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Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes

Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes . Lecture outline: mass balance Raleigh distillation 3) the hydrological cycle 4) δ D and δ 18 O variability. spectrometer light intake. Chalk cliffs formed in Cretaceous. Mass balance of stable isotopes.

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Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes

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  1. Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes • Lecture outline: • mass balance • Raleigh distillation • 3) the hydrological cycle • 4) δD and δ18O variability spectrometer light intake Chalk cliffs formed in Cretaceous

  2. Mass balance of stable isotopes Principle: stable isotopes are CONSERVED, unlike radioactive isotopes Therefore, if one reservoir is enriched, the other reservoir must be depleted R (reservoir size) is expressed in moles ‘d’ represents the delta value for a given reservoir, expressed in per mil Example: What was the glacial-interglacial sea level change? Given: G-I δ18O change = +1.3‰ (SMOW) present-day ocean = 0‰ (SMOW) glacial ice caps averaged -35‰ Δ sea level = 140m

  3. Continual fractionation in a closed system: Raleigh distillation ex: rainfall from cloud IF PRODUCT REMOVED (cannot re-equilibrate w/ parent liquid) original vapor first drop enriched phase (α ? 1) equilibrium vapor becomes lighter rain becomes lighter next vapor next drop enriched phase equilibrium TIME TIME next vapor next drop enriched phase equilibrium final vapor final drop enriched phase NO FRACTIONATION FOR LAST DROP. . . Why?

  4. liquid vapor δ18O (‰ Raleigh distillation model • We can track the progression • of the vapor-rainfall if we know: • the initial isotopic ratio of the vapor • the fraction of vapor remaining • where • RV is the isotopic ratio of the vapor • RV0 is the initial isotopic ratio of the vapor • f is the fraction of vapor remaining • α is the fractionation factor We can also derive the formula for the Rrain as a function of α: After Dansgaard, 1964 If the α of vapor to liquid is 1.0092, what is the α of liquid to vapor? NOTE: fractionation increasing because T(cloud) decreasing

  5. Raleigh distillation in the real world If the tropics are the source of all cloud moisture, then the δ18O of rainfall _________ from equator to pole. What also happens as you move from equator to pole? This effect would ________ the δ18O of rainfall at the poles. What other natural systems might be characterized by Raleigh fractionation?

  6. The Hydrosphere How do 18O, 16O (δ18O) and 2H, 1H (δD) move through this system?

  7. Water Isotopic Variations NOTE: water isotopes are always reported with respect to SMOW Ocean δ18O = 0 ± 2‰ δD = 0 ± 16‰ Lake Michigan δ18O = -7‰ δD = -54‰ Lake Chad δ18O = -20‰ δD = -110‰ Dead Sea δ18O = +4.4‰ δD = 0‰ What processes explain these variations?

  8. Water Isotopic Fractionation – review from last lecture Reminder: Oxygen and hydrogen isotopes are strongly fractionated as they move through the hydrological cycle, because of the large fractionation associated with evaporation/condensation. This fractionation is temperature-dependent. GNIP – global network of isotopes in precipitation Rainwater samples are routinely collected for δ18O and δD analysis all over the world. The data are stored and managed by GNIP, and used to study the processes that fractionate water isotopes.

  9. Water Isotopic Fractionation – some data δ18O of rain near SMOW in tropics, highly depleted in high-latitudes δ18O of rain decreases far from vapor source (Raleigh) and is heavier during winter (temperature) Rozanski, 1993

  10. Temperature effect on the δ18O of precipitation holds for both spatial T variability and temporal variability Rozanski, 1993

  11. But what if we add all the GNIP global δ18Oprecip data? A bit more complicated, but generally strong relationship. However, what is happening at higher temperatures? Rozanski, 1993

  12. The so-called “amount” effect: more rain, heavier d18O NOTE: only in tropics (<30° N and S), where “deep convection” takes place Empirical relationship – meaning….? It would be difficult to explain a vapor source at +1‰, when the tropical oceans are ~0‰. Thought to be linked to increased evaporation of raindrop in dry, under-saturated environment… (i.e. vapor is -9‰ ish, but the raindrop is enriched as it falls from the sky) Mechanism still unknown – need atmospheric modeler’s help. Dansgaard, 1964 Rozanski, 1993

  13. Surface Water Salinity-δ18O relationship - general Global precipitation So δ18O of surface waters, like salinity, is also correlated to evaporation – precipitation.

  14. Surface Water Salinity-δ18O relationship - tropics Fairbanks et al., 1997 Slope of δ18O-salinity relationship is 0.273 in the deep tropics (<5° N and S), vs. 0.45 elsewhere. Why?

  15. The “Global Meteoric Water Line” – what happens to δ18O happens to δD, but with a different α annual mean dD vs. d18O of precipitation But month-to-month variations at a given site fall off this line – “deuterium excess” Craig, 1961 Rozanski, 1993

  16. 1-3km Water Why don’t all waters fall on the GMWL? Or…. why do different “source” waters have different ‘deuterium excess’ values? Fact: water vapor above the ocean is -13‰ in δ18O, not the -9.2‰ expected from equilibrium fractionation. Why? • Planetary boundary layer • the layer where exchange occurs • between the surface and the free • atmosphere • evaporation not purely • equilibrium process • what other type of fractionation • is involved? Given the potential for complicated boundary layer physics, it’s a wonder that the GMWL exists at all!

  17. Deuterium excess Humid regions will show smaller departures from GMWL than arid regions. Generally interpreted as a proxy for the “source” of the moisture.

  18. Modeling water isotopes in the hydrosphere Full atmospheric General Circulation Model (GCM) with water isotope fractionation included. Noone, D., 2002 Goal: quantify physical processes associated with water isotope variability Applications: atmospheric mixing, vapor source regions, impact of climate variability on hydrological cycle, interpretation of paleoceanographic records

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