Ocean in the Earth System E-Book

Patrick Prouzet

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

Seitenzahl: 393

Veröffentlichungsjahr: 2014

Beschreibung

Complexity is an intrinsic property of natural systems. In the oceanic system, it is linked to many interactions with the atmosphere, geosphere and biosphere with which it exchanges energy and matter.

Complexity of the ocean system has, at different spatial and temporal scales, hydrodynamic mechanisms of these exchanges and dynamics of elements and compounds, they are involved in biogeochemical cycles or used as tracers.

By its pedagogical approach, it defines the terms, methods, techniques and analytical tools used. Then, it analyzes the consequences of climate change, future projections, human impact and the concept introduced with planktonic pelagic ecosystem component.

Leseprobe

Contents

Foreword

1 The Ocean in the Earth System: Evolution and Regulation

1.1. The Earth system and its components

1.2. The ocean, from its origins

1.3. The ocean, oxygen and the evolution of life forms

1.4. The regulation of the greenhouse effect by the ocean

1.5. Oceanic photosynthesis regulates itself on a short timescale

1.6. Conclusion

1.7. Acknowledgments

1.8. Bibliography.

2 The Ocean and the Climate System

2.1. Introduction

2.2. Climate change

2.3. Physics and dynamics

2.4. Some key elements for understanding the ocean’s role in the climate

2.5. Some questions for the future.

2.6. Bibliography.

3 Ocean–Atmosphere Interactions

3.1. Introduction: what are ocean–atmosphere interactions?

3.2. Interface processes and their role in the coupled system

3.3. Examples of energy exchanges

3.4. Conclusion

3.5. Bibliography.

4 Marine Biogeochemical Cycles

4.1. Introduction: geochemistry, biogeochemistry and marine biogeochemistry

4.2. A fundamental characteristic of the Earth’s system: biogeochemical cycles

4.3. Carbon: at the heart of living matter

4.4. Oxygen: a poison that Earth cannot do without

4.5. Nitrogen: a chemical element over which countries have fought in the past

4.6. Phosphorus: a chemical element over which countries may fight in future

4.7. Biogeochemical equilibria and human societies: problems.

4.8. Bibliography.

5 Ocean Acidification and its Consequences

5.1. Introduction

5.2. Observations.

5.3. Projections

5.4. Impacts of ocean acidification

5.5. What are the solutions?

5.6. Conclusion

5.7. Acknowledgments

5.8. Appendix

5.9. Bibliography.

List of Authors

Index

First published 2014 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USAwww.wiley.com

© ISTE Ltd 2014

The rights of André Monaco and Patrick Prouzet to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Control Number: 2014950495

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-84821-701-0

Foreword

We have been asked by ISTE to stimulate work in the area of the environment. Therefore, we are proud to present the “Seas and Oceans” set of books, edited by André Monaco and Patrick Prouzet.

Both the content and the organization of this collection have largely been inspired by the reflection, initiatives and prospective works of a wide variety of national, European and international organizations in the field of the environment.

The “oceanographic” community, in France and internationally – which is recognized for the academic quality of the work it produces, and is determined that its research should be founded on a solid effort in the area of training and knowledge dissemination – was quick to respond to our call, and now offers this set of books, compiled under the skilled supervision of the two editing authors.

Within this community, there is a consensus about the need to promote an interdisciplinary “science of systems” – specifically in reference to the Earth’s own “system” – in an all-encompassing approach, with the aim of providing answers about the planet’s state, the way it works and the threats it faces, before going on to construct scenarios and lay down the elementary foundations needed for long-term, sustainable environment management, and for societies to adapt as required. This approach facilitates the shift of attention from this fundamental science of systems (based on the analysis of the processes at play, and the way in which they interact at all levels and between all the constituent parts making up the global system) to a “public” type of science, which is finalizable and participative, open to decision-makers, managers and all those who are interested in the future of our planet.

In this community, terms such as “vulnerability”, “adaptation” and “sustainability” are commonly employed. We speak of various concepts, approaches or technologies, such as the value of ecosystems, heritage, “green” technologies, “blue” chemistry and renewable energies. Another foray into the field of civilian science lies in the adaptation of research to scales which are compatible with the societal, economic and legal issues, from global to regional to local.

All these aspects contribute to an in-depth understanding of the concept of an ecosystemic approach, the aim of which is the sustainable usage of natural resources, without affecting the quality, the structure or the function of the ecosystems involved. This concept is akin to the “socio-ecosystem approach” as defined by the Millennium Assessment (http://millenniumassessment.org).

