Discussion 2 Paper due to me Sept 28-pdf and complete citation Find a paper using isotope(s) as tool for understanding biogeochemical processes They or it may be radiogenic or stable Written paper same requirements 1.5 pages. Summarize and critique the paper. Describe how the isotope (s) are used to understand a biogeochemical cycle Oral presentation October 8 same information 5 minutes Everyone will select a paper this time. 2018 assignment Try to keep your presentation simple and to 5 minutes. 1- title and author of your paper can be on your slide (I always think it is interesting to note where the scientists are in terms of location) You may have 1 to 2 data slides. 2- isotope chosen 3- how it was used to understand some component of a biogeochemical cycle. 4 why this research is important and how use of the isotope gave insightful information. In your short paper 1.5 pgs that you turn in before class Discuss the above and also describe any limitations you feel existed in the work and give complete citation. 1
Basics Examples Isotopes as tools 2
Isotopes Equal places Stable versus unstable/radiogenic (anthropogenic vs natural) N/Z = Neutrons/Protons when get to > 1.5 become unstable. ½ life Radioactive used for dating U-Pb (U235-700 million/ U238 4.5 billion) 147 Sm- 143 Nd 1.06 * 10 11 yr K-Ar 1.3 by 14 Carbon - 5370 y 2 protons and 2 neutrons like 4 2He electron Figure 2.1 Porcelli and Baskaran (2011) 3
Isotopes in Environmental Studies 75 elements with useful stable isotopes Figure 2.1 and 2.2 from Porcelli and Baskaran (2011) Radiogenic-Lithogenic-natural rock mineral decay Decay series Cosmogenic-reactions with cosmic rays (largely protons) 14 C, 7 Be, 10 Be, Anthropogenic-nuclear explosions Lithogenic tracers Uranium and thorium-dust and contamination due to human activities (i.e. mining, nuclear energy production) Table 2.5 Porcelli and Baskaran 2011 4
Atmospheric-Cosmogenic nuclides Table 28.1 Lal and Baskaran (2011) 7 Be (53 days) and 10 Be (1.4 my) Formed by natural cosmogenic ray spallation of N 2 and O 2 in the atmosphere delivered to the surface by wet and dry deposition Near constant deposition Used as tracers of stratosphere-troposphere air exchange, mixing air masses, atmospheric circulation, removal rate of aerosols Because constant rate of deposition to soils, vegetation, waters, ice and sediment can be applied to understand processes that operate over weeks to millions of years Radioactive decay Figure 2.3 Porcelli and Baskaran, 2011) 5
Anthropogenic Isotopes Generated through nuclear reactions Not present naturally so can be measured at low concentrations Table 2.4 Porcelli and Baskaran Measurement Digestion and concentration Quantification using radiation counting of alpha, beta, and gamma radiation Quantification using ICPMS Have well defined standards 6
Light STABLE ISOTOPES Hydrogen 1 1H, 2 1Deuterium ( 3 1Tritium 12.3 yrs) Carbon - 12 6C, 13 C ( 14 C radioactive cosmic ray spallation of N 2 ) Nitrogen - 14 7N, 15 N Oxygen - 16 8O, 17 O, 18 O Sulfur - 32 16S, 33 S, 34 S, 36 S Table 2.6 commonly used stable isotopes Different isotopes of an element have different abundance influencing what we measure Convert to gas and analyzed by gas source MS Others! Li B Si Cl Mo Ag Hg 7
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 100 C 2 H 2 16 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 8
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 Weiss et al. 2008/Ehleringer et al 2015 paper/albararede 2015, Measurement stable and radiogenic isotopes 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 9
Molybdenum Table. Stables isotopes of molybdenum. Table. Stables isotopes of molybdenum. Isotope 92 Mo 94 Mo 95 Mo 96 Mo 97 Mo 98 Mo 100 Mo Mass /Da 91.906809 (4) 93.9050853 (26) 94.9058411 (22) 95.9046785 (22) 96.9060205 (22) 97.9054073 (22) 99.907477 (6) Natural abund. (atom %) 14.84 (35) 0 9.25 (12) 0 Nuclear spin (I) Nuclear magnetic moment (μ/μ N ) 15.92 (13) 5 / 2-0.9142 16.68 (2) 0 9.55 (8) 5 / 2-0.9335 24.13 (31) 0 9.63 (23) 0 Reporting 18 R O sample R R standard standard 13 C 18 16 18 16 O O sample 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 Ratio + more of the heavy isotope in the sample/ratio more of the lighter isotope in the sample 3 10
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 From Clark and Fritz (1997) 11
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 better we can measure heavy isotope fractionation Mass dependent and independent fractionation MIF-not dependent on mass Molecular symmetry Nuclear volume effect Magnetic isotope effect Anomalies of odd relative to light isotopes Light reactions 12
MDF (Wiederhold) Kinetic-different reaction rates heavy and light isotopes (vaporization, diffusion, biotic processes) Equilibrium heavier isotopes enriched in stronger bonds 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 13
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. 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 14
Temperature dependence-example from Faure CO 2 air- ocean H 2 O CO2 H 2O O 18 16 g g 18O 18 16 l O O O l 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 15
Read Tripple paper for next class 16
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). warm cold 17
Figure 26. 4 Faure 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 18
Fractionation driven by ocean temperature- colder more fractionation and 18O likely to remain in the ocean Steen Larsen paper JGR v 116 2011 Higher T 9 change in δd results in 1 o C change in T 40 /9= 4.5 o C http://www.agu.org/sci_soc/vostok.html 19
1.8 x 1.7 = 3.06 o C δ 18 O of O2 in the atmosphere Atmosphere O 2 δ 18 O +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 with 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 plants and organically bound C retaining 16O during photosynthesis 20
From Clark and Fritz 1997 21
C on surface of the earth δ 13 C mantle= earth total = -5 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 Note this is not up to date 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 C isotopes in plants Fig 27.1 Faure Why are CAM plants different? 22
Nitrogen Not common in minerals but important in biosphere and atmosphere Ocean is N 2 in equilibrium with atmosphere Nitrogen --Important in food web studies and as tracer Understanding of fractionation in processes is becoming clearer From Clark and Fritz 1997 23
Isotopic composition of N Table 28.2 Faure Organisms preferentially excrete N14 this is more easily used in catabolic reactions that generate energy 24
Faure-food web Fig 27.4 20 15 N ( 0 /00) 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 DD PR FP BWL RWL 25
SULFUR ISOTOPES The stable sulfur isotopes are: 32 S, 33 S, 34 S, 35 S 34 S 34 32 34 32 S S sample 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-+ 2(CH2O) 2CO2+ H2S + 2 H2O Reduction of 32 S is faster than 34S 3 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. 26
From Clark and Fritz (1997) Archean and Early Proterozoic (to about 2 by ago) No fractionation- 2H+ + SO42-+ 2(CH2O) 2CO2+ H2S + 2 H2O 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 ~ 2 by consistent 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 27
Mercury (see Bergquist and Blum, 2009 Elements) 7 isotopes span 4% mass difference 196 Hg(0.16%), 198 Hg(10.0%), 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 Mercury MDF Redox transformations Biological cycling Volatilization- Hg isotope in volatilized lighter 28
Elements; December 2009; v. 5; no. 6; p. 353-357; DOI: 10.2113/gselements.5.6.353 Mineralogical Society of America 29