CHEMICAL OCEANOGRAPHY Light Stable Isotope Geochemistry Lecture 1

Stable Isotope geochemistry is concerned with variations of the isotopic compositions of elements arising from physicochemical processes (vs. nuclear processes). Characteristics of a useful stable isotope system: 1.large relative mass difference between stable isotopes (  m/m) 2.abundance of “rare” isotope is high (0.1-1%) 3.element forms variety of compounds in natural system Examples: 2 H/ 1 H, 13 C/ 12 C, 15 N/ 14 N, 18 O/ 16 O, 34 S/ 32 S fractionation refers to the change in an isotope ratio that arises as a result of a chemical or physical process. Occurs during: - isotopic exchange reactions in which the isotope are redistributed among different molecules containing that element - unidirectional or incomplete reactions - physical processes like evaporation/condensation, melting/crystallization, adsorption/desorption, diffusion

Outline Principles Mass spectrometery Isotopes fractionation in water (ò 18 O; òD) Water isotopes and the hydrologic cycle Oxygen isotopes in the ocean Oxygen isotopes in carbonates Paleo-applications Water isotopes in ice

Fractionation types There are three types of isotope fractionation: 1.equilibrium fractionation 2.kinetic fractionation 3.mass-independent fractionation (far less important) Equilibrium fractionation - arises from the translational, rotational, and vibrational motions of 1. molecules in gases and liquids 2. atoms in crystal lattices - energy of these motions is mass-dependent - systems will move to the lowest energy configuration - usually largest in covalent bonds, minimal in ionic bonds From William White’s (Cornell) upcoming Geochemistry textbook most imp.

Equilibrium fractionation (cont) So why does equilibrium fractionation occur? -a molecule with a heavy isotope sits at a lower zero point energy level than the same molecule with all light isotopes -bonds with high potential energies are broken more readily -bond strengths vary for light and heavy isotopes of an element What about temperature? -the difference in zero point energies for light vs. heavy molecules decreases with increasing T -bond strengths converge at high T, fractionation factor goes to 1 at high T zero point energy Effect of vibrational E in harmonic oscilllator model E=1/2h

NOTE:  is close to 1 because ratios differ by parts per thousand  approaches 1 as temperature increases

Kinetic fractionation - arises from fast, unidirectional, incomplete reactions (many biologically-mediated rxns) Consider two molecules of CO 2 : 12 C 16 O 2 (mass = *16 = 44) and 13 C 16 O 2 (mass = *16 = 45) if their energies are the same, then: and the ratio of their velocities is: SO… 12 C 16 O 2 can diffuse 1.1% further than 13 C 16 O 2 in a given amount of time In reality, gas are not ideal, velocity difference is reduced by collisions, reduced fractionation assuming ideal gas This can be observed as gas moves through a fine capillary tube ( 12 C 16 O 2 arrives first). 1.Velocities of gas molecules are different - kinetic energies of molecules of ideal gas are equal - so differences in mass (heavy vs. light isotopes) must be compensated for by velocity

Example of isotopic effects PropertyH D 2 16 O Melting Point 0oC0oC3.81 o C Boiling Point 100 o C o C Max Density3.98 g/cc11.23 g/cc Kw x Ex. The vapor pressure of heavy water is less than for "normal" light water and therefore light water evaporates more relative to heavy water, whereas the reverse occurs in precipitation.

IMPORTANT  ’s are typically very small, and reflect as shown earlier the differences in ground- state energies of the different isotopes. Two important facts on isotope fractionation (1) Regardless of the magnitude of the isotopic effect, there will be no fractionation if all the reactants are transformed to products (e.g., a complete distillation of water)  Because the ground-state energies are temperature dependent,  ’s approach unity as temperature increases and there is no isotopic fractionation. This is more relevant in High-T geochemistry than in seawater with the exception of hydrothermal processes. 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.  varies inversely with T So at colder temperatures, isotopes will be more heavily fractionated.

