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The difference in methane concentrations recorded in Greenland and Antarctic ice cores indicates that most of the increase into the Bølling warm period came from the tropics rather than the boreal wetlands7. Rosen et al.3 use this recent finding to suggest that the phase lag between Greenland climate and methane also applies to Greenland and tropical climate. However, they recognise that emission histories are possible in which the northern methane sources react first, even if they contribute only 20% of the rise; in this case, the tropical lag can be as long as 40 years.

In some ways, a near-zero (within uncertainty) lag is tantalizing. A clear lead of either the north or the tropics relative to Greenland temperature would have given a straightforward pointer to the trigger of the rapid change. As it is, a synchronous change still allows the trigger to be in the north, south or tropics, provided the signal can be transmitted around the world rapidly. An example of a potentially rapidly transmitted northern change is the resumption or strengthening of the AMOC, which would result in a retreat of sea ice in the North Atlantic. This would alter the position of the intertropical convergence zone, and thus rapidly affect precipitation and wetland emissions in the tropics. However, other triggers in the tropics or Southern Hemisphere cannot be ruled out: for example, a meltwater pulse in the Southern Hemisphere could cause southern cooling and sea-ice growth, as well as push the intertropical convergence north, where it could reinvigorate Asian wetland emissions of methane.

The cause of the Bølling–Allerød warming may still be up for debate, but the synchroneity of the changes does present an opportunity for resolving its timing — and improving the age model for the rest

of the ice core. The Greenland ice cores have been dated by counting annual layers, a method that allows the uncertainty on absolute dates to grow large with depth.

The dating of Dansgaard–Oeschger events in speleothems is based on radiometric ages, so it is more precise. If these dates could be transferred to the Dansgaard–Oeschger events as recorded in ice cores, it would reduce uncertainty4, but such a transfer relies on the unproven assumption that the changes recorded in speleothems and ice cores are synchronous.

The work of Rosen et al .3 demonstrates that temperature change in Greenland was nearly synchronous with changes in tropical methane sources at the onset of the Bølling–Allerød. If the same synchroneity holds true for other Dansgaard–Oeschger events, then it would provide a theoretical basis to assume that changes recorded in subtropical speleothems are also synchronous with temperature and atmospheric chemistry changes recorded over Greenland, and originating at high and low latitude respectively. Given that methane can be measured in both Greenland and Antarctic ice cores, this might open the door to much more precise age models across sites, and make it possible to link tropical and polar climate records over many glacial cycles. ❐

Eric W. Wolff is in the Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. e-mail: ew428@cam.ac.uk

References1. Johnsen, S. J. et al. Nature 359, 311–313 (1992).2. Wolff, E. W., Chappellaz, J., Blunier, T., Rasmussen, S. O. &

Svensson, A. Quat. Sci. Rev. 29, 2828–2838 (2010).3. Rosen, J. L. et al. Nature Geosci. 7, 459–463 (2014).4. Barker, S. et al. Science 334, 347–351 (2011).5. Blunier, T. et al. Nature 394, 739–743 (1998).6. Severinghaus, J. P. & Brook, E. J. Science

286, 930–934 (1999).7. Baumgartner, M. et al. Biogeosciences 9, 3961–3977 (2012).

Published online: 4 May 2014

CARBON CYCLE

Sequestration in buried soilsRapid deposition of wind-borne silt after the end of the last glacial period buried a large reservoir of organic carbon in the deep soil. Geochemical analyses suggest that this sequestered soil carbon could be released to the atmosphere if exposed to decomposition.

William C. Johnson

Soils are considered to be one of the major terrestrial reservoirs of organic carbon1. However, we seldom give

much thought to the carbon reservoir contained within soils that have been buried beneath the modern landscape.

Ancient soils are buried by processes such as aeolian activity (loess and sand), alluvial and colluvial deposition,

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Figure 1 | Air trapped in ice. Analyses of air bubbles in Greenland ice cores show that atmospheric methane concentrations rose at the start of the Bølling–Allerød warming 14,700 years ago6. Rosen et al.3 use the isotopic composition of nitrogen contained within the same bubbles to show that Greenland temperatures rose nearly synchronously with the change in atmospheric methane concentrations. Because methane concentrations reflect emissions from wetlands in the mid to low latitudes, Rosen et al. conclude that the abrupt warming was rapidly transmitted throughout the Northern Hemisphere.

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volcanic eruptions, glaciation, accretion of permafrost, and prehistoric cultural activity2, and can contain large quantities of carbon. These buried soilscapes are far more common than generally appreciated, and in some regions they form important stratigraphic features near the surface. The issue at hand is whether or not this soil organic carbon was degraded during burial — if it remains un-degraded, then it should be available as a dynamic source for decomposition to carbon dioxide following exhumation. Writing in Nature Geoscience, Marin-Spiotta and colleagues3 report a characterization of the organic carbon from a body of soil buried about 10,000 years ago, and show that even though the carbon in this soil is between 11,300 and 12,000 years old, much of it has yet to fully decompose — and would therefore be reactive if re-exposed to the atmosphere.

