10-m Layer Research Articles

Episodic riverine influence on surface DIC in the coastal Gulf of Maine

Anomalously high precipitation and river discharge during the spring of 2005 caused considerable freshening and depletion of dissolved inorganic carbon (DIC) in surface waters along the coastal Gulf of Maine. Surface pCO 2 and total alkalinity (TA) were monitored by repeated underway sampling of a cross-shelf transect in the western Gulf of Maine (GOM) during 2004–05 to examine how riverine fluxes, mixing, and subsequent biological activity exert control on surface DIC in this region. Most of the variability in surface DIC concentration was attributable to mixing of low DIC river water with higher DIC, saline GOM waters, but net biological uptake of DIC was significant especially during the spring and summer seasons. The extent and persistence of the coastal freshwater intrusion exerted considerable influence on net carbon dynamics. Integrated over the 10-m surface layer of our study region (∼5 × 10 4 km 2), net biological DIC uptake was 0.48 × 10 8 mol C during April–July of 2004 compared to 1.33 × 10 8 mol C during April–July of 2005. We found the temporal signature and magnitude of DIC cycling to be different in adjacent plume-influenced and non-plume regions. Extreme events such as the freshwater anomaly observed in 2005 will affect mean estimates of coastal carbon fluxes, thus budgets based on short time series of observations may be skewed and should be viewed with caution.

Peculiarities of diurnal variations of the vertical gradient of the atmosphere refraction index in Eastern Russia

In this paper, diurnal variations of the vertical gradient of the atmosphere refraction index in the lower 300-m layer are computed and analysed in detail for a vast territory of Eastern Siberia and Far East of Russia by using 10-year observation data obtained at 32 aerological stations. In particular, it was found out that diurnal variation of the gradient is more diverse in summer and less diverse in fall. In the most north-easterly regions, unlike other climate zones where maximum gradient changes occur in summer and in winter, the greatest diurnal fluctuations of gn are observed in spring. In the studied region, amplitudes of diurnal variations of gn are significant and quite comparable with those of inter-seasonal oscillations of gn.

Velocity Structure of the West-Bengal Sedimentary Basin, India along the Palashi-Kandi Profile Using a Travel-time Inversion of Wide-angle Seismic data and Gravity Modeling – An Update

The 2-D shallow velocity structure along the north-south Palashi-Kandi profile in the West Bengal sedimentary basin has been updated by travel-time inversion of seismic refraction, wide-angle reflection and gravity data. A six-layer shallow model up to a depth of about 7 km has been derived. The first layer, which has an average velocity of 2.0 kms−1, represents the alluvium deposit, which rests over the shale formation with average velocity of 3.0 kms−1. The thin (200 m) Sylhet limestone, observed at a nearby Palashi well, remains hidden in the present data set. Hence a 200-m thin layer with a velocity of 3.7 kms−1, corresponding to the Sylhet limestone, has been assumed to be present throughout the profile. The fourth layer with a velocity of 4.5–4.7 kms−1 at a depth of 1.7–2.4 km represents the Rajmahal traps. The ‘skip’ phenomenon and rapid amplitude decay of first arrivals indicate a low-velocity zone (LVZ) in the study area. Using the ‘skip’ phenomena and wide-angle reflection data, identified on seismograms, the LVZ with a velocity of 4.0 kms−1, indicating the Gondwana sediments, has been delineated below the Rajmahal traps. The next layer with a velocity 5.4–5.6 kms−1 overlying the crystalline basement (5.8–6.25 kms−1) may be associated with the Singhbhum group of meta volcanic rock that has been exposed in the western part of the basin. The basement lies at a variable depth of 4.9 to 6.8 km. The overall uncertainties of various velocity and boundary nodes are ± 0.15 kms−1 and ± 0.5 km, respectively. The elevated basement feature in the north might have acted as a structural barrier for the deposition of Sylhet limestone during the Eocene epoch. The seismically derived shallow structure correctly explains the observed Bouguer gravity anomaly along the profile. The addition of reflections in the present analysis provides a stronger control on the depths and velocities of basement and overlying sedimentary formations, compared to the earlier model derived mainly by the first arrival seismic data.

