Isotope signatures associated with early meteoric diagenesis

Abstract

The environments in which carbonate diagenesis proceeds have been documented in previous studies of Holocene and late Pleistocene sediments and limestones on Barbados, West Indies. Variations in the carbon and oxygen isotopic composition of limestones, produced during early freshwater diagenesis, have been observed in this study to occur in specific patterns. Six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework: (1) the subaerial exposure surface is characterized by strongly ¹²C-enriched limestones. δ¹³C compositions of underlying limestones grow progressively heavier with increasing depth; (2) the subaerial exposure surface may also be marked by slight ¹⁸O-enrichment; (3) an abrupt shift in δ¹⁸O values may differentiate sediments above the exposure surface from those below; (4) sediments altered in the marine-meteoric mixing zone may be characterized by positive covariance between their δ¹⁸O and δ¹³C compositions; (5) the vadose-phreatic boundary may be marked by a sharp increase in δ¹³C values in the seaward portions of a fresh groundwater system; and (6) samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of δ¹⁸O and a wide range of δ¹³C compositions. Analysis of samples from five Palaeozoic and Mesozoic formations, which contained petrographic evidence of early freshwater diagenesis, showed that isotope patterns similar to those observed in Barbados limestones have been preserved in rocks as old as Mississippian. These isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks. They might be used to identify limestones diagenetically altered in meteoric environments, to identify mixing zone cements and dolomites, and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations.