Western Shelf Research Articles

Tidal and subtidal currents in the Strait of Sicily

Current measurements, from shipboard and moored instruments, from the Strait of Sicily are examined to evaluate the importance of tidal and subtidal currents during summer 2000. The results show that the principal tidal components (M2, K1, S2, and O1) are particularly important in the western shelf and sill region but are significantly less important over the eastern shelf and almost negligible in the central part of the strait. There is a significant internal tide in the narrowest passage through the western sill. The residual (nontidal) currents describe the main pathways and local structures of Atlantic Water as it crosses the strait. The residual currents are compared with the geostrophic velocity calculated from simultaneous hydrographic measurements.

Seasonal circulation on the western shelf of the Gulf of Mexico using a high‐resolution numerical model

The seasonal circulation on the western shelf of the Gulf of Mexico is studied using a high‐resolution numerical simulation, historical hydrographic data, sea level data, and satellite images. Three regions are distinguished, the Tamaulipas‐Veracruz (TAVE) shelf, the Louisiana‐Texas (LATEX) shelf, and the western Campeche Bank. On the TAVE shelf there is a swift reversal of the along‐shelf current, downcoast from September to March and upcoast from May to August when there is upwelling due to offshore Ekman transport. Circulation on the western Campeche Bank is upcoast throughout the year. The LATEX shelf has a cyclonic circulation, except during summer months when the flow is eastward. During spring‐summer the upcoast current on the TAVE shelf reaches the southern Texas shelf where it encounters a downcoast coastal current favoring offshore transports. In the fall‐winter, the downcoast current reaches the southern Bay of Campeche where it meets an opposing along‐shelf current, generating seasonal offshore transports. During fall and winter, cool low‐salinity water from the Mississippi and Atchafalaya Rivers is advected westward along the LATEX shelf onto the TAVE shelf, developing along‐shelf fronts and temperature inversions commonly observed over the outer shelf and shelf break. The main forcing over the western shelf of the gulf is the along‐coast wind stress component. The existence of the cross‐shelf transports in the confluence regions is supported by chlorophyll a data. Up to 80% of the seasonal sea level variability is explained by the along shelf currents and the low‐frequency variability of the atmospheric sea level pressure.

Platform-basin transect of a middle to late Jurassic large-scale carbonate platform system (Shotori mountains, Tabas area, east-central Iran)

Following a phase of predominantly siliciclastic sedimentation in the Early and Middle Jurassic, a large-scale, low-latitude carbonate depositional system was established in the northern part of the Tabas Block, part of the central-east Iranian microplate, during the Callovian and persisted until the latest Oxfordian/Early Kimmeridgian. Running parallel to the present eastern block margin, a NNW/SSE-trending carbonate platform developed in an area characterized by reduced subsidence rates (Shotori Swell). The growth of this rimmed, flat-topped barrier platform strongly influenced the Upper Jurassic facies pattern and sedimentary history of the Tabas Block. The platform sediments, represented by the predominantly fine-grained carbonates of the Esfandiar Limestone Formation, pass eastward into slope to basin sediments of the Qal'eh Dokhtar Limestone Formation (platform-derived allochthonites, microbialites, and peri-platform muds). Towards the west, they interfinger with bedded limestones and marlstones (Kamar-e-Mehdi Formation), which were deposited in an extensive shelf lagoon. In a N−S direction, the Esfandiar Platform can be traced for about 170 km, in an E-W direction, the platform extended for at least 35–40 km. The width of the eastern slope of the platform is estimated at 10–15 km, the width of the western shelf lagoon varied considerably (>20–80 km). During the Late Callovian to Middle Oxfordian, the Esfandiar Platform flourished under arid climatic conditions and supplied the slope and basinal areas with large amounts of carbonates (suspended peri-platform muds and gravitational sediments). Export pulses of platform material, e.g. ooids and aggregate grains, into the slope and basinal system are interpreted as highstand shedding related to relative sealevel variations. The high-productivity phase was terminated in the Late Oxfordian when the eastern platform areas drowned and homogeneous deep water marls of the Upper Oxfordian to Kimmeridgian Korond Formation onlapped both the Qal'eh Dokhtar Limestone Formation and the drowned Esfandiar Limestone Formation. Tectonic instability, probably caused by faulting at the margins of the Tabas Block in connection with rotational movements of the east-central Iranian block assemblage, was responsible for the partial drowning of the eastern platform areas. In some areas, relicts of the platform persisted to produce shallow-water sediments into the Kimmeridgian.

