Carbonate dissolution biogenic carbonates

Walter and Morse (1984) were able to document the relative importance of microstructure for the dissolution of biogenic carbonates. Biogenic magnesian calcites are structurally disordered and chemically heterogeneous. Both these factors play a role in the reactivity of these minerals in natural systems. 

The sulfete and chloride minerals in evaporites (gypsum, anhydrite, halite) undergo congruent dissolution to produce Ca (aq), S04 (aq), Na (aq), and Cl (aq). The dissolution of evaporite and biogenic carbonates (limestone, dolomite, and calcite) generates... 

Figure 2.15. Log rate constant, k, as a function of log reciprocal diameter for dissolution of various biogenic carbonates a comparison with the geometric model. (After Walter and Morse, 1984b.)...

Figure 7.6. Relative dissolution rates of biogenic carbonates in seawater undersaturated with respect to calcite. (After Walter, 1985.)...

Figure 1. CO2 partial pressure control on carbonate dissolution A) effect of temperature on biogenic soil pCOz and total soil pCOz, soil pC-Oi is lower than biogenic pCOi due to inhibition effects of high CO2 or low oxygen levels (Drake, 1983) and, B) equilibrium concentrations of Ca with respect to calcite for open, closed, and restricted air volume systems Equilibrium pCOi values below atmospheric equilibrium is...

The interaction between the carbonate cycle and the organic carbon cycle takes place under a variety of circumstances. At one end of the spectrum, carbonate chemistry may be under direct enzymic control (see Chapter 2.2). It may take place within cells, within organisms, or within micro-environments in close contact with living tissues (e.g., molluscan mantle). At the other extreme, where products of metabolic activities modify the overall chemistry of the environment, carbonate dissolution or precipitation may be influenced indirectly. The closer the contact between the organism and the substrate, the more specific are the biogenic dissolution and crystallization patterns that remain as traces of biological activity in sediments. 

Throughout Earth s history life has continuously modified the environment so that it is unusual today to encounter any chemical process that is not in some way influenced by it. Biogenically increased concentrations of CO2, oi anic acids, and chelating agents all affect carbonate dissolution in natural waters. 

Several studies support the idea that the chemical composition of biogenic carbonate is modified by partial dissolution under the infiuence of bottom waters or porewaters that are undersaturated with respect to calcium carbonate (e.g. Rosenthal et al. 2000). In a study of planktonic foraminifera conducted by Brown Elderfield (1996), artificial partial dissolution of G. tumida tests resulted in a decrease in both Mg/Ca and Sr/Ca, but there was no significant change for G. sacculifer. The cause of this reduction is thought to be preferential dissolution of high-Mg inner (chamber) calcite, which forms in warmer waters than the low-Mg calcite crust (keel). In theory, it should be possible to correct for such effects if the extent of dissolution can be quantified. A number of dissolution indicators, including foraminiferal test weights (e.g. Broecker Clark 2001) and calcite crystallinity (Bassinot et al. 2004), have been tested, but none have been widely applied to date. 

The incorporation of oxygen isotopes into biogenic carbonate is complex and as Wefer et al. (1999) point out, the 8 0 value of a carbonate is a function of temperature, ice volume, selective growth in space and time, vital effects and post-depositional dissolution. The carbonate chemistry and pH of a benthic foraminifera s microenvironment or microhabitat also plays an important role in test isotopic composition (e.g. Spero et al. 1997 Bemis et al. 1998 Bijma et al. 1999 Wolf-Gladrow et al. 1999), with a 0.2 increase in pH resulting in a 8 0 shift of approximately — 0.22%o (Zeehe 1999). 

Escanaba Trough has a range from -12.2 to -3.9%o, and 5 C values increase with CO2 content, presumably due to increased magmatic contributions (Taylor 1992). The vent fluids at Middle Valley show a range from -38.9 to -10.6%o, indicating substantial contributions from pyrolysis of organic carbon in the sediments and dissolution of detrital biogenic carbonates (Taylor 1992). 

Murray RW, Leinen M, Isem AR (1993) Biogenic flux of A1 in the central equatorial Pacific Ocean Evidence for increased productivity during glacial periods. Paleoceanogr 8 651-670 Murray RW, Knowlton C, Leinen M, Mix AC, Polski CH (2000) Export production and carbonate dissolution in the central equatorial Pacific Ocean over the past 1 Ma. Paleoceanogr 15 570-592 Nancollas GH, Amjad Z, Koutsoukas P (1979) Calcium phosphates-speciation, solubility, and kinetic considerations. Am Chem Soc Symp Ser 93 475-497... 

Compaction of mudstones is, as for sand, mostly a function of effective stress at low temperatures (<80°C) (Aplin et al. 1999, Bjprlykke 1997). Carbonate cementation sourced by biogenic carbonate may, however, occur at rather shallow depth so that carbonate concretions or layers can be rendered almost incompressible at shallow depth. The precipitation of quartz cement in mudstones follows much the same principles as in sandstones and is primarily controlled by the temperature. Dissolution of biogenic silica has a strong effect as a source of silica on quartz cementation at temperatures exceeding about 80 C. When modelling the compaction of mudstones the... 

Li XG, Song JM, Yuan HM, Li FY, Sun S (2005) High contents of biogenic silicate in Jiaozhou Bay sediments-evidence of Si-hmitation to phytoplankton primary production. Oceanol Limnol Sin 36(6) 92-98 (in Chinese with English abstract) Li XJ, Chen F, Liu J, Huang XH (2004) Distribution and its dissolution of carbonate in seafloor surface sediment in the western South China Sea. Geochimica 33(3) 254-260 (in Chinese with English abstract)... 

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