Letters in 2013

The influence of inherited tectonic-plate strength on the structure of mountain belts is debated. Analysis of geological data collected from mountain belts worldwide shows that the style and amount of deformation in a mountain range are strongly influenced by the age and strength of the colliding plates.

Mantle minerals in faults above a subducting slab can become aligned. Laboratory analyses show that this mineral alignment can also generate direction-dependent friction that can cause faults to slip both seismically and aseismically, depending on the direction of movement.

Earth’s crust formed from melted mantle, yet the earliest record of this process is recorded only in crustal rocks. Isotopic dating of mantle rocks in the Ujaragssuit Nunât intrusion, southwest Greenland, identify melting events that occurred up to 4.36 Gyr ago, providing a mantle record of ancient melting to complement the crustal record.

Low levels of the micronutrient iron limit primary production and nitrogen fixation in large areas of the global ocean. Measurements in the South Atlantic suggest that slow-spreading submarine ridges serve as a significant oceanic iron source in these waters.

During glacial periods, ice sheets covered continental margins through much of Arctic North America, Greenland and western Eurasia. Marine structures suggest that an ice sheet up to a kilometre in depth periodically covered the East Siberian continental shelf as well.

The thermal state of the Earth’s surface is usually influenced more by climate than by heating from the Earth’s interior. Numerical models show that in the oldest and thickest part of the Greenland Ice Sheet, geothermal heat flux through an anomalously thin lithosphere leads to strong regional variations in basal melting.

The magnitude and rate of seismicity differ between subduction zones. Calculations of background seismicity rates, based on a global model of subduction zone seismicity, reveal a positive correlation between relative plate velocity and background seismicity, yet only the seismically quieter zones seem capable of generating magnitude 9 earthquakes.

Between about 50 and 10 thousand years ago, almost 100 genera of large animals went extinct. Mathematical analyses suggest that the extinctions in Amazonia have led to a reduction in the lateral flux of the limiting nutrient phosphorus—by transport of dung and bodies—by 98%.

Methylmercury is a neurotoxin that poses significant health risks to humans. Laboratory experiments suggest that the activity of methylating and non-methylating bacteria may together enhance the formation of methylmercury in anaerobic environments.

Greater Himalayan glaciers are retreating and losing mass. A combination of the latest ensemble of climate models combined with a glacio-hydrological model suggests that in two contrasting watersheds in the Greater Himalaya, glaciers will recede but net glacier melt runoff is on a rising limb until at least 2050.

The mode of carbon storage in Earth’s mantle is unclear. High-pressure laboratory experiments on mantle analogue materials reveal that significant quantities of carbon can be stored in tiny defects within the minerals, providing an efficient mechanism for carbon storage in the mantle.

Sediment grains in rivers are often bound together and stabilized by bacterial films. Experiments and mathematical models show that sediments bound by biofilms behave like a single elastic membrane that can rip catastrophically if the river flows fast enough.

Methane is abundant in marine sediments. Analysis of sediment cores and seismic images of marine sediments obtained off the coast of Pakistan show fracturing of gas hydrates and an increase in upward methane flux in the decades following a large earthquake in the Arabian Sea in 1945, suggesting that quakes can trigger hydrocarbon seepage.

Submarine seeps release substantial amounts of methane into the overlying water column at continental margins, leading to the formation of calcium carbonate deposits. Analyses of methane-derived carbonate build-ups on the Nile Delta suggest that their formation coincided with the development of deep-water anoxic or suboxic conditions.

The precise location of the mantle plume upwelling beneath Hawaii is debated. Seismic data reveal a thick layer of melt in the mantle beneath western Hawaii, implying that the upwelling plume may be deflected around an ancient, resistive root beneath the island.

The Lusi mud eruption in Indonesia has been ongoing since 2006. Numerical simulations show that a parabolic-shaped layer in the rock surrounding the site of the Lusi eruption could have amplified and focussed incoming seismic energy from an earthquake, which then triggered the mud eruption.

Iceberg calving—implicated in the retreat of ice shelves—is a complex process constrained by few observations. Numerical simulations suggest that the pattern of iceberg calving is controlled by the geometry of the glacier, and that regions of Greenland and Antarctica may be particularly vulnerable to catastrophic calving-driven retreat.

The East Antarctic ice sheet is considered to be largely insensitive to temperature changes in the Southern Ocean. Marine sediment records indicate the East Antarctic ice sheet repeatedly retreated by several hundred kilometres during intervals of Pliocene warmth.

The Greenland and Antarctic ice sheets have been reported to be losing mass at accelerating rates. Comparison of mass loss trends over the past decade with reconstructions of past mass loss indicates that the existing satellite record is too short to separate long-term mass loss trends from natural variability.

During the last deglaciation, the North Pacific Ocean was characterized by a spike in primary productivity, which has been attributed to iron input. Marine sediment analyses suggest that, instead, the productivity may have been fuelled by deep convection and subsequent stratification.