Letters in 2010

The climate of early Mars could have supported a complex hydrological system. Analysis of ancient deltaic deposits and valley networks reveals the presence of a planet-wide equipotential surface in the northern lowlands, indicative of the existence of a vast ocean on Mars 3.5 billion years ago.

An episode of climate warming 200 Myr ago was associated with catastrophic environmental changes. Experimental and palaeontological data suggest that a climate-driven shift to more flammable leaf shapes contributed to increased fire activity in East Greenland at this time.

The diversity of marine life has varied throughout the past 500 million years. Statistical analyses suggest that fluctuations in the availability of marine nutrients has been one important regulator of rates of origination during this time.

The decay of soil and sedimentary organic matter yields organic compounds with a high molecular weight, termed humic substances. Sediment-incubation experiments suggest that microbial reduction of solid-phase humics can accelerate iron(III) oxide reduction in wetland soils.

The Earth formed through accretion of many planetary embryos that were probably differentiated into a metallic core and a silicate mantle. The metals and silicates were assumed to fully mix during accretion, but models of Earth’s formation that assume only partial mixing are found to be equally compatible with geochemical observations.

Vertical motions of the rocky margins of Greenland and Antarctica respond to mass changes of their respective ice sheets, but these motions can be obscured by ancient episodes of glacial advance or retreat. An analysis of the acceleration of vertical motion indicates that accelerated ice loss in western Greenland started in the late 1990s.

Lake Tanganyika has become warmer, increasingly stratified and less productive over the past 90 years. Analyses of lake sediments show that this recent warming is unprecedented within the past 1,500 years.

The ratio of nitrogen to phosphorus in phytoplankton varies greatly with taxa and growth conditions. An ecosystem model suggests that the relative abundance of fast- and slow-growing phytoplankton controls the amount of new nitrogen added to the ocean.

Climate-change projections suggest that European summer heatwaves will become more frequent and severe during this century. An analysis of a set of high-resolution regional climate simulations reveals consistent geographical patterns in these changes, with the most severe health impacts in southern European river basins and along the Mediterranean coasts.

The global geoid is characterized by a semi-continuous belt of minima that surround the Pacific Ocean. Simulations with mantle flow models suggest that these geoid lows are correlated with high-velocity anomalies near the base of the mantle and low-velocity anomalies in the mid-to-upper mantle.

An acceleration of ice-mass loss has been observed near the margin of the Greenland ice sheet, partly as a result of faster ice motion. Observations by GPS receivers reveal high seasonal variability in ice motion, with summer motion up to 220% higher than winter background levels.

Nitrous oxide is a potent atmospheric greenhouse gas that is thought to be produced in soils through biological processes. Field measurements reveal nitrous oxide fluxes near Don Juan Pond, Antarctica — of comparable magnitude to those found in tropical soils — which may result from abiotic water–rock reactions.

Deep western boundary currents east of the Antarctic Peninsula and the Kerguelen plateau are important pathways for transporting deep Antarctic water masses to the global ocean. An array of moored current meters, used to quantify the water transport in this system, reveals a flow that is stronger than any measured in a deep western boundary current at similar depths so far.

The loss of carbon dioxide from soils increases initially under climate warming, but tends to decline to control levels within a few years. Simulations of the soil-carbon response to warming with a microbial-enzyme model show that a decline in both microbial biomass and the production of degrading enzymes can explain this attenuation response.

Natural petroleum seepage emits large volumes of oil and methane to the oceans every year, accompanied by the formation of asphalt volcanoes on the sea floor. The discovery of seven asphalt volcanoes off the coast of southern California may help to explain high methane emissions recorded during the late Pleistocene.

Deformation of the Himalaya and Tibet is thought to relate to flow within a weak crustal channel at depth. Magnetotelluric imaging of the Earth’s subsurface reveals a complex pattern of deformation, with two distinct weak crustal channels at 20–40 km depth.

The 100,000-year glacial cycles are generally thought to be driven by the eccentricity of the Earth’s orbit. Statistical analyses of climate variability and orbital forcing over the past five million years indicate that the glacial cycles are the result of an internal climate oscillation phase locked to the 100,000-year eccentricity cycle.

The impact of thawing permafrost on the nitrogen cycle is uncertain. Laboratory experiments using permafrost cores from northeast Greenland reveal that rewetting of thawed permafrost increases nitrous oxide production over 20-fold.

The sedimentary deposits at Meridiani Planum on Mars were formed in acidic surface waters. Geochemical calculations show that the oxidation of dissolved iron and the precipitation of oxidized iron minerals in the surface waters could be sufficient to generate the inferred acidity.

The cause of the Laramide phase of mountain building remains uncertain. Modelling and plate reconstructions show that Laramide events coincide with subduction of the Shatsky oceanic plateau, implicating surface rebound after removal of the subducting plateau in Laramide uplift.