Geochemistry of Carbonate Cements in Surficial Alluvial Conglomerates and their Paleoclimatic Implications, Sultanate of Oman

Abstract

ABSTRACT 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 drainage areas underlain by the Semail Ophiolite, groundwaters have Mg2+/Ca2+ ratios greater than two, and cements are often dolomite or high-magnesium 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 calcite. The oxygen and carbon stable isotopic ratios of the cements vary widely, from -10.6 to + 3.0 PDB and from -10.0 to + 0.7 PDB, respectively. Cement d18O values principally reflect variation in rainfall d18 over a time scale of several thousand years. Rainfall and cement d18O values probably are inversely correlated with the mount of rainfall, which is related to the frequency and intensity of the Indian Ocean monsoon. Thus, cement d18O is potentially a proxy indicator of relative rainfall and monsoon activity. For each of three sampling areas, d13C is positively correlated to d18O. Cement d13C 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 organic material in conglomerate deposits results in lower cement d13C values compared to periods of lesser plant activity.