Chemostratigraphy

Stable Isotopes

Isotopes of ElementsChart of the NuclidesDelta NotationIsotope Fractionation Equilibrium Kinetic Raleigh

When the universe was formed 15 billion years ago (the “Big Bang”) light elements of H (99%), He (1%) and trace amounts of Li were formed.Subsequent reactions during star formation created the remaining elements,

Isotopes of Elements

The chemical characteristic of an element is determined by the number of protons in its nucleus. Atomic Number = number of Protons (Z) = the number of protons found in the nucleus of an atom of that element, and therefore identical to the charge number of the nucleus. The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons. Different elements can have different numbers of neutrons and thus atomic weights (the sum of protons plus neutrons). Atomic Weight = protons + neutrons = referred to as isotopes

There are 92 naturally occurring elementsSome are stable; some are Radioactive

Isotopes of one element have the same protons number (and obviously of electrons) but a different neutrons number.

So, they have the same protonic number Z but a different mass number (protons+neutrons).

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.

Number of neutrons tendsto be greater than the number of protons

1:1 line

Full Chart of the Nuclides

1:1 line

Examples for H, C, N and O:

Atomic Protons Neutrons % AbundanceWeight (Atomic Number) (approximate)

Hydrogen H 1P 0N 99.99 D 1P 1N 0.01Carbon 12C 6P 6N 98.89

13C 6P 7N 1.1114C 6P 8N 10-10

Nitrogen 14N 7P 7N 99.6 15N 7P 8N 0.4Oxygen 16O 8P 8N 99.76 17O 8P 9N 0.024 18O 8P 10N 0.20

All Isotopes of a given element have the same chemical properties, yet there are small differencesdue to the fact that heavier isotopes typically form stronger bonds and diffuse slightly slower % Abundance is for the average Earth’s crust, ocean and atmosphere

2H

1/2 = 5730 yr

- Unstable isotopes like 235U and 238U breakdown over time. Stable isotopes (of which there are many), do not.

- They last essentially forever. As such, they are totally useless for establishing the absolute ages of rocks through radiometric techniques, but they do have amazing uses for other applications.

- For one, stable isotopes are useful for establishing the evolutionary history of pore water. This means you can do paleohydrology etc.

- You can also determine paleotemperatures, establish the geochemical variation of sea water over time, and map out sea level oscillations.

There are several isotopes that are useful to geologists. Here are some of them and the rocks/materials that can be studied using them:- Hydrogen: 1H, 2H (2D) (water)- Carbon: 12C, 13C (organic materials, petroleum, meteorites, carbonate systems)- Oxygen: 16O, 17O, 18O (water, ice, carbonate minerals, meteorites, hydrothermal systems)- Nitrogen: 14N, 15N (biological systems, mantle rocks; diamonds -trace gases)- Sulphur: 32S, 33S, 34S, 36S (meteorites, biological systems, magmatic rocks)

Whenn neutron/n proton is about 1Stable isotopes(non radioactive)It nuclear composition is not altered by timeDifferent weight:Same chemical propertiesSame physical propertiesUseful in the climatic studies

Whenn neutron/n proton is much different from 1 Unstable isotopes(radioactive)They transform themselves spontaneouslyuntil it took astable configurationHave other applications,such datings(14C, U/Th)

Mass Spectrometer – Basic Schematics

Isotopes are measured as ratios of two isotopes by various kinds of detectors.Standards are run frequently to correct for instrument stability

Magnetic fielddeflects ion beam

Gases ionizedGases accelerated

Detectors

1. Input as gases2. Gases Ionized3. Gases accelerated4. Gases Bent by magnetic field5. Gases detected

high vacuum

Isotope-ratio mass spectrometry (IRMS) is a specialization of mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of isotopes in a given sample.This technique has two different applications in the earth and environmental sciences. The analysis of 'stable isotopes' is normally concerned with measuring isotopic variations arising from mass-dependent isotopic fractionation in natural systems. On the other hand, radiogenic isotope analysis involves measuring the abundances of decay-products of natural radioactivity, and is used in most long-lived radiometric dating methods.

https://en.wikipedia.org/wiki/Isotope-ratio_mass_spectrometry

δ (in ‰) = [(Rsample - Rstandard) / R standard ] x 1000or

R / Rstd = if δ is in ‰

Define H = heavy L = light

NomenclatureReport Stable Isotope Abundance as ratio to Most Abundant Isotope (e.g. 13C/12C) - Why? The Ratio of Isotopes is What is Measured Using a Mass Spectrometer The Ratio Can Be Measured Very Precisely. The isotope ratio of a sample is reported relative to a standard using δ (“delta”) notation – usually with units of ‰ because the differences are typically small.

Example: 13C (in %o) = [ (13C/12C)sample / (13C/12C) standard ] – 1 x 1000

Example: If (13C/12C) sample = 1.02 (13C/12C) std

13C = 1.02 (13C/12C) std / (13C/12C) std - 1 x 1000

= 0.02 x 1000 = 20 %o

Standards Vary

Equilibrium isotopic fraction: it depends by the differences in the thermodynamics properties of the molecules characterized by different isotopes.

