Hydrochemistry, origin and evolution of sedimentary subsurface fluids; II, Early diagenetic pore-water evolution in high sedimentation-rate offshore basins

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Authors:Hesse, Reinhard
Author Affiliations:Primary:
McGill Univ., Dep. Geol. Sci., Montreal, QC, Canada
Volume Title:Geochemistry (English Language Edition)
Source:Geochemistry (English Language Edition), 6(1), p.1-18. Publisher: Science Press, Beijing, China. ISSN: 0253-486X
Publication Date:1987
Note:In English. 43 refs.; illus. incl. 1 table
Summary:High sedimentation-rate basins are characteristic of the continental margins, where sedimentation rates up to 500m/Ma and more are no exception. The sediments are rich in organic matter and undergo a well-established sequence of bacterial oxidation reactions, starting at the sediment/water interface with: (1) the oxidation zone (absent in euxinic basins), which is followed downwards by (2) the nitrate reduction zone, (3) the sulfate reduction zone, (4) the carbonate reduction zone, (5) the fermentation zone and (6) the decarboxylation zone, which marks the onset of thermo catalytic reactions characteristic of the intermediate stage (and deeper stages) of diagenesis. This sequence of bacterially mediated organic-matter decomposition reactions in rapidly deposited sediments on the continental margins is well reflected in the vertical trends of pore-water chemistry. Carbon isotopic fractionation permits to distinguish between zones (1) to (3) with negative δ13C values for the CO2 generated, and between zones (4) and (5) with increasingly heavier carbon-isotopes in CO2 (up to +15 to +25per mill δ13C). The formation of gas-hydrates causes characteristic chemical and isotopic signatures in pore waters retrieved from hydrate-beating sediments. After hydrate decomposition (in the sampling process or at the base of hydrate zones) marked chlorinity decreases accompanied by increases in heavy oxygen isotopes are observed. These are distinctly different from the effects of meteoric water influx, which are not accompanied by positive δ18O values. The opposite trend of increasing salinity at shallow burial depths, rather than pore-waters freshening, is observed in the vicinity of evaporites. [R.A.H.]
Subjects:Aerobic environment; Anaerobic environment; Basins; Biogenic processes; C-13/C-12; Carbon; Chemically precipitated rocks; Controls; Diagenesis; Early diagenesis; Effects; Evaporites; Geochemistry; Isotopes; Marine environment; Marine sediments; O-18/O-16; Organic compounds; Oxidation; Oxygen; Pore water; Porosity; Reduction; Salinity; Sediment-water interface; Sedimentary basins; Sedimentary rocks; Sedimentation; Sedimentation rates; Sediments; Stable isotopes; Evolution; High rates; Organic materials
Abstract Numbers:88M/0813
Record ID:1987059949
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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