How Bad Was The Great Oxidation Event?

All over Earth, the history of life and the planet itself is written in its rocks. Fossils large and small are embedded in overlapping rocks, and by breaking through these layers now exposed on the surface, paleontologists can take a journey through time. About 66 million years ago, rock layers around the world preserve a remarkable transition from a world ruled by giants to one reeling from their sudden absence. The rock layers over 66 million years old belong to the Cretaceous period, the third and last era of dinosaurs, incredibly huge fossils. They are the remains of incredibly gigantic creatures that roamed the Earth's surface and the depths of the ocean grazing hunting living every lifestyle imaginable on this ancient land, but rocks less than 66 million years old tell a very different story here, when beginning of the Paleogene, the fossils are

great

ly diminished both in number and size, they speak of a world devoid of dinosaurs completely stripped of their giants.

In this new world no four-legged animal that weighs more than 25 kilos lives. They are mostly small ones, among which are our own ancestors and it is a quiet and empty place. world in which only 25 species of the cretaceous remain a symbol of how brutal life is on our planet in 1979 with hammer and chisel they extracted and examined a sample of the Paleogene Cretaceous frontier in the Italian Aponines, half of a rock the size of a fist belonging to the era of the dinosaurs and the other to a world of small mammals. In the middle of it, a thin, dark, undulating line was the only remnant of a catastrophic

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that had ended the reign of the dinosaurs in that delicate layer.

Walter and Lewis discovered inflated concentrations of iridium, a metal that is extremely rare on Earth but which they theorized was delivered by an asteroid 66 million years ago, an 80-kilometer-wide asteroid crashed into the Yucatan Peninsula in Mexico, destroying itself in the process and spreading iridium in a thin layer throughout the planet. It triggered a series of catastrophic climatic and tectonic changes that in just 30,000 years decimated life on Earth. The dinosaurs died, never to return, along with 75 of all species on the planet. The living world was completely transformed, and yet this brutal impact and its subsequent horrors were nowhere near the worst mass extinction

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the earth had seen in nearly 200 million years. deeper in time at the end of the permian period the earth's biodiversity plummeted in the blink of a geological eye this time there was no external agent of destruction the earth itself conspired to bring about its armageddon huge volcanic eruptions spewed clouds of dust lava and gases that transformed the planet The changes drove ancient species from many different families to extinction, including trilobites, ammonites, and some of the first land mammals and reptiles. 96 percent of all marine species and 70 percent of terrestrial vertebrates were lost, known colloquially as the

great

dying.

This has been widely considered the deadliest of Earth's many extinction events, but there is one more candidate for this spooky corona billions of years deeper in time - the evidence may not be as clear as the boundary rock that Walter Álvarez held it in his hand, but much closer to the formation of our planet. In the world there are records of an event so catastrophic that it almost caused the loss of all life on Earth in the archaic world at least two and a half billion years ago the microbial ecosystem thrives in blissful ignorance of the coming apocalypse life is limited to Various bacteria cluster around hydrothermal vents that dot the ocean floor and feed on the various minerals spewed out by these smoldering vents, but these are crowded microcosms, these microbes can only stray so far from the life-giving chemistry they provide. their metabolisms depend and hydrothermal vents can only supply a limited amount.

So new metabolisms arise through mutations of the genetic molecule, bacteria learn to do new things with the chemicals around them, not only that, but they can also exchange their new abilities with their sisters in a genetic handshake of the that only these simple life forms are Able to gently collide with each other in the crowded metropolis of vents, two microbes can briefly join their membranes and transport small packets of genetic material back and forth, constantly exchanging genes and creating new combinations within their own cells. This was the source of amazing diversity in these gloomy seas. and the mechanism that gave rise to an entirely new innovation, which would ultimately transform the face of the planet, was that of pigments, tiny specks that would appear purple to our eyes, but are capable of absorbing specific wavelengths of light. and capture the suddenly microbial energy.

Life forms have a new source of energy available from sunlight. Sunlight is inexhaustible and easily accessible as long as you can survive the waves and tides lurking in the shallow waters to the first light-loving microbes. In a chemical world their innovation opened the doors to a new type of existence these photographic containers could eat the sun this was the birth of photosynthesis but it was not the photosynthesis that we learn in school it was a strange and harmful type of chemistry with exotic ingredients Fed by purple bacteria in the sun's infrared light beyond the visible spectrum, they used hydrogen and dissolved sulfides to complete their reactions and produced solid yellow sulfur and corrosive sulfuric acid as byproducts.

The early photosynthetic world was a toxic, smelly sludge, but this was not the cause of the The catastrophe would come even after bacteria learned to eat the sun. Innovation continued through microbial gene sharing and many iterative trial and error improvements were made to the new metabolism. New green pigments absorb red and blue light and capture more energy than ever and a new simpler type. A new and improved form of photosynthesis is developed, energy from the sun is combined with carbon dioxide and water to fuel the metabolism of cells. This is the successful way that has been passed down through billions of years. years and on which green plants and algae depend to this day, but this tremendously successful new pathway had a dark side: it created a waste product more deadly and harmful than any sulfur before a toxic waste product that would bring disasters even to organisms that oxygen may not seem so terrible, in fact, we can hardly imagine a world without it, all animals on Earth today depend on oxygen for their own metabolism, but at the end of the archaic and beginning of the proterozoic two and Five hundred million years ago it was a novelty to which the entire planet was not accustomed.

