Letters in 2009

The sedimentary wedge in the Nankai Trough off the coast of Japan is cut by a landward-dipping thrust fault, which is an important plate boundary structure in this region. Geological and seismic reflection data indicate that the fault initiated ∼1.95 million years ago and underwent uplift and reactivation ∼1.55 million years ago.

The earliest fossils of cyanobacteria have been controversially reported from the 3,465-million-year-old Apex chert in Western Australia. Mineralogical analyses of the rock near the fossil site indicate that the site experienced repeated alterations at low and medium temperatures, and is therefore unlikely to have preserved any early forms of life.

The Australian–Indonesian summer monsoon affects rainfall variability across the Indo–Pacific region. Reconstructions of monsoon strength from stalagmites show that precipitation increased from 11,000 to 7,000 years ago, as rising global sea level caused the flooding of the Indonesian continental shelf.

The micronutrient molybdenum is necessary for nitrogen fixation, but is very rare and highly soluble in soils. X-ray spectroscopy analysis of forest soil samples indicates that most of the molybdenum in the litter layer binds to organic matter.

The Salton Sea is an evolving pull-apart basin located between the San Andreas and Imperial faults in Southern California. Seismic and geological data reveal a rapidly subsiding southern sub-basin that is bounded by a hinge zone to the north, and northwest-dipping normal faults to the south.

Sea level fluctuated substantially over the past 22,000 years. A simple model based on these fluctuations estimates between 7 and 86 cm of sea-level rise by the end of the twenty-first century—in agreement with climate model projections.

It has been proposed that hydrocarbons could be produced abiogenically under the high pressure, high temperature conditions characteristic of the upper mantle. In situ Raman spectroscopy indicates that methane forms saturated hydrocarbons, containing two to four carbons, when exposed to upper-mantle conditions.

Following the crystallization of a magma ocean, the martian mantle probably underwent an overturning event, but its initiation, timing and geochemical consequences are poorly constrained. Isotopic data for martian meteorites and numerical simulations provide strong evidence for early overturning in the martian mantle.

Mineral dust can be transported long distances in the lower atmosphere. Satellite measurements and model simulations show that dust generated during a storm in the Taklimakan Desert, China, in 2007 was transported more than once around the globe.

Phosphorus is frequently the limiting nutrient in marine and terrestrial ecosystems. Analysis of lightning-derived glassy compounds from North America, Africa and Australia suggests that cloud-to-ground lightning increases the bioavailability of this nutrient.

About 55 million years ago global surface temperatures increased by 5–9 ^∘C within a few thousand years, following a pulse of carbon released to the atmosphere. Analysis of existing data with a carbon cycle model indicates that this carbon pulse was too small to cause the full amount of warming at accepted values for climate sensitivity.

The flux of methane—a greenhouse gas—from submarine hydrocarbon seeps to the atmosphere is not well quantified. Direct measurements of methane concentrations and isotopic depth profiles in deepwater hydrocarbon plumes indicate that a significant amount of methane from deep-ocean sources could reach the surface ocean.

Global sea-level rise, reduced sediment supply and subsidence threaten the stability of the Mississippi Delta. Calculations of riverine sediment load and storage indicate that 10,000–13,500 km² of the delta could be submerged by AD 2100.

Some aspects of the Earth system—such as global mean temperatures, and sea-level rise due to thermal expansion or melting of large ice sheets—continue to respond to climate change long after the stabilization of radiative forcing. Simulations with a coupled climate–vegetation model show that similarly ecosystems may be committed to significant change after climate stabilization.

Marine-terminating outlet glaciers control the stability of ice sheets. Exposure ages and radiocarbon dates show that an Arctic outlet glacier of the Laurentide ice sheet rapidly retreated about 9,500 years ago, and imply strong feedbacks between bathymetry and ice movement.

Ammonia is a significant atmospheric pollutant, accelerating the formation of particulate matter and damaging aquatic and terrestrial ecosystems. Infrared measurements of ammonia concentrations, obtained by the IASI/MetOp satellite, suggest that ammonia emissions in the Northern Hemisphere have been markedly underestimated.

Sea level has varied by over one hundred metres across glacial–interglacial cycles over the past 520,000 years. An extended sea-level reconstruction shows a strong coupling between these sea-level changes and Antarctic surface temperatures over the past five glacial cycles.

The mechanisms for localization of black-smoker systems at mid-ocean ridges remain to be fully understood. Seismic data for a segment of the Juan de Fuca ridge with long-lived black-smoker vents reveal ongoing magma recharge into the crustal magma chamber, thereby providing an explanation for the localization.

For the past few centuries, multidecadal climate variability in North Atlantic sea surface temperatures has been modulated by the Atlantic Multidecadal Oscillation (AMO). A coral-based temperature reconstruction reveals that the AMO is a transient climate feature that only became significant after AD 1730.

Over the past two decades, seasonal periods of rapid atmospheric mercury deposition over Antarctica have been described. Ice core records show that similar events have occurred during previous glacial periods, probably as a result of interactions between sea salts and mineral dust in the polar atmosphere.