Geoscience Reference
In-Depth Information
8.6 Diagenetic transformation of organic material
Mud deposited on the ocean floor normally contains enough dead organic matter to provide
food for a burrowing fauna of fish, worms, and mollusks. Even so, almost all these creatures
depend on oxygen being available in the interstitial water. Only the top few centimeters of
the mud, known as the bioturbated layer, will provide such an oxygenated environment.
This layer is recognizable from its burrows and its chaotic character. The top part of the
oxygenated sediment is thus mixed to the extent that the finer sedimentary structures dis-
appear. Imagine for a moment that we are observers on the sea floor, resisting burial, and
living long enough to observe the sediment sinking beneath our feet. We would see the
bioturbated layer acting like a moving average on the sedimentary record (electronics afi-
cionados might liken it to a low-pass filter). If, however, the bottom water is particularly
poor in oxygen, usually because of a great abundance of organic matter, burrowing animals
cannot breathe, bioturbation is impossible, and the sedimentary record conserves even the
finest details down to the infra-millimetric scale (the rocks are then often referred to as
laminites).
Below the bioturbated layer, the dissolved oxygen of the interstitial water is fully used
up. Other organisms, microbes, take over. To maintain their metabolism they are able to
oxidize organic matter by reducing other dissolved components, such as nitrates to nitro-
gen (denitritrification), solid MnO
2
and Fe(OH)
3
to soluble Mn
2
+
and Fe
2
+
, and sulfates
to hydrogen sulfide. These reactions are also known as dissimilatory reactions because
they do not involve assimilation of their products by cellular material. These biochemi-
cal reactions occur in the space of a few tens of centimeters below the water-sediment
interface. The sequence of early diagenetic reactions in the uppermost layers below the
sediment-water interface is controlled by the redox potential of the possible diagenetic
reactions. We are going to assume that dead organic matter is represented for simplicity
by CH
2
O (formaldehyde), a very simple compound which could be a precursor of sugars
and which is oxidized into HCO
3
, the stable form of carbon in solution. Another form
of “food” is the acetate ion released by acetogenic bacteria, probably the most abundant
organic substrate in oxygen-poor sediments. The dominance plot of the different redox
species is shown in
Fig. 8.8
for a pH
7: it indicates the sequence of reduction reactions,
which is simply that of decreasing pe. Right below the interface, the oxic layer is well
ventilated by the bioturbation and organisms use normal respiratory oxidative mechanisms
to turn organic matter into carbon dioxide plus water, but also ammonium into nitrites fol-
lowed by nitrates, a process known as nitrification. When all the dissolved oxygen is used
up and cannot any longer be replenished by burrowing animals, bacteria turn to increas-
ingly stable electron acceptors, nitrate, Mn
3
+
,Fe
3
+
, and SO
2
4
to oxidize organic matter.
The sequence of the most important dissimilatory reactions is shown in
Fig. 8.9
and is
determined by the energy they liberate. It can be seen from this sequence how intersti-
tial solutions become increasingly reducing with depth. Under these conditions, nitrate is
reduced to nitrogen (denitrification), which is lost to the ocean through pore waters, while
ferric iron and manganese accumulated on the sea floor as hydroxides are progressively
reduced and dissolved. Most of the reduced Mn
2
+
≈
is also returned to bottom seawater