Water Isotopic Fractionation 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. Remember  m/m For 16 O- 18 O it is 2/16=1/8 For H-D it is 1/1=1 therefore    D NOTE: water isotopes are always reported with respect to SMOW

Water Isotopes in the Hydrosphere How do 18 O, 16 O (  18 O) and 2 H, 1 H (  D) move through this system?

liquid vapor  18 O (‰ Raleigh distillation model After Dansgaard, 1964 We can track the progression of the vapor-rainfall if we know: 1.the initial isotopic ratio of the vapor 2.the fraction of vapor remaining where R f is the isotopic ratio of the vapor R i is the initial isotopic ratio of the vapor  is the fraction of vapor remaining  is the fractionation factor NOTE: fractionation increasing because T(cloud) decreasing

Raleigh distillation in the real world

Temperature effect on the  18 O of precipitation Note: closed basins where E>>P fall off the Meteoric Water Line (MWL)

Sea Surface Water Salinity-  18 O relationship So  18 O of surface waters, like salinity, is also correlated to evaporation – precipitation. Global precipitation ∆  18 O/∆S≈0.5

Consider: E, P, river and glacier inputs, sea ice freezing/melting

Carbonate  18 O – introduction Minerals (e.g. carbonate, quartz, barite, etc) form from super-saturated solution.  18 O of these minerals is a fxn of  18 O of solution and temperature of solution Remember: the  18 O of a solid phase is usually reported in PDB (heavy standard) while  18 O of liquid phase is usually reported in SMOW (light standard) interconversion equation:  18 O CaCO3 (SMOW)=  18 O CaCO3 (PDB)+0.27‰ Important: You need to know the  18 O of the solution to derive temperature from  18 O solid The ocean  18 O is defined as 0‰ (Standard Mean Ocean Water).

The relationship between water-  18 O, temperature, and the equilibrium  18 O of calcite was determined empirically by (i) Sam Epstein et al., (1953) and later modified by (ii) Craig (1965): (i) T= (  c-  w)+0.14 (  c-  w) 2 (i)T= (  c-  w)+0.10(  c-  w) 2 Where:  c is the  18 O of CaCO 3 and  w is the  18 O of the water NOTE CHANGE  w must be wrt PDB if  c is good for low T, paleoceanography A rule of thumb  18 O=0.22‰ / °C Carbonate  18 O – temperature relationships

Shackelton benthic foraminifera: T=16.9-4(  c-  w) Erez planktonic foraminifera: T= (  c-  w)+0.03(  c-  w) 2

growing glaciers deep-sea foraminifera Isotopes in deep-sea carbonates The “Ice Volume” effect- Light isotope removed from ocean, locked into large ice sheets. Ocean  18 O shift (+1.5‰) recorded in marine carbonates that grew during glacial. SPECMAP – standard benthic  18 O record, used to date marine sediments of unknown age

Glacial-Interglacial foraminifera  18 O Data from deep-sea (benthic) foraminifera show +1.5‰  18 O shift during LGM LGM The main question in paleoceanography: How much of this shift was due to ice volume (sea level change) and how much was due to temperature change? Current studies suggest an “ice volume effect” of  18 O water (~ ‰), so we have 0.5‰ left over for temperature change. How much did bottom water temperatures change during the LGM? (problem set) Or you could measure temperature (trace metal concentrations in carbonates e.g., Mg/Ca), and obtain a “residual”  18 O that gives you the  18 O SW change.

In order to reconstruct surface temperatures from carbonate  18 O formed during the LGM, you need to 1)remove the ice-volume effect 2)constrain the  18 O of your water mass 3)apply the paleo-temperature equation Glacial-Interglacial climate reconstruction However, people can use other proxies to get at temperature: 1)foraminifera assemblage data (CLIMAP) 2)tree lines and snow lines will be lower during cold times 3)trace metals in carbonates (Mg/Ca and Sr/Ca) 4)alkenones (saturation index of long-chained alkanes in coccolithophores)

Hydrogen and oxygen isotopes in ice A paleothermometer Empirically and theoretically, isotopic composition of precipitation and site temperature are strongly correlated in time and space; colder places and colder times have isotopically lighter precipitation. Because water molecules containing heavier isotopes exhibit a lower vapor pressure, when the temperature falls, the heavier water molecules will condense faster than the normal water molecules. The relative concentrations of the heavier isotopes in the condensate indicate the temperature of condensation at the time, allowing for ice cores to be used in global temperature reconstruction.

Ex: EPICA – a new LONG Antarctic ice core  D variations why?... Augustin, L. et al Water isotopes in ice cores Also applied to: Greenland  18 O – GISP, Jouzel et al., Andes  18 O, Lonnie Thompson Alaska  18 O, Ken Moore and others