Basics Examples Isotopes as tools http://ethomas.web.wesleyan.edu/ees123/isotope.htm Isotopes Equal places Stable versus unstable/radiogenic (artificial vs natural) N/Z = Neutrons/Protons when get tt to >1.5 become unstable. 1
½ life Radioactive used for dating U-Pb (U235-700 million/ U238 4.5 billion) Sm-Nd 50 by K-Ar 1.3 by Carbon 14-5370 y STABLE ISOTOPES Hydrogen 1 1H, 2 1Deuterium ( 3 1Tritium 12.3 yrs) Carbon - 12 6C, 13 C( 14 C radioactive) Nitrogen - 14 7N, 15 N Oxygen - 16 8O, 17 O, 18 O Sulfur - 32 16S, 33 S, 34 S, 36 S Different isotopes of an element have different abundance influencing what we measure 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 16 2 O has a boiling point of 100 C 2 H 16 2 O has a boiling point of 101.4 C Implications the slightly different properties of the isotopes result in: (1) a temperature dependence for many reactions (2) chemical 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) Biological fractionation 3
Applications 1. Temperature (e.g. paleoclimate, ore deposits, petrology) 2. Processes (e.g. vaporization, oxidation/reduction, photosynthesis, trophic transfer) 3. Sources (e.g. groundwater contaminants, avian migration, forensic geochemistry) 4. Tracers (natural and artificially-enriched compounds EST Paper, Measurement Interested in comparing variation not actual concentrations -Bend a beam of charged ions in a magnetic field -Separate isotopes that then go to faraday cup collectors -Each cup measures that isotope simulateously-get a ratio -Switch between sample and standard to compare 4
Reporting 18 R O sample R R standard standard 13 C 18 16 18 16 O Osample O O 18 16 O O 3 standard 3 10 10 13 12 13 12 C C sample C C 13 12 C C standard standard standard 10 3 COMMONLY USED STANDARDS Oxygen - standard mean ocean water (SMOW). Hydrogen - SMOW Carbon - calcite from a belemnite fossil from the Peedee Formation in South Carolina (PDB). Sometimes PDB is also used for carbonate oxygen. Nitrogen - atmospheric N 2. Sulfur -sulfur in troilite (FeS) of the Canyon Diablo Meteorite from Meteor Crater, Arizona (CDT). Relatively light elements so mass differences easy to measure EST article is on using heavy isotopes 5
From Clark and Fritz (1997) ISOTOPE FRACTIONATION Isotope fractionation: the development of differences in isotopic composition as a result of physical and chemical processes. The degree of fractionation depends on the relative weights of the isotopes. Commonly fractionated: H, C, N, O, S Somewhat fractionated: Si, Fe, Cl As our methods get tbetter we can measure heavy isotope fractionation 6
Factors causing fractionation Phase changes lighter isotope greater vibrational frequency due to less mass VF=1/(mass) 1/2 bonds more easily broken with lighter The Rule of Bigeleisen (1965) - The heavy isotope goes preferentially into the compound with the strongest bonds Heavier goes into compound with higher oxidation state Lighter isotopes form weaker bonds in compounds, react faster. concentrated in the products. At high temperatures, the equilibrium constant for isotopic exchange tends towards unity, i.e., because small differences in mass are less important when all molecules have very high kinetic and vibrational energies. 7
THE ISOTOPE FRACTIONATION FACTOR The isotope fractionation factor is defined as: a Ra b Rb where R a = the ratio of heavy to light isotope in phase a; R b = the ratio of heavy to light isotope in phase b. For example, consider: H 2 O(l) H 2 O(v) at 25 C 18 16 l Rl O O v( O) 18 16 R O O v l v 1.0092 CO2 air- ocean H2O 18 16 CO 2 g O O 18 16 H 2Ol O O 18 l O v 1.04 CO2 (g) + H2O (l) H2 CO3 CO2 (g) + H2O (l) At 25 o C δ 18 O CO2 in air is 40 While that of ocean is 0 8
Temperature dependence-example from Faure 9
Because both H and O occur together in water, δ 18 O and δd are highly correlated in meteoric water, yielding the meteoric water line Meteroric water is derived from atmospheric water vapor Figure 1. (Clark and Fritz 1997, p. 37, as compiled in Rozanski et al. 1993, modified by permission of American Geophysical Union). 10
warm cold Figure 26. 4 Faure 11
Seasonal variations Climatic variations Ice core data Use relationships established for δ 18 O and temperature of specific locations δ 18 O=0.695Tannual -13.6 δ 18 O=0.33Tmonthly -11.9 1 decrease for 1.7 o C decrease Fractionation driven by ocean temperature- colder more fractionation and 18O likely to remain in the ocean Steen Larsen paper JGR v 116 2011 12
