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Chapter I
GENERAL INTRODUCTION
1.1. OCEANS
The oceanic system, which covers about 71% of the earth
crust, contains four major constituent sub systems the seawater,
suspended particulate material, sediment and the biota. The oceanic
part of the world has an area of about 361 million sq km, an average depth
of about 3,730 m and a total volume of about 1,347,000 million cubic km.
The deepest part of the oceans is the Mariana Trench (11,516 m) in
the Pacific Ocean. Compare this with the Mount Everest, 8,849 m
above sea level of the highest peak. The study of the world ocean plays
a major role in the understanding of various aspects of the physics
and chemistry of the earth.
Oceans are a huge warehouse of resources like minerals
(metals, oil, natural gas, chemicals, etc.), food (fish, prawns, lobsters, etc.)
and energy (waves, water currents, tides, etc.). We have been using
oceans for transporting goods (in ships and oil tankers) and for
recreation purposes (beaches, water sports, etc.). We have also been
using oceans to dump all municipal waste, industrial effluents,
pesticides used in agriculture, etc., resulting from activities of the
ever-growing population. In addition, oceans control weather and
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climate and thus considerably influence the environment. Even the
quality of air that we breathe depends greatly on the interaction
between the oceans and the atmosphere. Oceans have served as
channels of adventure and discovery. From expeditions to seas far and
near, we have understood how mother earth works, how the seafloor
is formed and, how parts of the continents have moved thousands of
kilometers over a long period. Thus, there are many reasons to study
the oceans and benefit from it.
The oceans act as a major reservoir in global geochemical
cycles. In particular, marine sediments are the ultimate sink for most
of the material derived from the continents, either from natural or
anthropogenic sources, being transferred to the oceans on a globalscale via riverine and atmospheric transport, with glacial transport
and hydrothermal inputs to sediments being important locally.
The other important source of sediments is that of material formed by
biogenic production in the water column (pelagic production).
1.2. SEDIMENT
Sediment is an integral and dynamic part of river basins,
including estuaries and coastal zones. Sediment originates from the
weathering of minerals and soils upstream and is susceptible to
transport downstream by the river water (Forstner, 2004).
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Marine sediments result from accumulated autochthonous
organic
compounds
derived
from
biological
productivity
and
continental materials supplied through aeolian and alluvial processes
(Valdes et al., 2005; Libes, 1992; Emelyanov, 2001).
From a practical viewpoint, four functions of aquatic
sediments can be distinguished
x
Memory effect, mainly in dated sediment cores from lakes,
reservoirs, and marine basins, as historical records reflect
variations of pollution intensities in a catchment area.
x
Life support, i.e., sediment has ecological, social and economic
value, as an essential part of aquatic ecosystem by forming a
variety of habitats and environments. A systematic approach is
needed, comprising bio tests and effect-integrating measurements,
because chemical analysis is inefficient in the assessment of
complex pollution.
x
Secondary source, mobilization of contaminated particles and
release of contaminants after natural or artificial resuspension of
sediments.
x
Final storage quality, the ability of a sediment body for long-term
immobilization of potentially hazardous substances; e.g., this
can be achieved by transfer into practically insoluble pollutant
species (Forstner, 2003).
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The seafloor is covered by layers of sediment (except on
youngest crust at mid-ocean ridges). The oldest sediment rests on
oceanic crust (and is the same age as oceanic crust). The youngest
sediment is at the top. Sediment is not distributed uniformly on the
seafloor. There is very little sediment cover on mid-ocean ridges and
upto 10,000 m beneath continental rises. The average thickness
of sediments is about 500 m. Rates of sediment accumulation
(sedimentation rates) also vary. Sedimentation rates are typically
measured in units of cm per 1,000 years. The sedimentation ranges
are as follows
x
Deep Ocean (average) : 0.5-1.0 cm/1000 year
x
Continental margins : 10-50 cm/1000 year
x
Major river deltas, some bays and estuaries: as high as 500 cm/
1000 year
Ocean sediments are important for a variety of reasons
i.
