Robert A. Berner

Sedimentary pyrite formation : An update

Sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time. The amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter, dissolved sulfate, and reactive detrital iron minerals. Organic matter appears to be the major control on pyrite formation in normal (non-euxinic) terrigenous marine sediments where dissolved sulfate and iron minerals are abundant. By contrast, pyrite formation in non-marine, freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic-rich fresh water shales from marine shales. Under marine euxinic conditions sufficient H2S is produced that the dominant control on pyrite formation is the availability of reactive iron minerals. Calculations, based on a sulfur isotope model, indicate that over Phanerozoic time the worldwide average organic carbon-to-pyrite sulfur ratio of sedimentary rocks has varied considerably. High CS ratios during Permo-Carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time. Low CS ratios, compared to today, during the early Paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean. The latter could have been due to lower oceanic oxygen levels and/or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time.

TARGET ATMOSPHERIC CO2: WHERE SHOULD HUMANITY AIM?

Paleoclimate data show that climate sensitivity is ~3 deg-C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6 deg-C for doubled CO2 for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO2 fell to 450 +/- 100 ppm, a level that will be exceeded within decades, barring prompt policy changes. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.

The use of chromium reduction in the analysis of reduced inorganic sulfur in sediments and shales

Abstract A sulfur analysis scheme employing the use of chromium reduction for the determination of reduced inorganic sulfur compounds (pyrite + elemental sulfur + acid volatile monosulfides) in modern sediments and shales is presented. Exhaustive testing shows that chromium reduction does not reduce or liberate either organic sulfur or sulfate sulfur; making the method specific only to reduced inorganic sulfur phases. The high degree of specificity, ease of sample analysis, and excellent analytic precision should make this technique ideally suited for routine analysis of modern and ancient sediments.

A New Geochemical Classification of Sedimentary Environments

Because of the relative constancy of pH in most subaqueous sediments and the general lack of measurability of Eh, these parameters are not practically useful for classifying sedimentary environments. In their place a new classification is offered which, derived from studies of modern sediments, is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation. Sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen. Anoxic environments, in turn, are divided into sulfidic and non-sulfidic depending upon the presence of measurable dissolved sulfide. Anoxic-nonsulfidic environments are further divided into postoxic, or resulting from oxygen removal without sulfate reduction (weakly reducing), and methanic , or resulting from complete sulfate reduction with consequent methane formation (strongly reducing). The environments are shown to succeed one another during early diagenesis in the order: oxic, post-oxic, sulfidic, methanic. Iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record.

Burial of organic carbon and pyrite sulfur in sediments over phanerozoic time: a new theory

In present day marine sediments, almost all of which are deposited in normal oxygenated seawater, rates of burial of organic carbon (C) and pyrite sulfur (S) correlate positively and bear a constant ratio to one another (C/S ∼- 3 on a weight basis). By contrast, calculations, based on the isotopic model of Garrels and Lerman (1981), indicate that at various times during the Phanerozoic the worldwide burial ratio must have been considerably different than the present day value. This ratio change is caused by the requirement that, increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur, in order to maintain a roughly constant level of O2 in the atmosphere. Such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments, as at present, to non-marine freshwater, or to euxinic environments, in the geologic past. A shift to predominantly freshwater burial can help explain predicted high C/S ratios in Permo-Carboniferous sediments, and a shift to euxinic environments can help explain predicted low C/S ratios during the early Paleozoic. It is demonstrated that the three environments today exhibit distinguishably different average C/S ratios.

The role of magnesium in the crystal growth of calcite and aragonite from sea water

Abstract The seeded precipitation (crystal growth) of aragonite and calcite from sea water, magnesium-depleted sea water, and magnesium-free sea water has been studied by means of the steady-state disequilibrium initial rate method. Dissolved magnesium at sea water levels appears to have no effect on the rate of crystal growth of aragonite, but a strong retarding effect on that of calcite. By contrast, at levels less than about 5 per cent of the sea water level, Mg has little or no effect on calcite growth. Extended crystal growth on pure calcite seeds in sea water of normal Mg content resulted in the crystallization of magnesium calcite overgrowths, containing 7–10 mole % MgCO 3 in solid solution. This suggests that the rate inhibition by Mg is due to its incorporation within the calcite crystal structure during growth, which causes the resulting magnesian calcite to be considerably more soluble than pure calcite. The standard free energy of formation of 8.5 mole% Mg calcite calculated on this assumption is in good agreement with independent estimates of magnesian calcite stability. From the work of Katz ( Geochim. Cosmochim. Acta 37 , 1563–1586, 1973), Plummer and Mackenzie ( Amer. J. Sci . 273 , 515–522, 1974), and the present paper, it can be predicted that the most stable calcite in Ca-Mg exchange equilibrium with sea water contains between 2 and 7 mole%MgCO 3 in solid solution. Likewise, calcites containing more than 8.5 mole% MgCO 3 are less stable, and those containing less than 8.5 mole% MgCO 3 are more stable than aragonite plus Ca and Mg in sea water.