Microbiological and biogeochemical processes in a pockmark of the Gdansk depression, Baltic Sea

Comprehensive microbiological and biogeochemical investigation of a pockmark within one of the sites of gas-saturated sediments in the Gdansk depression, Baltic Sea was carried out during the 87th voyage of the Professor Shtokman research vessel. Methane content in the near-bottom water and in the underlying sediments indicates stable methane flow from the sediment into the water. In the 10-m water layer above the pockmark, apart from methane anomalies, elevated numbers of microorganisms and enhanced rates of dark CO2 fixation (up to 1.15 micromol C/(1 day)) and methane oxidation (up to 2.14 nmol CH4/(1 day)) were revealed. Lightened isotopic composition of suspended organic matter also indicates high activity of the near-bottom microbial community. Compared to the background stations, methane content in pockmark sediments increased sharply from the surface to 40-60 ml/dm3 in the 20-30cm horizon. High rates of bacterial sulfate reduction (SR) were detected throughout the core (0-40 cm); the maximum of 74 micromol/(dm3 day) was located in subsurface horizons (15-20 cm). The highest rates of anaerobic methane oxidation (AMO), up to 80 micromol/(dm3 day), were detected in the same horizon. Good coincidence of the AMO and SR profiles with stoichiometry close to 1:1 is evidence in favor of a close relation between these processes performed by a consortium of methanotrophic archaea and sulfate-reducing bacteria. Methane isotopic composition in subsurface sediments of the pockmark (from -53.0 to -56.5% per hundred) does not rule out the presence of methane other than the biogenic methane from the deep horizons of the sedimentary cover.

Modeling distributions of flying insects: Effective attraction radius of pheromone in two and three dimensions

The effective attraction radius (EAR) of an attractive pheromone-baited trap was defined as the radius of a passive “sticky” sphere that would intercept the same number of flying insects as the attractant. The EAR for a particular attractant and insect species in nature is easily determined by a catch ratio on attractive and passive (unbaited) traps, and the interception area of the passive trap. The spherical EAR can be transformed into a circular EAR c that is convenient to use in two-dimensional encounter rate models of mass trapping and mating disruption with semiochemicals to control insects. The EAR c equation requires an estimate of the effective thickness of the layer where the insect flies in search of mates and food/habitat. The standard deviation ( SD) of flight height of several insect species was determined from their catches on traps of increasing heights reported in the literature. The thickness of the effective flight layer ( F L ) was assumed to be SD · 2 · π , because the probability area equal to the height of the normal distribution, 1 / ( SD · 2 · π ) , times the F L is equal to the area under the normal curve. To test this assumption, 2000 simulated insects were allowed to fly in a three-dimensional correlated random walk in a 10-m thick layer where an algorithm caused them to redistribute according to a normal distribution with specified SD and mean at the midpoint of this layer. Under the same conditions, a spherical EAR was placed at the center of the 10-m layer and intercepted flying insects distributed normally for a set period. The number caught was equivalent to that caught in another simulation with a uniform flight density in a narrower layer equal to F L , thus verifying the equation to calculate F L . The EAR and F L were used to obtain a smaller EAR c for use in a two-dimensional model that caught an equivalent number of insects as that with EAR in three dimensions. This verifies that the F L estimation equation and EAR to EAR c conversion methods are appropriate.

Assessing bottom structure influence on fish abundance in a marine hill by using conjointly acoustic survey and geographic information system

Acoustic survey and a geographic information system (GIS) were used conjointly to estimate the significance of bottom substrate to explain fish abundance distribution in the complex topography context of a marine hill. The survey was conducted in the Hachirigase hill, off south- western Honshu Island in the Sea of Japan. Analysis of acoustic data focused on a 10-m thick layer beginning at the bottom, assuming that bottom influence does not exceed this distance. Area backscattering strength (Sa) values were rescaled in order to reflect real differences before being analyzed. Results show that the influence of the different parameters taken into account (bottom depth, bottom substrate and water temperature) varies widely from one place to another. Bottom substrate is above all important in the zone located to the west of the peak. The kriging and stratification methods were applied to the Sa distribution for estimating the total Sa in the hill. Differences between results are discussed. Conjoint use of acoustic survey and GIS is an effective method for analyzing and estimating fish abundance distribution in a marine hill.

