Introduction

Sediments from the North Atlantic and its subarctic basins preserve exceptional archives of changing environmental conditions. High-resolution studies of North Atlantic sediments have shown that magnetic measurements are particularly sensitive to millennial-scale variations and can make significant contributions to our understanding of climatic and environmental variability. Until recently, measurements of magnetic susceptibility have been most widely used and reported in this environmental context. However, significantly more information can be obtained by using a variety of magnetic measurements. Magnetic methods provide a highly sensitive, high-resolution, rapid and non-destructive technique for quantifying lithologic variability which, in marine sediments, is often a direct response to changing environmental conditions. Within the dynamic context of the North Atlantic region, magnetic measurements can also provide information about sediment source and depositional mechanism. High-resolution magnetic studies are now becoming increasingly common, employing a range of magnetic parameters to provide a wealth of new information about the dynamics and inter-relationships of the Earth's environmental system. The growth and decay during the Quaternary of large, mid-latitude continental ice sheets have been linked to changes in the amount and seasonal distribution of incoming solar radiation. These insolation changes display periods of approximately 100, 41 and 23 kyr, due to changes in the Earth's eccentricity, tilt, and precession relative to the sun, respectively (Chapter 10 and e.g. Hays et al., 1976). However, the cryosphere/climate system is not completely linearly forced by these orbital variations, but varies as a complex response to many interactions between the ice sheets, oceans and atmosphere. Superimposed on these long period orbital variations are more rapid environmental changes that occur on much shorter time periods (102–103 years).

Introduction

Iron sulphides have attracted the attention of geologists owing to their frequent occurrence in modern sediments, sedimentary rocks and ore deposits. They are of importance in studies of environmental pollution and in economic contexts: e.g. as ‘sinks’ for toxic heavy metals (Dekkers & Schoonen, 1994), as raw materials for sulphuric acid production (Lin, 1997) and as indicators of the presence of gold-bearing ores (Menyah & O'Reilly, 1991). Additionally, however, iron sulphides are attracting increasing attention within the contexts of environmental magnetism and palaeomagnetism. One reason for this interest is the increasing frequency with which the presence of authigenic magnetic iron sulphides is being reported, particularly in lake and brackish water sediments. These occurrences highlight the need for careful evaluation of sedimentary magnetic records in terms of palaeoenvironmental inferences (see Chapter 7). Magnetic iron sulphides have also been shown to play an important role in palaeomagnetic investigations of Quaternary (and older) sediments. For example, in sediments bereft of any detrital magnetic minerals, ‘syn’-depositional formation of authigenic greigite or pyrrhotite can provide a means of reliable remanence acquisition and hence contribute a robust and useful palaeomagnetic record. Conversely, as discussed below, post-depositional formation of such minerals may result in unreliable and ambiguous palaeomagnetic directions.

The formation of iron sulphides in sedimentary environments frequently involves geochemical processes that are mediated by micro-organisms, in particular bacteria responsible for the decomposition of organic matter. Sulphur isotope studies of Archean and Proterozoic rocks have revealed that sulphate-reducing bacteria have played an important role in the state of the global sulphur and oxygen cycles since an early stage in Earth's geological history (Ohmoto et al., 1993).

The Quaternary geological period, beginning roughly two million years ago, is the most recent, and ongoing, period of Earth's history. It comprises a particularly dynamic timespan, characterized by major shifts in climate, expansion and contraction of continent-sized ice sheets, rises and falls in global sea level, migrations and extinctions of fauna and flora, and not least, the evolution and exponential rise in population of modern humans. These sequences of dramatic changes in the Earth's climate and biosphere have frequently imprinted themselves within natural proxy records – as physical, chemical and isotopic variations within sediments or ice. Such proxy records, if they can be retrieved and deciphered, allow us to identify the timing, rate and mechanisms of past changes in climate and environment. This information is increasingly critical. It provides the context and perspective for both our present understanding and future prediction of climate, at a time when human modification of the climate system, via greenhouse gas emissions and pollutant aerosols, appears significantly under way.

