Eastern Transect Research Articles

Ice keel seabed features in marine channels of the central Canadian Arctic Archipelago: evidence for former ice streams and iceberg scouring

The study area lies within the central part of the Canadian Arctic Archipelago; a region that was covered by the Laurentide Ice Sheet during the Late Wisconsinan glaciation and earlier. Multibeam imagery from widely spaced transects indicates the presence of linear groove and ridge features on the seabed at several localities in Peel Sound, Franklin Strait, northern Larsen Sound, and within M’Clintock Channel. These lineations resemble features in Antarctica and in several formerly glaciated regions that have been interpreted to be sole marks emplaced beneath fast-flowing ice streams. Based on these analogies, a similar origin is inferred for the lineations on the channel floors within the study area The lineations are oriented parallel to the channel axes and margins. They occur on all transects within the bathymetrically deeper area at the junction of Franklin Strait and Peel Sound. Northward in Peel Sound they occur extensively on the western and central transects, and more locally on the eastern transect. Their north-south orientation is normal to that of glacial flow features on Somerset Island and most of eastern Prince of Wales Island, which border Peel Sound to the east and west, respectively. The trend of the lineations is northeasterly (parallel to the channel axis) in Franklin Strait and mainly northerly in Larsen Sound and M’Clintock Channel. Elsewhere, the seabed imagery commonly displays scours of various sizes and orientations created by the keels of icebergs. Seabed sediments revealed by 3.5 kHz sub-bottom profiles are interpreted to consist primarily of ice-contact sediments, that in part are thinly mantled by draped water lain sediments. The age of the lineations has not been established. Possibly their formation was coincident with an ice stream in the M’Clintock Channel – eastern Victoria Island region, which formed an ice shelf in Viscount Melville Sound that grounded on southern Melville and Byam Martin islands at ca. 10.4–9.6 14C ka BP Alternatively, they could result from later glacial events.

Hydrostratigraphy as a control on subduction zone mechanics through its effects on drainage: an example from the Nankai Margin, SW Japan

Geofluids (2010) 10, 114–131AbstractAt many subduction zones, accretionary complexes form as sediments are offscraped from the subducting plate, and excess pore pressures commonly develop as low‐permeability marine sediments undergo rapid tectonically driven loading. Mechanical models demonstrate that pore pressure controls the overall geometry of these systems by modifying shear strength both within the accretionary wedge and along its base. At the Nankai margin offshore SW Japan, the taper angle of the accretionary wedge varies markedly along‐strike, from ∼4° along an eastern (Muroto) transect, to 8–10°along a western (Ashizuri) transect. Sediment stratigraphy on the subducting plate also varies: along the Ashizuri transect, the lowermost part of the section includes abundant sandy turbidites, whereas along the Muroto transect it is composed of monotonous hemipelagic mudstone. Here, I use a numerical model of fluid flow, together with laboratory measurements that constrain the bulk mudstone permeability, to quantitatively test the hypothesis that the turbidite‐rich section along the Ashizuri transect allows drainage at the base of the accretionary complex, resulting in differences in mechanical strength sufficient to cause the differences in taper angle. My results demonstrate that if the turbidite‐rich units are 2–100 times more permeable than the mudstone units, the variation in stratigraphy can indeed explain the observed taper angles. In contrast, permeability anisotropy within the turbidite‐rich units has only a minor effect; anisotropy ratios of ∼1000:1 would be required to cause the differences in taper angle. Along the Ashizuri transect, simulated pore pressures result in a basal shear strength ranging from a few MPa at the trench to ∼20 MPa by 30 km arcward; along the Muroto transect shear strength is substantially lower, reaching only ∼5 MPa by 30 km. This work shows that lithostratigraphy can strongly influence the mechanical behavior of subduction zone faults, through its control on the distribution and magnitude of excess pore pressure.

