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Oceanic nitrogen concentrations are controlled by the balance between nitrogen fixation and denitrification. Examination of nutrient concentrations in the North and South Atlantic Ocean suggests that nitrogen fixation is controlled by the supply of dissolved iron.
Many mountain ranges have asymmetric topography and drainage patterns. Laboratory experiments show that tectonic uplift combined with a precipitation gradient will cause the drainage divide to migrate towards the drier side of the mountain range, thereby triggering the splitting of drainage basins on the dry side of the range.
The Eocene Thermal Maximum 2 occurred 53.5 million years ago in response to elevated atmospheric carbon dioxide levels. Geochemical and microfossil analyses of Arctic sediments show that the during this event the surface of the Arctic Ocean warmed and freshened, and the coldest month mean temperatures did not fall below 8 ∘C.
The cause of high electrical conductivity in the middle crust beneath the Pacific Northwest region of the US is not clear. New electrical-resistivity data reveal a connection between this regional conductor and a localized conductor beneath a prominent volcano in the region, suggesting that the anomalous conductivity is due to the presence of partial melts.
Sea ice is a critical component of the climate and oceanographic system in the North Atlantic Ocean. A biomarker record reveals millennial-scale and glacial–interglacial fluctuations in sea-ice coverage in the northernmost Atlantic Ocean over the past 30,000 years.
Sprite discharges above thunderclouds, at altitudes of 40–90 km, are usually created by a strong positive cloud-to-ground lightning flash. A numerical discharge model of the process suggests that sprite streamers are generated through the collapse of a downward-propagating screening-ionization wave in the lower ionosphere.
Magnesium silicate perovskite, the dominant mineral in the lower mantle, is thought to transform into a post-perovskite phase in the mantle’s lowermost region. Laboratory experiments suggest substantial weakening could occur during the transformation from perovskite to post-perovskite, which could explain the anomalous physical properties of the lowermost mantle.
Past interglacials can be thought of as a series of natural experiments in which boundary conditions varied considerably. Examination of the palaeoclimate record of the past 800,000 years reveals a large diversity among interglacials in terms of their intensity, duration and internal variability.
Both core formation and the late addition of extraterrestrial material have been invoked to explain the abundances and relative proportions of iron-loving elements in the Earth’s mantle. High-temperature experiments suggest that the concentration of gold is consistent with core formation, but the amounts of osmium and iridium require later inputs of extraterrestrial material.
Changes in the sea surface temperature of equatorial waters have critical effects on the large-scale atmospheric circulation. Shipboard measurements of turbulence kinetic-energy dissipation rate indicate that seasonal surface cooling in the central equatorial Pacific may be largely caused by mixing induced by tropical instability waves.
Banded iron formations are plentiful in the rocks representing early Earth, but the mechanisms by which they formed remain controversial. Geochemical modelling indicates that the hydrothermal leaching of low-aluminium ocean crust and subsequent chemical reactions in iron- and silica-rich hydrothermal fluids could have triggered the alternating deposition of iron and silica-dominated sediments.
Geophysical data reveal that at subduction zones oceanic plates could be pervasively hydrated for several kilometres below the crust–mantle boundary. Numerical experiments suggest that such deep hydration is facilitated by negative pressure gradients that lead to the downward pumping of water along bending-related normal faults.
Numerical simulations that assume realistic core-fluid viscosities have been unsuccessful in fully reproducing the unique characteristics of the Earth’s geomagnetic field. An evaluation of boundary conditions suggests that the prescription of a uniform heat flux at the core’s surface could generate a more Earth-like magnetic field.
