Marine Geochemistry E-Book

Roy Chester

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 1040

Veröffentlichungsjahr: 2012

Beschreibung

Marine Geochemistry offers a fully comprehensive and integrated treatment of the chemistry of the oceans, their sediments and biota. The first edition of the book received strong critical acclaim and was described as ‘a standard text for years to come.’ This third edition of Marine Geochemistry has been written at a time when the role of the oceans in the Earth System is becoming increasingly apparent.

 Following the successful format adopted previously, this new edition treats the oceans as a unified entity, and addresses the question ‘how do the oceans work as a chemical system?’  To address this question, the text has been updated to cover recent advances in our understanding of topics such as the carbon chemistry of the oceans, nutrient cycling and its effect on marine chemistry, the acidification of sea water, and the role of the oceans in climate change. In addition, the importance of shelf seas in oceanic cycles has been re-evaluated in the light of new research.

Marine Geochemistry offers both undergraduate and graduate students and research workers an integrated approach to one of the most important reservoirs in the Earth System.

Additional resources for this book can be found at: www.wiley.com/go/chester/marinegeochemistry.

Leseprobe

Table of Contents

Cover

Companion Website

Title page

Copyright page

Dedication

Preface to the third edition

1 Introduction

1.1 Setting the background: a unified ‘process-orientated’ approach to marine geochemistry

Part I: The Global Journey: Material Sources

2 The input of material to the ocean reservoir

2.1 The background

3 The transport of material to the oceans: the fluvial pathway

3.1 Chemical signals transported by rivers

3.2 The modification of river-transported signals at the land–sea interface: estuaries

4 The transport of material to the oceans: the atmospheric pathway

4.1 Material transported via the atmosphere: the marine aerosol

4.2 The chemistry of the marine aerosol

4.3 Material transported via the atmosphere: the air–sea interface and the sea-surface microlayer

4.4 The atmospheric pathway: summary

5 The transport of material to the oceans: the hydrothermal pathway

5.1 Hydrothermal activity: high-temperature basalt–seawater reactions

5.2 Hydrothermal activity: low-temperature basalt–seawater reactions

5.3 The hydrothermal pathway: summary

6 The transport of material to the oceans: relative flux magnitudes

6.1 River fluxes to the oceans

6.2 Atmospheric fluxes to the oceans

6.3 Hydrothermal fluxes to the oceans

6.4 Relative magnitudes of the primary fluxes to the oceans

6.5 The continental shelf

6.6 Relative magnitudes of the primary fluxes to the oceans: summary

Part II: The Global Journey: The Ocean Reservoir

7 Descriptive oceanography: water-column parameters

7.1 Introduction

7.2 Some fundamental oceanographic properties of seawater

7.3 Oceanic circulation

7.4 Tracers

7.5 An ocean model

7.6 Characterizing oceanic water-column sections

7.7 Water-column parameters: summary

8 Air–sea gas exchange

8.1 Introduction

8.2 The exchange of gases across the air–sea interface

9 Nutrients, oxygen, organic carbon and the carbon cycle in seawater

9.1 The nutrients and primary production in seawater

9.2 The distribution of nutrients in the deep oceans

9.3 Dissolved oxygen in seawater

9.4 Ocean carbon cycle

9.5 Dissolved carbon dioxide in seawater: dissolved inorganic carbon and the dissolved CO2 cycle

10 Particulate material in the oceans

10.1 The measurement and collection of oceanic total suspended matter

10.2 The distribution of total suspended matter in the oceans

10.3 The composition of oceanic total suspended matter

10.4 Total-suspended-matter fluxes in the oceans

10.5 Down-column changes in the composition of oceanic TSM and the three-layer distribution model

10.6 Particulate material in the oceans: summary

11 Trace elements in the oceans

11.1 Introduction

11.2 Oceanic residence times

11.3 The vertical distribution of dissolved trace elements in the oceans

