Scientists recently concluded an expedition aboard the research vessel JOIDES Resolution to learn more about Atlantis Massif, an undersea mountain, or seamount, that formed in a very different way than the majority of the seafloor in the oceans.
Unlike volcanic seamounts, which are made of the basalt that’s typical of most of the seafloor, Atlantis Massif includes rock types that are usually only found much deeper in the ocean crust, such as gabbro and peridotite.
The expedition, known as Integrated Ocean Drilling Program (IODP) Expedition 340T, marks the first time the geophysical properties of gabbroic rocks have successfully been measured directly in place, rather than via remote techniques such as seismic surveying.
With these measurements in hand, scientists can now infer how these hard-to-reach rocks will “look” on future seismic surveys, making it easier to map out geophysical structures beneath the seafloor.
“This is exciting because it means that we may be able to use seismic survey data to infer the pattern of seawater circulation within the deeper crust,” says Donna Blackman of the Scripps Institution of Oceanography in La Jolla, Calif., co-chief scientist for Expedition 340T.
“This would be a key step for quantifying rates and volumes of chemical, possibly biological, exchange between the oceans and the crust.”
As the tectonic plates separate, new crust is formed at the spreading center and a combination of stretching, faulting and the intrusion of magma from below shape the new seafloor.
Periods of reduced magma supplied from the underlying mantle result in the development of long-lived, large faults. Deep portions of the crust shift upward along these faults and may be exposed at the seafloor.
This process results in the formation of an oceanic core complex, or OCC, and is similar to the processes that formed the Basin and Range province of the Southwest United States.
“Recent discoveries from scientific ocean drilling have underlined that the process of creating new oceanic crust at seafloor spreading centers is complex,” says Jamie Allan, IODP program director at the U.S. National Science Foundation (NSF), which co-funds the program.
“This work significantly adds to our ability to infer ocean crust structure and composition, including predicting how ocean crust has ‘aged’ in an area,” says Allan, “thereby giving us new tools for understanding ocean crust creation from Earth’s mantle.”
Atlantis Massif is a classic example of an oceanic core complex.
Because it’s relatively young–formed within the last million years–it’s an ideal place, scientists say, to study how the interplay between faulting, magmatism and seawater circulation influences the evolution of an OCC within the crust.
“Vast ocean basins cover most of the Earth, yet their crust is formed in a narrow zone,” says Blackman. “We’re studying that source zone to understand how rifting and magmatism work together to form a new plate.”
The JOIDES Resolution first visited Atlantis Massif about seven years ago; the science team on that expedition measured properties in gabbro.
But they focused on a shallower section, where pervasive seawater circulation had weathered the rock and changed its physical properties.
For the current expedition, the team did not drill new holes.
Rather, they lowered instruments into a deep existing hole drilled on a previous expedition, and made measurements from inside the hole.
The new measurements, at depths between 800 and 1,400 meters (about 2,600-4,600 feet) below the seafloor, include only a few narrow zones that had been altered by seawater circulation and/or by fault slip deformation.
The rest of the measurements focused on gabbroic rocks that have remained unaltered thus far.
The properties measured in the narrow zones of altered rock differ from the background properties measured in the unaltered gabbroic rocks.
The team found small differences in temperature next to two sub-seafloor faults, which suggests a slow percolation of seawater within those zones.
There were also significant differences in the speed at which seismic waves travel through the altered vs. unaltered zones.
“The expedition was a great opportunity to ground-truth our recent seismic analysis,” says Alistair Harding, also from the Scripps Institution of Oceanography and a co-chief scientist for Expedition 340T.
“It also provides vital baseline data for further seismic work aimed at understanding the formation and alteration of the massif.”
The Integrated Ocean Drilling Program (IODP) is an international research program dedicated to advancing scientific understanding of the Earth through drilling, coring and monitoring the subseafloor.
The JOIDES Resolution is a scientific research vessel managed by the U.S. Implementing Organization of IODP (USIO). Texas A&M University, Lamont-Doherty Earth Observatory of Columbia University and the Consortium for Ocean Leadership comprise the USIO.
