tracers in the sea
TRANSCRIPT
Tracers in the SeaTRACERS IN THE SEA w. S. Broecker and T.-H. Peng
TRACERS
AND
A PUBLICATION OF THE LAMONT-DOHERTY GEOLOGICAL OBSERVATORY,
COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964
Printed in the United States of America
Tsung-Hung Peng's work at Oak Ridge was sponsored jointly by the National Science Foundation's Ecosystem Studie~ Program under In teragency Agreement No. DEB 8115316 and the Carbon Dioxide Re search D1vision, Office of Energy Research, U.S. Department of En ergy, under contract W-740S-eng-26 with Union Carbide Corporation.
A decade has passed since Chemical Oceanography, the prede cessor to this book, was written. During this time our knowledge of the chemistry of the oceans has mushroomed. The Geochemical Ocean Sections program (GEOSECS) fleshed out our sketchy picture of the distributions of chemical, isotopic and radiochemical tracers in the sea. The discovery of ridge crest hydrothermal systems has added a new dimension to ocean chemistry. The results of sediment trap deployments, of pore water measurements, and of in situ filtrations of particulate matter have expanded our knowl edge of the cycle of particulate matter in the sea. Hence, rather than revising Chemical Oceanography, we decided to write a new book, Tracers in the Sea. While this new book follows the outline of its predecessor and contains material from its predecessor, about 75% of the material is new. Unlike Chemical Oceanography, which could be read from cover to cover in a few sittings, Tracers in the Sea has far more meat and cannot be so quickly digested.
The objective of this book is to demonstrate how the distri bution of tracers in the sea and its sediments can be used to aid us in deciphering the operations plan and history of operation for the ocean as a chemical plant. To do this requires the integra tion of chemical information with information based on studies of ocean sediments, of organisms living in the sea, and of ocean cur rents. This integration leads to a description of how the ocean "works" from the point of view of a geochemist. Such integrations are not common to the field of oceanography because only in at tempting to understand the chemistry of the sea is it necessary to consider the results from all the sister disciplines.
One dominant theme which recurs in the book is the operation of the earth's carbon cycle. Carbon is a key element in organisms and their shells. It is a constituent of the key greenhouse gas in our earth's atmosphere. It has two isotopes of great impor tance as tracers, namely, 14C and 13 C• In the book we explore not only the present day distribution of this key element but also how this distribution may have differed during the last glacial period and how it will differ during the coming fossil fuel C02-induced superinterglacial period.
While aimed both at the professional and at the student, the book has the format of a text. As such it puts more emphasis on explanation than on documentation. To counter the somewhat thin referencing in the body of the text a rather elaborate reference list is given at the end of the book. This list contains over 700 entries and is organized by topic. Thus, a reader interested in finding all the papers which bear on, let's say, the distribu-
tion of 3He or of lead in the sea has only to look up the sections on these properties in the reference list to find a fairly com plete list of papers written on these subjects. As the papers on each topic are listed in order of publication date and as they are annotated with a sentence or two stating their contribution, the reader can get some sense of development of the data base and thinking in a given area by looking through this reference lis t. Hence, the reference numbers in the text and in the figure and table captions serve a dual purpose. They give the reader the source of specific information used in the book and they lead him to the section in the references which lists related papers not specifically mentioned in the text.
Every text book in science needs problems. They reinforce the concepts in the text and force the student to think carefully about the quantitative aspects of the sUbject. We include both the usual short problems and also what we call superproblems. The latter give the student a better in-depth view of key material and also good training in box modeling which plays such an important role in marine geochemistry. As the superproblems require 4-10 hours of work, the instructor should consider haVing students work in pairs on the problems and having each student work on only 1 or 2 of the problems during a semester. We have found that presentations of the solutions by the students doing the work to the rest of the class proves very effective. Solutions to both the problems and superproblems can be obtained by writing to the authors.
Much of this book was written during a sabbatical leave in Germany. I (WSB) would like to thank the Alexander Von Humboldt Foundation for providing fellowship support for a nine month visit to Heidelberg and to Karl Otto Munnich and Wolfgang Roether and their colleagues in the Umweltphysik group for their friendship and help during this time. The drafting for the book was done by Patty Catanzaro and the typing by Vicky Costello, Rae Pochapsky and Andrea Radigan. Ellen C:oxe acted as liaison between these production efforts at Lamont and the writing and modeling ac tivities by T.-H. Peng at Oak Ridge National Laboratory and by W.S. Broecker in Heidelberg. The sketch of Arnold Bainbridge and the cover design were done by Dee Breger, a member of the technical staff in geochemistry at Lamont-Doherty. The cover photo, a rosette cast from the Woods Hole Oceanographic Institution research vessel KNORR, was taken by David Chipman of the Lamont-Doherty Geochemistry group.