Higher T http://www.agu.org/sci_soc/vostok.html δ 18 O of O2 in the atmosphere Atmosphere O 2 δ 18 O+235 +23.5 Ocean water is 0 Why is the atmosphere so positive? Complicated from Hoefs Stable isotope geochemistry 6 th ed O2 in the air produced by photosynthesis without fractionation Also if there was equilibrium between atmospheric oxygen and water on the surface at 25 o C the atmosphere would be + 6 Idea is that the more positive value is due to respiration and oxygen more consumed during respiration is the lighter isotope resulting in 18 O remaining in the atmosphere Or plants and organically boun d C retaining 16O 13
From Clark and Fritz 1997 C on surface of the earth δ 13 C mantle= earth total = -5 If on the surface If on the surface ¼ buried as carbon with value -28 ¾ left in ocean in equilibrium with ocean carbonates 0 See evidence of great oxidation event at 2.4 by prior to this carbonates in the ocean have slightly positive δ 13 C Atmosphere C in CO2 is -7.7 Coal is -27 Attribute value to being less than -5 due to fossil fuel input with ½ being dissolved in ocean and assimilated by plants 14
C isotopes in plants Fig271Faure 27.1 Faure Why are CAM plants different? Nitrogen Not common in minerals but important in biosphere and atmosphere Ocean is N 2 in equilibrium with atmosphere 15
Nitrogen --Important in food web studies and as tracer Fractionation in processes not so clear From Clark and Fritz 1997 Isotopic composition of N Table 28.2 Faure 16
) 15 N ( 0 /00) 20 18 16 14 12 10 8-35 -30-25 -20-15 13 C ( 0 / 00 ) Carbon and nitrogen ratios in fish of the Truckee River and Steamboat Creek CP Ve M We Lo EP PR FP BWL RWL Hg in Truckee River fish 0.7 0.6 PR 0.5 PR PR ug Hg/g wet wt. 0.4 0.3 0.2 0.1 Lo EP EP EP EP EP EP PR Lo EP 0 0 5 10 15 20 25 30 35 40 45 length cm 17
Faure-food web Fig 27.4 SULFUR ISOTOPES The stable sulfur isotopes are: 32 S, 33 S, 34 S, 35 S 34 S 34 32 34 32 S Ssample S S 34 32 S S standard standard 10 The most important cause of S-isotope fractionation is the reduction of sulfate by anaerobic bacteria 2H++SO42+ + SO42-+ 2(CH2O) 2CO2+ H2S + 2 H2O Reduction of 32 S is faster than 34S 3 18
SULFIDE/SULFATE FRACTIONATION The extent of fractionation of S-isotopes between sulfate and sulfide by biological processes depends on: 1) The rate of metabolism by bacteria. 2) Composition and abundance of food supply. 3) Size of sulfate reservoir. 4) Temperature. 5) The rate of removal of H 2 S. From Clark and Fritz (1997) Archean and Early Proterozoic (to about 2 by ago) No fractionation- 19
Sulfur isotopes and oxygenation of the atmosphere Sulfur isotope signature important for understanding presence of abundant O in the atmosphere δ34s sulfide sulfate 0 no biogenic fractionation 3.8 to 2.7 by begin to see fractionation with negative values in sulfide minerals-not uniform-anaerobic /aerobic basins basins ~ 2 by consistent t fractionation in marine sediments Mass independent versus dependent fractionation Dependent chemical and physical processes operating to separate Independent-photochemical or electron spin driven MIF of S prior to 2.4 by suggests S in atmosphere being impacted by UV light Advances EST Feature p655 Multi collector ICP-MS Ion source-hi T plasma Mass analyzer Detection unit New trends Cr reduction of Cr(VI) to Cr(III) mass dependent (kinetic control) Tracing Cr pollution 20
Cu- tracing acid mine drainage Zn-uptake by plants favors the light isotope Se- largest fractionation with redox reactions Mercury (see Bergquist and Blum, 2009 Elements) 7 isotopes span 4% mass difference 196 Hg(0.16%), 198 Hg(10.0%), 0%) 199 Hg(16.9%), 200 Hg(23.1%), 201 Hg(13.2%), 202 Hg(29.7%), 204 Hg(6.8%) Tracing sources? MIF vs MDF MIF due to nuclear field effect- nuclear volume and nuclear charge radius that does not scale linearly with number of neutrons magnetic spin associated with odd neutrons that have non zero nuclear spin, magnetic moments Photochemical reduction 21
MDF Mercury Redox transformations Biological cycling Volatilization- Hg isotope in volatilized lighter Published in: Abir Biswas; Joel D. Blum; Bridget A. Bergquist; Gerald J. Keeler; Zhouqing Xie; Environ. Sci. Technol. 2008, 42, 8303-8309. DOI: 10.1021/es801444b Copyright 2008 American Chemical Society 22
nomenclature =deviation of the measured isotope ratio from the theoretical predicted by MDF From Bergquist and Blum, 2009 Elements 23
Elements; December 2009; v. 5; no. 6; p. 353-357; DOI: 10.2113/gselements.5.6.353 Mineralogical Society of America More papers Science 2011- Abiotic Fe fractionation pyrite formation produces a large Fe isotope fractionation Science 2011 The oxygen isotopic composition of the sun inferred from captured solar wind. 24