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  • Geobiology 2013 Lecture 5

    Biogeochemical Tracers

    Isotopics #2: C, H, O and N

    Acknowledgements: John Hayes, Karen Casciotti, Steve

    Macko, John Hedges

    Assigned Reading • Stanley 2nd Ed Chapter 10, pp 221-240

    • Hayes JM 2001 Fractionation of the isotopes of carbon and hydrogen in biosynthetic processes. Reviews in Mineralogy Stable Isotopic Geochemistry, John W. Valley and David R. Cole (eds.)

    • Hayes Concepts and Calculations

    1

  • Geobiology 2012 Lecture 5

    Biogeochemical Tracers

    Isotopics #2: C, H, O and N

    Need to Know:

    Ballpark delta values of C, O, N, H in marine and terrestrial

    biomass

    How H&O are fractionated in the hydrological cycle

    How (roughly) C&H fractionation occurs in organic matter

    How (roughly) N is fractionated on land and in the ocean

    2

  • Sources of hydrogen and oxygen isotopic

    fractionation

    3

  • % 0

    Fraction of remaining vapor

    1.0 0.75 0.50 0.25 0

    0

    -5

    -10

    -15

    -20

    -25

    Condensate

    Vapor

    9

    11

    % 0

    % 0

    20 15 10 0 -20 -30

    Cloud temperature (oC)

    Figure by MIT OpenCourseWare.

    The major nonbiological fractionation process affecting stable hydrogen (and oxygen) isotopes is the hydrologic cycle in which water molecules containing lighter isotopes

    Image removed due to copyright restrictions. (1H & 16O) are preferentially evaporated and retained in a Graph of deuterium per mil over oxygen-18 per mil. cloud (vs. 2H & 18O).

    The net result of this fractionation process is that precipitation at increasingly inland, higher altitude (cooler) sites is depleted in both 18O and 2H (D).

    4

    -13%0 Vapor

    -15%0 Vapor

    -17%0 Vapor

    -3%0 Rain

    -5%0 Rain

    Continent Ocean

    0

    Figure by MIT OpenCourseWare.

  • Courtesy of John Hayes. Used with permission.

    In general, tree sap isotopically resembles local meteoric water, whereas leaf water is isotopically enriched. Organic matter is depleted in D versus the leaf water from which it is biosynthesized (values given above are typical for N. American plants).

    In general, lipids are depleted in D versus lignin and cellulose from the same plant, whereas cellulose is slightly more depleted than coexisting lignin (Rundle et al., 1989).

    Cellulose is usually analyzed in nitrated form (NO3 replacing OH on each C), so that only the nonexchangeable H directly bound to C is analyzed.

    Because the D in cellulose nitrate reflects local water, woods can be used as a proxy for the hydrogen isotope composition of past environments in which the wood was made

    5

  • Hydrogen isotopic signatures

    Fd-NADP reductase (2.4)Photosynthesis H2O + hv � O2 + NADPH

    2 reduced ferredoxin + NADP+ + H+ 2 oxidized ferredoxin + NADPH

    Insertion of a non-exchangeable H- from NADPH into pyruvate.

    Reactions like this responsible for setting δD of organics

    H going from NADPH to pyruvate becomes H in lipid

    6

  • WATER

    0

    Cellulose

    Reproc.

    Production & Exch.

    of NADPH

    -100 Starch

    Exported Photosynthate

    Reproc. & Exch.

    D = ?

    Photosynthate

    Cytosol -200 Chloroplast ?

    Relationships between the hydrogen isotopic compositions of initial photosynthate and related products and of water available within plant cells. The uncertainty indicated on the pathway between water and photosynthate reflects the possibility of offsetting fractionations in the production of NADPH and the subsequent transfer of H - to photosynthate.

    Hydrogen isotopic

    signatures inherited

    from water through

    reactions of

    NADPH

    Figure by MIT OpenCourseWare.

    7

  • -150 -100 -50 0

    -340

    -320

    -300

    -280

    -260

    -240

    -220

    -200

    -180

    =197

    HO

    Figure by MIT OpenCourseWare.