We thank Heinrich Holland for encouraging us to follow a rather unconventional plan for publication. The Lamont-Doherty Geological Observatory served as publisher. Author-ready copy was prepared using the geochemistry group word processor under the supervision of Patty Catanzaro. The printing was jobbed out to Edwards Brothers Incorporated, Ann Arbor, Michigan. The advertis ing and distribution were handled by Observatory staff. We thank IT. Barry Raleigh, director of the Observatory, for his courageous backing of this dangerous experiment.
CHAPTER 1 INTERNAL CYCLING AND THROUGHPUT
Pathways from River Mouth to Sea Floor
Introduction 1
Constituent Classification 6
Composition of Particulate Matter Caught in Sediment Traps 13
A Simple Model for Biologically Utilized Constituents 15
The Distributions of Biointermediate CDnstituents 22
Estimation of Input Rates 25
Horizontal Segregation of Constituents in the Deep Sea 28
Summary 40
Factors Influencing the Distribution of Sedimentary Constituents
Introduction 45
Opal Solution on the Sea Floor 49
Distribution of Qalcite in Marine Sediments 58
Degree of Calcite Saturation 61
Variation in the Carbonate Ion Qontent of Sea Water 62
Spacial Variations in the CaC0 3 Saturation of Sea Water 71
Factors Controlling the Rate of Calcite Solution 84
Thickness and Shape of the SUblysocline Transition Zone 89
Variation of Sediment Type with Time 94
Manganese Nodules 98
The Cycles of Gases within the Sea
Introduction 110
The Rate of Gas Exchange 113
Stagnant Film Thickness Derived from Natural Radiocarbon 118
Stagnant Film Thicknesses Determined by the Radon Method 122
Oxygen Concentrations in Surface Ocean Water 127
Oxygen Deficiencies in the Deep Sea 131
The Marine NzO Cycle 139
Excess Helium 147
Origin of the Equatorial Pacific COz Anomaly 158
Summary 161
The Cycle of Metals in the Sea
Introduction
166
168
The Distribution of Lead-210
The Distribution of Polonium-210
The Distribution of Radium-226
Toward a Model of Metal Transport
Distributions of Stable Metals in the Sea
Stable Isotope Ratios in Reactive Metals
Transport of Iron and Manganese in the Sea
Lessons from Controlled Ecosystem Studies
Distribution Coefficients
Rates of Vertical Mixing and Sediment Accumulation
Introduction
Rate of CDntinental Runoff
CDmparison of Model and Observed Rates of CaC03 Solution 269
Summary 271
Control Mechanisms Operating in the Sea
Introduction
Nitrate Controls
Factors Influencing Nutrient Gradients in the Deep Sea
Summary
The Movement of Water Through the Deep Sea
Introduction
Tracers for Diapycnal and Isopycnal Mixing
Mixing Rates Based on Radon-222 and Radium-228
The Distribution of Helium-3 in the Deep Pacific
Sources of Deep Water
Feed for NCW Production
Ventilation of the Deep Pacific and Indian Oceans
The Grand Cycle of Radiocarbon in the Deep Ocean
Biological Short-Circuiting
Argon-39
Summary
The Movement of Water Through the Oceanic Thermocline
Introduction
Temporal Trends in Tritium
318
324
325
331
334
335
341
343
343
349
359
365
368
372
374
377
379
383
383
388
399
402
406
Bomb Carbon-14 Distribution within the Thermocline 412
Explanations for Low Equatorial Bomb Carbon-14 Inventories 422
Implications of Equatorial Upwelling to the Tritium Budget 425
An Upwelling Rate Based on the Equatorial C02 Anomaly 427
Helium-3 Distribution in the Main Oceanic Thermocline 432
Purposeful Tracers 438
Introduction 444
Formation and Destruction of Organic Materials 451
Changes in CaC03 Storage 456
Evidence for an Early Post-Glacial Lysocline Change 457
Changes in Phosphate Concentration 466
The Combined Evidence from Deep Sea Cores 472
Cause of the Oceanic Phosphate Change 479
An Alternate Scenario 480
The Oxygen Record 484
Glacial to Interglacial Lysocline Changes 485
Changes in the Distribution of Nutrients in the Deep Sea 491
Summary 496
CHAPTER 10 CAN MAN OVERRIDE THE CONTROLS?
