# Planetary science: A nickel for your planet's thoughts

Paolo A. Sossi

► To cite this version:

Paolo A. Sossi. Planetary science: A nickel for your planet's thoughts. Nature Geoscience, 2017, 10, pp.249-251. 10.1038/ngeo2909. insu-03748875

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
in these climates. On the other hand, Wagner-Riddle et al. only estimated emissions from croplands. Substantial emissions during winter or after soil freezing and subsequent thawing have been observed for managed and natural grasslands, forests, and drained organic soils and natural wetlands (Fig. 1). Owing to this phenomenon's broad occurrence, a revision of the attribution of sources within the entire global N₂O budget is needed.

To improve estimates of N₂O emission from managed and natural terrestrial ecosystem sources, it is necessary to undertake continuous N₂O emission measurements in long-term observation networks, such as the Integrated Carbon Observation System or National Ecological Observatory Network. N₂O flux measurements at long-term research sites around the world can provide the high temporal resolution of emission measurements needed to capture hot-moments and the standardized measurements (for example, soil carbon, soil texture and soil inorganic N) that are needed

to understand what drives emissions and their temporal variation.

Our understanding of the drivers and processes underlying N₂O emissions related to freeze–thaw events is still poor. Key uncertainties in simulating freeze–thaw N₂O emission are the development of the snow cover, the effect of subsurface ice formation on soil gas diffusion and water flow during thaw, as well as the dynamics of soil microbial processes and communities involved in the production and consumption of N₂O during winter⁷. An improved understanding of the controls over freeze–thaw N₂O emissions and more realistic process based models could help provide insight into potential mitigation options for reducing greenhouse gas emissions, and provide important assistance for policymakers as they refine their national contributions as part of the Paris Agreement.

The high spatio-temporal heterogeneity in N₂O emissions from agricultural soils has led many researchers to try to constrain emissions associated with specific management activities such as fertilization,

irrigation and tillage. Wagner-Riddle and colleagues³ suggest that at least in croplands that freeze, it may be even more important to understand microbial processes in soils that are freezing and thawing.

Klaus Butterbach-Bahl and Benjamin Wolf
*are at the Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research (IMK-IFU), Kreuzeckbahnstraße 19, 82467 Garmisch-Partenkirchen, Germany.*
e-mail: klaus.butterbach-bahl@kit.edu;
benjamin.wolf@kit.edu

References

1. WMO Greenhouse Gas Bulletin: The State of Greenhouse Gases in the Atmosphere Based on Global Observations through 2015 (World Meteorological Organization, 2016).

2. Syakila, A. & Kroeze, C. *Greenh. Gas Meas. Manag.* **1**, 17–26 (2011).

3. Wagner-Riddle, C. et al. *Nat. Geosci.* **10**, 279–283 (2017).

4. Alexandratos, N. & Ursinsma, J. *World Agriculture Towards 2030/2050: The 2012 Revision* ESA Working Paper No. 12–03 (Food and Agriculture Organization, 2012).

5. Herrero, M. et al. *Nat. Clim. Change* **6**, 452–461 (2016).

6. Abalos, D. et al. *Glob. Chang. Biol.* **22**, 1244–1255 (2016).

7. Wang, C. et al. *Glob. Chang. Biol.* **22**, 2963–2978 (2016).

Published online: 6 March 2017

PLANETARY SCIENCE

# A nickel for your planet's thoughts

Variability of iron isotopes among planetary bodies may reflect their accretion or differentiation histories. Experiments suggest nickel may be the ingredient controlling iron isotope signatures, supporting fractionation during core formation.

Paolo A. Sossi

Understanding the formation and subsequent evolution of the terrestrial planets remains a fundamental question in Earth science¹. Stable isotopes of heavy elements shed light on these questions because they are sensitive to high-temperature processes that separate — or fractionate — isotopes according to their mass. Iron is an abundant element in planetary reservoirs, which include the crusts, mantles and cores of rocky bodies. On the planetary scale, small variations in the relative abundances of the different iron isotopes may arise from their volatilities during accretion. Additionally, iron isotopes may be fractionated when iron is incorporated into metallic cores during differentiation of planetary bodies. As they report in *Nature Geoscience*, Elardo and Shahar² have performed experiments that simulate core formation in planetary bodies and found that nickel, a major component of all planetary cores, plays a more important role in controlling iron isotope fractionation than previously thought, potentially

explaining the compositions of planetary materials through core formation.

