Stable Isotopes & Biogeochemical Cycles NRES765, Fall 2011 Dr. Mae Gustin

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1 Stable Isotopes & Biogeochemical Cycles NRES765, Fall 2011 Dr. Mae Gustin isotope: from iso (same) and topos (place) specific combination of protons and neutrons in an atomic nucleus e.g. carbon, # protons = 6 # neutrons = 6 12 C stable # neutrons = 7 13 C stable # neutrons = 8 14 C radioactive

2 So What? the same atomic # means that isotopes have basically the same properties the different atomic weights of isotopes means that they have just slightly different properties (e.g. physical characteristics, rates of chemical reactions) e.g. 1 H 2 16 O has a boiling point of C 2 H 2 16 O has a boiling point of C 1 H 2 18 O has a boiling point of C the dependence of properties upon mass is known as mass dependent fractionation (MDF), and is responsible for virtually all isotope fractionation effects on Earth (fractionation = change in isotope composition during a reaction)

3 Implications the slightly different properties of the isotopes results in: (1) a temperature dependence for many reactions (2) isotope fractionation (a change in isotopic composition) between products and reactants, for an incomplete reaction (3) differentiation of isotopic composition between elemental reservoirs with different properties (physical, chemical, biological)

4 Applications 1. Temperature (e.g. paleoclimate, ore deposits, petrology) 2. Processes (e.g. vaporization, oxidation/reduction, photosynthesis) 3. Sources (e.g. groundwater contaminants, avian migration, forensic geochemistry) 4. Tracers (using artificially-enriched compounds, e.g. 90% 13 C vs. 1.1% 13 C-natural)

5 M & Ms

6 What Elements? C, H, O, N, S (Cl) - The Classics water, organic compounds, atmosphere, some inorganic compounds, virtually all minerals analyses typically performed by gas source, electron-impact, magnetic sector isotope ratio mass spectrometry (lab available at UNR) new spectroscopy technique can perform analyses for some elements, for some sample types (not available at UNR)

7 (2004) What About the Rest of the Periodic Table? recent instrumentation development allows for stable isotope analysis of many other elements e.g. Se, Cd, Cr, Cu, Pb, Ni, Zn, Fe, Mo, Hg VERY active area of research analyses performed by multicollector - inductively coupled plasma - mass spectrometry (MC-ICP-MS) no suitable lab at UNR

8 Biogeochemical Cycles virtually all of the elements and compounds covered in NRES 765 (P and As excepted) can be AND ARE studied using stable isotope techniques stable isotope analysis provides an additional, independent analytical tool to help decipher the complex biogeochemical behavior of these elements and compounds

9 Notation δ 13 C = {[( 13 C/ 12 C) sample /( 13 C/ 12 C) reference ] -1} x 1000 units of (i.e. tenths of %) heavy = relatively high 13 C/ 12 C ratio light = relatively low 13 C/ 12 C ratio isotope fractionation refers to a change in isotopic composition associated with a reaction e.g. Δ 13 C vapor-liquid = δ 13 C vapor - δ 13 C liquid Δ product-reactant = δ product - δ reactant

10 Sources of Nitrate

11 Open vs. Closed Systems, & Rayleigh Fractionation A : liquid water - open system D: liquid water - closed system B: water vapor - open system E: water vapor - closed system

12 The Hydrologic Cycle (part I) water with H ( 1 H) or D ( 2 H) D 2 O liquid + H 2 O gas = D 2 O gas + H 2 O liquid K = a(h 2 O) liquid a(d 2 O) gas /a(h 2 O) gas a(d 2 O) liquid or K = c(h 2 O) liquid c(d 2 O) gas /c(h 2 O) gas c(d 2 O) liquid Then K = (D/H) gas /(D/H) liquid = α the equilibrium fractionation factor values of α are usually close to 1, and are a function of T, e.g. α Η = at 20 C α Η = at 100 C

13 The Hydrologic Cycle (part II)

14 The Hydrologic Cycle (part III) normal precipitation evaporation

15 The Hydrologic Cycle (part IV)

16 The Hydrologic Cycle (part V)

17 Archaean Atmospheres & the Rise of Oxygen when did the oxygen concentration in the Earth s atmosphere begin to rise? MAJOR scientific question consensus opinion is that O 2 began to rise at 2.4 Ga small (but very vocal) opinion that O 2 concentration began to rise much earlier than this (approx. 4 Ga)

