Diagenesis pore water chemistry

Reaves (1986) described another study specifically aimed at relating calcium carbonate preservation to chemical diagenesis in sediments. He compared Mercinaria shells and pore water chemistry in intertidal mud flat and tidal creek... 

Budd D.A. (1988) Aragonite-to-calcite transformation during fresh-water diagenesis of carbonates Insights from pore-water chemistry. Geol. Soc. Amer. Bull. 100, 1260-1270. 

While all of these reactions are favored thermodynamically, they are almost always enzymatically catalyzed by bacteria. It has been observed from the study of pore waters in deepsea sediments (e.g., Froelich et al., 1979) and anoxic basins (e.g., Reeburgh, 1980) that there is an ordered sequence of redox reactions in which the most energetically favorable reactions occur first and the active electron acceptors do not overlap significantly. Bacteria are energy opportunists. Using estimates of the stoichiometry of the diagenesis reactions (Table 2) one can sketch the order and shape of reactant profiles actually observed in sediment pore-water chemistry... 

Simpson, G. Hutcheon, I. (1995) Pore-water chemistry and diagenesis of the modem Fraser River Delta. J. sediment. Petrol., A65, 648-655. 

Because diagenesis may alter only a small fraction of the solid phases, its impact may be difficult to detect from studies of solid phases alone. Pore water chemistry is much more sensitive to such changes. For example, in a sediment of 80% porosity, dissolution... 

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (see previous discussion). However, during the early stages of sulfate reduction (—2-35%), this reaction may not cause precipitation, but dissolution of carbonate minerals, because the impact of a lower pH is greater than that of increased alkalinity (Figure 4). Carbonate ion activity decreases rapidly as it is titrated by CO2 from organic matter decomposition leading to a decrease in pore-water saturation state. This process is evident in data for the Fe-poor, shallow-water carbonate sediments of Morse et al. (1985) from the Bahamas and has been confirmed in studies by Walter and Burton (1990), Walter et al. (1993), and Ku et al. (1999) for Florida Bay, Tribble (1990) in Checker Reef, Oahu, and Wollast and Mackenzie (unpublished data) for Bermuda sediments. 

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