1. NEWS & VIEWS RESEARCH
called macrostratigraphy5. This method has are. The common-cause hypothesis is one e-mail: stratum@uga.edu
been instrumental in moving palaeobiology of the most interesting developments in
away from a preoccupation with sampling macroevolution, and advances in strati­ 1. Peters, S. E., Kelly, D. C. & Fraass, A. J. Nature 493,
398–401 (2013).
biases and towards seeking mechanisms of graphy and analytical methods will continue 2. Raup, D. Paleobiology 2, 289–297 (1976).
diversity change. The evidence in foraminif- to have an important role in testing it6. ■ 3. Smith, A. B. Phil. Trans. R. Soc. B 356, 351–367 (2001).
era for the common-cause hypothesis adds 4. Peters, S. E. Proc. Natl Acad. Sci. USA 102,
12326–12331 (2005).
to previous evidence from marine inverte- Steven M. Holland is in the Department 5. Peters, S. J. Geol. 114, 391–412 (2006).
brates, but raises the question of how wide- of Geology, University of Georgia, 6. Patzkowsky, M. E. & Holland, S. M. Stratigraphic
spread these macroevolutionary relationships Athens, Georgia 30602-2501, USA. Paleobiology (Univ. Chicago Press, 2012).
E ARTH SCIENCE Another way in which a high nickel content
might be found in olivine crystallizing from
Plumbing nickel a primary magma would be for olivine not to
have been present in the mantle source rock
at all6. Currently, this is a leading explanation
from the core
for the unusually high nickel abundance found
in olivine from Hawaii and other magma
locales lying within plates rather than at plate
boundaries7, although here again, tempera-
Magmas that have erupted at Earth’s surface reveal a potential new mantle ture effects have been invoked as an alterna-
source. This source, which is rich in nickel and has a primordial helium isotopic tive explanation5. In this ‘olivine-free’ source
content, may have originated at great depth in the mantle. See Letter p .393 model, partial melts extracted from recycled
oceanic crust react with the normal mantle
and dissolve away the olivine. Herzberg et al.
M I C H A E L J . W A LT E R magmas almost never make it to the surface contend that this process does not work for
because, as they rise and cool, they crystallize their magmas because chemical diagnostics
M
elting of source rocks in the mantle and so differentiate. This means that petrolo- involving the iron, manganese and calcium
generates magmas that erupt at gists must devise models that can eliminate content of olivine simply do not fit the model.
Earth’s surface. Identifying chemi- the effects of crystallization. Olivine is the first If temperature effects and olivine removal are
cally unique mantle source rocks is a big deal mineral to crystallize from most mantle melts. not the culprits, Herzberg and colleagues are
because they can reveal much about Earth’s Fortunately, the co-variation of olivine’s nickel effectively left with only one alternative — a
internal structure, which in turn informs content and ‘magnesium number’ (the ratio new high-nickel-content source rock.
our understanding of plate tectonics and of the molar concentration of magnesium to So where does the excess nickel come from?
planetary evolution. By far the best-defined magnesium plus iron) means that the amount Herzberg et al. propose that it originates from
mantle source is the part of the mantle that is of crystallization a magma experiences can be the most nickel-rich reservoir known —
extensively tapped by magmas called MORBs, tracked, and this provides a way of estimating Earth’s core. It has been conjectured that, early
which erupt at plate boundaries along mid- the composition of the primary mantle melt. in Earth’s history, a primordial basal magma
ocean ridges. Two basic properties of the Once the primary melt has been reconstructed, ocean existed at the core–mantle boundary8,
source rock for these MORBs are that it con- the composition of the mantle source rock can presumably in chemical equilibrium with the
tains about 2,000 parts per million of nickel also be deduced. It is this kind of modelling that iron- and nickel-rich metallic liquid core. A
and that its helium isotopic composition is a underpins Herzberg and colleagues’ identifica- solidified or partially molten vestige of this
result of radio­active decay and is thus not pri- tion of a unique, nickel-rich mantle source. basal magma may have survived to the present,
mordial1,2. The MORB source rock defines the However, accounting for the effects of crys- existing as a unique mantle reservoir.
