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Lecture # 5: Radiocarbon

Lecture # 5: Radiocarbon. A new dimension to Organic Geochemistry. D 14 C can be used as an isotopic tracer of the carbon cycle. C- Gigatons. I. Introduction. Why learn about radiochemical tracers?

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Lecture # 5: Radiocarbon

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  1. Lecture # 5: Radiocarbon A new dimension to Organic Geochemistry D14C can be used as an isotopic tracer of the carbon cycle C- Gigatons

  2. I. Introduction • Why learn about radiochemical tracers? • Radioisotopes provide dependable clocks, by which the dynamics of different natural processes can be determined, along with the ages of materials. • Many isotopes are used in earth/ocean sciences (eg 234U, Th, Ra) but only ONE organic element has useful radio-isotope: • C!!

  3. Application of Radiocarbon to Marine Studies Radiocarbon (14C) has two sources (“source functions” Natural (Upper Atmosphere) Anthropogenic aka “Bomb-14C”

  4. Application of Radiocarbon to Marine Studies Tracer and Chronometer (seasonal to millennial time-scales) Inorganic: Ocean Circulation & Time-scale of Ventilation Water mass tracer Bomb 14C: seasonal – multi-decadal ventilation Longer term changes in circulation (natural variability, “pre-bomb” 14C) Carbon Cycle air-sea exchange (ocean uptake) biogeochemical processes (eg. DIC-POC-DOC-SOC)

  5. 14C - A quick primer • Unstable or “Radioactive” 14C • Discovered by W.F Libby in early1940s (Nobel Prize in 1960) • What does radioactive decay mean? • a finite probability of spontaneous decay such that given n-atoms over fixed period of time 1 will decay. • represented by either the half-life or the mean life of the isotope in question.

  6. Formation of 14C - • Formed primarily in the stratosphere from N (1° slow or thermal neutron capture by 14N) • protons & alpha particles interact with other atoms to produce (2°) neutrons.

  7. Formation of 14C - • Production of 14C is influenced by: • Bkg galactic flux of cosmic rays • Geomagnetic shielding • Solar wind (sunspot activity levels) • Rapidly oxidized to 14CO2 - so atm CO2 is really source pool False color image of Earth’s magnetic field

  8. 14C - A quick primer • 14C Half life(S) • “Libby half-life” 5568±30 yrs (Libby, Anderson, & Arnold, Science 1949) • Subsequently it has been determined that this half-life is not quite correct. • “Cambridge” half-life 5730±40 yrs • More correct half-life is about 3% larger than the Libby age (ie., with conventional 14C age 3% older)

  9. Radio-isotope Standard exponential decay equation: Which gives rise to the familiar exponential-decay curves: for A = Ao e – λt Half life (t½)  time required for ½ of original radionuclides to decay. (happens to work out to  t½ = 0.693/ ) Mean life ()  average time that a radioisotope exists before decay. (happens to work out to  = t½/0.693 )

  10. Notes on practical use of 14C: • 3 x t1/2 ~ 17000 yrs, 4 x t1/2 ~ 23, 000 • so: 14C is only useful for modern processes. • Its Based on assumption that Carbon-pool in OM is in equilibrium with Atmospheric source.

  11. 14C: Complications & Complexities “Count rates, representing the rate of 14C decay, are the basic data obtained in a 14C laboratory. The conversion of this information into an age or geochemical parameters appears a simple matter at first.” “ However, the path between counting and suitable 14C data reporting causes headaches to many.” (Stuiver & Polach, 1977) (Probably a vast understatement)

  12. 14C: Complications & Complexities Origin of 14C in OM is incorporation of 14CO2 into organic matter via photosynthesis. In order to get an accurate “date” back in time, you thus have to assume you know the “source” amount at time = zero. • However, both Natural changes and human activities have changed this source function! Why…?

  13. Complication 1: What is wrong with this picture? Why? Radiocarbon -Calendar “Calibration” (IntCal-98) Source function is actually NOT constant. (ie, solar intensity, shielding, etc)

  14. Complication 2: Fossil Fuel Burning • “Suess” effect of 14C-free CO2 due to fossil fuel combustion since the late 1800s Fossil fuel –generated CO2 has a 14C content of… Zip. (its “dead”) • Thus, as we pump more and more “dead” CO2 into the atm, are slightly decreasing the source function. • Thus: a possible anthropogenic “tag”?