Ocean sediments contain valuable resources: petroleum and
natural gas, minerals, etc.
ii.
Sediments record evidence of past processes occurring on land
and in the ocean. These include:
x Rates of continental erosion and transport by rivers
x Down-slope movements of turbidity currents
x Biological activity in surface waters
x Volcanic eruptions
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1.2.1. Classification of marine sediments
Basically marine sediments have been classified into two
categories
i) Descriptive classification
ii) Genetic classification
i) Descriptive classification
Descriptive
classification
distinguishes
sediments
by
differences in texture or composition. Useful textural properties of
sediments include grain size, grain shape and grain roundness.
These classifications consider differences in mineral content, chemical
composition, or in the case of biogenic sediments, the most abundant
biological constituent. Particle size is important in determining the
methods of transportation and accumulation of sediments within the
oceans. Size classes of sediments are given in Table 1.1.
Table 1.1. Size classes of sediment
Particle description
Particle size (mm)
Cohesive properties
Cobble
256-64
Non-cohesive sediment
Gravel
64-2
Non-cohesive sediment
Very coarse sand
2-1
Non-cohesive sediment
Coarse sand
1-0.5
Non-cohesive sediment
Medium sand
0.5-0.25
Non-cohesive sediment
Fine sand
0.125-0.063
Non-cohesive sediment
Silt
0.062-0.004
Cohesive sediment
Clay
0.004-0.00024
Cohesive sediment
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The grain size of sediment gives an indication of the energy
of the environment where the grains were transported and deposited.
The smallest grains (clay) sink very slowly through water column, and
can remain suspended by slight turbulence in flowing water.
They tend to accumulate only under conditions where water is not
flowing rapidly. They can also be transported large distances by wind.
By contrast, larger grains (gravel, pebbles, etc.) sink rapidly, and can
only be pushed along the bottom by fast flowing water, such as might
be found in a fast flowing stream or where waves break against a
beach. These larger grains can thus accumulate in relatively high
flow-energy environments. Sand, silt, being intermediate in size, can
be moved by moderate flows. Sand, like gravel, sinks relatively quickly
and is mostly transported along the bottom of the water column. Silt
and clay can move in suspension (within the water). Both can also be
transported to limited distances by strong winds. The terrigenous
sediments deposited on the deep seafloor far from continental margins
are fine grained materials (silt or clay-sized) transported by the wind,
that have fallen out of the air and settled slowly through the water
column. The biogenous fraction of the sediment on the deep-ocean
floors, however, ranges in size from clay to sand. Coarser terrigenous
materials (sand-sized and larger grains) are mostly restricted to
marginal
areas
adjacent
to
the
continents,
where
rates
of
sedimentation are much higher. Figure 1.1 shows a graph that
describes the relationship between stream flow velocity and particle
erosion, transport and deposition. The entrainment of silt and clay
needs greater velocities than larger sand particles.
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Fig. 1.1. The relationship between stream flow velocity and particle erosion,
transport, and deposition
Sediments play an important role in elemental cycling in
the aquatic environment. They are responsible for transporting a
significant proportion of many nutrients and contaminants. They
also mediate their uptake, storage, release and transfer between
environmental compartments. Most sediment in surface waters
derives from surface erosion and comprises a mineral component,
arising from the erosion of bedrock, and an organic component arising
during soil-forming processes (including biological and microbiological
production and decomposition). An additional organic component may
be added by biological activity within the water body.
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Knowledge of the size gradient of particles that make up
suspended load is a prerequisite for understanding the source,
transportation and, in some cases, environmental impact of sediment.
Although particles of sizes ranging from fine clay to cobbles and
boulders may exist in the marine environment.
