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aerosols to total absorption is based on some assumptions. However, the researchers provide reassuring arguments to counter the point that black carbon absorption could simply have been misattributed to organic carbon. First, the experiments with diesel would have suffered from the same flaws, yet show no dependency on the BC-to-OA ratio. And the findings are qualitatively robust to large increases in the estimated contribution of black carbon. This method may not capture the full uncertainty of the calculations, however. Future additional insights into the differences between the properties of organics emitted by biomass and fossil-fuel combustion will be particularly welcome.

The next step is to reassess the climate impact of aerosols from biomass burning in light of these findings. Current climate

models simulate a neutral climate impact from these aerosols on a global average, because the gain of energy as a result of black carbon absorption is compensated by the loss of energy from the reflective organic component4. Yet, according to calculations by Saleh et al., made on the basis of simplified assumptions, biomass-originated organic aerosols are in fact contributors to global warming, if they are as absorbing as these experiments suggest.

The absorption properties of organic aerosols from biomass burning as derived by Saleh and colleagues3 will now need to be incorporated into climate models that can assess their climate impact in a larger context, including interactions between aerosols and their environment, especially clouds. If the results are confirmed, lasting

reductions in the fires set to clear forests and fertilize fields in many parts of Africa, South America, and Southeast Asia would become a way to slow down climate change, as has been suggested previously5. ❐

Nicolas Bellouin is at the Department of Meteorology, University of Reading, Reading, UK. e-mail: n.bellouin@reading.ac.uk

References1. Boucher, O. et al. in Climate Change 2013: The Physical Science

Basis (eds Stocker, T. F. et al.) 571–657 (Cambridge Univ. Press, 2013).

2. Bond, T. C. et al. J. Geophys. Res. Atmos. 118, 5380–5552 (2013).3. Saleh, R. et al. Nature Geosci. 7, 647–650 (2014).4. Myhre, G. et al. Atmos. Chem. Phys. 13, 1853–1877 (2013).5. Ramanathan, V. & Xu, Y. Proc. Natl Acad. Sci. USA

107, 8055–8062 (2010).

Published online: 3 August 2014

BIOGEOCHEMISTRY

Oxygen burrowed awayMulticellular animals probably evolved at the seafloor after a rise in oceanic oxygen levels. Biogeochemical model simulations suggest that as these animals started to rework the seafloor, they triggered a negative feedback that reduced global oxygen.

Filip J. R. Meysman

When Charles Darwin visited the family estate of his future wife Emma in 1837, her father,

Josiah Wedgwood, showed Darwin an agricultural field where just 15 years earlier, the surface had been covered with lime and cinders1. These materials were now buried at considerable depth below the surface, and Wedgwood believed that earthworms were the responsible agents2. This discussion with his future father-in-law triggered a lifelong interest in the mixing of soil by animals — bioturbation — and, in later years, Darwin devoted an entire monograph to the subject, stating that “Worms have played a more important role in the history of the world than most persons would at first suppose”3. Boyle et al.4 now take this idea to such a scale and level, that even Darwin — not afraid of a grand idea himself — would most likely be genuinely surprised. Writing in Nature Geoscience, these authors suggest that the evolution of bioturbation some 540 million years ago affected the oxygenation of oceans and the atmosphere.

Until now, ideas about the co-evolution of life and the Earth typically put geochemistry and microbiology in the driver’s seat, with interactions between microbes and rocks determining global

biogeochemistry, and setting the stage upon which biological evolution unfolds. Yet, since the emergence of multicellular life in the Neoproterozoic era, microbes are not the only players around, and so the question can be raised whether the evolution of plants or animals induced any feedback on global geochemistry, steering the co-evolution of life on earth. So far, the best studied ‘macro-organism’ feedback is the evolution of vascular plants in the Devonian, which introduced a potent biotic feedback on global biogeochemistry, increasing atmospheric O2 levels by stimulating the burial of carbon in oceanic sediments through the production of refractory lignin5. But what about that other defining moment in the evolution of life, the rise of animals?

Animals may not have been passive amidst changing atmospheric or oceanic conditions, either. It is generally held that an increase of oceanic oxygen at the end of the Neoproterozoic allowed the development of multicellular organisms with higher metabolic demands. This biotic expansion culminated in the Cambrian explosion 540 million years ago, when a wide variety of animals appeared in the fossil record over a relatively short time period6.

Originally living on top of the seafloor, early animals soon colonized the subsurface as well, looking for shelter from predators and tapping into organic food resources buried in the sediments. These activities dramatically altered the appearance of the ocean floor2 (Fig. 1). Before the rise of bioturbation, most of the seafloor was covered with microbial mats. As animals dug into and under these mats, the regular layers of matted sediments were replaced by a heterogeneous sediment layer intersected with animal burrows. In recent years, new speculations have yielded scenarios to explain how this ‘burrowing revolution’ could have influenced atmospheric or oceanic variables. One idea is that the onset of bioturbation may have increased oceanic sulphate concentrations in the ocean by stimulating sulphide re-oxidation in marine sediments7.

