PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view ·...

18
Stable Isotopes – Raleigh distillation 10/4/1 and water isotopes ture outline: mass balance Raleigh distillation the hydrological cycle δD and δ 18 O variability spectrometer light intake Chalk cliffs formed in Cretaceous

Transcript of PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view ·...

Page 1: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

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

Lecture outline:1) mass balance

2) Raleigh distillation

3) the hydrological cycle

4) δD and δ18O variability

spectrometer lightintake

Chalk cliffs formed in Cretaceous

Page 2: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

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

0t

it R i iR d R d

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‰

go*0‰ * 35‰ + V *1.3‰o iV V

i o goV V V Δ sea level = 140m

Page 3: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

Continual fractionation in a closed system: Raleigh distillation

originalvapor

firstdrop

next vapor

enriched phase (α ? 1)

nextdrop

enriched phase

next vapor nextdrop

enriched phase

final vapor finaldrop

enriched phase

TIM

E

TIM

E

NO FRACTIONATION FOR LAST DROP. . . Why?

vapor becomeslighter

rainbecomes lighter

IF PRODUCT REMOVED(cannot re-equilibrate w/ parent liquid)

equilibrium

equilibrium

equilibrium

ex: rainfall from cloud

Page 4: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

liquid

vapor

δ18O

(‰

Raleigh distillation model

After Dansgaard, 1964

We can track the progressionof the vapor-rainfall if we know:1. the initial isotopic ratio of the vapor2. the fraction of vapor remaining

where RV is the isotopic ratio of the vaporRV0 is the initial isotopic ratio of the vaporf 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:

0

1V

V

Rf

0

1R

V

R fR

αα

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

Page 5: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

Raleigh distillation in the real world

What other natural systems might be characterized by Raleigh fractionation?

If the tropics are the source of all cloud moisture, thenthe 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.

Page 6: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

The Hydrosphere

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

Page 7: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

Water Isotopic Variations

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‰

NOTE: water isotopes are always reportedwith respect to SMOW

What processesexplain thesevariations?

Page 8: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

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 precipitationRainwater 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.

Page 9: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

Rozanski, 1993

δ18O of rain near SMOWin tropics, highlydepleted in high-latitudes

δ18O of rain decreasesfar from vapor source (Raleigh)and is heavier during winter (temperature)

Water Isotopic Fractionation – some data

Page 10: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

Temperature effect on the δ18O of precipitation

holds for both spatial T variability

and temporal variability

Rozanski, 1993

Page 11: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

But what if we add all the GNIP global δ18Oprecip data?

However, what is happening athigher temperatures?

A bit more complicated,but generally strongrelationship.

Rozanski, 1993

Page 12: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

Dansgaard, 1964Rozanski, 1993

The so-called “amount” effect: more rain, heavier δ18O

Empirical relationship – meaning….?

It would be difficult to explaina 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 raindropis enriched as it falls from the sky)

Mechanism still unknown – needatmospheric modeler’s help.

NOTE: only in tropics (<30° N and S), where “deep convection” takes place

Page 13: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

18 0.45* 15.5O Sδ

Surface Water Salinity-δ18O relationship - general

So δ18O of surface waters, like salinity,is also correlated to evaporation – precipitation.

Global precipitation

Page 14: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

Fairbanks et al., 1997

18 0.273* 9.4O Sδ

Surface Water Salinity-δ18O relationship - tropics

Slope of δ18O-salinity relationship is 0.273 in the deep tropics (<5° N and S), vs. 0.45 elsewhere. Why?

Page 15: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

The “Global Meteoric Water Line” – what happens to δ18O happens to δD, but with a different α

188* 10D Oδ δ

Craig, 1961Rozanski, 1993

annual mean δD vs. δ18O of precipitation

But month-to-month variationsat a given site fall off this line – “deuterium excess”

188*d D Oδ δ

Page 16: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

Why don’t all waters fall on the GMWL?Or…. why do different “source” waters have different ‘deuterium excess’ values?

Planetary boundary layerthe layer where exchange occursbetween the surface and the freeatmosphere

-evaporation not purely equilibrium process

-what other type of fractionationis involved?

Water

Fact: water vapor above the ocean is -13‰ in δ18O, not the -9.2‰ expected from equilibrium fractionation. Why?

1-3k

m

Given the potential for complicated boundary layer physics, it’s a wonder that theGMWL exists at all!

Page 17: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

Deuterium excess

Humid regions will show smaller departures from GMWL than arid regions.

Generally interpreted as a proxyfor the “source” of the moisture.

Page 18: PowerPoint Presentationshadow.eas.gatech.edu/~kcobb/isochem/l… · PPT file · Web view · 2013-02-06Stable Isotopes – Raleigh distillation 10/4/12 and water isotopes Lecture

Noone, D., 2002

Modeling water isotopes in the hydrosphere

Full atmospheric General Circulation Model (GCM) with water isotope fractionation included.

Goal: quantify physical processes associated with water isotope variability

Applications: atmospheric mixing, vapor source regions, impact of climatevariability on hydrological cycle, interpretation of paleoceanographic records