In this context, where the complexity of natural systems is compounded with the complexity of societies, it has been difficult (if only because of how specialized the experts are in fairly reduced fields) to take into account the whole of the terrestrial system. Hence, in this editorial domain, the works in the “Seas and Oceans” set are limited to fluid envelopes and their interfaces. In that context, “sea” must be understood in the generic sense, as a general definition of bodies of salt water, as an environment. This includes epicontinental seas, semi-enclosed seas, enclosed seas, or coastal lakes, all of which are home to significant biodiversity and are highly susceptible to environmental impacts. “Ocean”, on the other hand, denotes the environmental system, which has a crucial impact on the physical and biological operation of the terrestrial system – particularly in terms of climate regulation, but also in terms of the enormous reservoir of resources they constitute, covering 71% of the planet’s surface, with a volume of 1,370 million km3 of water.

This set of books covers all of these areas, examined from various aspects by specialists in the field: biological, physical or chemical function, biodiversity, vulnerability to climatic impacts, various uses, etc. The systemic approach and the emphasis placed on the available resources will guide readers to aspects of value-creation, governance and public policy. The long-term observation techniques used, new techniques and modeling are also taken into account; they are indispensable tools for the understanding of the dynamics and the integral functioning of the systems.

Finally, treatises will be included which are devoted to methodological or technical aspects.

The project thus conceived has been well received by numerous scientists renowned for their expertise. They belong to a wide variety of French national and international organizations, focusing on the environment.

These experts deserve our heartfelt thanks for committing to this effort in terms of putting their knowledge across and making it accessible, thus providing current students with the fundaments of knowledge which will help open the door to the broad range of careers that the area of the environment holds. These books are also addressed to a wider audience, including local or national governors, players in the decision-making authorities, or indeed “ordinary” citizens looking to be informed by the most authoritative sources.

Our warmest thanks go to André Monaco and Patrick Prouzet for their devotion and perseverance in service of the success of this enterprise.

Finally, we must thank the CNRS and Ifremer for the interest they have shown in this collection and for their financial aid, and we are very grateful to the numerous universities and other organizations which, through their researchers and engineers, have made the results of their reflections and activities available to this instructional corpus.

André MARIOTTI

Professor Emeritus at University Pierre and Marie Curie

Honorary Member of the Institut Universitaire de France

France

Jean-Charles POMEROL

Professor Emeritus at University Pierre and Marie Curie

France

1

The Ocean in the Earth System: Evolution and Regulation

1.1. The Earth system and its components

It is very common today for our planet to be called an “Earth system”. The scientific meaning is both a necessary tool of communication and an area of potential confusion. Therefore, let us first endeavor to probe this concept and define how the Earth is a “system” of which the ocean is one of the “components”.

A system is classically defined as a group of elements (or components), each of them interacting with the others through certain principles or rules. This definition seems trivial and merits some comments and clarifications.

1.1.1. A system is a set of objects whose limit is arbitrary, but pertinent

First, the word “set” implies a grouping within a boundary, defined in a subjective and arbitrary manner by the observer. In what circumstances is the arbitrary choice of a grouping relevant? For the scientific observer, the grouping is most often relevant because it corresponds to the category of “an area of study”, generally a characteristic of an academic field. This might be a cell for a cytologist, a multicellular organism for a physiologist, a population or settlement for a population biologist, the ecosystem for an ecologist, the society for a sociologist or for an economist, the area beneath the Earth’s surface for a tectonician or volcanologist, the Earth’s surface and atmosphere for a climatologist or an oceanographer, etc. For the non-scientific observer, the intuitive grouping is that which corresponds best to a visual entity, whether that be observed by eyesight alone or through images furnished by means of modern technology (from the microscope that allows cells to be seen to satellites that allow the Earth to be seen from space). It is not surprising that the disciplines in the natural sciences, for the most part, are derived from the visual perceptions of the “man on the street” and from technological progress.

The arbitrary identity of a system, through a grouping, is therefore a form of categorization of the type that biologists use for very broad families of life forms (e.g. bacteria and archaea, protist eukaryotes, multicellular or metazoan eukaryotes). We note, however, that this is different from phylogenetic categorization (modern day taxonomy), as this is founded upon similarities in the attributes and in the genetic proximity amongst a range of objects: that of individual living beings.

In the case of a living being, the definition is, in general, relevant because of its physical limits (e.g. cuticles and skin) and its autonomy. Note that the idea of autonomy can after all be very weak. Is an ant autonomous without its anthill? Is a cell autonomous in a multicellular organism? Is a man or woman autonomous on a reproductive level?

The definition of a system is, in retrospect, very arbitrary when the proportion of non-living components of the system is important (e.g. ecosystems and societies). Such a system should therefore be the subject of a precise and rigorous description within the framework of the limits that have been arbitrarily fixed upon it. Too often, the notion of an ecosystem is, unfortunately, employed as a generalization, each user implicitly conferring a different typology on it.

