Environmental Chemistry Volume 1

A Review of the Recent Literature Concerning the Organic Chemistry of Environments Published up to Mid-1973

By G. Eglinton

The Royal Society of Chemistry

Copyright © 1975 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-755-7

Contents

Chapter 1 Stable Isotope Studies and Biological Element Cycling By J. W. Smith, 1,
Chapter 2 Environmental Organic Chemistry of Rivers and Lakes, Both Water and Sediment By P.A. Cronwell, 22,
Chapter 3 Environmental Organic Chemistry of Bogs, Marshes, and Swamps By P. H. Given, 55,
Chapter 4 Environmental Organic Chemistry of Oceans, Fjords, and Anoxic Basins By R.J. Morris and F. Culkin G. Eglinton, 81,
Chapter 5 Hydrocarbons in the Marine Environment By J. W. Farrington and P. A. Meyers, 109,
Chapter 6 The Fate of DDT and PCB’s in the Marine Environment By M. M. Rhead, 137,
Chapter 7 Environmental Organic Chemistry of 2,4-Dichlorophenoxyacetic Acid By J. E. Allebone, R. J. Hamilton, and B. Ravenscroft, 160,
Author Index, 191,

CHAPTER 1

Stable Isotope Studies and Biological Element Cycling

BY J.W. SMITH

1 Introduction

Natural biological, physical, and chemical processes operating over geological time have resulted in the establishment of recognizable patterns in the distribution of the stable isotopes of many of the light elements. This knowledge and an increasing understanding in detail of the many individual processes involved in the creation of this pattern now allow the sources and previous histories of light elements in many geological systems to be determined with considerable certainty. Urey first demonstrated the connection between the environment and isotopic ratios and developed the oxygen thermometer for the evaluation of palaeotemperatures. Since these early experiments the method has acquired increasing recognition and application. Very recently2 the value of isotope-ratio measurements in revealing otherwise unobservable relationships and effects has been demonstrated in studies of the distribution of the light elements in returned lunar samples.

For the purpose of this discussion it must be assumed that the organic geochemist is primarily concerned with the isotopic composition of those organic compounds currently present, or being created or destroyed, in order that the biogeochemistry of natural processes may be better understood. However, much of the organic material in these three categories has recently been introduced into the present environment by man and it is therefore essential to know the extent and effect of such additions if a meaningful interpretation of experimental data is to be made. In this respect, the role of fossil fuels can rarely be ignored, a situation well demonstrated by the very considerable interest which continues to be paid to the effects on the environment of the direct release of either fossil fuels or the by-products resulting from their utilization in the chemical industry and power production. Even when due regard is paid to these effects, a meaningful understanding and interpretation of isotopic data can scarcely be made if interest is solely limited to organic molecules. Very often in Nature the immediate precursor of an organic compound is an inorganic molecule, an example being the photosynthesis of sugars from carbon dioxide, and, since the isotopic composition of the product is dependent on that of the reactant, it becomes essential in environmental studies to give some consideration to such inorganic portions of the element cycle. Perhaps the greatest benefit to be gained from isotopic measurements is the ability to determine both the precursors and decomposition products of materials of interest and as a result, biogeochemical studies commonly include not only investigations of the distribution and isotopic composition of existing organic compounds, but also of related inorganic species, e.g. sulphate, sulphide, and carbon dioxide, which may be of significance in the biological assimilation and cycling of the elements.

In a Report which is primarily concerned with organic materials, a full discussion of all those processes, both organic and inorganic, which result in a fractionation of the isotopes cannot be entertained. Accordingly, only those inorganic processes which most obviously and directly affect the distribution and isotopic composition of organic compounds are considered. It is understood, however, that all reactions which result in isotopic fractionation probably modify the isotopic ratios in organic compounds to some extent, even if this is not directly detectable. Not excluded are those conversions by microorganisms in which both the reactants and products are inorganic compounds and the organisms in fact provide little more than a pathway for the completion of thermodynamically favoured reactions. In the case of the dissimilatory bacterial reduction of sulphate, whilst at any stage the quantities of sulphur organically bound within cellular material are probably negligible when compared with the large quantities of sulphide produced, the major role played by this process in the sulphur cycle and the marked isotopic fractionations which result make the inclusion of such metabolic conversions essential.

2 Carbon

Since several excellent reviews of the geochemistry of the stable carbon isotopes are available, it is sufficient that only brief mention be made here of the processes responsible for isotopic fractionation. Either directly or indirectly, biological materials result almost entirely from photosynthesis. Carbon in the forms of gaseous and dissolved CO2 or as bicarbonate in solution may be utilized in the photosynthetic process; however, since at equilibrium the bicarbonate in solution is considerably enriched in 13C*> relative to CO2 in solution or in the gaseous state, the isotopic composition of photosynthesized materials will vary with the source of carbon available. In Nature these two major sources of carbon are the atmosphere and bicarbonate in solution in the oceans and, in general, materials derived from these two reservoirs may be distinguished by their 13C content. However, since the degree of isotopic fractionation between the two reservoirs decreases with temperature and the quantity of dissolved CO2 relative to bicarbonate in solution decreases with pH, estimates of the environment during photosynthesis based on isotopic measurements are not always precise. Differences in isotopic composition also arise between the carbon source and the products during photosynthesis. This fractionation has been attributed to the relative collision rates of the CO2 molecules with the leaf surface. Detailed studies of the process indicate that the major fractionation stage, which results in the photosynthetic product being enriched in 12C by some 17[per thousand] relative to atmospheric CO2, commonly occurs during the enzymatic fixation of dissolved CO2 as 3-phosphoglyceric acid.

