Late Cretaceous magnetic anomalies in the North Atlantic

An identification of anomalies 31–34 is presented for the North Atlantic. North of the Azores-Gibraltar Ridge this implies a revision of the identification of the magnetic anomalies older than anomaly 26. DSDP site 10 in the western North Atlantic appears to be located on the old end of anomaly 33. The relative spacings of anomalies 29–34 in the North and South Atlantic, North and South Pacific and Indian Oceans are compared and the estimated relative widths of the magnetic polarity intervals in the Late Cretaceous are revised.     

Geoid anomalies over Gorringe Ridge,North Atlantic Ocean

Gorringe Ridge is a strong uplifted block of oceanic crust and upper mantle lying at the eastern end of the Azores-Gibraltar plate boundary. The geoid over this structure derived from Seasat altimeter data exhibits a 9-m height anomaly with a north-south lateral extension smaller than 200 km. An attempt is made to interpret this geoid together with the gravity anomalies and with the seismicity, which has been compiled as a function of depth.It is first shown that the flexure of the oceanic lithosphere due to the ridge loading does not provide a good fit of the geoid anomalies and probably should be discarded, as it assumes a continuous unfractured elastic plate.Models involving local heterogeneities are then tested. The comparison of the observed geoid anomalies with the anomalies due to the uncompensated relief indicates that the topographic high has no shallow compensation.Uncompensated models, previously proposed to explain the gravity anomalies, are tested using the geoid. One model (Purdy and Bonnin, in Bonnin [11]), which involves an uplift of upper mantle material at depth, generates too strong geoid anomalies and must be discarded. Another model, which represents a nascent subduction zone (Le Pichon et al. [25]), fits both the gravity and geoid anomalies, but leads to difficulties in explaining the deep seismicity north of Gorringe Ridge.A model in isostatic equilibrium is also able to fit both gravity and geoid anomalies. This model involves a deep root of density 3.0 g cm^?3, as has been previously proposed for many oceanic ridges and plateaus. This model is compatible with the deep seismicity, but the origin of this low-density material at great depth is up to now an unresolved question.More likely, dynamical models taking into account the forces induced by the convection related to the slow plate convergence in this area will have to be considered.     

Specific features of the genesis of the Azores-Gibraltar fault zone (North Atlantic)

Maps of the main structures and the degree of geothermal studies of the Azores-Gibraltar and Iceland regions are constructed. Computer modeling of the coefficient of velocity of thermal subsidence into the asthenosphere of the Azores segment of the Mid-Atlantic Ridge is performed. According to the results of modeling, this velocity exceeds the mean velocity of thermal subsidence of the mid-ocean ridges of the world ocean by a factor of ∼ 1.5. The high velocity of subsidence of the Azores segment of the ridge is caused by the influence of the hot substance of the plume on the process of subsidence. The distributions of the heat flow in the Horseshoe basin, Alboran Sea, and southern part of the Iberian Peninsula are analyzed. A zone of increased heat flow and seismicity, extending from the Madeira-Torre Ridge through the Horseshoe basin, and farther to the east through the southern parts of Spain and France, is identified for the first time. The identified geothermal zone marks the northern branch of the diffuse boundary between the Eurasian and African plates. In the region of the Alpine chain, this zone joins the southern boundary between the Eurasian and African plates, which runs along the northern part of Africa and the Apennine Peninsula. The European and African plate boundaries outline the identified Western-Mediterranean plate, which mainly consists of the oceanic and thinned continental lithospheres of the Algerian-Provence and Tyrrhenian basins.     

South Atlantic sea surface temperature anomalies and air-sea interactions: stochastic models

Data on the South Atlantic monthly sea surface temperature anomalies (SSTA) are analysed using the maximum-entropy method. It is shown that the Markov first-order process can describe, to a first approximation, SSTA series. The region of maximum SSTA values coincides with the zone of maximum residual white noise values (sub-Antarctic hydrological front). The theory of dynamic-stochastic climate models is applied to estimate the variability of South Atlantic SSTA and air-sea interactions. The Adem model is used as a deterministic block of the dynamic-stochastic model. Experiments show satisfactorily the SSTA intensification in the sub-Antarctic front zone, with appropriate standard deviations, and demonstrate the leading role of the abnormal drift currents in these processes.     

