Letters in 2009

Spatially extensive evidence for nitrogen-induced stimulation of forest growth has been lacking. Analysis of forest inventory data from the northeastern and north-central US collected during the 1980s and 1990s indicates that nitrogen deposition enhanced above-ground carbon storage by 61 kg per kg of nitrogen deposited.

Water movement in upland humid watersheds from the soil surface to the stream is often described using the concept of translatory flow, which assumes that water at any soil depth is well mixed. A study of water isotopes in an Oregon watershed instead suggests that trees and streams tap into separate water reservoirs.

Earth-system climate sensitivity includes the effects of long-term feedbacks such as changes in continental ice-sheet extent and terrestrial ecosystems. A reconstruction of atmospheric carbon dioxide levels 4.5 million years ago suggests that Earth-system climate sensitivity is significantly higher than that estimated from global climate models, which includes only fast feedback mechanisms such as changes in clouds and sea ice.

Delta morphology is thought to be controlled by factors such as river discharge, tides and waves. Numerical modelling shows that sediment cohesion also strongly influences the development of a delta’s characteristics.

The concentration of chemicals that absorb low-frequency sound in the ocean has declined as a result of anthropogenic increases in ocean acidity. Model simulations predict large reductions in sound absorption in the high latitudes and areas of deep-water formation over the twenty-first century.

Antarctic Intermediate Water is an oxygen-rich water mass that spreads throughout the Southern Hemisphere oceans. Marine sediment chemistry indicates that the water mass was produced at higher rates and had higher oxygen concentrations during the Last Glacial Maximum.

The collision history of asteroids in the early Solar System is difficult to reconstruct. A study of plagioclase breakdown using X-ray diffraction measurements under increasing pressures and temperatures suggests that peak pressures in collisions that formed shocked meteorites have been overestimated.

The movement of tectonic plates before the Cretaceous period is poorly understood. A global mantle tomography model suggests that the longitude of oceanic subduction zones up to 300 million years ago was offset by up to 18^∘ compared with tectonic reconstructions for the same period.

The generation of fluids as mountain belts form is thought to be related to the heating that accompanies rock burial. Mineral-equilibria modelling suggests that the exhumation of greywacke — a common rock type in mountain-forming regions — following heating also generates new fluid.

A set of lakes filled with liquid hydrocarbon have been discovered in the high latitudes of Saturn’s moon Titan. A quantification of these lakes shows a dramatic hemispheric asymmetry in their occurrence, which could be a result of the orbital configurations of Titan and Saturn.

Accurate quantification of Antarctic ice-sheet mass balance and its contribution to global sea-level rise remains challenging. Gravity Recovery and Climate Experiment data spanning the period April 2002 to January 2009 confirm earlier estimates of ice loss for Antarctica and indicate that East Antarctica started losing mass in about 2006.

Zircon is a common mineral in continental crustal rocks, and is often used in the reconstruction of geological processes. Analyses of the isotopic and trace element compositions of large zircons of gem quality from the Bohemian Massif reveal that zircons can remain intact in the upper mantle under specific chemical conditions for about 20–60 million years.

A significant number of the early aftershocks following large shallow earthquakes are missing in existing earthquake catalogues. An analysis of the waveforms of 3,647 relocated earthquakes along the Parkfield section of the San Andreas fault reveals 11 times more aftershocks within three days of the 2004 Parkfield earthquake than listed in a standard catalogue.

Exposed rocks on underwater mountains and ridges host abundant and diverse microbial communities. X-ray and microscopic analyses of volcanic rocks associated with Loihi seamount in Hawaii suggest that seafloor microbes may commonly be sustained by energy inputs from the water column.

Surface water temperatures in Lake Superior have warmed faster than air temperature during the thermally stratified summer season. Analyses of a range of observations show that as a result surface wind speeds above the lake have been increasing by nearly 5% per decade.

Earthquakes are far more common along the San Jacinto fault in California than the nearby southern San Andreas fault. An analysis of seismic and geodetic data suggests that the excess seismicity along the San Jacinto fault is due to a creeping motion along the deeper parts of the fault.

The Slumgullion landslide in the western United States has been moving almost continuously for over 100 years. Observations and numerical modelling show that air pressure changes from atmospheric tides trigger daily movement.

Oceanic nitrogen concentrations are controlled by the balance between nitrogen fixation and denitrification. Examination of nutrient concentrations in the North and South Atlantic Ocean suggests that nitrogen fixation is controlled by the supply of dissolved iron.

Many mountain ranges have asymmetric topography and drainage patterns. Laboratory experiments show that tectonic uplift combined with a precipitation gradient will cause the drainage divide to migrate towards the drier side of the mountain range, thereby triggering the splitting of drainage basins on the dry side of the range.

The Eocene Thermal Maximum 2 occurred 53.5 million years ago in response to elevated atmospheric carbon dioxide levels. Geochemical and microfossil analyses of Arctic sediments show that the during this event the surface of the Arctic Ocean warmed and freshened, and the coldest month mean temperatures did not fall below 8 ^∘C.