Great Moments in Earth History: Antarctic GlaciationPosted: October 10, 2013
Buried under thousands of feet of hard, ancient ice lies the solid earth of the Antarctic continent. For some 34 million years, vast glacial plains have ebbed and flowed over this rocky land. But the initiation of Antarctic glaciation—the point in time when conditions became right for snowfall to exceed snowmelt year after year—began suddenly and enigmatically.
The growth of glaciers on Antarctica marks the end of the geologic epoch known as the Eocene—an epoch actually known for some of the hottest global temperatures in Earth’s geologically recent history. High CO2 punctuated by extreme bursts of even more CO2 caused significant warming for the early part of the Eocene’s 22 million year span. Fossil records show that the Antarctic continent was not only ice free then, but that it supported rainforests and crocodiles!
So the transition from a lush tropical landscape to a barren ice covered wasteland is a mystery that scientists have yet to fully explain. Cooling began gradually around the middle Eocene, and it made a pronounced and sudden shift at the Eocene’s conclusion 34 million years ago.
At that time, CO2 levels plummeted. In a geological instant—400,000 years—Antarctica was covered in ice. Some sort of threshold must have been passed, geologists reason. Cooling can beget more cooling because ice reflects incoming heat from the Sun back into space. This undoubtedly happened. But something else had to have occurred to cause the drop in CO2 that allowed the world to become cool enough to form glaciers in the first place.
The answer to this mystery may lie at the intersection of geology, oceanography, climatology, and biology. Around 41 million years ago, tectonic changes allowed for the formation of something called the Drake Passage—an oceanic opening between Antarctica and South America that allows a current to flow, unobstructed, around the entire circumference of Antarctica.
Some researchers believe that this flow would have carried nutrient-rich water from deep on the ocean floor to the surface of the ocean. When nutrient-rich waters meet sunlight, primary producers—organisms that photosynthesize—can flourish. Organisms that photosynthesize take in CO2 to create the organic matter they need to live and grow. That CO2 ultimately comes from the atmosphere.
When these organisms die, they—along with the carbon they removed from the atmosphere—can sink to the bottom of the ocean, effectively removing significant quantities of carbon from the climate system. This transport of carbon from the atmosphere, to the ocean, to organisms, and finally to sediments on the ocean floor is known as the biologic pump. Its strength has varied throughout geologic history.
The opening of the Drake Passage, some scientists argue, may have driven the biologic pump into overdrive by creating a primary producer feeding frenzy. But not all primary producers are created equal, and it would have taken an organism very good at taking in carbon and transporting it to the hidden depths of the ocean for this to be an explanation for such a large drop in atmospheric CO2. One obvious candidate has emerged: diatoms.
Diatoms, today, account for over 40% of all primary productivity in the world’s oceans. They are powerful drivers of the biologic pump not only because they take in carbon, but because they act as ballasts for other organic matter. They create beautiful glassy shells from silica, which organic matter from all kinds of organisms can stick to. These organic covered glass shells readily sink to the base of the ocean, taking plenty of carbon with them.
So the big question is this: was there a major expansion of diatoms 34 million years ago and could such an expansion be the reason for such a precipitous drop in CO2? A new study published in the journal Earth and Planetary Science Letters says there might be something to this theory.
The team of researchers, led by Katherine Egan at Oxford, studied the silica content of deep ocean cores in the Antarctic Ocean. These cores preserve a record of sedimentary layers from the end of the Eocene to the initiation of Antarctic glaciation. These cores are the geologists’ equivalent to an ancient primary text—filled with information about the past, but in need of a skilled and knowledgeable reader.
The team looked specifically for the chemical signatures of diatoms—subtle variations in the chemistry of silica deposited on the ocean floor at that time. They wanted to see not only if diatoms were around in the time leading up to glaciation, but also if there was a dramatic increase in diatoms that occurred when CO2 levels dropped dramatically.
All evidence, it turns out, points to a significant increase in the population of diatoms. Even more compelling, the timing lines up perfectly with the formation of the Atlantic Circumpolar Current, which occurs just before the dramatic drop in atmospheric CO2. Their study lends significant credence to the idea that the opening of the Drake Passage created the conditions for a hyperactive biologic pump that forced global CO2 levels below the threshold necessary for ice sheets to form on Antarctica.
It is remarkable to think—and it should give us humans pause—that an organism so tiny that it needs to be viewed under a microscope could have transformed the world so profoundly. The transition from a world without ice to a world with ice is certainly one of the most significant events in our planet’s long history. It is a transition, like all major events in Earth’s history, inexorably linked to the organisms that inhabit it.