INSIGHTS | PERSPECTIVES
depth—>7% over a limited depth range—
ruling out temperature as the cause of the
wave speed variations.
This globally present, rapid wave speed
decrease is coincident with the intersection of the temperature-depth profile and
the carbon-bearing, mantle solidus (9),
suggesting that it is caused by 1% partial
melt. The deeper thermal layer also likely
contains partial melt and may interact
chemically and physically with the underlying mantle. Moreover, this layer of melt
at the base of the cratonic plate may aid
in isolating the overlying strong, buoyant
mantle from the convecting mantle beneath, giving increased importance to driving forces at plate edges.
Imaging techniques over the past decades that have focused on secondary seismic arrivals (also referred to as reflectivity
analysis, receiver functions, or converted arrivals) suggest that the bases of the tectonic
plates are not controlled by temperature
but by secondary factors, including water
content, presence of melt, or composition
(10). The results presented by Tharimena
et al., implicating a carbon-associated melt,
add to a growing set of findings suggesting
that plates are controlled by secondary features that have a first-order effect on rheology, the defining characteristic of plates.
Recent work highlights that the cratonic
lithosphere is not as simple as once thought.
Although it may be dry and chemically dis-
tinct, relative to the convecting mantle, in-
ternal structures such as mid-lithosphere
discontinuities (11), dipping reflectors (12),
possible strong layering (8), and the xeno-
lith constraints demonstrate strong internal
variation. The length scales and details of
these variations may play a decisive role in
advancing our understanding of the craton’s
origin, longevity, and deformation.
What was the process that chemically
strengthened the cratonic lithosphere? What
is the role of the cratonic lithosphere’s thermal boundary? How do the geochemical,
seismological, and geodynamic properties
vary within cratons, and over what scales?
Solving these questions will require detailed examination of the cratons, but doing so within a global context. The results of
Tharimena et al. do just that, by proposing a
stronger role for chemical changes than for
thermal history in defining tectonic plates,
by explicitly requiring the presence of in situ
melt, globally. In the context of recent seismic
and xenolith data, these results demonstrate
that the cratonic lithosphere is much richer
and more complex than a massive, homogeneous, impenetrable body. New and striking
complexities within the cratonic lithosphere
are crucial to unraveling the question of how
the continents were made. j
1. S. Tharimena, C. Rychert, N. Harmon, Science357, 580
2. P. Tackley, Science 288,2002(2000).
3. A. Lenardic, L.-N. Moresi, H. Mühlhaus, J. Geophys. Res.
108, 2303 (2003).
4. G. Hirth, D. Kohlstedt, Earth Planet. Sci. Lett.144, 93
5. T.H.Jordan, Nature 274, 544(1978).
6. H. Zhu, E. Bozdağ, D. Peter, J. Tromp, Nat. Geosci.5, 493
7. B.Savage, B.M.Covellone, Y.Shen, Earth Planet. Sci. Lett.
459, 394 (2017).
8. H. Yuan, B. Romanowicz, Nature 466, 1063 (2010).
9. R. Dasgupta, M. Hirschmann, Nature 440, 659 (2006).
10. K. M. Fischer, H. A. Ford, D. L. Abt, C. A. Rychert, Annu. Rev.
Earth Planet. Sci. 38, 551 (2010).
11. E. Rader et al ., Geochem. Geophys. Geosyst .16, 3484
12. M. S. Miller, D. W. Eaton, Geophys.Res.Lett. 37, 18 (2010).
40 to 175 km
175 to 250 km
> 250 km
New molecular communica-
tion in type III CRISPR-Cas
systems has been identified
By Gil Amitai and Rotem Sorek
The CRISPR-Cas (clustered regularly interspaced short palindromic re- peats–CRISPR-associated protein) system is known to protect bacteria gainst foreign invading DNA, usu- ally from phages (viruses that infect bacteria) or plasmids (circular DNA
found in the cytoplasm of bacteria). Since
the first demonstration of CRISPR-Cas
functionality a decade ago (1), mechanistic understanding of CRISPR-Cas has not
only enabled genome editing but also revolutionized our appreciation of bacterial
defense against their viruses. CRISPR-Cas
systems show a high degree of sophistication in providing immunity against phages,
including elaborate mechanisms to accurately identify the invading DNA, safety
checks to prevent self-targeting (2), and
high diversity of target destruction mechanisms among different types of CRISPR-Cas systems (3). Kazlauskiene et al. (4),
on page 605 of this issue, and a study by
Niewoehner et al. (5) report the discovery
of an unexpected aspect of CRISPR-Cas immunity: intracellular signaling.
Kazlauskiene et al. and Niewoehner et
al. show that in type III CRISPR-Cas systems, identification of phage nucleic acids
by the CRISPR-Cas efector complex leads
to the generation of a small molecule called
cyclic-oligoadenylate (cOA). This molecule
then activates a CRISPR-associated ribonuclease (RNase), the function of which
was previously unclear. Both papers demonstrate that the RNase, once activated,
cleaves cellular RNA nonspecifically, probably leading to dormancy or death of the
Department of Molecular Genetics, Weizmann Institute
of Science, Rehovot 76100, Israel.
Cross sections of Earth
The full path of the SS and SS precursors along with details at the surface bounce point where the craton and its
melt layer at the base of the chemical craton is identified globally. The S wave is direct, the fastest or minimum
time, shear wave speed arrival, whereas the SS wave includes a single, surface bounce. SS precursors propagate
along with SS but reflect, or bounce, off of Earth’s strong contrast, internal boundaries, and arrive just before SS.