Boyle et al.4 now suggest that the effects of the burrowing revolution went even further. They propose that the onset of bioturbation triggered a series of feedbacks, which counteracted the global oxygenation in the late Neoproterozoic, and subsequently stabilized atmospheric O2 at lower levels. To support this idea, they simulated the effects of sediment mixing on phosphorus and organic carbon burial in a simplified

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model of global biogeochemistry. In their simulations, bioturbated sediments retain more phosphorus relative to carbon compared with unmixed sediments, a signature also observed in present-day bioturbated sediments8. Higher organic phosphorus burial reduces the availability of phosphate and hence decreases oceanic productivity, which leads to reduced organic carbon burial and lower oceanic and atmospheric O2 concentrations. This subsequently promotes the expansion of ocean anoxia, which then restricts

the distribution of bioturbating animals. The overall result is a negative feedback process that stabilizes atmospheric oxygen at a lower level, compared with the initial Neoproterozoic concentrations.

Boyle et al. also show that redox conditions, as reconstructed from proxy data, are largely consistent with the expansion of ocean anoxia that their model predicts. Still, it should be noted that there are few proxy datasets with sufficiently high temporal resolution, and that there is considerable debate on how bioturbation developed

through the early Phanerozoic. For example, the current model simulations assume a rapid increase in the depth and intensity of bioturbation in the early Cambrian, which may not necessarily have been a global phenomenon9. Indeed, much is still unknown about the redox structure of the ocean and atmosphere during the late Neoproterozoic and early Cambrian (800 to 400 million years ago) and its relationship with evolving life.

Boyle et al.4 now propose that the advent of digging and burrowing animals set up a feedback loop that regulated atmospheric oxygen and oceanic phosphorus levels. Although we are still far from knowing to what extent worms and their ilk influenced the geochemical history of our planet, this is a novel and testable hypothesis, which will inspire novel thinking and incite engaging discussions — maybe even with your prospective father-in-law. ❐

Filip J. R. Meysman is at the Royal Netherlands Institute of Sea Research (NIOZ), Korringaweg 7, 4401 NT Yerseke, The Netherlands. e-mail: filip.meysman@nioz.nl

References1. Desmond, A. & Moore, J. Darwin: The Life of a Tormented

Evolutionist (Gardners Books, 1992).2. Meysman, F. J. R., Middelburg, J. J. & Heip, C. H. R.

Trends Ecol. Evol. 21, 688–695 (2006).3. Darwin, C. The Formation of Vegetable Mould, Through the Action

of Worms, with Observation of their Habits (John Murray, 1881).4. Boyle, R. et al. Nature Geosci. 7, 671–676 (2014).5. Beerling, D. J. & Berner, R. A. Proc. Natl Acad. Sci. USA

102, 1302–1305 (2005).6. Canfield, D. E., Poulton, S. W. & Narbonne, G. M. Science

315, 92–95 (2007).7. Canfield, D. E. & Farquhar, J. Proc. Natl Acad. Sci. USA

106, 8123–8127 (2009).8. Slomp, C. P., Thomson, J. & de Lange, G. J. Mar. Geol.

203, 141–159 (2004).9. Tarhan, L. G. & Droser, M. L. Palaeogeogr. Palaeoclimatol. Palaeoecol.

399, 310–322 (2014).

Published online: 3 August 2014

DEEP EARTH

Post-perovskite at tenIn 2004, a phase transition was discovered in the most abundant lower-mantle mineral. A decade of focused experiments, computations and seismic imaging stimulated by this discovery has revealed previously unknown complexities in Earth’s deep mantle.

Sang-Heon Shim and Thorne Lay

Deep within the Earth, at a depth of about 2,900 km, the rocky mantle meets the metallic core. Here,

heat flux from the core creates a thermal boundary layer in the lowermost mantle where temperature increases steeply

to nearly 4,000 K. Over time, dense materials may have accumulated above the core/mantle boundary and interacted with this hot thermal boundary layer. This region could therefore hold the key to understanding the dynamic evolution of

our planet. The enormous pressures in the lowermost mantle cause dramatic changes in mineral structures and properties. In 2004, it was discovered that the most abundant mineral in the lower mantle, a magnesium-silicate mineral with a perovskite-type

Figure 1 | Animal burrows as preserved in early Cambrian rocks. With increased oxygenation of the ocean at the Ediacaran/Cambrian boundary, mobile animals started to burrow and rework the sediment of the ocean floor. Boyle et al.4 use a simplified biogeochemical model to support their proposal that this expansion of bioturbation resulted in a negative feedback loop that lowered atmospheric oxygen levels.

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