Fine grained sediment (silt + clay) is responsible for a
significant proportion of the annual transport of metals, phosphorus,
chlorinated pesticides and many industrial compounds such as
polynuclear aromatic hydrocarbons, polychlorinated biphenyls, dioxins
and furans. Of the 128 priority pollutants listed by the United States
Environmental
Protection
Agency,
65%
are
found
mainly,
or
exclusively, in association with sediment and biota.
ii) Genetic classifications
Genetic classifications distinguish sediments according to
the process by which they originate. Marine sediments originate by
three basic processes viz., biological, chemical and physical. According
to the origin, further the marine sediments have been classified into
four categories (Table 1.2)
x Terrigenous
x Biogenous
x Hydrogenous
x Cosmogenous
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Terrigenous
Most
terrigenous
sediments
are
minerals
and
rock
fragments derived from weathering of the continents. Those found
near the continents are mostly delivered to the oceans by rivers, but
they also come from wave erosion of coastal rocks and sediments.
These sediments are transported along the continental shelf by waves
and near shore currents. Eventually, they can be transported down
the continental slope by gravity flows (slumps, slides, turbidity currents).
At high latitudes, glacial marine sediments are deposited at the fronts
of glaciers, or in the deep ocean when icebergs drop sediment as they
melt (known as ‘ice-rafted’ sediment). Volcanogenic sediment is
volcanic debris deposited near sites of volcanism, such as near
convergent-margin volcanic arcs.
Biogenous
Sediments in which the grains were formed by the action of
a living organism like shells, and other hard parts secreted by
organisms that fall to the bottom of the ocean and slowly accumulate.
When the biogenic component makes up more than 30% of the
sediment the sediment is called ooze.
Oozes composed of the hard
parts of various organisms occur in the deep ocean. They are not very
abundant on the continental margins due to dilution by terrigenous
sediments. Oozes dominate 62% of Deep Ocean.
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The most common biogenous sediments include:
Siliceous oozes (SiO2nH2O; silicon dioxide)
Siliceous oozes are made up of the tests of floating
(planktonic) organisms that extract silica from seawater to make their
hard parts. The biogenic form of silica is opal, while the inorganic form
is quartz. Opal contains significant amounts of water bound up in its
structure.
Calcareous oozes (CaCO3; calcium carbonate; calcite and aragonite)
Many marine organisms constitute skeletons of the calcium
carbonate mineral ‘calcite’. Aragonite (Mother of Pearl) is a less common
biogenic form of CaCO3. Aragonite and calcite are polymorphs of CaCO3,
meaning they have different crystal structures but the same chemical
composition. Aragonite, although common in the shells of planktonic
mollusks (the pteropods), is easily dissolved by seawater and is not
commonly preserved in deep-ocean sediments. Aragonite (pteropod)
oozes are only preserved in relatively shallow, warm, tropical waters.
Phosphates (Ca5(PO4)3(OH, F); calcium phosphate)
The common skeletal mineral composing the bones and
teeth of vertebrates (e.g., fish, marine mammals, birds) is biogenic
apatite. In contrast to aragonite, apatite is a very stable mineral that
does not degrade easily under normal conditions. Apatite is usually a
minor component of deep-sea sediments.
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Hydrogenous
Sediments
formed
by
chemical
precipitation
of
the
components dissolved in seawater. The most common hydrogenous
sediments are ‘manganese nodules’. These are black, lightweight
objectives that show concentric layering. They are commonly found on
the deep seafloor in regions of slow sedimentation (e.g., the deepocean basins). The nodules, on average, are composed of 64% MnO2,
33% Fe2O3, and 3% of mixed Ni, Co, and Cu.
Another important type of hydrogenous sediment is
hydrothermal sediment. Hydrothermal sediments are produced at
mid-ocean ridges. Cold seawater percolates through fissures near the
ridge crest. This water is then heated by hot rocks under the ridge,
and it leaches metals out of the basaltic oceanic crust. These
hydrothermal fluids then flow back out of the ridge through fissures
and vents. Temperatures of these fluids have been measured at
greater than 300qC, and they are known to support unusual biological
communities that live at the interface between cold ocean water and
the hot vent fluids. As the metal-rich hydrothermal fluids mix with
seawater and cool, oxides of Mn and Fe precipitate and are deposited
at the ridge crest. These deposits also contain economical deposits of
gold (Au) and other important metals.