Whilst the above situation holds in general for the majority of higher plants (that is, those which use the Calvin cycle in photosynthesis), evidence has been gathering to show the existence of other synthetic pathways for enzymatic fixation of carbon which give rise to different 13C/12C ratios in the final plant products. A study of 104 selected species of plants has revealed a much wider variation in the 13C contents than might previously have been expected; many terrestrial mono- and di-cotyledons and one gymnosperm have δ13C values greater than -18[per thousand]. Plants within this category included many from desert, salt-marsh, and tropical environments; where less favourable conditions for plant growth prevail it is suggested that the high 13C contents in these plants may reflect the utilization of other more efficient photosynthetic cycles under these harsher conditions. Considerable variations in the 13C/12C ratios between sub-species growing in different environments are reported in support of the view that physiological adaptations to the environment have been made by the plants.

Variations in the 13C contents of the products of photosynthesis also occur and commonly appear as isotopic differences between the extractable lipid portion of the plant and its main structure, or within particular classes of chemical compounds, e.g. carbohydrates, fatty acids, and amino-acids.

Since the major sources of carbon for photosynthesis are of inorganic form, although they may have been immediately derived from organic materials, it is essential that reference be made in this review to those investigations in which efforts to relate organic and inorganic carbon are made. Systems in which organic forms of carbon are not immediately involved will not be discussed here.

In attempts to determine the origins of naturally occurring organic compounds, isotopic comparisons are frequently made with other organic compounds which have resulted from the biological utilization of either atmospheric carbon dioxide or those carbon forms that are in solution in sea water. In many instances such comparisons have proven to be rewarding, and consequently the continued interest in this approach results in fresh additions being frequently made to the already sizeable literature on this aspect of isotope chemistry. Thus, whilst it has long been recognized that humic acids in non-marine sediments result from the degradation of the lignin in land plants, only comparatively recently has it been shown that humic acids constitute a very considerable fraction of the organic matter in marine sediments. Whether these marine acids are composed largely of transported continental materials, whether they are autochthonous and result from the recombination of the decomposition products of plankton, or whether they may be of dual origin is not fully resolved, although the general evidence favours the last view. Since terrigeneous plants are usually enriched in 12C relative to marine plankton, and it has been shown that the isotopic composition of the organic matter in marine sediments varies from δ13C -19 to -22[per thousand], and largely reflects that of the plankton in the water, several investigators have measured the 13C/12C ratios of marine and non-marine organic residues in attempts to determine the sources of carbon in each and to differentiate between these. Much of these data and those from their own studies of the humic acids from a wide range of marine, coastal, littoral, and continental sediments and soils has recently been combined by Nissenbaum and Kaplan in an effort to resolve this problem finally. δ13C values in the 20 marine samples examined range from -17.2 to -27.4[per thousand], with these extreme values relating to materials from the Cariaco Trench and the Santa Monica Basin, respectively. The high 12C content of the latter is explained by a large influx of land-plant material, but no explanation for the other anomalous extreme value is offered. When these two samples are excluded, an average value of -22.2[per thousand], results, with a standard deviation of l.O[per thousand]. The 12 coastal and littoral samples were found to have δ13C values of from – 19.1 to -27.3[per thousand], with the 3 samples from tidal marshes being most enriched in 13C and having values of -19.1, -19.3, and -21.2[per thousand]. The average value for the remaining 9 samples is -25.3[per thousand], with a standard deviation of l.0[per thousand]. The 14 continental samples exhibited the greatest variation in 13C contents, with values of from -14.8 to -29.1[per thousand], being reported. The highest 13C content related to soil from a sugar-cane plantation in Hawai. Carbon fixation in cane is via the Hatch-Slack pathway, and inclusion of plant debris in the soil probably accounts for the high 13C/12C ratio. No reason for the high 13C content of a Hula peat sample is given (-19.2[per thousand],). The sharp isotopic difference between the sediment (-21.0[per thousand],) in land-locked Lake Haruna and the soil (-28.2[per thousand],) from the lake shores shows that the former originates from a lacustrine biota rather than land-plant materials. When the three isotopically ‘heavy’ samples are excluded, the remainder have an average value of -26.O[per thousand], and a standard deviation of 1.5[per thousand],.

Although a general, if not well-defined, differentiation between marine, coastal, and continental humic acids can be made on the basis of absolute isotopic composition, the significant number of samples which are not easily accommodated into these three classifications suggest that either the processes determining the isotopic composition of the samples examined are insufficiently understood, or additional processes are operating.

