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

AUGUST 2005 FINAL REPORT
During La Niña, Guam experiences a deficit in rain during the dry and rainy season (86% and 87% of
historical averages, respectively) (Pacific ENSO Applications Center 1995). During June to September
rainfall amounts exceed historical averages (104% of average). The Northern Mariana Islands also
experience a surplus of rainfall throughout the year during La Niña (104 to 139% of historical averages
with excess rainfall peaking in March, April and May) (Pacific ENSO Applications Center 1995).
2.1.3 Pacific Decadal Oscillation
The PDO is a long-term climatic pattern capable of altering SST, surface wind, and sea level pressure
(SLP) (Mantua 2002; Mantua and Hare 2002). The PDO is a long-lived El Niño-like pattern of Pacific
climate variability and experiences both warm and cool phases. However, the PDO has three main
characteristics separating it from ENSO events. First, PDO events can persist for 20 to 30 years which
contrasts with the relatively short duration of ENSOs (up to 18 months). Second, climatic effects of the
PDO are more prominent in ecosystems outside the tropics. Third, the mechanisms controlling the PDO
are unknown, while those forces creating ENSO variability have been resolved (Mantua and Hare 2002).
During warm phases of the PDO, the western tropical Pacific experiences periods of increased SLP while
the opposite is true during cold periods of the PDO. However, the effect of the PDO is weak in tropical
areas, such as the Marianas OPAREA, and thus climatic anomalies are most likely due to ENSO forcings
(Mantua 2002; Mantua and Hare 2002).
2.2 MARINE GEOLOGY
The study area is located at the intersection of the Philippine and Pacific crustal plates, atop what is
believed to be the oldest seafloor on the planet dating to the Jurassic era (Handschumacher et al. 1988).
The collision of the two plates has resulted in the subduction of the Pacific Plate beneath the Philippine
Plate forming the Mariana Trench (Kennett 1982; Figure 2-1). The Mariana Trench is over 2,270 km long
and 114 km wide. The deepest point in the trench and on Earth, Challenger Deep (11,034 m; 11°22’N,
142°25’E), is found 544 km southwest of Guam in the southwestern extremity of the trench (Fryer et al.
2003).
The seafloor of the study area region is characterized by the Mariana Trench, the Mariana Trough, ridges,
numerous seamounts, hydrothermal vents, and volcanic activity. Two volcanic arcs, the West Mariana
Ridge (a remnant volcanic arc than runs from approximately 21°N 142°E to 11°30’N 141°E) and the
Mariana Ridge (an active volcanic arc) are separated by the Mariana Trough (Baker et al. 1996; Figure 2-
1). The Mariana Trough formed when the oceanic crust in this region began to spread between the ridges
as recently as four million years ago. Currently the Mariana Trough is spreading at a rate of less than 1
centimeter per year (cm/yr) in the northern region and at rates up to 3 cm/yr in the center of the trough
(Yamazaki et al. 1993). The Mariana archipelago is located on the Mariana Ridge, 160 to 200 km west of
the Mariana Trench subduction zone. The Mariana archipelago is comprised of fifteen volcanic islands:
Guam, Rota, Tinian, Saipan, FDM, Aguijan, Anatahan, Sarigan, Guguan, Alamagan, Pagan, Agrigan,
Asuncion, Maug, and Farallon de Pajaros (listed from south to north) (Figure 2-2). Approximately 800 km
separate Guam from Farallon de Pajaros (Eldredge 1983; DoN 1998; DoN 2003a).
The islands north of FDM are located on an active volcanic arc ridge axis and were formed between 1.3
and 10 million years ago (Randall 1985, 1995, 2003; Fryer personal communication). The six southern
islands (Guam to FDM) are on the old Mariana fore-arc ridge axis and formed about 43 million years ago
(Eocene) (Randall 1985, 2003; Birkeland 1997). The young volcanic active ridge axis is offset 25 to 35 km
west of the southern arc ridge axis (Randall 1995). The islands on the southern ridge consist of a volcanic
core covered by thick coralline limestone (up to several hundreds of meters) (DoN 2003a). The
subsidence of the original volcanoes in the southern islands allowed for the capping of the volcanoes by
limestone. Limestone covers the northern half of Guam (limestone plateau height: 90 to 180 m above
mean sea level [MSL]) while volcanic rock and clay are exposed on the southern half of the island (DoD
1999). Tinian consists of rocky shoreline cliffs and limestone plateaus with no apparent volcanic rock
(DoD 1999). Similar to Tinian, the uplifted limestone substrate of FDM is bordered by steep cliffs (DoN
2004).
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