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Chemical Oceanography - 19 Introduction to isotopic tracers Gitai Yahel The School of Marine Sciences Ruppin Academic Center Gitai Yahel (Yahel@Ruppin.ac.il), Tel.(052)291 8007, Skype gitaiyahel, Web http://Moodle.Ruppin.ac.il Partly after Schmidt, Heip, Hungate,Johnson, & Middelburg

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Isotopic tracers are a widely used in oceanography Radioactive vs. stable isotopes Stable isotopes are used to: Trace the sources and sinks of material in the environment Determine the extent and type of biogeochmical processes which have acted on materials Provide information on paleooceanographic conditions Experimentally trace specific elements using stable isotope tracers (e.g., 15 N). Radioactive active isotopes are used for most of the above + dating Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (2)

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Isotopes Isotopes are atoms that contain the same number of protons but differ in the number of neutrons. Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (3) 6 protons 6 neutrons 6 protons 7 neutrons protons electrons neutrons Carbon-12Carbon-13

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Radioactive Isotopes Some isotopes are stable, while others are unstable, or radioactive. (6P + 6N) (6P + 7N) (6P + 8N) Stable isotopes Radioactive isotope Carbon-12Carbon-13Carbon-14 R. Doucett Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (4)

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Radioisotopes Radioisotopes provide the clocks that are used to measure rates of ocean processes oPresent in a variety of elements with different chemistry oHave a variety of half lives, and a variety of sources oCan be measured at very low concentration and extremely accurately oRadioisotope tracers are both natural ( 10 Be, 14 C, 238 U, 230 Th) and anthropogenic ( 14 C, 90 Sr, 240 Pu steady state tracers transient tracers Usage in chemical oceanography oAge of water and sediment oRates of scavenging and production oRates of gas exchange * Very important as tracers in biological oceanography (primary production, nitrogen sources, etc) Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (5)

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Nuclear Radiation Radioactivity 1896: Antoine Henri Becquerel found that uranium salts would fog photographic film plates Marie and Pierre Curie showed the uranium caused the fog on the plates. Marie named this radioactivity. The penetrating rays/particles were called radiation. Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (6)

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Nuclear Radiation First scientist to win 2 Nobel Prizes Only scientist to win Nobel Prizes in 2 sciences *Physics (1903) "in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel *Chemistry (1911) "for her discovery of radium and polonium Died in 1934 from leukemia due high exposure to radioactive elements Marie Curie Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (7)

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Nuclear Reactions Nuclei of unstable isotopes gain stability by undergoing changes. These changes release a lot of energy! Not affected by temperature, pressure, or catalysts. Not affected by the compounds that they are present in. Cannot alter the rate of chemical reactions* * Not entirely true We will return to this issue later Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (8)

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Nuclear Reactions Disproves Daltons theory that atoms are indivisible. Radioisotopes: Unstable isotopes Too many or too few neutrons relative to the number of protons in a nucleus make a nucleus unstable. Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (9)

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Nuclear Radiation - Nuclear Decay Three types of Radiation: 1)Alpha: a He nucleus (2 protons and 2 neutrons) Atomic number reduced by 2 and atomic mass reduced by 4 Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (10)

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2) Beta: an electron (a neutron is transformed into a proton) Nuclear Radiation - Nuclear Decay Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (11)

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3) Gamma: high energy photon (no mass and no charge) *Often emitted with alpha or beta particles *Extremely penetrating and very dangerous! Nuclear Radiation - Nuclear Decay Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (12)

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Nuclear Stability and Decay About 17% of known nuclei are stable and do not change Stability: depends on proton-to-neutron ratio Higher atomic numbers demand more neutrons per proton for stability 12 C 1:1 206 Pb 1.5:1 Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (13)

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Nuclear Transformations Nuclear Stability and Decay Neutron-to-proton ratio determines the type of decay that occurs! *Beta emission: nuclei with too many neutrons per proton *The result is a decrease in the # of neutrons and an increase in the # of protons Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (14)

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Nuclear Transformations Nuclear Stability and Decay Neutron-to-proton ratio determines the type of decay that occurs! *Positron emission: nuclei with too few neutrons per proton Positron: particle with the mass of an electron, but a positive charge *The result is an increased # of neutrons and a decrease in the # of protons Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (15)

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Nuclear Transformations Nuclear Stability and Decay All elements with atomic numbers greater than 83 are radioactive! *Alpha emission: nuclei with too many neutrons and protons *The result is a more stable (increased) neutron-to-proton ratio Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (16)

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Half-Life The time required for one-half of the nuclei of a radioisotope to decay 1005025 13 Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (17)

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Half-Life Half lives can be fractions of seconds or billions of years Calcium-37: 175 ms Radon-222: 3.8 days Carbon-14: 5.73 x 10 3 years Uranium-238: 4.46 x 10 9 years Thus Radioisotopes can be useful as clocks Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (18)

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The uranium series Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (19)