Estimates of the transition zone temperature in a mechanically mixed upper mantle

Partial melting of mantle peridotite generates a physically and chemically layered oceanic lithosphere that is cycled back into the mantle in subduction zones. Stirring times of the mantle are too long to allow for complete re-homogenization of subducted basalt and harzburgite, given the low chemical diffusivity of the solid mantle. This suggests that the Earth's mantle is a mechanical mixture of basaltic and harzburgitic components. Using a recently developed thermodynamic formulism we determine the phase equilibria and the seismic properties of a mantle comprised of a mechanical mixture of basalt and harzburgite (MM) and a homogeneous mantle (EA) with identical pyrolitic bulk chemistry. We use the theoretical shear velocity profiles as a new thermometer of the mantle below the magma-genetic zone by modeling the difference ΔT_410-660 between traveltimes of shear wave reflections off the 410-km and 660-km with the potential temperature T_P. ΔT_410-660 are measured from waveform stacks. They indicate that, over 1000+ km wave lengths, the temperature varies by about 200 K. Lowest and highest temperatures are resolved for the western Pacific subduction zones and the central Pacific, respectively. This variation is similar for the EA and MM and is in excellent agreement with estimates of transition zone thickness and shear velocity variations. The median value of T_P for the EA is 1720 K. It is about 1625 K for the MM, a value that is in better agreement with the Normal-MORB values of 1610 ± 40 K inferred from olivine-liquid equilibria given that our sampling region encompasses the Western Pacific subduction zones and the oldest parts of the Pacific Plate. We argue therefore that a mechanical mixed mantle, with generally higher velocities and steeper velocities gradients, represents a better physical reference model than a model based on a fully equilibrated assemblage.     

A high-resolution satellite-derived sea surface temperature climatology for the western North Atlantic Ocean

Long-term and high-resolution (∼1.2 km) satellite-derived sea surface temperature (SST) fields of a monthly mean time series for the 1985–1999 period, and a daily climatology have been calculated for the North West Atlantic Ocean. The SST fields extend from 78°W to 41°W in longitude, and 30°N to 56°N in latitude, encompassing the region off Cape Hatteras, North Carolina, to the southern Labrador Sea. The monthly mean time series, consists of 180 cloud-masked monthly mean SST fields, derived from a full-resolution NOAA/NASA Pathfinder SST data set for the 1985–1999 period. The satellite-derived monthly mean SST fields, as compared with in situ monthly mean near-surface ocean temperatures from buoys located in the western North Atlantic, yield an overall RMS difference of 1.15 °C. The daily climatology, which consists of 365 fields, was derived by applying a least-squares harmonic regression technique on the monthly mean SST time series for the full study period. The monthly mean and daily climatological SST fields will be useful for studying inter-annual variability related to climate variability of SST over the study domain.     

白天混合层顶部夹卷层厚度的特征研究

本文首先对Deardorff的一阶模型给予解释,在此基础上分析对流边界层湍流动能方程,分析机械湍流和对流湍流对边界层发展的贡献,提出一个新的速度尺度,混合层顶速度尺度,定义了全理查森数,给出夹卷层厚度的参数化方案,并用Boers和Elotanta的雷达观测数据进行验证。参数化方案与实验数据符合得很好。当夹卷层厚度表示为夹卷速度或夹卷理查森数的函数时,该函数曲线随边界层发展通常表现为磁滞回线形状现象,利用本文的理论进行了解释。     

Internal-wave-induced and double-diffusive nutrient fluxes to the nutrient-consuming surface layer in the oligotrophic subtropical North Atlantic

In the literature, an inconsistency exists between estimates of biotically-effected carbon export inferred from large-scale geochemical studies (Jenkins 1982; 47 gC m^–2 a^–1) and local measurements of turbulent nutrient supply (Lewis et al. 1986; 4 gC m^–2 a^–1) in the eastern subtropical North Atlantic. Nutrient supply to the upper ocean by turbulent mixing is reexamined using local standard oceanographic measurements and high-resolution vertical profiles of nutrients averaged over a large region directly comparable to that investigated by Jenkins (1982).Responsible Editor: Franciscus Colijn     

Neogene crustal emersion and subsidence at the Romanche fracture zone,equatorial Atlantic