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Cosmogenous
Inorganic
sediments
which
are
originate
by
the
accumulation of materials from outer space. Two main types of
cosmogenous sediments are as follows:
i.
‘Cosmic
spherules’
form
when
sand-sized
particles
of
interplanetary dust melt as they enter the upper atmosphere at
speeds of about 12 km/sec. These small spherical objects can be
removed from deep-sea sediments with strong magnets.
ii.
Impact deposits form when large asteroids or comets impact the
earth at speeds of 15 to 60 km/sec., the enormous explosions
that blast meteorite material from great distances. One such an
impact occurred 65 million years ago (that killed all the dinosaurs
and many other species) left a sediment layer over the entire
surface of the earth.
Terrigenous sediments are by far the most abundant by
volume and mass, followed by the biogenous sediments. Hydrogenous
materials are found to be only a small portion of marine sediments,
and cosmogenous materials are very rare except near ancient
meteorite impacts. Distribution of principal type of sediment on the
ocean floor is shown in Fig. 1.2.
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Table 1.2. Classification of marine sediments by source of particles
Sediment
type
Source
Examples
Distribution
% of all
ocean floor
area covered
Terrigenous
Erosion of land,
volcanic eruptions,
blown dust
Quartz sand,
clays,
estuarine mud
Dominant on continental
margins abyssal plains,
polar ocean floors
~ 45
Biogenous
Organic; accumulation
of hard parts of some
marine organisms
Calcareous
and siliceous
oozes
Dominant on deep-ocean
floor (siliceous ooze
below about 5 km)
~ 55
Hydrogenous
Precipitation of
dissolved minerals
from water, often by
bacteria
Manganese
nodules,
phosphorite
deposits
Present with other, more
dominant sediments
Cosmogenous
Dust from space,
meteorite debris
Tektite
spheres,
glassy nodules
Mixed in very small
proportion with more
dominant sediments
<1
0
Fig. 1.2. Distribution of principal type of sediment on the ocean floor (Kennett, 1982)
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1.3. MINERALS
Minerals are everywhere around us. Silicon and oxygen are
the most abundant crustal elements, together comprising more than
70% by weight. It is therefore not surprising that the most abundant
crustal minerals are the silicates (e.g., olivine, Mg2SiO4), followed by
the oxides (e.g., hematite, Fe2O3). The silicate minerals make up the
largest and most important class of rock-forming minerals, constituting
approximately 90% of the crust of the earth (Fig. 1.3). They are
classified based on the structure of their silicate group. Other important
types of minerals include: the carbonates (e.g., calcite, CaCO3) the
sulfides (e.g., galena, PbS) and the sulfates (e.g., anhydrite, CaSO4).
Most of the abundant minerals in the earth's crust are not of
commercial value.
Fig. 1.3. Classification of silicates (Bailey, 1980)
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Sandstone is a sedimentary rock composed mainly of sand
sized minerals such as quartz and/or feldspar (quartz and feldspar
are the most common minerals in the earth’s curst). The formation of
sandstone involves two principal stages. First, a layer or layers of sand
accumulates as the result of sedimentation, either from water (as in a
stream, lake, or sea) or from air (as in a desert). Typically,
sedimentation occurs by the sand settling out from suspension; i.e.,
ceasing to be rolled or bounced along the bottom of a body of water.
Finally, once it has accumulated, the sand becomes sandstone when
it is compacted by pressure of overlying deposits and cemented by the
precipitation of minerals within the pore spaces between sand grains.
The environment where it is deposited is crucial in
determining the characteristics of the resulting sandstone, which, in
finer detail, including its grain size, sorting, and composition and, in
more general detail, include the rock geometry and sedimentary
structures. Principal environments of deposition may be split between
terrestrial (rivers, alluvial fans, glacial outwash, lakes and deserts)
and marine environments (deltas, beaches, delta flats, offshore bars,
storm deposits and turbidities).
