Variability of the western Galician upwelling system (NW Spain) during an intensively sampled annual cycle. An EOF analysis approach

The key features of the western Galician shelf hydrography and dynamics are analyzed on a solid statistical and experimental basis. The results allowed us to gather together information dispersed in previous oceanographic works of the region. Empirical orthogonal functions analysis and a canonical correlation analysis were applied to a high-resolution dataset collected from 47 surveys done on a weekly frequency from May 2001 to May 2002. The main results of these analyses are summarized bellow. Salinity, temperature and the meridional component of the residual current are correlated with the relevant local forcings (the meridional coastal wind component and the continental run-off) and with a remote forcing (the meridional temperature gradient at latitude 37°N). About 80% of the salinity and temperature total variability over the shelf, and 37% of the residual meridional current total variability are explained by two EOFs for each variable. Up to 22% of the temperature total variability and 14% of the residual meridional current total variability is devoted to the set up of cross-shore gradients of the thermohaline properties caused by the wind-induced Ekman transport. Up to 11% and 10%, respectively, is related to the variability of the meridional temperature gradient at the Western Iberian Winter Front. About 30% of the temperature total variability can be explained by the development and erosion of the seasonal thermocline and by the seasonal variability of the thermohaline properties of the central waters. This thermocline presented unexpected low salinity values due to the trapping during spring and summer of the high continental inputs from the River Miño recorded in 2001. The low salinity plumes can be traced on the Galician shelf during almost all the annual cycle; they tend to be extended throughout the entire water column under downwelling conditions and concentrate in the surface layer when upwelling favourable winds blow. Our evidences point to the meridional temperature gradient acting as an important controlling factor of the central waters thermohaline properties and in the development and decay of the Iberian Poleward Current.     

Copepod diversity in the water masses of the southern East China Sea north of Taiwan

The warm oceanic current Kuroshio and the continental shelf water of the East China Sea meet in the western North Pacific, north of Taiwan and form an upwelling when they converge. The intrusion of the Kuroshio westward over the East China Sea shelf thus results in complicated exchanges of waters between these two water masses. We studied the copepods in the plankton collection taken from an east–west transect crossing these waters in April 1995 when the intrusion of the Kuroshio over the East China Sea shelf was beginning to retreat. The taxonomy of copepods was carefully treated and erroneous species records reported in the literature were guarded against. We evaluated the copepod diversity, the association of copepod species, and the association of stations in these water masses.     

Hydrographic cruises off northwest Africa: the Canary Current and the Cape Ghir region

We present hydrographic data for several sections located along the African coastline and off Cape Ghir, carried out at times of weak surface winds (October 1995 and September 1997). The main sections are near the continental slope, at mean distances between 100 and 150 km from the coastline. North of Cape Ghir (31°N) the geostrophic transport (referenced to 650 m) of North Atlantic Central Water through these sections is 3.7 and 2.0 Sv for 1995 and 1997, respectively. This confirms that a major fraction of the water transport by the Canary Current flows east, into the continental slope off northwest Africa, at latitudes above Cape Ghir. Most of this flow continues south past Cape Ghir, along the coast and probably through the eastern passages of the Canary Archipelago. A significant fraction, however, may escape through surface Ekman transport (0.3–0.5 Sv during the early fall season) and by offshore flow at Cape Ghir (1.1 Sv in September 1997, referenced to 650 m). Despite the weak winds the Cape Ghir filament was clearly visible, characterized by localized coastal upwelling associated to a cyclonic shallow structure and cold (and fresh) waters stretching offshore as a very shallow feature (50–100 m deep). The satellite images show that the surface temperature field is highly variable, in rapid response to the surface winds, always with a core region of relatively cold water and commonly with one or two associated eddies. Our results support the existence of two recirculation cells in the area: a horizontal one that connects the interior eastern boundary currents with the coastal region and a vertical one related to both wind-induced and filament upwelling. The data also show a salinity subsurface maximum at the root of the filament, linked to water inflow from northern latitudes, and a subsurface anticyclonic eddy over the Agadir canyon, likely related to the poleward slope undercurrent.     