Dissolution and pyritization of magnetite in anoxie marine sediments

Concentrations of magnetite were determined, with depth, in sediments of varying H2S content taken from Long Island Sound (FOAM, NWC, and Sachem sites) and from a site in the rapidly depositing subaqueous portion of the Mississippi Delta. For the three Long Island Sound sites, dissolution of magnetite during burial is clearly demonstrated, and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide. By combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide, we find that magnetite dissolution is well described by the following rate law: dCmagdt = -1.1× 10−5C0.5BCmagAmag where Cmag is the concentration of magnetite (grams per gram of sediment), Cs is the concentration of dissolved sulfide (mM), and Amag is the specific surface area of magnetite in the sediment (cm2 per gram of magnetite.) This equation means that, for a typical range of magnetite surface areas and dissolved sulfide concentrations, the “half-life” of magnetite in anoxic marine sediments ranges from about 50 to 1000 years. SEM observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite. This only occurs if magnetite is in contact with high concentrations of H2S (>1 mM) for relatively long periods of time (several hundred years). With low concentrations of H2S (<1 mM) magnetite dissolution occurs without identifiable pyrite replacement. We suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator.

AUTHIGENIC APATITE FORMATION AND BURIAL IN SEDIMENTS FROM NON-UPWELLING, CONTINENTAL MARGIN ENVIRONMENTS

Abstract Evidence for precipitation of authigenic carbonate fluorapatite (CFA) in Long Island Sound and Mississippi Delta sediments suggests that formation of CFA is not restricted to environments of active coastal upwelling. We present porewater data suggestive of CFA formation in both these areas. Application of a sequential leaching procedure, designed specifically to separate authigenic carbonate fluorapatite from other phosphorus-containing phases, including detrital apatite of igneous or metamorphic origin, provides strong supporting evidence for authigenic apatite formation in these sediments. The size of the authigenic apatite reservoir increases with depth, indicating continued formation of CFA during early diagenesis. This depth increase is mirrored by a decrease in solid-phase organic P at both sites, suggesting that CFA is forming at the expense of organic P. Mass balance considerations, application of diagenetic models to interstitial water nutrient data and the saturation state of the interstitial water are consistent with this interpretation. Diagenetic redistribution of phosphorus among the different solid-phase reservoirs is observed at both sites and results in near perfect retention of P by these sediments over the depth intervals sampled. Formation of CFA in continental margins which do not conform to the classically defined regions of phosphorite formation renders CFA a quantitatively more important sink than has previously been recognized. Including this reservoir as a newly identified sink for reactive P in the ocean, the residence time of P in the modern ocean must be revised downward. The implication for ancient oceans of CFA formation in continental margin sediments other than phosphorites is that phosphorite formation may be less a representation of episodicity in removal of reactive P from the oceans than of localized concentration of CFA in phosphatic sediments by secondary physical processes.

Methane production in the interstitial waters of sulfate-depleted marine sediments

Methane in the interstitial waters of anoxic Long Island Sound sediments does not reach appreciable concentrations until about 90 percent of seawater sulfate is removed by sulfate-reducing bacteria. This is in agreement with laboratory studies of anoxic marine sediments sealed in jars, which indicate that methane production does not occur until dissolved sulfate is totally exhausted. Upward diffusion of methane or its production in sulfate-free microenvironments, or both, can explain the observed coexistence of measurable concentrations of methane and sulfate in the upper portions of anoxic sediments.

C/S method for distinguishing freshwater from marine sedimentary rocks

In organic-rich sediments laid down in fresh water, much less diagenetic pyrite is formed than in analogous marine sediments because of the much lower concentrations of dissolved sulfate found in most fresh waters as compared to seawater. As a result, modern organic-rich freshwater sediments exhibit a much higher organic carbon-to-pyrite sulfur ratio (C/S) than marine sediments with similar organic contents. On this basis, C/S ratios can be used to distinguish ancient marine from freshwater (or slightly brackish) sedimentary rocks. This is demonstrated here for several Carboniferous shales and siltstones. The C/S technique cannot distinguish brackish-water sediments deposited under salinities greater than half that of seawater from marine sediments, as demonstrated by analyses of modern Chesapeake Bay sediments. Also, the method is not applicable to nearly pure limestones or to rocks low in organic matter (less than about 1% organic carbon). Saline (high sulfate) phases of ancient lakes can be distinguished from nonsaline phases using the C/S method.