The Hydrogeochemistry of Early Meteoric Diagenesis in a Holocene deposit of Biogenic Carbonates

Abstract

ABSTRACT This study presents the results of a quantitative hydrogeochemical investigation of early meteoric diagenesis in a deposit of biogenic, mixed-mineralic Holocene carbonate sediments at Ocean Bight in the southeastern Bahamas. Ground-water samples were collected from a network of 27 wells arranged into 4 transects across the 1.5 km2 strand-plain deposit. Four wells provided multi-depth samples at 0.5 m intervals, thus allowing for the construction of vertical chemical profiles. Analyses of the water consisted of temperature, pH, alkalinity, CI- , Ca2+, Mg2+, SR2+, SO42-, and 13C. Data derived from these analyses were used in a series of equilibri m and mass-balance equations to calculate mineral saturation levels and magnitudes and rates of mineral transformation reactions. Interpretation of measured and calculated parameters provided a detailed picture of patterns and processes of meteoric diagenesis at Ocean Bight. Four features of meteoric diagenetic processes operating at Ocean Bight are distinct from processes documented at previously studied non-biogenic and aragonite dominated Holocene systems: 1) lighter cementation with respect to magnitudes of calcite precipitation, 2) diminished magnitudes of aragonite dissolution, 3) elevated rates of mineral stabilization, and 4) lower efficiencies of transformation reactions. Another distinction of this study, which is not thought to be attributed solely to the presence of mixed biogenic carbonates, is the isolation of transformation reactions to the vadose and upper phreatic zone. This result contradicts the popularly held belief that metastable carbonate deposits undergo rapid equilibration throughout the freshwater phreatic zone. This distribution of diagenetic reactions, along with the analytical results, characterizes a chemical environment dominated by carbonate mineral transformation reactions linked mainly to the oxidation of organic matter and not only to the relative solubility of the different carbonate minerals. This finding may prove important in attempts to explain the preservation of metastable minerals in ancient geologic sequences despite repeated exposure to freshwater phreatic zones. This finding also questions the assumption that the extent of paleo-phreatic lenses is equal to the observed extent of freshwater phreatic alteration.