Euxinic conditions

Arsenic is a biogenic deposit in the paleochannels of the Bhagirathi and Padma rivers under euxinic conditions (P. Chakrabarty, West Bengal State Remote Sensing Board 1999, personal communication to authors). 

In the two Mesozoic examples shown in Figure 4, the light and uniform values that typify water-column pyrite formation under euxinic conditions, and are further favored by the comparatively slow rates of sediment accumulation, are well expressed. The Cretaceous data of Gauher (1986,... 

The accumulation of O2 in Earth s atmosphere and its influence on the global sulfur cycle between —2.4 Ga and near modem levels during the Phanerozoic has been the subject of much recent research. One model (Lyons et al., in press) calls for generally low oceanic 864 concentrations (—10% of modern value) in the Meso-proterozoic ocean (1.6-0.9 Ga). However, by —1.8Ga, sufficient 864 was delivered to the ocean to potentially result in pervasive euxinic conditions in the world ocean as proposed by Canfield (1998). Levels of atmospheric oxygen and seawater 864 may not have been reached until the Neoproterozoic (—900 Ma). 

Biomarker analysis is one of the best paleo-environmental tools to identify anoxic and euxinic conditions in the water column. Biomarkers of phototrophic sulfur bacteria such as isorenieratane (32) (Section 8.03.5.6.1) and 3-isobutyl-4-methyl-maleimide (49d) (Section 8.03.5.7) unambiguously indicate euxinic conditions within the photic zone of the water-sediment system. Other biomarkers that are often associated with sediments deposited beneath anoxic waters are 28,30-dinorhopane, 25,28,30-trisnorhopane (59) and gammacerane (60) (Section 8.03.5.10). 

Fig. 4.14 Plot of weight percent organic carbon vs. weight percent pyrite sulfur for normal-marine modern sediments. Each plotted point represents the average value of samples in a given core, taken at a sediment depth where contents of organic carbon and pyrite have attained quasi-steady-state values, i.e. where early diagenesis of carbon and sulfur is (essentially) complete. The dashed lines enclose data from a variety of other studies (after Berner and Raiswell 1983). Sediments deposited under anoxic (euxinic) conditions would plot above the trend line, freshwater sediments significantly below.

ScH.l. Depositional environment schematic sulfur cycle in marine water body and upper water-sediment interface. The redox line (gray dashed perpendicular) is placed at S and relates only to the redox state of the sulfur, not the oxic or euxinic conditions. The black dashed line separates between sediment and water bodies. For the present paper, we would like to emphasize the major 8 ratio changes caused by the dissimilatory reduction of sulfate. For more detailed explanations on the formation of sulfur rich OM, see Aizenshtat et al. (1983, 1995, 2004) and Krein and Aizenshtat (1993, 1994). 

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