PART 1
Changing Seas
CHAPTER 1
Four and a Half Billion Years
In a remote part of Western Australia there is a range of hills so smoothed by time that little more than undulations are left. The summer sun bakes the red earth, and sparse trees dig deep to quench their thirst. Close up, jumbled slabs reveal beds of smoothed pebbles brought here by ancient floods—very ancient, for the Jack Hills’ rocks were laid down three billion years ago. This landlocked desert may seem like a strange place to begin a book about the oceans, but these stones, as unremarkable as they appear, have rewritten the earliest history of our planet.
Zircons are pyramid-shaped crystals that coalesce from cooling magma. They are incredibly resilient and survive through repeated remelting and hardening, as the crust of our restless planet is recycled by plate tectonic movements. Zircons form gems of many hues. The name is thought to come from the Persian zargun, which means golden, probably in reference to the translucent yellow stones that were once traded from Sri Lanka through Persia to Europe and to China. Brilliant sky blue crystals were mined from river mud by the people who built the temple city of Angkor Wat in Cambodia.
Jack Hills’ zircons are less showy. In fact, they are so small they can hardly be seen with the naked eye. It took over two hundred pounds of Jack Hills’ rock to yield less than a thimble of zircons.1 But it was worth the effort. Each crystal was found to contain traces of uranium, which decays over time into lead, making it a geological chronometer that ticks away the eons. You can date the time of crystal formation to within a few tens of millions of years by measuring the ratio of uranium to lead. The most ancient Jack Hills’ crystal is 4.4 billion years old, which takes us to within a whisker of the formation of Earth (well, a 170-million-year whisker, but that is close!). It lets us glimpse a planet in its infancy.
We can never be sure of all the details of Earth’s birth and childhood; no one was there to see it, and we can’t rewind time to have a look. The best we can do is search the world for ancient rocks and fossils and probe them for their secrets. We spin theories that fit the evidence and rework them when new data find them wanting. These days, new ways of interrogating the past are developed with breathtaking speed, so the picture is coming into clearer focus. Although many uncertainties remain in the story I will tell in this chapter, the broad pattern of events is well established. The Earth coalesced from the rotating disk of dust and debris that would become our solar system about 4.57 billion years ago.2 The Sun is around the same age, so our planet formed when the match was struck to light the solar system. It grew hotter as debris pounded the growing planet into shape, until the rocks melted. Seen from the darkness of space, our world would have glowed like a faint sun. A planet the size of Mars barreled into this new world sometime around 4.53 billion years ago and smashed off pieces that became the Moon. The impact was so enormous that it vaporized rock, and the primeval world was shrouded by a thick atmosphere of rock and other gases. As the planet cooled a time came when minerals condensed, and for two thousand years the skies rained droplets of molten rock onto an ocean of magma below. The atmosphere remained thick with other vapor and gases even after this thousand-degree deluge ended, and the atmospheric pressure at the surface of the flaming sea would have been hundreds of times higher than today.3
We owe a lot to this collision. It knocked Earth’s axis of rotation askew, which gives us seasons.4 Over the vast plain of geological time the Moon has slowed and stabilized the Earth’s rotation, giving us longer days. A billion years ago a day was just eighteen hours long, and the year lasted 480 days. The Moon was much closer to the Earth early on, and would have loomed large in the sky. Its gravitational pull gives us much greater tides than the more distant Sun alone, and the flood and ebb of tides would have been violent as they rose higher and fell faster in the short days.5
The searing heat of fireball Earth is called the Hadean. For a long time we thought these hellish conditions lasted for the first half billion years of Earth history, but a zircon from Jack Hills has recently transformed that view. The 4.4 billion year old crystal and tiny particles of other materials trapped within it formed at temperatures characteristic of coalescing granite in contact with liquid water.6 The continents are built from granite, so we can see within this insignificant speck the imprint of a cooler world that had land and water long before we suspected such a place existed. Instead of comprising seething oceans of fire, the world might have been more like a large, steamy sauna.
