16

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 global-

scale 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).

18

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

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.

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.

Secondary source, mobilization of contaminated particles and

release of contaminants after natural or artificial resuspension of

sediments.

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

Deep Ocean (average) : 0.5-1.0 cm/1000 year

Continental margins : 10-50 cm/1000 year

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:

Rates of continental erosion and transport by rivers

Down-slope movements of turbidity currents

Biological activity in surface waters

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)

Terrigenous

Biogenous

Hydrogenous

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 deep-

ocean 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 300 C, 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

< 1

Cosmogenous Dust from space, meteorite debris

Tektite spheres, glassy nodules

Mixed in very small proportion with more dominant sediments

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 rock-

forming minerals, the silicates, are diamagnetic or paramagnetic. In

general, silicate minerals which contain magnetic ions such as Fe2+,

Fe3+ or Mn2+ are paramagnetic.

39

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

40

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

41

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.