11.4 Surface water distributions

11.5 Scavenging mechanisms

11.6 Trace-element speciation

11.7 Trace elements in seawater: summary

12 Down-column fluxes and the benthic boundary layer

12.1 Down-column fluxes

12.2 Evolution of particle flux with depth

12.3 The benthic boundary layer: the sediment–water interface

12.4 Down-column fluxes and the benthic boundary layer: summary

Part III: The Global Journey: Material Sinks

13 Marine sediments

13.1 Introduction

13.2 The formation of deep-sea sediments

13.3 A general scheme for the classification of marine sediments

13.4 The distribution of marine sediments

13.5 The chemical composition of marine sediments

13.6 Chemical signals to marine sediments

13.7 Marine sediments: summary

14 Sediment interstitial waters and diagenesis

14.1 Early diagenesis: the diagenetic sequence and redox environments

14.2 Organic matter in marine sediments

14.3 Diagenesis: summary

14.4 Interstitial water inputs to the oceans

14.5 Interstitial water inputs to the oceans: summary

15 The components of marine sediments

15.1 Lithogenous components

15.2 Biogenous components

15.3 ‘Hydrogenous’ components: halmyrolysates and precipitates

15.4 Cosmogenous components

15.5 Summary

16 Unscrambling the sediment-forming chemical signals

16.1 Definition of terminology

16.2 The biogenous signal

16.3 The detrital and authigenic signals

16.4 Unscrambling the detrital and authigenic signals

16.5 Signal spikes

16.6 The ocean-wide operation of the sediment-forming signals

16.7 Unscrambling the sediment-forming chemical signals: summary

Part IV: The Global Journey: Synthesis

17 Marine geochemistry: an overview

17.1 How the system works

17.2 Balancing the books

17.3 The role of the ocean margins

17.4 Conclusions

Index

COMPANION WEBSITE:

This book has a companion website:

www.wiley.com/go/chester/marinegeochemistry

with Figures and Tables from the book

This edition first published 2012 © 2012 by Roy Chester and Tim Jickells

Blackwell Publishing was acquired by John Wiley & Sons in February 2007. Blackwell’s publishing program has been merged with Wiley’s global Scientific, Technical and Medical business to form Wiley-Blackwell.

Registered office: John Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK

Editorial offices: 9600 Garsington Road, Oxford, OX4 2DQ, UK

The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK

111 River Street, Hoboken, NJ 07030-5774, USA

For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com/wiley-blackwell.

The right of the author to be identified as the author of this work has been asserted in accordance with the UK Copyright, Designs and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher.

Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought.

Library of Congress Cataloging-in-Publication Data

Chester, R. (Roy), 1936–

 Marine geochemistry / Roy Chester and Tim Jickells. – 3rd ed.

p. cm.

 Includes bibliographical references and index.

 ISBN 978-1-118-34907-6 (cloth) – ISBN 978-1-4051-8734-3 (pbk.) 1. Chemical oceanography. 2. Marine sediments. 3. Geochemistry. I. Jickells, T. D. (Tim D.) II. Title.

 GC111.2.C47 2012

 551.46–dc23

2012010712

A catalogue record for this book is available from the British Library.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.

Cover image: Blue ocean waves from underwater.©Solvod/Shutterstock.com

Cover design by: Simon Levy Associates

This book is dedicated with great affection to

John Riley and Dennis Burton.

Two great pioneers in the field of Marine Chemistry

Preface to the Third Edition

This edition of Marine Geochemistry has been created at a time when the role of the oceans in the Earth System is becoming ever more evident. The central role of ocean processes in climate change, and indeed in all aspects of global change, is increasingly important to all society. The scientific understanding of the role of the oceans and of how they function has developed sufficiently over recent years to justify a new edition of this book. The revisions incorporated in this new edition are the result of the collaboration between the two authors following Roy’s retirement.