Diatom distribution in southeastern Pacific surface sediments and their relationship to modern environmental variables

The quantitative analysis of diatom assemblages preserved in 52 samples from the Bellingshausen and the Amundsen Seas provides the first comprehensive view on the distribution of diatoms in surface sediments of the eastern and central Pacific sector of the Southern Ocean. On a latitudinal transect along 120°W, diatom valve accumulation rates (AR) reach maximum values (8–10 × 10 8 valves m − 2 yr − 1 ) in a zone extending over ca. 900 km between the Antarctic Polar Front and the maximum average winter sea ice extent and exceed those ARs obtained from an eastern transect along 90°W by one order of magnitude. Lowest diatom concentrations (1–3 × 10 6 valves g − 1 ) were encountered in sediments of the Sea Ice Zone, affected by winter and summer sea ice. The accumulation rate pattern of the most abundant diatom Fragilariopsis kerguelensis (> 50% abundance in 47 samples) mirrors the pattern of the total diatom valve AR and the biogenic silica (BSi) AR, making F. kerguelensis the major contributor to the BSi preserved at the sea floor. Relative abundances of diatom species and species groups were statistically compared with a selection of environmental variables, such as the mean summer sea surface temperature and salinity, mean annual surface nutrient concentration (nitrate, phosphate, silicon), mean annual water column stratification, mixed layer depth in summer, and mean summer and winter sea ice concentrations. Polynomial canonical redundancy analysis (RDA) revealed the biogeographic distribution of diatom species had the strongest relationship with summer sea surface temperature (SSST) out of the nine tested environmental variables. This relationship accounted for 69.6% of the total variance of the diatom distribution, with 29.7% explained by the first gradient (significantly correlated to SSST with r 2 = 0.941) and 15.6% explained by the second gradient (correlated to both summer and winter sea ice and silicon concentration). Azpeitia tabularis, Hemidiscus cuneiformis and Roperia tesselata were associated with warmer water conditions (> 4 °C), whereas Fragilariopsis curta, F. separanda, F. rhombica and Thalassiosira gracilis were correlated with cold SSST (< 1.5 °C). Under the second gradient relationship, Actinocyclus actinochilus and F. curta were the most important diatoms representative of the diatom distribution in relation to the observed mean summer and winter sea ice concentrations. Confirming these environmental relationships is crucial for the development of reference data sets used in quantitative estimations of palaeoclimatic and palaeoceanographic conditions with statistical methods. This new data set represents the first modernised treatment of diatom remains from the SE Pacific Ocean and generally supports the use of a circum-polar database for the determination of summer SST, sea ice and potentially biogenic silica distribution of the Southern Ocean back into the Late Quaternary.

Residual flow over a bump in Quellón Bay

Measurements of velocity and density profiles were used to describe the tidal and residual flow structure in Quellon Bay, southern Chile (43 o 10'S), an eastward opened bay with ca. 6 km width (north-south) at its mouth and a low river discharge at its head, associated to a productive marsh- like branch. Current profiles were obtained in winter and spring with a 307.2 kHz Acoustic Doppler Current Profiler (ADCP) during two semidiurnal cycles along a repeated triangular circuit. The eastern side of the triangle consisted in an across- bay transect oriented in the north-south direction, and two along-bay (aprox. east-west oriented) transects formed the other sides identified as northern and southern transects. The triangle circumscribed a small bump of ca.10 depth, surrounded by bathymetry of the order of 20-30 m. Density profiles were obtained at the westernmost corner of the triangle. The longitudinal mean flow in the eastern transect at the mouth showed a lateral exchange structure that consisted in inflow (westward) over the northern side and outflow (southeastward) over the southern side. Lateral residual flow showed increases of the southward component that seems to be related to the presence of the bump. The longitudinal residual flow was mostly westward in the southern transect and eastward in the southern transect, suggesting that flow enters the bay through the northern side and exits through the southern side. Over the southern transect there were evidences of flow divergence over the bump which promotes upward velocities over this feature. Upward velocities over the bump and downward velocities over the eastern flank were estimated using the continuity equation. There were evidences that divergence is induced by flow separation yielded by a pair of counter-rotating vortex over this feature. Nonlinearities seem to dominate the tidal dynamic balance. The intra tidal variability of flow and density at the repeated station showed no significant differences between the low and high rainy seasons, suggesting that dynamics of the system is less affected by river discharge. Small increases of salinity were associated to the effect of tidal flood phase. There were no significant differences between samplings on July and November, nor in residual flow patterns or in density field.

