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Seawater is a complex solution of inorganic ions and organic molecules in contact with solid phases. Because of its reactivity and sorptive properties, some of the organic matter (OM) will directly affect the kinetics (and possibly the equilibria) of inorganic reactions by modifying the rates and types of reactions occurring between the inorganic ions and the solids. The interaction of natural OM with CaCO3 systems occurs in two ways: (1) adsorption of the OM to CaCO3 surfaces, and (2) complexation or chelation of free cations by dissolved or adsorbed OM. Both processes involve polar functional groups on the organic molecules, with the carboxylate anion (-COO-) being the most likely interacting species, although other functional groups may also be important.
OM associated with a variety of skeletal and nonskeletal CaCO3, including skeletal organic matrix, OM within ooids, and OM extracted from carbonate grain surfaces, was studied for chemical characterization, adsorption phenomena, and cation-binding ability. Skeletal OM is largely protein, whereas ooid and adsorbed OM are humic substances with proteinaceous components comprising about one-third of the composition. Aspartic acid is the most abundant amino acid in both skeletal protein and the humic substances. Conversely, OM associated with noncarbonate
sediments is poor in proteinaceous constituents and relatively depleted in aspartic acid.
Aspartic acid-rich protein and humic substances bind or complex with metal ions in proportion to the concentration of carboxyl groups present. Blockage of carboxyl groups to make them inactive destroys the ability of the OM to bind metal ions. Many, if not most, of the carboxyl groups available for metal-ion complexation in both calcified protein and aspartic acid-rich humic substances are on aspartic acid. Thus, this amino acid provides a significant portion of the metal-binding ability of the different types of OM.
Aspartic acid-rich OM is preferentially adsorbed by calcite compared to quartz. Again, the carboxyl group is the likely function to be involved in this adsorption. Blockage of carboxyl groups significantly reduces the ability of humic substances to adsorb to calcite. The similarity in geometry, charge, and composition enables the carboxylate anion (-COO-) to substitute for the carbonate anion (CO3=) in complexing calcium ion or adsorbing to calcite surfaces.
Competition between organic and inorganic ions for dissolved species and surface adsorption sites is driven by the requirement of the system to remain electroneutral. Concentration variations in dissolved organic and inorganic ions in the pore waters brought about by bioturbation and organic and inorganic diagenesis result in variations in the tendency of OM to affect the chemistry of the system. Most of the CaCO3 formed in the marine environment consists of skeletal material. This CaCO3 contains proteinaceous OM that is thought to be involved in formation of the mineral phase (biological calcification). By analogy, naturally-occurring OM of somewhat similar composition and properties and with identical functional groups may also be involved in the precipitation f CaCO3 in the sedimentary environment (geological calcification).
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