Geoscience Reference
In-Depth Information
ion, such as the charge, the ionic radius, the pH, the concentration of the ligand, the redox
potential, multiple complexation, and even the spin of the element. The cell utilizes this
complex arsenal to manipulate the crystallization pathway of some particular minerals.
A distinction can be made between minerals formed as a by-product of cell metabolism
or activity (biologically induced mineralization) and minerals that are an integral part of
the organism physiology (biologically controlled mineralization):
1. Biologically induced mineralizations. Cyanobacteria and their stromatolite constructs
are one of the earliest examples of an organism precipitating calcite. Precipitation of
iron and manganese oxyhydroxides is frequent as a consequence of bacteria creating
a reducing environment which concerts Fe 2 + and Mn 2 + into low-solubility Fe 3 + and
Mn 4 + . It is also known that bacteria play a strong role in the precipitation of silica
from hydrothermal springs. Apatite forms upon decay of biological matter. A form of
pyrite, called framboidal for its raspberry-like appearance, is an example of biologically
mediated sulfide precipitation.
2. Biologically controlled mineralizations. The most important biominerals of this type
are carbonates (foraminifera) and silica (diatoms and radiolarians), which cover broad
expanses on the sea floor. These minerals are mostly used to confer protection and
mobility to the organisms. A spectacular case is intracellular precipitation of nicely
aligned magnetite crystals in so-called magnetotactic bacteria.
8.5 Biological controls on the ocean-atmosphere system
Consequences of this biological activity on the chemistry of the ocean-atmosphere sys-
tem are very significant. Today, the alkalinity flux through the ocean is regulated by the
precipitation of organic calcite. Phytoplankton acts on the atmosphere by the photosyn-
thetic production of oxygen, which is balanced by the respiration of both the phyto- and
the zooplankton. What limits the process is not very well understood: it is believed, as sug-
gested by John Martin, that the availability of micronutrients, notably Fe and Zn, may keep
productivity in check. The story goes that during a seminar at Woods Hole, John Martin
rose and said “Give me a half tanker of iron, and I will give you an ice age.” The project
IRONEX, designed to test this assumption, supports this view: south of the Galapagos
Islands, in the Eastern Pacific, in waters naturally rich in major nutrients because of the
very low latitude, addition of 445 kg of iron triggered a strong algal bloom with a clearly
visible change in chlorophyll levels and measurable effects on dissolved CO 2 .
Export of organic matter towards the sediment acts as a CO 2 pump and a source of O 2 for
the ocean-atmosphere system. The fate of buried carbon controls the long-term evolution
of atmospheric oxygen. If sedimentary C is oxidized, either because it is oxidized during
diagenesis or because the sediment is shoved up against continental margins, reaction with
atmospheric oxygen converts it back to carbon dioxide. This phenomenon can be seen at
work today in the natural oil seeps of the Persian Gulf. If sediments are subducted with
oceanic lithospheric plates, carbon is injected into the mantle and the atmosphere inherits
the leftover oxygen irreversibly.
 
 
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