In the extreme case of a planetary system, the limit again becomes pertinent (the summit of the atmosphere, if there is one, or the soil surface, or the sea surface), since there is a great contrast between a medium where, under the effect of gravitational attraction, the density of the matter is high (the interior of the system, atmosphere included) and a medium where the density of matter is close to zero (the planet’s exterior, interplanetary space). However, unlike living beings, such a system is not “reproducible” in a biological sense; in other words, it has no equivalents. Despite that, it can, like living beings, have global characteristics essentially invisible to the non-scientific observer, notably in terms of self-organization, regulation and adaptive evolution.

1.1.2. One system is necessarily built into another

A multicellular organism is, as the name suggests, an organized living being; in other words, it is composed of organs that interact with each other in that they maintain the existence of the organism as a whole. But the organisms themselves are composed of specialized (somatic) cells that have interactions between them. It is therefore evident that the arbitrary limit of the internal components of a system, which amount to subsystems but not necessarily to organs, depends directly on its external definition, that is to say on the identity that we attribute to it.

Another theoretical intermediate limit distinguishes isolated or closed systems from open systems. It is this point that we will now tackle.

1.1.3. The Earth is a “closed” system

In thermodynamics, a system is called “isolated” when it does not exchange energy nor matter with the outside world. The term “matter” is theoretically useless here, when we consider that from a physicist’s point of view, according to Einstein’s theory of general relativity, matter is only a particular form of energy. This only accounts for the principle of conservation of energy, matter being only an approximation for the scales of time and space relevant to the life of mankind or the history of humanity.

By virtue of the second law of thermodynamics, an isolated system naturally evolves toward a situation of equilibrium, which is known as “thermodynamic death”. In our own perceptions, such a system does not exist because any attempt to observe it would, in fact, break its isolation. It is therefore only theoretical and can only give rise to non-refutable hypotheses whose scientific character can be discussed [POP 62] (such as the cosmological hypothesis of multiverses where universes evolve in isolation from one another).

Conversely, a system is called “open” when it exchanges energy and matter with the outside world. In this case, the system can depart from a situation of thermodynamic equilibrium to reach, eventually, a stationary situation without equilibrium (regulation) whose maintenance is assured by the transformation of a flux of energy during its passage to the heart of the system. This transformation is the price to be paid in order for the level of organization of the system to be maintained or increased. This is what occurs during photosynthesis in plants by directly transforming light energy from the Sun, but also more indirectly, during respiration in animals, plants or bacteria. By doing so, these reuse, more or less directly, the chemical energy that the products of the initial photosynthesis contain; that is to say organic matter (reduced carbon) and oxygen (O2).

A stable system does not accumulate energy in the long-term. Apart from a few insignificant and temporary fluctuations, such as phases of lipid stocking and the development of living creatures, the entirety of the adsorbed energy is sent back outside the system, only with a modification in quality (an increase in entropy). This is, for example, the case with changes in quality of the energy contained in organic matter (chemical energy) which, after being used by a living consumer (via respiration, by using oxygen or another oxidizing compound), is partly transformed into heat that is then transferred into the surrounding fluid. In this sense, this flux of energy can be considered a mediator that reduces the entropy to the system. As entropy is a physical concept of the measurement of chaos, this reduction results in the system maintaining or increasing its “order”, otherwise known as its level of organization.

The Earth, during its accretion phase (around 4.5 billion years ago), which lasted around 100 million years, was a largely open system which received a very significant influx of matter relative to the mass already accumulated. It was in the next phase, during this continued influx of matter, that terrestrial water, which we know today in the form of oceans, ice, rivers, lakes, groundwater, clouds and vapor, accumulated on the new planet. Simple organic molecules, formed in space, also arrived on Earth during this period. It remained a very open system around 4 billion years ago during a phase of late and intense meteoric bombardment.

But since this very distant epoch, the Earth has not experienced any further significant accumulation of matter. Of course, the current average inward and outward fluxes of matter, at the boundary of the Earth system, are not null – in the order of several hundreds of thousands of tons per year – but they are entirely negligible compared to the fluxes of matter that are exchanged between the internal components of the Earth system. The former, therefore, has almost no influence on the latter. Of course, the exceptional impact of a large meteorite occurs from time to time with important consequences for evolutionary dynamics, the most well known of them being the extinction of the dinosaurs, with the exception of the ancestors of birds, 65 million years ago. However, such an event, even if it heavily impacts the nature of present and future species, has no long-term effect on the Earth system’s general biogeochemical dynamics. Once the event and its climatic consequences have passed, the system regains the normal course of its regulations and evolutions, which we will describe in this chapter and which are also addressed in Chapter 4, and in Chapter 2 of [MON 14b].

