A comparative mineralogical study of gas-related sediments of the Gulf of Cádiz

The Gulf of Cádiz area has been extensively surveyed in recent years and several gas-related fluid escape seafloor structures have been identified. In this study, gravity cores, collected during the ANASTASYA/00 and ANASTASYA/01 cruises, on mud volcanoes, hemipelagic sediments and dredged material from diapiric structures, have been studied. A comparative mineralogical analysis by XRD and SEM of samples from different areas has been performed in order to determine whether there is a characteristic mineralogy related to these fluid escape structures, and also to determine the origin of the mud matrix and constrain the depth of the parent units. The mineralogical analysis reflects the different origins of the different units described in the cores: hemipelagic material of the slope, clays that underlie the mud volcanoes and are discharged at the sea bottom surface, and authigenic and diagenetic minerals possibly involved in the anaerobic oxidation of methane in the mud volcano sediments.     

Supply and precipitation of hydrothermal iron in the rift valley of the Mid-Atlantic Ridge at 26° and 29° N

This publication considers the probing data on aquatic anomalies (hydrothermal plumes) in the areas of 26° and 29° N of the Mid-Atlantic Ridge (MAR). The mass of the hydrothermal iron supply and the intensity of the iron sedimentation onto the bottom were estimated by means of sediment traps. It was found that the plume of the TAG hydrothermal vent 6 km³ in volume contained about 67 t of suspended Fe; the plume of the Broken Spur field (up to 8.24 km³ in volume) contained 23.5 t or less because of the lower concentration. The data on the sedimentary matter fluxes showed that 0.3–0.5% of the hydrothermal iron was precipitated immediately from a plume of neutral buoyancy onto the bottom; the bulk of the iron was dissipated into the environment. From the dimensions of the plumes, the flow dynamics, the iron concentrations in the plumes, and the amounts of iron supplied by hydrothermal vents, it was found that the resident time of the plumes considered was from 5 to 10 days.     

Intense Arctic mesocyclones

Characteristics of polar mesoscale cyclones (PMCs) over the North European Basin are analyzed using cloud cover distribution data for 1981–1995. Special features of the annual cycle and interannual variations in characteristics of Arctic mesocyclones with a spiral and comma-shaped cloud structure are discussed. Against the background of large interannual variations, no statistically significant trends were found in the characteristics of Arctic mesocyclones over the North European Basin in the late 20th century. It is shown that the cumulative frequency distribution of PMCs is well approximated by an exponential function in a size range for Arctic mesocyclones from 50 to 400 km. The applicability of the Weibull distribution as an approximation of the PMC size distribution of the number of cyclone days is analyzed. It is shown that the correspondence between the real distribution and the Weibull distribution became worse in the 1990s than in the 1980s, especially the first half of the 1980s. Much of this was due to an increased local maximum in the 1990s in the distribution of polar mesocyclones with diameters about 400 km. This local maximum was found for all types of mesoscale vortices and for all analyzed five-year subperiods during 1981–1995. A large discrepancy between the frequency distribution functions for PMCs depending on their diameters was shown to exist for different types of Arctic mesocyclones.     

Population dynamics of Callichirus major (Say, 1818) (Crustacea, Thalassinidea) on a beach in northeastern Brazil

We describe the structure, reproductive cycle, fecundity, growth, and mortality of a harvested population of the ghost shrimp Callichirus major. Samples were collected at monthly intervals from September 1999 to October 2000 on an urban sandy beach (08°11′S 34°55′W) in northeastern Brazil. During this period the sex ratio did not differ significantly from 1:1 (0.98 M: 1 F). Minimum and maximum sizes of the Dorsal Oval were 2.59 and 12.19 mm for males and 4.46 and 12.62 mm for females, respectively. Ovigerous females were found throughout the period, except between August and September 2000. Maximum lifespan was estimated as 3.3 and 3.4 years for females and males, respectively. This northeastern population differed from others previously studied in southern and southeastern Brazil, in regard to sex ratio, maximum attained size, maturation size, period and duration of the reproductive cycle, and fecundity. We interpret these regional differences as evidence for over-fishing at the study site, and suggest that large-scale management plans for callianassid populations should use regional population parameters.     

