Up until around 500 million years ago, the continents of Earth were practically lifeless, harboring – at most – slimy mats of bacteria on rocky, barren wastelands. Around this time plants began to creep out of the oceans, gradually developing adaptations that allowed them to expand further and further inland over millions and millions of years. But there is a dark side to this story: the increasing success of plants on land may have contributed to one of the largest set of extinctions known to the fossil record.
Plants colonized land over a period as long as tens to hundreds of millions of years. But there were a number of evolutionary advances that brought about swift change. Each advance allowed plants to either expand to new habitats or grow larger. And with each advance, the roots of these pioneering plants broke more and more earth apart. To Tom Algeo, a geologist at the University of Cincinnati, this process may have created a chain of events that removed massive quantities of oxygen from the ocean.
Although the first land plants evolved around 500 million years ago, they remained close to the waters edge and did not grow very large for around 100 million years. But these early plants paved the way for the future success of larger plants. This later success is largely due to lignin, tissue that gives plants structure and support.
Author’s note: This post is the first in a series of great Earth history moments. Stay tuned for a new post every other week.
Around 6 million years ago, the Mediterranean Sea became separated from the Atlantic. Cut off from the world’s oceans, it began to evaporate. By 5.3 million years ago, there was literally no sea left. 1000 years later, it was refilled in a geologic instant.
A number of discoveries led to the conclusion that the Mediterranean dried out completely sometime in the past. The first came in the 1960s, when seismic studies of the floor of the Mediterranean revealed a unique layer – dubbed the M reflector – across the whole basin. Scientists interpreted it to be a large layer of salt distributed evenly across the seafloor.
Later, in 1970, a leg of the Deep Sea Drilling Project cored deep into the Mediterranean seabed. They found what the seismic data predicted: a hard layer of evaporites – rocks composed of salts.
The only way to get evaporite rocks at the base of a sea is to evaporate water until it becomes so concentrated with salts that they can no longer be dissolved. This forces them to precipitate into a solid form.
Just as enigmatic as the salt layer, engineers mapping the base of the Nile River in preparation for the construction of the Aswan Dam around this time found that carved deep beneath the silty floor of the Nile was a canyon whose ancient base was well below sea level.
The only way for a canyon to be carved into bedrock is for a river to flow through it. But a river won’t cut lower than sea level. This deep canyon meant that Medteranian sea level must have been dramatically lower in the past.
In 1972, Kenneth Hsu, the primary investigator on the Deep Sea Drilling Leg that cored the Mediterranean, authored a paper in Nature concluding that the sea must have evaporated nearly completely to produce such an anomalous layer of evaporite minerals and to have cut canyons so deep. In the paper he admitted it was a “preposterous idea,” but stated that no other explanation presented itself. Read the rest of this entry »