10. RELATIONSHIP BETWEEN POROSITY, ELECTRICAL CONDUCTIVITY, AND CATION EXCHANGE CAPACITY IN BARBADOS WEDGE SEDIMENTS 1Pierre Henry 2 |
ABSTRACTMeasurements of grain densities in a shore-based laboratory show that the high dispersion of measurements obtained on board the JOIDES Resolution is probably related to insufficiently precise volumetric measurements, but that the effect of this imprecision on porosity determinations is generally less than 5%. A correlation is found between the total water content of the samples and their cation exchange capacity (CEC). This correlation confirms that smectites strongly influence the ability of the sediment to retain both adsorbed and pore water. Chemical analysis show that (1) interlayer cations are dominantly Na, (2) divalent cations (Mg, Ca) take up to 30%-40% of the surface charge, and (3)1/5 to more than 1/3 of the water present in the sample is chloride free. This chloride-free water corresponds to smectite interlayer water and to water adsorbed on external surfaces. Electrical resistivity logs as well as measurements on samples indicate a sharp decrease in resistivity in the transition from lithologic Units II to III. Lithologies with the highest smectite content (and highest CEC) have the lowest electrical conductivities at a given porosity. This result may in part be explained by water and ion adsorption in the smectite interlayer spaces but also leads to an unsolved question: Do 0.6-nm smectite interlayer spaces have the same conductance as the external surfaces of the particles? |
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