Letters in 2011

Eutrophication increases the acidity of coastal waters. Model simulations suggest that the drop in pH in coastal waters of the northern Gulf of Mexico since pre-industrial times is greater than that expected from eutrophication and ocean acidification alone.

Current estimates of carbon dioxide evasion from inland waters are based on incomplete spatial coverage. Streams and rivers in the United States release 97 Tg of carbon to the atmosphere each year, according to an analysis of chemical and morphological data.

Pronounced warming in the Arctic region is an important feature of observed and modelled climate change. Simulations with a coupled climate model show that the thermal inversion at the surface that predominates in Arctic winter amplifies Arctic warming by lowering the ability of the warming surface layer to radiate to space.

The Ganges–Brahmaputra drainage basin represents one of the largest sources of terrestrial biospheric carbon to the ocean. Radiocarbon analyses suggest that 20% of the carbon exported from this system has an average age of more than 15,000 years.

The stability over time of the zonal jets on the giant planets has been debated. An analysis of observations from the Cassini spacecraft reveals an acceleration of wind velocities in Saturn’s high-altitude equatorial jet between 2004 and 2009, by 20 m s⁻¹ at tropopause level and by 60 m s⁻¹ in the stratosphere.

An influence of solar irradiance variations on Earth’s surface climate has been repeatedly suggested. Simulations with a climate model driven by satellite measurements of solar ultraviolet irradiance show an atmospheric response to the solar minimum that resembles the negative phase of the North Atlantic Oscillation.

Ocean circulation patterns during the Late Cretaceous greenhouse interval remain disputed. Marine sediment geochemistry suggests that the end of the greenhouse interval coincided with the onset or intensification of deep-water formation in the North Atlantic Ocean.

The early Eocene was marked by a series of abrupt warming events. Numerical modelling suggests that the events were the result of nonlinear interactions between orbital forcing, ocean circulation and the carbon cycle.

The amount of radiocarbon-depleted carbon dioxide in the atmosphere rose dramatically during the last deglaciation. Estimates of the radiocarbon content of water at 2.7 km depth in the northeast Pacific Ocean over the past 24,000 years suggest that this water mass was not a significant source of this carbon.

Tropical Atlantic sea surface temperatures and surface winds vary together in a pattern termed the Atlantic Meridional Mode. Simulations with an ocean general circulation model that is driven by a record of dust radiative forcing suggest that dust emissions over West Africa substantially influence the state of the tropical Atlantic Ocean on interannual to decadal scales.

The volatile organic compound isoprene — a precursor to the air pollutant ozone — is produced by many plant species. Canopy-scale measurements in Malaysia, combined with model simulations, suggest that isoprene emissions are under circadian control.

Retreating ice caps overlying volcanoes are thought to relieve surface pressure, causing enhanced magma generation and volcanic activity. Analysis of crustal stress during a dyke intrusion event associated with retreat of Iceland’s largest ice cap indicates that ice retreat could instead promote storage of magma in the crust, rather than eruption at the surface.

Lavas erupted from volcanoes in the north of the Hawaiian volcanic island chain have a different geochemical signature from those in the south. Analysis of the geochemistry of lavas erupted in the Samoan and Marquesas volcanic chains reveals similar trends, implying that the lowermost mantle beneath the southern Pacific is isotopically enriched.

Microbes were thought to be the dominant reef constructors following the end-Permian mass extinction. Sponge–microbe reef deposits formed in the Early Triassic from the western United States suggest that instead, metazoan-reef building continued immediately following the extinction wherever marine conditions allowed.

The mechanisms by which carbon is transported from subducted oceanic crust into the overlying mantle wedge are poorly understood. Geochemical analyses of diamond-bearing fluid inclusions found in the western Alps indicate that carbon dissolution, driven by fluids released from the subducting plate, provides an efficient mechanism to transport carbon into the mantle.

Riverbed cutoffs affect the river-channel geometry and flood-plain morphology. Field studies following two cutoff events in the Wabash River, USA, show that these events also trigger the release of large volumes of sediment, much of which is deposited immediately downstream.

Whether sea level was stable during the last interglacial remains a subject of debate. An analysis of the U–Th ages of coral reefs in the Bahamas, corrected for open-system behaviour, confirms the presence of at least one oscillation within the sea-level highstand.

The effect of expanded glaciation on the relief of mountain ranges is debated. Chronometric and model data from the European Alps suggest that relief in the Rhône Valley was enhanced as a result of the increased incision of the valley following the mid-Pleistocene climate transition.

The mantle transition zone beneath China is anomalously hydrated. Geochemical analyses of basalts erupted above the mantle transition zone in northeast China indicate that water may have been added to this zone during the dehydration of subducted slabs, on two separate occasions over the past one billion years.

During the last interglacial period, the volume of the Greenland ice sheet was up to 60% smaller than today. Climate and ice-sheet modelling suggests that about 55% of this change was caused by higher ambient temperatures and the remaining 45% was a result of higher insolation and the associated climate feedbacks.