If the Earth has had an exchange with the universe in the pst 4 billion years, it is likely to have been through a flux in radiative energy. For thermodynamics, this means a non-isolated, but “closed”, system. The Earth receives a certain quantity of radiative energy from the Sun and from space and sends an equal quantity back into space, but increases the average wavelength of its radiation; that is to say it shifts its spectrum to infrared. The individuality of the Earth system is thus partly due to the fact that it is a closed system and its perimeter is not arbitrary since it corresponds to the pragmatic image that we have of it as an object, well separated from the exterior universe. Moreover, we will show later that the maintenance of an atmosphere, oceans, a climate and life, for 4 billion years, is largely due to the fact that the Earth is a closed system through which energy travels and is transformed. This is an exceptional situation in the solar system, since we can observe other neighboring planets, formed at the same time, such as Mars and Venus, where these characteristics are not present simultaneously.

The atmosphere of Mars is extremely thin and liquid oceans do not currently exist on Mars nor Venus. The condition of a “closed planetary system” is therefore necessary, but not sufficient, to explain the preservation of the principal characteristics of Earth – atmosphere, liquid water, life – over the course of 4 billion years. The other conditions stem from adaptive interactions that have been established progressively, on the one hand between the larger components of the Earth system (e.g. geophysical, geochemical and biogeochemical exchanges) and on the other hand between those components and life forms that constitute them (flux due to the metabolisms of life forms). The objective of this chapter is to describe these interactions.

1.1.4. The major components of the Earth system

The general physics and biogeochemistry of the Earth system show that the most important internal fluxes of matter and energy only occur within a few particular interfaces. The interface that separates the “solid” Earth from external envelopes of fluids (the ocean and the atmosphere) is found at the location of volcanic emissions, erosion, sedimentation and the subduction of tectonic plates. The interface between the ocean and the atmosphere gives rise to evaporation, precipitation and exchanges of gas. The interface between the continents and oceans is the location of transfer of continental matter eroded into the oceans, in particulate and dissolved forms, that partly fuels the sedimentation of the oceans.

A simplified but pertinent description of the Earth system therefore consists of defining the internal “base” components through a few very large compartments (or reservoirs), which include the geosphere, ocean and atmosphere. They are like the vital organs of an animal between which fluxes of matter and energy travel. In this sense, they are physical compartments, whose role in the Earth system is “organic”.

Improvements in our knowledge of these compartments, through active research over the last few decades, have, of course, led to the refinement of the understanding of the system. It is in this way that we are able to make distinctions between the different reservoirs of water on earth (the hydrosphere) – the oceans, continental fresh water, ice and water vapor. However, further distinctions between subcompartments have proven to be necessary. For example, the ocean’s surface and its interior do not have the same physical properties and are separated by a discontinuity in temperature (the thermocline) but also a discontinuity in density. These two subcompartments exchange matter and energy between themselves (via thermohaline oceanic convection and turbulence in the thermocline), but also experience exchanges with the atmosphere at its upper surface, in the case of the ocean surface, and with the seafloor, in the case of ocean’s interior (underwater volcanic activity, sedimentation, early diagenesis, circulation of hydrothermal fluids, etc.). Similar distinctions exist between the upper and lower atmosphere, or between the geosphere in contact with the atmosphere (soils, outcrops, lavas and basalts on continents and islands) and that which is in contact with the ocean (sediments on continental margins and abysses, lavas and basalts).

1.1.5. What is the biosphere?

The notion of a “biosphere”, in parallel with the large components that we have just described above, brings us back to considering the sum of all life forms as simply an additional component. Evidently, this would not correspond to the reality and would moreover be fundamentally dismissive in regards to the importance and complexity of the role of life in the functioning and evolution of our planetary system.

Firstly, life forms are neither homogenous nor continuous. They have effectively colonized a great number of niches, whether that is the surface of the geosphere (sediment), the oceans, continental water or the surfaces of continents. Moreover, although they represent only a slight mass (biomass) compared to that of the large compartments of the Earth system, life forms are the mediators of an important part of the flux of matter between compartments through the phenomena of photosynthesis and respiration, the mobility of organisms, the phenomenon of bio-mineralization leading to the construction of internal skeletons and shells, and the production of organic and mineral waste (senescence, death, production of feces and urine, etc.). A life form is therefore not, strictly speaking, a reservoir of the Earth system, but rather a motor, or mediator, of its internal interactions. But the definition of the biosphere does not stop there. When we speak of the evolution and regulation of the Earth system across large scales of time, life is also characterized by its endlessly renewed diversity, from which its adaptive plasticity develops under the pressure of natural selection. It is this plasticity of life forms that, via its role in the exchange of matter, allows the entirety of the Earth system to be evolutionarily flexible.

Thus, we acknowledge that the description of an ecosystem, whatever its perimeter, does not just come down to its life forms. It cannot limit itself to describing instantaneous states, but should also describe the dynamics and retroactions, where living and non-living entities enter simultaneously into play. We will, therefore, conclude here that the term “biosphere” is equivalent to that of the Earth system (or global planetary ecosystem) from the advent of early life on Earth (around 4 billion years ago) to today, as such, for the greater part of its history. But in order to avoid any confusion, we will restrict ourselves to using a single expression: “Earth system”.