Estimating Seasonal Changes in Volumetric Soil Water Content at Landscape Scales in a Savanna Ecosystem Using Two-Dimensional Resistivity Profiling

Abstract Water distributed in deep soil reservoirs is an important factor determining the ecosystem structure of water-limited environments, such as the seasonal tropical savannas of South America. In this study a two-dimensional (2D) geoelectrical profiling technique was employed to estimate seasonal dynamics of soil water content to 10-m depth along transects of 275 m in savanna vegetation during the period between 2002 and 2006. Methods were developed to convert resistivity values along these 2D resistivity profiles into volumetric water content (VWC) by soil depth. The 2D resistivity profiles revealed the following soil and aquifer structure characterizing the underground environment: 0–4 m of permanently unsaturated and seasonally droughty soil, less severely dry unsaturated soil at about 4–7 m, nearly permanently saturated soil between 7 and 10 m, mostly impermeable saprolite interspaced with fresh bedrock of parent material at about 10–30 m, and a region of highly conductive water-saturated material at 30 m and below. Considerable spatial variation of these relative depths is clearly demonstrated along the transects. Temporal dynamics in VWC indicate that the active zone of water uptake is predominantly at 0–7 m, and follows the seasonal cycles of precipitation and evapotranspiration. Uptake from below 7 m may have been critical for a short period near the beginning of the rainy season, although the seasonal variations in VWC in the 7–10-m layer are relatively small and lag the surface water recharge for about 6 months. Calculations using a simple 1-box water balance model indicate that average total runoff was 15–25 mm month−1 in the wet season and about 6–9 mm month−1 in the dry season. Modeled ET was about 75–85 mm month−1 in the wet season and 20–25 mm month−1 in the dry season. Variation in basal area and tree density along one transect was positively correlated with VWC of the 0–3-m and 0–7-m soil depths, respectively, during the wettest months. These multitemporal measurements demonstrate that the along-transect spatial differences in soil moisture are quasi-permanent and influence vegetation structure at the scale of tens to hundreds of meters.

Distribution of alkalinity and dissolved calcium in the Sea of Okhotsk

During the summer seasons of 2002 and 2004, the total alkalinity (TA) and dissolved calcium (Ca) were studied at 41 stations in different areas of the Sea of Okhotsk: the Kuril depression, Deryugin Basin, the slopes of the Kamchatka Peninsula and Sakhalin Island, and in Sakhalin Bay. It was shown that the distributions of the TA and Ca in the water mass of deep sea areas are determined by the processes of CaCO3 formation and dissolution according to the relation Δ Ca = 0.5 Δ TA (1). The variations of the TA and Ca values observed in the upper 10-m layer and in the near-bottom layers of local depressions in the Deryugin Basin do not satisfy relationship (1). Probable reasons for this discrepancy are considered: organic matter mineralization, mixing of water masses with different preform TA and Ca values, sea ice melting, runoff from land, and sea bottom effects. It is shown that the enrichment in the alkalinity and calcium is caused by the Amur River runoff in the desalinated sea surface layer and by the high geochemical activity in the Deryugin Basin in the near-bottom 200-m layer of local depressions.

Long-term variability of oxygen and mineral phosphorus concentrations in Lake Dal’nee (Kamchatka)

Many-year data on the concentrations of O2 and mineral phosphorus in a fattening-spawning water body for the Pacific salmon (red salmon) are presented; the seasonal and year-to-year dynamics of these con-centrations are analyzed. It is found that the lake water mass features a high concentration of O2, the upper 20-m layer being most aerated. It is shown that the year-to-year variability of the concentration of O2 dissolved in the lake water depends on the amount of organic matter coming with groundwater and surface runoff. The oxygen and phosphorus regimes of the lake at low and abundant entries of puberal red salmon for spawning are considered.

