Hi! I’m Bob Stern and I’d like to talk about a wonderful opportunity for international,
interdisciplinary research on Earth’s last frontier – its oceanic trenches. People have
always been inspired by great explorations and science has also benefitted tremendously.
Amundsen’s trek to the South Pole and back from 1911 to 1912, the astronauts round-trip visit to
the moon in 1969, and NASA’s rover exploration of Mars going on today are some of the most famous
of the great explorations. Oceanic trenches are key geodynamic phenomenon, formed where oceanic
lithosphere sinks back into the mantle, forming the deepest parts of the oceans.
In spite of this importance, trench research lags behind exploration of other seafloor regions.
This is mostly because of technological challenges
posed by the great pressures and the lack of obvious sites to concentrate research.
Trenches are just as important as mid-ocean ridges and submarine arc volcanoes, but the
latter are better understood because they are much shallower and have abundant young lavas with many
hydrothermal vents that attract scientists like bees to flowers. These realities have discouraged
research in the deepest parts of the ocean, known as the Hadal Zone in the past. But we now have the
technology to explore the oceanic trenches and it is time that we got organized to do this.
Oceanic trenches are of great scientific interest to a wide range of marine scientists including
geoscientists, marine chemists, marine biologists, and physical oceanographers.
So, what are some of the great scientific problems that Hadal Zone research could address?
There are many, but as a geoscientist, I’ll focus on three big geoscientific opportunities,
using the southern Mariana trench as an example. The first opportunity concerns
what comes out of the great normal faults on the downgoing plate, formed by plate bending.
Earthquakes tell us that these faults on the outer trench slope cut the oceanic lithosphere down to
50 km, well into Earth’s mantle. These deep faults provide great pathways for seawater to flow down
and interact with lithosphere then back up and for magmas to migrate to erupt on the seafloor.
If we study the area around where these faults intersect the seafloor, we are likely to
find two things. The first are low-temperature hydrothermal vents, reflecting serpentinization
in the upper mantle caused by seawater flowing down these faults. Some of these fluids must
return to the surface after being extensively modified by serpentinization reactions.
These vents are likely to be similar to the Lost City vent field on the Mid-Atlantic Ridge.
Such “serpentothermal” vents differ from the many high-temperature hydrothermal vents found
along spreading ridges in that they are cool, low energy, high-Ph, “low flow” systems that cannot be
found by traditional surface ship investigations such as Tow-Yo CTD and turbidity casts. We know
that such vents exist in some convergent margins because serpentinite mud volcanoes of the Mariana
forearc grew up around them and because we found the Shinkai Deep Field in the southern Mariana
forearc southwest of Guam. Discovering the first such vent on the downgoing plate requires seafloor
observations with manned submersible or ROV. When we find the first serpentothermal vents on
the downgoing plate, the aqueous geochemists and biologists will want to study the fluids and life.
We may also find small, young volcanic fields, similar to the “Petit Spot”
volcanic fields found east of the Japan Trench. The second geoscientific research opportunity
focuses on studying the mantle exposed in the inner trench wall. Because of
this and the next objective, we should focus on convergent margins with little sediment cover,
so as much of the inner trench wall is exposed as possible. In the southern Marianas SW of Guam,
a US-Japanese team found the Moho exposed between about 4 and a half and 5.9 km deep,
meaning that 5 to 6 km of intact upper mantle is exposed in situ between this and the trench floor.
Focused study of a mantle transect from bottom to top, including determining the abundance
of peridotite, pyroxenite, intrusions, and other rock types followed up by lab
studies of variations in mantle compositions and fabrics present would result in a “gold standard”
for our understanding of the upper mantle. The third geoscientific research opportunity
concerns the nature of the subduction interface, where the downgoing and overriding plates meet.
This is not just a plane but is a zone known as the subduction channel
where rocks from both plates mix to make mélange, some of which is returned to the surface.
The base of the inner trench wall is where the subduction channel intersects the seafloor.
The thickness of the subduction channel is likely to be tens to hundreds of meters thick.
Some or all of the exposed subduction channel may be buried beneath even thin
surface sediments, but we won’t know unless we go down and look for it.
For reasons noted before, we should focus on convergent margins with as little sediment cover
as possible. These are common among Western Pacific trenches and convergent margins.
We cannot study every trench in the world and, following best practices from the very productive
MARGINS and GeoPRISMS initiatives, we should focus resources on as few localities as possible,
optimally one site where the international research community can focus effort.
The obvious place to concentrate such effort is around the Challenger Deep just south of
the southern Marianas. A coordinated joint effort, organized and led by scientists from China, Japan,
and the United States, is the obvious way to do this, with efforts from other nations welcomed.
The Challenger Deep is more firmly embedded in the public’s imagination than any other trench
and for this reason is the stand-out target; it is also conveniently located near all three nations.
Given that we have strong scientific objectives and can agree about the
best place to center this effort, what is the next step after we have started the discussion?
Establishing an informal coordinating committee seems like a useful first step, perhaps with two
scientists – one senior and one junior – from each of the “Big Three” plus other nations.
This committee would collect and share information about upcoming cruises and post this information
and links to related information on a website. Then they would develop plans to move the
initiative forward. I would be happy to help get this off the ground, just send me an email
if you are interested or if you have any other ideas. Thanks for listening!