Distribution of dinoflagellate cyst assemblages in surface sediments from the northern and western shelf of Iceland

In order to provide calibration for palaeoceanographic investigations, 54 surface sediment samples from the northern and western margin of Iceland and Greenland have been analysed for their dinoflagellate cyst assemblages. Relatively high diversity was observed with a total of 28 taxa. Cysts of Pentapharsodinium dalei are dominant accompanied by Operculodinium centrocarpum and Nematosphaeropsis labyrinthus. Three groups of assemblages have been recognised based on multivariate statistical analyses and these are related to surface water masses and currents. The first association, Group I, located west of Iceland, is characterised by high abundance of O. centrocarpum and N. labyrinthus accompanied by significant occurrence of the heterotrophic taxa Brigantedinium spp., Selenopemphix quanta, cysts of Polykrikos schwartzii and cysts of Protoperidinium americanum. This region is under the influence of the Irminger Current (IC) and contains the most productive waters around Iceland. The second association, Group II, comprising high relative abundance of cysts of P. dalei accompanied by significant occurrence of O. centrocarpum and a very high concentration (up to 256 920 cysts/g), is situated in the northern Icelandic shelf, in the Polar Front realm, i.e. the marginal zone between the East Icelandic Current (EIC) and the northern branch of the IC. The third group (Group III) is in the region influenced by the East Greenland Current and the EIC and characterised by the co-dominance of cysts of P. dalei and O. centrocarpum, relatively high representation of Impagidinium pallidum and low concentrations (from 370 to 56 220 cysts/g). This investigation demonstrates that dinocyst distribution is clearly related to upper water mass physical factors. Advection of temperate species around Iceland suggests significant transport by surface currents, especially the IC; however, the apparent boundaries between the associations and decreasing percentages related to environmental gradients indicate that dispersal by surface transport is limited by the ecological requirements of the dinoflagellate cells. Bottom currents may play a role in the accumulation rate of cysts.

Focused arrivals in shallow water propagation in the Straits of Florida

Observations of multipath on the western shelf area of the Florida Straits have consistently revealed an intense arrival associated with refracted bottom reflected (RBR) rays/modes. The ‘late’ or ‘focused’ arrival is 15 to 20 dB higher than other arrivals. Parabolic Equation and Normal Mode propagation models and the shallow water invariant, beta, are used to explain the focusing. The sound speed profile that produces the focusing comes about from a combination of a strong downward refracting layer resulting from the geostrophic equilibrium of the Florida current on top of a well mixed turbulent bottom boundary layer. The resulting c(z) resembles cosh(g(1−z/D)), which produce perfect focusing at every range, that is, all RBR modes have the exact same group velocity.

A coupled ocean‐ecosystem model of the Ross Sea: 2. Iron regulation of phytoplankton taxonomic variability and primary production

A model of the Ross Sea pelagic ecosystem has been developed by incorporating biological and nutrient dynamics into an ocean general circulation model. Surface heat and salt fluxes are calculated in the model using sea ice concentrations that are specified daily using the 20 year climatology derived from passive microwave satellite data. Surface spectral irradiance is calculated and propagated through both the water column and the sea ice. The biological state variables include two phytoplankton groups (diatoms and Phaeocystis antarctica), two nutrients (NO3 and Fe), detritus, and zooplankton. Nutrients are supplied to the sea surface from upwelling of nutrient‐rich deep waters, and in the case of Fe, from melting sea ice and snow which have accumulated aeolian‐derived Fe during the preceding autumn and winter. Phytoplankton growth rate is computed as a function of light or nutrient limitation. The model is able to accurately simulate the temporal evolution of the two well‐described phytoplankton blooms that dominate the southwestern Ross Sea: the diatom bloom in the shallow mixed layer of the western continental shelf (including Terra Nova Bay) and the P. antarctica bloom in the more deeply mixed Ross Sea polynya region. The simulated temporal progression of both phytoplankton blooms as well as their magnitude of primary production are in good agreement with time series estimates made using satellite ocean color data as validated by in situ observations. Model results suggest that Fe availability controls annual primary production in the Ross Sea, with only about two thirds of the available macronutrients in surface waters being consumed during the course of the bloom, but that Fe has little role in determining phytoplankton community composition. Rather, the taxonomic composition of the phytoplankton blooms is determined by the differential responses of shade‐adapted P. antarctica and high light‐adapted diatoms to the different mixing (and hence light) regimes within the Ross Sea. The model also predicts the development of a previously undescribed phytoplankton bloom along the eastern continental shelf, just north of the Ross Ice Shelf. The physical environment of this third phytoplankton bloom differs from that of the western shelf and the Ross Sea polynya region in that it receives a larger input of sea ice‐derived Fe and experiences complete exhaustion of surface NO3. Consequently, annual production on the eastern continental shelf is higher than anywhere else in the Ross Sea and supports an unusually large grazer population.