Distribution depth of the transforming stage larvae of myctophid fishes in the subtropical–tropical waters of the western North Pacific

We describe the day–night vertical distribution patterns of 18 species or types of myctophid fish larvae at the transforming stage based on discrete depth sampling from the surface down to 1000-m depth in the subtropical–tropical western North Pacific. A total of 551 transforming stage larvae were collected at the 19 sampling stations. Except for the Diaphus species and Notolychnus valdiviae, all of the transforming stage larvae (including genera Benthosema, Bolinichthys, Centrobranchus, Ceratoscopelus, Diogenichthys, Hygophum, Lampanyctus, Lobianchia, Myctophum, Symbolophorus, and Triphoturus) were collected in the lower mesopelagic zone from 600- to 900-m depth during both day and night, showing no diel vertical migration (DVM). On the contrary, the Diaphus species and N. valdiviae larvae undergo DVM during the transforming stage, occurring below 200-m layer during the daytime and migrating up to the upper 150-m layer at night, i.e., they show earlier adaptation to juvenile–adult behaviors. Most myctophid fish larvae are known to undertake substantial ontogenetic vertical migration (OVM) from the epipelagic to mesopelagic zones during their early life stage. Although considerable sampling effort was carried out in this study, transforming larvae, except for the above two migratory ones, were not collected in the epipelagic and upper mesopelagic zones, strongly suggesting that their sinking speed would be high. It would be advantageous for survival to spend their highly vulnerable transforming stage in the lower mesopelagic zone, where predation pressures are lower and physical conditions are more stable than in the upper layers.

Intraseasonal Variability of Equatorial Indian Ocean Zonal Currents

Abstract New satellite and in situ observations show large intraseasonal (10–60 day) variability of surface winds and upper-ocean current in the equatorial Indian Ocean, particularly in the east. An ocean model forced by the Quick Scatterometer (QuikSCAT) wind stress is used to study the dynamics of the intraseasonal zonal current. The model has realistic upper-ocean currents and thermocline depth variabilities on intraseasonal to interannual scales. The quality of the simulation is directly attributed to the accuracy of the wind forcing. At the equator, moderate westerly winds are punctuated by strong 10–40-day westerly wind bursts. The wind bursts force swift, intraseasonal (20–50 day) eastward equatorial jets in spring, summer, and fall. The zonal momentum balance is between local acceleration, stress, and pressure, while nonlinearity deepens and strengthens the eastward current. The westward pressure force associated with the thermocline deepening toward the east rapidly arrests eastward jets and, subsequently, generates (weak) westward flow. Thus, in accord with direct observations in the east, the spring jet is a single intraseasonal event, there are intraseasonal jets in summer, and the fall jet is long lived but strongly modulated on an intraseasonal scale. The zonal pressure force is almost always westward in the upper 120 m, and changes sign twice a year in the 120–200-m layer. Transient eastward equatorial undercurrents in early spring and late summer are associated with semiannual Rossby waves generated at the eastern boundary following thermocline deepening by the spring and fall jets. An easterly wind stress is not necessary to generate the undercurrents. Experiments with a single westerly wind burst forcing show that apart from the intraseasonal response, the zonal pressure force and current in the east have an intrinsic 90-day time scale that arises purely from equatorial adjustment.

Influence of the dynamics of currents on the hydrophysical structure of the waters and the vertical exchange in the active layer of the Black Sea

An analysis of the data of measurements of the fine structure and microstructure fluctuations of hydrophysical fields in the upper 200-m layer of the Black Sea carried out using CTD profilers and a Baklan free falling microstructure and turbulence profiler revealed the existence of a positive correlation between the intensity of the fine structure and microstructure fluctuations and the dynamics of the currents. On the other hand, the level of the fine structure and microstructure fluctuations reflects the rate of the vertical turbulent exchange. It was shown that, in the case of the absence of the Black Sea Rim Current (BSRC) jet or clearly manifested mesoscale eddy structures, the vertical turbulent exchange in the pycnocline is weak, while, in the opposite case, it is stronger. The results obtained support the supposition that the interbasin dynamics play an important role in the maintenance of the rate of small-scale mixing in the pycnocline and halocline and provide the vertical transport of dissolved oxygen from the cold intermediate layer into the deeper layers of the sea.

