GEOCHEMISTRY OF CARBONATE CEMENTS IN SURFICIAL ALLUVIAL CONGLOMERATESAND THEIR PALEOCLIMATIC IMPLICATIONS, SULTANATE-OF-OMAN

Early diagenetic carbonate cements are a common feature of Quaternary alluvial conglomerates in Oman. Cements are formed in the vadose and, more commonly, phreatic zones from near-surface groundwaters. In drain age areas underlain by the Semail Ophiolite, groundwaters have Mg2+/Ca 2+ ratios greater than two, and cements are often dolomite or high-mag nesium calcite in addition to low-magnesium calcite. In drainage areas underlain by limestone, groundwaters have Mg2+/Ca2+ ratios of around one or less and cement mineralogy is nearly always low-magnesium calci te. The oxygen and carbon stable isotopic ratios of the cements vary w idely, from -10.6% to +3.0% PDB and from -10.0% to +0.7% PDB, respecti vely. Cement delta(18)O values principally reflect variation in rainfa ll delta(18)O over a time scale of several thousand years. Rainfall an d cement delta(18)O values probably are inversely correlated with the amount of rainfall, which is related to the frequency and intensity of the Indian Ocean monsoon. Thus, cement delta(18)O is potentially a pr oxy indicator of relative rainfall and monsoon activity. For each of t hree sampling areas, delta(13)C is positively correlated to delta(18)O . Cement delta(13)C values are also related to rainfall amount because rainfall controls the plant population. Greater plant respiration of isotopically depleted CO2 to shallow groundwaters and burial of organi c material in conglomerate deposits results in lower cement delta(13)C values compared to periods of lesser plant activity.