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Early Diagenesis

Early Diagenesis. Carbonates Madelon van den Hooven. Overview. Introduction Dissolution of CaCO 3 Carbonate cycle Main depocenters for CaCO 3 Last comments. The sediment zones. Oxidation by O 2 Oxidation by NO 2 Oxidation by SO 2 Oxidation by Mn and Fe. Dissolution of CaCO 3 (1).

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Early Diagenesis

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  1. Early Diagenesis Carbonates Madelon van den Hooven

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

  3. The sediment zones • Oxidation by O2 • Oxidation by NO2 • Oxidation by SO2 • Oxidation by Mn and Fe

  4. 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 ΣCO2. • Conversely, in anoxic sediment layer, production of alkalinity by sulfate reduction.

  5. 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.

  6. 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.

  7. 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.

  8. 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

  9. Carbonate cycle Contact of the ocean with the atmosphere: CO2 + H2O  H2CO3Contact 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

  10. 24.5 Shelf Carbonate Production 5 Slope Carbonate Production 60-90 Pelagic Carbonate Production River Input Surface Waters 5 Ocean Alkalinity Deep Sea Sea Floor 11 Pelagic Sediments 14.5 Shallow-water Carbonates 6 Sloop Sediments Production and accumulation of marine inorganic carbon in the modern ocean (x 1012 mol yr-1) Inorganic carbon cycle in the ocean

  11. Shallow water environments Reefs are the most productive

  12. Shallow-waters • Continental shelves: very little knowledge, because sediments are a mixture of modern and relictic components. • CaCO3 production at: • Reefs: 7*1012 mol yr-1 accumulate, rest (2*1012 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.

  13. 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 C:N:P ratio

  14. 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 120*1012 mol yr-1, but is subject to great uncertainty due to the complexity of the processes.

  15. 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.

  16. 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.

  17. 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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