Intrusions of eastern North Atlantic central waters and phytoplankton in the north and northwestern Iberian shelf during spring

The influence of intrusions of eastern North Atlantic central water (ENACW) in the north and northwestern Iberian shelf on phytoplankton composition and abundance and on particle-size distributions of seston was analyzed using data collected on three extensive cruises during spring 1991 and 1992. Water with temperature and salinity values between 12.20 and 13.86 °C and between 35.66 and 35.98 psu, respectively, characteristics of the subtropical type of ENACW (ENACW_t), was detected in the upper 100 m of the water-column in all cruises, but particularly in the western coast in 1992. The highest salinity values of this water were found near the surface (0–100-m depth) and in early spring 1992, while minimum salinity values, and also minimum geographical extension, were found in late spring in both years. Phytoplankton blooms concentrated in frontal areas between different water types, with maximum intensity and extension in early spring.Using temperature and salinity characteristics, samples were classified in four groups corresponding to the major water types found in the region: Bay of Biscay central water (BBCW), two segments of ENACW of different salinity and surface water influenced by continental runoff. This classification was significantly confirmed by three independent discriminant analyses using hydrographic and chemical (dissolved nutrients and chlorophyll) variables, phytoplankton species abundance variables and particle-size concentration of seston variables. Phytoplankton blooms related to the presence of saline waters were characterized by the dominance of either chain-forming diatoms or a mixture of diatoms and phytoflagellates and high concentrations of seston. The diatom species dominating in saline waters were typical of upwelling-induced blooms occurring generally during summer. Blooms occurring in waters influenced by runoff also contained diatoms but in lower numbers than those of saline waters. Nutrients were not exhausted in the region, suggesting that phytoplankton populations were still in active growth. These results are interpreted taking into account the known variability in water-mass formation and in the poleward current driving ENACW_t along the shelf, and indicate that saline intrusions are a major feature affecting the distribution and composition of plankton in the spring in the southern Bay of Biscay, thus enlarging to a wider spatial scale their reported influence on the pelagic ecosystem.     

Hydrographic conditions affecting two fishing grounds of Mallorca island (Western Mediterranean): during the IDEA Project (2003–2004)

This paper describes the hydrographic conditions observed during six surveys carried out during 2003 and 2004, in the framework of the “IDEA Project” (acronym for “Influence of oceanographic structure and dynamics on demersal populations in waters of the Balearic Islands”). The surveys were developed on the shelf and slope of Mallorca Island, in particular in two fishing grounds at the north and south of the Mallorca channel. Periodic movements of the fishing fleet between these two areas have been regularly reported, suggesting a seasonal variability of the resources which could be in turn associated with the hydrodynamic variability. With this motivation, water masses affecting these grounds have been identified and their seasonal variability has been studied. Different oceanographic and environmental conditions have been found between the two fishing grounds. These differences are related to the presence of mesoscale structures, associated with the Western Mediterranean Intermediate Water (WIW) at the north of the Ibiza channel and big gyres detached from the Algerian Current. The former has been shown to have influence on the regional oceanic circulation and the latter could affect the progress of fresh Atlantic Water (AW) towards the channels and make possible the presence of high salinity values at intermediate waters at the south of Mallorca Island. Historical data from other oceanographic cruises carried out in the region are finally used to discuss the interannual variability of these mesoscale structures.     

From suspended particles to strata: The fate of terrestrial substances in the Gaoping (Kaoping) submarine canyon

