Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
2.6.2 Hydrothermal Vents
Deep-sea hydrothermal vents occur in areas of crustal formation near mid-ocean ridge systems both in
fore-arc and back-arc areas (Humphris 1995). Seawater permeating and entrained through the crust and
upper mantle is superheated by hot basalt and is chemically altered to form hydrothermal fluids as it rises
through networks of fissures in newly-formed seafloor (Humphris 1995; McMullin et al. 2000). The
temperature of the hydrothermal fluid is characteristically 200° to 400°C in areas of focused flows and
less than 200°C in areas of diffuse flow. Other than being hot, hydrothermal fluids are typically poor in
oxygen content, and contain toxic reduced chemicals including hydrogen sulfide and heavy metals
(McMullin et al. 2000). As the hot hydrothermal fluids come in contact with seawater overlying the vent,
heavy metals precipitate out of the fluid and accumulate to form chimneys and mounds. In complete
darkness, under the high ambient pressure of the deep sea, in nutrient-poor conditions, and under
extreme thermal and chemical conditions, metazoans (multicellular animals) are able to adapt and
colonize these sites. Chemosynthetic bacteria use the reduced chemicals of the hydrothermal fluid
(hydrogen sulfide) as an energy source for carbon fixation and generate a chemosynthetic-based primary
production. In turn, vent organisms (metazoans) consume the chemosynthetic bacteria or form symbiotic
relationships with them, and use numerous morphological, physiological, and behavioral adaptations to
flourish in this extreme deep-sea environment. These chemosynthetic organisms produce communities
typically characterized by a high biomass and low diversity.
A number of hydrothermal vents have been located in the study area (Figure 2-3). Evidence of active
hydrothermal venting has been identified near more than 12 submarine volcanoes and at two sites along
the back-arc spreading center off of the volcanic arc (Kojima 2002; Embley et al. 2004) with the potential
for more systems yet to be discovered. Hydrothermal vents located in the Mariana Trough experience
high levels of endemism due to their geographic isolation from other vent systems, with at least 8 of the
30 identified genera only known to occur in western Pacific hydrothermal vent systems (Hessler and
Lonsdale 1991; Paulay 2003). Hydrothermal vents at Esmeralda Bank, one of the active submarine
volcanoes in the Marianas MRA study area, span an area greater than 0.2 km 2 on the seafloor and expel
water with temperatures exceeding 78°C (Stüben et al. 1992). West of Guam and on the Mariana Ridge,
there are three known hydrothermal vent fields: Forecast Vent site (13°24’N, 143°55’E; depth: 1,450 m),
TOTO Caldera (12°43’N, 143°32’E), and the 13°N Ridge (13°05’N, 143°41’E) (Kojima 2002; Figure 2-3).
The gastropod Alviniconcha hessleri is the most abundant chemosynthetic organism found in
hydrothermal vent fields of the Mariana Trough. Vestimentiferan tube worms are also found in these sites
west of Guam (Kojima 2002).
2.6.3 Abyssal Plain
The Mariana Trough is comprised of a large relatively flat abyssal plain with water depths ranging
approximately from 3,500 to 4,000 m (Thurman 1997; Figure 2-2). Very little data regarding the Mariana
Trough within the study region has been investigated. However, in general abyssal plains can be
described as large and relatively flat regions covered in a thick layer of fine silty sediments with the
topography interrupted by occasional mounds and seamounts (Kennett 1982; Thurman 1997). It is host to
thousands of species of invertebrates and fish (Mariana Trench 2003).
2.6.4 Mariana Trench
The seafloor contains numerous hydrothermal vents formed by spreading tectonic plates (Mariana Trench
2003). Away from the hydrothermal vents, the seafloor is covered with soft brown sediments devoid of
rock formations (Kato et al. 1998). Sediments that lack carbonate and silica shells appear to be
dissolving, suggesting that the ocean floor lies below the carbonate compensation depth (CCD) and at or
near the silicate compensation depth (SCD) (Ogawa et al. 1997). In addition, sediments appear to be
affected by local currents, which can transport sandy or silty sediments along the trench floor (Ogawa et
al. 1997). The trench is host to numerous hydrothermal vent systems supporting a wide variety of
chemosynthetic organisms. In addition, the deep waters of the Mariana Trench support barophilic
organisms capable of surviving in the cold, dark, high pressure environment. One mud sample taken from
Challenger Deep by oceanographers yielded over 200 different microorganisms (Mariana Trench 2003).
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