MICROFACIES AND MICROFOSSIL ASSEMBLAGES (SMALLER FORAMINIFERS, ALGAE, PSEUDOALGAE) OF THE HUECO GROUP AND LABORCITA FORMATION (UPPER PENNSYLVANIAN-LOWER PERMIAN) SOUTH-CENTRAL NEW MEXICO, USA

During the latest Carboniferous and earliest Permian (Virgilian-Wolfcampian), the eastern margin of the Orogrande Basin in south-central New Mexico was rimmed by a narrow, tectonically unstable shelf, on which sediments of the Laborcita and Abo formations were deposited. Sediments of the Hueco Group accumulated on the more stable western margin of the Orogrande Basin. On the eastern shelf, the Laborcita Formation of the northern Sacramento Mountains represents a transition from marine to terrestrial facies and is composed of clastic-carbonate cycles. Limestones accumulated during relative sea-level highstands in a shallow marine shelf environment. Clastic sediments were deposited during relative lowstands when there was strong clastic influx in a nearshore to terrestrial environment. The overlying Abo Formation is composed of terrestrial red beds. On the western shelf, the Shalem Colony Formation of the Hueco Group, equivalent to the Laborcita Formation, is composed of mostly normal marine shallow shelf limestones with only minor interbedded clastics. The Robledo Mountains Formation and Hueco Group reflects a trend from a restricted shallow shelf and tidal flat clastics facies in the lower part to more open conditions in the upper part, which also prevailed during deposition of the overlying Apache Dam Formation. Limestones of the Laborcita Formation and Hueco Group contain smaller foraminifers, algae and problematic carbonate microfossils. All taxa except two pseudoalgae in open nomenclature ( Litostroma (?) sp. and “problematicum gen. 1”) are taxa already described. Compared with the Carnic Alps (Austria/Italy), the assemblages of smaller foraminifers of the Laborcita Formation and Shalem Colony Formation are very similar to those of the Auernig and Carnizza Formations (Auernig Group), and Lower Pseudoschwagerina Limestone (Rattendorf Group), indicating an Orenbugian (“Bursumian”) to Asselian age. Smaller foraminifers of the Robledo Mountains Formation allow correlation with the Grenzland Formation and Upper Pseudoschwagerina Limestone (Rattendorf Group) of the Carnic Alps, dated as Asselian to Sakmarian. Smaller foraminifers of the Apache Dam Formation suggest an Artinskian age, correlating with the Wolfcampian, or the Trogkofel Group of the Carnic Alps.

Structure and eastward extension of the Changjiang River plume in the East China Sea

The hydrographic structure and offshore extension of freshwater plume discharged from the Changjiang (also known as the Yangtze) River in the northern East China Sea were investigated by analyzing conductivity‐temperature‐depth (CTD) data and drifter trajectories collected during the summers of 1997 and 1998. From June to early September, when southerly winds prevail, the plume tends to move northeast in the Chinese coastal area and then separates from the coastal zone to travel eastward over 400 km offshore across the western shelf of the northern East China Sea. During other seasons, when northerly winds prevail, the plume is confined to the Chinese coast. In the summer the plume in the midshelf, confined to a thin surface layer 10 to 15 m thick, extends eastward in the form of patches of low‐salinity water rather than spreading as a tongue‐shaped pattern from the Changjiang mouth. The eastward movement of patches in the western shelf is primarily due to upwelling favorable southerly winds. Upon reaching the vicinity of Cheju‐do, an island in the middle of northern East China Sea, the patches are advected to the Korea/Tsushima Strait by either the Cheju Warm Current or a northward‐flowing mean current of the Kuroshio Branch Current and then finally flow into the East/Japan Sea.