This edition has been updated to reflect recent advances in the field of marine geochemistry. In particular new insights into nutrient cycling and the carbon cycle have led to a large scale reorganisation of Chapters 8 and 9 compared to previous editions. The relatively recent recognition of the key role of iron as a nutrient is discussed in Chapters 9 and 11. In addition, a section on shelf seas has been added in Chapter 6 to draw together the new understanding of processes in these regions which are now evidently of considerable importance to the marine geochemical cycle, as well as being both of great societal value while also under considerable pressure from human activity.

We are grateful to our publishers for their patience and support, to Phil Judge for producing the new diagrams for this edition and to our respective institutions for allowing us the opportunity to develop this and previous editions. Our special thanks go to colleagues around the world who have published the science we attempt to summarize here.

Finally we would also like to thank Alison Chester and Sue Jickells for their help and support with this endeavour, and for so much more – including keeping us reasonably sane.

Roy Chester and Tim Jickells

1

Introduction

The fundamental question underlying marine geochemistry is, ‘How do the oceans work as a chemical system?’ At present, that question cannot be answered fully. The past four decades or so, however, have seen a number of ‘quantum leaps’ in our understanding of some aspects of marine geochemistry. Three principal factors have made these leaps possible:

1 advances in sampling and analytical techniques;

2 the development of theoretical concepts;

3 the setting up of large-scale international oceanographic programmes (e.g. DSDP, MANOP, HEBBLE, GEOSECS, TTO, VERTEX, JGOFS, SEAREX, WOCE), which have extended the marine geochemistry database to a global ocean scale.

1.1 Setting the Background: a Unified ‘Process-Orientated’ Approach to Marine Geochemistry

Oceanography attracts scientists from a variety of disciplines, including chemistry, geology, physics, biology and meteorology. A knowledge of at least some aspects of marine geochemistry is an essential requirement for scientists from all these disciplines and for students who take courses in oceanography at any level. The present volume has been written, therefore, with the aim of bringing together the recent advances in marine geochemistry in a form that can be understood by all those scientists who use the oceans as a natural laboratory and not just by marine chemists themselves. Furthermore, the oceans are a key component of the Earth System, so an understanding of ocean geochemistry is central to understanding the functioning of the Earth as an integrated system (Lenton and Watson, 2011). One of the major challenges involved in doing this, however, is to provide a coherent global ocean framework within which marine geochemistry can be described in a manner that cannot only relate readily to the other oceanographic disciplines but also can accommodate future advances in the subject. To develop such a framework, it is necessary to explore some of the basic concepts that underlie marine geochemistry.

Geochemical balance calculations show that a number of elements that could not have come from the weathering of igneous rocks are present at the Earth’s surface. It is now generally accepted that these elements, which are termed the excess volatiles, have originated from the degassing of the Earth’s interior. The excess volatiles, which include H and O (combined as H2O), C, Cl, N, S, B, Br and F, are especially abundant in the atmosphere and the oceans. It is believed, therefore, that both the atmosphere and the oceans were generated by the degassing of the Earth’s interior. In terms of global cycling, Mackenzie (1975) suggested that sedimentary rocks are the product of a long-term titration of primary igneous-rock minerals by acids associated with the excess volatiles, a process that can be expressed as:

(1.1) 

As this reaction proceeds, the seawater reservoir is continuously subjected to material fluxes, which are delivered along various pathways from external sources. The oceans therefore are a flux-dominated system. Seawater, however, is not a static reservoir in which the material has simply accumulated over geological time, otherwise it would have a very different composition from that which it has at present; for example, the material supplied over geological time far exceeds the amount now present in seawater. Further, the composition of seawater appears not to have changed markedly over very long periods of time; at least the last few million years and probably longer. Rather than acting as an accumulator, therefore, the flux-dominated seawater reservoir can be regarded as a reactor. Elements are intensively recycled within the vast oceans by biological and chemical processes, although the extent of this recycling and the associated lifetime of components of the chemical system within the oceans vary enormously. It is the nature of the reactions that take place within the reservoir, that is the manner in which it responds to the material fluxes, which defines the composition of seawater via an input → internal reactivity → output cycle. The system is ultimately balanced by the Earth’s geological tectonic cycle that subducts ocean sediments into the Earth’s interior and returns them to the land surface.