This book aims to examine the Quaternary palaeoclimatic and environmental information recorded by magnetic proxies. Magnetic grains, dominantly iron oxides and sulphides, occur virtually ubiquitously in Quaternary sediments, soils, dusts and organisms, albeit often in minor or trace concentrations. As is widely known, such grains may act as palaeomagnetic recorders of the Earth's ancient magnetic field. Additionally, however, they may act as sensitive magnetic recorders of palaeoclimatic and palaeoenvironmental change. Changes in climate produce changes in the environment, including sedimentary and soil-forming environments. The mineralogy, concentration, magnetic grain size and morphology of magnetic grains may all vary according to the origins of the magnetic grains and their subsequent post-depositional sedimentary history.

Introduction

Earth's ecosystems are dynamic and can adapt to those environmental factors capable of causing injury, disease or mortality. These factors, termed stresses, can be of a biotic and/or abiotic nature. Among abiotic stresses, air pollution ranks as the most harmful (Fig. 8.1). Atmospheric particulate pollutants, especially primary pollutants (i.e. those released directly into the troposphere by natural or anthropogenic processes) include heavy metals and other elements. ‘Heavy metals’ (or trace or toxic metals) is a term applied to a large group of trace elements of both industrial and biological importance, with atomic densities greater than 6 g cm-3. Dust containing heavy metals is dispersed globally by atmospheric circulation and becomes a significant component of sediments, soils, and the hydrosphere. This has a major impact on Earth's ecosystems, owing to the rates and mechanisms by which atmospheric pollutants are (a) transferred to Earth's surface, (b) made available to receptor organisms, and (c) taken up by organisms. Some studies report that dust particles may contribute via dry deposition from 20% to as much as 90% of the total annual atmospheric deposition to forest canopies (Linbergh & Harris, 1981). Annually, wet and dry deposition contributes significant fractions of the total flux to forest floors; for Pb (70 to 100%), Zn and Cd (each ∼ 40%) and Mn (∼ 10%).

Introduction

Quaternary and older geological deposits are a natural archive for past climate-driven changes in the sedimentary environment and thus form a context for understanding present-day and future changes. Changes in climate produce variations in the sedimentary sequences which make up the geological archive. Any change may be reflected in many sedimentological and geochemical parameters. These can include sediment colour, density, grain size, carbonate content, and clay mineralogy, as well as geochemical elements, such as sulphur or barium, which respond to palaeoredox conditions or palaeoproductivity (e.g. van Os et al., 1994). All these sedimentological and geochemical factors can also influence the magnetic properties of sediments. Conversely, magnetic properties can be used as proxy parameters for many environmental processes. Such application of magnetic principles is called ‘environmental magnetism’. The magnetic properties of sediments are, naturally, the result of many complex and non-linear processes, such as diagenesis which involves dissolution, mobilization, and precipitation. The discipline of environmental magnetism must thus necessarily utilize data from other branches of the earth sciences. Also, to further our understanding of the sedimentary and geochemical processes involved, a precise chronostratigraphic framework is mandatory. Accurate time control is essential for constraining all kinds of processes, in particular their timing and rates of change.

A major breakthrough in Quaternary chronology – with a resolution and accuracy of several kyr – has come from establishing astronomical (polarity) time-scales (APTS).

Introduction

Lake sediments are natural archives of environmental information. Material settling on a lake bed may have originated from the atmosphere, the surrounding catchment or the lake itself, and samples of accumulated sediment may be examined in a variety of ways to reconstruct past environmental conditions. Since 1975, the sediments from over 100 different lakes world-wide have been analysed for their magnetic properties. This chapter is a review of the approaches which have been developed to interpret magnetic measurements and a summary of the environmental information which has been gained from magnetic records in different contexts – recent pollution, human and climate impact on hydrological processe and climate change. The focus is on the Holocene period but records which span glacial–interglacial timescales are briefly considered.

Lough Neagh revisited

When presented with a pollen diagram (Fig. 7.1) from Holocene lake sediments which displayed the same features as the sediment's record of magnetic susceptibility, a sediment property for which at the time little was known, there were alternative responses. A bizarre but unimportant coincidence? Or a correlation suggesting new insight, which demanded a closer look? In choosing the latter response, Frank Oldfield and Roy Thompson triggered not only new interest in the magnetic properties of environmental materials but also led a path of discovery along which completely novel applications of using magnetic measurements rapidly appeared.