Quartz framework grains are the dominant minerals in
most sedimentary rocks; this is because they have exceptional
physical properties, such as hardness and chemical stability. These
physical properties allow the quartz grains to survive multiple
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recycling events, while also allowing the grains to display some degree
of rounding. Feldspathic framework grains are the second most
abundant mineral in sandstones. Feldspar can be divided into two
smaller subdivisions: alkali feldspars and plagioclase feldspars. Alkali
feldspar is a group of minerals in which the chemical composition of
the mineral can range from KAlSi3O8 to NaAlSi3O8, this represents a
complete solid solution. Plagioclase feldspar is a complex group of
solid solution minerals that range in composition from NaAlSi3O8 to
CaAl2Si2O8 (Boggs, 2006).
Clay minerals are hydrous aluminium phyllosilicates,
sometimes with variable amounts of iron, magnesium, alkali metals,
alkaline earths, and other cations. Clay minerals are common
weathering products (including weathering of feldspar) and low
temperature hydrothermal alteration products. Clay minerals are very
common in fine grained sedimentary rocks such as shale, mudstone,
and siltstone and in fine grained metamorphic slate and phyllite.
Clays are ultrafine-grained (normally considered to be less than 2
micrometers in size on standard particle size classifications) and so
require special analytical techniques. Clay minerals include the
following groups:
Kaolin group includes the minerals kaolinite, dickite,
halloysite, and nacrite (polymorphs of Al2Si2O5(OH)4). Some sources
include the kaolinite-serpentine group due to structural similarities.
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Smectite group includes dioctahedral smectites such as
montmorillonite, nontronite and trioctahedral smectites for example
saponite.
Illite group includes the clay-micas. Illite is the only
common mineral.
Chlorite group includes a wide variety of similar minerals
with considerable chemical variation.
Clay minerals are aqueous silicates with layered or chain
lattices comprising layers of silicon-oxygen tetrahedral formed into
hexagons and united by octahedral layers. Usually, the clay minerals
are represented by very fine particles and possess the latter’s qualities
of plasticity and absorption. The sizes of the particles show a
significant range of variation; for example, kaolinite particles range in
size from 1 to 100 µm. The majority of clay minerals in sediments are
confined to the fraction < 1 µm. Apart from clay minerals, finely
ground primary minerals such as quartz, amphibole, goethite, and so
forth, are also found in this fraction (Lisitsyn and Kennett, 1996).
1.4. PETROLEUM HYDROCARBONS IN MARINE ENVIRONMENT
Ocean sediments are repository of vast oil (petroleum) and
natural gas deposits. They form when organic matter of dead
microorganisms is buried by mud on the seafloor. Due to high
temperatures and pressures at great depths, this organic matter is
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converted into oil and natural gas. Petroleum hydrocarbons consist of
a very large number of compounds that by definition are found in
crude oil as well as other sources of petroleum such as natural gas,
coal, and peat. Petroleum geochemistry is the study of geochemical
processes that lead to the formation, migration, accumulation and
alteration of crude oils and natural gas (Hunt, 1995). Crude oil is a
complex mixture of thousands of organic compounds, formed through
processes i.e., deposition, thermal and bacterial alteration of organic
matter (OM), catalytic effects of clastic minerals, oxidation and
reduction in sedimentary environment for millions of years (Tissot and
Welte, 1984).
Hydrocarbons enter the marine environment via three
general processes (Farrington, 1980):
i. Biosynthesis (biogenesis hydrocarbons)
Marine organisms can a) synthesize their own hydrocarbons,
b) obtain from their food sources, or c) convert precursor compounds
obtained with their food. These hydrocarbons may be released during
metabolism or upon the death and decomposition of the organism.
ii. Geochemical processes
There are a number of geochemical processes introducing
hydrocarbons into the marine environment. The natural seepage of oil
is an obvious example of this category. Weathering of ancient
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sediments and associated ancient hydrocarbons to the marine
environment by fluvial or Aeolian processes can result in introduction
of an assemblage of hydrocarbons and other processes, forest fires
and early diagenesis of organic matter deposited to surface sediments,
must also be considered (Farrington, 1980).