Isotopic measurements are also used to illustrate the fact that although a contribution of terrigeneous humic acids to marine deposits often occurs close to continental margins, in general these acids are seldom transported far into the oceans, except where high-energy turbidity currents are involved.

In contrast to this broad survey, the U.C.L.A. group have recently reported their findings from a detailed ‘in depth’ study of the forms of carbon in samples of sediments and interstitial waters from several locations in Saanich Inlet, a fjord in British Columbia. The reported δ13C values of -19.2[per thousand], for the plankton, -26.6[per thousand], for the humus-rich soil in the Inlet surroundings, and -20.1[per thousand], to -22.5[per thousand], for the marine sediments suggest a dual origin for the organic matter in the sediments, a view which is further confirmed by the distribution of lipid constituents in these. Measurements on various classes of extractable compounds in the sediments, soils, and plankton gave a consistent isotopic pattern (Table 1). In every case the products derived from the plankton were enriched in 13C relative to the average values for the sediment and the products from the soils were depleted in 13C content relative to the sediment, thus confirming the value of this approach in this case and the dual origin of the sedimentary material.

As much as 150 mg1-1 of dissolved organic matter consisting of high-molecular-weight polymers of amino-acids and carbohydrates was extractable from the interstitial waters. The chemical and isotopic composition (δ13C -20 to -21[per thousand],) of this material, which is believed to be the precursor of fulvic and humic acids, indicates that it results largely from the recombination of plankton degradation products, a conclusion which is in marked contrast to the widely held view that humic acids are derived from the lignin and cellulose derivatives of higher plants. Differences between the 13C contents of these acids and the more highly condensed insoluble organic residues are thought to be largely due to the loss of isotopically ‘heavy’ CO2 during decarboxylation reactions.

The distribution and isotopic composition of the other forms of carbon present in the Saanich Inlet samples are particularly interesting. 613C values for the sediment carbonates range from +l.0[per thousand], at the surface to -3.5[per thousand] at depth, a change which is attributed to the production of biogenic CO2 in the deeper anoxic regions of the basin. However, δ13C values for the dissolved CO2 in the corresponding interstitial waters vary from – 11[per thousand] near the surface (one value of -37[per thousand] is reported) to +l8[per thousand], at depth. If these high 13C contents arose from a preferred utilization of the lighter isotope, both CO2 and 12C contents should decrease with depth, as in continental-shelf sedinients. Since this is not so, an explanation other than dependence on a simple kinetic effect is required.

The formation of isotopically ‘heavy’ CO2 as the result of exchange between this and the methane present in the system is not an acceptable explanation, since this exchange is extremely slow relative to that between CO2 and carbonate, and equilibration between the latter compounds was not established. It has been shown that CH4 and CO2, the latter strongly enriched in 13C, can be produced by the fermentation of acids but, in view of the large quantities of CO2 involved, the authors favour the reduction of preformed biogenic CO2 (δ13C -20[per thousand]) resulting from the diagenesis of the organic material present, by methane-forming bacteria using the molecular or organically available hydrogen in the system. Reduction of CO by such methods has been experimentally demonstrated20 and the degree of isotopic fractionation is in agreement with kinetic data.

Similar measurements have been less helpful in determining the origin of the extractable organics in the Dead Sea. The 13C contents of the lake sediments (-23.8 to -24.3[per thousand],), surface plankton (-24.8[per thousand],), surrounding soil (-24.3[per thousand],), closely associated oil shale (-28.7[per thousand],), and asphalt (-26.0[per thousand],) indicate that the contribution of carbon from the two latter possible sources is insignificant, but still do not allow the origin to be determined.

Parker, in an earlier study of shallow marine systems, has commented on the variations in isotopic composition which exist between organisms and between different compounds from the same organism using the same carbon source. The organic carbon in the individual organisms ranged in 13C content, relative to the inorganic carbon in the seawater, from 0 to -20[per thousand],, and in every case the lipids or fatty acids were depleted in 13C by from 4 to 15[per thousand], relative to the total organic carbon in the organism. The author suggests that in view of these results caution must be exercised when attempts are made to relate biogenic residues to particular growth environments on isotopic evidence alone. In the same system, diurnal variations of 4[per thousand], in the 13C content of the sea water were observed to correspond directly with the preferred utilization of 12CO2 during photosynthesis by day and the respiration of 12C-enriched carbon dioxide throughout the hours of darkness. Similar changes have been described in the atmosphere over densly wooded areas and grasslands, where both 12C and carbon dioxide contents fall during the day and rise at night. Decreasing 13C contents in city atmospheres as the result of vehicle exhaust-gas pollution have also been reported, as have been changes in the isotopic composition of wood samples with age as a result of increasing contributions of CO2 from the combustion of fossil fuels. CO production from combustion appears not to be of general significance. The photo-oxidation of methane is clearly the principal source of CO, although seasonal and local variations due to the autumnal death of plants, increased domestic heating, etc., occur. Five sources of CO with δ13C values from -22 to -30×0 are listed.

(Continues…)Excerpted from Environmental Chemistry Volume 1 by G. Eglinton. Copyright © 1975 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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