The river–sea system consisting of the Gaoping (new spelling according to the latest government's directive, formerly spelled Kaoping) River (KPR), shelf, and Submarine Canyon (KPRSC) located off southern Taiwan is an ideal natural laboratory to study the source, pathway, transport, and fate of terrestrial substances. In 2004 during the flood season of the KPR, a system-wide comprehensive field experiment was conducted to investigate particle dynamics from a source-to-sink perspective in the KPRSC with the emphasis on the effect of particle size on the transport, settling, and sedimentation along the pathway. This paper reports the findings from (1) two sediment trap moorings each configured with a Technicap PPS 3/3 sediment trap, and an acoustic current meter (Aquadopp); (2) concurrent hydrographic profiling and water sampling was conducted over 8 h next to the sediment trap moorings; and (3) box-coring in the head region of the submarine canyon near the mooring sites. Particle samples from sediment traps were analyzed for mass fluxes, grain-size composition, total organic carbon (TOC) and nitrogen (TN), organic matter (OM), carbonate, biogenic opal, polycyclic aromatic hydrocarbon (PAH), lithogenic silica and aluminum, and foraminiferal abundance. Samples from box cores were analyzed for grain-size distribution, TOC, particulate organic matter (POM), carbonate, biogenic opal, water content, and ²¹⁰Pb_ex. Water samples were filtered through 500, 250, 63, 10 µm sieves and 0.4 µm filter for the suspended sediment concentration of different size-classes.Results show that the river and shelf do not supply all the suspended particles near the canyon floor. The estimated mass flux near the canyon floor exceeds 800 g/m²/day, whose values are 2–7 times higher than those at the upper rim of the canyon. Most of the suspended particles in the canyon are fine-grained (finer than medium silt) lithogenic sediments whose percentages are 90.2% at the upper rim and 93.6% in the deeper part of the canyon.As suspended particles settle through the canyon, their size-composition shows a downward fining trend. The average percentage of clay-to-fine-silt particles (0.4–10 µm) in the water samples increases from 22.7% above the upper rim of the canyon to 56.0% near the bottom of the canyon. Conversely, the average percentage of the sand-sized (> 63 µm) suspended particles decreases downward from 32.0% above the canyon to 12.0% in the deeper part of the canyon. Correspondingly, the substrate of the canyon is composed largely of hemipelagic lithogenic mud. Parallel to this downward fining trend is the downward decrease of concentrations of suspended nonlithogenic substances such as TOC and PAH, despite of their affinity to fine-grained particles.On the surface of the canyon, down-core variables (grain size, ²¹⁰Pb_ex activity, TOC, water content) near the head region of the canyon show post-depositional disturbances such as hyperpycnite and turbiditic deposits. These deposits point to the occurrences of erosion and deposition related to high-density flows such as turbidity currents, which might be an important process in submarine canyon sedimentation.     

Estimating salinity to complement observed temperature: 1. Gulf of Mexico

This paper and its companion [Thacker, W.C., Sindlinger, L., 2007-this issue. Estimating salinity to complement observed temperature: 2. Northwestern Atlantic. Journal of Marine Systems. doi:10.1016/j.jmarsys.2005.06.007.] document initial efforts in a project with the goal of developing capability for estimating salinity on a region-by-region basis for the world oceans. The primary motivation for this project is to provide information for correcting salinity, and thus density, when assimilating expendable-bathythermograph (XBT) data into numerical simulations of oceanic circulation, while a secondary motivation is to provide information for calibrating salinity from autonomous profiling floats. Empirical relationships between salinity and temperature, which can be identified from archived conductivity–temperature–depth (CTD) data, provide the basis for the salinity estimates.The Gulf of Mexico was chosen as the first region to explore for several reasons: (1) It's geographical separation from the Caribbean Sea and the North Atlantic Ocean makes it a “small ocean” characterised by a deep central basin surrounded by a substantial continental shelf. (2) The archives contain a relatively large number of CTD data that can be used to establish empirical relationships. (3) The sharp fronts associated with the Loop Current and its rings, which separate water with different thermal and haline characteristics, pose a challenge for estimating salinity. In spite of the shelf and the fronts, the relationship between salinity and temperature was found to be sufficiently regular that a single empirical model could be used to estimate salinity on each pressure surface for the entire Gulf for all seasons. In and below the thermocline, root-mean-square estimation errors are small — less than 0.02 psu for pressures greater than 400 dbar, corresponding to potential density errors of less than 0.015 kg/m³. Errors for estimates nearer to the surface can be an order of magnitude larger.     