The Buildup of Fossil Fuel CO 2 in the Atmosphere and Oceans
Introduction
Capacity of the Sea for Fossil Fuel CD2 Uptake
Utilizable Capacity - Simplified Calculation
Utilizable Capacity - Rigorous Calculation
Kinetics of Fossil Fuel CO 2 Uptake by the Sea
Numerical Model
Prediction of Future C02 Levels
Solution of Sea Floor Calcite
Summary
500
500
505
511
513
517
524
526
534
550
552
564
Introduction to the References 569
SUbject Outline for the References 569
Annotated Reference List 574
Frequently Used C.onstants 680
Abbreviations 682
Index 683
etta-pte/!- I
INTERNAL CYCLING AND THROUGHPUT PATHWAYS FROM RIVER MOUTH TO SEA FLOOR
INTRODUCTION
The sea is a way station for the products of continental ero sion. All substances received by the sea are ultimately passed a long to the sediment and rock lining its floor. The great tecton ic forces that continually modify the geography of the earth's surface eventually push the material buried in this way back above sea level where it becomes subject to erosion. Then another trip through the sea begins.
The greater proportion of the products of erosion enter the sea in particulate form. They are dropped by the winds to the sea surface or disgorged by rivers into coastal waters. These rock frag'rnents and soil residues are chemically qui te inert. They travel only as far as the currents can carry them before reaching the sea floor. They play a minor role in the story we have to tell.
Of great interest to us are those substances that dissol ve during erosion and are carried to the sea in ionic form. They constitute the sea's salt. As long as they remain dissolved, gravity cannot influence them. Al though the processes at work in the sea ul timately " reprecipi tate" these ions, most cons ti tuents of sea sal t wander through the sea long and far before becoming entombed in the sediment. The temporary entrapments in particu late matter experienced by these wandering ions will prove of par ticular interest to us.
The composition of sea salt reflects not only the relative abundance of the dissolved substances in river water but also the ease wi th which a given substance becomes entrapped in the sedi ments. Sodium, for example, is both abundant in the dissolved matter in ~ivers and sparingly reactive in the sea. This combina tion is reflected by its high concentration in sea salt. Calcium, al though even more abundant in river water than sodium, is an im portant ingredient in the shells of marine organisms. Because of this special mode of entrapment, the abundance of calcium in sea water is far lower than that of sodium.
Many components of sea salt show little variation in concen tration within the sea. Others show very large changes in concen tration from place to place. As we shall see, these differences are largely the result of cycling by organisms. Plants live only in surface waters, from which they extract certain constituents
1
needed to construct their tissues. While much of this plant mat ter is consumed by animals living in the surface ocean, some in solubles and indigestibles (i.e., fecal matter) move into the deep sea under the influence of gravi ty - and so the life cycle leads to chemical segrega tiona Des true tion of plant rna tter occurs, on the average, at greater depths than formation. The interaction of this life cycle with the large-scale water circulation pattern in the sea resul ts in inhomogeneities in the distribution of these species, not only within the sea itself but also in the sediments deposited on the ocean floor.
One of the aims of this book is to point out the factors that influence the average concentrations of the various components of sea sal t and the fac tors tha t produce chemical inhomogenei ties wi thin the sea and its sediments. The approach might be termed "inverse chemical engineering". The ocean is a great chemical plant that processes the dissolved matter added from rivers and dispenses it as sediment. Unlike most chemical plants, the sea has no advance operational blueprint. As chemical oceanographers, we wander through the plant measuring inputs, outputs, and intern al compositions - trying to reconstruct the missing design. As in most chemical plants, the two critical features of the ocean are the manner in which the ingredients are mixed and the manner in which the ingredients react wi th one another. Oceanic mixing is accomplished by a complex system of currents and a host of turbu lent eddies. Many of the chemical reactions are catalyzed by the enzymes in living organisms. Thus any study of the chemistry of sea water is heavily dependent on knowledge derived from physical oceanographic and marine biologic studies.