Submarine and coastal land oil-seeps release petroleum
hydrocarbons to the marine environment. Weathering of soil and
sediments and transport of some of the hydrocarbons in these
sediment to the marine environment should also be considered as an
input, although probably small when compared to other sources
because of a slow degradation of the hydrocarbons during the
weathering process.
iii. Anthropogenic inputs (petroleum contamination)
The oil entering the sea from anthropogenic activities,
mostly (65.2%) originates from discharges of municipal and industrial
waste, urban and river runoff, ocean dumping, and atmospheric
fallout. An additional 26.2% of the oil derives from discharges related
to transportation (e.g., tanker accidents, deballasting, and dry docking).
Only about 8.5% of the anthropogenic input is attributable to release
from fixed installations (e.g., coastal refining, offshore production facilities,
and marine terminals). The total oceanic input of petroleum hydrocarbons
from man’s activities is approximately 2.37 million tonnes (mt)/year,
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which far exceeds that from natural source (0.25 mt/year) such as oil
seeps (Kennish, 1997). Petroleum hydrocarbons contamination in the
aquatic environment is controlled by their characteristics and site
condition as well. The fate of petroleum hydrocarbons in aquatic
environment is shown in Fig. 1.4.
Fig. 1.4. The fates of petroleum hydrocarbons in aquatic environment
Most of the weathering processes, such as evaporation,
dispersion, dissolution and sedimentation, lead to the disappearance
of oil from the surface of the sea, whereas others, particularly the
formation of water-in-oil emulsions and the accompanying increase in
viscosity, promote its persistence. The speed and relative importance
of the processes depend on factors such as the quantity and type of
oil, the prevailing weather and sea conditions, and whether the oil
remains at sea or is washed ashore.
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Ultimately, the marine environment assimilates spilled oil
through the long-term process of biodegradation. Physical, chemical
and biological fates of petroleum in water are illustrated in Fig. 1.5.
Fig. 1.5. Physical, chemical, and biological fates of petroleum to aquatic system
1.5. METALS IN MARINE SEDIMENTS
Some metals enter the sea from the atmosphere, e.g.,
natural inputs of metals, such as aluminium (Al) in wind blowing dust
of rocks and shales, and mercury (Hg) from volcanic activity. Lead (Pb)
inputs in the atmosphere from industrial and vehicular exhaust are
much greater than natural inputs. Some metals are deposited by gas
exchange at the sea surface, by fallout of particles (dry deposition) or
are scavenged from the air column by precipitation (rain) which is
called wet deposition. Rivers make a major contribution of metals in
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the marine environment. The nature of metals depends on ore-bearing
deposits in the catchment area and the discharge of human waste and
discharges when the river passes through urban areas. Dredging of
shipping channels produce large quantities of metal pollution. Much
smaller quantities of metals are added to the sea by direct discharges
of industrial and other waste and the dumping of sewage sludge.
An important characteristic of marine sediments is that they contain
enhanced (excess) concentrations of certain trace or heavy metals,
such as Cu, Cr, Ni, Pb, and Zn. Heavy metals entering the marine
environment are removed from surface waters by either physical
(i.e., water movement) or chemical and biological (scavenging)
processes and transported to the sediment surface in a variety of solid
phase associations, in the form of biogenic detritus, clay minerals and
hydrogenous precipitates. Detrital elements are associated with a
crystalline mineral matrix and are usually immobile with respect to
early diagenesis. In contrast, elements which have been transported in
a dissolved form and have been incorporated into sediments from
solution (the non-detrital or non-residual or authigenic fractions) are
associated with a variety of non-crystalline phases. The non-residual
elements have the potential to be environmentally mobile and can be
involved
in
the
particulate/dissolved
reactions
of
the
aquatic
biogeochemical cycles. During early diagenesis, dissolution, remobilization
and migration of heavy metals occurs at and below the sediment/
water interface; the latter resulting in a supply of heavy metals to the
upper portions of deep-sea sediments.
