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Researchers show link between fault activity and amount of water seepage into mantle
With the presence of tectonic faults and plate boundaries, the seemingly solid ground we walk on is actually interwoven with cracks, gaps and holes. The same is true for the seabed. It is not completely impermeable. Seawater can find its way into the lower layers of the mantle, where it can affect the mantle's composition.
An international team of researchers, led by the University of Southampton and with collaboration from the GEOMAR Helmholtz Centre for Ocean Research Kiel, has shown a direct link on a geological timescale between fault activity and the amount of water entering the earth's mantle along the faults.
To measure the amount of water entering the crust, sound waves were used to map the distribution of serpentinite. Serpentinite is formed when seawater that seeps down from the seabed interacts with peridotite, a dry rock which makes up most of the mantle beneath the crust.
Over a four-month period, special sound waves were directed at the seabed from the US research vessel MARCUS G. Langseth (National Science Foundation). As they travelled through the mantle, the sound waves could be detected by sensitive instruments placed at the seabed. The amount of time needed for the signals to travel from an acoustic seismic source to the seabed instruments shows how fast sound travels in the rocks, and indicates the amount of serpentinite present.
The GEOMAR research vessel POSEIDON recorded the signals received by the seabed instruments, using 78 seismometers. In this way, the scientists got a 3-D image of the seabed and the deeper structures up to 12 kilometres, said co-author Dr Cord Papenberg from GEOMAR.
The research showed that that the amount of serpentinite at the bottom of the fault was directly related to the size and duration of fault movement. Co-author Dr Dirk Klaeschen, also from GEOMAR, said that they already knew that the seabed was full of faults and about the presence of serpentinite, but they had not known about the distribution or relationship of the magnitude of the disturbances.
The research was conducted over a 86km by 22km area of seafloor at the Deep Galicia Margin, west of Spain. Here, during the formation of the Atlantic Ocean about 150 million years ago, Portugal and Spain separated from Newfoundland. Traces of the processes can be found at the location, said co-author Dr Klaeschen. The findings of the research were published in the international journal Nature Geoscience.
In the course of the research, the scientists were able to estimate the approximate amount of seawater that penetrated the faults at the earth's mantle. These amounts were comparable to those at other tectonic environments like mid-ocean ridges. Dr Klaeschen explained that the super-heated water there released numerous substances from the rock and deposited them at the seafloor where they would later become black smokers. He added that there were obviously other similar fault zones, but they were as yet currently unknown, thus, more investigation was needed to find out how widespread such occurrences were in the oceans.
Link to study: www.nature.com/.../ngeo2671.html