Interannual variability of the shelf-slope front position between 75° and 50° W

The shelf-slope front (SSF) is a continuous shelf-break front running from the Tail of the Grand Banks to Cape Hatteras, North Carolina, separating colder and less-saline continental shelf waters from warmer and more saline slope waters. Time series containing mean monthly SSF positions were produced along each of 26 longitude lines between 75° and 50°W by workers located at Bedford Institute of Oceanography by digitizing individual frontal charts and computing mean monthly latitudinal positions over a 29-year (1973–2001) period. After removing seasonal variability at each longitude, interannual variability (IAV) of the SSF position at each longitude was computed as the annual mean of all monthly SSF position anomalies for each year over the 29-year period. Despite some missing data, a longitude-time plot reveals alternating bands of offshore (late-1970s, late-1980s, late-1990s) and onshore (early-1980s, early-1990s, early-2000s) annual mean SSF anomaly values, exhibiting a period of approximately 10 years. Annual mean SSF anomaly amplitudes are largest in the east, with maxima of O (± 100 km) located east of 60° W for years when data are available. Empirical orthogonal function (EOF) modes 1–4 (accounting for > 90% of the variance) form a set of basis functions that describe the SSF anomaly data and allow reconstruction of the entire data set since missing data are relatively few (14%). A complex empirical orthogonal function (CEOF) analysis using the “reconstructed” data reveals a wavelength scale of approximately 20° of longitude, a distance nearly equal to the entire study domain, along with steady, westward phase propagation of SSF anomalies over approximately the same distance. Speed calculations for the westward-propagating features yield a value of approximately 1.2 to 2.4 cm s^− 1 (1 to 2 km d^− 1), with annual mean SSF anomalies thus requiring about 4 years to propagate from the Tail of the Grand Banks in the east to Cape Hatteras, North Carolina, in the west. This propagation speed and the timing of the SSF positional anomalies at the Tail of the Grand Banks for the 29-year study period are in agreement with speeds computed for the propagation of quasi-decadal salinity anomalies through the Labrador Sea and the time of their arrival at the Tail of the Grand Banks. The small westward SSF anomaly propagation speed is an order of magnitude smaller than the associated currents, in agreement with a highly damped flow-through system originating from both Davis Strait and the West Greenland Current as discussed by other workers. Observations from both southern and northern portions of the study domain, within both continental shelf and slope waters, show that interannual changes in the volume of shelf water along with shelf water bulk properties exhibit a strong relationship with IAV of the SSF position over long time periods.     

Nutrient and organic matter patterns across the Mackenzie River, estuary and shelf during the seasonal recession of sea-ice

Suspended material, nutrients and organic matter in Mackenzie River water were tracked along a 300 km transect from Inuvik (Northwest Territories, Canada), across the estuarine salinity gradient in Kugmallit Bay, to offshore marine stations on the adjacent Mackenzie Shelf. All particulates measured (SPM, POC, PN, PP) declined by 87–95% across the salinity gradient and levels were generally below conservative mixing. Organic carbon content of suspended material decreased from 3.1% in the river to 1.7% in shelf surface waters while particulate C:N concurrently decreased from 17.1 to 8.6. Nitrate and silicate concentrations declined by more than 90% across the salinity gradient, with nitrate concentrations often below the conservative mixing line. Phosphate concentrations increased from 0.03 μmol/L in the river to 0.27 μmol/L over shelf waters, thereby changing the inorganic nutrient regime downstream from P to N limitation. Dissolved organic carbon decreased conservatively offshore while dissolved organic N and P persisted at high levels in the Mackenzie plume relative to river water, increasing 2.7 and 25.3 times respectively. A deep chlorophyll-a maximum was observed at two offshore stations and showed increases in most nutrients, particulates and organic matter relative to the rest of the water column. During river passage through the Mackenzie estuary, particulate matter, dissolved organic carbon and inorganic nutrients showed sedimentation, dilution and biological uptake patterns common to other arctic and non-arctic estuaries. Alternatively, inorganic content of particles increased offshore and dissolved organic N and P increased substantially over the shelf, reaching concentrations among the highest reported for the Arctic Ocean. These observations are consistent with the presence of a remnant ice-constrained (‘stamukhi’) lake from the freshet period and a slow flushing river plume constrained by sea-ice in close proximity to shore. Nutrient limitation in surface shelf waters during the ARDEX cruise contributed to the striking deep chlorophyll-a maximum at 21 m where phytoplankton communities congregated at the margin of nutrient-rich deep ocean waters.     

Autumn net copepod abundance and assemblages in relation to water masses on the continental shelf of the Yellow Sea and East China Sea

Copepod species diversity, abundance and assemblages in relation to water masses over the continental shelf of the Yellow Sea (YS) and East China Sea (ECS) were studied extensively based on the net plankton samples in autumn 2000. Multivariate analysis based on copepod assemblage resulted in recognition of five groups (Groups 1−5) corresponding to the water masses. Groups 1 and 2 delineated from inshore stations with low salinity YS Surface Water, and offshore stations with YS Cold Water in the YS. Group 3 located in the joint area of YS and ECS mainly with Mixed Water. Groups 4 and 5 in the ECS delineated two assemblages mainly from inshore and shallow stations with ECS Mixed Water in the southeastern ECS, and offshore stations along the ECS shelf edge controlled by saline Kuroshio Water. Salinity and temperature were more important in characterizing copepod assemblage of the continental shelf than chlorophyll a.     