Two lead agencies support the IODP: the U.S. National Science Foundation and Japan’s Ministry of Education, Culture, Sports, Science and Technology.
Additional program support comes from the European Consortium for Ocean Research Drilling, the Australia-New Zealand IODP Consortium, India’s Ministry of Earth Sciences, the People’s Republic of China’s Ministry of Science and Technology, and the Korea Institute of Geoscience and Mineral Resources.
Mediterranean bottom currents and the sediment deposits they leave behind offer new insights into global climate change, the opening and closing of ocean circulation gateways and locations where hydrocarbon deposits may lie buried under the sea.
A team of 35 scientists from 14 countries recently returned from an expedition off the southwest coast of Iberia and the nearby Gulf of Cadiz. There the geologists collected core samples of sediments that contain a detailed record of the Mediterranean’s history. The scientists retrieved the samples by drilling into the ocean floor during an eight-week scientific expedition onboard the ship JOIDES Resolution.
The group–researchers participating in Integrated Ocean Drilling Program (IODP) Expedition 339: Mediterranean Outflow–is the first to retrieve sediment samples from deep below the seafloor in this region.
Much of the sediment in the cores is known as “contourite” because the currents that deposit it closely follow the contours of the ocean basin.
“The recovery of nearly four kilometers of contourite sediments deposited from deep underwater currents presents a superb opportunity to understand water flow from the Mediterranean Sea to the Atlantic Ocean,” says Jamie Allan, program director at the National Science Foundation (NSF), which co-funds IODP.
“Knowledge of this water flow is important for understanding Earth’s climate history in the last five million years.”
“We now have a much greater insight into the distinctive character of contourites, and have validated beyond doubt the existing paradigm for this type of sedimentation,” says Dorrik Stow of Heriot-Watt University in the United Kingdom and co-chief scientist for Expedition 339.
The world’s oceans are far from static. Large currents flow at various depths beneath the surface. These currents form a global conveyor belt that transfers heat energy and helps buffer Earth’s climate.
Critical gateways in the oceans affect circulation of these major currents.
The Strait of Gibraltar is one such gateway. It re-opened less than six million years ago.
Today, deep below the surface, there is a powerful cascade of Mediterranean water spilling out through the strait into the Atlantic Ocean.
Because this water is saltier than the Atlantic–and therefore heavier–it plunges more than 1,000 meters downslope, scouring the rocky seafloor, carving deep-sea canyons and building up mountains of mud on a little-known submarine landscape.
The sediments hold a record of climate change and tectonic activity that spans much of the past 5.3 million years.
The team found evidence for a “tectonic pulse” at the junction between the African and European tectonic plates, which is responsible for the rising and falling of key structures in and around the gateway.
This event also led to strong earthquakes and tsunamis that dumped large flows of debris and sand into the deep sea.
At four of the seven drill sites, there was also a major chunk of the geologic record missing from the sediment cores–evidence of a strong current that scoured the seafloor.
“We set out to understand how the Strait of Gibraltar acted first as a barrier and then a gateway over the past six million years,” says Javier Hernandez-Molina of the University of Vigo in Spain and co-chief scientist for Expedition 339. “We now have that understanding and a record of a deep, powerful Mediterranean outflow through the Gibraltar gateway.”
The first drill site, located on the west Portuguese margin, provided the most complete marine sediment record of climate change over the past 1.5 million years of Earth history.
The sediment cores cover at least four major ice ages and contain a new marine archive to compare against ice core records from Greenland and Antarctica, among other land-based records.
The team was surprised to find exactly the same climate signal in the mountains of contourite mud they drilled in the Gulf of Cádiz.
Because these muds were deposited much faster than the sediments at the Portuguese margin site, the record from these cores could prove to yield even richer, more detailed climate information.
“Cracking the climate code will be more difficult for contourites because they receive a mixed assortment of sediment from varying sources,” Hernandez-Molina says.
“But the potential story that unfolds may be even more significant. The oceans and climate are inextricably linked. It seems there is an irrepressible signal of this nexus in contourite sediments.”
The team also found more sand among the contourite sediments than expected.