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Carbon-14 ( 14 C) dating half-life = 5730 years Exponential decay Spallation- cosmic rays constantly produce 14 C in the atmosphere 14 C is consumed by plants Nearly constant amounts throughout history Compares 14 C levels in sample to the current atmospheric levels where 14 C is the quantity and is the decay constant: The solution to this equation is: Where, for a given sample of carbonaceous matter: o 14 C 0 = number of radiocarbon atoms at t = 0, i.e. the origin of the disintegration time o 14 C = number of radiocarbon atoms remaining after radioactive decay during the time t o = radiocarbon decay or disintegration constant Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (20)

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Radiocarbon dating Raw (not calibrated) radiocarbon age: t 1/2 = 5568 years Methods: oNumber of disintegration per time Scintillation counting Gas proportional counting oDirect counting Accelerated Mass Spectroscopy (AMS) Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (21) F Years 1/2 Drops to 0.5 in 5730 years ( 1/2 ) Drops to 0.25 in 2* 1/2 years

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Nuclear Transformations Transmutation Reactions Transmutation: conversion of an atom of one element into another Two instances of transmutation: 1) Radioactive Decay 2) Particles bombard the nucleus of an atom Ernest Rutherford (1919): first artificial transmutation Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (22)

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Nuclear Transformations Transmutation Reactions Trans uranium Elements: Atomic numbers above 92 *All are artificial and radioactive *Produced in nuclear reactors and accelerators *Np and Pu first synthesized in 1940 *More than 20 elements synthesized since Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (23)

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Nuclear Transformations Nuclear Fission Splitting of the nucleus into smaller fragments U-235 and Pu-239 are the only fissionable isotopes 1 kg of U-235 releases the energy of 20,000 tons of dynamite Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (24)

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Nuclear Transformations Nuclear Fission Chain Reaction: neutrons produced react with other fissionable atoms Bombs are uncontrolled chain reactions Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (25)

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Nuclear Transformations Nuclear Fission Reactors use controlled fission to produce energy Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (26)

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Chemical Oceanography, Yahel@Ruppin.ac.il (27) Stable isotopes Most elements in the periodic table have more than one stable isotope. 12 C 13 C 2H2H 1H1H 36 S 32 S 33 S 34 S 14 N 15 N R. Doucett Wednesday, 9 January 2013

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Chemical Oceanography, Yahel@Ruppin.ac.il (28) One isotope is always more common than the others The mass ratio difference is more pronounced for lighter elements ElementIsotopesAbundance Mass ratio Hydrogen 1 H, 2 H 1 H = 99.985% 2 H = 0.015% 2 Carbon 12 C, 13 C 12 C = 98.89% 13 C = 1.11% 1.083 Nitrogen 14 N, 15 N 14 N = 99.633% 15 N = 0.366% 1.071 Oxygen 16 O, 17 O, 18 O 16 O = 99.759% 17 O = 0.037% 18 O = 0.204% 1.125 1.058 Sulfur 32 S, 33 S, 34 S, 36 S 32 S = 95.00% 33 S = 0.76% 34 S = 4.22% 36 S = 0.014% 1.125 1.062 1.031 R. Doucett Commonly used stable isotopes Wednesday, 9 January 2013

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Chemical Oceanography, Yahel@Ruppin.ac.il (29) Measurement Stable isotope ratios (e.g., 13 C/ 12 C) are measured using a mass spectrometer. Three masses of CO 2 (44/45/46) are measured to determine the amount of 13 C and 12 C in a sample. 12 C+ 16 O+ 16 O = 44 13 C+ 16 O+ 16 O = 45 12 C+ 18 O+ 16 O = 46 R. Doucett Wednesday, 9 January 2013 o Convert element of interest into a stable gas. o Purify/separate gas analyte from contaminants (off-line or on-line) o Measure isotopic ratios on an isotope ratio mass spectrometer (IRMS )

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13 C/ 12 C = 0.011225 13 C/ 12 C = 0.011071 13 C/ 12 C = 0.010918 Raw ratios are converted into delta ( ) permil values: Chemical Oceanography, Yahel@Ruppin.ac.il (30) Mass spectrometers can measure very small differences in isotope ratios. 13 C sample = x 1000 13 C/ 12 C sample - 13 C/ 12 C standard 13 C/ 12 C standard R. Doucett Wednesday, 9 January 2013

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Standards Vary Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (31)

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Manipulating units o The units are not the same as molarity or Atom % notation, but are ratios. o The units must be used only for comparison. o Many mathematical calculations should be made by converting units to atom %. o The units must be on exactly the same scale to be comparable (e.g., V-SMOW for 18 O). Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (32)

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Natural ranges of isotope ratio values -8 Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (33)

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Isotopic Fractionation Two types of isotopic fractionation that cause changes in isotopic ratios: Kinetic isotope fractionation: oOne isotope reacts, diffuses, or evaporates faster than the other oCan be due to chemical, physical, or biological processes oUsually, the lighter isotope reacts or diffuses faster oMagnitude of the isotopic fractionation effect is temperature, reaction rate, and species dependent oe.g., 13 C in photosynthesis Equilibrium isotope fractionation: oExchange reactions in which a single atom is exchanged between 2 species (with isotopic preference) oBidirectional (reversible) chemical reactions oTemperature dependent oE.g., 18 O in water and air Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (34)