Surface chlorophyll in the Black Sea over 1978–1986 derived from satellite and in situ data

Absolute values of chlorophyll a concentration and its spatial and seasonal variations in the Black Sea were assessed by using satellite CZCS and in situ data. Since the satellite CZCS had operated for the 1978–1986 period, CZCS data was used for assessing the past state of the Black Sea just before the onset of drastic changes observed in late 1980s. The approach used for the calculation of the absolute values of chlorophyll a concentration from CZCS data was based on the direct comparison of in situ chlorophyll a data and those of CZCS and by applying the algorithm developed for the transformation of CZCS data into chlorophyll a values. CZCS Level 2 data related with pigment concentration having a spatial resolution of 1 km at nadir were used. The daily Level 3 files were derived by binning Level 2 values into 4-km grid cells and the monthly and seasonal Level 3 files were created by averaging the daily Level 3 files over the corresponding period. In situ chlorophyll a data were obtained by spectrophotometric and fluorometric methods in 15 scientific cruises over the 1978–1986 period. Total number of ship-measured data used for the comparison with those CZCS values was 590. Chlorophyll a concentration (Chl) was derived from CZCS values ( C) with regression equations Chl= kC; the coefficient of transformation k was calculated from six different data sets by taking into account distinctions between subregions and seasons. The reasons for difference in the k values have been analyzed. Statistical comparison of the chlorophyll a values measured in situ and those derived from CZCS data was based on log-transformed data and gave the following results: regression slope=0.842, regression intercept=−0.081, coefficient of determination ( R 2)=0.806, root–mean–square error=0.195. The mean monthly chlorophyll a distributions derived from CZCS data over 1978–1986 have been constructed and the mean seasonal chlorophyll a values in different regions have been calculated and analyzed. The significant difference in chlorophyll concentration between the western shelf regions and the open part of the Black Sea has been demonstrated, especially in warm season. At almost all seasons, the highest chlorophyll concentration is observed in the western interior shelf region which is under strong influence of Danube. The summer mean chlorophyll concentration in this region is ∼18 times higher than that in the open parts and about nine times higher than in the eastern shelf region. The greatest seasonal variations are observed in the open part of the Black Sea: chlorophyll concentration in cold season is four to six times higher than in summer and three to five times higher than in April and October. To the contrary, in the western interior shelf regions, the concentration is higher in May–October (about twice than that in November–March). Seasonal variations in the western outer shelf regions are smoothed out as compared with both the western interior shelf and the open regions.

Glaciomarine sedimentation and its paleoclimatic implications on the Antarctic Peninsula shelf over the last 15 000 years

Analyses of sedimentological, geochemical and micropaleontological parameters from radiocarbon-dated sediment cores retrieved from the Antarctic Peninsula’s western continental shelf reveal a detailed paleoclimatic and/or paleoceanographic history over the last 15 000 radiocarbon years. Deglaciation of the outer shelf off Anvers Island commenced prior to at least 15 000 yr BP, marked by the deposition of distal glaciomarine diamicton (facies 2) beneath a floating ice shelf, and lasted for 3800 years with increasing diatom abundance and total organic carbon (TOC) over time. A return to colder conditions occurred between 12 800 and 11 600 yr BP with a drop in TOC content and diatom abundance, which is coincident with the Younger Dryas event in the North Atlantic region. At this time, an abrupt increase in percentage sea-ice taxa as well as in the ratio of ( Fragilariopsis curta+Fragilariopsis cylindrus)/ Thalassiosira antarctica suggests renewed ice-shelf advance. In contrast, the inner shelf was deglaciated somewhat later about 11 000 yr BP, that is, 3000 years after the outer shelf. Prior to 11 000 yr BP, deposition of proximal glaciomarine diamicton (facies 1) close to the grounding line under a floating ice shelf and/or persistent sea ice may have occurred on the inner shelf. After this date, deposition of distal glaciomarine diamicton (facies 2) followed. A climatic optimum is recognized between 6000 and 2500 yr BP, coinciding with a ‘mid-Holocene climatic optimum’ from several other Antarctic sites, e.g. the Palmer Deep. During this time, as the glacial system receded from the shelf, greatly enhanced primary productivity occurred in open marine conditions, resulting in the deposition of diatomaceous mud (facies 3) and causing post-depositional dissolution of calcareous benthic and planktonic foraminifers in sediment. Around 2500 yr BP (the onset of the Neoglacial), diatomaceous sandy mud (facies 4), characterized by a decrease in TOC and diatom abundance, reflects the formation of more extensive and seasonally persistent sea ice, as evidenced by an increase in percentage of sea-ice taxa and in the ratio of ( F. curta+F. cylindrus) /T. antarctica. Our results provide evidence of climatic change on the Antarctic Peninsula’s western shelf that helps to refine the existence and timing of late Pleistocene and Holocene millennial-scale climatic events in the Southern Hemisphere.