Partly phosphatized, oolitic-biogenic limestones were recovered 950–1300 m below sea level from two sites near the crest of a transverse ridge running parallel and adjacent to the Romanche fracture valley (equatorial Atlantic). Some of the biogenic contituents of the limestones (the benthonic foraminiferal genusAmphistegina; corals of the genusStylophora sp., scaphopods, etc.); their paleofacies assemblage (including echinoderms, gastropods, calcareous algae, etc.); and the presence of a well-developed oolitic facies, indicate that the limestones formed in very shallow water close to sea level. In addition, several features of the limestones (including the presence of stromatolite-like laminae, and dissolution features typical of subaerial diagenesis) suggest that the summit of the transverse ridge might have undergone episodes of emersion. The limestones were formed on a shallow carbonate bank at around the Miocene-Pliocene boundary, i.e., 5 ± 1 Myr ago, as determined by the age of fossil planktonic foraminifera and corals. The transverse ridge must have subsided since that time at an average rate of 0.2 mm/yr. It is unlikely that the vertical motions of the Romanche transverse crustal block were caused solely by accreting plate margin- or mantle plume-related volcanic and/or tectonic processes. It is suggested instead that such motions are related to vertical tectonism typical of large oceanic fracture zones.     

Particulate aluminium, iron and manganese chemistry at the deep Atlantic boundary layer

Scanning electron microscopy and instrumental neutron activation analyses of filtered suspended matter from the Atlantic Ocean show that particulate aluminium (Al_p) is a sensitive measure of bottom derived or resuspended material. The proportion of Al_p in suspended particulate matter (SPM) increases slightly between surface and intermediate depths but shows large and steady increases in deep waters with approach to the bottom.

Fe_p/Al_p andMn_p/Al_p ratios are always higher than the crustal ratios throughout the water column. We show that the processes which can explain such enrichments are different for particulate matter in surface waters (scavenging, incorporation in biogenic particles) than for resuspended material (precipitation from interstitial waters on surficial sediments).

Close to the Mid-Atlantic Ridge, the bottom suspended matter exhibits higher Fe_p/Al_p andMn_p/Al_p ratios than in abyssal plains. A ridge crest source must be invoked to explain the striking enrichment of Mn_p. This source could also explain the enrichment of Fe_p, although primarily because the resuspended flux is small in that region, one cannot exclude the contribution of particles from the mid-water column.     

Morphology,magnetic anomalies and basalt magnetization at the ends of the Galapagos high-amplitude zone

Detailed bathymetric and magnetic data, complemented by nine dredge stations, define the eastern and western limits of a belt of high-amplitude magnetic anomalies associated with the Galapagos hot spot. The hypothesis of “magnetic telechemistry” was tested and locally confirmed. High amplitudes correspond to high remanence, susceptibility, FeO^T, TiO₂, and presumably titanomagnetite concentration. The average remanence of surface samples in the high-amplitude zone is 0.027 emu/cm³ (range, 0.009–0.085 emu/cm³), about 4 times that of the normal-amplitude zone. Magnetic amplitudes are only 2–2.5 times higher, however. If the greater TiO₂/FeO^T ratio of high-amplitude zone basalts also characterizes the titanomagnetites, remanence in the high-amplitude zone may fall off more rapidly with depth in the crust as a result of reheating. Alternatively, small pillows of high remanence are more common than larger pillows at the top of the high-amplitude zone crust; FeTi basalt may also be concentrated in the upper part of the crust. Anomaly amplitudes are highest at the ends of the zone, particularly in the east. As asthenosphere crystal slushes presumably flow away from the Galapagos plume, progressive crystal fractionation may enrich residual magmas in FeO^T and TiO₂. The Galapagos FeTi zone terminates abruptly against transform fractures at both ends, perhaps because subaxial flow is dammed at the transforms. The FeTi-producing crystal slushes have advanced east and west at speeds up to 10 cm/yr since they first appeared at the spreading axis at least 6.6 m.y. B.P. Their progressive advance was connected with the progressive southward jumps of the spreading axis east of the Galapagos hot spot, and northward jumps to the west.     

Time variations of the relationships between the North Atlantic Oscillation and European winter temperature and precipitation

The degree of stationarity of relationships between the NAO index and long European temperature and precipitation series in winter is quantified by running correlations with a time window of 31 years at 29 and 27 stations in Europe, respectively. They indicate major nonstationarities in the NAO-to-surface climate relationships at most stations. The temporal course most common for correlations with temperature is a slight change prior to about 1950, followed by an increase; for precipitation, a typical course is a decrease in the first half of the 20^th century, followed by an increase. The temporal variations in correlations do not result from the presence of trends in the time series. The periods of high correlations with temperature are accompanied with an eastward shift of both NAO action centres; the eastward shift is thus at least partly responsible for the time variations in correlations. huth@ufa.cas.cz     

Plastic ingestion in the North Atlantic fulmar

Fulmars found dead on the Dutch coast, and fulmars collected in arctic colonies have considerable quantities of plastic in their stomachs. The average number of plastic items ingested is almost twelve in Dutch fulmars, and four to five in arctic fulmars. User-plastics and industrial plastics are about equally abundant. Ingestion of user-plastics suggests a stronger impact of toxic chemicals from plastics than generally assumed.     