The Earth is unique within the solar system for its liquid oceans and seas. Where did all this water come from? There was water in the disk of dust and gas from which the solar system formed, but some scientists believe that the inner region of this disk, where the Earth formed, was too hot for water.7 They contend that icy comets and asteroids delivered water from far reaches of the solar system long after Earth’s formation. This barrage goes on today; every few seconds a “snowball” the size of a large truck melts into the outer atmosphere. But measurements of the isotopic composition of these snowballs suggest that they contributed only a few percent of the planet’s water.8 Meteorites were assumed to have delivered most of the rest, but recently a comet has been found with water having an isotopic composition very similar to Earth’s oceans, so comets could have contributed a bigger share than once thought.9 Another view is that water molecules drifting in space stuck to particles of dust, and therefore rock and water came together at the same time. Our world actually has enough water for five to ten oceans, perhaps more: most remains trapped within the bedrock. About half a percent of basalt rock is water, by weight, trapped within the mineral lattice.10 As the early world heated up and rocks melted, water boiled off into the atmosphere. For over a hundred million years the oceans were in the sky as a dense shroud of vapor that churned above the glowing surface of our planet.
A new rain began to fall when the Earth cooled down a bit, this time of scalding water. The downpour lasted for thousands of years, and would be repeated several times over the next half billion years, as giant impacts from extraterrestrial debris boiled off the upper layers of the seas. The Earth continued to be struck by asteroids for a half billion years more, ones far bigger than those that killed off the dinosaurs sixty-five million years ago. Few traces of these impacts remain, for our crust is continuously reworked into the planet’s interior, but we can read their fury from the Moon, whose cratered surface recorded the bombardment and has remained immobile since it cooled.
To begin with, oceans covered most of the world; their volume could have been twice that of the present seas.11 Islands cropped up where blocks of the Earth’s crust collided and volcanoes built ash and lava mountains, but there were no continents. Those came later and formed slowly. The world today is broken up into continents and sea because the crust is made up of two kinds of rock. It is made of basalt below the sea and is slightly denser than the granites that make up the continents. Repeated remelting in the furnace of our young world separated off the lighter granitic rocks that form today’s continents. Both kinds of rock float on a sea of hot, viscous magma in the mantle below, but the continents float higher. As with icebergs, how much you see above water depends on how much lies unseen. Continents have deep roots, so they float higher in the mantle than oceanic crust. This is why the average height of land is 2,770 feet above sea level, while the average depth of the oceans is 12,140 feet; it is a consequence of the different densities of their rocks.12
The sticky magma of the Earth’s mantle keeps the world’s surface in constant motion. Hot magma rising from deep down creates new crust in some places. Since the world is not getting bigger, that creative force is matched by destruction somewhere else, as crust slides back into the mantle. The Earth’s surface is thus divided into blocks, called plates, that each moves in a slow-motion geological dance through time. Here again the density difference between oceanic and continental crusts comes into play. Because oceanic crust is thin and d
ense (three to six miles thick, compared to the twenty to twenty-five miles of most continental granite), it is recycled quickly into the mantle, about ten times faster than that of the continents. (None of the oceanic crust is greater than 200 million years old, whereas about 7 percent of the land is older than 2.5 billion years.13) Think of the continents as the froth bobbing on a pool beneath a waterfall. The water pours on beneath, but the froth endures.
Continents began to form very early on. Crust was recycled quickly at the beginning on a hot Earth, but the rate slowed as the planet cooled, and intense meteorite bombardment came to an end four billion years ago. Continents grew over time and reached roughly their present landmass two and a half billion years ago. Since then they have been recycled at about the same rate as they are created. Today the plates creep slowly; the Atlantic Ocean is opening at an inch a year, a little slower than fingernails grow.
