Ocean Drilling Expedition in Portuguese Plate

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.

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Source: geology.com

The Boundary Between Magma and Water

Scientists and drillers recovered a remarkable suite of heat-tempered basalts that provide a detailed picture of the rarely seen boundary between magma and seawater. These samples were collected during a return to ODP Hole 1256D, one of the deepest “hard r

ock” penetration sites of scientific ocean drilling. ODP Hole 1256D has been stabilized, cleared to its full depth, and primed for further deepening.

Panama City, Panama – Integrated Ocean Drilling Program (IODP) Expedition 335 Superfast Spreading Rate Crust 4 recently completed operations in Ocean Drilling Program (ODP) Hole 1256D, a deep scientific borehole that extends more than 1500 meters below the seafloor into the Pacific Ocean’s igneous crust – rocks that formed through the cooling and crystallization of magma, and form the basement of the ocean floor.
An international team of scientists led by co-chief scientists Damon Teagle (National Oceanographic Center Southampton, University of Southampton in the UK) and Benoît Ildefonse (CNRS, Université Montpellier 2 in France) returned to ODP Hole 1256D aboard the scientific research vessel, JOIDES Resolution, to sample a complete section of intact oceanic crust down into gabbros.

A granoblastic basalt (the hardest material ever drilled in ocean drilling) viewed under the microscope (picture is 2.3 mm across).

This expedition was the fourth in a series and builds on the efforts of three expeditions in 2002 and 2005.

Gabbros are coarse-grained intrusive rocks formed by the slow cooling of basaltic magmas. They make up the lower two-thirds of the ocean crust. The intrusion of gabbros at the mid-ocean ridges is the largest igneous process active on our planet with more than 12 cubic kilometers of new magma from the mantle intruded into the crust each year. The minerals, chemistry, and textures of gabbroic rocks preserve records of the processes that occur deep within the Earth’s mid-ocean ridges, where new ocean crust is formed.

“The formation of new crust is the first step in Earth's plate tectonic cycle,” explained Teagle. “This is the principal mechanism by which heat and material rise from within the Earth to the surface of the planet. And it’s the motion and interactions of Earth’s tectonic plates that drive the formation of mountains and volcanoes, the initiation of earthquakes, and the exchange of elements (such as carbon) between the Earth's interior, oceans, and atmosphere.”

“Understanding the mechanisms that construct new tectonic plates has been a major, long-standing goal of scientific ocean drilling,” added Ildefonse, “but progress has been inhibited by a dearth of appropriate samples because deep drilling (at depths greater than 1000 meters into the crust) in the rugged lavas and intrusive rocks of the ocean crust continues to pose significant technical challenges.”

ODP Hole 1256D lies in the eastern equatorial Pacific Ocean about 900 kilometers to the west of Costa Rica and 1150 kilometers east of the present day East Pacific Rise. This hole is in 15 million year old crust that formed during an episode of “superfast” spreading at the ancient East Pacific Rise, when the newly formed plates were moving apart by more than 200 millimeters per year (mm/yr).

“Although a spreading rate of 200 mm/yr is significantly faster than the fastest spreading rates on our planet today, superfast-spread crust was an attractive target,” stated Teagle, “because seismic experiments at active mid-ocean ridges indicated that gabbroic rocks should occur at much shallower depths than in crust formed at slower spreading rates. In 2005, we recovered gabbroic rocks at their predicted depth of approximately 1400 meters below the seafloor, vindicating the overall ‘Superfast’ strategy.”

Previous expeditions to Hole 1256D successfully drilled through the erupted lavas and thin (approximately one-meter-wide) intrusive “dikes” of the upper crust, reaching into the gabbroic rocks of the lower crust. The drilling efforts of Expedition 335 were focused just below the 1500-meter mark in the critical transition zone from dikes to gabbros, where magma at 1200°C exchanges heat with super-heated seawater circulating within cracks in the upper crust. This heat exchange occurs across a narrow thermal boundary that is perhaps only a few tens of meters thick.

In this zone, the intrusion of magma causes profound textural changes to the surrounding rocks, a process known as contact metamorphism. In the mid-ocean ridge environment this results in the formation of very fine-grained granular rocks, called granoblastic basalts, whose constituent minerals recrystallize at a microscopic scale and become welded together by magmatic heat. The resulting metamorphic rock is as hard as any formation encountered by ocean drilling and sometimes even tougher than the most resilient of hard formation drilling and coring bits.

Expedition 335 reentered Hole 1256D more than five years after the last expedition to this site. The expedition encountered and overcame a series of significant engineering challenges, each of which was unique, although difficulties were not unexpected when drilling in a deep, uncased, marine borehole into igneous rocks.

The patient, persistent efforts of the drilling crew successfully cleared a major obstruction at a depth of 920 that had initially prevented reentry into the hole to its full depth of 1507 meters. Then at the bottom of the hole the very hard granular rocks that had proved challenging during the previous Superfast expedition were once more encountered. Although there may only be a few tens of meters of these particularly tenacious granoblastic basalts, their extreme toughness once more proved challenging to sample– resulting in the grinding down of one of the hardest formation coring bits into a smooth stump.

A progressive, logical course of action was then undertaken to clear the bottom of the hole of metal debris from the failed coring bit and drilling cuttings. This effort required the innovative use of hole-clearing equipment such as large magnets, and involved over 240 kilometers of drilling pipe deployments (trips) down into the hole and back onto the ship. (The total amount of pipe “tripped” was roughly equivalent to the distance from Paris to the English coast, or from New York City to Philadelphia, or Tokyo to Niigata). These efforts returned hundreds of kilograms of rocks and drill cuttings, including large blocks (up to 5 kilograms) of the culprit granoblastic basalts that hitherto had only been very poorly recovered through coring. A limited number of gabbro boulders were also recovered, indicating that scientists are tantalizingly close to breaking through into the gabbroic layer.

Expedition 335 operations also succeeded in clearing Hole 1256D of drill cuttings, much of which appear to have been circulating in the hole since earlier expeditions.

“We recovered a remarkable sample suite of granoblastic basalts along with minor gabbros, providing a detailed picture of a rarely sampled, yet critical interval of the oceanic crust,” Ildefonse observed. “Most importantly,” he added, “the hole has been stabilized and cleared to its full depth, and is ready for deepening in the near future.”

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Fonte: geology.com