Small-scale convection beneath oceans and continents

Small-scale convection supplies heat flow of ～17 mW m-2 to the base of stable continents where xenolith studies resolve the geotherm.However,effects of small-scale convection are difficult to resolve in ocean basins.On first pass,most seafloor appears to subside to an asymptote compatible with ～40 mW m-2 convective heat flow.These common regions are tracked by hotspots so uplift associated with ponded mantle material is an attractive alternative.Unaffected seafloor in the North and South Atlantic continues to subside with the square root of age as expected from pure conduction.The theory of stagnant-lid convection provides good scaling relationships for heat flow.For linear viscosity,heat flow is proportional to the underlying "half-space" viscosity to the 1/3 power and the temperature to change viscosity by a factor of e to the 4/3 power.The formalism is easily modified to represent convection beneath a lid of highly viscous and buoyant cratonal lithosphere and to represent transient convection beneath thickening oceanic lithosphere.Asthenospheric mantle with linear,strongly temperature-dependent,and weakly depth-dependent viscosity is compatible with both oceanic and continental data.More complicated rheology may allow vigorous small-scale convection under most but not all old ocean basins.Still viable hypotheses require poorly understood global features,including lateral variations of asthenospheric temperature.Seismological studies have the potential to resolve the lithosphere-asthenosphere boundary,including local variations of its depth associated with small-scale convection.     

A newly discovered Roseobacter cluster in temperate and polar oceans

Bacterioplankton phylotypes of alpha-Proteobacteria have been detected in various marine regions, but systematic biogeographical studies of their global distribution are missing. Alpha-Proteobacteria comprise one of the largest fractions of heterotrophic marine bacteria and include two clades, SAR11 and Roseobacter, which account for 26 and 16% of 16S ribosomal RNA gene clones retrieved from marine bacterioplankton. The SAR11 clade attracted much interest because related 16S rRNA gene clones were among the first groups of marine bacteria to be identified by cultivation-independent approaches and appear to dominate subtropical surface bacterioplankton communities. Here we report on the global distribution of a newly discovered cluster affiliated to the Roseobacter clade, comprising only as-yet-uncultured phylotypes. Bacteria of this cluster occur from temperate to polar regions with highest abundance in the Southern Ocean, but not in tropical and subtropical regions. Between the south Atlantic subtropical front and Antarctica, we detected two distinct phylotypes, one north and one south of the polar front, indicating that two adjacent but different oceanic provinces allow the persistence of distinct but closely related phylotypes. These results suggest that the global distribution of major marine bacterioplankton components is related to oceanic water masses and controlled by their environmental and biogeochemical properties.     

No oceans on Titan from the absence of a near-infrared specular reflection

With its substantial atmosphere of nitrogen, hydrocarbons and nitriles, Saturn's moon Titan is a unique planetary satellite. Photochemical processing of the gaseous constituents produces an extended haze that obscures the surface. Soon after the Voyager fly-bys in 1980 and 1981 photochemical models led to the conclusion that there should be enough liquid methane/ethane/nitrogen to cover the surface to a depth of several hundred metres. Recent Earth-based radar echoes imply that surface liquid may be present at a significant fraction of the locations sampled. Here we present ground-based observations (at near-infrared wavelengths) and calculations showing that there is no evidence thus far for surface liquid. Combined with the specular signatures from radar observations, we infer mechanisms that produce very flat solid surfaces, involving a substance that was liquid in the past but is not in liquid form at the locations we studied.     

Temperature excludes N2-fixing heterocystous cyanobacteria in the tropical oceans

Whereas the non-heterocystous cyanobacteria Trichodesmium spp. are the dominant N2-fixing organisms in the tropical oceans, heterocystous species dominate N2 fixation in freshwater lakes and brackish environments such as the Baltic Sea. So far no satisfactory explanation for the absence of heterocystous cyanobacteria in the pelagic of the tropical oceans has been given, even though heterocysts would seem to represent an ideal strategy for protecting nitrogenase from being inactivated by O2, thereby enabling cyanobacteria to fix N2 and to perform photosynthesis simultaneously. Trichodesmium is capable of N2 fixation, apparently without needing to differentiate heterocysts. Here we show that differences in the temperature dependence of O2 flux, respiration and N2 fixation activity explain how Trichodesmium performs better than heterocystous species at higher temperatures. Our results also explain why Trichodesmium is not successful in temperate or cold seas. The absence of heterocystous cyanobacteria in the pelagic zone of temperate and cold seas, however, requires another explanation.     

Widespread magma oceans on asteroidal bodies in the early Solar System

Immediately following the formation of the Solar System, small planetary bodies accreted, some of which melted to produce igneous rocks. Over a longer timescale (15-33 Myr), the inner planets grew by incorporation of these smaller objects through collisions. Processes operating on such asteroids strongly influenced the final composition of these planets, including Earth. Currently there is little agreement about the nature of asteroidal igneous activity: proposals range from small-scale melting, to near total fusion and the formation of deep magma oceans. Here we report a study of oxygen isotopes in two basaltic meteorite suites, the HEDs (howardites, eucrites and diogenites, which are thought to sample the asteroid 4 Vesta) and the angrites (from an unidentified asteroidal source). Our results demonstrate that these meteorite suites formed during early, global-scale melting (> or = 50 per cent) events. We show that magma oceans were present on all the differentiated Solar System bodies so far sampled. Magma oceans produced compositionally layered planetesimals; the modification of such bodies before incorporation into larger objects can explain some anomalous planetary features, such as Earth's high Mg/Si ratio.     

