Our thesis: Life has retained planetary water.
We champion the poorly developed Gaian view that life has vigorously helped maintain abundant water on the Earth’s surface over the last three and a half thousand million years. We defend the idea that life’s populations persist and continue to expand on Earth not because of any “lucky accident” that has situated our moist planet at an optimal distance from the sun, but rather that water has been actively retained by communities of living organisms acting to maintain wet local surroundings. The result has been the retention of moist habitability over geological time. We suggest that without life’s involvement in complex geological, atmospheric and metabolic processes, Earth would long ago have lost its water, becoming a dry and barren world much like Mars and Venus. Theoretical interpolation of a lifeless planet Earth between that of Mars and Venus shows that our planet now would be a dry, carbon dioxide-rich world with a temperature primarily determined by steady increase in solar luminosity (Lovelock 2000).
Without continuous flows of carbon, hydrogen, nitrogen, sulfur, phosphorus and other essential elements, primarily as compounds in watery solution, no life form continues to thrive. The purpose of life, much like other thermodynamic systems open to the flow of matter and energy, is to dissipate chemical and thermal gradients (differences across distances) as elegantly detailed by Schneider and Sagan (2006). The assurance of energy and matter flows in appropriate amounts, rates and useable chemical form is a sine qua non of the living state. Life is not a “thing” but a growth process (Day, 2007) that tends to overgrow its bounds. All living beings are invariably limited by appropriate availability of energy and matter. The many limitations to life’s intrinsic capacity for growth and diversification is the process Charles Darwin (1809-1882) recognized as “natural selection”.
In recognition that “independence” from the biosphere is death and that life is a powerful geological force, V.I.Vernadsky (1863-1945) explained that all life is connected through Earth’s fluid phase (Sagan, 2007). This comprises the atmosphere (air, including that in soil, caves and dissolved in water) and the hydrosphere (oceans, lakes, rivers, streams, springs, and so forth). Early in the 20th century, the Harvard University scholar L.J. Henderson (1958), presented a persuasive but nearly forgotten argument that life would not exist on this planet without the water that sustains and supports it. He reviewed the salient features of life’s “universal solvent system” in his chapter dedicated to the physics and chemistry of water. The thermal properties of water (its specific heat, latent heat, thermal conductivity, expansion before freezing) and its action upon other substances (as a solvent, and by virtue of its ionization and surface tension properties) are unique amongst solvents and are utterly required by the physiological and ecological systems of life on our planet. The eclipse of Henderson’s virtually unknown work coupled with the hagiographic attitude toward Darwin is probably attributable to the tendency in the literature of evolutionary biologists to overlook environmental chemistry in general and, in this case, especially the chemistry and biochemical involvements of water, which seem irrelevant to them. What is remarkable is the fact that Henderson’s analysis is not at all obsolete – on the contrary – we find it germane to any Gaian analysis of the water anomaly on Earth relative to the other inner planets. In the spirit of Ian McHarg’s remarks (Margulis et. al., ed. 2007) we recommend to the readers of this book that a modern detailed reappraisal of Henderson’s concept of the “fitness of the environment” be undertaken. McHarg adds Henderson’s concept of the environmental importance of water to Darwin’s work on evolution in his search for understanding the creative survival of the living. For McHarg, there is a criterion by which living (and other) processes can be evaluated for their creativity (and destruction). He calls it “creative fitting in health” and contrasts it with “reductive misfit revealed in pathology” (McHarg 2006, p.25). He points out that whereas Darwin emphasized that the organism “is fit for the environment” Henderson, who McHarg admires as much as he does Darwin, said “The actual environment, the actual world constitutes the fittest possible abode for life”. McHarg (2006, p. 23) unites the propositions of Darwin and Henderson when he concludes that “there is a requirement for any system – whether sub-cellular, cell, tissue, organism, individual, family, institution–to find the most fit of all environments and to adapt both the environment and the system itself”. McHarg insists that the survivors adapt in the sense that they actively and continuously change their environment to accomplish fitting in a thermodynamically creative way. The sum of active and incessant local environmental alteration, in this case by the movement of the water and matter with which life interacts, we recognize as “Water Gaia”.
Here, we expand the insights of our predecessors by elucidating the tight correlations between life and water. Life, aptly called “animated water” by Vernadsky and colleagues, is mandated by the presence and properties of water. Life ensures its own continuity by retaining and interacting with liquid water on our planet’s surface.
