Stable Isotopes OUTLINE Reading: White Ch 9.1 to 9.7.1 (or digital p370-400) Exercise answer? What does the salt do? Today 1. 2 leftovers 2. Stable Isotopes for hydrologic and climate applications 1
CaCO 3 preserving sediments vs depth Generic depth distribution pattern aragonite calcite Not so much calcite GG325 L24, F2013 Sequence on a moving plate: B A D C Siliceous oozes (opal) found in the sediments beneath high productivity zone (equator; can be above / below the CCD). 2
The Nature of Matter - Elements Element cannot break up into simpler substances by ordinary chemical processes Atomic Number (Z) = # Protons, H=1, Fe=26, U=92 Mass Number (A) = # Protons + # Neutrons Clicker question: isotopes To make different isotopes of a given element, you need to have different numbers of: a) protons b) electrons c) neutrons e.g.: 14 6C 3
The Nature of Matter - Elements # Protons is fixed, # Neutrons can vary (isotopes with different mass #) Hydrogen (1,2,3) (Average in nature is 1.008) Iron (54,56,57,58) (Average in nature is 55.85) Uranium (234,235,238) (Average in nature is 238.03) 88 Naturally occurring elements - many have >1 isotopes What do the different average masses reflect? Isotope proportions Isotopes in Nature Two types: Stable Mass-dependent fractionation Can you name example? Oxygen: d( 18 O/ 16 O) ratios between water and water vapor Radio-active/radiogenic (get to these later) Decay: parent -> daughter, can be chemically fractionated, followed by decay 4
Common low T Stable Isotopes: Common elements whose isotopes are used in low T geochemistry What is the relative mass difference between 1H- 2H? Aka deuterium-> How about 32S-33S? Notation and measurements Hard to measure absolute amounts (counting atoms), instead we measure ratios, expressed in delta notation: By definition: d or delta units: 18 16 18 16 ( O/ O) -( O/ O) spl 18 16 ( O/ O) é ù 18 std d O = ê ú*1000 ê ú ë std û most deltas are named after the heavy isotope So ratios are compared to a standard value (=isotope ratio in standard) 5
More stable isotope notation δ is in units of (multiplied by 1000 to bring out differences) 18 16 18 16 ( O/ O) -( O/ O) spl 18 16 ( O/ O) é ù 18 std d O = ê ú*1000 ê ú ë std û Fractionation factor given by: R a A - B= R A B R a and R b = stable isotope ratios of 2 materials: Þisotopes are stable, yet ratios change between materials: isotopes are fractionated wrt each other: process-gauge Types of Isotope Fractionation of Interest Here a. due to overall molecular mass differences b. due to mass-dependent bond energy differences c. kinetic effects arising from either of the above Molecular ex: Water molecule combinations (H, D, 16 O and 18 O) lightest molecule: H- 16 O-H H- 16 O-D D- 16 O-D mass: 18 19 20 molecule: H- 18 O-H H- 18 O-D D- 18 O-D mass: 20 21 22 heaviest ~ 22% heavier 6
Molecular mass differences and speed Bulk mass differences mean molecular velocity by: ( ) v = 8k T ½, where T = temperature, m = mass, and π m k = the boltzman constant. So velocity ~ 1/m : heavy = slow, light = fast => Light species equilibrate faster (=get differences) = kinetics Kinetic Fractionation Enhances molecular fractionation Consider evaporation H 2 O 16 > H 2 O 18 ~~~~~~~ ~~~~~~~~~ ~~~~~~~~~~~ H 2 O 16 < H 2 O 18 H 2 O 16 moves faster than H 2 O 18 : attains equilibrium faster, and enhances H 2 O 16 enrichment during evaporation 7
Equilibrium fractionation and bond energy Effect of vibrational E in harmonic oscilllator model Cause of equilibrium fractionation? - Bonds with heavy isotopes have lower potential E - higher potential E = easier bond to brake => So bond strengths vary for light vs heavy isotopes What about temperature? - As Tñ, difference between different pairs = less => Same bond strengths, no fractionation Example Fractionation Ranges and Standards Common isotope standards (R STD ) Element Standard Isotopes per mil isotopic variation H C O Standard Mean Ocean Water (SMOW) Calcite PD Belemnite (PDB) SMOW, or PDB 2 H/ 1 H (D/H) rocks/minerals 13 C/ 12 C -35 to +5 natural waters -180 to +20-410 to +50 18 O/ 16 O -2 to + 36, or -32 to +5-50 to +15, or -78 to -15 8
Temperature dependence of fractionation a ~ 1/T 2 : a goes up as T goes down, with a = R a /R b => constrain temperatures of processes (today and in the past) Example: CaCO 16 O 16 O 16 + H 2 O 18 H 2 O 16 + CaCO 16 O 16 O 18 Parameterization rewritten to: T = 16.9-4.20 (d C - d W ) + 0.13(d C - d W ) 2 d C = d 18 O calcite d W = d 18 O water The range of isotopic compositions of H, C and O on the earth δd (δ 2 H) shows the greatest range, as expected from the large D-H mass difference. 9
The range of isotopic compositions of H, C and O on Earth d 18 O in rocks decreases with increasing formation temperature (Oxygen in Si-O-Si bonds, as in chert, is more enriched in 18 O than in Si-O-Al or Si-O-Mg bonds.) Generally increasing formation temperature for sedimentary, metamorphic, and igneous rocks. mantle GG325 L25, F2013 The range of isotopic compositions of H, C and O on Earth inorganic materials heavier C than organic ones. Organic matter = isotopically light: biotic metabolisms prefer breaking the higher energy (easier to break) 12 C-X bonds. GG325 L25, F2013 10
H and O isotopic composition in Natural waters on Earth In hydrologic cycle extent of isotopic fractionation during liquid vapor exchange = f(t) Liquid-vapor exchange over a wide temperature range causes isotopic heterogeneity Evaporation of water favors the lighter isotopes at all temperatures, but more so at lower temperature. O and H behaves similarly Processes with effect on 18 O / 16 O ratios The oxygen isotopic composition of seawater (d 18 O w ) is controlled by fractionation effects due to: evaporation and precipitation at sea surface freezing of ice in Polar Regions admixing of water masses with different ratios (melt water, river run-off) global isotopic content of the oceans 11
Processes through time & 18 O / 16 O ratios Large amounts of H2O in ice changes composition of the ocean Ice cores then record temperatures through time (next lecture) H and O isotopic composition in waters on Earth Temperatures from equator to pole decrease Temperature gradient from equator to poles causes variations in both the isotopic ratios of surface ocean waters and of water vapor in the overlying atmosphere. T do dd heavy temperature at the earth's surface meteoric water line light equator poles 12
H and O isotopic composition in waters on the earth Meteoric and atmospheric waters from around the world follow the dd vs. d 18 O relationship shown here, which is known as the meteoric water line : δd SMOW = 8 δ 18 O SMOW + 10 H and O isotopic composition in waters on Earth latitudinal (temperature) effect, and the rain-out effect The rain-out effect: precipitation enriched in heavy isotope, compared to vapor =>water condenses from vapor that is continually growing lighter, as more rain falls, further inland 13
H and O isotopic composition: reaction with rock Water rock reactions clear in H-O isotopes: values off the meteoric water line Interactions shift δ 18 O without much change in δd (because rocks contain little exchangeable H). X 14