Traditionally, there have been two schools of thought on the overall nature of the processes that operate to control the composition of seawater.

1 In the equilibrium ocean concept, a state of chemical equilibrium is presumed to exist between seawater and sediments via reactions that are reversible in nature. Thus, if the supply of dissolved elements to seawater were to increase, or decrease, the equilibrium reactions would change in the appropriate direction to accommodate the fluctuations.

2 In the alternative steady-state ocean concept, it is assumed that the input of material to the system is balanced by its output, that is, the reactions involved proceed in one direction only. In this type of ocean, fluctuations in input magnitudes would simply result in changes in the rates of the removal reactions, and the concentrations of the reactants in seawater would be maintained.

At present, the generally held view supports the steady-state ocean concept. Whichever theory is accepted, however, it is apparent that the oceans must be treated as a unified input–output type of system, in which materials stored in the seawater, the sediment and the rock reservoirs interact, sometimes via recycling stages, to control the composition of seawater.

It is clear, therefore, that the first requirement necessary to address the question ‘How do the oceans work as a chemical system?’ is to treat the seawater, sediment and rock reservoirs as a unified system. It is also apparent that one of the keys to solving the question lies in understanding the nature of the chemical, geological and biological (biogeochemical) processes that control the composition of seawater and how these interact with the physical transport within the ocean system, as this is the reservoir through which the material fluxes flow in the input → internal reactivity → output cycle. In order to provide a unified ocean framework within which to describe the recent advances in marine geochemistry in terms of this cycle, it is therefore necessary to understand the nature and magnitude of the fluxes that deliver material to the oceans (the input stage), the reactive processes associated with the throughput of the material through the seawater reservoir (the internal reactivity stage), and the nature and magnitude of the fluxes that take the material out of seawater into the sinks (the output stage).

The material that flows through the system includes inorganic and organic components in both dissolved and particulate forms, and a wide variety of these components will be described in the text. In order to avoid falling into the trap of not being able to see the wood for the trees in the morass of data, however, it is essential to recognize the importance of the processes that affect constituents in the source-to-sink cycle. Rather than taking an element-by-element ‘periodic table’ approach to marine geochemistry, the treatment adopted in the present volume will involve a process-orientated approach, in which the emphasis will be placed on identifying the key processes that operate within the cycle. The treatment will include both natural and anthropogenic materials, but it is not the intention to offer a specialized overview of marine pollution. This treatment does not in any way underrate the importance of marine pollution. Rather, it is directed towards the concept that it is necessary first to understand the natural processes that control the chemistry of the ocean system, because it is largely these same processes that affect the cycles of the anthropogenic constituents.

Since the oceans were first formed, sediments have stored material, and thus have recorded changes in environmental conditions. The emphasis in the present volume, however, is largely on the role that the sediments play in controlling the chemistry of the oceans. The diagenetic changes that have the most immediate effect on the composition of seawater take place in the upper few metres of the sediment column. For this reason attention will be focused on these surface deposits and their role in biogeochemical cycles. The role played by sediments in recording palaeooceanographic change will be touched upon only briefly. It is, however, important to recognize that the oceans play a key role in the Earth System, a role that evolves over geological time, and the oceans also record the history of the evolution of the Earth System and its climate (e.g., Emerson and Hedges, 2008; Lenton and Watson, 2011).