The scientists found this sand filling the contourite channels, deposited as thick layers within mountains of mud, and in a single, vast sand sheet that spreads out nearly 100 kilometers from the Gibraltar gateway.
All testify to the strength, velocity and duration of the Mediterranean bottom currents. The finding could affect future oil and gas exploration, the researchers believe.
“The thickness, extent and properties of these sands make them an ideal target in places where they are buried deeply enough to allow for the trapping of hydrocarbons,” Stow explains.
The sands are deposited in a different manner in channels and terraces cut by bottom currents; in contrast, typical reservoirs form in sediments deposited by downslope “turbidity” currents.
“The sand is especially clean and well-sorted, and therefore very porous and permeable,” says Stow. “Our findings could herald a significant shift in future exploration targets.”
IODP is an international research program dedicated to advancing scientific understanding of the Earth through drilling, coring, and monitoring the subseafloor.
IODP is supported by two lead agencies: the U.S. National Science Foundation and Japan’s Ministry of Education, Culture, Sports, Science, and Technology. Additional program support comes from the European Consortium for Ocean Research Drilling, the Australia-New Zealand IODP Consortium, India’s Ministry of Earth Sciences, the People’s Republic of China (Ministry of Science and Technology), and the Korea Institute of Geoscience and Mineral Resources.
The JOIDES Resolution is a scientific research vessel managed by the U.S. Implementing Organization of IODP (USIO). Texas A&M University, Lamont-Doherty Earth Observatory of Columbia University, and the Consortium for Ocean Leadership comprise the USIO.
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The two “islands” were found on the remote sea floor in international waters 1,600 kilometres (1,000 miles) west of Australia during a surveying trip last month.
Their rocks contained fossils of creatures found in shallow waters, meaning they were once part of the continent at or above sea level rather than created by undersea volcanic activity, said Sydney University geophysicist Jo Whittaker.
Whittaker, one of the key researchers, said she was particularly interested in exploring India’s drift first northwest and then sharply north, where its northeast coast, once joined to Australia, smashed into Eurasia, forming the Himalayas.
“We have a fairly good idea where those continents were but we don’t exactly know, the eastern Indian Ocean is one of the more poorly explored parts of the world’s oceans in terms of tectonics,” she told AFP.
“So it will help us figure out the plate kinematic motions that led to India moving away from Australia and heading up off to crash into Eurasia.”
Samples of sandstone and granite dredged from a steep cliff on one of the islands, about 2,000 metres (6,600 feet) below the ocean surface, are to be dated but the research team believe they are up to one billion years old.
The rocks will also be compared with samples from Australia’s west coast to try to determine where exactly the islands broke away from.
Similar matching was not possible with India because the relevant coast was now “smashed into the Himalayas somewhere,” said Whittaker.
India’s east coast was once adjacent to what is now modern-day Antarctica.
She likened the continental separation to pulling something “a bit gooey” apart and said the fragments, which are a fraction of the thickness of normal continental crust and combined about the size of Scotland, were the “little pieces that got left behind.”
“These pieces are probably not as thick as (continental crust) so they sit a little bit lower in the water, like something floating in the bath essentially,” she said.
Whittaker added that the fossil find was extremely lucky given the vastness of the area they were dredging.
“We’re excited to actually get some really good samples and very clear cut continental rocks which show that (the islands) are little fragments of Gondwana that were left behind as India moved away from Australia,” she said.
Plate tectonic theory is a relatively young science which was only recognised in the 1950s and experts were still trying to establish what made the continents move and change direction, she added.
Australia was moving northwards at a speed of about seven centimetres (2.75 inches) a year, likely due to a subduction zone along the Indonesian coastline where two plates met that was linked to the destructive 2004 earthquake and tsunami.
Antarctica, on the other hand, was not moving at all and Whittaker said discoveries like the Gondwana islands were critical.
“It’s very significant, it’s not every day you discover two large continental fragments on the ocean floor,” she said.
“Together with some of the other data this has the potential to change how we’ve been modelling that part of the world and that timeframe.”
- ‘Lost’ continent Gondwana sheds light on formation of world today (telegraph.co.uk)
- Secret of ghost alps of Antarctica revealed (telegraph.co.uk)