Beschreibung: Surface area measurements as well as organic carbon, nitrogen and phosphorus analyses on various grain size fractions of carbonate mud samples confirm that in natural environments of carbonate deposition, surface sorption processes take place which are similar to those described earlier for dissolved organics and artificially suspended calcite particles in both seawater and synthetic solutions. The specific surface area of the sediment increases from 1.8m2/g for the coarse-grained fraction to 12.5 m2/g for the fine material; likewise organic carbon, nitrogen and phosphorus increase with increasing surface area so that there are 1.20 mg C, 0.175 mg N and 0.06–0.20 mg P associated with every square meter of carbonate surface irrespective of the mineralogy of the sediment particles. It appears that the organic matter in these sediments is similar in composition, structure and quantity to the organic layers produced in sorption experiments. With their apparently defined structure and ubiquitous nature, these layers could determine the mineralogy and orientation of submarine carbonate cement or could even be a prerequisite to calcification in general.

Beschreibung: Humic acid dissolved in artificial seawater influenced the morphology, internal structure, and composition of aragonite when precipitation was induced with dilute Na2CO3 solution. At sodium humate concentrations of around 20 mg/1, numerous brownish spherical aragonite bodies developed within one day at 25 °C. The spheres ranged in size from 10–100 microns and resembled natural marine ooids. They formed with gentle agitation of the solution as well as with no water movement at all. The typical structure of natural ooids consisting of concentric alternating aragonite and organic laminae was experimentally duplicated as layers of aragonite crystals alternating with humate membranes; however, in contrast to natural ooids, the individual aragonite crystals here were oriented radially with their c-axes. The aragonite of the spheres contained about 20 wt. % more strontium than the aragonite precipitated experimentally without the addition of sodium humate, and organic carbon content of the spherical aragonite was about 7% by weight.

