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Stable Isotopes in Geochemistry and Chemical Oceanography

Stable Isotopes in Geochemistry and Chemical Oceanography. January 21, 2014 S. Nemiah Ladd. Catalyst (A quick warm-up to get us started) . Take a minute to write down answers to the following questions on your own sheet of paper: What is an isotope?

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Stable Isotopes in Geochemistry and Chemical Oceanography

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  1. Stable Isotopes in Geochemistry and Chemical Oceanography January 21, 2014 S. Nemiah Ladd

  2. Catalyst (A quick warm-up to get us started) Take a minute to write down answers to the following questions on your own sheet of paper: What is an isotope? What are some ways that stable isotopes are used in geochemistry and chemical oceanography?

  3. What is an isotope? • Atomic Number = # Protons = defines which element • Atomic Mass= protons + neutrons = defines which isotope of an element • An element can have different numbers of neutrons and thus atomic weights. • Example: Carbon can exist as 12C, 13C, 14C • How many protons and neutrons in each of the C isotopes? • 1 chemical, many isotopes!

  4. How are isotopes used in geochemistry and chemical oceanogrpahy? • Paleoclimate (Past temperature, rainfall, ice volume, ocean pH) • Origin of organic matter in the ocean • Rates of primary productivity in the ocean • Sources of CO2 and CH4 in the atmosphere • Water use efficiency and drought tolerance in plants (and past plant communities) • Ages of rocks, organic material, deglaciations • And many more…

  5. Isotopes key questions: What are isotopes? What are the types of isotopes? How do we express measurements of isotopes? What is isotope fractionation and how do we express it? What is equilibrium isotope fractionation? What is kinetic isotope fractionation? What is Raleigh distillation? What are some applications of stable isotopes?

  6. Types of Isotopes Isotopes can be categorized into 2 categories: Stable isotopes – Isotopes that do not decay over the timescale of earth history (4.5 billion years) Radioactive isotopes – Isotopes that spontaneously convert into other nuclei at a discernable rate (Jim will talk more about radioactive isotopes tomorrow)

  7. The chart of the nuclides (protons versus neutrons) for elements 1 (Hydrogen) through 12 (Magnesium). Valley of Stability Most elements have more than one stable isotope. b decay 1:1 line X X a decay # of Neutrons tends to be greater than # of protons

  8. Examples for H, C, N and O: % Abundance is for the average Earth’s crust, ocean and atmosphere

  9. Stable isotopes of light isotopes can be measured precisely, but not accurately We can overcome poor accuracy by comparing our measurements to a standard of known isotopic value that is measured concurrently

  10. Isotope standards

  11. Nomenclature – δ Notation Isotope ratios are measured relative to a standard because this allows for accurate measurements among different instruments. Differences in the isotope ratio can be very very small so we use δ(“del”) notation δ = “delta” units are per mil (‰) Where H = moles of heavy isotope L = moles of light isotope R = H/L δ tells us how much the sample deviates from the standard

  12. Quick check Where H = moles of heavy isotope L = moles of light isotope R = H/L If a sample has more 13C relative to the 12C than VPDB, will its δ13C value be: Positive? (Hold up your pink card) or Negative? (Hold up your green card)

  13. The sign ofδ Sample has relatively more of the heavy isotope than the standard Sample has relatively less of the heavy isotope than the standard Compared to the standard

  14. Example 1: The isotope standard for C is PDB (13C/12C = 0.011237) Your sample has an 13C/12C = 0.010957. What is δ13C in ‰for the standard? For the sample?

  15. Isotopic Fractionation Small differences in the distribution of the isotopes in materials because heavier isotopes form stronger bonds and move slightly slower A heavier mass = stronger bond You would need a stronger string to hold two bowling balls together than you would need to hold two golf balls together Isotope Fractionation = process that results is differences in delta values in products and reactants If all isotopes of the same element behaved the same, they wouldn’t provide much geochemical information. Different behavior by molecules containing different isotopes of the same element(fractionation) results in different distributions of isotopes among pools.

