Geoscience Reference
In-Depth Information
Deposition is driven by a range of physico-
chemical and biologically mediated processes,
the latter in association with microbial mats that
facilitate local reductions in CO
2
and thus car-
bonate precipitation (Pedley 2000). These deposits
are termed
tufa
where deposition occurs under
ambient environmental conditions, or
travertine
where the deposits are associated with thermal
activity (Ford & Pedley 1996) and have been
shown to have potential as recorders of palaeo-
climatic information (Andrews et al. 1994).
Organic inputs, derived from plant material,
can also contribute abundant material to sedi-
ment substrates. This is particularly the case in
salt marshes (see Chapter 7) and mangroves (see
Chapter 9). Along mangrove-colonized shorelines,
where external inputs of sediment (siliciclastic
or carbonate) are minimal, this organic mater-
ial can be the main substrate contributor and
leads to the development of mangrove peat
(Woodroffe 1983). Biogenic, but non-carbonate
sediment, contributors such as diatoms are also
important within, for example, lake environ-
ments (Chapter 4). The progressive accumula-
tion of such microfossils within lake sediments
has proved to be an effective long-term recorder
of a range of environmental parameters such
as effective moisture, i.e. lake water levels, and
of temperature (Battarbee 2000) and have thus
been widely used as proxy records of climatic
and hydrological change (e.g. Bradbury 1997).
properties distinct from natural sediment grains
and, as a consequence, interacts with the envir-
onment in a different manner.
Another significant component of modern
sediment, mostly absent from pre-industrial age
sediments, are contaminants. A
contaminant
is commonly defined as a substance released
into the environment without a known impact
(Farmer 1997), or the presence of elevated con-
centrations of substances in water, sediments or
organisms (GESAMP 1982). In neither of these
definitions is the potential to cause environmental
harm attributed to a contaminant. This is in con-
trast to a
pollutant
, which is more specifically
defined as a substance that either causes harm
to the environment or exceeds an environmental
standard. Contaminants in sediments take a vari-
ety of forms, including metals, inorganic elements,
nutrients, organic compounds and radionuclides,
and the major sources of these contaminants are
highlighted in Chapter 3 (Table 3.2). It is import-
ant to be aware that many of these contaminants
can be sourced from natural processes as well as
anthropogenic activities, although in most cases
anthropogenic inputs tend to dominate.
Contaminant sources to sediments may be
of particulate, dissolved or gaseous form, but
for most contaminants the particulate form
is dominant (Horowitz 1991). Although con-
taminant sources are predominantly particulate,
there are important exceptions. Contaminants
from sewage treatment works (e.g. Zn and P)
can be predominantly in solute form, and metals
from acid mine drainage are also in dissolved
form at source owing to the low pH of the
waters. These dissolved contaminants, however,
commonly become associated with the sedi-
ment phase, via mineral precipitation or surface
adsorption, as solution concentration, pH and
Eh change through mixing with dilute river water
(Boult et al. 1994).
Contaminant sources may take one of two
general forms, point source and diffuse (non-
point) source. Point sources of pollution origin-
ate from a single location and include mines and
mine waste, landfill sites, factories, waste water
treatment works and bedrock mineralization.
Diffuse sources of contaminants originate from
1.2.3 Anthropogenic particles and compounds
Increasingly important in many sedimentary
systems are inputs of anthropogenically sourced
sediments. These include both sediment grains
that come from material that is anthropogenic
in origin (e.g. building material, industry) and
sedimentary materials that have been heavily
impacted by anthropogenic activity. A good
example of anthropogenic-derived sediment is
that present within urban environments (see
Chapter 6). In this environment, as well as soil
and vegetation sources, sediment is sourced
from vehicle wear, building material, combus-
tion particles and industrial material. All of
this material has chemical and mineralogical