Water on Venus, Earth and Mars
Scientists concur: all three inner planets Venus, Earth and Mars prior to the Archean Eon, over 3500 million years ago began with meteoric and probably subsurface water in abundance. Geomorphological observation of erosion by water, steady bombardment by water-rich comets, asteroids and meteorites and other evidence attests to copious quantities of early water on Earth (Robert 2001). Water must have out-gassed from ancient tectonic activity as all these planets and their moons were bombarded by the water-rich bolides of the early Solar System. The surface of Venus, closer to the Sun and that of Mars, beyond Earth’s orbit, reveal riverine, lacustrine or marine features that suggest vast quantities of open water flowed on pristine active lithospheres of our early “sister planets”. Whereas much, perhaps even an ocean’s-worth or more of water, was lost from both our neighbors, the early Earth apparently retained its primordially wet conditions.
Our hypothesis that water retention is a Gaian phenomenon is testable. Venus probably lost its water because its proximity to the Sun meant that even early in the history of the solar system it would have received 40% more solar radiation than the modern Earth. This high radiative flux would have evaporated huge amounts of water vapor into the atmosphere of Venus that set in train a catastrophic positive feedback on warming due to the powerful greenhouse effect of water vapor. This idea is the so-called ‘runaway greenhouse’. Abundant water vapor in the stratosphere would have been photo-dissociated by ultraviolet radiation in processes that led to massive quantities of hydrogen loss to space (Kump et. al. 2004).
Although Mars receives some 43% less solar radiation than the Earth, it likely once had sufficient greenhouse gasses in its atmosphere to generate temperatures high enough to liquefy water on its surface. Carbon stripped out into carbonate rocks would not have been returned to the atmosphere because of the absence or early demise of plate tectonics on the planet (Kump et. al. 2004). Some of this water would then have evaporated into the thin Martian atmosphere, followed by photo-dissociation of water vapor and hydrogen loss to space. The extent to which water ice exists in the Martian north pole, the south pole or trapped under large areas of the Martian surface is the subject of vigorous current research.
Rather than the 10 centimeters or fewer precipitable water measured today on dry and barren Mars and Venus the Earth is shockingly wet. More than 104 times the quantity of water expected on an Earth without life is still here. From reconstruction of its past history scientists conclude that throughout the geological eons our planet has been watery. Today water on our planet is found mostly in its liquid phase within the global oceans which cover some 70% of our planet’s surface. Quantitatively small but climatically crucial amounts of water also exist in the gas phase as clouds and water vapor. In the solid phase as sea and glacial ice, as frost, hailstones and especially snow water augments the Earth’s albedo (greater reflectivity of solar energy to space). The movements of water between these and other reservoirs constitutes the hydrological cycle and is “the largest movement of any substance on Earth” (Cahine 1992). The hydrological cycle has massive effects on climate because of how water determines the exchange of heat and moisture between the atmosphere and the planet’s surface. One way in which contemporary organisms actively configure the Earth’s climate into a state suitable for water (and thus the perpetuation of life) is by influencing the hydrological cycle through the process of evapotranspiration in large trees and plants. Evapotranspiration involves massive movements of water against gravity from the entire root-zone (rhizosphere) up a few to over 30 meters into the air. The flow of water up through tree trunks and plant stems is powered by solar energy. Water is released at height as vapor through the stomata ; these active pores that open and close on the undersides of leaves . Organisms also influence the hydrological cycle in important ways by retaining water in soils and by emitting a variety of cloud-seeding chemicals over land and ocean (Hayden 1998, Bonan 2002). Furthermore, bacteria such as Pseudomonas syringae that are commonly swept up into clouds in large numbers exert a massive influence on the hydrological cycle. Proteins on the outer surfaces of these bacteria facilitate the formation of ice crystals that eventually return significant quantities of water to the Earth’s surface as rain and snow(Christner et. al. 2008).