Discovery of a submerged relic reef and shoreline off Grand Cayman: further support for an early Holocene jump in sea level

Abstract Drilling close to the base of a submerged sea-cliff on the terraced eastern shelf of Grand Cayman has revealed a relic Acropora palmata reef at a depth of 21 m below msl. Ten cores from its crest are principally composed of cobble-sized clasts of A. palmata set in a matrix of cemented skeletal grainstone. The clasts have a distinctive succession of encrusters that indicate rapid burial: a photophilic association of crustose coralline algae, foraminifera and vermetid gastropods superimposed by a cryptic association of sclerosponges, foraminifera and serpulids. In addition to rapid burial, U–Th thermal ionization mass spectrometer (TIMS) dating of coral clasts within 1 m of the relic-reef surface indicates minor temporal mixing with ages between 8.9 and 8.1 ka. Such mixing and rapid burial is consistent with a hurricane deposit and is identical to deposits found on the crests of modern reefs. In relation to its age, the preservation of a −18.5-m intertidal notch cut into the submerged sea-cliff on the western shelf of Grand Cayman implies that the crest of the relic reef has been lowered 1.5–2 m by marine abrasion/bioerosion at a rate of ∼0.25 mm yr−1. Reconstructing this eroded section using average Holocene accretion rates indicates that the reef likely ceased accreting at ∼7.6 ka at a depth of ∼19 m. Comparing these data with other relic reefs in the Caribbean indicates that the relic reef on Grand Cayman died within 160 years of relic reefs on Barbados, St. Croix, St. Thomas and north Florida. This narrow interval of reef demise also coincides with the time when modern reefs were establishing themselves some 4–9 m higher upslope—a fact that can only be resolved by invoking a rapid 6-m jump in sea level ∼7.5 ka ago. Such a jump would also account for two unexplained events around this time: the restricted interval of global delta initiation and the catastrophic flooding of the glacially lowered Black Sea.

Chemical speciation of copper and zinc in surface waters of the western Black Sea

The chemical speciation of Cu and Zn was investigated by voltammetric titration methods in the surface waters (∼10 m) of the western Black Sea during an Istanbul–Sevastopol cruise conducted in November 1998. Supporting parameters (temperature (T), salinity (S), pH, alkalinity (Alk), suspended particulate matter (SPM) and dissolved and particulate 234Th) were obtained in order to distinguish hydrographic features against involvement of the metals in biogeochemical processes. In the Turkish continental slope region, the cruise track intersected a narrow vein of colder water originating on the western shelf. The core of this cold water vein was characterised by a relatively low salinity, higher specific alkalinity and higher metal (especially Cu) and metal-binding ligand concentrations. A very large portion of Cu (93–99.8%) and Zn (82–97%) was organically complexed. The degree of complexation was highest in shelf waters and lowest in the central gyre. Titration data for Cu were modelled by two classes of organic binding ligands characterised by (CL1=3–12 nM, log K1′=13.1–13.9) and (CL2=20–70 nM, log K2′=9.4–11.2). These ligands occurred mainly in the ‘dissolved’ phase, as defined by 0.4-μm filtration. The stronger Cu-binding ligand seemed to be produced in situ in response to Cu concentration, whereas the weaker Cu-binding ligand appeared to be derived from terrestrial sources and/or reducing shelf sediments. Titration results for Zn were generally represented by one class of ligands (CL1=8–23 nM, log K1′=9.4–10.2), which were almost uniformly distributed between the ‘dissolved’ (78±8%) and the particulate phase (22±8%). The concentration of these strong Zn-binding ligands showed a very good correlation with SPM (r2=0.64), which improved when the dissolved ligands alone were considered (r2=0.78). It is hypothesised that these ligands were produced in situ by the bacterial breakdown of particulate organic matter.