Normally, the the product will be lighter than the reactant.

Kinetic fractionation is an isotopic fractionation process that separates stable isotopes from each other by their mass during unidirectional processes. Biological processes are generally unidirectional and are very good examples of "kinetic" isotope reactions. All organisms preferentially use lighter isotopic species, because "energy costs" are lower, resulting in a significant fractionation between the substrate (heavier) and the biologically mediated product (lighter). As an example, photosynthesis preferentially takes up the light isotope of carbon 12C during assimilation of an atmospheric CO

2 molecule. This kinetic isotope fractionation explains why

plant material (and thus fossil fuels, which are derived from plants) is typically depleted in 13C by 25 per mil (2.5 per cent) relative to most inorganic carbon on Earth.

Rain is enriched in 18O due to the equilibrium isotope effect as 18O has stronger bonds in H2Owater than H2Ovapour

example

Ad esempio nel passaggio di stato da H2Oaq ad H2Ovap: H2Ovap sarà più omeno ricco di 16O rispetto all’acqua di partenza? Nella trasformazione da liquido a gas l’acqua si arricchisce di 16O (diventa più negativa, più leggera) per frazionamento cinetico. Nel ritrasformarsi in liquido (condensazione), la pioggia prende 18O (più pesante) per frazionamento di equilibrio, poiché 18O ha un legame più forte con H in H2Oaq. Il vapore rimanente è ancora più negativo.

Isotopic Fractionation

Fractionation Factor = A-B = RA / RB where R = ratio of two isotopes in materials A or B

often

= Rproducts / Rreactants

The state of unequal stable isotope composition within differentmaterials linked by a reaction or process is called “isotope fractionation”

Two kinds of Isotope Fractionation Processes

1. Equilibrium Isotope effects Equilibrium isotope fractionation is the partial separation of isotopes between twoor more substances in chemical equilibrium. Usually applies to inorganic species. Usually not in organic compoundsDue to slightly different free energies for atoms of different atomic weight Vibrational energy is the source of the fractionation. Equilibrium fractionation results

from the reduction in vibrational energy when a more massive isotope is substituted for a less massive one. This leads to higher concentrations of the heavier isotope in substances where the vibrational energy is most sensitive to isotope fractionation (e.g., those with the highest bond force constants)

If molecules are able to spontaneous exchange isotopes they will exhibit slightlydifferent isotope abundances at thermodynamic equilibrium (their lowest energy state)

For example: exchange reactions between light = Al, Bl and heavy = Ah, Bh

aA1 + bBh ↔ aAh + bB1

The heavier isotope winds up in the compound in which it is bound more strongly.Heavier isotopes form stronger bonds (e.g. think of like springs). If α = 1 the isotopes are distributed evenly between the phases. Example: equilibrium fractionation of oxygen isotopes in liquid water (l) relative to water vapor (g). H2

16O(l) + H218O(g) ↔ H2

18O(l) + H216O(g)

At 20ºC, the equilibrium fractionation factor for this reaction is: α = (18O/16O)l / 18O/16O)g = 1.0098

Example: The carbonate buffer system involving gaseous CO2(g), aqueous CO2 (aq), aqueous bicarbonate HCO3

- and carbonate CO32-.

An important system that can exhibit equilibrium isotope effects for bothcarbon and oxygen isotopes

13CO2(g) + H12CO3- ↔ 12CO2(g) + H13CO3

-

The heavier isotope (13C) is preferentially concentrated in the chemical compound with the strongest bonds. In this case 13C will be concentrated in HCO3

- as opposed to CO2(g). For this reaction has the form: H/L = (H/L)product / (H/L)reactants = (H13CO3

- / H12CO3-) / (13CO2 / 12CO2)

H/L = 1.0092 at 0ºC and 1.0068 at 30ºC

2. Kinetic FractionationNon-equilibrium – during irreversible reactions like photosynthesisOccurs when the rate of chemical reaction is sensitive to atomic mass Results from either differential rates of bond breaking or diffusion rates Compounds move at different rates due to unequal masses.Light are always faster. For kinetic fractionation, the breaking of the chemical bonds is the rate limiting step. Essentially all isotopic effects involved with formation / destruction of organic matter are kinetic There is always a preferential enrichment for the lighter isotope in the products. 12CO2 mw = 44 These must have the same kinetic energy (Ek = 1/2mv2)13CO2 mw = 45 so 12CO2 travels 12% faster than 13CO2. All isotope effects involving organic matter are kineticExample:12CO2 + H2O = 12CH2O + O2 faster13CO2 + H2O = 13CH2O + O2 slowerThus organic matter gets enriched in 12C during photosynthesis (13C becomes negative)

OXYGEN ISOTOPES

B.C. SchreiberU. Washington

Dept. Earth & Space Sciencemodified

Standards Vary

OXYGEN ISOTOPES

Oxygen isotope chemostratigraphyRelative concentrations (stable isotopes): 16O = 99.76%17O = 0.38% 18O = 0.21% Urey and Emiliani (1947) discovered that oxygen isotopes fractionate, depending largely on temperature. They examined shells of foraminiferans throughout the Pleistocene and O isotopes in the shells appeared to respond to the temperature changes associated with the ice ages, withoceanic sediments becoming isotopically heavy during glaciations and lighterduring warming.