In September 1844, the southern shores of Lake Superior were a wilderness of sturdy white pines casting long shadows across a gently rolling landscape of ice-covered rocks. A small team of searchers. are making their way across this landscape led by one William Burt, a geologist and surveyor sent to Michigan's northern peninsula to search for new metal oars. Michigan already has a thriving copper industry and investors are increasingly looking to exploit the state's natural wealth. One September afternoon, as the sun casts a golden glow over the landscape, William Burt squints and looks at his compass. Something is not right.

The sun in his eyes tells him that he is facing west, but the compass needle points forward and turns north. look across the shimmering waters of the lake, but the needle still points resolutely toward the hillside. On Bert's orders, the surveyors camped here and began their investigations to discover the cause of the compass's strange behavior. Several days of digging, hacking and blasting, later, the searchers. Stand in front of a clean, intact rock surface the likes of which a small team has never seen before. The rocks are strikingly deep red and jet black arranged in gently alternating layers, some just millimeters thick, others half the height of a man.

Later tests will reveal that iron makes up more than thirty percent of these rocks compared to only five percent on average in the Earth's crust with little more than the needle of a compass William Burt had struck the black stony gold of Michigan these rocks Iron-rich formations would become known as banded iron formations and would bring Michigan into the industrial revolution, but banded iron formations have more to offer than just their mineral resources. These rocks now mined in northern Michigan were born in the Proterozoic oceans 2.4 billion years ago. They are the lucrative product of a world in crisis with the invention of oxygen from water-based photosynthesis being produced and emitted to the oceans for the first time in Earth's history.

Oxygen is a remarkably reactive element and will combine instantly with almost any other element creating entirely new compounds. In the Proterozoic oceans, newly created oxygen reacts with green iron hydroxide suspended through the water column transforming it into iron oxide, which spreads from the coasts where photosynthetic bacteria thrive, turning the waters an apocalyptic blood red. Over time iron oxide settles floating through miles of water to be deposited on the ocean floor becoming iron-rich crimson layers in banded iron formations, but something curious is happening in the shallow waters. While the first innovative microbes may have evolved to use the new, efficient form of photosynthesis, they have not yet developed resistance to unprotected oxygen.

The waste product builds up in their cells and reacts aggressively with the delicate chemical machinery that keeps them alive. The microbes have no protection because they have never needed it until now, so these new bacteria are suffocated by their own waste gases and die en masse just as they do. They reach the peak of their success when populations grow too large, the flow of oxygen in the ocean exceeds what can be removed by iron and all shallow waters become oxygenated, toxic oxygen sweeps like poison gas through the prostrate bacterial communities. in the sun, killing them where they lie with the sacrificed sun-loving microbes.

Oxygen production almost stops and normal service resumes in the deep ocean. Muddy shales and silica-rich cherts accumulate free of iron oxide covering red sediments, but oxygenated cleansing is not absolute, not all photosynthetic bacteria are. extinct life finds a way for the microbes to find their way back to the speckled coastal waters, their populations recover and they begin to pour oxygen back into the ocean once again, so the cycle repeats millions of years of flowering. and death with oxygen levels rising and falling. of red iron oxides alternating with black anoxic silicates gradually, pulse by pulse, banded iron formations accumulate.

They are an elegant deep-water record of a bitter battle for survival in the shallows. Banded iron formations like those discovered by William Burt in Michigan found today on nearly every continent formed in huge swaths of the deep Proterozoic ocean and recorded pulsating oxygen levels that lasted for 200 million years, but this is just the beginning. of the looming oxygen catastrophe. Geologists examining rocks from the earliest Proterozoic period note a striking transition around 2.4 billion years ago. makes black basil and its black sands and black, muddy riverbeds turn red for the first time the phenomenon is more widespread than just the banded iron formations of the deep sea this is a transformation that took over the entire world about 2.4 billion years ago the earth rusted but how did this happen?