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1.6. MAGNETIC MINERALS IN SEDIMENTS
Magnetic minerals can be produced, modified, transported
and deposited by a range of environmental and anthropogenic processes
(Thompson and Oldfield, 1986). The major sources and cycles of
magnetic minerals are summarized in Fig. 1.6. In some situations
these magnetic minerals provide very stable assemblages which may
be traced from source to sink (Walden et al., 1992), whilst in other
situations they may be modified by subsequent environmental
conditions (Snowball and Thompson, 1988). Thompson et al. (1980) have
summarized the principal sources of magnetic minerals displaying
ferromagnetic behavior within the environment and which may be
present within the soils and sediments of interest to the quaternary
scientist. These include detrital minerals derived from other rocks,
sediments or soils (transported by water or wind), authigenic/
diagenetic production, volcanic ash, in-situ pedogenic processes
(including both inorganic and organically driven production), cosmic
sources (generally only important near sites of meteor impacts),
anthropogenic pollution and magnetic bacteria.
Sedimentary rocks may have magnetic minerals as detrital
particles within them and, additionally, such minerals may be formed
with the rocks during or after lithification. The major groups of rockforming minerals, the silicates, are diamagnetic or paramagnetic. In
general, silicate minerals which contain magnetic ions such as Fe2+,
Fe3+ or Mn2+ are paramagnetic.
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Fig. 1.6. Major sources and cycles of magnetic minerals within the environment
(Thompson and Oldfield, 1986)
1.7. IMPORTANCE OF MINERAL, GEOCHEMICAL AND ENVIRONMENTAL
MAGNETIC STUDIES IN SEDIMENTS
Marine sediments are highly fractionated crustal materials
supplied to the ocean from a number of different sources. Most of the
near shore sediments are brought to the sea by the action of rivers,
thus their composition is determined largely by the lithology of the
contributing catchment area. Moreover, sediments are repositories of
heavy metals, organic carbon and petroleum hydrocarbons deriving
from anthropogenic activities, particularly near heavily populated
domestic centers. In the mobilization process, heavy metals may be
adsorbed by clays, can complex with organic compounds or may
co-precipitate with oxides and hydroxides. As many metals and
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hydrocarbons occur naturally in weathered materials and oil seeps
and drainage systems due to their presence in local rocks, the relative
influence of natural and anthropogenic sources on the geochemistry of
coastal sediments is not always clear. Therefore, for a better assessment
of metal and hydrocarbon distributions within such environment, it is
important to distinguish between pollutants released by natural
processes and those introduced by human-related activities.
The amounts of heavy metals and hydrocarbons in natural
systems can be of environmental significance because where elevated
they may contaminate surface and shallow groundwater. In addition,
marine organisms and vegetation in coastal environments can uptake
metals and hydrocarbons, increasing the potential for the entry of
some metals and hydrocarbons into the food chain. Furthermore,
sediment data is useful for describing metal and hydrocarbon
occurrence in assessing their distribution in coastal plains.
Petroleum hydrocarbon residues and heavy metals are
considered as priority pollutants; hence the concentration of
hydrocarbons and heavy metals in the sediment could be used to
define regions of polluted sediment. To map the spatial and temporal
extent of the polluted sediment using chemical analyses would be
impractical in terms of time and cost per analysis. To monitor
sediment contamination from industrial and other anthropogenic
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activities, there is a need of fast and cost effective screening and
monitoring tools for sediment pollution. Previous studies have
demonstrated that environmental magnetic methods not only can be
used for identification of sources of contaminants but also as an
approximate tool to detect and characterize environmental pollution.