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Environmental Geochemistry. January 26, 2007. What is geochemistry?. The study of -chemical composition of the Earth and other planets -chemical processes and reactions that govern the composition of rocks and soils

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what is geochemistry
What is geochemistry?

The study of

-chemical composition of the Earth and other planets

-chemical processes and reactions that govern the composition of rocks and soils

-the cycles of matter and energy that transport the Earth's chemical components in time and space

-and their interaction with the hydrosphere and the atmosphere.

outline of topics
Outline of Topics
  • Formation of the elements
  • Composition of Earth
  • Aqueous Solutions
  • Chemical Equilibrium
  • Acid-Base Equilibria
  • Redox
  • Biogeochemistry
  • Stable Isotopes

(with comments on weathering, sorption, pollution…)

aqueous solutions
Aqueous Solutions

Water is special


Ionic Strength

I = 1/2 ∑mz2

chemical equilibrium
Chemical Equilibrium

Exists when a system is in a state of minimum energy (G)

  • - Often not completely attained in nature (e.g., photosynthesis leaves products out of chemical equilibrium)
  • - A good approximation of real world
  • Gives direction in which changes can take place (in the absence of energy input.)
  • Systems, including biological systems, can only move toward equilibrium.
  • -Gives a rough approximation for calculating rates of processes because, in
  • general, the farther a system is from equilibrium, the more rapidly it will move
  • toward equilibrium; however, it is generally not possible to calculate reaction rates from thermodynamic data.
acid base equilibria





Acid-Base Equilibria

Bronsted-Lowry definition: acid donates H+; base accepts H+

In aqueous systems, all acids stronger that H2O

generate excess H+ ions (or H3O+); all bases stronger

than H2O generate excess OH-

acid base

Many reactions influence pH

Photosynthesis and respiration are acid base reactions.

aCO2(g) + bNO3- + cHPO42- + dSO42- + f Na+ + gCa2+ + hMg2+ iK+ + mH2O + (b + 2c + 2d -f -2g - 2h - i)H+<-----> {CaNbPcSdNafCagMghKiH2Om}biomass + (a + 2b)O2

Oxidation reactions often produce acidity.

Reduction reactions consume acidity

pH influences many processes

-weathering (Fe and Al more soluble at lower pH)

-cation exchange (leaching of base cations from soil due to acid rain)

-sorption(influences surface charge on minerals and therefore what sticks to them)

acid base14

Alkalinity ≈ ANC

Alkalinity = ∑(base cations) - ∑(strong acid anions)

Any process that affects the balance between base cations and acid anions must affect alkalinity.

  • The oxidation state of an atom is defined with the following
  • convention:
  • The oxidation state of an atom in an elemental form is 0.
  • In O2, O is in the 0 oxidation state.
  • When bonded to something else, oxygen is in oxidation
  • state -2 and hydrogen is in oxidation state of +1 (except for
  • peroxide and superoxide).
  • In CO32-, O is in -2 state, C is in +4 state.
  • The oxidation state of a single-atom ion is the charge on
  • the ion.
  • For Fe2+, Fe is in +2 oxidation state.

Redox reactions tend to be slow and are often out of thermodynamic equilibrium - but life exploits redox disequilibrium.

Oxidation - lose electrons

Reduction - gain electrons

Fe was oxidized, Mn was reduced

why do we care about redox rxns
Why do we care about redox rxns?

Oxidation state can impact

  • Sorption/desorption
  • Solubility
  • Toxicity
  • Biological uptake


Measure of oxidation-reduction potential gives us info about chemical species present and microbes we may find.

accumulation of o 2 in the atmosphere
Accumulation of O2 in the Atmosphere

Fe2+ = Fe(II) = slightly soluble in sea water with no O2 present

Add O2 - oxidizes Fe(II)-->Fe(III)

Very small [O2] required

Fe3+ = Fe(III) = extremely insoluble in water

Essentially all of the oxygen in the atmosphere came from photosynthesis



ammonia→ nitrite → nitrate


nitrate → nitrite → nitric oxide → nitrous oxide → N2

N Fixation

N2 →ammonia

what is an isotope
What is an isotope?
  • Isotope- line of equal Z. It has the same # protons (ie. they are the same element) but a diff. # of neutrons.








how did all this stuff get here
How did all this stuff get here?
  • 4 types of isotopes, based on how they formed:
    • Primordial (formed w/ the universe)
    • Cosmogenic (made in the atmosphere)
    • Anthropogenic (made in bombs, etc)
    • Radiogenic (formed as a decay product)
stable isotopes
Stable Isotopes

Light isotopes are fractionated during chemical reactions, phase changes, and biological reactions, leading to geographical variations in their isotopic compositions

FRACTIONATION: separation between isotopes on the basis of mass (usually), fractionation factor depends on temperature

Bonds between heavier isotopes are harder to break

stable isotope examples
Stable Isotope Examples
  • Rayleigh fractionation: light isotopes evaporate more easily, and heavy isotopes rain out more quickly

d= {(Rsample – Rstandard) / Rstandard} x 103

stable isotope examples27
Stable Isotope Examples
  • d18Ocarbonate in forams depends on d18Oseawater as well as T, S
  • d18Oseawater depends on how much glacial ice there is
    • Glacial ice is isotopically light b/c of Rayleigh fract.
    • More ice means higher


stable isotopes28
Stable Isotopes
  • C inorganic matter, fossil fuels, and hydrocarbon gases is depleted in 13C ==> photosynthesis
    • used as an indicator of their biogenic origin and as a sign for the existence of life in Early Archean time (~ 3.8 billion years ago)
  • N isotopic composition of groundwater strongly affected by isotope fractionation in soils plus agricultural activities (use of N-fertilizer and discharge of animal waste)
  • Particulate matter in ocean enriched in 15N by oxidative degradation as particles sink through water column
    • Used for mixing and sedimentation studies
  • S isotopes fractionated during reduction of SO42- to S2- by bacteria
    • didn’t become important until after ~2.35 Ga when photosynthetic S-oxidizing bacteria had increased sulfate concentration in the oceans sufficiently for anaerobic S-reducing bacteria to evolve (photosynthesis preceded S-reduction which was followed by O respiration)
stable isotope examples29
Stable Isotope Examples
  • Stable isotopes can also tell you about biology
  • Organisms take up light isotopes preferentially
  • So, when an organism has higher

30Si, it means that it was feeding from a depleted nutrient pool

stable isotopes30
Stable Isotopes
  • Boron isotopes measured in forams used for paleo-pH
    • d11B depends on pH
    • (Gary Hemming)
  • Nitrogen isotopes used for rapid temp. changes in ice cores
    • d15N depends on temp. gradient in firn
    • (Jeff Severinghaus)
  • Stable isotopes are also used to study magmatic processes, water-rock interactions, biological processes and anthropology and various aspects of paleoclimate






(WM White Geochemistry Ch9 - Stable Isotopes)

Isotopes: Principles and Applications - Faure & Mensing

How to Build a Habitable Planet - Wally Broecker