The bulk compositions of the terrestrial planets are thought to match those of chondritic meteorites because chondrites came from parent bodies that did not experience sufficient heating to differentiate into a core, mantle and crust. However, with no samples of planetary cores or mantles (other than upper mantle rocks from Earth), it is difficult to confirm that the terrestrial planets are, in fact, chondritic. As a result, elemental and isotopic fractionation processes occurring during planet formation can be inferred only from the composition of basaltic rocks — after taking into account the magmatic processes associated with their emplacement in planetary crusts. Basalts are thus used as indirect probes of planetary composition³.

Iron isotopes in basaltic rocks from inner Solar System bodies, such as the Earth and Moon, Mars, Vesta, and the parent body of the angrite meteorites, exhibit a range

of compositions (Fig. 1). This variation is significant compared with chondritic meteorites, which all possess the same iron isotope composition⁴. Martian⁵ and Vestan basalts⁶ have near-chondritic iron isotope compositions, whereas basaltic rocks from the Earth and angrite meteorites are slightly — but systematically — isotopically heavier⁶,⁷. On the Moon, however, clear distinctions exist between volcanic glasses, which have roughly chondritic iron isotope compositions, and basalts, which are isotopically heavier⁷.

Various mechanisms have been invoked to explain the heavy iron isotope composition of basalts from differentiated rocky bodies. The formation of planetary cores — the largest reservoir of iron in planetary bodies — was initially considered the process likely to be responsible. However, subsequent experimental work⁸ led to the prevailing consensus that core formation does not induce isotopic fractionation of iron. The effect of another significant component of planetary cores, nickel, was not systematically
investigated because of its solubility with iron in alloys at the conditions of core formation, hence the expectation that nickel behaves like iron⁹.

Elardo and Shahar² performed experiments in a piston cylinder at the high pressures and temperatures expected during core formation in growing planetary bodies to quantify the potential effect of nickel on iron isotope fractionation between metal and silicate. In contrast to the idea that nickel does not markedly affect the properties of iron metal, they document an increasing preference of the heavy isotopes of iron for Fe-Ni alloys relative to silicate melt, with increasing concentration of nickel in the alloy. Nickel is sufficiently abundant in planetary cores to induce significant iron isotope fractionation according to the experiments, and this should be expressed in the light isotope composition of planetary mantles relative to chondrites. As the affinity of nickel for iron metal decreases with temperature, smaller bodies, in which core formation occurs at lower temperatures, should preserve isotopically lighter mantles.

Although the iron isotope compositions of planetary mantles have not been directly measured, the compositions of mantle-derived basalts show either no fractionation or

are isotopically heavy in iron isotopes relative to chondrites. To reconcile the expectation of isotopically light mantles with chondritic or isotopically heavy basalts, Elardo and Shahar propose that partial melting or magma ocean differentiation results in the enrichment of heavier isotopes relative to their mantle sources (Fig. 1a). Although such enrichment may not be efficient under the reduced conditions of planetary mantles¹⁰, Elardo and Shahar argue that the range of iron isotope compositions in lunar magmas suggests otherwise. In combination, fractionation during core formation and partial melting ensures that the iron isotope composition of the bulk planet — core, mantle and crust — remains chondritic, while matching constraints from planetary basalts.

The conclusions of Elardo and Shahar² are challenged by another study tackling the same problem with a complementary method. Liu and co-workers¹¹ determined the iron bond strength in synthetic samples of basalt, metallic iron and iron-rich alloys. The iron bond strength in the structure of the solid or liquid controls the resulting fractionation of iron isotopes. Liu et al. conclude that iron isotope fractionation between metal and silicate is negligible under the conditions of Earth's core formation, and insensitive to the

amount of nickel in the alloy (Fig. 1b). In this model, Earth's mantle remains unfractionated relative to its core, implying a non-chondritic bulk planet. Therefore, the absence of iron isotope fractionation during core formation requires another explanation for the diversity of iron isotopic compositions of basalts across the terrestrial planets.