In order to rationalize the process-orientated approach, special attention will be paid to a number of individual constituents, which can be used to elucidate certain key processes that play an important role in controlling the chemical composition of seawater. In selecting these process-orientated constituents it was necessary to recognize the flux-dominated nature of the seawater reservoir. The material fluxes that reach the oceans deliver both dissolved and particulate elements to seawater. It was pointed out above, however, that the amount of dissolved material in seawater is not simply the sum of the total amounts brought to the oceans over geological time. This was highlighted a long time ago by Forchhammer (1865) when he wrote:

Thus the quantity of the different elements in seawater is not proportional to the quantity of elements which river water pours into the sea, but is inversely proportional to the facility with which the elements are made insoluble by general chemical or organo-chemical actions in the sea…

[our italics]. According to Goldberg (1963), this statement can be viewed as elegantly posing the theme of marine chemistry, and it is this ‘facility with which the elements are made insoluble’, and so are removed from the dissolved phase, which is central to our understanding of many of the factors that control the composition of seawater. This was highlighted more recently by Turekian (1977). In an influential geochemical paper, this author formally posed a question that had attracted the attention of marine geochemists for generations, and may be regarded as another expression of Forchhammer’s statement, that is ‘Why are the oceans so depleted in trace metals?’ Turekian concluded that the answer lies in the role played by particles in the sequestration of reactive elements during every stage in the transport cycle from source to marine sink.

Ultimately, therefore, it is the transfer of dissolved constituents to the particulate phase, and the subsequent sinking of the particulate material, that is responsible for the removal of the dissolved constituents from seawater to the sediment sink. The biological production and consumption of particles by the ocean microbial community and its predators is central to this process. It must be stressed, however, that although dissolved → particulate transformations are the driving force behind the removal of most elements to the sediment sink, the transformations themselves involve a wide variety of biogeochemical processes. For example, Emerson and Hedges (2008) and Stumm and Morgan (1996) identified a number of chemical reactions and physicochemical processes that are important in setting the chemical composition of natural waters at a fundamental physicochemical level. These processes included acid–base reactions, oxidation–reduction reactions, complexation reactions between metals and ligands, adsorption processes at interfaces, the precipitation and dissolution of solid phases, gas–solution processes, and the distribution of solutes between aqueous and non-aqueous phases. The manner in which reactions and processes such as these, and those specifically associated with biota, interact to control the composition of seawater will be considered throughout the text. For the moment, however, they can be grouped simply under the general term particulate ↔ dissolved reactivity. The particulate material itself is delivered to the sediment surface mainly via the down-column sinking of large-sized organic aggregates as part of the oceanic global carbon flux. Thus, within the seawater reservoir, reactive elements undergo a continuous series of dissolved ↔ particulate transformations, which are coupled with the transport of biologically formed particle aggregates to the sea bed. Turekian (1977) aptly termed this overall process the great particle conspiracy. In the flux-dominated ocean system the manner in which this conspiracy operates to clean up seawater is intimately related to the oceanic throughput of externally transported, and internally generated, particulate matter. Further, it is apparent that several important aspects of the manner in which this throughput cycle operates to control the inorganic and organic compositions of both the seawater reservoir and the sediment sink can be assessed in terms of the oceanic fates of reactive trace elements and organic carbon.

Many of the most important thrusts in marine geochemistry over the past few years have used tracers to identify the processes that drive the system, and to establish the rates at which they operate (Broecker and Peng, 1982). These tracers will be discussed at appropriate places in the text. The tracer approach, however, also has been adopted in a much broader sense in the present volume in that special attention will be paid to the trace elements and organic carbon in the source/input → internal reactivity → sink/output transport cycle. Both stable and radionuclide trace elements (e.g. the use of Th isotopes as a ‘time clock’ for both transport and process indicators) are especially rewarding for the study of reactivity within the various stages of the cycle, and organic carbon is a vital constituent with respect to the oceanic biomass, the down-column transport of material to the sediment sink and sediment diagenesis.

To interpret the source/input → internal reactivity → sink/output transport cycle in a coherent and systematic manner, a three-stage approach will be adopted, which follows the cycle in terms of a global journey. In Part I, the movements of both dissolved and particulate components will be tracked along a variety of transport pathways from their original sources to the point at which they cross the interfaces at the land–sea, air–sea and rock–sea boundaries. In Part II, the processes that affect the components within the seawater reservoir will be described. In Part III, the components will be followed as they are transferred out of seawater into the main sediment sink, and the nature of the sediments themselves will be described. The treatment, however, is concerned mainly with the role played by the sediments as marine sinks for material that has flowed through the seawater reservoir. In this context, it is the processes that take place in the upper few metres of the sediments that have the most immediate effect on the composition of seawater. For this reason attention will be restricted mainly to the uppermost sediment sections, and no attempt will be made to evaluate the status of the whole sediment column in the history of the oceans.

