Introduction

Geologists have studied most aspects of sediments in order to elucidate characteristics significant in determining the environment of deposition. Traditionally the lithotype + biotype = rock approach has led to the concept of lithotopes and biotopes (environmental derivatives of lithotype and biotype), and the very successful concepts of lithostratigraphy, biostratigraphy and chronostratigraphy. It is clear that no single parameter can be used to distinguish all depositional environments, but if a number of parameters are measured, chances of error may be reduced. Unfortunately, which parameters are of most importance is unclear. Much of this uncertainty exists because sampling of the environments has been inadequate, and available data bases are poorly defined. Moreover, conclusions are rarely based upon the principles of experimental design and hypothesis testing.

The accessibility of samples strongly influences the choice of a sampling design, and, for the determination of depositional environment this is a particularly significant constraint. If outcrops are studied, a wealth of information relating to paleoenvironments becomes available; if conventional cores are studied the quantity of possible information is reduced, and even further so by sidewall cores. Unfortunately, the most abundant kind of sample available to the subsurface geologist is the collection of cuttings from the rotary bore. The information available from cuttings is indeed minimal, and even with good samples the derived information may be of reduced value due to caving and other problems connected with taking samples. Unfortunately for the subsurface geologist well cuttings with occasional sidewall cores are the rule rather than the exception.

Introduction

The Triassic in its type area, the Germanic Basin, is characterized by a threefold lithologic subdivision: (1) the Buntsandstein, (2) the Muschelkalk, and (3) the Keuper. This facies development can be recognized in large parts of Europe, namely, Germany, Poland, Luxembourg, France, The Netherlands, the North Sea Basin and Switzerland. Muschelkalk carbonates are not represented in Great Britain. In other areas, towards the basin margins, the Muschelkalk is developed in a more clastic facies.

Although the marine standard stages of the Triassic were established in the Alpine Tethys area, the traditional lithologic units of the Germanic Basin are still often incorrectly employed as chronostratigraphic units. These lithologic units are at least partly diachronous (Kelber, 1990). The Buntsandstein and the Keuper are developed in a terrestrial facies. The marine Muschelkalk faunas are largely endemic. Hence, correlations between the Germanic and Tethyan Triassic are extremely difficult. Palynology is one of the very few methods which may enable interregional correlations of marine and nonmarine deposits.

In the last 15 years one of the major research projects of the Laboratory of Palaeobotany and Palynology (Utrecht) has focused on the palynological characterization of the Alpine Triassic standard stages and substages (e.g., Schuurman, 1976, 1977, 1979; Visscher & Krystyn, 1978; Visscher & Brugman, 1981; Van der Eem, 1983; Brugman, 1986). After palynological characterization of the marine standard stages was attempted, research concentrated on the Triassic in its classical area.

The availability of a number of continuously cored wells offered excellent opportunities for detailed palynological investigations. Studies of these well and outcrop sections in the Germanic Basin were carried out in cooperation with the Department of Geology of the Ruhr University Bochum (Germany).

Introduction

The title above is really the subject of this whole book. Some time ago, Egon Degens (1965) estimated that about 2% of the total volume of sedimentary rocks on Earth is organic matter, that is, 20 m of a total of 1000 m of sediment in the whole of Earth history. All but 5 cm (= coal and oil) of the 20 m (=19.95 m) is finely disseminated organic matter in shales, limestones and sandstones. Much of it is amorphous, much is from marine animals, algae and protists, either degraded or in recognizable particles. Since at least the Late Devonian, however, terrestrial plant biomass has been a (perhaps the) major source of the organic matter in sedimentary rocks (see discussion in Traverse, 1992). At present, approximately 150 × 106 metric tons of chemically resistant, particulate organic matter (POC) reaches the continental shelves from the major rivers of the world (Deuser, 1988; Ittekot, 1988). That can be compared with estimates of total sedimentation of 70 × 108 metric tons annually (Holland, 1978).

As a rather rash guess, I would say that modern sedimentary rocks probably contain something like 10 times as much resistant-walled pollen-and-spore-like material as do 375 × 106-year-old Devonian shales and sandstones (Schuyler & Traverse, 1990). Nevertheless, land-derived plant debris in microscopic particles has been a very significant factor in building the organic bankroll of the Earth for some hundreds of millions of years, and much of it is recognizable as to source. In addition, marine-derived organic matter has been and is making a sizable contribution.