  15. Complication 3: “The Big One” Big Bombs = Mucho 14C D14C in CO2 and Reconstructed in Trees NH Why the curve? Atmos D14C(‰) SH In “D” notation 1000‰ is 2X the amount of 14C

  16. Thermo-nuclear weapons testing = Giant Tracer Experiment The 14C content of atmospheric CO2 was almost doubled (100% excess) in the early 1960’s *1964= Testing Moratorium. The huge “pulse” has now decayed away significantly, but in the meantime it has worked its way down into the ocean, into surface sediments, into all organic matter made since 1950’s. (if your house is built today, the wood will be “hot”- it was built in 1950, the wood will be “pre-bomb” = much “colder”

  17. Finally: what about mass- dependent fractionation? • Mass dependent isotopic fractionation needs to be accounted for! • 14/12C fractionated ~ twice as much as 13/12C => Standard delta notation has this built in.

  18.  Finally: The  (delta) Notation for 14C: To account for all of above. 14C( ‰) = 14C – [(213C + 50)(1 + 14C/1000)] * Note: 14C, does not mean the same thing as  13C! Why the bizzare expression? Because of the preceding complications, this expression is “rigged” to give a 14C = zero for a wood (with 13C = -25.0 ‰) formed in 1850 that is not diluted by “dead” CO2 from fossil fuel combustion, nor bomb 14C.

  19. ~ actual modern OM (residual bomb) +100 14C “dead” (No radiocarbon) “modern” 1850 wood The 14C Scale: from 0 to -1000 -1000 -500 0

  20. -1000 -500 0 +100 (infinite) ~6 Kyr ~ 0 ypb ~14 Kyr ~2 Kyr 0.25 0 0.50 0.75 1.0 fraction modern ( %) But two other scales are also commonly used: -750 -250  14C 14C “age” FM

  21. Measurement of 14C activities and ages: Carbonaceous materials are combusted to CO2, then analyzed directly or converted into liquid (benzene) or solid (graphite) form for counting emitted - (requires >100 mg C). • Nowadays, measurements are often made by direct countingof individual 14C atoms in a graphite “target” using an accelerator mass spectrometer (AMS) (requires <100 g C).

  22. Simplified Accelerator Mass Spectrometry JS Vogel et al.,

  23. Accelerator Mass Spectrometry (Big and not so big toys) What does AMS buy you over conventional counting? In general, accuracy and precision are similar among the good counting & AMS labs. AMS: SIZE: 10’s of µg-C TIME for Analysis: minutes Conv. Counting SIZE: grams of carbon TIME: days to months per spl

  24. PART II: ORGANIC GEOCHEMICAL USES:SOURCE TRACERor“CLOCK”?

  25. PART II: ORGANIC GEOCHEMICAL USES • HUGE advantage: its in all organic CARBON  in principle it can be used to obtain an “age” on ALL organic matter! • 1. Gives you a direct measure of cycling rates. (as opposed to looking at a bunch of molecular parameters, and saying.. Gee.. That looks really degraded.. Therefore I guess it must be old.. Errr. Maybe…) However: due to ~ short half-life, it can only be used to date relatively “modern” material. ( ie < ~ 20k yrs). (however, on the upside, that means its very sensitive to modern time-scale processes).

  26. 14C USES • 2. Gives you a “tag” for source of identical compounds, that is tied to cycling rates. (EG: two identical molecules, one from this week, one preserved from the carboniferous..)

  27. Examples of 14C USES 1: “clock” for individual molecules, or OM pools.

  28. DOC component ages = cycling rates CARBOS (green) =major component YOUNGEST Amino Acids/proteins (yellow) = Medium, and similar to average “Lipid-like” (smallest component) = BY FAR Oldest! Carbos Black = total material

  29. Examples of 14C uses:2: Source Tracers – IF hypothesized source in an environment has a very diff. 14C signature

  30. Indigenous deep biosphere exist in crust? & C source to upper ocean? “old” POC/DOC export ? DIC = 13k yrs

  31. Examples of 14C uses:3: Bomb Tracer “experiments” : How fast do components of the carbon cycle turn over?

  32. Tracking deep circulation: • Biological Pump • DOC/POC sfc <> deep • Deepwater sources • N. Atlantic(NADW) • Antarctic (AABW) • Air-Sea (14)CO2 exchange Image courtesy of C. Charles

  33. End

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