1.2. The ocean, from its origins

1.2.1. Was there an ocean 4.4 billion years ago?

Isotopic studies carried out on zircons from Australia dating from 4.4 billion years ago [WIL 01] have given rise to the emergence of the hypothesis that liquid water could have been present on the primitive Earth from the end of its principal accretion phase, characterized by the presence of a magma ocean resulting from the transformation of gravitational energy into heat. The liquid water would have accumulated on a solid crust that had only just cooled.

Even if this hypothesis still remains widely disputed, it is generally acknowledged that the oceans were probably formed more than 4 billion years ago. Evidently, this early liquid water was in equilibrium with an atmosphere much denser than that of today, of which the pressure at ground level was several tens of times greater than that of today. The original oceans therefore probably experienced a very hot phase, in the order of several hundreds of degrees Celsius.

1.2.2. The origin of water on Earth (4.5 – 4 billion years ago)

It is self-evident to say that water on Earth has an extraterrestrial origin because this is true of all the elements that constitute our planet. Nothing is created from nothing in our observable universe! Nevertheless, the arrival of water, carried on meteorites and comets, was not in exact synchrony with the arrival of rocky elements and this is due to the history of the formation of the solar system. Fervent research activity carried out by planetologists and astrophysicists particularly endeavors to explain the differences in composition and orbit of objects in the current solar system (the Sun, telluric “rocky”, planets, giant “gas” planets, satellites, asteroid belts, comets, the Oort cloud). The explanatory model develops particularly in the form of a historical sequence of several important events such as the formation, differentiation and the progressive cleaning of the protostellar disk, the movement of volatile components (including water) associated with the thermonuclear ignition of the Sun, the formation of telluric planets, the formation of giant gas planets, the migration of Jupiter (the “hot Jupiter” hypothesis) and substantial impacts.

It is not possible to give here an overview of the state of current knowledge in this field; the interested reader can refer to more specialized works on the formation of the solar system. We can, however, emphasize a major point for this present chapter: most water, brought through influx asteroids and comets, accumulated late, after the principal phase of accretion by rocky material, but probably in the first 500 million years of the Earth’s history. It is this water that would constitute the majority of the current oceans (93% of water on earth) and the different reservoirs of fresh water (glaciers and ice caps above all, then lakes, rivers, groundwater, atmospheric water vapor and clouds).

1.2.3. The ocean and the end of the “Venus” phase of the Earth’s history (between 4.5 and 4 billion years ago)

During accretion, the heaviest elements were concentrated by the effect of gravity at the center of the Earth’s sphere (especially iron and nickel, but also other, rarer heavy elements), whereas the lightest elements became concentrated on its surface. Taking into account the initial conditions in temperature and radiation, the Earth’s very first atmosphere was probably dominated by gaseous and stable components, formed by the chemical combination of light elements such as hydrogen, carbon, oxygen and nitrogen – the most likely candidates being carbon dioxide (CO2), dinitrogen (N2) and water (H2O). This is the reason why, in the absence of natural archives from the era, the current theoretical models propose an initial atmosphere that was very dense, dominated, apart from water, by around 98% CO2 and 2% N2; these proportions can vary slightly depending on the models used [GAR 06]. Certain minor or trace components (such as gaseous SiO2) are not excluded from these hypotheses, particularly because of the very turbulent conditions caused by enormous, repeated meteoric impacts or by the mega-impact that gave rise to the formation of the dual Earth–Moon system.

The primordial atmosphere was probably very dense (from 60 to 160 times the current atmospheric pressure) because it contained, in the form of gaseous CO2, the carbon that is present today in the form of solids (carbonated rocks and sediments and organic matter, notably fossils). In such conditions, the greenhouse effect was enormous and it kept the Earth’s surface temperature at several hundred degrees Celsius. This phase is sometimes called the Earth’s “Venus” phase by analogy with the conditions that prevail today on Venus.

Water, which arrived on Earth progressively carried by meteorites and comets, progressively accumulated, at first in gaseous form, increasing the greenhouse effect, then in liquid form when the saturation pressure was reached. It is therefore very probable that the first reservoirs of liquid water contributed considerably to the reduction of the greenhouse effect [LIU 04] as they constituted chemical reactors by which gaseous carbon in the form of CO2 could be transformed into solid carbon in the form of carbonates (Metal2+CO32–), via the equilibriums between the different forms of carbon mineral dissolved that coexist in the aqueous phase, CO2, HCO3– and CO32– [ZEE 01].

