Letters in 2008

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.

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.

Changes in ocean chemistry that favoured the precipitation of aragonite or calcite are thought to have influenced the skeletal mineralogy of marine calcifyers. An investigation of the original skeletal mineralogy of large numbers of marine taxa suggests that the selective recovery of marine organisms from mass extinctions has a much greater influence on the overall percentage of aragonitic organisms than the Mg/Ca ratio of the oceans.

Monsoons are often viewed as planetary-scale sea-breeze circulations, caused by contrasts in the thermal properties between oceans and land surfaces. Numerical simulations suggest that instead feedbacks between large-scale extratropical eddies and the tropical atmospheric overturning circulation are essential for the development of monsoons.

The ancient Farallon plate subducted under North America in two distinct stages. High-resolution tomographic images show large pieces of the plate, including the currently active piece, which descends from the Pacific Northwest coast to 1,500 km depth, and its stalled predecessor, which now occupies the transition zone and lower mantle beneath the eastern half of the continent.

The Waiho Loop Moraine has been interpreted as evidence for Younger Dryas cooling in southern New Zealand, but recent dating and climatological studies have questioned this idea. A detailed analysis of the sedimentology of the moraine suggests it was formed after a large landslide onto the Franz Josef glacier triggered a glacial surge, independent of climate forcing.

The rifting of the Seychelles microcontinent from India involved two phases of extensional activity. The initial separation of the Laxmi Ridge from India was accompanied by extensive magmatism but the later separation of the Seychelles from the Laxmi Ridge was only weakly magmatic.

The surface waters of the North Atlantic subtropical gyre are depleted in phosphate, relative to the South Atlantic gyre. Despite this nutrient limitation, the two gyres have comparable rates of carbon fixation. Measurements of enzyme activity suggest that dissolved organic phosphorus may be fuelling northern productivity.

Although North Atlantic deep-water formation was greatly reduced during the last glacial maximum, bottom-water currents were as vigorous as at present. However, they were weakened during periods of North Atlantic surface freshening. A strong correlation can be seen between bottom-water-current strength and Greenland air-temperature records, thus confirming a close connection between ocean circulation and abrupt climate change.

Eclogites have been suggested as high niobium/tantalum reservoirs that complement the low niobium/tantalum ratios of the silicate Earth. However, the hafnium isotopic composition of eclogite fragments suggest that the high niobium/tantalum signature of eclogites is unlikely to be primary. Instead, it probably reflects chemical modification during residence in the subcontinental lithospheric mantle.

During the Cretaceous period, warm deep and intermediate waters filled the oceans. Evidence from benthic foraminferal δ¹⁸O and Mg/Ca ratios suggests that the intermediate water masses in the proto-Atlantic Ocean formed from high salinity waters sinking from shallow shelf seas.

Abrupt changes in the African Monsoon, which have been recorded throughout the late Pleistocene and early Holocene epochs, tend to coincide with changes in North Atlantic thermohaline circulation. A numerical simulation shows that the interaction between thermohaline circulation in the North Atlantic Ocean and wind-driven currents in the topical Atlantic Ocean contributes to the rapidity of African Monsoon transitions during abrupt climate change events.

Silicate weathering reactions remove carbon dioxide from the atmosphere and store it in carbonate minerals. During the high atmospheric carbon dioxide conditions of the Early Eocene Climatic Optimum, rates of chemical weathering, physical erosion and denudation in the western USA were equivalent to the highest recorded rates in the non-glacial Quaternary.

Surface waves that were generated by 12 out of 15 earthquakes with magnitudes greater than 7 since 1990 led to a global increase in the number of small earthquakes. This suggests that dynamic triggering of earthquakes is common and is independent of the tectonic environment.

Analogue modelling of caldera-forming eruptions suggests that sinking of the magma chamber roof is variable in space and time, leading to substantial stirring and mixing of magma. This can explain the common occurrence of geochemical zonation and magma mingling in deposits erupted from calderas.

Variations in the Earth’s magnetic field over a span of a few months can be resolved despite the potential filtering effects of the electrically conducting mantle, and are indicative of rapid flow in the Earth’s outer core.