By Brian Savage
Continents have tolerated billions of years of tectonic stresses and disfig- urement, yet they continue to survive. Compared with their oceanic coun- terpart, where a sinking demise is an almost certainty, continents and their
internal cores, or cratons, are much thicker
(>175 km), older (>2 billion years), colder,
and more buoyant. However, their basic attributes, such as size and shape, are a still
a matter of debate because of large uncertainties in deceivingly straightforward, but
entirely complicated, measurements. Continental cratons are rigid
bodies composed of
both crust and mantle,
and their thickness was
thought to be related
to temperature and extend to depths of 250 to
350 km. On page 580 of
this issue, Tharimena
et al. (1) use reflections
of seismic waves within
the cratons to constrain their thickness globally. The strength of the reflections suggests
that the base of the cratonic plate is defined
by a partial melt of carbon-laced silicate
mantle, not temperature.
Convection of Earth’s mantle causes the
development of thermal boundary layers and tectonic plates at the surface. In
addition, the stability of cratons requires
increased buoyancy and a substantial rheology contrast (2, 3) between the convecting mantle and the tectonic plates. These
variations in rheology and buoyancy are
not available solely from temperature gradients near Earth’s surface; the oceanic
plates are closer to true thermal boundary layers. A history of extensive previous
melting left the cratonic mantle chemically
distinct from surrounding mantle and
made it more buoyant and viscous (4, 5).
Fragments of deep cratonic rocks that
are brought to the surface by volcanism—
xenoliths—provide the best direct evidence
of its composition and thermal structure
near the volcanoes. Xenoliths originating
from depths of up to 200 km define a tem-
perature-depth profile that coincides with
observations of continental heat flow. Near
depths of 175 km, but variable by continent,
xenoliths show a conspicuous chemical
change, in which shallower samples reflect
more melt extraction than xenoliths from
greater depths. Melt extraction dehydrates
the mantle, leaving it more viscous. Within
the boundaries of the thermally defined
craton, this shallower mantle, up to depths
of 175 km, is strong and dry and underlain
by weaker and wetter
mantle that extends to
the base of the ther-
tomography has done
well at identifying the
the tectonic plates but
less well so at discerning increasingly important higher-order internal and bounding
structures, including the plate’s base—that
is, a rheological boundary. Imaging by using surface wave data tends to result in
smoothed images of plates 200 to 250 km
thick, whereas body wave data show very
thick plates extending up to 350 to 400 km.
A boom in data availability and improved
computational techniques are recognizing
higher-order features (6, 7), but they are
still difficult imaging targets. Anisotropic
seismic wave speed models, typically surface waves, suggest a layered structure similar to the aforementioned chemical layer
surrounded by the thermal layer (8).
By using seismic reflections associated
with surface bounces (SS precursors), Tharimena et al. identified sharp seismic wave
speed variations with depth, including the
crust-mantle transition (the Moho), internal plate structures (the mid-lithosphere
discontinuity), and the transition between
chemical and thermal layers of the continent (see the figure). Further examination of the reflection’s amplitudes shows a
rapid decrease in seismic wave speed with
cally induced intestinal inflammation.
These findings offer an essential contribution to the understanding of numerous
pathological situations associated with dysbiosis (11). Indeed, a higher abundance of
butyrate-producing bacteria in the gut is
associated with a lower risk of intestinal
inflammation (12) and gut barrier dysfunction, but also a lower risk of obesity and
type 2 diabetes mellitus (13, 14). However,
this does not mean that we can target intestinal PPAR-g to treat such diseases, because there are other butyrate receptors
and PPAR-g has numerous roles.
Several important questions remain un-answered. Can we directly extrapolate such
findings to humans? Additionally, we know
that numerous types of dysbiosis exist according to the type of antibiotic used. Thus,
it is unclear whether all antibiotics affect
butyrate–PPAR-g signaling, or if enough
evidence exists to directly link luminal
oxygen and nitrate levels to dysbiosis and
pathologies. For instance, metformin (used
to treat type 2 diabetes mellitus) or gastric
bypass surgery (to treat obesity) are associated with increased butyrate-producing
bacteria and improved health, but also a
higher abundance of Enterobacteriaceae
(13). Therefore, an increase in Enterobacteriaceae is not always a risk factor for poor
health, due to antibiotic use, or reduced butyrate abundance in the gut (13).
The study by Byndloss et al. highlights a
unique mechanism of symbiosis between
host and microbes, which directly interferes
with the metabolic capacities of both bacterial and host cells. Therefore, this study
starts to decipher the complex interactions
between host and microbes, which may
help identify future therapeutic strategies
targeting the gut microbiota. j
REFERENCES AND NOTES
1. A. Cabinian et al. , Gut 10.1136/gutjnl-2016-313214 (2017).
2. H. Chu, S. K. Mazmanian, Nat. Immunol. 14, 668 (2013).
3. T. S. Postler, S. Ghosh, Cell Metab.26, 110 (2017).
4. M. X. Byndloss et al., Science 357, 570 (2017).
5. W. E. Roediger, Gut 21, 793 (1980).
6. K. M. Maslo wski et al., Nature 461, 1282 (2009).
7. F. Rivera-Chavez, C. A. Lopez, A. J. Baumler,Free Radic.
Biol. Med. 105, 93 (2017).
8. A. M. Spees et al., mBio 4, e00430 (2013).
9. M.N.Xavier et al., Cell Host Microbe 14,159(2013).
10. A. Everard et al. , Nat. Commun. 5, 5648 (2014).
11. P. D. Cani, Nat. Rev. Gastroenterol. Hepatol. 14, 321
12. K. Machiels et al., Gut 63, 1275 (2014).
13. K. Forslund et al., Nature 528, 262 (2015).
14. F. F. Anhe et al ., Can. J. Diabetes 41, 439 (2017).
P.D.C. is a recipient of grants from Fonds de la Recherche
Scientifique, European Research Council Starting grant
2013 (336452-ENIGMO), Fund for Strategic Fundamental
Research–WELBIO, and the Funds Baillet Latour (Grant for
Medical Research 2015).
A seismic shift in
continental tectonic plates
Seismology provides an improved estimate for
thickness of the continents
Department of Geosciences, University of Rhode Island, 9 East
Alumni Avenue, 317 Woodward Hall, Kingston, RI 02881, USA.
“...the cratonic lithosphere
is much richer and
more complex than a