nov/8th/2011 | Dr. Dorrik Stow y Dr. Fº Javier Hernández-Molina
Scientific expedition IODP 339 sets sail from the Azores in mid-November, on the trail of past climate change beneath the deep seafloor.
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Scientific ocean drilling beneath the deep seafloor has fundamentally altered our understanding of the Earth. Over the past four decades hundreds of expeditions have been mounted to every ocean on the planet, involving thousands of research scientists and recovering many hundreds of miles of core samples. Cores of deep-sea sediment yield invaluable information about how our planet works – and has worked in the past. They provide indispensable evidence of past environmental change, the genesis of earthquakes, volcanoes and tsunamis, the nature and evolution of life, and much more.
On November 17th the world’s most sophisticated scientific drillship, JOIDES Resolution, will put into Ponta Delgada in the Azores, a remote island of the mid Atlantic Ocean, from where she will embark on yet another journey into the unknown. The expedition 339 of the Integrated Ocean Drilling Program (IODP) will be led by co-chief scientists Professor Dorrik Stow, from Heriot Watt University in Edinburgh, and Professor Javier Hernandez-Molina, from Vigo University in Spain. Both co-chiefs are eminent in the field of deep-sea exploration. They will be joined by IODP project manager, Dr Carlos Alvarez Zarikian, from Texas A&M University in the USA and an expert team of 34 scientists from 13 different nations around the world. JOIDES Resolution is 147 m long, has a drilling rig that towers 62.5 m above the water, and operates with a crew of about 111 people. There are four tiers of scientific laboratories furnished with state-of-the-art equipment and computer support.
The JOIDES Resolution will steam due east from the Azores towards Gibraltar. Its course will track above a deep suture line in the outer crust of Earth that runs from the Mid Atlantic Ridge (the birthplace of the Atlantic Ocean) to the Mediterranean Sea. This is suture line marks where the great African tectonic plate scrapes and rubs against Europe...and earthquakes abound. The mission of Expedition 339 is to explore beyond the deep seafloor at this boundary between Africa and Europe in the area of the Gulf of Cadiz.
Professor Stow explains: “We have mapped the seafloor of the Gulf of Cadiz in some detail now, and have found a rugged seascape of gentle mountains, strange gas-leaking mud volcanoes, and giant seafloor canyons. But it has never been drilled for scientific exploration. When JOIDES Resolution arrives above our first target drill site, kept exactly on station by her four mobile thrusters, we will be poised above the exciting and challenging unknown.” Professor Hernandez-Molina continues: “We have learned a great deal from 8 years of painstaking preparations for this moment, and have many exciting theories to test. Everything could be turned on its head by some unexpected finding in the next two months at sea.”
Both scientists are eager to explain something more of what their careful research has told them so far and what questions they hope to answer on Expedition 339.
Opening of the Gibraltar Gateway
• 6 million years ago the Straits of Gibraltar was closed and Europe was joined with Africa by a narrow strip of land (rather like the Panama isthmus between North and South America today). The former Mediterranean Sea evaporated entirely to dryness leaving behind a huge salt-lined basin. Sometime afterwards the Straits creaked open and Atlantic water flooded inwards to cover the salt pans. As the new Mediterranean filled, a heavy lower layer of salt-laden water slowly welled up and eventually reached the spill point across the Straits.
• Today, deep below the sea surface, there is a hugely powerful cascade of warm Mediterranean water spilling out through the Straits of Gibraltar – what oceanographers call the Gibraltar Gateway. It plunges over 1000 metres down slope into the Atlantic Ocean (a drop greater than the world’s greatest waterfall on land – the Angel Falls), scouring the rocky seafloor and then courses west and north along the margins of Spain and Portugal.
• Expedition 339 seeks to address several important questions: Exactly when did the opening and first outflow occur? What effects did the opening a new ‘gateway’ between ocean and sea have on global climate and seafloor architecture? And, has the flow pulsed, remained steady or even reversed in the eons since opening?
Impact on global climate
• Climate scientists have clearly demonstrated that the oceans and atmosphere are intricately coupled. Their joint power creates the weather and regulates Earth’s climate. The transfer of heat energy every second of every day via the ocean currents is quite staggering in its scale. They form a global conveyer belt in perpetual motion – a great flywheel that helps buffer Earth’s climate from the extremes.
