Geology Reference
In-Depth Information
Figure 8 Lead profile in a lake sediment in relation to depth and the year of incorporation
(From Davies and Galloway.
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)
newest at the top, in contact with the water. If burrowing organisms do
not provide too much disturbance (termed bioturbation), the sediment
can preserve a record of depositional inputs to the water body. An
example is provided by Figure 8 in which lead is analysed in sediment
core dated from its radioisotope content.
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The concentration rises from a
background around the year 1800, corresponding to the onset of indus-
trialisation. Considerably increased deposition is seen after 1930 due
to the introduction of leaded petrol. While some of the lead input is
via surface waters, the majority probably arises from atmospheric depo-
sition.
7.4 BIOGEOCHEMICAL CYCLES
A general model of a biogeochemical cycle appears in Figure 9. Al-
though biota are not explicitly included, their role is a very important
one in mediating transfers between the idealised compartments of the
model. For example, the role of marine phytoplankton in transferring
sulfur from the ocean to the atmosphere in the form of dimethyl sulfide
has been highlighted in Chapter 4. Biota play a major role in determin-
ing atmospheric composition. Photosynthesis removes carbon dioxide
from the atmosphere and replenishes oxygen. In a world without biota,
lightning would progressively convert atmospheric oxygen into nitrogen
oxides and thence to nitrate, which would reside in oceans. Biota also
exert more subtle influences. In aquatic sediments, micro-organisms
often deplete oxygen more quickly than it can be replenished from the
overlying water, producing anoxic conditions. This leads to chemical