To us, the oceans are immense. They cover 140 million square miles and fill a volume of 324 million cubic miles. It is hard to imagine a cubic mile. If you flooded all of New York’s Central Park with water to the height of a thirty-story building, the volume would be a little greater than one eighth of a cubic mile; it would take 2.66 billion Central Parks of water to fill the oceans, an almost unthinkable volume of water. Yet at the scale of the planet, the oceans form a layer only as thick as the skin of an apple.14
Zircons aside, the oldest rock in the world is the four-billion-year-old Acasta Gneiss in northern Canada. This formed deep underground, so it tells us little about what was going on at the surface. The oldest surface rocks are the highly metamorphosed Isua sediments of southern Greenland, which formed underwater and give us the first direct evidence of oceans.15 Remarkably, these deposits suggest that life had already evolved by the time they were formed. There are no fossils in Isua sediments, but the chemical composition of carbon buried in these rocks is characteristic of the presence of life.
The first life-forms evolved in the early Archean eon—the billion-and-a-half-year eon that followed the Hadean. It was a very different world from the one we live in today. There was almost no free oxygen, for a start, and the sun burned 25 percent less bright.16 Methane-producing microbes evolved sometime between 3.8 billion and 4.1 billion years ago, creating a greenhouse gas twenty-five times more powerful than carbon dioxide.17 Methane levels rose and the planet warmed.18 For over a billion years, until at least 2.5 billion years ago, the greenhouse shroud sustained these liquid seas as the Sun’s fire warmed the Earth. The oceans would have frozen if it hadn’t been for this dense blanket of gases in the atmosphere, and life might never have kindled, or it could have started only to be snuffed out early on.
There is not a trace or shadow of life for hundreds of millions of years after the first spark, beyond chemical alterations in the rocks.19 But spectacular recent advances in genetics and computing enable us to grow the tree of life backward, from its leaves to the tip of the root from which it sprung. Every living thing, from the humblest virus to the greatest of the whales, shares a common heritage that is written in their genes. That similarity tells us, just as Charles Darwin predicted, that everything living today is descended from life’s primordial spark. The genes that code for different metabolic functions can be placed in the sequence of their emergence on the tree of life, and show something of their timing. They tell us when life passed critical evolutionary milestones, and from them we can infer how the environment was changing. In some cases early life-forms had to respond to planetary upheavals, but often they themselves were responsible for changing the world around them.
Hot springs under intense pressure can be found in the deep sea, spewing forth water superheated to six or seven hundred degrees Fahrenheit and so laden with minerals that their plumes are opaque black or white. They deposit metal-rich compounds nearby that in the early oceans might have catalyzed important chemical reactions. Today these hot springs support rich communities based entirely on energy captured from chemical reactions undertaken by microbes. Here, then, is a vestige of how the earliest microbes might have made their living, perhaps even of the place where life itself was forged.
The planet was ruled by singleton cells and microbial slime for more than three billion years. It is hard to grasp such a vast number. Think of it this way: If every one of those years lasted just a second it would take ninety-five years for three billion to pass by. Microbes evolve fast, because they have fleeting generations. Even taking into account the likelihood that Archean oceans were far less productive than our own, there was time for hundreds of billions of generations to come and go.20 Every new generation offered the possibility of variations that are the raw material of evolutionary innovation. This was a time of extraordinary inventiveness, when the foundations were laid for almost everything life does today.
Microbes developed the ability to produce energy early on, by converting hydrogen sulfide—the gas that gives rotten eggs their smell and that pours forth from deep-sea hot springs—into sulfates.21 This was an essential step. Some developed complex chemical machinery to draw energy from sunlight to accomplish this conversion in shallow water, and photosynthesis was born. Microbes are not easily fossilized, but they are petrified into flinty rocks, called chert. The earliest fossils are microscopic threads locked into 3.45 billion year old Australian cherts, although their interpretation is controversial.22 They look like cyanobacteria, a group still common today. These were the creatures that would later on develop the capacity for photosynthesis, the method of generating energy from sunshine that dominates all primary production today. They use the sun’s energy to create carbon compounds—food, in other words—from carbon dioxide and water. Their waste product is oxygen.