The steps involved in the three-stage global journey are illustrated schematically in Fig. 1.1. This is not meant to be an all-embracing representation of reservoir interchange in the ocean system, but is simply intended to offer a general framework within which to describe the global journey. By directing the journey in this way, the intention therefore is to treat the seawater, sediment and rock phases as integral parts of a unified ocean system.

Fig. 1.1 A schematic representation of the source/input → seawater internal reactivity → sink/output global journey. The large open arrows indicate transport from material sources, and the large filled arrows indicate transport into material sinks; relative flux magnitudes are not shown. The small arrows indicate only that the strengths of the fluxes can be changed as they cross the various interfaces in the system; thus, g and n represent gross and net inputs or outputs, respectively. Material is brought to the oceans in both particulate and dissolved forms, but is transferred into the major sediment sink mainly as particulate matter largely by biological processes. The removal of dissolved material to the sediment sink therefore usually requires its transformation to the particulate phase. This is shown by the p↔d term. The intention here, however, is simply to indicate that internal particulate–dissolved reactivity occurs within the seawater reservoir, and it must be stressed that a wide variety of chemical reactions and physicochemical processes are involved in setting the composition of the water phase: see text. For convenience, coastal zones are not shown.

In addition to the advantages of treating the oceans as a single system, the treatment adopted here is important in order to assess the status of the marine environment in terms of planetary geochemistry. For example, according to Hedges (1992) there is a complex interplay of biological, geological and chemical processes by which materials and energy are exchanged and reused at the Earth’s surface. These interreacting processes, which are termed biogeochemical cycles, are concentrated at interfaces and modified by feedback mechanisms. The cycles operate on time-scales of microseconds to eons, and occur in domains that range in size from a living cell to the entire ocean–atmosphere system, and interfaces in the oceans play a vital role in the biogeochemical cycles of some elements. The chemistry of the vast oceans is ultimately profoundly shaped by their internal biological processes which are dominated by tiny organisms – microorganisms less than 1 mm in diameter. The carbon fixed from the atmosphere and transformed within the water column by these organisms affects the chemistry of the oceans and sustains most of the biological life within the oceans. The exchanges of CO2 associated with these processes also play a critical role in the global carbon cycle and in the habitability of the whole planet.

The volume has been written for scientists of all disciplines. To contain the text within a reasonable length, a basic knowledge of chemistry, physics, biology and geology has been assumed and the fundamental principles in these subjects, which are readily available in other textbooks, have not been reiterated here. As the volume is deliberately designed with a multidisciplinary readership in mind, however, an attempt has been made to treat the more advanced chemical and physical concepts in a generally descriptive manner, with appropriate references being given to direct the reader to the original sources. One of the major aims of marine geochemistry in recent years has been to model natural systems on the basis of theoretical concepts. To follow this approach it is necessary to have a more detailed understanding of the theory involved, and for this reason a series of Worksheets have been included in the text. Some of these Worksheets are used to describe a number of basic geochemical concepts; for example, those underlying redox reactions and the diffusion of solutes in interstitial waters. In others, however, the emphasis is placed on modelling a variety of geochemical systems using, where possible, actual examples from literature sources; for example, the topics covered include a sorptive equilibrium model for the removal of trace metals in estuaries, a stagnant film model for the exchange of gases across the air–sea interface, and a variety of models designed to describe the interactions between solid and dissolved phases in sediment interstitial waters.

Overall, therefore, the intention is to provide a unifying framework, which has been designed to bring a state-of-the-art assessment of marine geochemistry to the knowledge of a variety of ocean scientists in such a way that allows future advances to be understood within a meaningful context.