‘normal’ upper mantle, and possibly dominates tallization is a tricky business, because the way Nickel is a ‘siderophile’, which means that
the mantle as a whole. in which nickel partitions between olivine and it partitions strongly into the core relative to
In this issue, Herzberg and co-workers3 the melt can depend on the pressure, temper- the mantle. But partitioning is known to be
investigate the chemistry of ‘primary’ mantle- ature and composition of the differentiating pressure dependent, with increasing pressure
derived magmas from several locations away magma. Several competing empirical models increasingly favouring the mantle9. Herzberg
from plate boundaries*. (Primary magmas are for nickel partitioning are available, some of and colleagues surmise that, at the pressure
those obtained directly by melting of the man- which involve significant temperature depend- that exists at the core–mantle boundary
tle, and which have not undergone any change ence4,5. The nickel partitioning model used by (about 130 giga­ ascals), the mantle should
p
in chemical composition.) The authors iden- Herzberg et al. does not include a tempera- be enriched in nickel, whereas the rest of the
tify a new mantle source containing about 20% ture term, and so if considerable temperature mantle would have equilibrated with the metal
more nickel than that of normal upper-mantle dependence does exist, then the calculated at much lower pressures during core forma-
rock. This source also seems to have a primor- differences that the authors find in the nickel tion (about 50 GPa)9. This ancient remnant
dial helium isotopic composition. Herzberg et contents of primary magmas might simply be of basal magma would also retain a primor-
al. identify the location of this reservoir as the a temperature effect and not a reflection of a dial helium isotopic composition character-
core–mantle boundary — a primordial source distinct mantle source5. Herzberg et al. are ized by a high concentration of helium-3 to
that is sometimes piped to the surface through well aware of this pitfall, and provide cogent helium-4, and possibly other primitive iso-
mantle plumes. arguments to explain why temperature vari- topic features10. The authors speculate that this
Primary magma from the mantle can be used ation is an unlikely reason for the differences high-nickel-content source contributes to
to probe the composition of the mantle source in nickel content that they have calculated in upwelling mantle plumes that originate near
rock. But it is one of nature’s foibles that primary the primary melts. Nevertheless, among many the core–mantle boundary, and that it is tapped
*This article and the paper under discussion3 were aficionados, this temperature effect and its when the plumes melt near Earth’s surface.
published online on 9 January 2013. consequences remain an issue of active debate. Herzberg and colleagues’ model is
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2. RESEARCH NEWS & VIEWS
provocative and requires further verifica- Michael J. Walter is in the Department
tion. For example, additional experiments are of Earth Sciences, University of Bristol,
needed to clarify the role of temperature in the Bristol BS8 1RJ, UK.
partitioning of nickel between olivine and the e-mail: m.j.walter@bristol.ac.uk
melt. Also needed are experiments performed
1. Hilton, D. R. & Porcelli, D. in The Mantle and Core,
under the conditions present at the core–man- Treatise on Geochemistry Vol. 2 (ed. Carlson, R. W.)
tle boundary, because this could elucidate how 277–318 (Elsevier, 2005).
50 Years Ago siderophile elements partition between molten
mantle and liquid core in these extreme circum-
stances. Modelling of the abundances of sidero-
2. McDonough, W. F. & Sun, S. Chem. Geol. 120,
223–253 (1995).
3. Herzberg, C. et al. Nature 493, 393–397 (2013).
4 Li, C. & Ripley, E. M. Chem. Geol. 275, 99–104 (2010).
In arboriculture the word ‘mulch’ is phile elements other than nickel in primary 5. Putirka, K., Ryerson, F. J., Perfit, M. & Ridley, W. I.
usually associated with the compost melts, such as cobalt, may provide further tests J. Petrol. 52, 279–313 (2011).
6. Sobolev, A. V., Hofmann, A. W., Sobolev, S. V. &
of wet leaves, straw, etc., laid as a of the deep-mantle-reservoir model. Finally, the Nikogosian, I. K. Nature 434, 590–597 (2005).
protection to the roots of newly composition of the magmas studied should be 7. Sobolev, A. V. et al. Science 316, 412–417 (2007).
planted or young trees, especially on investigated using a range of radiogenic isotope 8. Labrosse, S., Hernlund, J. W. & Coltice, N. Nature
450, 866–869 (2007).
inherently dry soils … In the autumn tracers (for example, lead, neodymium and haf- 9. Walter, M. J. & Trønnes, R. G. Earth Planet. Sci. Lett.
number of the Esso Magazine nium), to further analyse the primordial nature 225, 253–269 (2004).