Integrated seismic, flexural and gravimetric modelling of the Coastal Cordillera Thrust Belt and the Guárico Basin, North-Central Region, Venezuela

We constructed two integrated geodynamic transects for the eastern and western areas of the Coastal Cordillera Thrust Belt and the Guárico Basin (North-Central region of Venezuela), based on information from depth-converted reflection seismic cross-sections, surface geology, Bouguer anomaly and deep refraction seismic profiling. These geodynamic transects were modelled in order to: (1) understand the geodynamic processes responsible for the formation of the Guárico Basin, (2) quantitatively determine the contribution of the Coastal Cordillera thrust sheet loading in the generation of the basin fill (i.e. the effective elastic thickness of the northern part of the South American lithosphere, the total shortening of the thrust belt and the maximum subsidence of the basin) and (3) characterize the crustal structure and Moho depth in North-Central Venezuela. We used flexural modelling to obtain important quantitative information related to the total Middle Eocene to Recent shortening of the Coastal Cordillera, the total subsidence observed in the Guárico Basin and the effective elastic thickness of the South American lithosphere in northern Venezuela. We conclude that the load of the Coastal Cordillera on the South American lithosphere is sufficient to generate the subsidence observed in the Guárico Basin. The total shortening and subsidence obtained in the western transect are 44 km and 5 km, whereas they are 10 km and 7 km, in the eastern transect, respectively. The effective elastic thickness of the South American lithosphere obtained from the modelling is 25 km. The seismically-constrained deep structure of North-Central Venezuela indicates that Moho reaches a maximum depth of 45 km under the Guayana Shield and shallows to the north, to 35 km beneath the Guárico Basin. According to the gravity models, Moho is deeper under the Coastal Cordillera Thrust Belt than expected from flexural modelling, pointing to the existence of a crustal root more than 35 km deep. The Moho gets shallower (30 km) towards the Caribbean Sea.

Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology: Discussion

The Xolapa Complex of southern Mexico is composed of mid-crustal arc-related gneisses of poorly resolved ages, intruded by undeformed Cenozoic calc-alkaline plutons. Twelve undeformed and deformed tonalitic/ granodioritic samples from three transects across the Sierra Madre del Sur (Acapulco, Puerto Escondido, and Puerto Angel) were chosen for U-Pb zircon analysis. The measurements were performed on single crystals of zircons, using a multiple-collector laserablation inductively coupled plasma–mass spectrometer (MC-LA-ICP-MS). About 20–30 crystals were measured from each sample. Three gneisses and migmatites from the eastern transect (Puerto Angel), located 30–42 km from the coast yielded Grenvilleaged zircons (970–1280 Ma), suggesting that the samples represent Oaxacan basement, not deformed Xolapa Complex. The central transect (Puerto Escondido) yielded Oligocene ages (25–32 Ma) on undeformed plutons as well as mid-Mesozoic and Permian ages on gneisses. Most samples along the Puerto Escondido transect contain inherited ca. 1.1 Ga xenocrystals of zircons. The western transect (Acapulco) yielded Late Jurassic–Early Cretaceous ages (160–136 Ma) on gneisses, and Paleocene (55 Ma) and Oligocene (34 Ma) ages on undeformed plutons, with no inherited Grenville ages. The older ages and xenocrystic zircons in arc-related Xolapa Complex mirror the crustal ages found in neighboring terranes (Mixteca and Oaxaca) to the north of the Xolapa Complex, suggesting an autochthonous origin of Xolapa with respect to its neighboring northbounding terranes. The new data and previously published ages for Xolapa suggest that metamorphism and migmatization of the deformed arc rocks took place prior to the Cenozoic. Eocene and Oligocene plutons representing renewed arc-related magmatism in the area are common throughout Xolapa, and probably represent the more deeply exposed continuation of the Sierra Madre Occidental arc to the northwest. The available U-Pb data argue against the previously proposed eastward migration of magmatism between Acapulco and Puerto Angel during the Oligocene.