The Influence of Sea Ice on Ocean Heat Uptake in Response to Increasing CO2

Abstract Two significant changes in ocean heat uptake that occur in the vicinity of sea ice cover in response to increasing CO2 are investigated with Community Climate System Model version 3 (CCSM3): a deep warming below ∼500 m and extending down several kilometers in the Southern Ocean and warming in a ∼200-m layer just below the surface in the Arctic Ocean. Ocean heat uptake caused by sea ice retreat is isolated by running the model with the sea ice albedo reduced artificially alone. This integration has a climate response with strong ocean heat uptake in the Southern Ocean and modest ocean heat uptake in the subsurface Arctic Ocean. The Arctic Ocean warming results from enhanced ocean heat transport from the northern North Atlantic. At the time of CO2 doubling, about 1/3 of the heat transport anomaly results from advection of anomalously warm water and 2/3 results from strengthened inflow. At the same time the overturning circulation is strengthened in the northern North Atlantic and Arctic Oceans. Wind stress changes cannot explain the circulation changes, which instead appear related to strengthened convection along the Siberian shelves. Deep ocean warming in the Southern Ocean is initiated by weakened convection, which is mainly a result of surface freshening through altered sea ice and ocean freshwater transport. Below about 500 m, changes in convection reduce the vertical and meridional temperature gradients in the Southern Ocean, which significantly reduce isopycnal diffusion of heat upward around Antarctica. The geometry of the sea ice cover and its influence on convection have a strong influence on ocean temperature gradients, making sea ice an important player in deep ocean heat uptake in the Southern Ocean.

ARGO Upper Salinity Measurements: Perspectives for L-Band Radiometers Calibration and Retrieved Sea Surface Salinity Validation

With the view of preparing the strategy for the calibration/validation of future L-band satellite radiometers, we examine the salinity variability recorded by Array for Real-Time Geostrophic Oceanography (ARGO) floats in the upper 10-m layer of the surface ocean. Using one year of ARGO measurements,we show that the surface salinity variability at ten days and 200-km scales is above /spl plusmn/0.1 psu for 30% of the drifters and that this variability is larger than 0.2 psu in tropical regions affected by strong river discharges and by precipitations, and in frontal areas characterized by strong mesoscale activity. Vertical gradient observed between 5-10-m depth is much lower than the horizontal variability but leads to systematic biases in the tropics. The South Pacific Ocean appears to be the less variable both vertically and horizontally.

Eddy-Induced Heat Transport in the Subtropical North Pacific from Argo, TMI, and Altimetry Measurements

Abstract Basin-scale heat transport induced by mesoscale oceanic eddies is estimated by combining satellite-derived sea surface height and temperature [temperature data are from the TRMM Microwave Imager (TMI)] data with Argo float temperature–salinity data. In the North Pacific Ocean subtropical gyre, warm (cold) temperature anomalies of mesoscale eddies are found to be consistently located to the west of high (low) SSH anomalies. The phase misalignment between the temperature and velocity anomalies, however, is largely confined to the seasonal thermocline, causing most of the eddy-induced heat transport to be carried in the surface 200-m layer. By establishing a statistical relationship between the surface and depth-integrated values of the eddy heat transport, the basin-scale eddy heat transport is derived from the concurrent satellite SSH/SST data of the past six years. In the Kuroshio Extension region, the meandering zonal jet is found to generate oppositely signed eddy heat fluxes. As a result, the zonally integrated poleward heat transport associated with the Kuroshio Extension is at a level O(0.1 PW), smaller than the previous estimates based on turbulent closure schemes. Large poleward eddy heat transport is also found in the subtropical North Pacific along a southwest–northeast-tilting band between Taiwan and the Midway Islands. This band corresponds to the region of the subtropical front, and it is argued that the relevant temperature field for identifying this band in the turbulent closure scheme models should be that averaged over the seasonal thermocline.

The role of interannual environmental variations in the geographic range of spawning and feeding concentrations of redfish Sebastes mentella in the Irminger Sea

Abstract The Irminger Sea is occupied by the most abundant population of the redfish, Sebastes mentella, in the North Atlantic. Results of Russian and international surveys of redfish in the Irminger Sea between 1982 and 2003 are summarized here. Interannual variations in redfish spatial distribution and oceanographic conditions are also analysed. Distribution patterns of spawning and feeding concentrations of redfish are established, and the role of oceanographic factors in the formation of concentrations and in the migrations of redfish are shown. The habitat of the mature redfish population in pelagic waters is confined within the Sub-polar cyclonic Gyre. Redfish spawning grounds extend along the western slope of the Reykjanes Ridge in the areas of intensive rise of intermediate waters. Seasonal variation in water temperature above the ridge slopes is one of the factors causing feeding redfish to migrate westwards, to the central area of the Sub-polar Gyre. Atlantic Water advection, intensified by the Irminger Sea, and water temperature increase in the upper 200-m layer in the second half of the 1990s produced a considerable shift of commercial concentrations of feeding redfish from their usual grounds westwards, to the NAFO Regulatory area.