The first chemical traces of oxygen-producing photosynthesis were found in 2.7 billion year old shales rich in organic matter.23 We must thank this innovation for the way our world works, because this kind of photosynthesis has produced essentially all of the free oxygen around us today. It took hundreds of millions of years for enough oxygen to be made for us to detect its traces. Dark shales from Mount McRae in Western Australia laid down 2.5 billion years ago give us the first whiff of oxygen.24
Soon after (at least, in geological terms, since it took another fifty million years) we begin to find evidence of oxygen in rocks all over the world. The next 150 million years is known to geologists as the Great Oxidation Event, because it heralds the first major step in the formation of the atmosphere we have today. But far from being a boon—that would come later—oxygen first plunged the Earth into a crisis. When oxygen and methane get together the result is carbon dioxide and water. Methane is twenty-five times more powerful a greenhouse gas than carbon dioxide, so the Earth’s comfort blanket thinned, and the planet froze.
Some scientists think this ice age was so severe that the sea and the continents iced over all the way to the tropics.25 When ice forms at latitudes lower than about thirty degrees, it reflects so much heat back into space that glaciation runs away with itself. This is because more of the sun’s heat is absorbed by the Earth at the tropics than the poles, so more heat is reflected into space by low-latitude ice than high-latitude ice. Some simulations of such a world suggest that the oceans could have frozen to over three thousand feet deep. The ice would have retreated only after millions of years of volcanic activity had added enough extra carbon dioxide to the atmosphere to warm Earth enough to melt it. It is hard to see how life could have survived “Snowball Earth,” so one suggestion is that the tilt of the planet in relation to the Sun must have been different at this time, so that the poles were warmer than the tropics and ice-free conditions persisted through the great freeze in some places.26 Another possibility is that weather systems kept some areas of ocean ice-free.
You would not have wanted to swim in the oceans of the past. In Hadean and Archean times they are thought to have been rich in dissolved iron, and anoxic (meaning oxygen-free). The iron came from deep-sea hot springs and weathered rock. Iron dissolves in the
absence of free oxygen and is easily washed to the sea, whereas if oxygen is present, iron oxides tend to stay put. Early microbes put this iron to work. They developed ways to use the power of sunshine to oxidize free iron and make food from carbon dioxide and water. Iron was precipitated to the bottom of these seas to form thick deposits known today as banded iron formations.27
A slice of two-and-a-half-billion-year-old seabed sits on my shelf at home. It is only a centimeter thick but surprisingly heavy. Wavy layers of rust brown, yellow, and orange silica alternate through the slab with dark black and gray stripes of magnetite. Thicker layers of shimmering tiger eye fill gaps where the rock was later twisted and deformed under pressure. It is beautiful. I find it amazing to run my finger across this fragment of our primordial ocean. Most banded iron formations have long since been recycled back into the Earth’s mantle, but bits of ancient seabed lie stranded in rock formations in Australia, Canada, Russia, and elsewhere to form some of the richest sources of iron ore in the world. Banded iron formations are largely confined to rocks older than 2.4 billion years. They disappear from the record for the next 400 million years. For a long time it was thought that free oxygen produced by cyanobacteria had dissolved in the ocean and stripped it of iron, but based on the chemistry of rock deposits, another possibility now seems more likely. Donald Canfield, a polymath geobiologist based in Denmark, thinks that the sea had no oxygen below a thin surface layer for hundreds of millions of years after oxygen first began to rise in the atmosphere. His idea is that oxygen reacted with sulfides in terrestrial rocks and washed into the sea as sulfate. It was the sulfate that stripped the oceans of their iron, not the oxygen. The Great Oxidation Event would not penetrate to the deep sea until a billion years later.
The Ocean of Life Page 2