Particulate matter spatial–temporal distribution and associated surface sediment properties: Thermaikos Gulf and Sporades Basin, NW Aegean Sea

Particulate matter (PM) distribution was studied in the NW Aegean Sea (Eastern Mediterranean) over a series of four seasons. PM concentrations in the surface and bottom nepheloid layers exhibit persistent maxima close to the river mouths; the rivers are identified as the major sources of PM. In general, higher particulate matter concentration (PMC) values appear in the shelf of the Thermaikos Gulf, whereas the deep Sporades Basin represents the area of lowest PM concentration. PMC during winter is much higher than in summer. This strong seasonal feature is attributed to the variation of the fresh-water supply, which is maximised during winter and spring, and almost negligible during the dry seasons. PM is mainly transported southwards and preferentially along the western coastline of the study area, due to a well-defined and dominant cyclonic circulation. Lateral transport of the PM prevails, whereas vertical particle transport is relatively limited. BNL formation is mainly attributed to sediment resuspension forced by the near-bottom currents. Surface sediment grain-size distribution is characterised by the presence of relict sands in the central and eastern shelf, while fine-grained muddy sediments cover the western shelf and the Sporades Basin. Relict sands were deposited during the Holocene sea-level rise. The general water mass circulation and the distance from the PM sources seem to favour PM deposition close to the river mouths and along the western coastline, while PM hardly reaches the eastern part of the area. As a result, relict sands remain uncovered or partly mixed with modern fine sediments. The shelf traps most of the PM, while some portion escapes to the deep Basin, probably from the western side of the continental slope.

Internal tides and baroclinicity in the southern Weddell Sea: 1. Model description

Near the continental shelf break in the southern Weddell Sea, Warm Deep Water and Western Shelf Water meet. Mixing mechanisms, such as internal tides, have the potential to mix these water masses and form Antarctic Bottom Water. A modified version of the Princeton Ocean Model was utilized to investigate the internal tidal fields generated by the interaction of the M2 barotropic tide with topography for transects across the continental shelf and slope in the southern Weddell Sea. Internal tides were generated over the upper continental slope as predicted by linear internal wave theory. Although the essentially two‐dimensional domain resulted in differences between the model elevations and the observations exceeding the observational uncertainties, the cross‐slope velocities agreed well with differences less than the uncertainties for 78% of the existing observations.

Holocene–late Pleistocene non-tropical carbonate sediments and tectonic history of the western rift basin margin of the southern Gulf of California

Abstract Using high-resolution seismic reflection profiling and dating of (1) shallow marine vibracores and (2) sediments collected from uplifted marine terraces we reconstruct the tectonic history and sediment accumulation patterns of Holocene to late Pleistocene warm-temperate to subtropical carbonates in the southern Gulf of California, Mexico. The study was conducted in the vicinity of La Paz where carbonates form along the fault bounded narrow western shelf of the tectonically active Gulf of California rift basin. The non-tropical nature of the setting is responsible for (1) poor cementation of the bioclastic carbonates, and (2) a composition which is dominated by rhodoliths (coralline red algae), corals and mollusks. Unrimmed carbonate flats forming in small pocket bays and a rhodolith bioherm, which has a surface area of more than 20 km2 and is up to 16 m thick, constitute the major carbonate factories. Holocene carbonate accumulation rates were deduced from seismic and core data and are highest on the rhodolith bioherm (260 cm/ka) and in subtidal zones of pocket bays (210 cm/ka), and lowest on the inner and middle shelf (100 cm/ka). Taken together, rates of carbonate accumulation are intermediate in magnitude between higher rates recorded in fully tropical carbonate settings and lower rates typical of cool-water carbonates. Seismic reflection profiles demonstrate that Isla Espiritu Santo in the center of the study area is a west dipping fault block, which is tectonically influenced by two distinct faults, the La Paz and Espiritu Santo faults. The latter fault accommodates at least 700 m of east-side down normal offset, and forms a steep eastern escarpment leading into the La Paz slope basin. Some of the sediments produced in the shallow carbonate factories of the narrow La Paz shelf are transported across this escarpment and are redeposited in the slope basin at a water depth of 750 m. Uranium-series dates of marine terraces exposed on Isla Espiritu Santo indicate that late Pleistocene uplift along the eastern side of the island could be as high as 310 mm/ka whereas downdropping along the western side of the island occurred at a rate of up to 15 mm/ka. Isla Espiritu Santo therefore constitutes part of the uplifted shoulder of the western margin of the Gulf of California rift basin. Patterns of vertical tectonic movements of Isla Espiritu Santo together with fault offsets on the surrounding seafloor and slump features on steep submarine slopes, point to continuous activity of the La Paz and Espiritu Santo faults. Results of this study will assist in recognizing and interpreting similar settings along ancient non-tropical rift basin margins worldwide and especially in the Neogene of the Gulf of California.

