Geology Reference
In-Depth Information
Fe and Mn oxides, respectively. Patterns of trace element enrichment in
the nodules are determined by mineralogy, the controlling mechanisms
being related to cation substitution in the crystal structure. Mn phases
preferentially accumulate Cu, Ni, Mo and Zn. Alternatively, Co, Pb, Sn,
Ti and V are enriched in Fe phases.
4.3.3 DIAGENESIS
Diagenesis refers to the collection of processes that alter the sediments
following deposition. These mechanisms may be physical (compaction),
chemical (cementation, mineral segregation, ion exchange reactions) or
biological (respiration). The latter are of particular importance as the
bacteria control pH and pe in the interstitial waters, which in turn affect
a wide range of chemical equilibria. These master variables influence the
composition of the interstitial water, and can exert a feedback effect on
the overlying seawater. Also, they can ultimately control the minera-
logical phases that are lost to the sediment sink.
Organic material accumulates with other sedimentary components at
the time of deposition. High biological productivity in surface waters and
rapid sedimentation ensures that most nearshore and continental margin
sediments contain significant amounts of organic matter. Biochemical
oxidation of this material exhausts the available O
2
creating anoxic
conditions. The oxic/anoxic boundary occurs at the horizon where the
respiratory consumption of O
2
balances its downward diffusion. Upon
depleting the O
2
, other constituents are used as oxidants leading to the
stepwise depletion of NO
3
,NO
2
and SO
4
2
. Thereafter, organic matter
itself may be utilised with the concurrent production of CH
4
.
This series of reactions causes progressively greater reducing condi-
tions, with consequent influences on the chemistry of several elements.
Metals are reduced and so are present in lower oxidation states. In
particular, Mn undergoes reductive dissolution from MnO
2s
to Mn
aq
21
.
The divalent state being much more soluble, Mn is effectively remobi-
lised under anoxic conditions and can be released back into overlying
seawater. As seen in the previous section, this is one pathway to
ferromanganese nodule formation. This can also be true for other
elements that had been deposited following incorporation into the Fe
and Mn oxide phases. In contrast, some elements can be preserved very
effectively in anoxic sediments. Interstitial waters in marine sediments,
in contrast to freshwater deposits, have high initial concentrations of
SO
4
2
. Bacterial sulfate reduction proceeds via the reaction
2CH
2
O
þ
SO
2
4
Ð
H
2
S
þ
2HCO
3
