From: Andy Soos, ENN
Published February 15, 2012 07:22 AM

Mariana Trench Clam Fields

Clam can be used in several different ways: one, as a general term covering all bivalve molluscs. The word can also be used in a more limited sense, to mean bivalves that burrow in sediment, as opposed to ones that attach themselves to the substrate (for example oysters and mussels), or ones that can swim and are migratory, like scallops. Scientists have long marveled at the unusual life forms thriving at high temperature hydrothermal vents of the deep ocean. Now the discovery of clam communities near the lower temperature vents in the Mariana Trench is providing information about both the biogeography of the clams and the extent of the serpentinite vents that sustain them. A team of scientists from the United States and Japan discovered the vesicomyid clams while conducting deep sea dives from the Japanese R/V Yokosuka to study the geology of the southern Mariana.

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Located east of Mariana Islands in the western Pacific and at the deepest part of Earth’s ocean, the 1,580-mile long trench is where the Pacific Plate is pushed under the Mariana Plate. The research team, which included University of Hawaii at Mānoa’s Fernando Martinez, was exploring an area on the inner trench slope of that convergent margin during September 2010.

The Mariana Trench is part of the Izu-Bonin-Mariana Arc geological boundary system that forms the boundary between two tectonic plates. In this system, the western edge of one plate, the Pacific Plate, is subducted beneath the smaller Mariana Plate that lies to the west. Because the Pacific plate is the largest of all the tectonic plates on Earth, crustal material at its western edge has had a long time since formation (up to 170 million years) to compact and become very dense; hence its great height-difference relative to the higher-riding Mariana Plate, at the point where the Pacific Plate crust is subducted. This deep area is the Mariana Trench proper. The movement of these plates is also indirectly responsible for the formation of the Mariana Islands (which are caused by volcanism as a result of subduction of water trapped in minerals).

The research team discovered abundant vesicomyid clam communities associated with a serpentinite-hosted hydrothermal vent system and dubbed the area Shinkai Seep Field for the Japan Agency for Marine-Earth Science and Technology manned submersible Shinkai 6500 used in the dives.

Serpentinite is a type of metamorphic rock formed in the deep-sea by alteration of mantle rock. It is named for its snakeskin-like appearance, characterized by a mottled color, with a waxy luster and curving, polished surfaces, explains Martinez, a researcher in the Hawaii Institute of Geophysics and Planetology.

The serpentinization of peridotite rock generates a hydrogen- and methane-rich environment that supports the biological communities.

Since magmatic heat contribution is unlikely and no serpentinite mud volcanoes were observed, the scientists believe the serpentinization of peridotite at the Shinkai Seep Field is triggered by the flow of heated fluids traveling through faults from the subduction zone.
Abundance of clams

The vesicomyid clam community is the first live example described from a low-temperature serpentinite-hosted hydrothermal system from either convergent or divergent margins, the researchers write in an article published February 9, 2012, in Proceedings of the National Academy of Sciences’ online edition.

For further information and photo: http://www.hawaii.edu/news/article.php?aId=4921

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