Environmental Geochemistry January 26 2007 What is geochemistry

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 -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 • • Formation of the elements Composition of Earth Aqueous Solutions Chemical Equilibrium Acid-Base Equilibria Redox Biogeochemistry Stable Isotopes (with comments on weathering, sorption, pollution…)

Formation of the Elements

Composition of Earth

Aqueous Solutions Water is special

Ionic Strength I = 1/2 ∑mz 2

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.

Reaction Rates/Equilibrium

Acid-Base Equilibria Bronsted-Lowry definition: acid donates H+; base accepts H+ In aqueous systems, all acids stronger that H 2 O generate excess H+ ions (or H 3 O+); all bases stronger than H 2 O generate excess OH- 2 3 3 3

Acid-Base Many reactions influence p. H Photosynthesis and respiration are acid base reactions. a. CO 2(g) + b. NO 3 - + c. HPO 42 - + d. SO 42 - + f Na+ + g. Ca 2+ + h. Mg 2+ i. K+ + m. H 2 O + (b + 2 c + 2 d -f -2 g - 2 h - i)H+<-----> {Ca. Nb. Pc. Sd. Naf. Cag. Mgh. Ki. H 2 Om}biomass + (a + 2 b)O 2 Oxidation reactions often produce acidity. Reduction reactions consume acidity p. H influences many processes -weathering (Fe and Al more soluble at lower p. H) -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-Base Alkalinity ≈ ANC Alkalinity = ∑(base cations) - ∑(strong acid anions) Any process that affects the balance between base cations and acid anions must affect alkalinity.

Redox 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 O 2, 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 CO 32 -, 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 Fe 2+, 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? Oxidation state can impact 1. Sorption/desorption 2. Solubility 3. Toxicity 4. Biological uptake etc. Measure of oxidation-reduction potential gives us info about chemical species present and microbes we may find.

Accumulation of O 2 in the Atmosphere Fe 2+ = Fe(II) = slightly soluble in sea water with no O 2 present Add O 2 - oxidizes Fe(II)-->Fe(III) Very small [O 2] required Fe 3+ = Fe(III) = extremely insoluble in water Essentially all of the oxygen in the atmosphere came from photosynthesis

Biogeochemistry

Nitrification ammonia→ nitrite → nitrate Denitrification nitrate → nitric oxide → nitrous oxide → N 2 N Fixation N 2 →ammonia

What is an isotope? 14 N 15 N 12 C 13 C 14 C 10 B 11 B • 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? • 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 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 • Rayleigh fractionation: light isotopes evaporate more easily, and heavy isotopes rain out more quickly = {(Rsample – Rstandard) / Rstandard} x 103

Stable Isotope Examples • 18 Ocarbonate in forams depends on 18 Oseawater as well as T, S • 18 Oseawater depends on how much glacial ice there is – Glacial ice is isotopically light b/c of Rayleigh fract. – More ice means higher 18 Oseawater

Stable Isotopes • C in organic matter, fossil fuels, and hydrocarbon gases is depleted in 13 C ==> 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 15 N by oxidative degradation as particles sink through water column – Used for mixing and sedimentation studies • S isotopes fractionated during reduction of SO 42 - to S 2 - by bacteria – didn’t become important until after ~2. 35 Ga when photosynthetic Soxidizing 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 Examples • Stable isotopes can also tell you about biology • Organisms take up light isotopes preferentially • So, when an organism has higher 30 Si, it means that it was feeding from a depleted nutrient pool

Stable Isotopes Boron isotopes measured in forams used for paleo-p. H 11 B depends on p. H (Gary Hemming) Nitrogen isotopes used for rapid temp. changes in ice cores 15 N 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

References http: //mineral. gly. bris. ac. uk/Geochemistry/ http: //mineral. gly. bris. ac. uk/envgeochem/ http: //www. soest. hawaii. edu/krubin/gg 425 -sched. html http: //geoweb. tamu. edu/courses/geol 641/notes. html http: //www. imwa. info/Geochemie/Chapters. HTML (WM White Geochemistry Ch 9 - Stable Isotopes) Isotopes: Principles and Applications - Faure & Mensing How to Build a Habitable Planet - Wally Broecker