Radiocarbon A new dimension to Organic Geochemistry

Transcription

1 Radiocarbon Cosmic Rays A new dimension to Organic Geochemistry 14N + n 14 C + p 14C + O 2 14 CO 2 CO Gt 610 CO 2 14C 14 N + β τ 1/2 = 5730 yrs CO Gt 1020 Gt Gt Gt E^6 Gt 14 C can be used as an isotopic tracer of the carbon cycle C- Gigatons

2 I. Introduction A. 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 radioisotope: C!!

3 Application of Radiocarbon to Marine Studies Radiocarbon ( 14 C) 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 14 C: seasonal multi-decadal ventilation Longer term changes in circulation (natural variability, pre-bomb 14 C) Carbon Cycle air-sea exchange (ocean uptake) biogeochemical processes (eg.. DIC-POC POC-DOC-SOC)

5 14 C - A quick primer Unstable or Radioactive 14 C 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 Radio-isotope Standard exponential decay equation: dn - = N (1) dt Which gives rise to the familiar exponential-decay curves: for A = A o e λt N No N = 1/2 No dn dt Half life (t½) time required for ½ of original radionuclides to decay. (happens to work out to t½ = 0.693/ ) t 1/2 T Mean life ( ) average time that a radioisotope exists before decay. (happens to work out to = t½/0.693 )

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

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

9 14 C - A quick primer 14 C Half life(s) Libby half-life 5568±30 yrs (Libby, Anderson, & Arnold, Science 1949) Subsequently it has been determined that this halflife 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)

10 Notes on practical use of 14 C: 3 x t 1/2 ~ yrs, 4 x t 1/2 ~ 23, 000 so: 14 C is only useful for modern processes. Its Based on assumption that Carbon-pool in OM is in equilibrium with Atmospheric source.

11 14 C: Complications & Complexities Count rates, representing the rate of 14 C decay, are the basic data obtained in a 14 C 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 14 C data reporting causes headaches to many. (Stuiver & Polach, 1977*) (*Probably a vast understatement)

12 14 C: Complications & Complexities Origin of 14 C in OM is incorporation of 14 CO 2 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? Source function is actually NOT constant. (ie, solar intensity, shielding, etc) Radiocarbon - Calendar Calibration (IntCal-98) Calendar Years Before Present

14 Complication 2: Fossil Fuel Burning Suess effect of 14 C-free CO 2 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. BUT: on upside, a possible anthropogenic tag?

15 Complication 3: The Big One Big Bombs = Mucho 14 C 14 C in CO 2 and Reconstructed in Trees 1000 NH 800 Atmos 14 C( ) Why the curve? SH Date In notation 1000 is 2X the amount of 14 C

16 Thermo-nuclear weapons testing as Giant Tracer Experiment! The C content of 80 Atmosphere atmospheric CO 2 was 60 almost doubled (100% 40 excess) in the early s Excess C-14, % 0 Ocean *1964= Testing Moratorium. Year 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? 13 C, o/oo Fractions of a marine plankton Pectin Protein Hemicellulose Total Carbohydrate If you want to know AGE.. Then Mass dependent isotopic fractionation needs to be accounted for! Total Organic Matter Cellulose 14/12C fractionated ~ twice as much as 13/12C Lignin-like material Lipids => BUT standard delta notation has this built in. Marine Sediment Degens, 1969

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

19 The 14 C Scale: from 0 to ~ actual modern OM (residual bomb) C dead (No radiocarbon) modern 1850 wood

20 But two other scales are also commonly used! 14C (infinite) 14C ~14 Kyr ~6 Kyr ~2 Kyr ~ 0 ypb age FM fraction modern ( %)

21 Measurement of 14 C activities and ages: Carbonaceous materials are combusted to CO 2, then analyzed directly or converted into liquid (benzene) or solid (graphite) form for counting emitted - (requires >100 mg C). 2. Nowadays, measurements are often made by direct counting of individual 14 C atoms in a graphite target using an accelerator mass spectrometer (AMS) (requires <100 g C).

22 HOW DO YOU MEASURE 14C? Because so few 14C atoms (vs. 13 and 12), you can t do it with typical IRMS instruments..

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..but Conventional Counting SIZE: grams of carbon TIME: days to months per spl AMS: SIZE: 10 s of µg-c TIME for Analysis: minutes

24 Simplified Accelerator Mass Spectrometry High Energy Mass Spectrometer C MV C 64 samples Negative Ion Source 13 C 4+ Faraday Cup Tandem Electrostatic Accelerator Low Energy Mass Spectrometer 14 C 4+ Ion Identification Detector Rigidity Filter Velocity (Wien) Filter JS Vogel et al.,

25 PART II: ORGANIC GEOCHEMICAL USES: CLOCK or SOURCE TRACER?

26 ORGANIC GEOCHEMICAL 14 C USES a) HUGE advantage: its in all organic CARBON in principle it can be used to obtain an age on ALL organic matter! 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).

27 14 C uses in organic geochemistry 1: clock for individual molecules, or OM pools 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 well.. maybe )

28 Compound-Specific 14 C USES 2. Gives you a tag for source of identical compounds, that is tied to cycling rates. Consider two identical (very stable..) molecules found in a sediment.. one settled from surface this week, one preserved from the carboniferous.. Prediction: 14 C values MATCH other OM in core 14 C = OC Extract, measure concentrations 14 C = OC? Interpret: Source organism types- (ASSUME: from overlying surface water?) Interpretation WRONG - must be other source weathered material? Other location? Chemosynthesis?

29 3. Bomb Tracer experiments : How fast do components of the carbon cycle turn over? Example: 14 C in ocean DIC pool Gives you direct way to understand rates at which Atm CO2 is cycled into different pools.

30 Examples of specific applications 1: DOC pool and the clock application

31 DOC: two pool: model- major paradigm since 1980 s. um 0 Depth (m) background ~6000 yrs old semi-labile ~modern Surface DOC works out to be Mix between old background, And new surface addition

32 Overall: 1) Subsurface DOM pool: 14 C age = years. (as opposed to estimates of ocean circulation of ~ years.) A pool of exceptionally stable and un-reactive material, that on average has survived 5-10 ocean mixing cycles!!! Problem: this is an average age.(why a problem?)

33 Big QUESTION: Is the year bulk DOC age (and thus our 3 decade old understanding of DOM cycling rates) actually SIMILAR for all main DOM components??

34 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

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

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

37 Indigenous deep biosphere exist in crust! & AGED C source to upper ocean!

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

39 Tracking deep circulation: Biological Pump DOC/POC sfc <> deep Deepwater sources Atlantic N. Atlantic (NADW) Antarctic (AABW) Air-Sea (14) CO 2 exchange Pacific Indian Image courtesy of C. Charles

40 End