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Calcium carbonate nodules in soils: 13O/16O and 13C/12C ratios and 14C contents

1981; Elsevier BV; Volume: 25; Issue: 3-4 Linguagem: Inglês

10.1016/0016-7061(81)90033-1

1872-6259

Mordeckai Magaritz, Aaron Kaufman, Dan H. Yaalon,

Methane Hydrates and Related Phenomena

δ18 O values, δ13C values and 14C ages were determined in ten CaCO3, nodule populations collected from soil and paleosols in the Israeli coastal plain. The selected soils were carbonate-free when formed and the nodules in them represent either reprecipitation of carbonate illuviated from overlying horizons (descending mode) or precipitated from a raised brackish water table (ascending mode). The 14C ages represent the times of migration (illuviation) or eustatic movement (both climate-related) and the stable isotopic conditions reflect the environmental conditions at such times. The δ 13C values of most populations were found to have a wide range and cannot be used to reconstruct former climates. This wide range is attributed to microvariations of pCO2 in the soil. Because the δ 18O ranges in the same samples are usually comparable to analytical uncertainties and because the δ 18O mean values follow an age-correlated pattern, they permit us to place the soils in one of three categories. In two cases where nodules were soft both turned out to be young (∼ 1000 years) and to have wide ranges in δ 18O values; the latter are apparently due to continual re-equilibration with new soil solutions. In three cases of the descending mode of nodule formation, 12,000–14,000 year-old hard nodules had very narrow δ 18O whose means reflect the rainfall value at that time. The period between 12,000–14,000 years B.P. is independently known to be one of higher precipitation/evaporation ratio and of more intensive soil formation. These ascending-mode populations of hard nodules had narrow ranges in δ 18O (reflecting a mixture of sea water and rainfall) and all turned out to be 3,000–4,000 years old. At this period the sea reached its highest elevation, resulting in raising the nearcoast water table.