They say life began at sea, in ancient primordial oceans. But are they the same ones we swim in today? Scientists say NO! The Atlantic and her sisters are actually much younger than the continents. Every day we learn more about the ocean floor and ancient oceans.
It was a chilly Thursday morning in November 1980 when, just before dawn. S, the drill bit from the Texaco oil rig on Lake Peigneur on Jefferson Island, La., punctured the roof of the Diamond Crystal salt dome beneath the lake. Water began pouring into the dome, slowly at first, then more and more rapidly, flooding the tunnels, dissolving the salt and forcing a hasty evacuation of the miners working there.
Within seven hours the entire 1,100-acre lake was empty and two drilling rigs, a tugboat, 11 barges from the canal, a barge loading-dock, 70 acres of Jefferson Island and its botanical gardens, parts of greenhouses, a house trailer, trucks, tractors, a parking lot and tons of mud and trees had disappeared into the sinkhole at the bottom of the lake. The salt dome was filled with lake water and the mining operations closed forever.
The events leading up to the 1980 disaster began about 150 million years ago, when the nearly landlocked sea that is now the Gulf of Mexico dried up over and over again, said Gary Byerly, chair of LSU’s department of geology and geophysics. The evaporation of the water left huge layers of salt, which were eventually covered by silt and clay. Pressure from overlying sediments deformed the flat layers of salt into dome shapes.
At the same time, the algae that would eventually become the oil and gas the Texaco rig workers were looking for were living in the Gulf. The climate in that period fluctuated wildly, with years so dry the Mississippi River would become a mud flat, followed by years of flash flooding, said geologist Ezat Heydari of LSU’s Basin Research Institute. These conditions also produced great algal blooms. As the organisms died and drifted to the bottom, they eventually covered the floor of the Gulf to a depth of 500 feet or more, Heydari said. Gradually they became covered with silt and sank deeper and deeper into the seab ed until they began to cook under the pressure and temperature.
“Oil and gas are formed by thermal processes,” said LSU geology professor Jeffery Nunn. “Organic material has to be cooked at a temperature of 100 degrees Centigrade before it will become transformed into oil, and it doesn’t get that hot till you get to a depth of three kilometers (almost 10,000 feet). It takes a long time for something to be buried that deep.”
“The oil deposits we are tapping under the Gulf of Mexico were laid down about 60 million years ago,” Nunn said, “but even today, organisms are dying and falling to the bottom, continuing the process of hydrocarbon formation.”
The salt domes and the oil deposits the rig was searching for are relatively recent geological phenomena. But even the earliest seas had a profound influence on the planet today, Byerly said.
Byerly’s field of expertise is the Archean Age, including oceans which formed at the dawn of time and gave rise to life on earth.
“There is a lot of overlap between oceanography and geology. Understanding the ancient oceans is a major part of understanding geology,” he said.
Scientists now believe that the oceans formed shortly after the earth itself and have always been just about as deep and just about as salty as they are today. They have not become more salty over the eons, Byerly said, because they are constantly drying up and re-forming elsewhere, leaving behind great deposits of salt.
“Scientists used to believe that the ocean basins were ancient and the continents were relatively young. Now we know just the opposite is true. Ocean basins are the most geologically active places on earth. The average age of the Atlantic, for instance, is between 75 and 150 million years. But you can find rocks 3.5 billion years old on the continents,” he said.
Although the earth itself is estimated to be 4.5 billion years old, no rocks have survived from that early date because geological forces have recycled them. Byerly took out a rock which had been formed on the edge of an Archean sea and pointed out layers of fine brown lines that made dome-like arches against the cream-colored rock background. “These were laid down by algae-like creatures,” he said.
But the little plants that left their mark in the 3-billion-year-old rock were not the earliest life forms. Life probably began in the warm, shallow waters of even earlier oceans but did not survive. “The earth was struck by large meteors for perhaps the first 700 million years of its history. Many of these would have been big enough to evaporate all the oceans, sterilizing life.”
Although the process of change — the shifting seas and drifting continents, the changing composition of the atmosphere, the advance and retreat of great ice sheets — continues to this day, there are some things that have not changed since the earth began. One of them is the interaction between the land, the atmosphere and the oceans.
“We used to think you could study the continents separately from the oceans and the atmosphere, but now we know that is not true. A change in one affects the others,” Byerly said. When the seas were warmer, for instance, they evaporated more rapidly, causing more thunderstorms and rainfall, which in turn eroded the land. Silt from erosion might eventually block or divert rivers feeding an inland sea, causing the sea to dry up.
The oceans also act as a planetary thermostat, keeping the earth more temperate than it would be without them, Byerly said.
The 150-million-year-old Diamond Crystal salt dome under Lake Peigneur has been partially opened again, but not for the mining of salt. It is now used to store 60-million-year-old oil.