Gaia theory and water
Earth’s abundant water with respect to its sister planets Mars and Venus lead us to a Gaian analysis of this ‘water anomaly’. Scientists tend to assume that environments are “physico-chemical givens” to which organisms must “adapt” in order to survive. We disagree. Unlike the prevalent belief that “life adapts to its environment” or, put another way, that life is passive, a mere passenger on Space Ship Earth, Gaia theory posits that life contributes to active regulation of biologically relevant aspects of Earth’s surface within habitable limits (Lovelock 1972, 2000, 2005, Lovelock and Margulis 1974). The theory specifically proposes that the regulation emerges from tightly co-ordinated feedback subsystems that intrinsically and continuously embed the biota in its abiotic surroundings (Lovelock 2005, Lenton 1998). In a masterful analysis of the Earth’s physico-chemical history that includes the results of decades of space exploration, Paul Lowman (2002), a geoscientist at NASA Goddard in Maryland, with Armstrong the astronaut shows that during the Archean eon the major influences were the same as those that prevailed on Mars and Venus. However, from the base of the Proterozoic eon 2500 million years ago until the present day, Gaia’s unique signature is writ large: Earth became the Gaian planet. Paucity of water, failure to detect granite, vastly slower geochemical cycles of elements such as oxygen, carbon and phosphorus and much other evidence testifies to the fact that neither Venus nor Mars are “Gaian” (Lowman and Armstrong, 2002).
Life does indeed adapt itself and its environment as Henderson and McHarg insist. Yet the term “adaptation” as generally used by biologists who ignore the emergent and intimate synergies between our planet’s physics, chemistry and biology hinders our understanding of the Earth as a complex system. In fact common claims of “adaptation”, a cliché. likely impede investigation of the evolution of the Earth’s environment through geological time. We recommend a re-examination and perhaps even the elimination from discourse of this ambiguous term. Usually biologists study specific correlations of behavior, morphology or chemistry of a given organism to its immediate environment. But the assertion that any ‘organism’ is ‘well-adapted to its habitat’ has little, if any, meaning since the adaptation is not measurable nor even estimable in a communicable way. All organisms alive today are ‘adapted’ by virtue of the implied continuation of their ancestors from the past to the present. We recommend for this reason that explicit statements that utilize broad and vague claims of “adaptation” (as if it sufficed as some sort of explanation) be replaced by McHarg’s and Henderson’s ideas of active fitting. Gaia emerges directly from active fitting writ large since all organisms impact each other and their surroundings. Through the exchange of heat, light, liquids, gases and a huge array of metals, salts, sugars and myriad other chemical compounds (usually dissolved in water) McHarg’s (2006) “creative fitting in health” as well as his “reductive misfit revealed in pathology” describe “adaptation” far more accurately than the cliché.
With respect to the hydrosphere, Gaia theory proposes a prospective research program: that organisms have actively retained water by thwarting its tendency to be lost. Without the involvement of life’s complex and often metabolic innovations (e.g., lipid monolayer biosynthesis, calcium ion extrusion that induces changes in carbonate, bicarbonate and CO2 equilibria, oxygenic photosynthesis, reversible protein absorption and release of water, etc.), Earth long ago also would have lost its water to space by atmospheric photolysis and hydrogen escape. We propose that life does not regulate the amount of water on the planet through a specific feedback process, but rather that it greatly reduces the rate of water loss by metabolic hydrogen capture and by regulation of relevant variables such as planetary temperature. Here we explore the major abiological processes that drive the loss of water from our planet, including the photo-dissociation of water and methane by solar UV radiation at the top of the troposphere and the chemical reactions in sea floor basaltic rocks that strip out oxygen atoms from water molecules. We then go on to outline the various ways in which life prevents such processes from drying out the planet. We include a discussion of how, by contributing to the regulation of the planetary carbon cycle over geological time, organisms have kept the planetary temperature suitable for the existence liquid water despite an ever brightening sun and ongoing outgassing of carbon dioxide from volcanic activity.
Modes of water retention by life
Any chemical or physical process that liberates hydrogen from water molecules in principle may lead to water loss from a planet. Hydrogen, H2 gas, has a mass so light that it reaches escape velocity from the Earth’s gravitational field.
