Volume 6 Issue 9, September 2013

The end of the Pleistocene epoch saw the extinction of large-bodied herbivores around the world. Numerical modelling suggests that continental-scale effects of this extinction on nutrient transport are ongoing.

The East Antarctic ice sheet is believed to be Earth's most stable ice sheet. Changes in geochemical composition of offshore sediments suggest that its margin repeatedly retreated by at least 350–550 kilometres inland between 5.3 and 3.3 million years ago.

The classical view of fluvial sediment transport considers only physical interactions between the river flow and riverbed particles. Experiments and theory suggest that microbial biofilms reduce sediment mobility by binding many grains together.

Climate change is affecting the cryosphere from above. Geothermal heat flux from below is also contributing to conditions at the base of Greenland's ice sheet, which sits atop a lithosphere of variable thickness.

Particles of organic matter in the ocean host diverse communities of microorganisms. These particles may serve as hotspots of bacterial gene exchange, creating opportunities for microbial evolution.

The Arctic is warming faster than any other region in the world. The resultant large-scale shift in sea ice cover could increase oceanic emissions of dimethylsulphide, a climate-relevant trace gas generated by ice algae and phytoplankton.

Photosynthetic microbes, collectively termed phytoplankton, are responsible for the vast majority of primary production in marine waters. A synthesis of the latest research suggests that two broad nutrient limitation regimes — characterized by nitrogen and iron limitation, respectively — dictate phytoplankton abundance and activity in the global ocean.

Coastal upwelling regimes associated with eastern boundary currents are the most biologically productive ecosystems in the ocean. A suite of human-induced changes could perturb primary production and nutrient cycling in these highly dynamic systems.

The flux of carbon out of the ocean surface is not sufficient to meet the energy demands of microbes in the dark ocean. A review of the literature suggests that non-sinking particles and microbes that convert inorganic carbon into organic matter could help to meet this deep-ocean energy demand.

The leakage of cold, methane-rich fluids from subsurface reservoirs to the sea floor sustains some of the richest ecosystems on the sea bed. These cold-seep communities consume around two orders of magnitude more oxygen than the surrounding sea floor as a result of the microbial consumption of seep methane.

The remote detection of surface water indigenous to the Moon has proved difficult because of alternative sources, such as the solar wind. Spectroscopic observations of hydroxyl-bearing materials in Bullialdus Crater by the Chandrayaan-1 spacecraft are consistent with indigenous magmatic water that was excavated by impact from the lunar interior.

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 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.

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.

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.

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%.

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.

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.

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.

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.

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.

Marine cyanobacteria supply much of the nitrogen that supports open ocean food webs and biogeochemical cycles. An experimental study suggests that the relationship between nitrogen fixation and carbon dioxide concentration varies significantly between cyanobacterial strains.

Sea level during the last interglacial period reached a peak of between 5 and 9 m above the present-day level. A detailed reconstruction of sea level and isostatic rebound from Western Australia indicates a prolonged period of sea-level stability at 3–4 m above present, followed by an abrupt sea-level rise of 5–6 m.

Microbes regulate the cycling of elements throughout the global ocean, from the icy surface waters that circulate at high latitudes, to the deep and surprisingly diverse vents that dot the continental seafloor. Human activities are starting to modify the way in which microbes mediate these cycles, at least in the relatively well-characterised waters of the upper ocean. The deeper layers of the ocean are probably less affected, at least at present, and are definitely less well explored. However, technological advances are starting to shed light on the cycling of elements at depth, revealing microbial systems that are quite different from those at the surface. In this Nature GeoscienceInsight we highlight some of the most intriguing advances in the microbial biogeochemistry of the oceans, a field that is very much in flux.