Geoscience Reference
In-Depth Information
100
Seawater
2
−
SO
4
10
H
2
S
(inferred)
+
NH
4
1
SiO
2
3
−
PO
4
0.1
0.01
0.001
0
5
10
15
20
25
30
Depth below seawater
sediment interface (cm)
−
Figure 10.2
Concentration of various species dissolved in interstitial water of the Saanich River fjord (western
Canada). With burial, sulfate is reduced to sulfide, organic nitrogen is reduced to ammonia
(denitrification), and phosphate and silica of organic origin are remobilized by diagenesis. Data
the form H
2
S, which eventually causes pyrite to precipitate, but in this core the sulfur data
are missing. Laboratory measurement of diffusion coefficients and estimates of sedimenta-
tion rate by radiochronometric methods allow us to determine, by resolving the diagenesis
equation, the rate at which sulfate is converted to sulfide and equally the rate of oxidation
of the organic matter. Similar behavior is observed for nitrates in seawater. Dissolution
of organic matter is reflected by a large increase in ammonia produced by the reduction of
nitrogen in proteins and other organic compounds (denitrification). The increase in remobi-
lized phosphate from phospho-organic compounds of soft matter and hard parts of certain
organisms (fish teeth, ichthyoliths) is responsible for diagenetic reprecipitation of calcium
phosphate (apatite), which may be intense enough to produce phosphate deposits of eco-
nomic value. The silica remobilized by dissolution of diatom and radiolarian tests is locally
reprecipitated as flint and the enclosing rocks can be further silicified as cherts. As sea-
water is highly undersaturated in SiO
2
, the expulsion of interstitial water also provides an
important source of silica for the ocean.
Until complete, these bacteria-controlled reactions are accompanied by substantial iso-
topic fractionation, of the order of 25 per mil for carbon and 20 per mil for sulfur, invariably
with organic matter and sulfide having a preference for the lighter isotope.
10.2 Hydrothermal reactions
This term is reserved for all medium temperature reactions, typically from 100 to 500
◦
C,
between aqueous solutions and the rock through which they circulate. These are largely,
but not exclusively, hydration reactions. Hydrothermal solutions and ores are the sources
of many metals of economic significance. The familiar thermal springs at the surface of