We summarize some chemical and biological processes that both liberate and capture free hydrogen over geological time in Table 1. They exemplify our habitation of an Earth with abundant water and serve as a guide to further detailed investigation. Geochemical processes that result in the liberation of molecular hydrogen began in at least the Archean eon and have continued until the present. They occur in basalt, the major rock type of the world ocean bottom. Basalt contains ferrous oxide (FeO) which, in the presence of carbon dioxide, strips out oxygen atoms from sea water. The net effect is to remove oxygen and place it in solid form in carbonate rock, a process that liberates hydrogen gas (reaction 1, Lovelock 2005). Hydrogen liberation via loss to space may entirely desiccate an inner planet within two billion years (Lovelock 2005). Bacterial metabolic pathways also liberate hydrogen (e.g., anoxygenic photosynthesis, anoxic decomposition of dead organic matter (fermentation, reaction 2), anaerobic glycolysis and many others release hydrogen on geologically instant time scales). The Earth has evaded desiccation by many means that inspire further investigation. Since Archean times bacterial communities have released oxygen into the sediments, water and air by oxygenic photosynthetic processes that split water (reaction 3), a reaction which to this day is limited to only three immensely talented inclusive taxa. In a purely abiological process, hydrogen gas (such as that released from reaction 1) combines with oxygen from photosynthesis thereby regenerating water (reaction 4). Oxygenic photosynthesis (reaction 3) also captures and retains hydrogen extracted from water for carbon dioxide reduction, thereby renewing organic matter in the making of food, body parts and energy storage molecules such as sugar and starch. New avenues of oxygen liberation were opened up during the Proterozoic eon some 1200 million years ago by photosynthetic algal protoctists and in the lower Phanerozoic eon about 450 million years ago by the first land plants. All these oxygenic photosynthetic processes continue today unabated. Even anti-Gaia scientists admit that chlorophyll a photosynthesis produced the oxygen rich atmosphere that permanently altered Earth and its evolutionary course. Without these bacterial metabolic innovations no animal would exist, and there be no ozone layer in the stratosphere to decrease the photodissociation of water by ultraviolet radiation in the lower atmosphere (Lovelock 2000).
Another bacterial contribution to hydrogen capture comes from the activities of bacteria such as Desulfovibrio that live in ocean sediments in sulfur-rich habitats. Desulfovibrio and its many relatives liberate hydrogen sulfide gas (reaction 5) as they reduce elemental sulfur, thiosulfate or the sulfate ion itself by “breathing”. Water is reconstituted when hydrogen sulfide is oxidized by aerobic chemoautotrophic bacteria such as Sulfolobus or Beggiatoa that abide at oxygen-rich seawater/sediment, caves, sulfur springs and other interfaces (reaction 6).
An important metabolic pathway in certain bacteria hardly seems possible in principle. These bacteria reconstitute water by reacting molecular hydrogen with carbon dioxide under conditions where oxygen gas is absent (reaction 7). Known as anaerobic chemoautotrophy, in this process hydrogen is used to reduce carbon dioxide to organic matter and water is reconstituted. Also in regions without any oxygen gas, methanogenic bacteria remove carbon dioxide and react it with free hydrogen to produce methane and water (reaction 8). Reactions 7 and 8 both require anoxic habitats, e.g., marine, lacustrine and riparian sediments, or the intestines of insects and mammals.
Table 1. A selection of key biological and abiological processes that influence the retention of water on planet Earth ( data from Smil 2003 and Lovelock 2005).
Reaction and domain in which it takes place
Effect on Earth´s water
Ferrous oxide in sea floor basalt reacts with carbon dioxide and water
Desiccates the Earth by liberating free hydrogen
Biological:fermenting bacteria in anoxic environments
Organic matter and water
Desiccates the Earth by liberating free hydrogen
Biological: oxygenic photosynthesis by bacteria, protoctists and plants
Carbon dioxide and water reacted by photosynthesisers. Organic matter and oxygen produced
Oxygen available for reaction with hydrogen. Potentially reconstitutes water.
Hydrogen and oxygen, producing water
Free oxygen from (3) reacts with free hydrogen: reconstitutes water.
Biological: bacterial reduction of elemental sulphur
Elemental Sulphur and hydrogen
Sequesters hydrogen into hydrogen sulphide gas.