Chemical and physical fronts in the Bohai, Yellow and East China seas

Associated with strong mixing and stirring, as well as enhanced bioproductivity and ecotones, oceanic fronts have garnered worldwide attention in recent years. Research into oceanic fronts, especially thermal fronts, has gained momentum since the advent of satellites and their increased accessibility. Yet, studies of salinity and nutrient fronts —particularly those that are subsurface are few and far between. This study reviews the most widely accepted facts about surface and subsurface temperature and salinity fronts in the Bohai, Yellow and East China seas and their seasonal variations. The distribution of nutrients in the surface and bottom waters are mapped and nutrient fronts, for the first time, are identified systematically.These fronts are generally strongest in winter when southward flowing coastal currents are influenced most by winter monsoons, and the contrasts between these cold, fresh, nutrient-rich currents and the northward flowing warm, saline but nutrient-poor Kuroshio are strongest. Surface fronts are generally weakest in summer when coastal currents may be weaker and temperature, salinity and nutrient contrasts are diminished. The existence of fronts and why some are disconnected are mainly related to oceanic features such as topography, boundaries between water masses and current flow patterns. Three latitudinal temperature and nutrient fronts in the southern East China Sea in winter may suggest eastward flowing currents. These currents have not been described previously.     

A 3D, finite element model for baroclinic circulation on the Vancouver Island continental shelf

This paper describes the development and application of a 3-dimensional model of the barotropic and baroclinic circulation on the continental shelf west of Vancouver Island, Canada. A previous study with a 2D barotropic model and field data revealed that several tidal constituents have a significant baroclinic component (the K1 in particular). Thus we embarked on another study with a 3D model to study the baroclinic effects on the residual and several selected tidal constituents.The 3D model uses a harmonic expansion in time and a finite element discretization in space. All nonlinear terms are retained, including quadratic bottom stress, advection and wave transport (continuity nonlinearity). The equations are solved as a global and a local problem, where the global problem is the solution of the wave equation formulation of the shallow water equations, and the local problem is the solution of the momentum equation for the vertical velocity profile. These equations are coupled to the advection-diffusion equation for density so that density gradient forcing is included in the momentum equations. However, the study presented here describes diagnostic calculations for the baroclinic residual circulation only.The model is sufficiently efficient that it encourages sensitivity testing with a large number of model runs. In this sense, the model is akin to an extension of analytical solutions to the domain of irregular geometry and bottom topography where this parameter space can be explored in some detail.In particular, the consequences of the sigma coordinate system used by the model are explored. Test cases using an idealized representation of the continental shelf, shelf break and shelf slope, lead to an estimation of the velocity errors caused by interpolation errors inherent in the sigma coordinate system. On the basis of these estimates, the computational grid used in the 2D model is found to have inadequate resolution. Thus a new grid is generated with increased accuracy in the region of the shelf break. However, even with increased resolution, spurious baroclinic circulation seaward of the shelf break and in the vicinity of Juan de Fuca canyon remained a significant problem when the pressure gradient terms were evaluated using the σ coordinate system and using a realistic density profile.With the new grid, diagnostic calculations of the barotropic and baroclinic residual circulation are performed using forcing from the observed σ_t (density) field and from the gradient of this field.     

Numerical simulations of seasonal and interannual variability of the Black Sea thermohaline circulation

The Black Sea general circulation is simulated by a primitive equation model with active free surface. The forcing is seasonally variable and is based on available climatic data. The model reproduces the main features of the Black Sea circulation, including the river discharge effects on the mean sea level and the Bosphorus outflow. Model results show that the simulated sea surface elevation increases in spring over the whole sea, reaching a maximum in the Danube delta area. In the same region, a minimum is observed in winter. The amplitude of the seasonal oscillations (about 8–12 cm over the whole basin) is of the same order of magnitude as the maximum horizontal variations (about 15–18 cm between the coastal areas and the basin interior). This strong signal formed mostly by river discharges, along with the seasonal variability in the other forcing functions and the local dynamics creates a well-pronounced interannual variability. The performance of the model in simulating the seasonal and interannual variability is critically analyzed, with a special attention on the cold intermediate water formation and the circulation in the upper 150 m. The simulations demonstrate that the source of intermediate waters is on the shelf, and that the water mass in the core of cold intermediate layer changes with time as a response to the periodic forcing at sea surface. This type of variability is characterized by pronounced interannual changes, proving that important differences could exist between water mass structure in different years, even when using identical atmospheric forcings each year.     