Near-surface mapping using SH-wave and P-wave seismic land-streamer data acquisition in Illinois, U.S.

Human activities have considerable impact on the upper 300 m of sediments of the earth. Conflicting activities in highly populated areas involving groundwater exploitation, aggregate extraction, and hazardous chemical-related production and storage induce stress on the environment. Nearly half the population living around the Great Lakes depends on shallow groundwater aquifers. A coalition of four Great Lakes states (Illinois, Indiana, Michigan, and Ohio), in conjunction with the U. S. Geological Survey, has initiated a program to map this sensitive upper 300-m layer. As part of this effort, the Illinois State Geological Survey (ISGS) has undertaken the task of preparing 3D quadrangle maps of the upper 100 m of the entire state at a scale of 1:24 000. Although large-scale maps indicating potential aquifer contamination already exist (Figure 1), very little is known about the groundwater pathways through the upper sedimentary column, which mainly consists of deposits from the Quaternary continental glacier...

Annual and Interdecadal Variability of the Available Potential Energy in the Black Sea

On the basis of the climatic data array on temperature and salinity including the data of observations carried out at 102,000 oceanographic stations in 1910–1998, we estimate the amount of available potential energy of large-scale processes in the 0–300-m layer of the Black Sea and study its annual variability. The annual average amount of available potential energy in the active 0–300-m layer is equal to 13.5×1014 J, its maximum amount attained in March is 25×1014 J, and its minimum amount attained in August is 8×1014 J. In the autumn–winter period, the density of available potential energy in the 0–50-m layer increases and its maximum is attained in December–January (30 J/m3). Beginning with January, the density of available potential energy increases in the layer of the main pycnocline (50–150 m). Its maximum is attained in March (50 J/m3) and then this quantity decreases till August (down to 14 J/m3). In August, we study the interdecadal variability of the available potential energy in the 50–300-m layer by using the data accumulated for decadal periods with five-year shifts in 1956–1995. The maximum variability is observed at a depth of 100 m. The maximum average (over the sea) amounts of available potential energy were observed in 1961–1970 and 1986–1995. The minimum amounts were recorded in 1976–1985.

Three-dimensional structure and avoidance behaviour of anchovy and common sardine schools in central southern Chile

Abstract We studied the avoidance behaviour and three-dimensional (3-D) structure of anchovy (Engraulis ringens) and common sardine (Strangomera bentincki) schools mixed in high concentrations in a coastal area of central southern Chile. Observations were carried out during an acoustic survey in January 2002 by means of a vertical echosounder and a multi-beam sonar. The sonar harvested around 900 series of 3-D school images, and 3000 2-D school images were collected with the echosounder. The results showed that all fish aggregations presented the same internal structure, but different global morphologies, from single small schools (with length three times the height) on the edges of the distribution to large dense layers (length more than seven times the height) in its centre. Observation of avoidance in the vertical and horizontal planes indicated that limited vertical diving occurred close to the ship (fish dive from the surface to the 5–10-m depth layer below the vessel), while no horizontal avoidance was observed.

EDDY-INDUCED HEAT TRANSPORT IN THE SUBTROPICAL NORTH PACIFIC FROM ARGO, TMI AND ALTIMETRY MEASUREMENTS

Abstract Basin-scale heat transport induced by mesoscale oceanic eddies is estimated by combining satellite-derived sea surface height and temperature [temperature data are from the TRMM Microwave Imager (TMI)] data with Argo float temperature–salinity data. In the North Pacific Ocean subtropical gyre, warm (cold) temperature anomalies of mesoscale eddies are found to be consistently located to the west of high (low) SSH anomalies. The phase misalignment between the temperature and velocity anomalies, however, is largely confined to the seasonal thermocline, causing most of the eddy-induced heat transport to be carried in the surface 200-m layer. By establishing a statistical relationship between the surface and depth-integrated values of the eddy heat transport, the basin-scale eddy heat transport is derived from the concurrent satellite SSH/SST data of the past six years. In the Kuroshio Extension region, the meandering zonal jet is found to generate oppositely signed eddy heat fluxes. As a result, the ...