Biological: Aerobic chemautorophic bacteria
Hydrogen sulphide from reaction (5) with oxygen from reaction (3)
Biological: Anaerobic chemautorophic bacteria
Carbon dioxide and hydrogen
Organic matter produced. Reconstitutes water
Biological: Anaerobic methanogenic bacteria
Carbon dioxide and hydrogen
Methane produced. Reconstitutes water
A physical process that is thought to have led to hydrogen escape during Earth’s geological history is the photo-dissociation of water by ultraviolet radiation in the lower stratosphere. However, relatively little hydrogen may have escaped via this route due the ‘cold trap’ in the tropopause (Catling et al 2001). Since Archean times, water vapor molecules have frozen out in this region of very cold air and fallen back into the lower atmosphere before they could be photo-dissociated by stratospheric ultraviolet radiation. Catling et al., suggest that the photo-dissociation of methane provided the main exit route for hydrogen during the Archean eon, and hypothesize that abundant methane was the major greenhouse gas that counteracted the early lower solar luminosity. Methane’s lower freezing point relative to water allowed it to transit into the stratosphere through the cold trap in gaseous form unaffected. There (much like the few water molecules that managed to reach the lower stratosphere above the cold trap) the methane was split by ultraviolet radiation, yielding molecular hydrogen that could escape to space, leaving carbon dioxide and oxygen in the atmosphere.
These reactions are simplified and summarized as follows (Catling et. al., 2001):
In this scenario (i.e. in reaction 9), the methane came from the bacterial decomposition of organic material in which hydrogen from water was originally fixed by oxygenic photosynthesis (reaction 3).
Reaction 9 may have led to the so-called ‘Great Oxidation Event’ (Catling et. al., 2001) that took place between 2,400 and 1800 million years ago during the Proterozoic eon. This event involved a relatively rapid transition to an oxidizing atmosphere, and may have ultimately produced the high levels of oxygen gas (circa 20%) in today’s atmosphere. The rise of atmospheric oxygen gas during the Proterozoic has been amply documented in the geological record, especially by world-wide deposits of Banded Iron Formations, or BIFs (Cloud, 1989). Apparently a relatively small increase in the burial rate of organic carbon may have triggered a non-linear switch to a high oxygen atmosphere at that time (Goldblatt et.al. 2006). The stratospheric ozone layer that resulted has significantly influenced the effectiveness of the cold trap to this day (Nisbet 1991).
Whatever led to the surplus of free oxygen gas in the Proterozoic, it is agreed that hydrogen loss via the photo-dissociation of methane would have declined significantly when oxygen became sufficiently abundant to oxidize methane to carbon dioxide and water via the following reaction:
As the Archean atmosphere probably contained a thousand times more methane than today’s value of 6-7 parts per million, the rate of hydrogen loss must have been approximately three hundred times greater than at present (Catling et. al. 2001). The modern biosphere’s effectiveness at preventing hydrogen loss and hence planetary desiccation is illustrated by the very low rate of hydrogen loss to space. The Earth currently loses a mere 50 tons of hydrogen from an atmosphere with a total mass of around 50 x1014 tons (Morton 2007).
The Great Oxidation Event marked a shift from methane to carbon dioxide as the Earth’s dominant greenhouse gas (Lovelock 2000). Other consequences for life and its effects on the planetary surface include the appearance of early eukaryotic cells and their obligate relation to oxygen respiration in symbiotic bacteria that became mitochondria (Margulis, et al. 2006) and a Gaian redistribution of many chemical elements such as manganese, copper, phosphorus, lead, tin, vanadium and so forth.
The metabolic versatility of bacteria permits oxidation of methane even in the absence of oxygen gas. Sulfate reducers, such as Desulfovibrio and some relatives use oxygen in sulfate ions, which are abundant in sea water, to reconstitute water from methane:
Might these reactions (10 and11) have produced water in sufficient quantity to increase the depth of the global ocean (S. Marashin, pers. comm.)?
Water and Earth’s temperature
Why has Earth retained both life and abundant liquid water since the Archean in spite of at least two strong external factors that have conspired to enhance the similarities between Mars, Venus and Earth. One is the increase of luminosity of the Sun whose energy output is 25% greater than it was 3500 million years ago, and the second is the continual eruption of carbon dioxide from volcanoes over the same period. These and other observations lead us to conclude that global temperatures have been actively regulated within the range suitable for liquid water by the Earth as a whole system. That the behavior, metabolism and physiology of organisms are essential to this regulation is a central tenet of the Gaia theory (Lovelock 2000; Margulis and Lovelock, 2007). Much remains to be learned, but now we can say with some confidence that organisms help to regulate the Earth’s temperature by manipulating the ratios of greenhouse gases in the atmosphere and by altering the planetary albedo (reflectivity), primarily by emitting cloud-seeding chemicals. Other effects on temperature and hence water retention by organisms involve the albedo of living beings themselves, such as the extensive cover of dark coniferous trees in the far northern latitudes that help to warm the modern Earth (Bonan 2002). Organisms can also changes the amount of surface water directly exposed to evaporation: elephant bodies carve out ponds and thus expose subsurface water to the surface; exudates of microbial mat organisms directly retard evaporation; caves made by water flowing through limestone, or the conversion of limestone to gypsum protect water flow beneath the rocks.