Introduction

Direct measurements of the Earth's magnetic field have been collected for several centuries (Merrill et al., 1996). Even from such short historical records it is clear that the magnetic field is not static but rather changes continuously on a variety of time scales. This variability is termed secular variation (SV). Palaeomagnetists, over the last several decades, have taken advantage of the natural remanent magnetization (NRM) recorded in rocks to extend our knowledge of the geomagnetic field secular variation backward into prehistoric time. Studies of palaeomagnetic secular variation provide a long-term view of geomagnetic field variability and are useful for aiding our understanding of the dynamo process which generates the Earth's magnetic field (e.g. Merrill et al., 1996) and for chronostratigraphic correlations (e.g. Creer et al., 1990).

It is well known from historical measurements that the Earth's magnetic field varies in both intensity and direction. But, since it is easier to recover high quality palaeomagnetic field directions than it is to arrive at reliable estimates of past field intensity (e.g. Tauxe, 1993), many palaeomagnetic studies have reported directional data only, even though intensity is an intrinsic component of secular variation. Reliable measurements of past intensity fluctuations should, however, yield added valuable information in the quest for a better understanding of the core dynamo process as well as provide an important additional chronostratigraphic correlation tool. For these reasons there has been considerable recent interest in estimating past field intensity changes (e.g. Mejia et al., 1996; Lehman et al., 1996).

Introduction

Analysis of terrestrial sedimentary sequences in the Alpine region resulted in the ‘four-glaciation’ model of Penck & Bruckner, a model that was the paradigm for Quaternary science from 1909 until the 1970s. That this model was wrong was demonstrated only when cores of undisturbed and continuously accumulating sediment were retrieved from the deep-sea floor and their multiple oscillations of oxygen isotope ratio revealed. Yet this revolution in our view of the Quaternary might instead have taken place in north-central China rather than at the bottom of the ocean. For here exists the closest land-based analogue of the deep sea, with quasi-continuous accretion of sediment over the entire span of the Quaternary – the interbedded layers of windblown dust (loess) and buried soils (palaeosols) of the Chinese loess plateau (see cover photograph and Fig. 3.1). Safely distal from the erosive glacial processes of the mid-latitudes, these sediments record thirty or more alternations in climate. Transport of dust from source areas and ensuing dust deposition on the loess plateau were at a maximum during cooler, drier climate stages; weathering and soil formation were enhanced during warmer, wetter stages. These soil–loess oscillations were initially investigated as long ago as the 1930s (Thorp, 1936; Teilhard de Chardin & Young, 1930) and field mapping of the units was carried out through the 1950s (Zhu, 1958; Liu, T.S., 1959). Until the 1980s, it was thought that the timespan represented by the sediments was of the order of one million years.

The environmental safety of a nation is determined by its environmental philosophy or a concise statement summarizing the set of official views and principles worked out by political leaders, a declaration of primary goals, trends, and ways a nation's activities can achieve an optimal interaction between society and nature.

To harmonize the interactions between society and nature it is essential to create conditions for environmental stability manifested as the ability of the ecosystems to sustain their structural and functional features under direct and indirect stress of anthropogenic impacts. Environmental problems that emerged in the Aral Sea basin are an integral part of the socio-economic policy pursued in the former USSR.They cannot be solved isolated from other acute issues facing society.

Creeping environmental phenomena in the Aral Sea basin have resulted from:

• orienting agriculture and industry toward maintaining the cotton independence of the former USSR at the cost of large-scale development of new (virgin) lands without improving the quality of those lands;

• widespread introduction of cotton and rice monocrops;

• large-scale non-dose-related ‘chemicalization’ of agriculture;

• use of water-wasting technologies and irrigation techniques resulting in enormous losses of irrigation water during its transport;

• irrational siting of production activities in locations with a deficit of water resources and without regard to air pollution.

These factors emerged under the then-dominant socio-economic and political conditions of the former Soviet Union, which can be characterized by the following factors:

• centralized command-and-control-type administration of the national economy based on state ownership of the means of production and natural resources with an absence of economic responsibility (i.e., penalties) for environmental damage;

• […]

Introduction

The challenging problems of the Aral Sea coastal region, which have arisen following striking changes in ecological conditions in the area, now seem to have been the logical result of the ill-considered planning of anthropogenic impacts on natural resources and human habitat.

