Volume 8 Issue 9, September 2015

The Earth underwent two snowball glaciation events between 720 and 635 million years ago. The preceding expansion of eukaryotic algae and a consequent rise in emissions of organic cloud condensation nuclei may have contributed to the dramatic cooling.

Braided channels are rare on ocean floors, but abundant on land. Experiments and theory suggest that deeper flows and rapid overbank deposition restrict braiding in underwater rivers relative to their terrestrial counterparts.

Tropospheric ozone is generated from precursor pollutants, but can be blown far afield. Satellite observations show rising ozone levels over China — and almost stable levels over western North America despite stricter regulations.

Saturn's F ring is chaperoned on both sides by the tiny moons Prometheus and Pandora. Numerical simulations show that this celestial ballet can result from the collision of two aggregates that evolved out of Saturn's main rings.

Evidence for a Neoproterozoic Snowball Earth in the sedimentary record has been controversial. A weathered horizon preserved in sedimentary rocks from Svalbard may provide a rare signature of prolonged global glaciation.

Sea surface temperatures have varied over the past 2,000 years. A synthesis of surface-temperature reconstructions shows ocean surface cooling from ad 1 to 1800, with much of the trend from 800 to 1800 driven by volcanic eruptions.

The material properties of the Earth’s core have been better constrained by recent technical and computational advances. The properties imply that the core was once hot, but is cooling quickly, and the inner core is young.

Lying beyond Saturn’s main rings, the F ring is shepherded by two small satellites. Simulations suggest that this enigmatic configuration can result from the partial disruption of satellites in collisions at the edge of the main ring system.

Tropospheric levels of ozone and its precursors have risen in Asia since 2000. Satellite observations and chemistry–transport simulations suggest that transport of these pollutants to North America partly offsets benefits from stricter regulation.

Rivers and streams are key sources of CO₂. Estimated emissions and aquatic productivity from across the US show that small streams predominantly emit CO₂ produced in soils, but the contribution from aquatic metabolism increases with river size.

Braided channels are rare in submarine environments, yet common in fluvial systems. Laboratory experiments suggest that the formation mechanisms are the same, but submarine channels are typically not wide enough to promote braiding.

The presence of dynamic ice sheets during Snowball Earth glaciations is controversial. Geological evidence and ice sheet modelling suggest that ice sheets may have responded to orbital forcing when $p_{{\text{CO}}_2}$ reached a certain threshold.

Faults are unlocked by earthquakes. Analysis of seismic data from the 2015 Nepal earthquake shows that only part of the Main Himalayan Thrust fault was unzipped by the quake, leaving much of the fault locked and ready to slip in a future event.

Plate tectonic motions cause faults to slip during earthquakes and through creep. Laboratory shear experiments on semi-brittle rock-analogue materials suggest that such slip could occur via growth of a series of fractures.

Long-term changes of the mountain glaciers in the Tien Shan, Central Asia, are not well constrained. Remote sensing data and glaciological models reveal a 27% decline of glacier mass from 1961 to 2012, linked to increased summer melting.

Ammonium levels in Greenland ice cores track changes in soil emissions and wildfires, primarily in North America. Ice-core ammonium records show abrupt increases in wildfire activity during brief warmings in the last glacial period.

How plate tectonic subduction is initiated is unclear. Analysis of sediments and rock cores taken from the Izu–Bonin–Mariana subduction zone trench suggests subduction started spontaneously in this region around 50 million years ago.