We conclude that liquid water would have left the Earth’s surface long ago if organisms had not regulated global temperatures by these and other means. Continued volcanic activity that puts methane, water vapor, carbon dioxide and other greenhouse gases in the atmosphere in the face of an ever brightening sun would long ago have led the Earth into a Venus-like runaway feedback on global heating. On the other hand, too little carbon dioxide would have caused the oceans to freeze over leading to albedo increase, plunging the planet into a permanent frozen state via positive feedback (Ward and Brownlee 2000).
A major way in which life contributes to the regulation of global temperature through its involvement in the long-term carbon cycle in which calcium carbonate from the weathering of basaltic and granitic (silicate) rocks is deposited in the oceans. (Table 2 reactions 12 and 13)
Table 2. Reactions in the long-term carbon cycle (Adapted from Kump et.al. 2004)
is wollastonite, a simple mineral representing the general chemical composition of all silicate rocks. Note that two carbon atoms are removed from the atmosphere for each calcium ion weathered out of the rock
Denotes the intracellular precipitation of calcium carbonate. Note that one carbon atom is released to the atmosphere for each calcium ion precipitated. The net effect of reactions 12 and 13 is thus to cool the Earth
Granite is regenerated, and carbon dioxide is liberated to the atmosphere via volcanoes, thereby warming the Earth
On the land reaction 12 is enhanced by organisms: roots and hydrophilic microbial chemical exudates physically fracture the rock and thereby increase its reactive surface area; microbial and plant root respiration increase carbon dioxide levels in the soil, and bioturbation of the soil increases the flow of water onto particles of rock, taking water into places it would not otherwise be able to access. This process, first proposed by Lovelock and Whitfield (1982) and now referred to as ‘biologically assisted silicate rock weathering’, amplifies the purely chemical weathering rate from x10 to x1000 depending on location (Schwartzman and Volk 1989) – it is greatest where high temperatures combine with abundant rainfall.
Thus carbon that once resided in the atmosphere finds itself in calcium bicarbonate flushed by rivers and groundwater into the oceans where it is precipitated intracellularly as calcium carbonate by coccolithophorids (haptophyte algae) and foraminifera in their scales and exoskeletons (reaction 13). When these organisms die, the calcium carbonate accumulates in ocean sediments. Their fate is lithification into chalk and other limestones. Huge quantities of carbon have been sequestered in this way over geological time – the stock of carbon in the contemporary calcium carbonate reservoir is 4 x107 GtC, almost four orders of magnitude greater than the carbon in present-day fossil fuel reserves (Kump et. al. 2004). Chalk and limestone also contain significant quantities of silica (from the silicic acid in reaction 12) that may be deposited as radiolarite (chert rock that come from remains of radiolarian skeletons), or diatom tests (shells) and glass sponge spicules (Lovelock 2005). Such dynamics imply negative feedback with respect to the carbon cycle (Lenton 1998) and hence surface temperatures suitable for liquid water: if surface temperature increases (because of volcanic inputs of carbon dioxide to the atmosphere, together with an ever-brightening sun) so does rainfall. In a wetter and warmer world biologically assisted silicate rock weathering transfers more carbon from the atmosphere to calcium carbonate in the ocean, which cools the Earth, potentially down to a stable but lifeless frozen state. However, in a cooler and hence drier world this fate is avoided because the terrestrial biosphere rapidly becomes less effective at weathering silicate rocks, and so carbon dioxide accumulates in the atmosphere from volcanoes, thereby raising the global temperature (Lovelock 2005). An emergent property of this feedback has been the regulation of planetary temperature within limits suitable for life (and hence liquid water) over geological time.
The carbon dioxide that returns to the atmosphere via volcanoes is regenerated when silica-rich carbonate sediments are subducted into the mantle as the raised portions of descending slabs (plates) of the sea floor (reaction 14, Table 2). Here, at high temperature and under immense pressure, the sediments metamorphose and produce carbon dioxide and fresh granitic material which floats on top of the denser mantle to become new continental land mass available to be weathered (Kump et. al. 2004). Without such recycling of Earth’s crustal materials no terrestrial biota would exist to enhance silicate weathering.