Upwelling mechanisms in the northwestern Alboran Sea

From April 1996 to July 1997, a series of hydrographic surveys were carried out in the Northwestern part of the Alboran Sea to investigate the upwelling that is an almost permanent feature in this area. Simultaneously a mooring line was deployed in the north part of the eastern section of the Strait of Gibraltar to monitor the variability of the Atlantic Jet (AJ). Two mechanisms are shown to be relevant for the upwelling dynamic in the region: the southward drifting of the AJ and wind stress. A linear relation between the angle under which the Jet enters the Alboran Sea and the distance from the coastline to the front associated with the Jet has been found. This angle that has been estimated from the low passed time series of current velocity measured by the uppermost instrument of the moored line has been then used to identify the onshore–offshore excursions of the Jet. Both upwelling mechanisms are identified from hydrographic data, because each of them has associated a different type of water mass, and they take place in different locations. Wind-driven upwelling dominates in coastal zones, on the shelf, while upwelling associated with southward drifting of the AJ prevails further offshore. The amount of sub-surface water brought up to the surface by each one is of the same order. However, wind-driven upwelling contributes to the fertilization of this region in a major extent because water upwelled by wind is richer in nutrient concentration.     

Seasonal upwelling underneath the Tsushima Warm Current along the Japanese shelf slope

Seasonal upwelling variations are examined in the eastward coastal boundary branches of the Tsushima Warm Current (TWC). The climatological pattern and the fundamental hydrographic structure of the seasonal appearance of cold water are revealed by analyzing the temperature profile data around the Japanese shelf area. Seasonal maps of temperature at the intermediate layer around 200 m depth show the rise of the main pycnocline along the Japanese coast due to seasonal subsurface cooling from May to September. The cold water areas appear around the strong curvature in the continental shelf break. These areas are confined to the south of the TWC thermal front, i.e., to the coastal boundary region. The seasonal appearance of the localized cooling areas implies that the seasonal upwelling is induced by horizontal variations in shelf topography and the intensifying TWC from May to September.     

Tidal current variability in the Central Great Barrier Reef

With tidal data from the literature and the field, a two-dimensional (depth-averaged) numerical model was formulated to simulate the dominant semi-diurnal tidal hydrodynamics of the Central Great Barrier Reef continental shelf. Importantly, the individual mesh dimensions of the numerical scheme were set at approximately 2 × 2 km which was sufficient resolution to incorporate the topography of each reef within the matrix. The model provided a new detailed understanding of the influence of the reef matrix on the tidal currents of the outer shelf. In particular, the model demonstrated that the spatial variability in the tidal current's speed and direction exists down to the scale of the 2 km grid size. The model also demonstrated the significant tidally-induced residual currents that result from the interaction with the complex topography of the reef matrix. The advective effect of these tidal currents would be significant as the tidally-induced residual currents are of similar magnitude to the non-tidal currents of the region. Further, the spatial variability in the modelled and observed tidal currents suggests highly spatially variable advective processes operate within the reef waters.     

High-resolution surveys of the biogeochemistry of the New England shelfbreak front during Summer, 2002