In this book, we will focus on the first-order processes operating within the sea. Until these are mastered, it is fruit less to proceed to the more complex "details". With this in mind, let us turn our attention to our first subject - the grand chemi cal balance existing in the sea.
DEPTH PROFILES OF SEA SALT COMPOSITION
The salt dissolved in sea water is remarkably uniform in its major constituents. This fact has greatly simplified the task of the physical oceanographer interested in mapping water density patterns within the sea. He needs only to measure the water temp erature, the water pressure and one major property of sea sal t (for example, the chloride ion content or the electrical conduct ivity) to make a very accurate estimate of the in situ density of a given sample of sea water. This task would be-extremely complex if the composition of sea salt were more varied.
Yet it is fortunate, too, that the compositional constancy of the major components of sea salt does not extend to all the minor components. If this were not the case, the oceanographer would lose one of his most powerful tools, for the varia tions in the minor consti tuents of sea wa ter bear important clues regarding mixing, biological, and sedimentary processes taking place within the sea. They are the tracers about which this book is written.
The major ion matrix of sea salt consists of the following constituents: Chlorine in the form of CI- ion; sulfur in the form
2
of SO 4 = ion· and magnesiwn, potassium, calcium and sodium in the form of Mg+q., K+, Ca++, and Na+ ions, respectively. These six ions dominate sea salt; their ratios, one to another, are very nearly constant. In fac t, only calciwn has been shown to vary measurably from place to place in its ratio to the other five con s ti tuents. Al though this cons tancy also extends to many of the lesser components of sea water (boron, bromine, fluorine, uranium, cesium, and others), it does not extend to all of them.
The best known of the varia tions in the composi tion of sea salt arise from the removal of constituents from surface sea water by plants and the subsequent destruc tion of plant-produced parti cles after downward movement. Deep water masses are richer in the constituents utilized by plants than surface water is. If the o cean were sterile, the chemical composition of sea sal t would be almost perfec tly uniform. The main differences in composi tion would be those resul ting from the transfer of gases between the atmosphere and surface waters of differing temperature would exis t.
Plants live only in surface water, where there is enough light to permi t photosynthesis. By the time the components of their debris are returned to the ionic form as a resul t of tissue oxida tion or hard part solu tion, downward movement under the in fluence of gravi ty or migra ting animals has occurred. It is not surprising, then, tha t the primary chemical differences observed in the ocean are all of the type just mentioned: deep water is en riched relative to surface water. The only major exception is dissolved oxygen.
Figures 1-1 and 1-2 show the dis tribu tion wi th depth of po tential temperature*, of salinity and of the concentrations of six biologically utilized sea salt constituents at a station in the North Pacific Ocean. In all cases, the most dramatic change oc curs in the upper thousand meters of the wa ter column. This so called main thermocline constitutes the zone of transition between the warm surface wa ters and the cold deep wa ters. The nu trient species nitrate (N03-), silicate (H 4Si0 4 ), and total dissolved in organic carbon (L:C02 = C02 + HC03- + C03=) show the deep water enrichment mentioned above. Alkalinity and barium show patterns similar (but not identical) to that of silicate. Dissolved oxygen gas, by contrast, shows a depletion. Unlike N03- and L:C02, which are released during respira tion, 02 is consumed during respira tion. Its pattern is also complicated by the fact that cold wa ters descending from the surface carry with them more dissolved gas than descending warm waters. The salinity minimum at about 600 meters depth is genera ted by the la teral intrusion of inter media te wa ters. Keep in mind tha t these frac tional salini ty dif ferences are quite small compared to the fractional differences in
*Sea water is heated by compression (i.e., about O.loC/km) as it descends from the surface. Potential temperatures are cor rected for this heating. Hence the potential temperature is the temperature a water sample would have were it returned to the sea surface.
