Geochemistry of meteoric calcite cements in some Pleistocene limestones

Abstract

ABSTRACT In this study, the stable isotope and trace element geochemistries of meteoric cements in Pleistocene limestones from Enewetak Atoll (western Pacific Ocean), Cat Island (Bahamas), and Yucatan were characterized to help interpret similar cements in ancient rocks. Meteoric calcite cements have a narrow range of δ 18 O values and a broad range of δ 13 C values in each geographical province. These Pleistocene cements were precipitated from water with stable oxygen isotopic compositions similar to modern rainwater in each location. Enewetak calcite cements have a mean δ 18 O composition of −6.5% 0 (PDB) and δ 13 C values ranging from −9.6 to +0.4%0 (PDB). Sparry calcite cements from Cat Island have a mean δ 18 O composition of −4.1% 0 and δ 13 C values ranging from −6.3 to + 1.1% 0 . Sparry cements from Yucatan have a mean δ 18 O composition of −5.7% 0 and δ 13 C values of −8.0 to −2.7% 0 . The mean δ 18 O values of these Pleistocene meteoric calcite cements vary by 2.4% 0 due to climatic variations not related directly to latitude. The δ 13 C compositions of meteoric cements are distinctly lower than those of the depositional sediments. Variations in δ 13 C are not simply a function of distance below an exposure surface. Meteoric phreatic cements often have δ 13 C compositions of less than —4.0% 0 , which suggests that soil‐derived CO 2 and organic material were washed into the water table penecontemporaneous with precipitation of phreatic cements. Concentrations of strontium and magnesium are quite variable within and between the three geographical provinces. Mean strontium concentrations for sparry calcite cements are, for Enewetak Atoll, 620 ppm (σ= 510 ppm); for Cat Island, 1200 ppm (σ= 980 ppm); and for Yucatan, 700 ppm (σ= 390 ppm). Equant cements, intraskeletal cements, and Bahamian cements have higher mean strontium concentrations than other cements. Equant and intraskeletal cements probably precipitated in more closed or stagnant aqueous environments. Bahamian depositional sediments had higher strontium concentrations which probably caused high strontium concentrations in their cements. Magnesium concentrations in Pleistocene meteoric cements are similar in samples from Enewetak Atoll (mean =1.00 mol% MgCO 3 ; σ= 0.60 mol% MgCO 3 ) and Cat Island (mean = 0.84 mol% MgCO 3 ; σ= 0.52mol% MgCO 3 ) but Yucatan samples have higher magnesium concentrations (mean = 2.20 mol% MgCO 3 : σ= 0.84mol% MgCO 3 ). Higher magnesium concentrations in some Yucatan cements probably reflect precipitation in environments where sea water mixed with fresh water.