Letters in 2011

Africa’s topography is characterized by large-scale uplifted domes and subsided basins. Numerical simulations of mantle flow suggest that high topography along Africa’s eastern margin formed as a result of the northward migration of the tectonic plate over the African superplume during the past 30 million years.

Phyllosilicate minerals are rare in the Noachian-aged crust of the northern lowlands of Mars, compared with the tropical highlands. Geochemical and climate modelling suggest that this dichotomy is consistent with the presence of a cold ocean fringed by cold-based glaciers.

Geochemical evidence suggests that sulphur-metabolizing bacteria were present at least 3.5 billion years ago. Geochemical and petrological analyses of microstructures from 3.4-billion-year-old rocks in Western Australia suggest they are the remains of early sulphur-reducing and sulphur-disproportionating bacteria.

Saturn’s moon Titan exhibits an active weather cycle that involves methane. An analysis of cloud observations and simulations with a general circulation model reveals that convection in Titan’s atmosphere is organized through an interplay of two wave modes, leading to local rates of precipitation of up to twenty times the average.

The El Niño–Southern Oscillation is the largest source of interannual variability in the tropical Pacific Ocean. A multidecadal reconstruction of the Southern Oscillation Index for the past 2,000 years shows three prolonged periods of El Niño-dominated conditions.

Stretching of the continental crust can double its surface area, but it is unknown whether similar amounts of extension occur at depth. Seismic results from the central Basin and Range province, western USA, reveal a thick root of lithospheric mantle that has not been extended and indicates that crustal stretching is decoupled from extension at depth.

Ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in Alaska, Greenland and Antarctica. High-frequency measurements of ice speed and basal water temperatures from a calving glacier in Patagonia show that changes in basal water pressure by a few per cent can significantly affect ice flow speed.

Movement of the down-going oceanic plate in subduction zones is accommodated by earthquakes, slow slip and free slip with increasing depth. Analysis of accompanying tremor reveals a continuum of slow-slip events in the Cascadia subduction zone, which suggests that deep free slip of the subducted plate may cause stress to be gradually transferred up the plate interface towards the seismogenic zone.

Additions of reactive nitrogen to terrestrial ecosystems have more than doubled since 1860 as a result of human activities. Model simulations suggest that the climatic benefits of nitrogen-induced carbon sequestration are offset by increased nitrous oxide emissions.

Reservoirs emit significant amounts of greenhouse gases. An analysis of data from 85 globally distributed hydroelectric reservoirs indicates that about 48 Tg carbon is emitted as carbon dioxide and 3 Tg carbon as methane, and that carbon emissions are correlated with reservoir age and latitude.

Faults are generally assumed to be more complicated at the surface than at depth. Analysis of the 2010 El Mayor–Cucapah earthquake, in contrast, reveals that the surface trace is nearly straight but the fault must be highly segmented at depth, thus the characteristics of this earthquake could not have been anticipated from surface geology.

The Australian–Indonesian monsoon is an important component of the climate system in the tropical Indo-Pacific region. High-resolution records of monsoon-controlled austral winter upwelling during the past 22,000 years reveal that glacial–interglacial variations in the Australian–Indonesian winter monsoon have been in phase with the Indian summer monsoon system.

During continental breakup, the onset of seafloor spreading is thought to be marked by the first occurrence of a magnetic anomaly. Analysis of seismic and magnetic data from the Iberia–Newfoundland continental-rift system suggests that the first magnetic anomaly observed here instead represents a magmatic event that pre-dates seafloor spreading.

Fragments of ancient continental lithosphere, entrained in the shallow oceanic mantle, have been found in a number of locations in the Southern Hemisphere. Isotopic analyses of material from Quaternary volcanic centres in Hispaniola indicate that the lavas are derived from an ancient lithospheric fragment with affinities to the supercontinent Gondwana.

Hot mantle upwelling in the Icelandic plume has caused episodic uplift of sedimentary basins located off the northwest coast of Europe. Reconstruction of river profiles on an ancient buried landscape constrains the history of surface uplift and suggests that pulses of hot plume material spread out at velocities of 35 cm yr⁻¹.

Oceanic uptake of carbon dioxide reduces the rate at which anthropogenic carbon accumulates in the atmosphere. Trends in oceanic and atmospheric carbon dioxide concentrations converge on a multidecadal timescale, according to an analysis of North Atlantic surface waters.

World demand for rare-earth elements and the metal yttrium is rapidly increasing. An analysis of more than 2,000 seafloor sediment samples suggests that deep-sea mud constitutes a highly promising giant resource for these elements.

Accurate projections of global sea-level rise require information of future ocean warming in the vicinity of the large ice sheets. An analysis of 19 climate model projections suggests that subsurface ocean warming near both polar ice sheets will be substantial, with the potential to lead to significant increases in ice-mass loss.

Earth’s largest earthquakes occur at the boundary between subducting oceanic crust and the overriding plate, yet the position of the plate boundaries remains uncertain. Analysis of zones of low seismic wave velocities beneath the northern Cascadia subduction zone implies that the plate boundary here may be deeper than previously thought.

The ice shelf buttressing Antarctica’s Pine Island Glacier has been melting rapidly. Observations taken between 1994 and 2009 show that meltwater production has increased by about 50% since 1994, as a result of a stronger circulation below the ice shelf.