Modelling the impact of atmospheric and terrestrial inputs on the Black Sea coastal dynamics

Abstract. The dynamics on the North Western Shelf area of the Black Sea are examined, with an emphasis on the circulation induced by buoyancy due to the land drained fresh waters and by the interaction with the atmosphere, notably wind stress. A three-dimensional, multi-layer hydrodynamic model is employed with realistic topography and parameterisation of river plume physics. We focus on the seasonal patterns of transport of the river induced low-salinity waters within the Coastal Low Salinity Band and the conditions that influence their removal toward the shelf interior. The numerical simulations show that coastal circulation is greatly influenced by river runoff and especially in the case of the Danube, which is excessively high with monthly aver-aged values ranging from 5000 to 10000 m3 /s. A significant contribution of runoff comes from the neighbouring rivers. At the same time, the North Western Shelf is quite broad, so that the coastal dynamics are largely sheltered from the conditions in the deeper sea. Buoyancy due to river runoff thus dominates, creating a southward coastal current that is the predominant pathway for the land-drained inputs. As in all shelf areas, wind stress is a major circulation forcing mechanism and it modifies the buoyancy induced flow. It is shown that the seasonal variability in river runoff and wind stress, in combination with the shelf topography, determines the different pathways for the terrestrial inputs. Implications on the overall basin circulation are drawn, as the availability of low-salinity waters of river origin affects the upper Black Sea layer. Consequently, the formation of distinct water masses (such as the Cold Intermediate Layer) and the properties of the outflow toward the Mediterranean are also influenced.Key words. Oceanography: general (continental shelf processes; numerical modelling) – Oceanography: physical (air-sea interactions)

Sea-ice transport of riverine particles from the Laptev Sea to Fram Strait based on clay mineral studies

The aim of this study was to identify pathways and processes of modern sediment transport from the Siberian hinterland to the Laptev Sea and further to the Arctic Ocean. Clay mineral analyses were performed on riverine suspended particulate material (SPM), surface sediments of the Laptev Sea shelf, and sea-ice sediments (SIS). Material collected during seven expeditions was included in this study. Clay mineral assemblages are used to decipher the distribution of riverine sediments on the shallow Laptev Sea shelf, the entrainment of fine particles into newly forming ice, and the transport of SIS from the Laptev Sea towards the ablation areas. A cluster analysis of our data set shows that the clay mineral assemblages of Laptev Sea shelf sediments and SIS are controlled mainly by the input of riverine SPM supplied by the Khatanga, Lena, and Yana Rivers. Whereas the western shelf clay-mineral province is characterized by enhanced smectite concentrations supplied by the Khatanga River, the eastern Laptev Sea is dominated by illite discharged through the Lena and Yana Rivers. The SIS smectite concentration serves as an indicator for sediment source areas on the circum-Arctic shelves. Subsequently, the Transpolar Drift can be distinguished into a Siberian Branch fed from the eastern Kara Sea and the western Laptev Sea, and a Polar Branch originating from the eastern Laptev Sea.

