Geology Reference
In-Depth Information
under steady-state conditions, when a mass balance between inputs and
outputs is achieved. An imbalance could signify that an important
source or sink has been ignored. Alternatively, the system may be
perturbed, possibly mediated through anthropogenic activities, and
therefore be changing towards a new equilibrium state.
Processes within the reservoir that affect the temporal and spatial
distribution of a given component are transportation and transforma-
tions. Both physics and biology within the system play a role. Clearly,
transport effects are dominated by the hydrodynamic regime. Although
transformations involve both chemical (dissolution, redox reactions,
speciation changes) and geological (sedimentation) processes, biological
activity generally controls nutrient and trace metal distributions. Fur-
thermore, the biota influences concentrations of O
2
and CO
2
, which in
turn determine the pH and pe (i.e., the redox potential), respectively. For
these reasons, some fundamental aspects of descriptive physical and
biological oceanography are included in this chapter.
In terms of biogeochemical cycling, the ocean constitutes a large
reservoir. The surface area is 361.11
10
6
km
2
, encompassing nearly
71% of the earth's surface. The average depth is 3.7 km, but depths in
the submarine trenches can exceed 10 km. The ocean contains about
97% of the water in the global hydrological cycle. A schematic repre-
sentation of the oceanic reservoir is presented in Figure 1. The material
within it can be operationally defined, usually based on filtration, as
dissolved or particulate. For simplicity here, the ocean is divided into
two layers, with distinct surface and deepwaters. The boundary regions
are also distinguished, as the composition in these regions can be quite
different to that of bulk seawater. Furthermore, interactions within
these environments can alter the mass transfers across the boundary.
The rationale for such features will be presented in subsequent sections.
Material supplied to the ocean originates from the atmosphere, rivers,
glaciers and hydrothermal waters. The relative importance of these path-
ways depends upon the component considered and geographic location.
River runoff commonly constitutes the most important source. Trans-
ported material may be either dissolved or particulate, but discharges are
into surface waters and confined to coastal regions. Hydrothermal waters
are released from vents on the seafloor. Such hydrothermal waters are
formed when seawater circulates into the fissured rock matrix, and under
conditions of elevated temperature and pressure, compositional changes in
the aqueous phase occur due to seawater - rock interactions. This is an
important source of some elements, such as Li, Rb and Mn. The atmos-
phere supplies particulate material globally to the surface of the ocean. In
recent years, this has been the most prominent pathway to the World