3
300
6 ......- ......----......--'--.....--'
Figure 1-1. Plots of temperature, salinity, dissolved oxygen con tent and ni trate content as a function of water depth at GEOSECS station 214 in the North Pacific (32°N, 176°W). Potential tem perature rather than the temperature measured in situ is given. The salini ty profile in this region of the ocean--sh"ows a pro nounced minimum at a depth just over 600 meters. This intermed iate water .C as it is called) forms in the northern Pacific and sinks and flows laterally beneath the waters of the warm temperate ocean. The level of this minimum is shown…
TRACERS
AND
A PUBLICATION OF THE LAMONT-DOHERTY GEOLOGICAL OBSERVATORY,
COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964
Printed in the United States of America
Tsung-Hung Peng's work at Oak Ridge was sponsored jointly by the National Science Foundation's Ecosystem Studie~ Program under In teragency Agreement No. DEB 8115316 and the Carbon Dioxide Re search D1vision, Office of Energy Research, U.S. Department of En ergy, under contract W-740S-eng-26 with Union Carbide Corporation.
A decade has passed since Chemical Oceanography, the prede cessor to this book, was written. During this time our knowledge of the chemistry of the oceans has mushroomed. The Geochemical Ocean Sections program (GEOSECS) fleshed out our sketchy picture of the distributions of chemical, isotopic and radiochemical tracers in the sea. The discovery of ridge crest hydrothermal systems has added a new dimension to ocean chemistry. The results of sediment trap deployments, of pore water measurements, and of in situ filtrations of particulate matter have expanded our knowl edge of the cycle of particulate matter in the sea. Hence, rather than revising Chemical Oceanography, we decided to write a new book, Tracers in the Sea. While this new book follows the outline of its predecessor and contains material from its predecessor, about 75% of the material is new. Unlike Chemical Oceanography, which could be read from cover to cover in a few sittings, Tracers in the Sea has far more meat and cannot be so quickly digested.
The objective of this book is to demonstrate how the distri bution of tracers in the sea and its sediments can be used to aid us in deciphering the operations plan and history of operation for the ocean as a chemical plant. To do this requires the integra tion of chemical information with information based on studies of ocean sediments, of organisms living in the sea, and of ocean cur rents. This integration leads to a description of how the ocean "works" from the point of view of a geochemist. Such integrations are not common to the field of oceanography because only in at tempting to understand the chemistry of the sea is it necessary to consider the results from all the sister disciplines.
One dominant theme which recurs in the book is the operation of the earth's carbon cycle. Carbon is a key element in organisms and their shells. It is a constituent of the key greenhouse gas in our earth's atmosphere. It has two isotopes of great impor tance as tracers, namely, 14C and 13 C• In the book we explore not only the present day distribution of this key element but also how this distribution may have differed during the last glacial period and how it will differ during the coming fossil fuel C02-induced superinterglacial period.
While aimed both at the professional and at the student, the book has the format of a text. As such it puts more emphasis on explanation than on documentation. To counter the somewhat thin referencing in the body of the text a rather elaborate reference list is given at the end of the book. This list contains over 700 entries and is organized by topic. Thus, a reader interested in finding all the papers which bear on, let's say, the distribu-
tion of 3He or of lead in the sea has only to look up the sections on these properties in the reference list to find a fairly com plete list of papers written on these subjects. As the papers on each topic are listed in order of publication date and as they are annotated with a sentence or two stating their contribution, the reader can get some sense of development of the data base and thinking in a given area by looking through this reference lis t. Hence, the reference numbers in the text and in the figure and table captions serve a dual purpose. They give the reader the source of specific information used in the book and they lead him to the section in the references which lists related papers not specifically mentioned in the text.
Every text book in science needs problems. They reinforce the concepts in the text and force the student to think carefully about the quantitative aspects of the sUbject. We include both the usual short problems and also what we call superproblems. The latter give the student a better in-depth view of key material and also good training in box modeling which plays such an important role in marine geochemistry. As the superproblems require 4-10 hours of work, the instructor should consider haVing students work in pairs on the problems and having each student work on only 1 or 2 of the problems during a semester. We have found that presentations of the solutions by the students doing the work to the rest of the class proves very effective. Solutions to both the problems and superproblems can be obtained by writing to the authors.
Much of this book was written during a sabbatical leave in Germany. I (WSB) would like to thank the Alexander Von Humboldt Foundation for providing fellowship support for a nine month visit to Heidelberg and to Karl Otto Munnich and Wolfgang Roether and their colleagues in the Umweltphysik group for their friendship and help during this time. The drafting for the book was done by Patty Catanzaro and the typing by Vicky Costello, Rae Pochapsky and Andrea Radigan. Ellen C:oxe acted as liaison between these production efforts at Lamont and the writing and modeling ac tivities by T.-H. Peng at Oak Ridge National Laboratory and by W.S. Broecker in Heidelberg. The sketch of Arnold Bainbridge and the cover design were done by Dee Breger, a member of the technical staff in geochemistry at Lamont-Doherty. The cover photo, a rosette cast from the Woods Hole Oceanographic Institution research vessel KNORR, was taken by David Chipman of the Lamont-Doherty Geochemistry group.