Beschreibung: Dissolved organic compounds interact with the surface of calcium carbonate minerals and effect simple inorganic equilibration between solution and solid. Organo-carbonate associations form between stearic acid and calcite and dolomite, and between albumin and aragonite, calcite and Mg-calcite. When stearic acid interacts with these minerals in hexane solution, there is evidence that the association results in a complete monolayer on the calcite surface and in half of a layer on the dolomite surface. Interaction is restricted to the calcium sites of the solid and the carboxyl group of the stearic acid. The same relationship exists between calcium and reactive organic sites when stearic acid and carbonate minerals interact in aqueous solution; however, the amount adsorbed is not sufficient to form a complete monolayer of pure stearic acid. Instead, the ion concentration products of pCa++ + pHCO3- + pStAc and pCa++ + pOH- + pStAc indicate that the aqueous solution is perhaps in equilibrium with one of two surface complexes of the above composition. Hydrated surface complexes appear to be large enough to completely cover the surface of carbonate minerals in aqueous solutions. At concentrations of approximately 25 mg of albumin per liter of water, carbonate minerals appear to adsorb sufficient albumin so that a complete monolayer is likely to form. At high concentrations and with increasing time and pH, multilayers or unoriented aggregates of albumin form on calcite and aragonite surfaces. Dissolved alanine does not associate with carbonate minerals at pH 6.12 and pH 9.87. Dissolved organic compounds interact with the surface of calcium carbonate minerals and effect simple inorganic equilibration between solution and solid. Organo-carbonate associations form between stearic acid and calcite and dolomite, and between albumin and aragonite, calcite and Mg-calcite. When stearic acid interacts with these minerals in hexane solution, there is evidence that the association results in a complete monolayer on the calcite surface and in half of a layer on the dolomite surface. Interaction is restricted to the calcium sites of the solid and the carboxyl group of the stearic acid. The same relationship exists between calcium and reactive organic sites when stearic acid and carbonate minerals interact in aqueous solution; however, the amount adsorbed is not sufficient to form a complete monolayer of pure stearic acid. Instead, the ion concentration products of pCa++ + pHCO3- + pStAc and pCa++ + pOH- + pStAc indicate that the aqueous solution is perhaps in equilibrium with one of two surface complexes of the above composition. Hydrated surface complexes appear to be large enough to completely cover the surface of carbonate minerals in aqueous solutions. At concentrations of approximately 25 mg of albumin per liter of water, carbonate minerals appear to adsorb sufficient albumin so that a complete monolayer is likely to form. At high concentrations and with increasing time and pH, multilayers or unoriented aggregates of albumin form on calcite and aragonite surfaces. Dissolved alanine does not associate with carbonate minerals at pH 6.12 and pH 9.87. Organo-carbonate associations affect the calcium carbonate equilibrium in solution by physically isolating the mineral surface and by reducing the surface free energy of the solid. Selective association with calcium sites and equilibration of the surface complex with the solution both could prevent simple dissolution reactions. In the natural environment, adsorbed unspecified surface-active compounds cause the total dissolved carbon concentration of the uppermost sea surface layer to increase up to fivefold as compared to the bulk surface sea water. Between 10 and 15% of this surface-active matter can be removed from seawater by adsorption on calcite surfaces. Organic compounds containing phosphorus are involved in this solid seawater interaction. At low calcite to seawater ratios, the adsorbed surface-active organic matter is visible on the surface of calcite and can be stained by Methylene Blue. The artificially produced organic coatings look the same as those observed on natural suspended carbonate minerals. At intermediate and high ratios, the seawater is depleted of surface-active organic matter. This allows uninhibited inorganic equilibration between seawater and carbonate minerals. Surface seawater, deep water, and interstitial water show inorganic equilibration at different calcite-to-seawater ratios. These ratios are constant for any seawater sample whether equilibrium is approached from under- or oversaturation. Recent carbonate sediments from the Bermuda platform show stainable organic coatings. The sediments do not interact with undersaturated seawater but dissolve after organic matter has been removed by H2O2. The natural stability conditions for organic coatings are unknown, but Pleistocene, Jurassic and Mississippian carbonate rocks contain stainable organic matter. In samples from the Bermuda Pleistocene, organic matter is localized on the surface of non-cemented carbonate particles and in pore spaces of carbonate rocks. The organic matter is stained with Methylene Blue in the micrite envelopes of an Upper Jurassic limestone sample from France and in pore space linings of a sample of Mississippian dolomite from Indiana. All organic matter encountered in pre-Recent rocks is associated with clay minerals that interfere with the Methylene Blue staining technique. Because of this interference, conclusions concerning effects of organic coatings on limestone diagenesis are tenative. It is believed that organic coatings as well as organo-clay associations on the surface of carbonate mineral particles are responsible for lack of equilibration. This is evident from the textures and mineralogy of carbonate rocks. Lack of grain enlargement (recrystallization), persistence of metastable carbonate phases (aragonite), and blocking of calcite precipitation nucleii during cementation of limestones are the major diagenetic processes which are affected by organo-carbonate associations.

Beschreibung: Carbon dioxide, ammonia, and reactive phosphate in the interstitial water of three sediment cores of the West African continental margin result from oxidation of sedimentary organic matter by bacterial sulfate reduction. The proposed model is a modification of one initially suggested by Richards (1965) for processes in anoxic waters: (CH2O)106 (NH3)8 (H3PO4) (0.7-0.2) + 53 SO4**2- =106 CO2 + 106 H20 + 8 NH3 + (0.7 - 0.2) H3PO4 + 53 S**2- The amount of reduced interstitial sulfate, the carbon-to-nitrogen-to-phosphorus atomic ratio of the sedimentary organic matter, as well as small amounts of carbon dioxide, which precipitated as interstitial calcium carbonate, are included in the general oxidation-reduction reaction. Preferential loss of nitrogen and phosphorus from organic matter close to the surface was recorded in both the interstitial water and sediment composition. It appeared that in deeper sections of the core organic carbon compounds were oxidized which were probably in an even lower oxidation state than that indicated by the proposed model. An estimated 2 % of the amount of organic matter still present was oxidized after it became incorporated into the sediment; whereas sulfide sulfur contents indicate that a much larger percentage (15-20%) seemed to have been subject to bacterial oxidation during the Pleistocene period, when a very thin oxidizing layer on the sediment allowed the above decomposition process to start relatively early favoured by almost fresh organic matter, and by almost unrestricted exchange of sulfate with the overlying water.