Depletion of lighter iron isotopes in planetary bodies due to differences in volatility may be a feasible way to produce the heavy iron isotopic compositions of planetary basalts (Fig. 1b). According to this hypothesis, lighter isotopes were lost preferentially to space from the vapour produced by large planetary impacts, such as the one thought to have formed the Moon, leaving behind an isotopically heavy residue⁷. However, the lack of fractionation observed between the Earth and Moon for other elements with similar volatility to iron suggests that isotopic fractionation occurred prior to the formation of the Moon, during earliest accretion in the solar nebula¹².

Regardless of timing, the correlation between iron isotope composition and chemical evidence for volatile depletion in basalts suggests a causative link between the two⁵. Such volatility-controlled hypotheses are enticing because they render fractionation

a

Core formation and partial melting

<figure><img src="image_2.png" /></figure>

b

Volatile depletion

<figure><img src="image_3.png" /></figure>

**Figure 1** | Influence of planetary processes on iron isotope compositions. With data from only chondrites and basalts available, the main phase of planet building is open to interpretation. **a**, Elardo and Shahar² propose that nickel-bearing core formation during differentiation enriches the mantle in light isotopes, whereas the core is comparatively heavy. Fractionation during partial melting is then required to enrich basaltic rocks in heavy iron relative to their isotopically light mantle sources. **b**, Alternatively, volatile depletion during accretion may control iron isotope fractionation. Here, bulk planetary compositions are enriched in heavy iron isotopes owing to either volatile loss during condensation in the solar nebula⁵,¹² or evaporation following giant impacts⁷, with limited fractionation due to core formation and partial melting, as supported by the findings of Liu *et al.*¹¹.
during magmatic processes unnecessary
in producing the observed compositions
of basalts. However, these models hinge
upon knowledge of how elements volatilize
and fractionate during accretion, for which
experimental data is sparse.

Elardo and Shahar² suggest that the different iron isotope compositions of planetary materials can be explained by fractionation during core formation in the presence of nickel. Liu *et al.*¹¹ argue that core formation should not fractionate iron isotopes significantly and volatility-controlled fractionation mechanisms may need to be invoked. The contrasting results may arise from the difficulty of detecting iron isotope fractionation in the nickel-poor alloys studied

by Liu *et al.*, compared with the nickel-rich
alloys studied by Elardo and Shahar, although
it is not yet clear why the outcomes of the
two studies differ. The different models are
not mutually exclusive at this stage, and
more data and better constraints on the iron
isotope compositions of planetary interiors
are needed to quantitatively assess these
hypotheses. So, a nickel for your thoughts?

Paolo A. Sossi is at the Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, F-75005 Paris, France.
e-mail: sossi@ipgp.fr

References

1. O'Neill, H. S. C. & Palme, H. Phil. Trans. R. Soc. A 366, 4205-4238 (2008).

2. Elardo, S. M. & Shahar, A. *Nat. Geosci.* **10**, 317–321 (2017).

3. Papike, J. J. *Am. Mineral.* **90**, 277–290 (2005).

4. Sossi, P. A., Nebel, O. & Foden, J. *Earth Planet. Sci. Lett.* **452**, 295–308 (2016).

5. Sossi, P. A., Nebel, O., Anand, M. & Poitrasson, F. *Earth Planet. Sci. Lett.* **449**, 360–371 (2016).

6. Wang, K. et al. *Geochim. Cosmochim. Acta* **89**, 31–45 (2012).

7. Poitrasson, F., Halliday, A. N., Lee, D., Levasseur, S. & Teutsch, N. *Earth Planet. Sci. Lett.* **223**, 253–266 (2004).