References

Broecker, W.S. and Peng, T.-H. (1982) Tracers in the Sea. Eldigio Press, New York: Columbia University.

Emerson, S.R. and Hedges, J.I. (2008) Chemical Oceanography and the Marine Carbon Cycle, Cambridge University Press.

Forchhammer, G. (1865) On the composition of sea water in the different parts of the ocean. Philos. Trans. R. Soc. London, 155, 203–262.

Goldberg, E.D. (1963) The oceans as a chemical system, in The Sea, M.N. Hill (ed.), Vol. 2, 3–25. New York: John Wiley & Sons, Inc.

Hedges, J.I. (1992) Global biogeochemical cycles: progress and problems. Mar. Chem., 39, 67–93.

Lenton, T. and Watson, A. (2011) Revolutions that Made the Earth. Oxford University Press.

Mackenzie, F.T. (1975) Sedimentary cycling and the evolution of the sea water, in Chemical Oceanography, J.P. Riley and G. Skirrow (eds), Vol. 1, 309–364. London: Academic Press.

Stumm, W. and Morgan, J.J. (1996) Aquatic Chemistry, 3rd edn, New York: John Wiley & Sons, Inc.

Turekian, K.K. (1977) The fate of metals in the oceans. Geochim. Cosmochim. Acta, 41, 1139–1144.

Part I

The Global Journey: Material Sources

2

The Input of Material to the Ocean Reservoir

The World Ocean may be regarded as a planetary dumping ground for material that originates in other geospheres, prior to its tectonic recycling, and to understand marine geochemistry it is necessary to evaluate the composition, flux rate and subsequent fate of the material that is delivered to the ocean reservoir.

2.1 The Background

The major natural sources of the material that is injected into seawater are the continental crust, the oceanic crust and the atmosphere. Primary material is mobilized directly from the continental crust, mainly by low-temperature weathering processes and high-temperature volcanic activity. In addition, secondary (or pollutant) material is mobilized by a variety of anthropogenic ‘weathering’ processes, which often involve high temperatures. The various types of material released on the continents during both natural and anthropogenic processes include particulate, dissolved and gaseous phases, which are then moved around the surface of the planet by a number of transport pathways. The principal routes by which continentally mobilized material reaches the World Ocean are via river, atmospheric and glacial transport. The relative importance of these pathways, however, varies considerably in both space and time. Rivers and glaciers enter the oceans at particular locations and impact particularly coastal regions, while atmospheric transport disperses material more widely with fluxes decreasing away from source regions. Water in the form of ice can act as a major mechanism for the physical mobilization of material on the Earth’s surface. The magnitude of the transport of this material depends on the prevailing climatic regime. At present, the Earth is in an interglacial period and large-scale ice sheets are confined to the Polar Regions. Even under these conditions, however, glacial processes are a major contributor of material to the oceans. For example, Raiswell et al. (2006) estimated that at present ∼29 × 1014 g yr−1 of crustal products are delivered to the World Ocean by glacial transport, of which ∼90% is derived from Antarctica. Thus, ice transport is second only to fluvial run-off in the global supply of particulate material to the marine environment, although it is less important as a source of dissolved material because it is frozen and hence has reduced chemical weathering. The impacts of material fluxes associated with glaciers are seen predominantly in the high latitudes, par­ticularly in the shelf seas of the Arctic and Southern Oceans.

Material also is supplied to the oceans from processes that affect the oceanic crust. These processes involve low-temperature weathering of the ocean basement rocks, mainly basalts, and high-temperature water–rock reactions associated with hydrothermal activity at spreading ridge centres. This hydrothermal activity, which can act as a source of some components and a sink for others, is now known to be of major importance in global geochemistry; for example, in terms of primary inputs it dominates the supply of dissolved manganese to the oceans. Although the extent to which this type of dissolved material is dispersed about the ocean is not yet clear, hydrothermal activity must still be regarded as a globally important mechanism for the supply of material to the seawater reservoir.