Lateral variability of flow over a sill in a channel of southern Chile

Measurements of velocity and density profiles were used to describe the tidal and mean flow structure across and along a sill in Refugio Channel, a fjord-like inlet in Southern Chile (43.9°S). These are the first oceanographic measurements of any kind effected in Refugio Channel. Current profiles were obtained with a 307.2-kHz acoustic Doppler current profiler during two semidiurnal cycles along a repeated triangular circuit. Two along-channel transects formed the sides of the triangle that crossed the sill and were identified as the western and eastern transects. One cross-channel transect, the base of the triangle, was located on the seaward side of the sill. Density profiles were obtained at the corners of the triangle. The longitudinal mean flow in the western transect showed a two-layer exchange structure over the landward side of the sill. The structure of net seaward flow at the surface and landward flow at depth was disrupted by the sill in such a way that over the seaward side of the sill, only seaward flow was observed throughout the water column. This likely resulted from the blocking of landward net flow by the sill. In the eastern transect, two-layer exchange dominated over most of the transect and was consistent with the observed density profiles. Over the seaward side of the sill, a surface layer, ∼10m deep, flowed landward as a third layer. This feature should have been caused by river input further seaward (to the north) and produced a surface convergence region over the sill. In terms of tidal flows, the greatest tidal current amplitudes were 40cm s−1 over the sill as the flow accelerated through the reduced cross-sectional area of the channel. Near-surface flow convergences were identified over both along-channel transects.

Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology

Research Article| July 01, 2004 Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology Mihai N. Ducea; Mihai N. Ducea 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar George E. Gehrels; George E. Gehrels 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Sarah Shoemaker; Sarah Shoemaker 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Joaquin Ruiz; Joaquin Ruiz 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Victor A. Valencia Victor A. Valencia 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Mihai N. Ducea 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA George E. Gehrels 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Sarah Shoemaker 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Joaquin Ruiz 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Victor A. Valencia 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Publisher: Geological Society of America Received: 06 Aug 2003 Revision Received: 06 Dec 2003 Accepted: 12 Jan 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2004) 116 (7-8): 1016–1025. https://doi.org/10.1130/B25467.1 Article history Received: 06 Aug 2003 Revision Received: 06 Dec 2003 Accepted: 12 Jan 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Mihai N. Ducea, George E. Gehrels, Sarah Shoemaker, Joaquin Ruiz, Victor A. Valencia; Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology. GSA Bulletin 2004;; 116 (7-8): 1016–1025. doi: https://doi.org/10.1130/B25467.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Xolapa Complex of southern Mexico is composed of mid-crustal arc-related gneisses of poorly resolved ages, intruded by undeformed Cenozoic calc-alkaline plutons. Twelve undeformed and deformed tonalitic/granodioritic samples from three transects across the Sierra Madre del Sur (Acapulco, Puerto Escondido, and Puerto Angel) were chosen for U-Pb zircon analysis. The measurements were performed on single crystals of zircons, using a multiple-collector laser-ablation inductively coupled plasma–mass spectrometer (MC-LA-ICP-MS). About 20–30 crystals were measured from each sample. Three gneisses and migmatites from the eastern transect (Puerto Angel), located 30–42 km from the coast, yielded Grenville-aged zircons (970–1280 Ma), suggesting that the samples represent Oaxacan basement, not deformed Xolapa Complex. The central transect (Puerto Escondido) yielded Oligocene ages (25–32 Ma) on undeformed plutons as well as mid-Mesozoic and Permian ages on gneisses. Most samples along the Puerto Escondido transect contain inherited ca. 1.1 Ga xenocrystals of zircons. The western transect (Acapulco) yielded Late Jurassic–Early Cretaceous ages (160–136 Ma) on gneisses, and Paleocene (55 Ma) and Oligocene (34 Ma) ages on undeformed plutons, with no inherited Grenville ages. The older ages and xenocrystic zircons in arc-related Xolapa Complex mirror the crustal ages found in neighboring terranes (Mixteca and Oaxaca) to the north of the Xolapa Complex, suggesting an autochthonous origin of Xolapa with respect to its neighboring north-bounding terranes. The new data and previously published ages for Xolapa suggest that metamorphism and migmatization of the deformed arc rocks took place prior to the Cenozoic. Eocene and Oligocene plutons representing renewed arc-related magmatism in the area are common throughout Xolapa, and probably represent the more deeply exposed continuation of the Sierra Madre Occidental arc to the northwest. The available U-Pb data argue against the previously proposed eastward migration of magmatism between Acapulco and Puerto Angel during the Oligocene. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Pigment adaptations in surface phytoplankton along the eastern boundary of the Atlantic Ocean