Water and plate tectonics
Thus the long term carbon cycle cannot operate without volcanic activity, itself an integral component of the colossal processes of plate tectonics, with its mountain chains, subduction zones and large granitic continents afloat on giant rafts of spreading sea floor basalt. These tectonic processes, which are essential for the maintenance of organic life itself, cannot take place without huge quantities of liquid water.
Water infiltrates the laterally moving sea floor basalt, changing its chemical nature so that it is pliable enough to sink into the Earth’s mantle when it collides with the edge of a continent at a subduction zone. Sea floor basalt becomes extensively hydrated at the mid-oceanic ridges. Here, magma chambers act as heat sources that drive local-scale convective systems that force hot sea water through fractures in the basalt. For it to be effective at hydration of sea floor basalt, the process requires an overlay of large amounts of water (Campbell and Taylor 1983). At subduction zones water-rich slabs of sea floor basalt are carried deep into the mantle where the material melts to produce vast amounts of granitic magma which rises up to form the continents. This process adds to the granite generated by the metamorphism of silica-rich calcium carbonate sediments beneath subduction zones mentioned earlier. The volatility of limestone produces watery carbon dioxide-rich lubricant which enhances the rates of plate tectonic activity. A vast amount of water has been required to generate the Earth’s continents, which have almost certainly covered some 30% of its surface since the beginning of the Proterozoic some 2.5 billion years ago (Ward and Brownlee 2000).
Without subduction, plate tectonics would stop because there would no closure of the convective cycle that reaches down to the planet’s outer core, in part driven by the decay of radioactive materials in the Earth’s depths (Kump et. al. 2004). Without plate tectonics, the return of carbon to the atmosphere would be severely curtailed or perhaps completely shut off. In tens of millions of years all the Earth’s land masses would be removed by weathering, with no new granite to replace this loss. The long term carbon cycle would cease, and the Earth would perhaps be plunged into a permanently frozen state (Ward and Brownlee 2000). We therefore observe an interesting and appropriately circular Gaian dynamic here: No life, no water. No water, no plate tectonics. No plate tectonics, no life.
Water and Culture
From the facts of a watery Gaian Earth can be inferred knowledge and wisdom that extends far beyond science (Harding, 2006). Recognition of the complex relationship between water, life and Earth history has recently became available in two over-sized and gorgeous books: Water (also published identically as Agua in 2006) and Water Voices from around the world (Marks, ed. 2007). The frontispiece of the first states “,..we need to create a new culture that acknowledges and respects the value of water. The survival of future generations of humans and all other species on this planet depends on such a new culture”. The second is dedicated to “our ancestor: water.” It bears testimony of citizens from fifty countries worldwide. Nobel laureates figure in both books and the color photographs at all levels from satellite to microscopic are remarkable. In Water Voices we learn about Lake Sarez in Tajikistan formed by the 1911 earthquake’s landslide and kept in place by the largest natural dam in the world. Tajikistan’s reverence for fresh water is palpable. The song of this Central Asian country is joined by many human and other messages and voices: a cayman from Cuba most of whose close relatives have been extinguished, a Red Eye tree frog from a Central American rain forest, wild salmon from Kamchatka, clown fish and corals and the tail of a humpback. The spectacular photographs in Voices and those of Antonio Vizcaino in Agua need no admonishment to induce us to protect our home planet. We commend to your attention both these “magnum opuses”; they speak louder than our words in search of Water Gaia. They represent the first step, coupled with the other chapters in this volume, towards actions that will help our planet . We end with a suggestion: that we properly rename our third-from-the-sun inner rocky planet after the humble, crucial chemical compound that sustains us: Water!
We thank Richard Betts, Tim Lenton, James Lovelock, James MacAllister, Sergio Maraschin, Will Provine and Bruce Scofield for useful discussion pertinent to the writing of this chapter. LM thanks The Tauber Fund, Abe Gomel and the University of Massachusetts Graduate School for support.
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Excerpted with permission from the forthcoming book In Search of Gaia, edited by Eileen Crist and Bruce Rinker, to be published by MIT Press, by Stephan Harding, Resident Ecologist, Schumacher College & Lynn Margulis, Distinguished University Professor, Department of Geosciences University of Massachusetts-Amherst