Zonation of authigenic carbonates within gas hydrate-bearing sedimentary sections on the Blake Ridge: offshore southeastern North America

Abstract

Authigenic carbonate mineral distributions are compared to pore-water geochemical profiles and used to evaluate diagenesis within sedimentary sections containing gas hydrates on the Blake Ridge (Ocean Drilling Program Sites 994, 995, and 997). Carbonate mineral distributions reveal three distinct diagenetic zones. (1) Carbonate minerals in the upper 20 m are primarily biogenic and show no evidence of diagenesis. The δ13C and δ18O values of calcite within this zone reflects marine carbonate (~0‰ Peedee belemnite [PDB]) formed in equilibrium with seawater. (2) Between 20 and 100 mbsf, calcite δ13C values are distinctly negative (as low as –7.0‰), and authigenic dolomite is common (~2–40 wt%) with δ13C values between –3.6‰ and 13.7‰. (3) Below 100 mbsf, dolomite abundance decreases to trace amounts, and disseminated siderite becomes the pervasive (~2–30 wt%) authigenic carbonate. Both siderite textures and stable isotope values indicate direct precipitation from pore fluids rather than dolomite replacement. The δ13C and δ18O values of siderite vary from 5.0‰ to 10.9‰ and 2.9‰ to 7.6‰, respectively. Comparisons between the δ13C profiles of dissolved inorganic carbon (DIC) and pore-water concentration gradients, with the δ13C and δ18O values of authigenic carbonates, delineate a distinct depth zonation for authigenic carbonate mineral formation. Coincidence of the most negative δ13C DIC values (≤–38‰) and negative δ 13C values of both calcite and dolomite, with pore-water alkalinity increases, sulfate depletion, and decreases in interstitial Ca2+ and Mg2+ concentrations at and below 20 mbsf, suggests that authigenic calcite and dolomite formation is initiated at the base of the sulfate reduction zone (~21 mbsf) and occurs down to ~100 mbsf. Siderite formation apparently occurs between 120 and 450 mbsf; within, and above, the gas hydrate–bearing section of the sediment column (~200–450 mbsf). Siderite δ13C and δ18O values are nearly uniform from their shallowest occurrence to the bottom of the sedimentary section. However, present-day pore-water 13C DIC values are only similar to siderite δ 13C values between ~100 and 450 mbsf. Furthermore, calculated equilibrium δ18O values of siderite match the measured δ18O values of siderite between 120 and 450 mbsf. This interval is characterized by high alkalinity (40–120 mM) and low Ca2+ and Mg2+ concentrations, conditions that are consistent with siderite formation.