Mechanical energy input to the world oceans due to atmospheric loading

MECHANICAL ENERGY INPUT FROM THE ATMOSPHERE IS ONE OF THE MOST IMPORTANT SOURCES OF ENERGY DRIVING THE OCEANIC GENERAL CIRCULATION[1,2]. THIS ENERGY IS TRANS- FERRED TO THE OCEANS, MOSTLY THROUGH SEA SURFACE WIND STRESS AND ATMOSPHERIC PRESSURE PERTURBATI…     

Vigorous exchange between the Indian and Atlantic oceans at the end of the past five glacial periods

The magnitude of heat and salt transfer between the Indian and Atlantic oceans through 'Agulhas leakage' is considered important for balancing the global thermohaline circulation. Increases or reductions of this leakage lead to strengthening or weakening of the Atlantic meridional overturning and associated variation of North Atlantic Deep Water formation. Here we show that modern Agulhas waters, which migrate into the south Atlantic Ocean in the form of an Agulhas ring, contain a characteristic assemblage of planktic foraminifera. We use this assemblage as a modern analogue to investigate the Agulhas leakage history over the past 550,000 years from a sediment record in the Cape basin. Our reconstruction indicates that Indian-Atlantic water exchange was highly variable: enhanced during present and past interglacials and largely reduced during glacial intervals. Coherent variability of Agulhas leakage with northern summer insolation suggests a teleconnection to the monsoon system. The onset of increased Agulhas leakage during late glacial conditions took place when glacial ice volume was maximal, suggesting a crucial role for Agulhas leakage in glacial terminations, timing of interhemispheric climate change and the resulting resumption of the Atlantic meridional overturning circulation.     

黑潮对中国近海热盐输运的季节和年际变化

应用垂向混合坐标系海洋模式（HYCOM）,对赤道以及北太平洋进行了气候态模拟和1990-1999年的模拟.给出了黑潮对我国近海热量和盐量输运的定量结果,并对其季节和年际变化特征进行了分析.气候态模拟的结果表明,黑潮对东海的热量输运与该区域的海表面热通量同量级,且呈反向变化,热量输运冬季最大,夏季最小,7月为-9×1013W,2月为2.5×1014W,春、秋为过渡季节.黑潮向南海输运的热量除夏季为负值外,其它季节均为正值.黑潮年均向东海输运热量1.67×1014 W,向南海输运热量1.5×1014W.盐量输运的变化趋势与热量一致.对1990-1999年的模拟结果进行小波分析表明,黑潮对东海热量和盐量输运具有4~7年的显著周期,且与厄尔尼诺现象密切相关.厄尔尼诺年,黑潮向东海输运的热量和盐量均有明显减小.黑潮对南海热量和盐量输运的周期约为3~4年.黑潮对南海的热量输运也与厄尔尼诺现象存在负相关关系.     

Analyses of the predicted changes of the global oceans under the increased greenhouse gases scenarios

A new climate model (ECHAM5/MPI-OM1) developed for the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC) at Max-Planck Institute for Meteorology is used to study the climate changes under the different increased CO2 scenarios (B1, A1B and A2). Based on the corresponding model results, the sea surface temperature and salinity structure, the variations of the thermohaline circulation (THC) and the changes of sea ice in the northern hemisphere are analyzed. It is concluded that from the year of 2000 to 2100, un- der the B1, A1B and A2 scenarios, the global mean sea surface temperatures (SST) would increase by 2.5℃, 3.5℃ and 4.0℃ respectively, especially in the region of the Arctic, the increase of SST would be even above 10.0℃; the maximal negative value of the variation of the fresh water flux is located in the sub- tropical oceans, while the precipitation in the eastern tropical Pacific increases. The strength of THC de- creases under the B1, A1B and A2 scenarios, and the reductions would be about 20%, 25% and 25.1% of the present THC strength respectively. In the north- ern hemisphere, the area of the sea ice cover would decrease by about 50% under the A1B scenario.     

Phylogenetic relationship and phenotypic comparison of Psychrobacter species isolated from polar oceans

To investigate the phylogenetic relationship and biogeography of bacterioplankton in polar oceans, four Psychrobacter strains, BSw10170, BSw20352, BSw20370, and BSw20461, isolated from seawater of the Bering Sea, the Chukchi Sea, and the Prydz Bay, were characterized by 16S rDNA sequencing and physiological and biochemical testing. Results demonstrated that close relationships existed between the Arctic and Antarctic strains with sequence similarities higher than 97%. These four Psychrobacter strains not only showed almost identical phenotypic characteristics among them, but also shared a lot of similarities with those related Psychrobacter species, indicating that psychrotolerance and halotolerance of Psychrobacter strains may be among the reasons for their bipolar, even global distribution in marine environments at the genus level.