• The Mediterranean Outflow is an important part of this energy transfer. It spreads right across the Atlantic Ocean and also veers northwards, sweeping along the continental slope off Spain and Portugal, and reaching passed Scotland to as far north as west of Norway. It is from here that the North Atlantic overturning circulation system begins, which is the main driver of the global conveyer belt. Recent data showed a slow-down in this overturn. If it were to shut-down completely there would almost certainly be major repercussions, and possibly the runaway global warming that some of the models invoke.
• The questions we will ask on Expedition 339 include: What were the climatic effects of the sudden addition of large volumes of warm, salt-charged Mediterranean Water to the North Atlantic some 5 million years ago? And just how important is this outflow in maintaining the climate flywheel in good working order?
Sea level and sediment architecture
• So powerful is this underwater current that it has spread a vast sand sheet for more than 100 km away from the Gibraltar Gateway. This is like covering the cities of Edinburgh and Glasgow, and the whole of the Midland Valley of Scotland with sand, or roughly 4,000 times the size of Rio’s magnificent Copacabana Beach. We believe this is the largest single expanse of sand anywhere in the deep sea. It is known as a contourite sand sheet as this type of deep-sea flow is called a contour current.
• As the current roars onwards it carves canyons across the seafloor, constructs fields of sand waves and then gradually slows. By the time it reaches the southern slope off Portugal it constructs gigantic mounds of mud and silt. These contourite drifts, as they are called, hold an amazingly detailed record of changes in flow intensity and speed over the past 5 million years. Such changes are directly linked to past variation in climate and sea-level.
On Expedition 339, we plan to drill into and at least 1500 metres through the contourite sand sheet – but how thick will it be? Were the currents that deposited the first sands after Gibraltar Gateway opened more powerful than those of today? And is this type of sand body potential a new type of reservoir target for oil and gas exploration? We will then drill through several muddy contourite drifts. Decoding these past records of natural climate change will be one of our principal challenges.
Recent tectonic activity and margin evolution
• The Earth’s crust in this region around the great Azores-Gibraltar suture line is under intense pressure caused by the collision and slippage of the African and European tectonic plates. The seafloor of the Gulf of Cádiz and off southwest Portugal provides an excellent example of how recent tectonic activity has markedly affected submarine topography.
• The deeper layers of rock (several kilometres below the surface) begin to deform under these pressures, and softer muddy sediment is squeezed upwards between the layers. In some places the mud reaches the seafloor and oozes out, often associated with seepage of natural gas. Here it builds up mud volcanoes and ridges. Strangely unique communities of deep-sea organisms are attracted to such habitats.
• On Expedition 339, we want to understand when and how exactly the recent tectonic affects the region. Has the tectonic activity controlled changes in the distribution of branches of Mediterranean Outflow and their influence in the North Atlantic? How was the early history of this flow affected by tectonic opening of the Gibraltar Gateway and how might it be in future?
Marine archive of climate change
• The deepest water site off the south-western Portuguese margin will be dedicated entirely to providing the most complete marine record of climate change through the past (Quaternary) ice-age – the last 2.6 million years of Earth history. This will provide a marine archive for comparison with ice core records from the Greenland and Antarctic ice sheets, and with the numerous land-based records – from tree rings, cave stalactites, varves in lake sediments, and others.
• Expedition 339 intends to document and investigate periods of very rapid climate change in the past, comparable to rapid global warming today, and hence understand more clearly what we might expect in our future.
The science is hugely challenging and the team of scientists on board will be working around the clock – 24 hours a day, seven days a week. Even Christmas Day and New Year’s Day will be no exception! By the time JOIDES Resolution arrives in Lisbon after two months at sea, we will have answers to some of our questions but by no means all. These first exciting results will be published in prestigious journals such as Nature and Science. The work will continue over the next 2-3 years in shore-based laboratories of the shipboard scientists, as well as another shore-based team who will join the research effort following the expedition. Each scientist will be working on different sets of samples taken from the cores, using the specialist techniques and equipment in their different laboratories. The co-chiefs will keep track of the whole scientific story as it emerges, and ensure its publication in the open scientific literature.
Both co-chiefs wish to express their thanks to REPSOL and TGS-NOPEC for allowing them to use a large unpublished data set of seismic records and well results from the Gulf of Cádiz and offshore west Portugal. They are also very grateful to the Spanish Hydrographic Institute (Cádiz, Spain), the Portuguese Hydrographic Institute (Lisbon, Portugal), and the TELEFONICA Company, all of which have helped them in sending information related to the location of submarine cables, former explosives dumping grounds, and other matters related to safe drilling operations.
tags» research global change
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