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Stable C isotope 13 C Stable isotope ( 13 C) Fractionation: Kinetic effects: 13 C reacts more slowly than 12 C 13 CO 2 diffuses more slowly than 12 CO 2 Equilibrium effects: 12 CO 2 + 13 CO 3 2- +H 2 O = 13 CO 2 + 12 CO 3 2- +H 2 O 13 C will partition into the species where overall energy is lowest Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (35)

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Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (36) Availability of substrate affects fractionation Beggars cant be choosers If substrate is non-limiting (and constantly renewed) maximum fractionation will take place. If substrate is limiting (and virtually all is used, with slow replacement), fractionation will be low Examples CO 2 limitation of phytoplankton affects 13 C Nitrate availability affects phytoplankton 15 N

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Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (37) NO 3 - concentration (M) 15 N of plankton biomass (o/oo) 0 +5 +10 +15 5 10 High N availability significant fractionation Low N availability Little fractionation N-fixing organisms Seawater NO 3 - 15 N value 15 N values of plankton depend on the source of N (e.g NO 3 - vs. N 2 ) and availability of the nutrient.

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Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (38) Isotopes in food web studies You are what you eat! Consumer isotopic composition reflects the isotope composition of the food source. Little fractionation along trophic levels for Carbon or Sulfur Some trophic enrichment of 15 N with higher trophic levels (+3 to +4 o/oo per trophic level) due to preferential excretion of light isotope.

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Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (39) Typical values for del 13 C Sea water DIC +2 o/oo Atmospheric CO 2 -7 o/oo Marine POC-20 to -22 o/oo Terrestrial plants -27 o/oo Marsh grasses (C4)-14 o/oo Benthic algae -17 o/oo Values for biogenic materials are approximate, and subject to variation depending on factors such as temperature and availability of substrates (e.g. CO 2 ) New data are emerging all the time!

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Unlike stable isotopes, radiocarbon is constantly created and destroyed Total number of 14 C atoms (N) Production in the stratosphere Loss by radioactive decay - N Total amount of radiocarbon on Earth can (and does) vary Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (40)

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http://www.iup.uni-heidelberg.de/institut/forschung/groups/kk/14co2.html Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (41)

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Application: calculating the ocean ventilation time Radiocarbon concentrations in per mille deviation from a reference value approximately equal to atmospheric radiocarbon before the industrial revolution. The upper ocean waters are contaminated with bomb radiocarbon. Data are from the Western Atlantic and Pacific GEOSECS sections. Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (42)

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Application: calculating the ocean ventilation time Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (43) Figure 2.5.6 Radiocarbon concentrations along the core of the deep waters in the three ocean basins and Antarctic (Stuiver et al. [1983]).

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Temperature proxy Preserved air- bubbles Annual layers Rapid changes can be studied Specific events can be investigated and links to other climate systems established ice cores

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Example: Core S100 from coastal Antarctica. Sseasonal peaks in the 18 O, DEP and ECM records between 70-80 m depth used for establishing an annual chronology. Dating - annual if accumulation is high Kaczmarska, M., Isaksson, E., Karlf, K., Winther, J-G, Kohler, J., Godtliebsen, F., Ringstad Olsen, L., Hofstede, C M., van den Broeke, M.R., Van De Wal, R. S.W., and Gundestrup, N. 2004. Accumulation variability derived from an ice core from coastal Dronning Maud Land, Antarctica. Annals of Glaciology, 39, 339-345.

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Application: Using 18 O from ice cores to reconstruct past air temperature Evaporation: lighter 16 O evaporates more easily from a water body resulting atmospheric H 2 O vapor is poorer in 18 O than oceanic water Condensation: heavier 18 O are precipitated faster than lighter 16 O Precipitation will be depleted in 18 O relative to the standard (ocean water) Negative 18 O Coldest snow is lightest (less heavy 18 O isotopes, more lighter 16 O isotopes) Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (46)

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Temperature affects on air 18 O/ 16 O ratio: colder temperatures more negative values for the 18 O warmer temperatures 18 O values that are less negative (closer to the standard ratio of ocean water) Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (47)

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Application: reconstructing past air temperature General rule of thumb: the heavy isotope will be concentrated in the phase in which it is most strongly bound (or lowest energy state). Solid>liquid>water, covalent>ionic, etc. Ex: 18 O in carbonates - heavily enriched in carbonate because O tightly bonded to small, highly charged C 4+, vs. weaker H + - so 18 O cal-water = 18 Ocarb- 18 Owater = 30 Wednesday, 9 January 2013 Chemical Oceanography, Yahel@Ruppin.ac.il (48)

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Using specific events for dating ice cores Using specific events for dating ice cores Examples from Svalbard ice cores Kekonen and others, 2002Pinglot and others, 2003 Nuclear weapon tests Laki 1783 Volcanic eruptions

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Depthage relationship Ice cores have layer thinning due to pure shear which means that if sample size is consistant the number per time unit will decrease with depth

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Ice cores as climatic indicators Trapped air bubbles in the ice can provide us with information on the atmospheric greenhouse content 18 O as an air temp. proxy