Research articles

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.

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.

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.

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.

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.

The demise of the Laurentide ice sheet during the early Holocene epoch allows rates of ice sheet decay under natural conditions to be assessed. Analysis of terrestrial and marine records of the deglaciation along with a climate model reveal two periods of rapid melting during the final retreat of this ice sheet, with rates of sea level rise of up to 1.3 cm per year.

Determining stratospheric ozone levels from before instrumental records began has proved difficult. Measurements of the chemical composition of plant spore walls suggest that ultraviolet-B-absorbing compounds have the potential to act as a proxy for past changes in ultraviolet-B radiation and stratospheric ozone.

Despite important biological and biogeochemical consequences of extensive ocean anoxic events, their identification is controversial. The marine isotope geochemistry of molybdenum can help quantify the past oxygenation state of the ocean if the riverine input of Mo isotopes is known. Analysis of a set of rivers that account for 28% of global river runoff suggests more variable Mo isotopic ratios in rivers that are also isotopically enriched in the heavy isotopes, suggesting near-total anoxia in the Proterozoic ocean and during Mesozoic ocean anoxic events.

Thick alluvial fan sediments from the core of the Emeishan Large Igneous Province have been considered as critical field evidence in support of plume-induced pre-volcanic doming and uplift. These sediments are now reinterpreted as mafic hydromagmatic deposits emplaced at sea level, precluding dynamic pre-volcanic uplift as predicted by mantle plume models.

Dissolved organic matter and nutrients from high-latitude coastal watersheds stimulate microbial activity and primary productivity in near-shore ecosystems. A survey of southeast Alaskan watersheds suggests that the extent of glacial coverage may control the release of these nutrients to rivers and ultimately the oceans.

The Arctic soil organic-carbon pool is poorly constrained. Measurements of soil organic carbon in the North American Arctic reveal that the carbon store in this region is larger than previous estimates suggest, and highly dependent on landscape type.

Dissolved organic matter in the ocean constitutes one of the largest pools of reduced carbon on the Earth’s surface. An analysis of observations from the Pacific Ocean shows that as organic matter is oxidized biologically, fluorescent dissolved organic matter is produced in situ in the ocean interior and is resistant to biological degradation on centennial to millennial timescales.

Organic-rich sedimentary units called sapropels have formed repeatedly in the eastern Mediterranean Sea, but the mechanisms leading to the formation of these shale beds are still under debate. The analysis of a suite of sediment cores covering the Eastern Mediterranean basin reveals that across the entire basin preservation of sapropel S1 was different in characteristics above and below 1,800 m depth, which is a result of different redox conditions.

In the high latitudes, abrupt cooling events are thought to control mainly the winter temperatures, thereby increasing seasonality. Sea surface temperature reconstructions from the Gulf of Mexico suggest that over the past 300,000 years these events also enhanced seasonality in tropical regions.

The nature, activity and metabolism of microbes that inhabit the deep subsurface environment are a matter of ongoing debate. The analysis of oil samples from three different basins in South America, central Europe and the Middle East indicates the presence of intact phospholipids and suggests that indigenous bacteria inhabit petroleum reservoirs in sediment depths of up to 2,000 m.

Geophysical data for the Cocos Plate sea floor suggest that basement outcrops along mid-ocean ridge flanks can discharge very large quantities of heat and fluid. This is indicative of high crustal permeability at the regional scale.

The Younger Dryas cold reversal during the last glacial termination is one of the most abrupt climate change events observed in the Northern Hemisphere. Analyses of varved lake sediments from western Germany suggest that storminess in the North Atlantic region increased within a single year at this time, providing a mechanistic link between a decrease in the meridional overturning circulation and western European cooling.

On 13 June 2006, parts of the eastern flank of the Eiger peak in the Swiss Alps collapsed, leading to a major rock fall. The unstable spur on this flank is made up of several blocks that move relative to one another. Instability was initiated by a block at the rear that acted as a wedge and triggered motion of the other blocks.

Atmospheric oxygen levels on Earth rose in at least six distinct steps and an examination of the timing of the steps suggests that they coincided with the formation of supercontinents and supermountains. This leads to the hypothesis that increased erosion of these supermountains released large amounts of nutrients to the oceans, stimulating productivity and the release of oxygen to the atmosphere. The subsequent burial of organic carbon along with the mountain sediments would have sustained the increased oxygen levels.

Changes in precipitation extremes under greenhouse warming are commonly assumed to be constrained by the Clausius–Clapeyron relationship, implying an increase in extreme precipitation of 7% per degree of climate warming. An analysis of 99 years of observations along with simulations with a regional climate model show that short-duration precipitation extremes can instead increase in severity twice as fast, by 14% per degree of warming.