Carbonates Madelon van den Hooven Early Diagenesis Carbonates Madelon van den Hooven

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

The sediment zones Oxidation by O2 Oxidation by NO2 Oxidation by SO2 Oxidation by Mn and Fe Upper part of the sediment: microbially mediated oxidation by O2 and nitrate, produces acidity and may lead to the dissolution of CaCO3. Suboxic and anaerobic subsurface layers of the sediment: oxidation of organic matter by Mn and Fe produces alkalinity and may induce the precipitation of CaCO3.

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. The dissolution of CaCO3 is most strongly reflected by an increase in porewater calcium concentrations near the sediment-water interface. It creates a calcium gradient in the oxic sediment surface layer. Total and carbonate alkalinity also increase as a result of the dissolution and contribute to the net flux of alkalinity and ECO2 to the overlying waters. Conversely, in the anoxic portion of the sediments, the saturation state of the porewaters increases at depth in response to the production of alkalinity by sulfate reduction.

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. The saturation state of the porewaters increases to high values, well beyond aragonite saturation, or is poised at or near calcite or aragonite saturation; then CaCO3 precipitates from the porewaters, because of a decrease of the porewater calcium concentration at certain depth, in the sulfate reduction zone.

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. Sulfate reduction results in a small decrease in pH but a net production of alkalinity. The decrease in pH would normally promote the dissolution of CaCO3, but the buildup of alkalinity increases the saturation state of the porewaters and will lead to the precipitation of CaCO3. The reaction describes the microbially mediated organic matter oxidation by sulfate reduction. The supersaturated state of the porewaters is determined by a steady state between the precipitation rate of CaCO3 and the alkalinity production rate from sulfate reduction. The precipitation of CaCO3 in the sulfate reduction zone creates an additional source of CO2 to the porewaters but is an insignificant source to the overlying waters since most of this CO2 will be neutralized by CaCO3 as it migrates up through the oxic, CaCO3 undersaturated zone. There is a good linear correlation between the fraction of organic carbon oxidized by sulfate reduction and the porewater sulfate concentration gradient between the sediment-water interface and a depth of 30 cm.

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. CaCO3 dissolution in marine sediments: is the process which is responsible for determining the carbonate chemistry of ocean water. It creates the largest buffer for neutralizing anthropologically produced CO2. The CO2 produced by people can by transported to the sediments.

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 Factors which control CaCO3 dissolution are: Particulate rain rates of CaCO3 and Corg to the ocean bottom Degree of saturation of the minerals calcite and aragonite in bottomwaters overlying the sediments . The degree of undersaturation of calcite and aragonite in deep-ocean waters. Dissolution rates of these minerals in undersaturated waters and the reaction with respiratory carbon dioxide.

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 Reactions………… Upon making contact with the atmosphere, the partial pressure of carbon dioxide determines the concentration of carbonic acid in solution. The aqueous complex H2CO3 dissociates into protons and bicarbonate ions. The second step of dissociation is determined by: Furthermore, the dissociation of water needs to be included: The contact made by processes of dissolution and precipitation with the solid phase of calcite is described by the reaction: A system which is closed to CO2 exists wherever the final calcite-carbonate-equilibrium is reached without any concurrrent uptake or release of atmospheric CO2 in its process. This implies that the first reaction is no longer valid. In his stead, a balance of various C-species is related to calcium. This balance maintains that the sum of C-species must equal to calcium concentration in solution, since both can enter the solution only by dissolution of calcite or agaronite.

Inorganic carbon cycle in the ocean 24.5 Shelf Carbonate Production 5 Slope Carbonate Production 60-90 Pelagic Carbonate Production River Input Surface Waters 14.5 Shallow-water Carbonates 6 Sloop Sediments 11 Pelagic Sediments Sea Floor Deep Sea 5 Ocean Alkalinity Production and accumulation of marine inorganic carbon in the modern ocean (x 1012 mol yr-1) Main depocenters for CaCO3 are the shallow-water environments, Sloop sediments and the pelagic calcareous sediments. The shallow-water environments, which consists of reefs, carbonate platforms and continental shelves.

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 10*106 1.5 Reefs: occupy an area of 0.6*106 km2 and are considered as the most productive carbonates environments In the open ocean the production of the pelagic sediments can not be determined precisely. The estimates of several researchers are not near each other: 20 to 90 *1012 mol yr-1. Reefs are the most productive

Shallow-waters 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. The classical picture of shallow water carbonates was suggesting that most of their formation was restricted to trpical and subtropical resions within 22oC. But now it has evident that a significant amount of carbonayte can also be formed as cool-water carbonate banks and reefs in temperate and cond latitudes. The portion of cool-water carbonate on the total amount of CaCO3 accumulation in shallow water environments is still questionable. In contrast to reefs, the carbonate platforms production is mainly carried out by benthic red/green algae. Accumulation of platform carbonate is difficult to assess because a lot of it is dissolved or can be found as exported material in several 10 to 100 m thick sediment wedges of Holocene age at fringes of the platforms. Continental shelves: very little knowledge, because sediments are a mixture of modern and relictic components.

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 H2O Organic matter oxidation with Redfield C:N:P ratio The equation refers to organic matter oxidation with Redfield C:N:P ratio and complete reaction of respiratory CO2 with sedimentary calcium carbonate. The diagenetic reactions are most intense at the sediment-water interface and within the upper centimeters of the sediment column where the most labile components become rapidly mineralized. Therefore the determination between burial and recycling is made very close to the sediment-water interface. The driving forces of carbon release from the sediments are the degradation of organic matter and the dissolution of calcium carbonate which are in turn dependent on the supply of organic matter and calcium carbonate in form of carbonate skeletons.

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. Is estimated about 120 *1012 of the processes controlling carbon remobilization.

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.

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.

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)