It is helpful to note that this general process is constantly occurring today, but through biologically mediated processes (biomineralization of calcium shells and skeletons) and via the alteration of aluminosilicate rocks. This alteration consumes CO2 and some of this carbon is ultimately found in the ocean in the form of carbonate ions (CO32–) due to oversaturation. These will then be transformed into solid form by the biological processes that create shells and skeletons, then stored in the sedimentary reservoir. Details on these processes can be found in Chapter 4, and in Chapter 2 of [MON 14b].

It is unknown how much time the end of this “Venus” phase could have taken; many specialists on the primitive Earth now believe that conditions favorable to the emergence of life were already present 4 billion years ago, that is to say, before the late meteoric bombardment, and it is not impossible that an emergence of life may have taken place in the ocean from this era [GAR 06]. If it existed, did this life survive a late meteoric bombardment? Did other emergences, successful or abortive, take place later? These questions remain largely open.

Figure 1.1.Diagram of the Earth system at the end of accretion around 4.5 billion years ago (see color section)

COMMENTARY ON FIGURE 1.1.– The influx of evaporation into the atmosphere is not compensated for and the planet is surrounded by a very dense atmosphere mainly composed of carbon dioxide (CO2), dinitrogen (N2) and gaseous water, resulting from gravitational differentiation (the heaviest elements being concentrated as a core at the heart of the geosphere). The greenhouse effect is enormous and an ocean of water cannot form. This is the Earth’s “Venus” phase.

1.2.4. Why are there oceans on Earth and a “Venus inferno” on Venus?

Earth and Venus are two neighboring telluric planets, formed at the same time and of a very comparable size. How do we explain the fact that the conditions on their surfaces and in their atmospheres are so dissimilar today? 1

Aside from the conditions on their surfaces and the composition of their atmospheres, two important characteristics distinguish the two planets. First, there are no active plate tectonics on Venus, whereas Earth maintains one. We can observe this in everyday life through earthquakes, volcanic activity or, more poetically, through contemplating our mountainous countryside. The second characteristic is the presence of an important satellite, the Moon, which revolves around the Earth. The Moon and the Earth have a very important mass ratio (1/81), now known to us as the most important between two planetary bodies in orbit around their common center of gravity (which is naturally much closer to the center of the Earth than to the center of the Moon). This duo resulted from a giant impact between two planets in the process of formation just over 4.5 billion years ago. Because of its kinetic conditions (notably speed and angle of incidence), this impact threw out a very significant quantity of matter into space while still preserving the core of the larger of the two proto-planets. The result was the formation of an orbital disk, followed by the progressive cleaning of this disk to form the Moon [KOK 00].

However, another scenario could have involved the instantaneous introduction of a significant quantity of water into the magma mantle of this protoplanet, a precursor to Earth in the right conditions, an event that did not occur, or which occurred differently, in the formation of Venus [BIB 09]. As we now know, thanks to the study of plate tectonics on Earth, the filtration of light components (including water) into the mantle leads to a decrease in its viscosity [BIL 01, DIX 04] and to a chemical differentiation between the continental crust and the ocean along the subduction zones. Together, these two phenomena favor the maintenance of an active tectonic system, at least as we know it on Earth.

Some results of the exploratory mission Venus Express published in 2009 by the European Space Agency [ESA 09] suggest that a tectonic system nonetheless existed at the beginning of Venus’ history, but it would have ceased to develop due to the lack of ocean. The primordial water was in fact systematically vaporized with a positive retroactive effect on the greenhouse effect, leading to its enhancement and to the gravitational escape of hydrogen.

In the case of the Earth, one hypothesis is that a much more active tectonic system, initiated by the giant impact that led to the formation of the Earth–Moon duo [RUI 11], could have absorbed some of the primordial water via subduction, sufficiently slowing the enhancement of the greenhouse effect so that sizeable oceans could form. After that, the reduction of the initial greenhouse effect by the precipitation of carbonates could have become considerable and the first stage of abiotic regulations of the Earth system was established. This is illustrated in Figure 1.2.

Figure 1.2.Diagram illustrating the bases of the first abiotic regulations of the Earth system (see color section)

COMMENTARY ON FIGURE 1.2.– Due to an ocean being in place, the inward fluxes are from now on potentially compensated by outward fluxes for each of the reservoirs of matter. Short-term regulation is assured by the chemical equilibriums of the ocean (the detail of which will be addressed later), whereas the long-term regulation depends on the tectonic cycle. The tectonic activity of the Earth system has been maintained up until today, whereas that of Venus ceased to be very early on. It is probable that this difference arises from the permanent introduction of water into the mantle via the phenomena of subduction. Such a communication between the upper geosphere and a liquid ocean would be established on the Earth system while the early tectonic was still sufficiently active, perhaps because of a more significant initial hydration of the mantle resulting from the mega-impact that formed the Earth–Moon duo.