Introduction

Pollen preserved in lake and peat sediments has been used to reconstruct changes in vegetation induced by climatic events (Davis, Spear, & Shane, 1980; Webb, Cushing, & Wright, 1983; Davis & Jacobson, 1985) and by anthropogenic alterations to the landscape (Brugam, 1978; Burden et al., 1986). Only recently has the record from estuarine sediments been similarly exploited (Brush, 1986; McGlone, 1988). In this chapter, we describe the general pathways of pollen in an estuary, and show from laboratory experiments and field observations how the transport and deposition of pollen in the estuary results in distributions in the sediments that record regional vegetation, land use and the effect of land use on rates and patterns of estuarine sedimentation. Our area of study is the Chesapeake Bay estuary extending between latitudes 37° and 39° 30' N in the mid-Atlantic region of the USA (Fig. 3.1).

Estuarine circulation

First, we examine the fluid motion, sediment motion, and salinity characteristics of Chesapeake Bay. Chesapeake Bay and many of its tributaries are partially mixed estuaries (Dyer, 1973), characterized by vertical, lateral and longitudinal gradients in salinity, but with no abrupt change from fresh to salt water as is found, for example, in the salt wedge environments of many fjords. On the other hand, Chesapeake Bay estuary is neither completely mixed nor homogeneous, as are estuaries characterized by large tidal flows and small river inflows. An example of an idealized, partially mixed estuary is shown in Figure 3.2A. When river water enters the estuary and meets ocean water, partial mixing occurs and causes lines of equal salinity to deform.

Introduction

Although the total global reservoir of carbon is massive (more than 1019 kg), only a very small fraction of this carbon is actively involved in the fluxes of the carbon cycle, most of the Earth's carbon being locked away in sediments, in carbonate in oceans and in igneous rocks and fossil fuels. The Earth's active carbon pool consists of carbon in living organisms, carbon in the atmosphere and carbon in soil organic matter. It has been estimated that the mass of carbon in soil organic matter amounts to about 3 × 1015 kg and this is approximately five times the size of the atmospheric pool (Bohn, 1976). The latter pool is approximately the same size as that in living organisms.

Components of soil organic matter

Soil organic matter can, initially, be largely sub-divided into three main fractions:

1. (a) The decomposing residues of plants (as well as animals and microbes).

2. (b) The living soil biota – microorganisms, animals and plant roots.

3. (c) Resistant organic matter – chemically/physically protected.

Under most circumstances, more than 90% of soil organic matter is in resistant forms, although there is a continual flow of carbon from one pool to another. Figure 3.1 illustrates how this continual ‘processing’ of soil organic matter occurs. Generally, the system is in a state of dynamic equilibrium, with the organic matter pools remaining more or less constant with time.

Decomposing residues of plants

By far the most important input to soil organic matter is in the form of plant residues.

It doesn't matter who you are or where you are, your very existence and survival depend upon the chemical reactions taking place all the time in soil. Indeed, this book itself owes its existence to the same reactions. The paper originates from trees grown in soil, the ink is from chemicals traceable back to the soil, and the authors are mere links in a complex food chain passing through soil-based cycles. The longevity of their role depends upon soil-derived food and regulation of the composition of the atmosphere by chemical reactions controlling the growth of photo synthesising plants. Of all the scientific disciplines that interact to make up the complex web of environmental science, soil chemistry could therefore be regarded as the most central. It is worth elaborating upon this concept, because any vague and fuzzy images the reader might have of possible applications of soil chemistry should then spring sharply into focus.

Figure 1.1 is an attempt to demonstrate the central role of soil chemistry, and its major interactions with other aspects of science. The outer ring of topics is impressive in its scope, but is by no means exhaustive. We could, for example, include terrestrial and aquatic zoology, since soils affect plants and water quality, and hence animal life. Plants may influence climate on a local scale and, in the long term, on a global scale, so we could add meteorology.

Having considered at some length in Chapters 2 and 3 the nature and origins of both the inorganic and organic components of soil, we are now in a position to consider how these components interact with water to regulate the chemical composition of the soil solution. These chemical and physicochemical interactions govern the chemical composition of the plant root environment, and hence short-term nutrient availability to plants. They also regulate drainage water chemistry, and are therefore clearly of great importance. It is appropriate to begin by looking more closely at the cation exchange behaviour of soils.

