January 25. Biogeochemistry

CHEMICALS IN THE ENVIRONMENT: OCEAN CHEMISTRY

• to produce organic matter you need:
• CO₃^2-
• NO₃^1-
• PO₄^3-
• decomposition (oxidation) turns CO₃, NO₃ and PO₄ into H₂0, and O₂ into air
• production of organic matter takes place in the 'autotrophic' (photic) zone; top ~200 m, <5% of the ocean volume
• portion of organic matter exported down to 'heterotrophic' zone depletes autotrophic zone of CO₃, NO₃ and PO₄
• the existing relationship between the statistical proportions of C, N and P compounds entering the marine biochemical cycle and their relative availability in the water suggests that the organisms have produced and are maintaining these ratios: REDFIELD RATIOS
• CAUSE?:
• coincidence dependent on accidents of geochemical history
• vanishingly small probablity (and untestable)
• passive adaptation on part of organisms
• phytoplankton can vary their composition based on availability of N and P, but not O
• organic processes control proportions of these elements in seawater
• most likely explanation

CHEMICALS IN ORGANISMS: SKELETAL MINERALOGY

• the chemistry of fossil remains can be used as a proxy indicator of environmental conditions
• limited by diagenesis
• MAJOR BIOMINERALS
• carbonate polymorphs: calcite, aragonite
• opaline silicates
• phosphates (e.g. calcium phosphate)
• MINOR BIOMINERALS
• magnetite caps on chiton teeth
• dolomite caps on echinoid teeth
• fluorite gizzard plates in gastropods
• gypsum crystals in jellyfish statoliths
• many skeletons are mixtures of aragonite and calcite
• rare instances of mixtures of aragonite and opaline silicate (sclerosponges)
• phosphate fish skeletons have aragonite otoliths (ear bones)
• minor minerals are present as separate architectural elements, not blended with major minerals; suggests genetic control
• Why calcium carbonate is the most common biomineral

1. shallow sea water is saturated or supersaturated w/r/t CaCO₃; precipitates inorganically in some places
2. more readily controlled biologically; concentration of CO₃^2- strongly dependent on pH and concentration of CO₂; these are readily controlled by metabolic processes
3. dissolved Si^2- and PO₄^3- are present in much lower concentrations and require larger quantities of sea water to precipitate the same mass of mineral
4. shallow seawater not saturated w/r/t SiO₂ or Ca₃(PO₄)₂; must be strong biological intervention to precipitate opaline or phosphatic skeletons

RELATIVE STABILITY OF CARBONATES

• aragonite is unstable at surface temperatures and pressures because it has a higher free energy of formation than calcite
• the presence of Mg stabilizes aragonite; Mg is abundant in the marine environment
• MgCO₃ is often co-precipitated with calcite and aragonite
• high-Mg calcite is less stable (more soluble) than aragonite
• calcite has 6-fold coordination (each Ca ion is bonded to 6 other moieties)
• aragonite has 8-fold coordination; Ca is barely big enough to manage this, which is why aragonite is less stable than calcite
• Mg/Ca ratio is higher in calcite than aragonite under equal temperature, pressure etc. conditions because Mg fits into 6-fold configuration better

ARAGONITE VERSUS CALCITE

• Group 1: aragonitic organisms that are more common in the tropics; e.g. scleractinian corals
• Group 2: aragonitic organisms confined to tropics; e.g. calcareous green algae
• Group 3: combined aragonite/calcite skeletons with increasing aragonite with increasing temperature; e.g. molluscs, annelids, cnidarians, bryozoans

TRACE CHEMISTRY

• impurities in biominerals present in solid solution, adsorbed on crystal face, incorporated into organic matrix or as separate mineral phases
• concentration is a function of:

1. physical chemistry of skeletal formation process
2. environmental variables
3. physiology
4. diagenetic processes

• most research has been done on carbonates
• all other factors being equal, trace cation to Ca cation ratio in the biomineral will equal that ratio in the environment

Me=molar concentration of trace element

K= proportionality constant or distribution or partition coefficient

[Me/Ca]_skeleton = K [Me/Ca]_seawater

1. If [Me/Ca]_fossil and K are known, then [Me/Ca]_water can be determined
2. What if K is not constant, but temperature dependent? If [Me/Ca]_fossil and K are known, then paleotemperature may be determined. Can be complicated by kinetic and biochemical effects.