BUT

Normally, when water evaporates, molecules with 16O more readilyenter the vapor phase. Typically this makes no difference as the water soon condenses and is back in the oceans. The same principle applies to atmospheric water. Condensation preferentiallydraws H2

18O out of the atmosphere. Results in 16O-rich (or 18O-depleted) polar snow. During times of high glacial activity, ocean waters are very enriched in 18O Organisms that incorporate oxygen-bearing molecules (such as CaCO3)into shells or bones will also be enriched in 18O. Therefore, a δ18O curve is a direct result of ice volume. So the δ18O curve is a fairly good proxy for temperature changes

SPECMAP. δ18O/16O bentonic foraminifers curve: ratio 18O/16O high = glacial phases, ratio 18O/16O low = interglacial phases

Planktonic forams measure sea surface TBenthic forams measure bottom T

Assumptions:1. Organism ppted CaCO3 in isotopic equilibrium with dissolved CO3

2-

2. The δ18O of the original water is known3. The δ18O of the shell has remained unchanged

18O of planktonic & benthic foraminifera:piston core V28-238 (160ºE 1ºN) Planktonic and Benthic foraminifera differ due todifferences in water temperature where they grow.

Estimation of temperature in ancient oceansCaCO3(s) + H2

18O CaC18OO2 + H2O The exchange of 18O between CaCO3 and H2OThe distribution is Temperature dependent

Ehleringer & Hall, 1993 Fig 5.3from lab experiments

18O in CaCO3 varies with Temperature

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO

3). The other polymorphs are the minerals aragonite and

vaterite. Aragonite will change to calcite at 380–470 °C, and vaterite is even less stable.

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO

3 (the other form being

the mineral calcite). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.Aragonite forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm- and cold-water corals (Scleractinia).

https://en.wikipedia.org/wiki/Aragonitehttps://en.wikipedia.org/wiki/Calcite

Complication: Changes in ice volume also influence δ18OMore ice = higher salinity = more δ18O left in the ocean

18O increases with salinity

Simplified from data in Dansgaard, 1964 & Rozanski, 1993

Where Rvapor / R liquid = f (-1) where f = fraction of residual vapor = Rl/Rv

Example: Evaporation – Condensation/Processes18O in cloud vapor and condensate (rain)plotted versus the fraction of remaining vaporfor a Raleigh process. The isotopic compositionof the residual vapor is a function of thefractionation factor between vapor and waterdroplets. The drops are rich in 18O so the vaporis progressively depleted in 18O .

Any isotope reaction carried out so that productsare isolated immediately from the reactants will showa characteristic trend in isotopic composition.

Raleigh Fractionation - Combination of both equilibrium and kinetic isotope effectsKinetic when water molecules evaporate from sea surfaceEquilibrium effect when water molecules condense from vapor to liquid form

Fractionation increases withdecreasing temperature

Distillation of meteoric water – large kinetic fractionation betweenocean and vapor. Rain, forming in clouds, is in equilibrium with vaporand is heavier that the vapor. Vapor becomes progressively lighter.D and 18O get lower with distance from source.

Water evaporation has a kinetic effect. Vapor is lighter than liquid. At 20ºC the difference is 9‰ (see Raleigh plot).Also the energy required for vaporization of H2

18O is greater than for H216O

Air masses transported to higher latitudes where it is cooler. water lost due to rain raindrops are rich in 18O relativeto cloud. Cloud gets lighter

OCEANIC WATER VAPOR MODIFIED OVER LAND

NASA, Earth Observatory, figure on-line

Isotope Fractionation

Enrichment process (isotope fractionation): The lighter 16O evaporates more easily. The heavier 18O is easier to condense out.

Atmosphere

The concentration of 18O in precipitation decreases with temperature. This graphshows the difference in 18O concentration in annual precipitation compared to theaverage annual temperature at each site. The coldest sites, in locations such asAntarctica and Greenland, have about 5 percent less 18O than ocean water.

(Graph adapted from Jouzel et. al., 1994)

OXYGEN ISOTOPES AND CLIMATE CHANGE

18O= 1.6o/oo

18O= 0.0

Oxygen isotoperatios as a

thermometer

Precipitation has less18O than ocean, why? 18O content of precipitation at the given latitude decreases with decreasing temperature.Why? The less 18O found in theglacier ice, the colder theclimate.