  16. Example 2: Fractionation If water from the lake evaporates, will the δD value of the vapor have a higher δD value than the lake water? (hold up your pink card) or a lower δD value than the lake water? (hold up your green card) H2O H2O H2O H2O Evaporation HDO HDO HDO H2O H2O H2O H2O If water from the lake evaporates, will the δD value of the lake water increase? (hold up your pink card) or decrease? (hold up your green card)

  17.  and εNomenclature Fractionation Factor =  If  = 1, no fractionation If  >1, more heavy in product If  <1, more heavy in reactant  is unitless Difference fractionation Factor = ε If ε = 0, no fractionation If ε> 0 , more heavy in product If ε< 0 , more heavy in reactant ε is in permil (‰) Emerson & Hedges defines ε = δHproducts– δHreactants, but that is not the standard definition among isotope geochemists

  18. Two kinds of Isotope Fractionation Processes • Equilibrium Isotope effects • Occurs in equilibrium reactions (reactions can go both ways) if the system is in equilibrium • Chemical equilibrium • Phase changes (closed system) • Distributes isotopes in a system so that the total energy of the system is minimized • Heavier isotope equilibrates into the compound or phase in which it is most stably bound • Within a molecule (CO2vs HCO3-) • Between molecules (CO2(g) vs CO2(aq) ) Equilibrium fractionation is due to slightly different free energies for atoms of different atomic weight Usually temperature dependent!

  19. Example #3:Condensation of Water Vapor in a closed container H2O(g) <=> H2O(l) In a closed container: δ18O of H2O(l) = -1‰ δ18O of H2O(g)= -10‰ Discuss with a partner and be ready to share: Is this reaction an example of an equilibrium isotope effect? How can you tell? Does the 18O “prefer” to be in the gas or liquid phase? Why?

  20. 2. Kinetic Fractionation Occurs in unidirectional reactions reversible reactions that are not yet at equilibrium diffusion Heavier isotopes move more slowly (KE = ½ mv2) Therefore react more slowly Bonds between heavy isotopes are stronger Therefore more activation energy needed for them to react; reactions occur at different rates Isotopes effects involving organic matter are typically kinetic

  21. Examples of Kinetic Fractionation • Three types of kinetic fractionation: • 1. Unidirectional reactions • Example: • Carbon fixation via photosynthesis: • 12CO2 + H2O -> 12CH2O + O2 faster • 13CO2 + H2O -> 13CH2O + O2 slower • Organic matter gets depleted in 13C during photosynthesis (decreases in 13C) • 2. Reversible reactions that are not yet at equilibrium • Example: • Evaporation of water vapor if not in equilibrium (net evaporation ie: N .Atlantic) • H216O(l) -> H216O(g)faster • H218O(l) -> H218O(g)slower • Water vapor gets depleted in 18O during net evaporation (decreases in 18O) • 3. Diffusion • Example: • Diffusion of H2O across a cell membrane • H216O(l)outside cell -> H216O(l)inside cell faster • H218O(l) outside cell -> H218O(l) inside cell slower δ13CCO2 -7‰ δ13Cleaf -27‰

  22. Equilibrium Fractionation vs Kinetic Fractionation The difference depends on the reason for the fractionation Equilibrium fractionation occurs so that the total energy of the system is minimized via forming the most stable bonds possible Equilibrium is related to bond stability of the isotope Kinetic fractionation occurs because smaller molecules move faster than heavier molecules and therefore react more slowly Kinetic is related to the rate at which the isotope reacts

  23. Isotopes in the carbon cycle E & H Fig. 5.6

  24. d13C of atmospheric CO2 versus time Quick question: Why is the δ13C value of atmospheric CO2 decreasing?

