Hydrochemistry,origin and evolution of sedimentary subsurface fluids: III. Early diagenetic mineralization reactions in high sedimentation—rate basins |
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| Authors: | Reinhard Aesse |
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| Institution: | (1) Department of Geological Sciences, McGill University Montreal, H3A 2A7, Quebec, Canada |
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| Abstract: | The early diagenetic evolution of pore-water chemistry is closely linked to mineralization reactions which consume significant
portions of the metabolites released by bacterial organic matter decomposition. These reactions are most intense in high-sedimentation
rate basins and include the precipitation of iron-sulfides and various carbonates leading to concretion growth. Early diagenetic
pyrite is typically framboidal attesting to its recrystallization from precursor mackinawite, greigite or amorphous FeS which
are the favored phases at high supersaturation levels during the initial sulfate reduction stages. The sulfur isotopic composition
of early diagnetic pyrite can be used to differentiate diffusion-controlled, open-system conditions with isotopically light
sulfide (δ
34S = − 35 to − 20‰) from closed system conditions, under which Raleigh distillation produces increasingly heaver sulfide (δ
34 S = − 35 to + 18‰). Alabandite (Mn-sulfide) is a rare authigenic sulfide in Mn-rich environments such as certain restricted,
semi-stagnant basins (Baltic Sea).
pH-buffering by hydrogen sulfide and hydrogen ion uptake by the reduction of manganese and iron oxides and hydroxides in the
nitrate and sulfate reduction zones raise the pH sufficiently to cause supersaturation of the porewaters with respect to Ca-,
Mg-, Fe- and Mn-carbonates and complex solid solutions of them. Fe-carbonates cannot form in the sulfate reduction zone in
the presence of dissolved sulfide which competes for the dissolved iron. Likewise, dolomite formation appears to be inhibited
or slowed down in the presence of substantial dissolved sulfate. The appearance of siderite and ankerite therefore signals
carbonate precipitation below the sulfate reduction zone. Supporting evidence for the early diagenetic origin of many carbonate
concertions is provided by their high carbonate contents (70 to 90% reflecting the porosity existing at the time of precipitation,
called “minus-cement porosity”), isotopic composition, clay fabrics, and preservation of original bedding features including
the shapes of fossils and fecal pellets. In these environments increasing carbon isotope ratios (δ
13 C = − 20 to + 15‰) indicate concretion growth below the sulfate reduction zone, i.e., in the methane generation zones. Continuing
concretion growth at greater burial depth explains pore water profiles with constantly low Ca and downward decreasing Mg concentrations.
Dissolved ammonia and phosphate profiles reguire adsorption and ion-exchange reactions as additional removal machanisms (besides
apatite precipitation) in order to explain their downward decrease after they have reached maximum concentrations below the
alkalinity maximum.
Classification of early diagnetic environments into oxic and anoxic and further subdivision of the latter into sulfidic and
non-sulfidic (with suboxic or post-oxic and methanic as further subcategories of the non-sulfidic environment) according to
Berner (1981) is preferred over the previous classification in terms of pH/Eh fields. The temperature range of the early diagenetic
stage extends from O to about 75°C, at which temperature thermocatalytic organic matter decomposition replaces the earlier
bacterially mediated reactions and causes a whole set of new diagenetic reactions (such as feldspar dissolution, smectite
to illite transformation) to start. |
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