Research articles

Surface waters of most of the world’s oceans are supersaturated with respect to atmospheric methane. Measurements in seawater samples suggest that an aerobic methane production pathway, which involves the decomposition of phosphorus-containing organic compounds, may be responsible.

The rifting of the Seychelles microcontinent from India involved two phases of extensional activity. The initial separation of the Laxmi Ridge from India was accompanied by extensive magmatism but the later separation of the Seychelles from the Laxmi Ridge was only weakly magmatic.

The surface waters of the North Atlantic subtropical gyre are depleted in phosphate, relative to the South Atlantic gyre. Despite this nutrient limitation, the two gyres have comparable rates of carbon fixation. Measurements of enzyme activity suggest that dissolved organic phosphorus may be fuelling northern productivity.

Although North Atlantic deep-water formation was greatly reduced during the last glacial maximum, bottom-water currents were as vigorous as at present. However, they were weakened during periods of North Atlantic surface freshening. A strong correlation can be seen between bottom-water-current strength and Greenland air-temperature records, thus confirming a close connection between ocean circulation and abrupt climate change.

Eclogites have been suggested as high niobium/tantalum reservoirs that complement the low niobium/tantalum ratios of the silicate Earth. However, the hafnium isotopic composition of eclogite fragments suggest that the high niobium/tantalum signature of eclogites is unlikely to be primary. Instead, it probably reflects chemical modification during residence in the subcontinental lithospheric mantle.

During the Cretaceous period, warm deep and intermediate waters filled the oceans. Evidence from benthic foraminferal δ¹⁸O and Mg/Ca ratios suggests that the intermediate water masses in the proto-Atlantic Ocean formed from high salinity waters sinking from shallow shelf seas.

Abrupt changes in the African Monsoon, which have been recorded throughout the late Pleistocene and early Holocene epochs, tend to coincide with changes in North Atlantic thermohaline circulation. A numerical simulation shows that the interaction between thermohaline circulation in the North Atlantic Ocean and wind-driven currents in the topical Atlantic Ocean contributes to the rapidity of African Monsoon transitions during abrupt climate change events.

Silicate weathering reactions remove carbon dioxide from the atmosphere and store it in carbonate minerals. During the high atmospheric carbon dioxide conditions of the Early Eocene Climatic Optimum, rates of chemical weathering, physical erosion and denudation in the western USA were equivalent to the highest recorded rates in the non-glacial Quaternary.

Surface waves that were generated by 12 out of 15 earthquakes with magnitudes greater than 7 since 1990 led to a global increase in the number of small earthquakes. This suggests that dynamic triggering of earthquakes is common and is independent of the tectonic environment.

Analogue modelling of caldera-forming eruptions suggests that sinking of the magma chamber roof is variable in space and time, leading to substantial stirring and mixing of magma. This can explain the common occurrence of geochemical zonation and magma mingling in deposits erupted from calderas.

There has been a strong disagreement between model predictions of troposphere warming and observations of temperature trends from radiosondes and satellites. However, when tropospheric temperature reconstructions are generated from thermal-wind measurements and the thermal-wind equation for 1970–2005, the results show a strong tropospheric warming trend, in agreement with model predictions.

Variations in the Earth’s magnetic field over a span of a few months can be resolved despite the potential filtering effects of the electrically conducting mantle, and are indicative of rapid flow in the Earth’s outer core.

High-resolution data from the Mars Reconnaissance Orbiter are used to identify clay-rich fluvial-lacustrine sediments in an open lake basin on Mars near the 45-km-diameter Jezero crater. The basin contains sedimentary deposits of hydrous minerals sourced from a smectite-rich catchment in the Nili Fossae region, which are well suited for the sequestration and preservation of organic material.

Using projected boundary conditions for the end of the twenty-first century, the frequency of Atlantic tropical cyclones and hurricanes in a regional climate model of the Atlantic basin is reduced compared with observed boundary conditions at the end of the twentieth century. This is inconsistent with the idea that higher levels of atmospheric greenhouse gases will result in increased Atlantic hurricane activity.

Fjords line mountainous continental margins where icesheets and glaciers once stood. A two-dimensional model simulation suggests that fjords can be eroded within one million years, primarily in response to topographic ice steering and erosion from ice discharge. Subsequent glaciers that form on these landscapes are smaller and exhibit greater responses to climate change.

Overlapping subduction of the Pacific and Philippines Sea plates leads to an enhanced fluid flux to the mantle source of arc volcanoes in central Japan. Spatial variability in the amount of fluid that each plate contributes is determined by the configuration of the subducting plates.

Reconstructions of palaeosecular variation suggest that the Earth’s magnetic field reversed less frequently 2.82 to 2.45 billion years ago, relative to the Cenozoic era. This suggests a long-term trend of decreasing geodynamo stability since the Archaean eon.

On geological timescales, carbon dioxide enters the atmosphere through volcanism and organic matter oxidation and is removed through mineral weathering and carbonate burial. An analysis of ice-core CO₂ records and marine carbonate chemistry indicates a tight coupling between these processes during the past 610,000 years, which suggests that a weathering feedback driven by atmospheric CO₂ leads to a mass balance between CO₂ sources and sinks on long timescales.

Submarine groundwater discharge, estimated from a ²²⁸Ra inventory across the upper Atlantic Ocean, provides a flux of 2–4×10¹³ m³ yr⁻¹, equivalent to 80–160% of the influx from rivers into the Atlantic Ocean.

Tropospheric ozone contributes significantly to human-induced greenhouse warming. Calculations from satellite measurements of spectral radiance suggest that ozone in the upper troposphere caused an average reduction in clear-sky outgoing long-wave radiation over the oceans of 0.48±0.14 W m⁻² for the year 2006 between 45^∘ S and 45^∘ N.