  25. Raleigh Fractionation - Concept • Vapor depleted in 18O compared to ocean water • Air masses transported to higher latitudes where it is cooler. • Rain enriched in 18O, removed from system (cloud) • Cloud gets lighter • Rain enriched in 18O, removed from system (cloud), but less enriched

  26. Raleigh Fractionation – Characteristic trend • Example:Evaporation – Condensation Processes • d18O in cloud vapor H2O(g) and condensate (H2O(l) rain) • plotted versus the fraction of remaining vapor for a Raleigh process. Idealized: • 20ºC – All vapor -9‰ • Just colder than 20ºC – Condensate starts to form, more enriched in 18O, but is removed from the system (rained out) • The vapor continues to condense as the temperature decreases – becoming more and more depleted in 18O • Fractionation increases with decreasing temperature • Same pattern for D/H isotopes - different scale because more fractionation during the condensation (ε = +78‰ rather than +9‰) If it gets colder in Antarctica, δD and δ18O of newly falling snow would get heavier (pink card) get lighter (green card)

  27. Millennial Temporal Variability Interglacial warm periods: Less Rayleigh isotope fractionation in precipitation (IPCC, 2007) (in thousands of years) (Flipped scale!) Glacial periods: why are δ18O values of benthic foraminifera increasing?

  28. Example 4: Using isotope mass balance to determine sources of organic matter • Land plants have an average δ13C value of -27‰. • Phytoplankton have an average δ13C value of -22‰. • You have a sample of organic material from coastal surface sediments with a δ13C value of -25.5‰. • What % of the organic material in your sample came from land? What % came from phytoplankton? NOTE: Need to convert δ values to R in order to do accurate calculations!!!

  29. Isotopes key questions: What are isotopes? What are the types of isotopes? How do we express measurements of isotopes? What is isotope fractionation and how do we express it? What is equilibrium isotope fractionation? What is kinetic isotope fractionation? What is Raleigh distillation? What are some applications of stable isotopes?

  30. Example #4: Bicarbonate system The carbonate buffer system involving gaseous CO2(g), aqueous CO2(aq), aqueous bicarbonate HCO3- and carbonate CO32-. One step of that reaction: CO2(aq) + H2O ↔ HCO3- + H+ δ13C of CO2(aq) = 1‰  = 1.0092 at 0ºC and 1.0068 at 30ºC (The IRMS standard for C is PDB (13C/12C = 0.011237)) Is this reaction an example of an equilibrium isotope effect? How can you tell? What is the final δ13C of HCO3- at 0ºC at 30ºC? Is 13C more stable as CO2(aq)or HCO3-? Is there more or less fractionation at higher temperatures?

  31. Raleigh Fractionation • A combination of kinetic and equilibrium isotope effects • Kinetic when water molecules evaporate from sea surface (net evaporation b/c system is not in equilibrium) • Equilibrium effect when water molecules condense from vapor to liquid form • A isotope fractionation reaction where products are isolated immediately from the reactants will show a characteristic trend in isotopic composition.

  32. Case study: 18O of forams in sediment to reconstruct paleotemperature • HCO3- + Ca2+ ↔ CaCO3(s) + H+ • Fractionation of 18O is temperature dependent and well quantified in labs • The 18O of CaCO3 precipitated on forams reflects the temperature • Preserved in marine sediments • Complicated because although this • relationship is well defined, depends on a known 18O of water

  33. Case study: 18O of forams in sediment to reconstruct paleotemperature HCO3- + Ca2+ ↔ CaCO3(s) + H+ last interglacial d18O of planktonic and benthic foraminifera from sediment core (160ºE 1ºN) Planktonic and Benthic differ due to differences in water temperature where they grow. Planktonic forams measure sea surface T Benthic forams measure benthic T Holocene last glacial In order to find temperature from this data, we would need to know the d18O of the water in which it was formed

  34. Case study: 18O of forams in sediment to reconstruct paleotemperature • Does the 18O of water in the ocean change over time? • Large scale Raleigh distillation • Net transfer of water from ocean to continental ice sheets make ice very depleted in 18O and the oceans enriched in 18O, increasing the d18O of water about 1‰ In some cores, pore water can be measured directly, which gets around this issue.

  35. Case study 3 Profiles of DI13C and 18O to estimate primary productivity The profiles of DI13C and 18O can be used to estimate primary productivity More photosynthesis in surface results in a heavier DI13C, resulting in a more positive d13C in surface DIC During respiration, 16O is preferentially taken up,resulting in a more positive d18O “left over” in the water (obvious at O2 minimum) Why does the d13C decrease slightly at the O2 minimum? North Atlantic data

  36. Full Chart of the Nuclides Valley of Stability 1:1 line

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