INSIGHTS | PERSPECTIVES
1204 13 MARCH 2015 • VOL 347 ISSUE 6227 sciencemag.org SCIENCE
Earth’s sea floor is created along great volcanic ridges that spread at rates of a few centimeters per year beneath every ocean basin. The volcanic and tectonic processes at these mid-ocean ridges ultimately determine the characteristics of the sea floor and the oceanic crust
beneath it. These processes occur beneath
several kilometers of rock and seawater,
seemingly far removed from climatic variations above sea level. Thus, it is surprising
that both Crowley et al. (1), on page 1237 in
this issue, and Tolstoy (2) found ice age periodicity in hilly topography on the flanks of
ridges submerged beneath the Southern and
After its creation at the mid-ocean ridges,
Earth’s sea floor forms the abyssal hills (see
the figure). If we could view this terrain
without its cover of water, it would appear
as gentle rolling hills hundreds of meters
high and spaced kilometers apart. The
abyssal hills are oriented roughly parallel
to the ridges where they are formed and are
carried across the ocean basins by sea-floor
Over the course of tens of millions of
years, the abyssal hills become buried by
sediments, but they are nevertheless observable in bathymetric surveys across
vast expanses of sea floor (3). This coverage makes the abyssal hills the most extensive geological feature on Earth, but
their remoteness means that they remain
largely unexplored and poorly understood.
Detailed bathymetric surveys, obtained using sonar that images a swath of sea floor
beneath a ship, have only been obtained
for a small portion of the sea floor. Close
examination of these images informs our
understanding of how the abyssal hills are
created. In particular, extensional faults associated with mid-ocean ridge rifting are
thought to be important (4). Such faults are
observable in bathymetric surveys of newly
created abyssal hills near ridge crests and
are prevalent in computer models of rifting (5).
Comprehensive bathymetry of the Aus-
tralian-Antarctic ridge south of Tasmania,
observed from a South Korean icebreaker,
allowed Crowley et al. to examine an un-
usually long record of abyssal hill fabric.
They found that the abyssal hills exhibit
characteristic spacing of about 23,000,
41,000, and 100,000 years. Working sepa-
rately but concurrently, Tolstoy also de-
tected 100,000-year periodicity in the
abyssal hill fabric of the East Pacific Rise, a
ridge west of South America (2).
These periods are familiar to paleoclimatologists: They are the primary Milankovitch cycles of cooling and warming of
Earth’s surface climate. Caused by periodic
fluctuations in Earth’s rotation, obliquity,
and orbit, respectively, the Milankovitch cycles result from changes in the amount and
location of radiation reaching Earth’s surface and can produce large climate swings
between ice ages and warm interglacial periods. For example, Earth has warmed since
the last Ice Age, which peaked between
20,000 and 30,000 years ago. The ice sheets
that once covered much of North America
and Eurasia have melted, causing sea level
to rise by about 120 m.
How do Milankovitch cycles induce
topographic variations on the sea floor?
Oceanic crust is formed from magma produced as hot rocks rise toward the ridge
crest, where decreased pressure causes
them to melt. The eruption and refreezing
of this magma produces the oceanic crust,
How climate influences
By Clinton P. Conrad
Sea-floor hills show the same periodicity as glacial cycles
Department of Geology and Geophysics, School of Ocean and
Earth Science and Technology, University of Hawaii, Honolulu,
HI 96822, USA. E-mail: firstname.lastname@example.org
“…the diversity of abyssal
hill fabrics observed on the
sea floor may result from
a range of interactions
melting variations and
equivalent to organonickelII precursors
leads to oxidatively induced C–C reductive
By combining an organonickelII precursor supported by a ligand that coordinates
the nickel with three heteroatom donors
with a CF3+ oxidant, isolable organonickelIV complexes were obtained. These complexes were partners for formation of C–X
bonds (where X denotes oxygen-, sulfur-,
and nitrogen-based reagents) in excellent
yields and under mild reaction conditions.
Preliminary kinetic experiments support a
mechanistic pathway in which an organonickelIV bond is attacked by the heteroatom
nucleophile. The high oxidation state of the
metal serves to ensure the bound carbon
atom is highly electrophilic and susceptible
to facile C–X bond formation.
Although further mechanistic details remain to be unraveled, the involvement of
a thoroughly characterized organonickelIV
in carbon-heteroatom bond formation
opens vistas for constructing new types of
molecules and materials under mild conditions. Key aspects of these transformations
include the ability to use an economically
more attractive metal as the catalyst; exploitation of the lability of the nickel-ligand
bonds to promote selective reactions under
mild conditions; and the formation of new
C–X bonds in which the carbon atom is
sp3-hybridized, which constitutes the most
challenging reaction of this class. Future
opportunities will entail exploring and developing a fundamental understanding of
the range of ligands and oxidants that can
successfully partner to facilitate nickelIV
cross-coupling reactions. The utility of this
work will be greatly enhanced if the efficacy
of other oxidants, ideally O2 (13), could be
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