Early Diagenesis

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Early Diagenesis

• Carbonates
• Madelon van den Hooven

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Overview

• Introduction
• Dissolution of CaCO3
• Carbonate cycle
• Main depocenters for CaCO3
• Last comments

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The sediment zones

• Oxidation by O2
• Oxidation by NO2
• Oxidation by SO2
• Oxidation by Mn and Fe

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Dissolution of CaCO3 (1)

• Ca2 in porewater increases near the
  sediment-water interface.
• As a result, the alkalinity will also increase
  and that contributes to the net flux of
  alkalinity and SCO2.
• Conversely, in anoxic sediment layer, production
  of alkalinity by sulfate reduction.

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Dissolution of CaCO3 (2)

• Carbonate saturation state of the porewaters
• to high values.
• CaCO3 precipitates from the porewaters, because
  of a
• of the porewater calcium concentration at depth,
  in the sulfate reduction zone.

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Sulfate reduction results in

• A small decrease in pH (dissolution of CaCO3),
• But a net production of alkalinity.
• (CH2O)106(NH3)16H3PO4 53 SO42- ? 106 HCO3- 16
  NH4 53 HS- HPO42- H
• Alkalinity increases the saturation state of the
  porewaters and will lead to the precipitation of
  CaCO3.

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Study of the CaCO3 cycle

• CaCO3 dissolution in marine sediments process
  responsible for determining the carbonate
  chemistry of ocean water.
• Known since 1952 changes in the pattern of CaCO3
  preservation.
• Changes in atmospheric CO2 content in glacial ?
  variations in the oceans carbon cycle.

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Factors which control CaCO3 dissolution

• Particulate rain rates of CaCO3 and Corg to the
  ocean bottom.
• Degree of saturation of calcite and aragonite in
  bottomwaters.
• Dissolution rates of these minerals in
  undersaturated waters.
• The reaction with CO2

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Carbonate cycle

• Contact of the ocean with the atmosphere
• CO2 H2O ? H2CO3 Contact without
  atmosphere
• H2CO3 ? H HCO3-
• HCO3- ? H CO32-
• H2O ? H OH-
• CaCO3,calcite ? Ca2 CO32-
• 2 Ca2 H OH- HCO3- 2 CO32- open system
• Ca2 H2CO3 HCO3- CO32- closed
  system

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Inorganic carbon cycle in the ocean
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Shallow water environments
Area Global CaCO3 production
106 km2 1012 mol yr-1
Reefs 0.6 9-24
Carbonate platforms 0.8 4
Continental shelves 10106 1.5
Reefs are the most productive
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Shallow-waters

• CaCO3 production at
• Reefs 71012 mol yr-1 accumulate, rest (21012
  mol yr-1 ) undergoes physical erosion and
  offshore transport, also biological destruction.

• CO32- platforms production is mainly carried
  out by benthic red/green algae. Accumulation is
  difficult to assess.

• Continental shelves very little knowledge,
  because sediments are a mixture of modern and
  relictic components.

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Deep-sea sediments

• Can be sinks and sources for inorganic and
  organic carbon introduced by different pathways
  to the sediment surface
• Export production from surface waters, mass flows
  and resuspension.
• Responsible for maintaining the low atmospheric
  CO2-level.(CH2O)106(NH3)16(H3PO4) 138 O2 124
  CaCO3 230 HCO3- 16 NO3- HPO42- 124 Ca2
  16 H2OOrganic matter oxidation with Redfield
  CNP ratio

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Last comments (1)

• The reservoir sizes in the world ocean and
  exchange fluxes between reservoirs in the
  carbonate system is not in steady state.
• The total carbon release from deep-sea sediments
  is about 1201012 mol yr-1, but is subject to
  great uncertainty due to the complexity of the
  processes.

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Last comments (2)

• Both bottom water undersaturation and organic
  matter decay are responsible for CaCO3
  dissolution in the sediments at more or less
  equal levels.
• Most organic matter degradation above 1000m.

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Last comments (3)

• Pressure dependence on the solubility product
  determines the change in the saturation carbonate
  ion value with depth (in the water column).
• Most CaCO3 dissolution in the ocean occurs on the
  bottom.

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Literature

• A. Mucci, B. Sundby, M. Gehlen, T. Araaki, S.
  Zhong, N. Silverberg, The fate of carbon in
  continental shelf sediments of eastern Canada,
  Deep-sea Research II 47 (2000) 733-760
• R.R. Schneider, H.D. Schultz, C. Hensen, Ch.9
  Marine Carbonates Their formation and
  destruction
• S.R. Emerson, D. Archer, Calcium carbonate
  preservation in the ocean, Phil. R. Soc. Lond. A
  331, 29-40 (1990)

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