SZO Seminar
Notes - April 8, 2015: Heat, fluid and methane on the
Washington segment of the Cascadia margin
Discussion leader: Paul
Johnson
The discussion highlighted
the many types of intertwined studies being conducted along
Cascadia margin. While
Paul focused on one topic, the distribution and evolution of
hydrates, the topic has direct and indirect links to numerous
subduction processes (e.g., faulting, accretionary wedge
structure, turbidites).
Paul gave an overview of gas
hydrates and why they are important, particularly with respect
to their impact on climate change. Hydrates release
methane, and the methane flux off the WA margin is same amount
per year as released in the Deepwater Horizon spill. At the current rates
of flux increase, by 2100 the annual rate of methane release
will be equivalent to 6 Deepwater Horizon spills. This would completely
change the biology of the entire coastal region. Evidence of such
massive impacts of methane release come from the Eocene, when a
massive plume event warmed all of the oceans, causing all the
hydrates to decompose and release their methane. This further raised
the planet’s temperatures, ultimately leading to massive
extinctions.
Cascadia’s hydrates -
background
Hydrates form cages of water
molecules that enclose guest gas molecules, mostly methane. When hydrates
decompose they release the gas, thus they sometimes are called
‘the ice that burns’. Hydrate stability depends on pressure,
temperature, and CH4 concentration. When temperature rises
it becomes unstable. Along
much of Cascadia margin the stability boundary is at ~500 m
depth.
Cascadia sediments have an
unusually high percentage of organic material, which are heated
by the warm young incoming plate, leading to hydrate formation. Methane-rich gases and
fluids travel upward from within accretionary wedge along
faults. Along much
of the Cascadia margin hydrates form 200-300 m thick layers that
coat the sediments until they reach the stability boundary at
~500 m depth where they decompose, releasing methane.
The above processes may lead
to over-steepened slopes that are stabilized by the hydrates,
and that may fail as they warm.
This could explain some of turbidites.
High latitudes warming more
than anywhere else. As
seabed temperature increase 2-3˚C over 30 years, hydrates become
unstable – big hydrate plums.
Cascadia’s hydrates and
climate change
Paul and colleagues analyzed
6,596 CTD (conductivity/temperature/salinity/depth) profiles
along Washington margin and inferred a 0.3˚C temperature rise over
the last 40 years at the hydrate stability depth of ~500 m. This warming is not
apparent at the surface, but its existence at depth makes sense
as deeper waters come from Sea of Okotsk. Data from Sea of
Okotsk show they have warmed significantly, consistent with
broader studies showing warming is most profound at polar
latitudes.
Their working hypothesis is
that the inferred increase of 0.3˚C is like moving hydrate
stability downward ~50 m and seaward ~1 km (depends on slope),
causing the hydrates between the old and new effective
boundaries to decompose and release methane via increased plume
activity. Methane
plumes exist at all depths above the stability boundary because
methane escapes on its own for other reasons than warming, so
this hypothesis predicts an increase in plume activity near the
stability boundary. To
demonstrate likely causality between warming waters and
increased activity requires independent evidence that the gases
released originate near the stability boundary and not from
greater depths.
Methane plumes can be
detected using imaging techniques and by following the fish;
i.e., fishermen report fishing hot spots, which when checked
turn out to be where plumes attract microbes that fish feed on. Plumes have been
mapped, after normalizing for density and activity (necessary
because of the high variability in slope), it appears that in
the interval between 400-500 m, the number of emission sites is
anomalously high. This
result is somewhat ambiguous because the sampling is
non-uniform. Nonetheless,
it is at least consistent with the hypothesis.
The final step in testing
this hypothesis is to show that the increased plume activity is
sourced by decomposing hydrates due to warming. One way to do this is
to show that the chemistry of emitted methane is consistent with
hydrate decomposition (e.g., the methane did not come from
deeper sources). This
is work in progress.