Atmosphere 750 GT (CO2) Terrestrial Primary Production 75 GT C/yr Marine Primary Production 60-75 GT C/yr River flux 0.5 GT C/yr Soil 2000 GT C POC 15 GT C Burial 0.1-0.2 GT C/yr DOC 700 GT C Carbonates 60,000,000 GT C Kerogen 20,000,000 GT C 150 GT C

  25. The ocean, a global carbon wastebasket? A small fraction of the DOC added to the ocean by rivers is colored (colored dissolved organic matter or CDOM) that can be tracked by remote sensing. It is believed that river DOM is remnant of OM cycling on land, and represents the material that cannot be degraded. Is the ocean filling up with terrestrial DOM?

  26. DOC transport through estuaries and the input of terrestrial organic carbon to the ocean. DOC concentrations are nearly always higher in rivers than in the ocean. Rivers add C to the ocean. In general, DOC displays conservative behavior wrt salinity in estuaries. Some estuaries add carbon, some remove it. Salinity (ppt)

  27. Utilization of DOM by bacteria at different salinities in the York River Raymond and Bauer AME v 22 (2000)

  28. What is the source of oceanic DOC ? Druffel et al., JGR 1992 Stable C isotopes Marine C -21‰ Terrestrial C C3 plants -27‰ C4 plants -15‰

  29. Production of reactive and non reactive DOC by phytoplankton and bacteria Microbial diversity is due to Changes in very reactive DOC CO2 Reactive DOC O2 Non reactive DOC ? Very reactive DOC Bacterial production is thought to be fuelled by very reactive DOC (simple, LMW organic compounds) that have half lives in seawater of hours to days.

  30. ….then why isn’t the ocean filled with river DOC, and where does the non reactive marine DOM come from?

  31. 15 m 900 m 0 20 40 60 80 100 120 DOC (µM) Data from Bob Chen and Jeff Bada

  32. DOC + light LMW carbonyls (C=O) C=O + fluorophore HPLC Ken Mopper and his group fwere able to show rapid photoxidation Of DOM in the presence of sunlight Filtered SW Whole SW Filtered SW Whole SW Not produced in dark controls,but are produced in sterile controls

  33. Production of LMW highly oxidized DOC with depth in the ocean DOC + light LMW photo-oxidation products Is photochemical degradation the long term sink for river (terrestrial) And therefore nonreactive DOM in the ocean ???

  34. H2C=O HOOCCOOH

  35. Highly oxidized LMW compounds are produced every day in seawater by photo-oxidation. They serve as a substrate for bacteria and therefore a sink for non-reactive DOC CH3C=O CH3(C=O)CH3 CH3CH2CH2OH

  36. …..then why is the ocean filled with (40 µM) marine DOC, and where does it go? Unpublished data removed

  37. Dissolved organic nitrogen and phosphorous Organic C-N-P are connected at the molecular level; what is true for C is (assumed to be) true for N and P. We cannot determine the age of DON directly. DOP residence times in surface water can be measured using cosmogenic 33P/32P. DON and DOP concentrations are higher in surface waters (where production > degradation), than in the deep sea, so we infer from DOC/DON/DOP ratios that there are “nonreactive” and “reactive” fractions of DON and DOP. DON and DOP are even more difficult to measure than DOC, there is no way to remove inorganic N and P from seawater prior to DON/DOP analyses, so all measurements of organic N/P are difference measurements.

  38. What drives microbial diversity in the ocean ? Karner et al. Nature 2001

  39. What is the relationship between organic matter composition, microbial diversity & production? How is microbial metabolism coupled to organic matter structure? How does DOM affect microbial adaptation & evolution? What sets the ocean inventory of organic carbon, and why is so much organic carbon preserved in seawater? Why do organic nitrogen and phosphorus accumulate in nutrient limited regions of the surface ocean where they are most needed, but disappear in the mesopelagic ocean where they are not needed? Karner et al. Nature 2001

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