Cation-exchange properties of soil clays

The existence of cation-exchange properties of smectites and vermiculite has already been mentioned in Chapter 2. So far, however, we have only considered cation exchange from negatively charged sites arising as a consequence of isomorphous substitution by cations of similar size but with lower charge, occurring in interlayer spacings. Generally the cations in the interlayer spacings are only labile when the spacing is ca 1.4 nm or more. In addition to negative charge arising as a result of isomorphous substitution, it also occurs at the edges of crystals where valences would otherwise be incompletely satisfied. The individual clay particles in a moist soil will thus be surrounded by a layer of hydrated, positively charged cations. This, in turn, creates a localised zone of positive charge, which will attract anions from the soil solution, creating an electrical double layer.

What is soil fertility?

The term ‘soil fertility’ is difficult to define precisely. Generally it describes an overall soil condition which is the combined product of various natural processes and management options. A ‘fertile soil’ is one which is capable of producing a desired crop with favourable yield and quality characteristics. Large areas of the world still suffer from severe restrictions to agricultural productivity as a result of soil chemical, physical and/or biological limitations. The example of excessive salt build up has been mentioned previously (Chapter 4). It has been suggested that approximately 35% of the world's irrigated land suffers from suboptimal productivity (Carter, 1982).

For any production system to be sustainable, nutrient inputs and outputs must be balanced over a reasonable timescale. The nutritional requirements of individual plant species vary considerably and therefore the fertility of a soil varies for individual plant species (Brown & Jones, 1977). Removal of nutrient elements with the harvested crop is a major potential cause of decline in soil fertility, and losses should therefore be replenished. Typical values for the amounts of major nutrients removed by a range of crops are shown in Table 5.1. While in the present context chemical aspects of fertility are of prime concern, it is difficult to separate these from the closely linked physical and biological processes.

Natural levels of fertility are seldom adequate to sustain a reasonable long-term degree of agricultural productivity.

One of the many vital roles played by soil is its function as a buffer regulating the quality of water in rivers and lakes and, to some extent, groundwaters. In the majority of catchments, unless outcropping rock is dominant, precipitation interacts to a substantial degree with soil. The precise fate of incoming precipitation, i.e. the pathway it follows through or over soil to a drain, stream or lake, depends upon the physical characteristics of the soil, the quality, duration and intensity of precipitation, prior climatic conditions, surface and subsurface topography, and the physical form of the precipitation (e.g. rain, hail, snow, mist etc.). The hydrological pathway followed, in turn, governs which zone or zones of the soil the water interacts with en route to open surface water or groundwater. The purpose of this chapter is to consider briefly how the interactions between draining water and the soil biota, atmosphere, minerals and organic matter ultimately influence water quality in natural ecosystems. We will then be in a better position to see how these natural processes cope with human activities such as modern agricultural and forestry practices and with pollution generally. The way in which the soil/water system copes with pollutants of other origins is discussed in Chapter 7.

Drinking water must be acceptable with respect to taste, colour, microorganisms, salinity, acidity and concentrations of organic pollutants, toxic heavy metals, aluminium, nitrate and nitrite.

Conscientious readers of the preceding seven chapters should, at this stage, have a reasonable grasp of the fundamental principles of soil chemistry and should be well aware of its importance in the context of environmental science as a whole. They might well be left asking themselves: ‘Where does the subject go from here? What, if anything, do we still need to do or to find out?’ Those whose studies or interests are motivated by career prospects might be wondering what the future holds in store for aspiring soil chemists. With this possibility in mind, the authors thought it would be appropriate to bring the book to a close by taking a brief look into the future, while at the same time seeing what lessons might be learned from past progress and mistakes.

The changing nature of research in soil chemistry

One recurring concept throughout this short volume has been that of the dynamic nature of soils. Soil scientists have been aware of this fundamental idea for many decades, and it is central to discussions of soil evolution in both the presence and absence of anthropogenic influences. Much of our understanding has been based on qualitative, or at best semi-quantitative, understanding of the fundamental chemical, physical and biological mechanisms involved in soil development and change. Over the past few years there has been an increasing tendency to try to put this understanding on a more quantitative basis.