Volume 1 Issue 10, October 2008

On 13 October 1908, Fritz Haber filed his patent on the “synthesis of ammonia from its elements” for which he was later awarded the 1918 Nobel Prize in Chemistry. A hundred years on we live in a world transformed by and highly dependent upon Haber–Bosch nitrogen.

Wildfires have been a natural part of the Earth system for millions of years. A new charcoal database for the past two millennia shows that human activity increased biomass burning after AD 1750 and suppressed it after AD 1870.

Carbon cycle–climate feedbacks are expected to diminish the size of the terrestrial carbon sink over the next century. Model simulations suggest that nitrogen availability is likely to play a key role in mediating this response.

Past variability in Sahel rainfall is closely linked to global sea surface temperature distributions in observations and models alike. Climate simulations for the 21st century suggest that additional influences may become important in the future.

The electronic configuration of iron impurities in lower-mantle minerals influences their physical properties, but it is not well constrained. New studies suggest that ferrous iron in silicate phases exists mainly in an intermediate spin state.

Saharan humidity has varied dramatically throughout the Pleistocene era. A new deep-sea sediment record reveals large and rapid hydrological shifts that are linked to the competing influences of low- and high-latitude climate processes.

The tropics sustain strong coherent variations in wind and precipitation on intraseasonal timescales of 30–60 days. These variations pace the active and break cycles of the monsoons, exerting a direct control on the livelihoods of large populations dependent on rain-fed agriculture. Emerging evidence suggests that heat fluxes from ocean to atmosphere play a fundamental role in driving these intraseasonal oscillations.

Observations over past decades show a sudden switch of Jakobshavn Isbræ—a large outlet glacier feeding a deep-ocean fjord on Greenland’s west coast—from slow thickening to rapid thinning in 1997. This switch is associated with a doubling in glacier velocity. Hydrographic data show a concurrent sudden increase in subsurface ocean temperatures along the entire west coast of Greenland, suggesting that the changes in Jakobshavn Isbræ were triggered by the arrival of relatively warm water originating from the Irminger Sea.

Climate change will have a significant impact on the hydrologic cycle, creating changes in freshwater resources, land cover and land–atmosphere feedbacks. Simulations using a groundwater flow model with integrated overland flow and land-surface model processes show that groundwater depth, which results from lateral water flow at the surface and subsurface, determines the relative susceptibility of regions to changes in temperature and precipitation.

Variability in northwest African humidity has been documented for the Holocene period, but less is known about the development of the hydrological balance during the Pleistocene period. Sedimentary records and numerical simulations for the past 120,000 years show abrupt millennial-scale changes in humidity, which may be related to changes in the strength of the North Atlantic meridional overturning circulation.

Swarms of non-volcanic tremor in southeastern Japan are associated with slow slip events and tend to occur with a periodicity of 12 or 24 h. This periodicity can be reproduced by a combination of stresses due to Earth tides and transient stress changes caused by slow slip events. Non-volcanic tremors may therefore be useful for understanding stress relaxation at the subduction-zone interface.

Chemical and isotopic variations in crushed material along the Chelungpu fault in Taiwan are suggestive of interactions at high temperatures with coseismically generated aqueous fluids. High fluid pressures along the fault zone during the magnitude 7.6 Chi-Chi earthquake in 1999 could have reduced friction and fault strength.

Iron has the ability to adopt different electronic configurations, and transitions in its spin state in the lower mantle can significantly influence mantle properties and dynamics. Experimental results for two lower-mantle perovskite compositions show that the intermediate spin state of iron is stable throughout the bulk of the lower mantle.

The Earth’s lowermost mantle displays an important mineralogical transition from perovskite to post-perovskite but the spin state of iron in these phases remains poorly known. Experimental results suggest that iron occurs in the intermediate spin state in both of these phases, which implies that changes in physical properties of the lower mantle must be governed by factors other than spin transitions in iron.

To understand the evolution of the inner core it is important to constrain the structure of its innermost part. Analysis of two types of seismic waves that traverse the inner core reveals seismic anisotropy of the innermost region and is consistent with the slowest direction of anisotropy being tilted away from the equatorial plane.

A compilation of wildfire records spanning six continents and 2,000 years reveals global patterns in biomass burning to be temporally linked with changes in climate, population and land use. An abrupt decline in biomass burning beginning about 150 years ago may be related to the expansion of intensive grazing, agriculture and fire management activities.

Precambrian banded iron formations contain thin alternating layers of iron- and silica-rich minerals. Microbial culture experiments show that these alternations could be caused by changes in the relative dominance of microbial iron oxidation and abiotic silica precipitation driven by fluctuating ocean temperature.

Rates of denitrification below some oceanic upwelling zones reach a maximum during periods when sea level rise was fastest throughout the Pleistocene glacial cycles. Nitrogen isotope data from North American shales deposited 300 million years ago during Palaeozoic glaciations are indicative of a similar pattern.

Catherine McCammon and colleagues surmounted experimental obstacles at the European Synchrotron Radiation Facility in France to unlock the secrets of iron in the Earth's lower mantle.

Nicole Posth and colleagues spent a month touring South African rock formations in their quest to understand the origin of ancient iron and silicate layers.