1.2.5. The ocean, cradle of the first living creatures (between 4.4 and 3.5 billion years ago)

What criteria can we use to distinguish a living state from a non-living state? This question is an inexhaustible source of impassioned conversations where different viewpoints clash, from those of scientists (the presence of a cellular metabolism, the presence of a genetic “code”, the presence of a capacity for “reproduction”, the presence of an open, self-regulated system, the presence of a steady state maintaining or increasing the level of organization, that is to say locally going against the second principle of thermodynamics, etc.), up to those beliefs of a spiritual, philosophical or religious order (essentialism, vitalism, creationism, etc.).

On a strictly scientific level, does this question have only one meaning? Is there a scientifically based reason for which it would be necessary to distinguish a living state from a non-living state, or is this only, once more, a question of definition and limits of an arbitrary nature?

To develop an argument in reply to this question here would take much too long and would not be relevant to this work. This is why we will restrict ourselves here to defining, arbitrarily, the “minimal” state of a life form as that of a biological cell possessing a metabolism, genetic information and a capacity for reproduction. This arbitrary definition should be distinguished from that of life in a general sense, which is richer and more complex, and to which we will return later.

A biological cell needs water, and the chemical prebiotic systems that preceded the first cells also needed it. The presence of liquid water, indeed of an aquatic environment, is therefore a necessary condition for the emergence of life. Researchers working on prebiotic conditions and the first life forms do not rule out the possibility that these could have appeared in multiple locations and on multiple occasions, in isolated lakes or seas that probably coexisted and succeeded each other in different places and at different stages of the initial evolution of our planet, even 4 billion years ago [GAR 06]. However, the geological evolution (primitive tectonics), the environmental variability (local climate, deposits of sediment and drought) and major hazards (volcanic activity, falling asteroids, notably before and during the last phase of meteoric bombardment around 4 billion years ago) render the survival of small, local autonomous ecosystems on timescales in the order of a million years or more almost impossible.

The question of the “origin” of life in the Earth system is therefore not only that of the processes by which the first living beings appeared in general, but it is also that of the conditions that permitted one of these life forms to become the common ancestor of all life today. How has life been able to develop into the lattest form, i.e. human beings, over billions of years, as phylogenetic trees indicate, which stretch back to a last common ancestor having lived 2 or 3 billion years ago (Last Universal Common Ancestor (LUCA)) and from the findings of paleontology, which stretch back to around 3.5 billion years ago (the age of the most ancient known bacterial fossils, in the form of stromatolites)?

The presence of a sufficiently developed ocean at different latitudes and in different climates is an element of enquiry that concerns the crucial phase of development of the first ecosystems. For life to have been able to “survive” hazards of all types in the long-term, it is necessary that its random destruction was only partial and rapidly compensated by a new colonization of the destroyed environments. It is necessary to underline here that the colonization is not a response to the necessity of increasing the chances of survival of the entirety of the population, but simply that of increasing the chances of survival of each individual by finding the resources that it needed elsewhere. The necessity is individual, but its effects extend to the scale of populations and ecosystems.

There is therefore every reason to think that the life from which human beings originated appeared and developed in an already significant ocean. That this origin may have been associated with deep hydrothermal environments through the use of chemical energy available at high temperature [REY 01, SHO 96], or rather in the surface environments through the use of solar energy, is a question that remains very open to research. Many of the life forms from deep hydrothermal environments, notably animals, have adapted to these extreme nutritional niches by natural selection, beginning from the ancestors that they shared with bacteria, cephalopods, fish, or crustaceans from other marine environments. However, it is unknown whether it is the same for thermophile and chemosynthetic bacteria or if these forms of life appeared locally.

1.3. The ocean, oxygen and the evolution of life forms

1.3.1. The essential characteristics had been selected in the ocean before the Cambrian period, over 540 million years ago

The work of the famous American paleontologist Charles Doolittle Walcott led him to discover, at the beginning of the 20th Century, the extraordinary diversity of fossils in the “Burgess Shale” (a fossil field in British Columbia, Canada) [WHI 80]. The corresponding organisms, dating from the Cambrian period (between 540 and 500 million years ago – even if the majority of fossils are in the later Cambrian period), were the most ancient forms of animal known at the time. They were marine animals, characterized by the presence of endoskeletons or exoskeletons and by a body organized into segments, such as we can still find today in arthropods, insects and their distant cousins: the vertebrates. This discovery formed the origin of the idea that an accelerated diversification of living species, known as “the Cambrian explosion”, occurred at this time. The exceptional preservation of these fossils includes not only their carapaces (toughened external body parts, e.g. an exoskeleton or shell), but also morphological traces of soft tissue situated in the area of the body. It was therefore legitimate to delve deep in the study of the diversification, functioning and behavior of these organisms.