Geomagnetic activity and the North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is the prominent pattern of winter climate variability that has a strong effect on weather in the North Atlantic region and the adjacent continents. At present, uncertainty prevails as to the mechanisms controlling the variability of the NAO. It is also difficult to explain why the positive phase of the NAO has prevailed over the past 37 years (1972–2008). We found high positive correlation coefficients between geomagnetic activity (used as a measure of solar wind intensity) and the NAO indices that equal 0.76 for 1962–1994 and 0.63 for 1961–2011. Positive correlations of the distribution of surface air temperature with the NAO and similarly with geomagnetic activity occur in the Northern Hemisphere. These results encourage our search for possible causes controlling the NAO. We have found that at times of high geomagnetic activity the NAO index is positive and magnetic reconnection may enable the solar wind to initiate downward winds in the magnetosphere. Wind anomalies originate at the edge of the stratospheric polar vortex and propagate downward through the troposphere taking part in the intensification of the vortex and of the westerlies. Stronger northerly winds over Greenland carry cold air southward and, together with the enhanced westerlies, advect the warm air from the Atlantic along the deep Icelandic low into Eurasia increasing temperatures there. On the other hand, at times of low geomagnetic activity, the NAO index is negative and the stratospheric polar vortex is weak. Warm air from the subtropics is carried into the Arctic and a rapid amplification of planetary waves propagating upward may cause displacement or even splitting of the weak vortex and sudden stratospheric warming. During this negative NAO phase the weakened westerlies allow more cold air to build up over North America and Eurasia.     

Changes and variation in the discharge regime of the Upper Suriname River Basin and its relationship with the tropical Pacific and Atlantic SST anomalies

Long‐term changes and variability in river flows in the tropical Upper Suriname River Basin in Suriname (2–6°N, 54–58°W) are analysed, including the relation to sea surface temperatures (SSTs) in the tropical Atlantic and Pacific Ocean. To analyse variability, lag correlation and statistical properties of the data series are used. Long‐term changes are analysed using parametric and non‐parametric statistical techniques. The analyses are performed for the period 1952–1985. The results show that both river discharge series at Semoisie and Pokigron are non‐stationary and have a negative trend. The negative rainfall trend in the centre of Suriname may be responsible for the negative trend in the annual river discharges in the basin. The highest correlation (Pearson's coefficient c) is obtained when the Tropical North Atlantic (TNA) SSTs lags the monthly discharges at Pokigron by 3–4 months (c = 0·7) and when the Tropical South Atlantic (TSA) SSTs lags the discharges by 4 months (c = ? 0·7). It also follows that the high (low) monthly flows, from April–August (September–March) are associated with increasing (decreasing) SSTs in the TNA and with decreasing (increasing) SSTs in the TSA. The results also reveal that years with low (high) discharges are more related to warmer (colder) SSTs during the year in the TNA region and a southward displacement of the Inter‐Tropical Convergence Zone (ITCZ). However, the Pacific El Niño (La Niña) events may also be responsible for low (high) flow years in this basin. Copyright © 2007 John Wiley & Sons, Ltd.     

Exploration of magnetic anomalies of the Baikal rift zone

The paper presents the results of an experimental study of natural magnetic field anomalies in the Baikal rift zone.     

Radium-228 profile at the second Geosecs intercalibration station, 1970, in the North Atlantic

²²⁸Ra concentrations were measured in 12 subsurface water samples collected during August–September 1970 in the North Atlantic at the Second Geosecs Intercalibration Station.²²⁸Ra concentrations ranged from 1.5 to 2.1 dpm/100 kg in the water layer between 100 and 600 m depth, above the main thermocline, but decreased with increasing depth to less than 0.5 dpm/100 kg at 1.5 km, below the main thermocline. The two deepest samples, collected below 4 km depth, indicated that the²²⁸Ra concentration increased again to at least 1.2 dpm/100 kg as the interface with the sea floor sediments was approached. Above the 4 km level, the²²⁸Ra profile was roughly similar to the tritium profile measured by Roether and Münnich.