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 281:13-26 (2004) - doi:10.3354/meps281013 Pigment adaptations in surface phytoplankton along the eastern boundary of the Atlantic Ocean R. G. Barlow1,*, J. Aiken2, G. F. Moore2, P. M. Holligan3, S. Lavender4 1Marine & Coastal Management, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa 2Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, UK 3Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK 4Institute of Marine Studies, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK *Email: rgbarlow@deat.gov.za ABSTRACT: Pigment and ocean colour data were used to characterise the phytoplankton at the surface along an eastern boundary transect of the Atlantic Ocean in May and June 1998. Five major regions were delineated, including the Benguela and Canary upwelling systems off SW and NW Africa, the temperate NE Atlantic, the northern oligotrophic gyre and a southern oligotrophic zone. Diagnostic indices were derived to assess the chemotaxonomic composition of the phytoplankton community, while photo-pigment indices were derived to investigate pigment adaptations across the regions. A large range in chlorophyll a concentrations was observed along the transect, varying from 0.03 to 30.6 mg m-3. The levels were high in the high productivity regions, with mean concentrations being 3-fold greater in the Benguela region compared to the Canary and temperate NE Atlantic. Chlorophyll a levels were low in the oligotrophic regions, but concentrations in the southern oligotrophic region were double those in the northern. Diagnostic indices revealed that microplankton dominated the community in the Benguela region, nanoplankton dominated the temperate NE Atlantic and a mixed community of micro- and nanoplankton were present in the Canary region. Picoplankton comprised the majority of the communities in the 2 oligotrophic regions. Photo-pigment indices indicated that variation in the chlorophyll a proportion of total pigments was small, averaging 45% across the transect, but the accessory pigment pool varied considerably. Chlorophyll c and photosynthetic carotenoids were prominent in the high productivity regions, resulting in photosynthetic pigments (chlorophylls plus carotenoids) accounting for 87 to 93% of total pigments. In contrast, the level of photoprotective carotenoids was high in oligotrophic regions, resulting in a photosynthetic pigment content of only 68%. KEY WORDS: Pigment indices · Ocean colour · Atlantic Ocean · Eastern transect Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 281. Online publication date: November 01, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Magnetostratigraphy of the Plio‐Pleistocene carbonate section of the Great Australian Bight

ODP Leg 182 drilled two north‐south transects in the western Great Australian Bight close to longitude 128°E. These transects penetrated a remarkably thick section of uppermost Pliocene and Pleistocene carbonate sediments, which are separated from Miocene sections by a major hiatus. In the eastern transect, at sites 1129, 1131 and 1127, the Brunhes‐Matuyama boundary (onset of C1n) was found at 343 m, between 280 and 300 m, and at 343 m below the sea floor, respectively. In the western transect, at sites 1132 and 1130, it was found between 170 and 181 m, and at 200 m, respectively. Within the Brunhes chron, inclination and intensity fluctuations and correlations between susceptibility and standard δ18O records were used to give age‐depth relations. These age relations from the palaeomagnetics are broadly consistent with the biostratigraphy and the δ18O results from Leg 182, but do not provide an entirely independent dataset. Rock magnetism stratigraphy at site 1131 revealed the principal bryozoan buildup to be at the time of the last glacial lowstand, as suggested by previous workers. At all sites the sedimentation rate increases from the basal unconformity up into the thick Upper Pleistocene section. In the western transect there appears to be a partial record of most of the Late Pliocene and Early Pleistocene chron boundaries (C1r1n to C2An3n) in a condensed section, but in the eastern transect only the Jaramillo (C1r1n) is observed.