(1962), an informative and well- of this potential mantle source rock10. ■ 10. Jackson, M. G. et al. Nature 466, 853–856 (2010).
illustrated account of a petroleum
mulch evolved by Esso Research is
given … Briefly, this type of mulch STEM C EL L S
is in fact an inexpensive emulsion
of petroleum resins, used as an
agricultural spray for improving
the growth and yield of crops of all
Surf the waves of
reprogramming
kinds. Its function is four-fold: to
warm the soil, to reduce evaporation,
to protect against soil-erosion, and
to retard the dispersal by rain-wash
and other agents of vital mineral Cellular reprogramming to a stem-cell-like state is inefficient and poorly
and chemical constituents in the soil understood, despite its biomedical potential. Detailed molecular analyses of
growth-zones wherever it is used … this process are now reported, and should help to overcome these limitations.
Tests on cotton plants in Arizona in
1960 revealed that where this mulch
was used, twelve days after planting, IGNACIO SANCHO-MARTINEZ & JUAN of the reprogramming process — until now.
121,900 plants per acre had emerged CARLOS IZPISUA BELMONTE In a comprehensive single-cell study,
as against 8,710 per acre on the Buganim et al. 2 investigated the dynam-
I
control area. The final yield in bales nduced pluripotent stem cells can generate ics of the total cellular transcripts (the tran-
per acre was 2.2 with mulch and 1.65 all tissues of the adult organism and — scriptome) underlying reprogramming. The
without, an increase of 34 per cent. unlike embryonic stem cells — their gener- authors analysed reprogramming-competent
From Nature 19 January 1963 ation does not have ethical implications1. They cells, which they could distinguish from other
are thus invaluable for research, and have great partially reprogrammed cells. They identified
potential for use in regenerative medicine and a hierarchical genetic network, which orches-
100 Years Ago drug discovery. The precise molecular mecha-
nisms that underlie the reprogramming of dif-
ferentiated cells to induced pluripotent stem
trated the acquisition of pluripotency, and
found previously unknown reprogramming
factors. Meanwhile, Polo et al.3 and Hans-
Sir Ray Lankester’s weekly cells remain largely unknown. Three papers2–4, son and colleagues4 have demonstrated that
contributions to The Daily Telegraph published in Cell and Cell Reports, provide the reprogramming dynamics can be monitored
represent the high-water mark most comprehensive analysis yet of the molec- in bulk cell populations. By comparing single-
of popular papers on scientific ular dynamics that bring about this process. cell data with cell-population results, Polo
subjects. The general public has in This work may not only guide the design of and co-workers could distinguish cells poised
recent years been infected with a improved reprogramming approaches, lead- to become iPS cells from reprogramming-
feverish desire for sensation; and as ing to increased efficiency, but also allow the resistant populations.
science can offer little to gratify that generation of higher-quality stem cells. Most Those authors’ findings3 indicate that,
appetite, thoughtful articles upon intriguingly, we think that the demonstration during reprogramming, cells transition
its achievements are now relatively that the cells transit through distinct inter­ through intermediate states that ultimately
much fewer in the periodical Press mediate states is an elegant indication that lead to the appearance of a population express-
than they were a generation or reprogramming proceeds in an inverse manner ing Oct-4. (Oct-4, along with three other
two ago … The papers are perfect to the process of embryonic development. transcription factors, Sox2, c-Myc and Klf4,
models of scientific exposition: In addition to the low efficiency of exist- mediate reprogramming to iPS cells and are
simple, yet not childish; informative, ing protocols for reprogramming to induced known as the Yamanaka factors1.) It is with the
but not tedious; bright without pluripotent stem (iPS) cells, a lack of molecular onset of Oct-4 expression that cells generally
being flippant; sparkling with markers hinders the monitoring of early stages become capable of generating iPS cells.
human interest and original always. of this process. Consequently, to unravel the Notably, further increase in the expression
From Nature 16 January 1913 molecular basis of reprogramming, research- of Yamanaka factors could rescue — at least
ers have focused on the initial and late phases to some extent — cells that were refractory to
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