A comprehensive analysis of the process of development of current ecological conditions (and its impacts on the way of life and on public health) is of interest from both a scientific and a practical point of view. In this regard the Aral Sea coastal region is an appropriate subject of research, because the pronounced and ecologically important natural processes and phenomena observed here were accompanied by sharp, adverse changes in the region's medical and demographic conditions.

The existence of ‘green’ and ‘brown’ ecological problems (Vogel, 1994), most often the result of creeping environmental changes (i.e., slowly changing, incremental, cumulative processes), is fairly obvious:

It is unclear, however, on what time scales the initially unmonitored processes are altered. To what extent and with what degree of degradation (or transformation) of the environment are they connected? Do these processes reach identifiable threshold values? Is it possible to change negative patterns and trends once we begin to speak of crisis? Perhaps we can establish thresholds (i.e., levels) of ecological change by assessing the character and importance of related processes.

The Aral Sea region is a classic one for investigation of the processes of anthropogenic desertification in the arid parts of Central Asia. The rapid dynamics of those processes, sparked by the decline of Aral Sea level, and the pollution of river and sea waters among other problems, have taken place in plain view during the span of one generation. Desertification is manifested in the intense degradation of natural resources up to the point of their complete depletion in a given area. Desertification processes observed on the dried part of the newly exposed seabed are especially highly dynamic, having changed within a relatively short period of time. Comprehensive studies of regional desertification processes are of special importance to the development of the concepts and methods needed to plan for the control of those processes on the basis of scientific research.

Scale of desertification

The most negative effect on the environment of the drying out of the Aral Sea is in the delta plains of the Amudarya and Syrdarya, and in a radius of influence of the atmosphere on relative humidity and the temperature regime at a distance of 150–200 km in the southwestern direction. At the same time, the impact of the drying sea has been insignificant on the composition of coastal sediments on the Ustyurt Plateau and in the eastern part of the sea's periphery (the northwest Kyzylkum part of the Aral Sea region).

This book has proven to be a labor of love. It began in 1994 with support from the United Nations Environment Programme (UNEP) Water Unit's director, Walter Rast. The idea was to document the incremental changes that have taken place in the Aral Sea basin in the past several decades. As is now well known, the Aral Sea has dropped in level about 17 meters in the short time span of three-and-a-half decades, and has dropped in volume by two-thirds. The Aral Sea's commercial fishing industry has collapsed. And as a result of chemical fertilizers and pesticides in the runoff from the fields to the rivers and the sea, human health in the region surrounding the Aral coastline (called the Priaralye) has been greatly affected.

The approach taken was to identify researchers who have spent years, if not decades, monitoring some aspects of environmental change in the Aral Sea basin. It therefore involved researchers from a variety of disciplines and countries who dedicated, and continue to dedicate, their professional lives to improving our understanding of environmental changes at the regional level. The environmental aspects presented include the following: landscape changes, changes in sea water quality and quantity, desertification processes, regional climate change, changes in the deltas, human health, political ideological changes related to the environment, streamflow variations, fisheries, and environmental impacts of the Karakum Canal.

The framework suggested as a guideline to these researchers in the preparation of their assessments was to enable them to view the changes that they were to write about as creeping environmental problems (or CEP) CEP are long-term, low-grade, incremental but cumulative environmental problems.

The Aral Sea is a part of a self-regulating hydrological system. It receives water from the two largest rivers in Central Asia, the Amudarya and Syrdarya, and the sea water evaporates into the atmosphere. Changes in this balance have a primary effect on the water level of the sea. These rivers flowing to the Aral have exhibited extremely uneven flow from one year to the next. For example, during a 60-year period, the Syrdarya's annual flow ranged from 22 to 57 km3, with a mean value of 34 km3; the Amudarya's flow ranged from 48 to 101 km3,with a mean value of 63 km3. A ‘periodicity’ of low-flow and high-flow periods has been observed to last 10–12 years. Clearly, the decrease of river flow to the sea results in a drop in sea level, in a reduction of sea surface area and volume, and in changes in other characteristics of the sea.

Irrigation farming in the Aral Sea basin began at least as early as 4000 BC. The local population of this region, like those in the valleys of the Tigris, Euphrates and Nile rivers, shared the experiences and knowledge accumulated over generations about the use of regional water resources, and about how to carry out irrigation farming in river floodplains and deltas in arid areas without disturbing the balance of nature.