8. Hin, R. C., Schmidt, M. W. & Bourdon, B. *Geochim. Cosmochim. Acta* **93**, 164–181 (2012).

9. Mao, H. K., Wu, Y., Chen, L. C., Shu, J. F. & Jephcoat, A. P. *J. Geophys. Res.-Sol. Ea.* **95**, 21737–21742 (1990).

10. Dauphas, N. et al. *Earth Planet. Sci. Lett.* **398**, 127–140 (2014).

11. Liu, J. et al. *Nat. Commun.*. http://dx.doi.org/10.1038/ncomms14377 (2017).

12. Dauphas, N., Poitrasson, F., Burkhardt, C. & Kobayashi, H. *Earth Planet. Sci. Lett.* **427**, 236–248 (2015).

Published online: 20 February 2017

VOLCANOLOGY

Vulcan rule beneath the sea

Over 70% of the volcanism on Earth occurs beneath an ocean veil. Now, robotic- and fibre-optic-based technologies are beginning to reveal this deep environment and identify subaqueous volcanoes as rich sources of sulfur, carbon dioxide and life.

Deborah Kelley

Seafloor is created by magmatism along the mid-ocean ridge system that, at roughly 70,000 km in length, wraps around the entire planet. New ocean crust is also formed at arc volcanoes above subduction zones, and at seamounts and ocean islands such as the Hawaiian island chain and Iceland, which form above mantle hotspots. Satellite data reveal more than 100,000 extinct and active seamounts that mark sites of past and present volcanic activity¹. At the American Geophysical Union Chapman Conference² titled ‘Submarine Volcanism: New Approaches and Research Frontiers’, held in Hobart, Tasmania in January 2017, geoscientists gathered to discuss how observations from submersibles, robotic vehicles and cabled observatories have revolutionized our understanding of oceanic eruptions.

Throughout recorded history oceanic
islands have been recognized as sites of major
volcanic events that significantly impact
nearby and distal populations. For example,
the 1815 Tambora eruption led to a 'year
without a summer' and the 1883 Krakatoa
eruption created an explosion that was heard
more than 4,800 km away, an atmospheric
shock wave that circled the globe at least
three times, and a tsunami 51 m high³. In
contrast, only within the last six decades
has it been recognized that the majority

of Earth's crust is formed by less explosive
volcanic activity beneath the oceans' surface.
Yet, it is here in this remote environment
that fundamental observations are providing
new insights into how our planet works, and
possibly keys to the origin of life itself.

The advent of plate tectonics theory
in the 1970s and growing use of deep
submergence vehicles to explore mid-ocean
ridge environments has forever changed the
way scientists view the deep ocean. Dives
using the human-occupied vehicle *Alvin* to
reach the Galápagos Rift in 1977 and the East
Pacific Rise in 1979 led to the spectacular
discovery of submarine hot springs emitting
380 °C, metal-laden acidic fluids that jetted
into the overlying water column⁴. Within
these extreme environments, life was found
to thrive in the absence of sunlight, supported
by heat, gases and chemicals extracted from
within the volcanoes themselves.

Numerous volcanic eruptions probably
occur daily along the extensive mid-ocean
ridge system, yet such events have never been
observed directly. Only rarely have sites been
visited in the days to months following an
eruption. Visits to the East Pacific Rise and
the Juan de Fuca Ridge reveal spectacular
glass-covered basaltic landscapes of collapsed
lava ponds, braided channels and thin sheet
flows that mark the rapid effusion of lava
onto the seafloor⁴.

At the workshop² geoscientists discussed how at both the East Pacific Rise and Juan de Fuca Ridge, billions of microbes have been observed streaming out of the seafloor for months after eruptions (Fig. 1), appearing like snow storms that blanket the newly formed lava flows in thick orange and white mats⁴ (D. S. Kelley, University of Washington, USA). These spectacular blooms are driven by the circulation of warm fluids through the recently formed cooling crust and supported by magmatically derived CO₂.

At the East Pacific Rise, as well as at Axial Seamount and the Endeavour Segment of the Juan de Fuca Ridge, autonomous underwater vehicles (AUV) with high-resolution sonars have enabled detailed imaging of the geometry of eruptive fissures, lava flows and hydrothermal vent sites (D. Clague, Monterey Bay Aquarium Research Institute, USA). Coupled with age dating and geochemical analyses of precisely sampled flows, these observations allow reconstruction of the detailed histories of changes in magmatic plumbing and sources, and confirmation of eruption frequencies as short as 5 to 15 year intervals. Following eruptions, hydrothermal vent temperatures rise and fresh vent fluids egress, and new chimneys form while others die off⁴.

Unlike mid-ocean ridge environments, arc
systems typically host more silicic magmas