In light of today’s knowledge, it is sensible to acknowledge this idea of an evolutionary explosion belonging to this era and to which we can associate the genesis of the entirety of living things. After close inspection, the fossilized organisms of the Cambrian era are for the most part animals; living beings already displaying certain fundamental characteristics in common, selected for over the course of numerous former diversifications. Some of them (on which there will be a new question later) are aerobic organisms: symbiotic cells with a differentiated nucleus (eukaryote) and a characteristic multicellular organisation. Furthermore, the development of carapaces is an evolutionary innovation of the Cambrian period which naturally favored the preservation of fossil traces of animals in possession of these attributes, and which focused the attention of paleontologists on the Cambrian and post-Cambrian periods. Indeed, we know today that many more ancient marine animals without carapaces existed 600 million years ago (the Ediacarian fauna, after the name of the Australian site where these fossils were discovered). This is evidence of the diversification of multicellular organisms in this era, plants as much as animals as no fauna is possible without flora. Some proterozoic fossils dated from 2.1 billion years ago have been found recently in Gabon [ELA 10]. Their morphological complexity intrigues paleontologists and leads to the belief that they could be either multicellular organisms (plant and/or animal) or colonies of prokaryotic cells (bacteria or archaea) possibly in possession of a rudimentary form of functional organization, highlighted by the work of microbiologists using modern examples [KEI 04]. Whatever they are, numerous important diversifications have occurred since the origin of terrestrial life forms, before the Cambrian period, to promote those that led to the selection of the molecular intracellular system that we know today (DNA/RNA/proteins) and of all its genomic and functional variants. The largest number of these diversifications occur in the domains of bacteria and archaea: unicellular prokaryotic microorganisms (cells without a differentiated nucleus), in contrast to eukaryotic cells whose diversity remains largely unexplored.

Figure 1.3 illustrates the principal evolutionary innovations that occurred, mainly in the ocean, between the beginning of life on Earth (over 3.5 billion years ago) and 400 million years ago (early Devonian Period). It also shows the major environmental evolutions, from an exclusively anoxic environment to a very heavily oxygenated environment, where only a few anoxic niches remained (organic sediments, digestive tubes and poorly ventilated deep zones of the ocean or certain lakes). These environmental evolutions matched evolutionary developments, such as the emergence of oxygenic photosynthesis, at the same time creating a new framework of parameters for later developments, such as the emergence of aerobic respiration.

Figure 1.3.The main evolutionary developments and the major environmental evolutions between the beginnings of life on Earth (over 3.5 billion years ago) and 400 million years ago (early Devonian Period) (see color section)

The links of cause and effect between the oxygenation of the environment and the major developments previous to the emergence of aerobic respiration are however less evident and still constitute working hypotheses for research. In the emergence of eukaryotic cells, it is not unreasonable to assume that prokaryotic cells, subjected to an increasingly oxygenated environment, were able to find mutual benefits by sharing respective innovations in the framework of endosymbiosis [MAR 91]; some taking the best part of their energy from aerobic respiration (mitochondria) and others furnishing the protective mechanisms of cytoplasm or DNA against the toxic effects of oxygen. As far as the emergence of multicellular organization is concerned, it is not unreasonable to assume that such an organization could have caused external contact with oxygen to be reduced. The aim of natural selection would then be to specialize the external cells in resisting the toxic effects of oxygen and to ensure an influx of matter into the internal cells. A major question for research is to discover how the change occurred from a malleable and temporary adaptation, responding to external conditions, to a biologically cemented situation, where it is the endosymbiotic cell or colonial cell group that reproduces itself as such.

Finally, the colonization of continental environments can also be linked to the history of oxygen. An atmosphere globally enriched with O2 is favorable to the formation of ozone (O3) because of the photochemical reactions that occur in its upper layers, which are subjected to very powerful rays from the Sun. The formation of a permanent ozone layer, even if subject to fluctuations with the seasons, certainly favored, indeed rendered possible, the selective adaptation of marine organisms in continental environments. The term “out of water” is too often used incorrectly here, since the organisms adapted first to environments from which water periodically retreated, such as in the intertidal zones of marshes, up to the point where they were capable of leaving the water for increasingly longer periods.

On a global scale, life forms and their environments have thus coevolved, through the device of retroactions, for over 3.5 billion years. Upon careful observation, we can confirm that the most fundamental characteristics of present day life forms (aerobic or anaerobic respiratory metabolism, endosymbiosis of eukaryotic cells and multicellularity) were all selected for in the ocean, before the start of the Cambrian period, over 540 million years ago.

1.3.2. How did oxygen accumulate?

Oxygen is an element very receptive to electrons; is it therefore very chemically reactive. The word “oxidize” comes moreover from this property since oxygen, in its O2 form, is capable of oxidizing almost all the other elements or chemical compounds. Only a few halogens, such as fluorine or chlorine, hyperoxygenated anions (permanganate MnO4–, dichromate Cr2O72–) or ozone (O3), have a greater oxidizing power.