Articles in 2008

Seismic anisotropy in Earth’s oceanic lithosphere and in the mantle wedge above subduction zones is associated with crystallographic preferred orientations of olivine. Experiments at high pressure and temperature suggest that a pressure of ∼3 GPa can induce the same changes in the crystal structure of olivine as high water activity at lower pressures.

Deep convection in the subpolar North Atlantic, an important component of the global ocean circulation, has been absent in recent years. Profiling float data from the Argo programme document the return of deep mixing to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008.

The formation and circulation of Antarctic Intermediate Water has varied over glacial–interglacial timescales. A neodymium record from the Atlantic Ocean basin suggests that changes in circulation may have been driven by changes both in Antarctic Intermediate Water formation in the Southern Ocean and in the strength of North Atlantic meridional overturning.

The response of ocean circulation in the Southern Ocean to changes in wind stress and surface buoyancy fluxes is under debate. An analysis of Argo data and historical measurements suggests that transport in the Antarctic Circumpolar Current and the meridional overturning circulation in the Southern Ocean are insensitive to decadal changes in wind stress.

Although the India–Eurasia collision initiated ∼50 Myr ago, major deformation and exhumation of the Himalaya did not begin until the early Neogene (∼23 Myr ago). This coincides with the increased intensity of the Asian monsoons, as indicated by weathering records from the South China Sea, Bay of Bengal and Arabian Sea, and hints at a dynamic coupling between climate and both erosion and deformation in the Himalaya.

Intense glaciation during the middle Pleistocene epoch led to focused denudation and mass redistribution within the St Elias orogen in southern Alaska, and resulted in structural reorganization of the orogen. The tectonic response of this orogen to climate change is consistent with predictions of numerical models.

On orbital timescales, Antarctic climate varies in phase with Northern Hemisphere insolation, but no physical mechanism for such a link is known. A new analysis suggests that at obliquity and precession timescales Antarctic climate may instead be responding to the duration of the local summer, which covaries with Northern insolation.

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.

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.

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.

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.

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.

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.

Pristine temperate rainforests are known to produce large amounts of bioavailable nitrogen, with only minimal loss. Tracing ¹⁵N in volcanic soils of a temperate evergreen rainforest in southern Chile helps to further unravel the retention mechanisms for bioavailable nitrogen in these ecosystems.

Arsenic contamination of groundwater resources threatens the health of millions of people worldwide, particularly in the densely populated river deltas of Southeast Asia. Maps of areas at risk of groundwater arsenic concentrations have been produced by combining geological and surface-soil parameters in a logistic regression model. They show that Holocene deltaic and organic-rich surface sediments are key indicators for arsenic risk areas and indicate elevated risks in Sumatra and Myanmar where no groundwater studies exist.

The Earth’s continents amalgamated into the supercontinent Pangaea 320 million years ago. After the supercontinent formed, structural deformation continued, which eventually resulted in the subduction of the ocean margin of Pangaea beneath the continental edge at the other end of the same plate.

Surface waters of most of the world’s oceans are supersaturated with respect to atmospheric methane. Measurements in seawater samples suggest that an aerobic methane production pathway, which involves the decomposition of phosphorus-containing organic compounds, may be responsible.

There has been a strong disagreement between model predictions of troposphere warming and observations of temperature trends from radiosondes and satellites. However, when tropospheric temperature reconstructions are generated from thermal-wind measurements and the thermal-wind equation for 1970–2005, the results show a strong tropospheric warming trend, in agreement with model predictions.

The Eocene–Oligocene transition is the largest global cooling in the Cenozoic period. A comparison of three independent proxies from the continental shelf and deep ocean reveals a three-step transition to cold glacial conditions, with ice sheets 25% larger than their present size.