We present the observations from a pair of field experiments at the New England shelfbreak front in June and August of 2002, each consisting of 14 cross-frontal surveys using the Lamont Pumping SeaSoar. Measurements of the front's physical, chemical, and bio-optical characteristics were made at high spatial and temporal resolution. The front, based on water-column hydrographic distributions, was found within a few km of the 200 m isobath during both cruises. We present here composite sections, based on averages of individual sections shifted in space to a common frontal location, of the cross-frontal distributions of these properties as a measure of the mean state of the front in both June and August. The observations show the familiar temperature, salinity, and density distributions of the summertime front, dominated by surface thermal heating. Nutrient and bio-optical distributions show the combined effects of water-mass exchange and biological processes. T, S, silicate, and phosphate distributions are suggestive of cross-frontal exchange of slope- and shelf waters, although transport mechanisms and pathways are not apparent. These properties, along with nitrate and optical measures of the suspended particle distributions, show vertical displacements of isopleths as the front is approached: property contours slope upwards toward the front from the shoreward side, and downwards toward the front from the seaward side. Again, actual water-movement pathways are not constrained by these suggestive patterns. Bio-optical distributions show elevated indicators of photosynthetic efficiency both seaward and shoreward of the front, but the front itself is a minimum in biomass. Accumulation of photosynthetic biomass appears to be controlled primarily by nitrate scarcity in waters within and above the pycnocline. At the base of the pycnocline, light limitation appears to be the controlling factor, although the base of the euphotic zone is deeper than the biomass maxima and the base of the pycnocline. Mechanisms explaining this phenomenon are unclear, but tenuous evidence suggests low stratification at the depth of the 1% light level may not allow phytoplankton to optimize for the low-light, high-nutrient conditions at depth. Cross-frontal differences in nutrient and bio-optical parameters, particularly in August, suggest distinct phytoplankton assemblages, and the presence of calcite-forming or nitrogen fixing groups to the community structure, especially in very shallow waters across the front and in pycnocline waters seaward of the front.     

Paleoclimatic evolution of the Galician continental shelf (NW of Spain) during the last 3000 years: from a storm regime to present conditions

A multiproxy study of the sedimentary record carried out on gravity core CGPL00-1 retrieved from the outer Galician continental shelf (NW of Spain) has allowed us to establish the main climate fluctuations affecting the region during the Upper Holocene. Grain size, TOC, C/N ratio, biogenic opal and planktonic foraminifera are the main analysed parameters. Lithology and grain size distribution lead to identify two sedimentary sequences: a lower half mainly composed by glauconitic sand and a muddy upper half. A chronology has been established based on three AMS radiocarbon ages, 907 cal. BC, 898 cal. BC and 1399 AD, and the aforementioned sedimentary sequences. The obtained radiocarbon ages are the first dated sediment samples for the Galician continental shelf. Geochemical markers show different trends in both sequences: low and/or fluctuating values in the sandy sequence and high and relatively constant values in the upper muddy sequence. The whole sandy interval is interpreted to be a nearly instantaneous deposit from a distal storm ebb current. The muddy interval was deposited in a stable and low-energy marine environment, similar to that found on the present outer shelf. The shift from a storm-dominated shelf to a low-energy environment took place synchronous with the Subboreal/Subatlantic transition, when an increase in storminess appears related to climatic instability. Transitional planktonic foraminiferal assemblage dominates the whole record, although a change to more abundant cold water species at 1420 AD, may relate to an intense upwelling pulse, probably reinforced by colder atmospheric temperatures during the Little Ice Age (LIA). Despite the presence of an upwelling regime since 1420 AD, lesser amount of opal has accumulated in the outer shelf due to enhanced offshore transport and stronger remineralization.     

Biogenic silica cycle in surface sediments of the Greenland Sea

In contrast to several investigations of biogenic silica (BSi) content and recycling in surface sediments of the Southern Ocean, little is known about the benthic cycle of BSi in high northern latitudes. Therefore, we investigated the silicic acid concentration of pore water and BSi content of surface sediments from the Greenland Sea. Low BSi contents of less than 2% were observed. High-resolution (2–5 mm) BSi profiles and comparisons to trap studies suggest that only relatively dissolution-resistant siliceous components reach the seafloor. Pore water investigations reveal BSi fluxes of more than 300 mmol m⁻² a⁻¹ only for a few sites on the shelf. A statistically significant relationship between water depth and BSi rain rate reaching the seafloor was not observed. Sampling along a transect perpendicular to the marginal ice zone (MIZ) revealed no enhanced rain rate of BSi reaching the seafloor in the vicinity of the ice edge. Although the MIZ of the Greenland Sea is characterized by the enhanced export of biogenic particles from surface waters, this feature is not reflected in the benthic cycle of biogenic silica. The lack of such a relationship, which is in contrast to observations of shelf and continental margin sediments in the southern South Atlantic, is probably caused by the enhanced dissolution of BSi in the water column and highly dynamic ice conditions in the Greenland Sea.