We thank Heinrich Holland for encouraging us to follow a rather unconventional plan for publication. The Lamont-Doherty Geological Observatory served as publisher. Author-ready copy was prepared using the geochemistry group word processor under the supervision of Patty Catanzaro. The printing was jobbed out to Edwards Brothers Incorporated, Ann Arbor, Michigan. The advertis ing and distribution were handled by Observatory staff. We thank IT. Barry Raleigh, director of the Observatory, for his courageous backing of this dangerous experiment.
CHAPTER 1 INTERNAL CYCLING AND THROUGHPUT
Pathways from River Mouth to Sea Floor
Introduction 1
Constituent Classification 6
Composition of Particulate Matter Caught in Sediment Traps 13
A Simple Model for Biologically Utilized Constituents 15
The Distributions of Biointermediate CDnstituents 22
Estimation of Input Rates 25
Horizontal Segregation of Constituents in the Deep Sea 28
Summary 40
Factors Influencing the Distribution of Sedimentary Constituents
Introduction 45
Opal Solution on the Sea Floor 49
Distribution of Qalcite in Marine Sediments 58
Degree of Calcite Saturation 61
Variation in the Carbonate Ion Qontent of Sea Water 62
Spacial Variations in the CaC0 3 Saturation of Sea Water 71
Factors Controlling the Rate of Calcite Solution 84
Thickness and Shape of the SUblysocline Transition Zone 89
Variation of Sediment Type with Time 94
Manganese Nodules 98
The Cycles of Gases within the Sea
Introduction 110
The Rate of Gas Exchange 113
Stagnant Film Thickness Derived from Natural Radiocarbon 118
Stagnant Film Thicknesses Determined by the Radon Method 122
Oxygen Concentrations in Surface Ocean Water 127
Oxygen Deficiencies in the Deep Sea 131
The Marine NzO Cycle 139
Excess Helium 147
Origin of the Equatorial Pacific COz Anomaly 158
Summary 161
The Cycle of Metals in the Sea
Introduction
166
168
The Distribution of Lead-210
The Distribution of Polonium-210
The Distribution of Radium-226
Toward a Model of Metal Transport
Distributions of Stable Metals in the Sea
Stable Isotope Ratios in Reactive Metals
Transport of Iron and Manganese in the Sea
Lessons from Controlled Ecosystem Studies
Distribution Coefficients
Rates of Vertical Mixing and Sediment Accumulation
Introduction
Rate of CDntinental Runoff
CDmparison of Model and Observed Rates of CaC03 Solution 269
Summary 271
Control Mechanisms Operating in the Sea
Introduction
Nitrate Controls
Factors Influencing Nutrient Gradients in the Deep Sea
Summary
The Movement of Water Through the Deep Sea
Introduction
Tracers for Diapycnal and Isopycnal Mixing
Mixing Rates Based on Radon-222 and Radium-228
The Distribution of Helium-3 in the Deep Pacific
Sources of Deep Water
Feed for NCW Production
Ventilation of the Deep Pacific and Indian Oceans
The Grand Cycle of Radiocarbon in the Deep Ocean
Biological Short-Circuiting
Argon-39
Summary
The Movement of Water Through the Oceanic Thermocline
Introduction
Temporal Trends in Tritium
318
324
325
331
334
335
341
343
343
349
359
365
368
372
374
377
379
383
383
388
399
402
406
Bomb Carbon-14 Distribution within the Thermocline 412
Explanations for Low Equatorial Bomb Carbon-14 Inventories 422
Implications of Equatorial Upwelling to the Tritium Budget 425
An Upwelling Rate Based on the Equatorial C02 Anomaly 427
Helium-3 Distribution in the Main Oceanic Thermocline 432
Purposeful Tracers 438
Introduction 444
Formation and Destruction of Organic Materials 451
Changes in CaC03 Storage 456
Evidence for an Early Post-Glacial Lysocline Change 457
Changes in Phosphate Concentration 466
The Combined Evidence from Deep Sea Cores 472
Cause of the Oceanic Phosphate Change 479
An Alternate Scenario 480
The Oxygen Record 484
Glacial to Interglacial Lysocline Changes 485
Changes in the Distribution of Nutrients in the Deep Sea 491
Summary 496
CHAPTER 10 CAN MAN OVERRIDE THE CONTROLS?