How did the photosynthetic microcosm emerge from its seemingly endless cycle of boom and bust with enough time? Photosynthesizers have the opportunity to innovate, exchange and evolve. They eventually evolved to resist their own waste gases. The oxygen no longer poisoned them immediately. They were able to do it. live long enough to increase their numbers, reinforce their population, and maintain their tenuous hold on the sunlit shallows, without periodic devastation, they could soak up the sun and belchoxygen without inhibitions, in a short time the seas ran out of iron to absorb the excess oxygen it brought with it. upon the end of widespread iron banding formations oxygen begins to fill the oceans dissolved in water starting at the coast and spreading inexorably to the darkest depths ocean currents bring tons of oxygenated water to the farthest reaches of the oceans Proterozoic silent oxygen deadly spread is catastrophic for the rest of the biosphere, those crowded microbial communities that cling to hydrothermal vents in the dark never change their ways, never exchange their genes with sun lovers and have no resistance to toxic oxygen, are decimated, even those that survive the initial sacrifice are deprived of the chemicals they depend on, the oxygen reacts with the iron, sulfur and manganese emitted by the vent, oxidizing it and locking it in reservoirs now inaccessible to these primitive microbes as oxygen spreads across the ocean, so microbial city after city falls gassed, poisoned, starved and rusted to death.

There was no escape: the photosynthesizers had created a new world order in which there was only room for those tolerant of their regime, but even this was not the end. The thin layer of iridium that Walter Álvarez and his father discovered in Italy in 1980 was a record of the asteroid that started the extinction kt the impact devastated everything for miles around, sent shock waves around the world, caused forest fires and filled the skies of smoke, but for most of the biosphere the death was much slower during a global winter that gripped the planet for years afterwards or during the intense warming that followed, it was the climatic aftermath that dealt the final blow, just as that with the great Proterozoic extinction in 1868, just across the Canadian border, from the then thriving iron mines in northern Michigan, exploring geologists discover another curious rock formation on the shores of Lake Huron here they discover a strange amalgam of pebbles and boulders laid in a fine gray cement-like matrix, from small sandy grains to boulders weighing several tons.

The stones are a colorful variety of many different types of rocks. White and gray speckled granites. Striped orange jasper and green stone. shining in the sun there are no source rocks to match these exotic pieces for hundreds and even thousands of miles the largest rocks are too heavy to be moved by wind or water to 19th century geologists their presence here is a mystery, it is as if they had been dropped from the sky. Today we know many more examples of these mysterious exotic rocks from later periods of Earth's history. They are called erratic and, in fact, they are dropped, although not from the sky but from the melting of glaciers.

Pebbles and boulders are torn from the bedrock and transported. along vast icy roads before being unceremoniously deposited when the ice melts Only the unstoppable advance of ice has the power to move so much rock so far from its source that the erratics around Lake Huron are exceptional in both their age and in their variety dating back to at least 2.1 billion years ago, they are the remains not only of a glacier but of a vast ice field that transports debris over thousands of kilometers. The so-called Huronian glaciation, named for these deposits, was an ice age like no other.

It was the ice catastrophe that followed the appearance of oxygen. catastrophe and the nail in the coffin for much of life on earth, the seas filled with oxygen and wiped out deep-sea microbial communities, but photosynthetic communities continued to thrive by pumping out more and more oxygen until the seas could no longer contain more, so residual oxygen began to seep into the atmosphere here it reacted with exposed rocks but it also reacted with other gases in the air, in particular it ripped apart methane molecules transforming them into carbon dioxide and, although today we know that carbon dioxide Carbon is a planet-warming greenhouse gas.

Early Earth's methane was a much more potent insulator as the heavens were stripped of their soft methane blanket. Global temperatures fell. The surviving greedy photosynthesizers continued to gobble up the carbon dioxide in the atmosphere, further depleting the planet's insulation. Then the Earth was submerged in ice. In a time of unimaginable proportions, the sun shone less brightly than it does today, and with the planet losing what little heat it once retained, lakes, rivers, and oceans began to freeze. The equator-facing ice shelves spread like icy corruption from the shallow waters over the deep oceans. The Earth was a snowball planet from head to toe wrapped in a monotonous layer of ice and remained that way for 300 million years with each continental shelf frozen and covered in ice a hundred meters thick.

Even the voracious photosynthesizers found their match. The shallow seafloor froze and buried the microbes. communities on solid ice the deepest waters that escaped freezing were black as night beneath the ice thick and opaque to sun-eating microbes this endless night was an inescapable death sentence after reaping devastation on life forms archaic in the depths of the ocean that are finally the same. agents of their own destruction together, the chemical revolution and the ensuing climate crisis combined to cause the largest mass extinction the Earth has ever seen; It is often overlooked in the face of more recent extinctions in which staggering numbers of species are lost or huge charismatic animals like The dinosaurs were gone forever, but the great oxygen extinction made primary productivity, the production of organic compounds by of life, plummeted.

Scientists estimate that 99% of all life on Earth was lost between 2.4 and 2 billion years ago. The abundant microbial oceans of the archaeans were wiped out forever and most of those ocean ecosystems would never recover just the one percent of life that clung on through the toxic ice age, refuges in the deep ocean that went untouched. by poisonous gas or in rare warm oases on the coasts that somehow escaped the global freeze. All life on Earth today descends from those. A lucky few who made it against all odds. Oxygenated photosynthesis, a chemical innovation created by tiny bacteria nearly two and a half billion years ago, is the fundamental root of most of our modern ecosystems, and yet its invention nearly led to the destruction of all life on Earth.

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