Rocks biogenic

Thick sedimentary pile from middle Miocene to late Pliocene is exposed in the Oga Peninsula, northern Honshu, Japan (Fig. 1.153). Age of the sedimentary rocks has been determined by microfossil data. Thus, the sedimentary rocks in the Oga Peninsula where type localities of Miocene sedimentary rocks in northern Japan are well exposed have been studied to elucidate the paleoenvironmental change of the Japan Sea (Watanabe et al., 1994a,b). Kimura (1998) obtained geochemical features of these rocks (isotopic and chemical compositions) and found that regional tectonics (uplift of Himalayan and Tibetan region) affect paleo-oceanic environment (oxidation-reduction condition, biogenic productivity). However, in their studies, no detailed discussions on the causes for the intensity and periodicity of hydrothermal activity, and temporal relationship between hydrothermal activity, volcanism and tectonics in the Japan Sea area were discussed. They considered only the time range from ca. 14 Ma to ca. 5 Ma. 

Biogenic structures Structures in fossils or rock formations that may have been formed by biological activity. 

The composition of the aeolian particles is temporally and spatially variable. These particles are typically fragments of weathered rocks, soil, or biogenic detritus, such as terrestrial plant fragments. Other biogenic particles include bacteria, phytoplankton, mold, fungal spores, seeds, and even insects. 

The chemical weathering of crustal rock was discussed in Chapter 14 from the perspective of clay mineral formation. It was shown that acid attack of igneous silicates produces dissolved ions and a weathered solid residue, called a clay mineral. Examples of these weathering reactions were shown in Table 14.1 using CO2 + H2O as the acid (carbonic acid). Other minerals that undergo terrestrial weathering include the evaporites, biogenic carbonates, and sulfides. Their contributions to the major ion content of river water are shown in Table 21.1. 

The other reason why the average salinity of seawater is 35%o lies in the fundamental chemistry of major ions. For example, the sevenfold increase in the Na /K ratio between river water and seawater (Table 21.8) reflects the lower affinity of marine rocks for sodium as compared to potassium. In other words, the sodium sink is not as effective as the one for potassium. Thus, more sodium remains in seawater, with its upper limit, in theory, being controlled by the solubility of halite. Likewise, the Ca /Mg ° ratio in seawater is 12-fold lower than that of river water due to the highly effective removal of calcium through the formation of biogenic calcite. 

The biogenic soft parts that become buried in the sediments are also transformed into sedimentary rocks, predominantly shale. Geologic uplift fallowed by chemical weathering leads to the oxidation of the organic carbon, i.e.. 

D.A. (1993) Rock magnetic criteria for the detection of biogenic magnetite. Earth Planetary Sci. Letters 120 283-300 Moukassi, M. Gougeon, M. Steinmetz, P. Dupre, B. Gleitzer, C. (1984) Hydrogen reduction of single crystals of wiistite doped with Mg, Mn, Ca, A1 and Si. Met. Trans. 15B 383-390... 

Processes and mechanisms responsible for recycling at the sediment-water interface cannot be explained by a single process, but are most likely a combination of many biogeochemical processes. Although pore-water HgT concentrations were higher than in lake waters, direct release of pore waters below about 2 cm could not totally account for the observed buildup in the hypolimnion of Little Rock Lake. Remineralization of recently deposited biogenic particulate matter and release of particle-bound Hg from this source most likely accounted for the observed water-column buildup. 

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