18 Mass Dependent Fractionation (MDF) & Mass Independent Fractionation (MIF) for elements with 3 of more stable isotopes, e.g. 32 S, 33 S, 34 S & 36 S isotope compositions (and isotope fractionation factors) are usually dependent on the differences in mass (MDF), e.g. δ 33 S 0.5 x δ 34 S, c.f. 33 S/ 32 S vs. 34 S/ 32 S i.e. 1 amu difference ( 33 S/ 32 S) vs. 2 amu difference ( 34 S/ 32 S) if this is not the case, then this is known as mass independent fractionation (MIF) MIF (deviations from MDF) given in the Δ notation, e.g. Δ 33 S = δ 33 S x δ 34 S (i.e. Δ 33 S = 0 means no MIF, but MDF instead)

19 Archaean S-MIF Farquhar et al. (2000) similar observations for Δ 36 S

20 What Produces S-MIF? virtually all known chemical, physical and biological processes produce S-MDF the only known process to produce significant S-MIF is the photolysis of SO 2 gas using ultraviolet (UV) light

21 So What? if there is significant O 2 in the atmosphere, then there will also be significant ozone (O 3 ) in the atmosphere ozone is a very strong absorber of UV light so, if there is no O 2 (and no O 3 ), then solar-uv light can photolyse SO 2, and produce S-MIF if there is O 2 (and O 3 is present), then the atmosphere absorbs solar-uv light, so there is no S-MIF hence, the presence (or absence) of S-MIF is strong evidence for the absence (or presence) of significant O 2 in the atmosphere

22 UV Experiments with Lasers peak at nm Solar UV spectrum ArF laser UV spectrum

23 Experimental S-MIF by UV Photolysis real effect, but does it represent natural conditions? what would be the effect of photolysis using a solar spectrum of light? if the S-MIF effect is an artefact of using a laser, do the experiments really tell us anything about the concentration of O 2 in Archaean atmospheres? if the S-MIF effect is not due to UV photolysis, what did cause the S-MIF effect, and why did the S-MIF effect disappear at 2.4 Ga?

24 Hg Stable Isotopes 7 stable isotopes: ( 196 Hg), 198 Hg, 199 Hg, 200 Hg, 201 Hg, 202 Hg and 204 Hg conventional wisdom: atomic masses of Hg isotopes too heavy for stable isotope fractionation relative mass difference (rmd) for 204 Hg vs. 198 Hg = 3.0% rmd 34 S vs. 32 S = 6.3% rmd 33 S vs. 32 S = 3.1% Hg has a range of redox states in nature (0 to +II)

25 Microbial Reduction of Hg(II) to Hg(0) α = Kritee et al. (2007) analytical uncertainty = 0.1 to 0.2

26 Photoreduction of Hg(II) to Hg(0) - part I Photoreduction of aqueous Hg 2+ and methyl-hg by natural sunlight: Bergquist & Blum (2007) quantifies the fractionation factor during photoreduction, can then be applied to natural samples

27 Photoreduction of Hg(II) to Hg(0) - part II Hg-MIF! Bergquist & Blum (2007)

28 Mass Independent Fractionation of Hg only for Hg isotopes with odd masses, and only for the photo-reduction experiments what the heck is going on?!

29 MIF-Hg #1: nuclear field shift effect due to differences in nuclear volume and nuclear charge radius, which do not scale linearly with # of neutrons predicted to be important for very heavy elements, e.g. Hg, U, Tl #2: magnetic isotope effect only odd isotopes have non-zero nuclear spin, magnetic moments, and hyperfine splitting dependent on nature of reaction intermediates predicted to be a bigger effect

30 MIF-Hg in Nature? YES!

31 Hg Stable Isotopes EXTREMELY active research area Kritee et al. (2007) ES&T, 49 citations in Web of Science Bergquist & Blum (2007) Science, 78 citations in Web of Science technique can be applied in very much the same way as classic stable isotope methodology mass independent fractionation (MIF) of Hg stable isotopes is an additional technique that can be used still early days - lots of work to be done yet

32 How about (insert your favorite element here?) YES! Virtually all elements that can be studied for stable isotopes have been studied for stable isotopes (at least to some extent) potential benefit for studying biogeochemical cycles is HUGE! the drawback is that the necessary instrumentation is expensive, and the analyses are technically demanding to perform

33 Conclusions it s a stable isotope world stable isotopes are our friends