Rocks on the floor of the Atlantic Ocean hold many secrets that can help scientists understand Earth and how life began there, so a group of researchers has embarked on the arduous task of drilling a hole more than three-quarters of a mile deep to extract a record-breaking rock core from the Earth’s mantle.
Scientists regularly take core samples – cylindrical samples of material taken from deep beneath the Earth’s surface – to study the composition of its various layers. The data acts as a window into the Earth’s past, providing information about climatic and environmental changes, or even the formation of the Earth itself. Drilling in the deep ocean poses unique challenges, so researchers are often forced to dredge rocks from the ocean floor. Analyzing the composition of these rocks can reveal valuable information. However, these rocks can be altered by exposure to ocean pressure and salt water.
The expedition took place from April to June 2023 in an area of the North Atlantic known as Atlantis Mountains, an underwater mountain that rises 14,000 feet (4,267 meters) above the ocean floor. The site was chosen because tectonic activity in the region has pushed rocks that are normally deep in the Earth’s mantle up quite close to the ocean floor, making them easier to retrieve. Still, deeper drilling was required to retrieve a nearly continuous, 4,160-foot-long (1,268-meter-long) core of peridotite, a type of igneous rock.
This extreme depth is much deeper than any previous attempt to drill into oceanic mantle rocks. According to the study, co-authored by C. Johan Lissenberg of Cardiff University, the scientists recovered 71 percent of the material they drilled, and almost completely recovered a long section of partially serpentinized harzburgite (i.e. rock that has been partially altered by water).
As described in a paper published in the journal ScienceBy analyzing the composition of minerals within the rocks, the researchers found evidence supporting a theory about how rocks that originate deep in the mantle rise to the surface: pressure melts the rock, forcing it upwards, where it mixes with magma in the Earth’s crust and erupts on the ocean floor.
The researchers also found intrusions of crystalline rocks called gabbro, which form when magma slowly cools. They believe gabbro plays a major role in regulating the minerals and gases in deep-sea vents, which some scientists believe are ideal places for primitive life to form. Learning more about the vents could lead to new theories about how life on Earth first began and how life could theoretically form on other planets.
In their study, the researchers acknowledged that further analysis of the drilled material is required. “The comprehensive rock record obtained by Expedition 399 provides a wealth of opportunities to fundamentally advance our understanding of the oceanic upper mantle,” they said.
“This depth is far beyond anything recorded in previous drilling and creates an opportunity to determine the structural and mineralogical characteristics of the mantle and how it interacts with the hydrosphere and biosphere,” Professor Erik Hellebrand of Utrecht University said in an accompanying article.
He also expressed hope that the drilling expedition would raise the bar in research into how the Earth was formed.
“Decades of seafloor sampling by dredging have painted a rough mineralogical picture of the mantle,” he writes, “but each new drilling mission reveals surprising facts about the formation of the mantle and oceanic crust. More ambitious drilling projects will reveal important information for understanding the biogeochemical influence of the oceanic mantle.”