Hydrochemistry,origin and evolution of sedimentary subsurface fluids: I. Early diagenetic pore-water evolution in low-sedimentation rate offshore basins |
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Authors: | Reinhard Hesse |
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Institution: | (1) Department of Geological Sciences, McGill University, H3A 2A7 Montreal, Quebec, Canada |
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Abstract: | Pore water studies enable (1) the detection of diagenetic reactions actively occurring in the sediment at the time of sampling,(2)
the distinction between principal modes of solute transport, i.e., between advection (convection) and diffusion, and (3) the
assessment of mineral-solution equilibria, Pore waters are, therefore, preferred diagnostic objects in the study of diagenesis,
particularly early diagenesis.
The single most important factor for pore-fluid evolution in modern offshore basins is sedimentation rate which is closely
correlated with organtic matter content of the sediment. Organic matter represents the most reactive sediment constituent
which, through bacterial decomposition, provides some of the main solutes involved in early diagenetic mineralization reactions.
On the basis of sedimentation rate and organic matter content, it is convenient to distinguish two end-members of basins (environments)
with respect to early diagenesis: I. Low to intermediate-sedimentation rate basins with convection or diffusion-controlled
pore-water profiles and II. high-sedimentation rate basins with reaction-controlled pore-water profiles.
The first group of basins or environments, which is the subject of this paper, is typically represented by pelagic sediments.
Three principally different trends of pore-water evolution occur: 1. convection-controlled profiles with no pronounced vertical
gradients for the dissolved species; 2. diffusion-controlled profiles with vertical gradients but linear correlations between
major ions, especially Ca and Mg, and 3. profiles with gradients but no linear correlation between the major ions. The later
are transitional to the trends seen in high-sedimentation rate basins. With respect to redox-potentials, the diagenetic environments
of low-sedimentation rate basins are generally oxic to suboxic. |
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