The Buildup of Fossil Fuel CO 2 in the Atmosphere and Oceans
Introduction
Capacity of the Sea for Fossil Fuel CD2 Uptake
Utilizable Capacity - Simplified Calculation
Utilizable Capacity - Rigorous Calculation
Kinetics of Fossil Fuel CO 2 Uptake by the Sea
Numerical Model
Prediction of Future C02 Levels
Solution of Sea Floor Calcite
Summary
500
500
505
511
513
517
524
526
534
550
552
564
Introduction to the References 569
SUbject Outline for the References 569
Annotated Reference List 574
Frequently Used C.onstants 680
Abbreviations 682
Index 683
etta-pte/!- I
INTERNAL CYCLING AND THROUGHPUT PATHWAYS FROM RIVER MOUTH TO SEA FLOOR
INTRODUCTION
The sea is a way station for the products of continental ero sion. All substances received by the sea are ultimately passed a long to the sediment and rock lining its floor. The great tecton ic forces that continually modify the geography of the earth's surface eventually push the material buried in this way back above sea level where it becomes subject to erosion. Then another trip through the sea begins.
The greater proportion of the products of erosion enter the sea in particulate form. They are dropped by the winds to the sea surface or disgorged by rivers into coastal waters. These rock frag'rnents and soil residues are chemically qui te inert. They travel only as far as the currents can carry them before reaching the sea floor. They play a minor role in the story we have to tell.
Of great interest to us are those substances that dissol ve during erosion and are carried to the sea in ionic form. They constitute the sea's salt. As long as they remain dissolved, gravity cannot influence them. Al though the processes at work in the sea ul timately " reprecipi tate" these ions, most cons ti tuents of sea sal t wander through the sea long and far before becoming entombed in the sediment. The temporary entrapments in particu late matter experienced by these wandering ions will prove of par ticular interest to us.
The composition of sea salt reflects not only the relative abundance of the dissolved substances in river water but also the ease wi th which a given substance becomes entrapped in the sedi ments. Sodium, for example, is both abundant in the dissolved matter in ~ivers and sparingly reactive in the sea. This combina tion is reflected by its high concentration in sea salt. Calcium, al though even more abundant in river water than sodium, is an im portant ingredient in the shells of marine organisms. Because of this special mode of entrapment, the abundance of calcium in sea water is far lower than that of sodium.
Many components of sea salt show little variation in concen tration within the sea. Others show very large changes in concen tration from place to place. As we shall see, these differences are largely the result of cycling by organisms. Plants live only in surface waters, from which they extract certain constituents
1
needed to construct their tissues. While much of this plant mat ter is consumed by animals living in the surface ocean, some in solubles and indigestibles (i.e., fecal matter) move into the deep sea under the influence of gravi ty - and so the life cycle leads to chemical segrega tiona Des true tion of plant rna tter occurs, on the average, at greater depths than formation. The interaction of this life cycle with the large-scale water circulation pattern in the sea resul ts in inhomogeneities in the distribution of these species, not only within the sea itself but also in the sediments deposited on the ocean floor.
One of the aims of this book is to point out the factors that influence the average concentrations of the various components of sea sal t and the fac tors tha t produce chemical inhomogenei ties wi thin the sea and its sediments. The approach might be termed "inverse chemical engineering". The ocean is a great chemical plant that processes the dissolved matter added from rivers and dispenses it as sediment. Unlike most chemical plants, the sea has no advance operational blueprint. As chemical oceanographers, we wander through the plant measuring inputs, outputs, and intern al compositions - trying to reconstruct the missing design. As in most chemical plants, the two critical features of the ocean are the manner in which the ingredients are mixed and the manner in which the ingredients react wi th one another. Oceanic mixing is accomplished by a complex system of currents and a host of turbu lent eddies. Many of the chemical reactions are catalyzed by the enzymes in living organisms. Thus any study of the chemistry of sea water is heavily dependent on knowledge derived from physical oceanographic and marine biologic studies.
