Identification:
Title:
Zonation of authigenic carbonates within gas hydrate-bearing sedimentary sections on the Blake Ridge; offshore southeastern North America
Year:
2000
Source:
In: Paull, Charles K., Matsumoto, Ryo, Wallace, Paul J., Black, Nancy R., Borowski, Walter S., Collett, Timothy S., Damuth, John E., Dickens, Gerald R., Egeberg, Per Kristian, Goodman, Kim, Hesse, Reinhard F., Hiroki, Yoshihisa, Holbrook, W. Steven, Hoskins, Hartley, Ladd, John, Lodolo, Emanuele, Lorenson, Thomas D., Musgrave, Robert J., Naehr, Thomas H., Okada, Hisatake, Pierre, Catherine, Ruppel, Carolyn D., Satoh, Mikio, Thiery, Regis, Watanabe, Yoshio, Wehner, Hermann, Winters, William J., Wood, Warren T., Miller, Christine M. (editor), Reigel, Ruth (editor), Proceedings of the Ocean Drilling Program; volume 164; scientific results; gas hydrate sampling on the Blake Ridge and Carolina Rise; covering Leg 164 of the cruises of the drilling vessel JOIDES Resolution, Halifax, Nova Scotia, to Miami, Florida, sites 991-997, 31 October-19 December 1995
Publisher:
Texas A & M University, Ocean Drilling Program, College Station, TX, United States
Volume:
164
Issue:
Pages:
301-312
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 delta (super 13) C and delta (super 18) O values of calcite within this zone reflects marine carbonate ( approximately 0 per mil Peedee belemnite [PDB]) formed in equilibrium with seawater. (2) Between 20 and 100 mbsf, calcite delta (super 13) C values are distinctly negative (as low as -7.0 per mil), and authigenic dolomite is common ( approximately 2-40 wt%) with delta (super 13) C values between -3.6 per mil and 13.7 per mil. (3) Below 100 mbsf, dolomite abundance decreases to trace amounts, and disseminated siderite becomes the pervasive ( approximately 2-30 wt%) authigenic carbonate. Both siderite textures and stable isotope values indicate direct precipitation from pore fluids rather than dolomite replacement. The delta (super 13) C and delta (super 18) O values of siderite vary from 5.0 per mil to 10.9 per mil and 2.9 per mil to 7.6 per mil, respectively. Comparisons between the delta (super 13) C profiles of dissolved inorganic carbon (DIC) and pore-water concentration gradients, with the delta (super 13) C and delta (super 18) O values of authigenic carbonates, delineate a distinct depth zonation for authigenic carbonate mineral formation. Coincidence of the most negative delta (super 13) C (sub DIC) values (< or =-38 per mil) and negative delta (super 13) C values of both calcite and dolomite, with pore-water alkalinity increases, sulfate depletion, and decreases in interstitial Ca (super 2+) and Mg (super 2+) concentrations at and below 20 mbsf, suggests that authigenic calcite and dolomite formation is initiated at the base of the sulfate reduction zone ( approximately 21 mbsf) and occurs down to approximately 100 mbsf. Siderite formation apparently occurs between 120 and 450 mbsf; within, and above, the gas hydrate-bearing section of the sediment column ( approximately 200-450 mbsf). Siderite delta (super 13) C and delta (super 18) O values are nearly uniform from their shallowest occurrence to the bottom of the sedimentary section. However, present-day pore-water (super 13) C (sub DIC) values are only similar to siderite delta (super 13) C values between approximately 100 and 450 mbsf. Furthermore, calculated equilibrium delta (super 18) O values of siderite match the measured delta (super 18) O values of siderite between 120 and 450 mbsf. This interval is characterized by high alkalinity (40-120 mM) and low Ca (super 2+) and Mg (super 2+) concentrations, conditions that are consistent with siderite formation.
Language:
English
Genre:
Serial
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