Seasonal abundance and development of Calanus finmarchicus in relation to phytoplankton and hydrography on the Faroe Shelf

Calanus finmarchicus is almost absent from the Faroe Shelf during winter and is advected into the shelf ecosystem during spring and summer. The species starts to spawn prior to the onset of the spring bloom. During the pre-bloom phase, reproduction is usually highest over the western and northern part of the shelf and the nauplii and copepodites are dispersed by the anticyclonic current throughout the shelf area. Gonad maturity evaluation revealed that females over the central shelf during 1997 were immature until early April, after which the proportion of mature females gradually increased and peaked at about 80–90% mature in May–June and August. Pre-bloom reproduction on the central shelf in 1997 was low and the recruits present then were most likely advected from the northern or western shelf. Two main periods with peaks of abundance were observed on the central shelf during 1997, the largest in May–June (∼250 individuals m−3) and another in late August (∼100 individuals m−3). Off the shelf, the abundance of C. finmarchicus in most years was significantly higher than on the shelf, and the proportional mix of stages differed.

Glacioeustatic Transgressive Reflux: Stratiform Dolomite in Pennsylvanian Bioherms of the Western Orogrande Basin, New Mexico

ABSTRACT Upper Paleozoic (Pennsylvanian) algal bioherms of the western Orogrande basin consist of multiple high-frequency sequences with stratiform dolomitization that preferentially affects peritidal facies and strata proximal to sequence boundaries. Field, petrographic, and geochemical analyses reveal two replacive dolomite phases, and several dolomite cements. The facies-selective (FS) replacive phase affects peritidal facies, and consists of micritic to finely crystalline, fabric-preserving dolomite with high Sr and Na concentrations and low to moderate Fe and Mn. The non-facies-selective (NFS) phase affects facies proximal to sequence boundaries, and consists of finely to medium crystalline dolomite that is both fabric-selective and commonly fabric-destructive. This phase has moderate to locally elevated Sr and Na concentrations, and moderate to high Fe and Mn. Dolomite cements line and occlude pores and molds, and consist of medium to coarsely crystalline rhombs with moderate Sr and Mn, low Na, and high Fe. Stable oxygen isotope values for bulk dolostones exhibit a wide range and include relatively low values, suggesting precipitation and recrystallization over a wide range of conditions. FS dolomitization occurred penecontemporaneously with peritidal deposition at glacioeustatic fall to lowstand. NFS dolomitization occurred early postdepositionally, by reflux of mesosaline to hypersaline brines during early-middle stages of each glacioeustatic transgression. Virgilian lowstand gypsum deposits of the southwestern Orogrande provided the Mg source for the transgressive brines. Dolomite cements precipitated largely under reducing conditions of the burial environment. Early dolomite in the western Orogrande basin and its absence in the eastern Orogrande basin reflects local and regional physiographic asymmetry. The western shelf was of low relief, with an archipelago of high-relief mounds, which favored fall-stage peritidal conditions with local restriction in the mounded region. In contrast, the eastern shelf was of high relief and relatively elevated, which precluded tidal-flat development and allowed marine inundation only at late transgressive to highstand stages, by nearly normal-marine waters. Development of both peritidal (penecontemporaneous) and transgressive reflux (early postdepositional) dolomitization was therefore restricted largely to the western Orogrande basin.

The effects of fishing on demersal fish communities of the Scotian Shelf

Since the 1970s, average individual weight of commercially targeted demersal fish on the Scotian Shelf (Canada) decreased by 51% on the eastern shelf and by 41% on the western shelf. For both systems, the integrated community size spectrum showed long-term declines in proportions of large fish and survey cpue of most targeted species. In the east, these changes were coincident with a doubling of fishing effort and a decline in bottom temperatures to the lowest value observed in 50 years. In the west, fishing effort more than doubled while bottom temperatures reached the highest value in 50 years. In both areas, declines in survey cpue and average weight were more prevalent for target species than for non-target species. Since introduction of the closure of the cod fishery on the eastern shelf in 1993 and the restrictions on landings on the western shelf, average weights and the community size structure have stabilized. In the east, this stability is associated with increasing bottom temperatures; in the west it is concurrent with high bottom temperatures. I conclude that both fishing and changes in bottom temperature have influenced demersal fish size but that the relative effects cannot be determined from current observations.