    8

  • Sources of nitrogen isotopic fractionation

    9

  • Simplified Terrestrial Nitrogen Cycle

    http://www.windows.ucar.edu/earth/climate/images/nitrogencycle.jpg

    Bottom line: In natural systems, new fixed nitrogen entering the system comes from bacterial nitrogen fixation which results in only small N-isotopic fractionations

    10

    Schematic figure of the nitrogen cycle removed due to copyright restrictions.

    http://www.windows.ucar.edu/earth/climate/images/nitrogencycle.jpg

  • Some Typical 15N values

    Atm. N2 ‘fixed’’ by terrestrial flora (Nif in legumes) ~ 0 per mil

    Atm. N2 ‘fixed’ by marine bacteria and Cb ~ 0 per mil

    Nitrate or ammonium utilized by terrestrial flora and converted to organic nitrogen ~0 per mil

    Dissolved nitrate in the ocean ~ +7 to +10 per mil

    Marine organisms high in the trophic structure (e.g. fish, whales)

    +10 to + 20 per mil

    11

  • Simplified Marine Nitrogen Cycle

    © Karen Casciotti OCW 12

    N2 fixation CO2 Atmosphericdeposition

    Dead organic matter Export production

    New Production

    Regenerated production

    NO3 -

    NH4 +

    NH4 + CO2 + CO2 +

    CO2 + DON, DOC

    Euphotic zone

    Aphotic zone

    Bacteria

    Phytoplankton Zooplankton

    Image by MIT OpenCourseWare.

  • 13

  • Major isotopic fractionation in the N-cycle occurs during denitrification (-20 to -40 per mil)

    Light N2 leaves the ocean causing the residual nitrate to become heavy

    14

    U. Weismann, “Biological nitrogen removal from wastewater”, Adv. Biochem.Engr. 51:113-154 (1994)

    Nitrification: NH4 + 1.5 O2 NO2 + H2O + 2H

    + + -

    -

    - -

    - lowers pH

    escapes

    NO2 + 0.5 O2 NO3

    5 HOAc + 8 NO3 4 N2 + 10 CO2 + 6 H2O + 8 OH

    Denitrification:

    MUST have carbon source, e.g., acetate raises pH

    Image by MIT OpenCourseWare.

  • 15N vs 13C Foodweb

    Trophic fractionation of carbon and nitrogen isotopes (Georges Bank)

    B. Fry (1988) Limnol. Oceanogr. 33, 1182-1190.

    15

    15

    16

    17

    18

    19

    20

    10

    11

    12

    13

    14

    5

    0 1

    2

    3

    4

    6

    7

    8

    9

    -25 -20-21-22-23-24 -15-16-17-18-19

    Phytoplankton

    δ 13C (13C )

    δ 15

    N (1

    5 N )

    POC

    Invertebrates

    Fish

    Image by MIT OpenCourseWare.

  • Trophic fractionation of carbon and nitrogen isotopes

    K. Yoshii, N. G. Melnick, O. A. Timoshkin, N. A. Bondarenko, P. N. Anoshko, T. Yoshioka, & E. Wada (1999) Limnol. Oceanogr. 44, 502-511.

    16

    δ1 5 N

    (

    )

    δ13C (% )

    1

    2

    3

    4

    -30.0 -29.0 -28.0 -27.0 -26.0 -24.0 -23.0-25.0

    10.0

    11.0

    12.0

    13.0

    14.0

    5.0

    6.0

    7.0

    8.0

    9.0

    4.0

    3.0

    15.0

    Pelagic sculpins

    Mesozooplankton Cyclops kolensis

    Comephorus baicalensis

    Cottocomephorus inermis

    Comephorus dybowskii

    Cottocomephorus grewingki

    Juvenile of pelagic sculpins

    Macrozooplankton Amphipod

    Macrohectopus branickii

    Mesozooplankton Epischura baicalensis

    Phytoplankton Aulacoseira baicalensis

    Trophic Level

    %

    Pelagic fish Coregonus autumnalis

    migratorius

    Seal phoca sibirica

    Image by MIT OpenCourseWare.

  • Biogeochemistry of the Stable Nitrogen Isotopes

    An important use of stable nitrogen isotopes is as an indicator of trophic level in natural systems with known nitrogen sources and relatively simple food webs. The following figure is from Schoeninger and DeNiro (1984) GCA 48, 625-639. Average 815N trophic offset is ~3 ‰ per trophic level

    Courtesy Elsevier, Inc., http://www.sciencedirect.com. Used with permission.

    17

    http:http://www.sciencedirect.com

  • Characterizing Organic Matter Using Bulk

    Isotopic Data

    18

  • -20 -19 -18 -17 -16 7

    8

    9

    10

    11

    α

    Figure by MIT OpenCourseWare.

    19

  • Organismal Variability in Bulk C-Isotopes

    20

    -20 -25 -30

    13C,‰ -15

    Fractions of a marine plankton

    Pectin

    Total carbohydrate

    Total organic matter

    Cellulose

    Lignin-like material

    Lipids

    Marine sediment

    Degens, 1969

    Hemicellulose

    Protein

    Image by MIT OpenCourseWare.

  • -7 -6 -5 -4 -3 -2 -1 0 1 -40

    -35

    -30

    -25

    -20

    -15

    Synthetic

    Biotechnological

    Mexico Comores

    Tahiti H

    H

    H

    H

    O

    OH OMe

    ?