Lecture # 4a- Stable Isotopes Part I

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1 Lecture # 4a- Stable Isotopes Part I 1) Stable Isotopes in Geochemistry: Background, etc 2) Intro to Carbon & N Isotopes

2 I. Introduction A. What are stable isotopes? Recall: Stable isotopes vary in mass, but chemistry is unchanged (ie outer shell electron configuration) Eg: Stable isotopes of Oxygen: Isotope # Protons #Neutrons Atomic Wt Atom% 16 O O O

3 Distribution and Occurrence 1. All the major bioactive elements (except P) have multiple stable isotopes. 2. Within this group, the light isotope (L) is consistently more abundant than the heavy (H) counterpart(s). 3. It is very small (ppt) differences in (H/L) that constitute the basis of using stable isotope signatures as geochemical source and process indicators Main Organic Stable Isotopes Isotope Atom% 1 H H (D) C C N N O O O S S S S 0.014

4 Why do useful differences occur in Nature? Reactions with kinetic component, always end up enriching in LIGHT isotope. Note: Natural processes: often may have both a passive (diffusion) and active (biochemical reaction) component. BOTH parts will fractionate for light isotope. In fact, most natural fractionations = chain of processes

5 A bit more background: Isotopes of different mass have slightly different bond strengths. This affects their: 1) speed - Lighter Isotopes tend to move faster rates of diffusive transport are different! (eg: 12 CO2(g) ~1% faster than 13 CO2(g)) Expression in enzymatic processes: Hays 2004, Fig. 2: enzymatic decarboxylation-

6 A bit more background: 2) Lesser effect: equilibrium distributions in a reversible reaction, where heavier isotope is typically concentrated in more strongly bonded form: 13 CO2(g) + H 12 CO3-(aq) = 12 CO2(g) + H 13 CO3-(aq) favored (1)

7 Main Organic Stable Isotopes RESULT: Virtually ANY and EVERY chemical process (both biological and abiotic) fractionates stable isotopes to some degree Eg: Photosynthesis, Respiration, Biochemical reaction steps, Evaporation, Gas dissolution, Precipitation, etc etc etc. OFTEN these fractionations are very predictable-

8 Thus Stable Isotopes represent an imbedded tag in every molecule for: 1) Source 2) Process history

9 Stable Isotope signatures of organic matter are among most powerful off all Organic geochemical tools! A few examples of uses/ processes: C 12/13: ultimate origin of fixed carbon N 15/14: Trophic level processing O & H - Hydrologic cycles: eg: PaleoRainfall S: ancient sulfer cycles. * C and N are most common (and easy) isotopes measured mainstays. Main Organic Stable Isotopes Isotope Atom% 1 H H (D) C C N N O O O S S S S 0.014

10 Some Terms and definitions: fractionation = The amount of change in light vs heavy (Measured in del notation) created by a given process. isotope effect (typically means kinetic isotope effect ) = fractionation occurs in the process of interest. 12 C 12 C C C An Isotope Effect (a phenomenon) Fractionation (observable quantity)

11 How isotope ratios are expressed: 1) The Del (δ) notation: measure of relative ratio of heavy vs light isotopes a) expressed as ( per mil, or part per thousand) relative to a (semi- arbitrary) standard. (why relative to a standard?) b) Written : δ 13 C of sedimentary bulk OC was 27.5 or δ 15 N of the bulk plankton tow from 100m was + 8 Formal Definition: H = (H/L)spl - (H/L)std x1000 (H/L)std where H= heavy isotope and L= light isotope

12 Reference Standards: Selected: To reflect the major source reservoir that processes are fractionating. EG: Ocean water, Atmospheric N, Carbon in deep space Primary Standards Isotope Ratios Ratios x 10-6 Standard mean ocean water 2 H/ 1 H O/ 16 O O/ 16 O 373 PeeDee belemnite (PDB) 13 C/ 12 C Air 15 N/ 14 N Canyon Diablo meteorite 32 S/ 34 S 22.22

13 How to make stable isotope measurements? Basic Problem: Differences are VERY SMALL! Recall: the per mil is not a direct measure, but is a relative measure- IE actual carbon Std Ratio 12 C/ 13 C = already a BIG excess Boils down to measuring absolute differences in a small populations of atoms of the Heavy Isotope. (Analogy: being able to measure relative differences in distance of a cm or two, accurately and reproducibly of the scale of here to Paris)

14 How its Done: Part I 1) As with CHN measures: Oxidize OM fully to CO2, NOx, etc. now, only have one kind of molecule to worry about detecting. 2) Now, need to measure RATIO of 13 CO 2 vs 12 CO 2, or the ratio of masses 44 vs 45- at exquisite sensitivity! Special kind of Mass Spec: Isotope Ratio Mass Spectrometer or IRMS

15 sample Roast (as w/ CHN) 1050 deg EA- IRMS setup: CO 2 CO 2 CO 2 CO 2 CO 2 CO 2 Molecular Leak Ion Source Ratio Mass Spectrometer 60 o Detectors Signal Processor Ionizing Electron Beam Magnet Analyzer Tube Precision attained: typically to better than 0.1!!!!

16 Intro to Stable C isotopes Major Use: as Original Source indicators Why: 1) Main δ 13 C signature is a measure of carbon fixation pathway 2) Further food-chain & Diagenetic transformations don t alter that signature all that much. For Carbon Isotopes: you are what you eat! (+/- ~ 1 )

17 Rau and Hedges, Science 1979:

18 Nitrogen Stable isotopes: a second dimension Major Use: Trophic Level indicators. Why: Unlike C Average δ 15 N trophic offset is ~3 per trophic level!

19 Basic use for N isotopes: derives from fact that light isotope is preferentially enriched in N excreted by ANIMALS (ammonia). Checkey, DSR you are what you eat + 3

20 Example of N isotopes and trophic levels Schoeninger and DeNiro (1984) GCA 48,

21 END PART I

22 below-moved to part II

23 A Basic Marine Example: Marine bicarbonate (in principle giant reservoir) Close to 0 Bulk Marine Organic Carbon Close to -21 (exact value depends..)

24 N B: In order to get this fractionation, you MUST have only a partial reaction! Why? What would a time vs. fractionation plot look like?

25 Thus Can get some unexpected effects: Eg: under conditions of very high production..observed fractionation falls! Marine biocarbonate (in principle giant reservior) Close to 0 Bulk Marine Organic Carbon << -20 (exact value depends..) Why? Consider: in extreme (theoretical) case, where 100% of biocarb is used fractionation must be 0! Other extreme: where starting material is infinite, fractionation free to approach maximum set by reaction series.

26 Q: What would a time vs. fractionation plot of a plankton bloom box model look like? (think box model.. Reactant/ product..)

27 Rau, 1998 DSRII N example

28 Aside: How can CO2 ever be limiting? Aside II: this sort of thing is not so central with C isotopes, where starting material is usually in excess.. but for example with N isotopes where starting material may often be totally used up- it becomes a key consideration.

Basic Background Review: Acid Base, Redox, and Stable Isotopes

Basic Background Review: Acid Base, Redox, and Stable Isotopes 1) Basic Physical Chemistry Thermodynamics Potential for reaction to proceed Expected state at equilibrium Kinetics Reaction rates Reaction