In this book, we will focus on the first-order processes operating within the sea. Until these are mastered, it is fruit less to proceed to the more complex "details". With this in mind, let us turn our attention to our first subject - the grand chemi cal balance existing in the sea.
DEPTH PROFILES OF SEA SALT COMPOSITION
The salt dissolved in sea water is remarkably uniform in its major constituents. This fact has greatly simplified the task of the physical oceanographer interested in mapping water density patterns within the sea. He needs only to measure the water temp erature, the water pressure and one major property of sea sal t (for example, the chloride ion content or the electrical conduct ivity) to make a very accurate estimate of the in situ density of a given sample of sea water. This task would be-extremely complex if the composition of sea salt were more varied.
Yet it is fortunate, too, that the compositional constancy of the major components of sea salt does not extend to all the minor components. If this were not the case, the oceanographer would lose one of his most powerful tools, for the varia tions in the minor consti tuents of sea wa ter bear important clues regarding mixing, biological, and sedimentary processes taking place within the sea. They are the tracers about which this book is written.
The major ion matrix of sea salt consists of the following constituents: Chlorine in the form of CI- ion; sulfur in the form
2
of SO 4 = ion· and magnesiwn, potassium, calcium and sodium in the form of Mg+q., K+, Ca++, and Na+ ions, respectively. These six ions dominate sea salt; their ratios, one to another, are very nearly constant. In fac t, only calciwn has been shown to vary measurably from place to place in its ratio to the other five con s ti tuents. Al though this cons tancy also extends to many of the lesser components of sea water (boron, bromine, fluorine, uranium, cesium, and others), it does not extend to all of them.
The best known of the varia tions in the composi tion of sea salt arise from the removal of constituents from surface sea water by plants and the subsequent destruc tion of plant-produced parti cles after downward movement. Deep water masses are richer in the constituents utilized by plants than surface water is. If the o cean were sterile, the chemical composition of sea sal t would be almost perfec tly uniform. The main differences in composi tion would be those resul ting from the transfer of gases between the atmosphere and surface waters of differing temperature would exis t.
Plants live only in surface water, where there is enough light to permi t photosynthesis. By the time the components of their debris are returned to the ionic form as a resul t of tissue oxida tion or hard part solu tion, downward movement under the in fluence of gravi ty or migra ting animals has occurred. It is not surprising, then, tha t the primary chemical differences observed in the ocean are all of the type just mentioned: deep water is en riched relative to surface water. The only major exception is dissolved oxygen.
Figures 1-1 and 1-2 show the dis tribu tion wi th depth of po tential temperature*, of salinity and of the concentrations of six biologically utilized sea salt constituents at a station in the North Pacific Ocean. In all cases, the most dramatic change oc curs in the upper thousand meters of the wa ter column. This so called main thermocline constitutes the zone of transition between the warm surface wa ters and the cold deep wa ters. The nu trient species nitrate (N03-), silicate (H 4Si0 4 ), and total dissolved in organic carbon (L:C02 = C02 + HC03- + C03=) show the deep water enrichment mentioned above. Alkalinity and barium show patterns similar (but not identical) to that of silicate. Dissolved oxygen gas, by contrast, shows a depletion. Unlike N03- and L:C02, which are released during respira tion, 02 is consumed during respira tion. Its pattern is also complicated by the fact that cold wa ters descending from the surface carry with them more dissolved gas than descending warm waters. The salinity minimum at about 600 meters depth is genera ted by the la teral intrusion of inter media te wa ters. Keep in mind tha t these frac tional salini ty dif ferences are quite small compared to the fractional differences in
*Sea water is heated by compression (i.e., about O.loC/km) as it descends from the surface. Potential temperatures are cor rected for this heating. Hence the potential temperature is the temperature a water sample would have were it returned to the sea surface.
3
300
6 ......- ......----......--'--.....--'
Figure 1-1. Plots of temperature, salinity, dissolved oxygen con tent and ni trate content as a function of water depth at GEOSECS station 214 in the North Pacific (32°N, 176°W). Potential tem perature rather than the temperature measured in situ is given. The salini ty profile in this region of the ocean--sh"ows a pro nounced minimum at a depth just over 600 meters. This intermed iate water .C as it is called) forms in the northern Pacific and sinks and flows laterally beneath the waters of the warm temperate ocean. The level of this minimum is shown…