Mixed layer variability in Northern Arabian Sea as detected by an Argo float

Seasonal evolution of surface mixed layer in the Northern Arabian Sea (NAS) between 17° N–20.5° N and 59° E-69° E was observed by using Argo float daily data for about 9 months, from April 2002 through December 2002. Results showed that during April - May mixed layer shoaled due to light winds, clear sky and intense solar insolation. Sea surface temperature (SST) rose by 2.3 °C and ocean gained an average of 99.8 Wm⁻². Mixed layer reached maximum depth of about 71 m during June - September owing to strong winds and cloudy skies. Ocean gained abnormally low ∼18 Wm⁻² and SST dropped by 3.4 °C. During the inter monsoon period, October, mixed layer shoaled and maintained a depth of 20 to 30 m. November - December was accompanied by moderate winds, dropping of SST by 1.5 °C and ocean lost an average of 52.5 Wm⁻². Mixed layer deepened gradually reaching a maximum of 62 m in December. Analysis of surface fluxes and winds suggested that winds and fluxes are the dominating factors causing deepening of mixed layer during summer and winter monsoon periods respectively. Relatively high correlation between MLD, net heat flux and wind speed revealed that short term variability of MLD coincided well with short term variability of surface forcing.     

Diagnosis of the interaction of tropical cyclones

At present the mutual influence of tropical cyclones (TCs) has been investigated to a much lesser extent than the cyclones themselves. Most frequently, such investigations are restricted to the study of the influence of the interaction of two TCs on their motion. However, actually, the mutual influence of TCs is much more diverse. For example, each cyclone alters the state of the medium (in particular, decreases the heat store of the upper ocean layer), i.e., affects the subsequent cyclones. Under conditions of a limited energy resource of the medium, cyclones, existing simultaneously, “compete,” to a degree, with each other. These and other similar circumstances have made it practical to consider tropical cyclones and the environment as interdependent elements of a complex open system. This study considers two versions of nonlinear models describing the interaction of TCs with the upper ocean layer and with one another similarly to a number of well-known and verified models of synergetics (interaction of populations, dynamics of optical quantum generators). It is shown that the models reproduce some very important qualitative features of the evolution of TC intensity.     

Steepness and spectrum of a nonlinearly deformed wave on shallow waters

The process of nonlinear deformation of a surface wave on shallow waters is investigated. The main attention is given to the relationship between the wave Fourier spectrum and the steepness of wave front slope. It is shown that an unambiguous relationship couples these quantities in the case of an initially sinusoidal wave, which allows estimation of the spectral composition of the wave field from the observed wave steepness.     

Estimation of Sea Ice Thickness from SARAL/AltiKa in Drifting Orbit Phase

Abstract

Intra and inter-annual variations in the sea ice thickness are highly sensitive indicators of climatic variations undergoing in the earth’s atmosphere and oceans. This paper describes the method of estimating sea ice thickness using radar waveforms data acquired by SARAL/Altika mission during its drifting orbit phase from July 2016 onwards yielding spatially dense data coverage. Based on statistical analysis of return echoes, classification of the surface has been carried out in three different types, viz. floe, lead and mixed. Time delay correction methods were suitably selected and implemented to make corrections in altimetric range measurements and thereby freeboard. By assuming hydrostatic equilibrium, freeboard data were converted into sea ice thickness. Results show that sea ice thickness varies from 4 to 5?m near ice shelves and 1 to 2.5?m in the marginal sea ice regions. Freeboard and sea ice thickness estimates were also validated using NASA’s Operation Ice Bridge (OIB) datasets. Freeboard measurements show very high correlation (0.97) having RMSE of 0.13. Overestimation of approximately 1–2?m observed in the sea ice thickness, which could be attributed to distance between AltiKa footprint and OIB locations. Moreover, sensitivity analysis shows that snow depth and snow density over sea ice play crucial role in the estimation of sea ice thickness.