On a global scale, therefore, the main pathways by which material is brought to the oceans are:

1 river and glacial run-off, which delivers material to the surface ocean at the land–sea boundaries;

2 atmospheric deposition, which delivers material to all regions of the surface ocean;

3 hydrothermal activity, which delivers material to deep and intermediate waters above the sea floor.

The manner in which these principal pathways operate is described individually in the next three chapters, and this is followed by an attempt to estimate the relative magnitudes of the material fluxes associated with them.

References

Raiswell, R., Tranter, M., Benning, L.G., Siegert, M., De’ath, R., Huybrechts, P. and Rayne, T. (2006) Contributions from glacially derived sediment to the global iron(oxyhyr)oxide cycle: implications for iron delivery to the oceans. Geochim. Cosmochim. Acta, 70, 2765–2780.

3

The Transport of Material to the Oceans: the Fluvial Pathway

Much of the material mobilized during both natural crustal weathering and anthropogenic activities is dispersed by rivers, which transport the material towards the land–sea margins. In this sense, rivers may be regarded as the carriers of a wide variety of chemical signals to the World Ocean. The effect that these signals have on the chemistry of the ocean system may be assessed within the framework of three key questions (see e.g. Martin and Whitfield, 1983).

1 What is the quantity and chemical composition of the dissolved and particulate material carried by rivers?

2 What are the fates of these materials in the estuarine mixing zone?

3 What is the ultimate quantity and composition of the material that is exported from the estuarine zone and actually reaches the open ocean?

These questions will be addressed in this chapter, and in this way river-transported materials will be tracked on their journey from their source, across the estuarine (river–ocean) interface, through the coastal receiving zone and out into the open ocean. Chemical fluxes of some components have also been substantially modified by human impact and the nature and scale of this impact will be considered.

In addition to riverine inputs, glacial flows also contribute inputs to the oceans. Chemical weathering in glacial environments is similar or slower than rates in fluvial catchments (Anderson et al., 1997). The largest glacial flows arise from Antarctica and Greenland, and hence inputs of anthropogenic materials from these systems are small compared to fluvial ones and organic matter will also be at low concentrations because of limited biological activity. Physical weathering in glacial systems is very substantial (Raiswell et al., 2006), but the chemical composition and behaviour of this material will be similar to that of fluvial particulate matter. Hence, the inputs of glaciers will not be treated separately but with river systems with differences noted where appropriate.

3.1 Chemical Signals Transported by Rivers

3.1.1 Introduction

River water contains a large range of inorganic and organic components in both dissolved and particulate forms. A note of caution, however, must be introduced before any attempt is made to assess the strengths of the chemical signals carried by rivers, especially with respect to trace elements. In attempting to describe the processes involved in river transport, and the strengths of the signals they generate, great care must be taken to assess the validity of the databases used and, where available, ‘modern’ (i.e. post around 1975) trace-element data will be used in the present discussion of river-transported chemical signals, since some earlier data sets may include overestimates of concentrations due to contamination problems during the sampling and analysis which were not recognized at the time.

3.1.2 The Sources of Dissolved and Particulate Material Found in River Waters

Water reaches the river environment either directly from the atmosphere or indirectly from surface run-off, underground water circulation and the discharge of waste solutions. Freshwater reaches the ocean predominantly via rivers with about 5% of the total arising directly via groundwater. The fluxes of groundwater and its composition are less well known. This component can be locally very important, for instance on limestone islands where there is often no surface freshwater, but in terms of global fluxes, rivers dominate and will be the focus of attention here. The sources of the dissolved and particulate components that are found in the river water include rock weathering, the decomposition of organic material, wet and dry atmospheric deposition and, for some rivers, human activity induced discharges. The source strengths are controlled by a number of complex, often interrelated, environmental factors that operate in an individual river basin; these factors include rock lithology, relief, climate, the extent of vegetative cover and the magnitude of pollutant inputs.