Of the array of geomorphic settings with the potential for soil burial, the thick loess accumulations in midcontinent North America, particularly the central Great Plains, provide an ideal site. Here, the ancient Brady soil lies buried within and under the loess mantle of the central Great Plains. The formation of the Brady soil started about 14,000 years ago within a grassland ecosystem4 and proceeded largely uninterrupted for up to about 4000 years, even during the Younger Dryas cold interval (roughly 12,900 to 11,500 years ago). It was finally buried around 10,000 years ago at the onset of arid conditions during the latest Pleistocene and early Holocene5, which mobilized fine sediments embedded within previously stabilized dune fields6. More-or-less continuous loess deposition occurred until about 6,500 years ago7, resulting in two metres of burial; episodic loess deposition thereafter allowed for the formation and burial of younger soils, and less well-developed surfaces. Ultimately, the Brady soil was buried to a depth of about six metres4.

Marin-Spiotta and colleagues3 assessed the age and nature of carbon within the Brady soil with geochemical, spectroscopic and isotopic measurements. They find that the buried carbon is comprised of a mix of black carbon and plant lipids, with lower contributions of carbohydrates and lignins. The Brady soil’s higher concentrations of black carbon relative to modern soils suggests that it is not simply burial that stabilized the soil carbon, but also the chemical alteration of the plant-derived carbon by wildfires. Burning increases the thermal stability of the soil organic matter

through the creation of highly condensed aromatic compounds.

Wildfires would probably have been commonplace in the grassland community of the Brady soilscape, especially given the dominance of warm-season grasses4. To the investigators’ credit, they sampled in triplicate at two Brady soil exposures located 200 m apart to provide replication and to avoid generalization from a series of single samples from a single profile. With their estimate of the Brady soil organic matter being 40% fire-derived, they calculate that, within the potential area of Brady soil distribution, as much as 2.7 Gt of organic carbon could be presently sequestered as black carbon and in more reactive forms.

The Brady soil is not the only prominent palaeosol with a well-expressed, carbon-rich topsoil horizon hidden within the loess deposits of the central Great Plains. The even thicker topsoil of the Gilman Canyon Formation is contained within a loess unit that accumulated during a period of mild conditions during the last glacial period8,9. Soil development within the Gilman Canyon Formation prevailed from about 40,000 to 25,000 years ago, and, similarly to the Brady soil, accumulated substantial amounts of organic carbon. Though stratigraphically below the Brady soil, the Gilman Canyon Formation soil appears close to the surface where erosion has occurred or where loess units are thin, particularly in the distal areas of the loess mantle (Fig. 1). This author happened upon a site in west-central Kansas where excavations for a house construction exposed the Gilman Canyon Formation soil; here this palaeosol was used as a ready-made soil for the lawn, thereby re-exposing the carbon contained in it to the atmosphere. Thus the challenge of assessing the potential carbon reservoir in other carbon-rich soils, such as the Gilman Canyon Formation, arises.

Marin-Spiotta and colleagues3 have convincingly argued that fire and burial of soil carbon can arrest decomposition, thereby stabilizing an archive of soil carbon, some of which could decompose if exhumed. Ongoing and expanding anthropogenic land disturbances are increasing the risk that these buried reservoirs of reactive carbon could be exposed to decomposition, releasing carbon to the atmosphere. ❐

William C. Johnson is in the Department of Geography, University of Kansas, Lawrence, Kansas 66045-7613, USA. e-mail: wcj@ku.edu

References1. Lal, R. Science 304, 1623–1627 (2004).2. Chaopricha, N. T. & Marin-Spiotta, E. Soil Biol. Biochem.

69, 251–264 (2014). 3. Marin-Spiotta, E. et al. Nature Geosci. 7, 428–432 (2014).4. Mason, J. A. et al. Quat. Sci. Rev. 27, 1772–1783 (2008).5. Williams, J. W., Shuman, B., Bartlein, P. J., Diffenbaugh, N. S. &

Webb III, T. Geology 38, 135–138 (2010).6. Mason, J. A., Jacobs, P. M., Hanson, P. R., Miao, X. & Goble, R. J.

Quat. Res. 60, 330–339 (2003).7. Miao, X., Mason, J. A., Johnson, W. C. & Wang, H.

Palaeogeogr. Palaeoclim. Palaeoecol. 245, 368–381 (2007).8. Johnson, W. C., Willey, K. L., Mason, J. A. & May, D. W.

Quat. Res. 67, 474–486 (2007).9. Muhs, D. R. et al. Geol. Soc. Am. Bull. 120, 1378–1407 (2008).10. Roberts, H. M., Muhs, D. R., Wintle, A. G., Duller, G. A. T. &

Bettis, E. A. Quat. Res. 59, 411–419 (2003).

Published online: 25 May 2014

Figure 1 | Buried soils of the central Great Plains. This excavation in central Kansas, which exposes both the Brady Soil (top arrow) and the soil of the Gilman Canyon Formation (bottom arrow), provides a comparison in soil horizon thicknesses between these two soils. Marin-Spiotta and colleagues3 demonstrated the presence of reservoirs of old organic carbon where the Brady soil is buried by six metres of loess. In the setting depicted here, both the Brady and Gilman Canyon Formation soils10 are found at much shallower depths than at the site used by Marin-Spiotta et al., and it is unlikely that the Brady soil carbon is isolated from the atmosphere. It would, however, require very little disturbance to bring the Gilman Canyon Formation soil — and its sequestered carbon — back into contact with the atmosphere. Scale is provided by the 1.2-metre-tall shovels.

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