The major disturbance of the long-lasting natural balance between ecological change in the Aral basin and the sea began early in the twentieth century as a result of human activities. Before then, the sea was abundant in water and even had a tendency to rise, despite the onset of intensive colonization of the region by Tsarist Russia.

Introduction

The Aral Sea lies in the center of the Turan Desert in a transitional belt between the ‘cold’ northern and ‘warm’ southern parts of the desert.The area immediately around the Aral Sea is called the ‘Priaralye’. Its external boundary is hypothetical and, according to the definition of Barykina et al. (1979), ‘coincides with a border of the area where the sea affects ecosystems and land use’. The newly exposed drying-out part of the seabed is, thus, a part of the Priaralye.

The ecosystem is an historically determined formation of living (e.g., biota) and non-living (e.g., soils, groundwater, microclimate) components transformed by biota that are in constant movement and interaction and are capable of self-regulation and reproduction. Landscape conditions in the Priaralye include a gypsum desert (the Ustyurt Plateau), high desert plains in the northern Priaralye, ancient delta plains and a modern plain in the eastern Priaralye, and ancient deltas and a modern (live) delta plain in the southern Priaralye (Figure 6.1).

About 200 basic ecosystems classified as a vegetation association or as a landscape are found on Priaralye territory.The composition of the ecosystem in each landscape is specific to a region. The floristic composition of vegetation communities in the Priaralye includes about 1400 species, which is a large percentage of the total flora (about 1700 species) of the Turan Desert. There are more than 150 species of birds and 80 species of mammals. In the Priaralye, northern desert plants and animals are replaced by subtropical and tropical species.

Encompassing an area over 2.3 million km2, the Aral Sea basin was one of the largest economic regions of the former USSR. In the early 1960s, a program of extensive development of irrigated farming was launched using water from the Amudarya and Syrdarya, the largest Central Asian rivers flowing into the Aral Sea. By the mid-1970s and early 1980s, the intense use of the downstream flow resulted in the drying out of the riverbed in different seasons at a distance of about two hundred meters from the sea. This distance increased steadily at a rate of about 1 meter per year. The irrigated area in the mid-1980s had reached 6.8 million ha compared with 2.9 million ha in 1959. In the former USSR, 95% of the raw cotton and 40% of the rice was produced there. However, these Soviet attempts to conquer nature in the Aral region were accompanied by adverse environmental impacts. Steadily accumulating adverse impacts and their eventual interactions resulted in an environmental crisis in the early 1980s that evolved into an environmental disaster a few years later. As a result, the situation in the Aral Sea ranks among the largest human-induced environmental disasters in the twentieth century, in terms of geographic scope and degree of severity.

Water resource problems in the Priaralye region (the region around the Aral Sea) are a result of the diminution of the natural water supply to the Amudarya and Syrdarya deltas and their vegetation. The reduced water supply generated soil degradation, solonchak (saline soils) development on the newly exposed dry seabed, and large-scale salt and dust transport to adjacent areas, which have destabilized terrestrial ecosystems.

In recent years the environmental crisis of the Aral Sea basin has attracted worldwide interest. International, governmental, and public organizations, as well as different agencies of the United Nations and the World Bank have tried to comprehend the reasons for the initiation of this crisis and to overcome its adverse effects. The adverse effects of the crisis on socio-economic processes in the region and on the health of the local population have been reviewed in the scientific literature (e.g.,Micklin, 1991; Glantz et al., 1993; Elpiner, this volume) and in popular science publications (Ellis, 1990). Meanwhile, when purely biological problems are considered in such publications, they have either been considered only superficially or have not been considered at all. The notion of creeping environmental changes per se in the Aral Sea proper has been considered only slightly in the scientific literature. This is even more true for aquatic communities because the studies of such communities are more labor-intensive compared with studies of terrestrial ecosystems.

This paper attempts to fill the gap in this area, based on field observations over the past 16 years.

In spite of insufficient detailed information on many of the ecosystems of the Aral Sea, it is possible to state with a great degree of certainty that creeping environmental changes began to appear in the mid-twentieth century. These changes were not initially connected with human-induced desiccation of the Aral Sea but were caused by large-scale acclimatization processes. For example, beginning in 1927, 18 fish species were introduced into the Aral Sea, of which 15 survived and inhabited the sea for some period of time (Karpevich, 1975).