The influence of Cracow urban pollution on small forest areas

The impact of Cracow conurbation on small forest ecosystems was studied during the last four years (1998-2001). Seven sites located at different distances from the city and from busy road were selected. Lichen Hypogymnia physodes from unpolluted areas were transplanted to each of investigated sites for 6-month period (winter 1998/1999, 1999/2000, summer 2000). The quantity of litter fall with percentage composition of each species was estimated, also the input of litter and heavy metals to the forest floor were evaluated. Metal concentration (Cd, Pb, Cu, Zn, Fe) in Hypogymnia physodes, leaves of common oak, hornbeam and litter fall were determined in IL 251 flame AAS. Acute injuries of lichen thalli as well as concentrations of Pb, Zn, Fe and S were higher after winter transplantation and in sites located close to Cracow conurbation. High concentration of heavy metals were noticed in tree leaves from eastern transect as the effect of steel works emission as well as in those sites from southern transect situated nearby the city. Although air pollution has generally declined in the Malopolska district over the last few years the metal concentration and input to the forest floor in 2001 remains at the same level than in the previous years.

Input of heavy metals to the forest floor as a result of Cracow urban pollution

The influence of Cracow's urban pollution on small forest ecosystems was studied during 1998–2000. Seven sites located at different distances from the Cracow conurbation and from busy road were selected. Two sites were situated 26 and 31 km from Cracow town along an eastern transect, the other five—3, 8, 14, 26 and 35 km respectively, from Cracow—along a southern transect. At the beginning of September, five litter traps were set at each site. The quantity of litter fall, along with percentage composition of each species, were estimated for an area of 1 m 2. The concentration of Cd, Pb, Cu, Zn and Fe in the litter fall and the input of these metals to the forest floor were determined. The input of cadmium ranged from 84 μg m −2 at Kornatka in 1998 to 382 μg m −2 at Bonarka in 2000. The input of lead was about 10 times higher than cadmium and varied from 406 μg m −2 at W e ̨ glówka in 1998 to 3717 μg m −2 at Bonarka in 2000. Input of Cu, Zn and Fe to the forest floor showed the similar tendency. During 1998–2000, the highest heavy metal concentrations in the litter fall and their impact on the forest floor were always found at sites close to the Cracow conurbation and, in the eastern transect, close to a steel works. Input of metals to the forest floor tended to decrease with increasing distance from the centre of Cracow.

Phytoplankton biomass and primary production in the marginal ice zone of the northwestern Weddell Sea during austral summer

During the austral summer of 1995, distributions of phytoplankton biomass (as chlorophyll a), primary production, and nutrient concentrations along two north-south transects in the marginal ice zone of the northwestern Weddell Sea were examined as part of the 8th Korean Antarctic Research Program. An extensive phytoplankton bloom, ranging from 1.6 to 11.2 mg m−3 in surface chlorophyll a concentration, was encountered along the eastern transect and extended ca. 180 km north of the ice edge. The spatial extent of the bloom was closely related to the density field induced by the input of meltwater from the retreating sea ice. However, the extent (ca. 200 km) of the phytoplankton bloom along the western transect exceeded the meltwater-influenced zone (ca. 18 km). The extensive bloom along the western transect was more closely related to local hydrography than to the proximity of the ice edge and the resulting meltwater-induced stability of the upper water column. In addition, the marginal ice zone on the western transect was characterized by a deep, high phytoplankton biomass (up to 8 mg Chl a m−3) extending to 100-m depth, and the decreased nutrient concentration, which was probably caused by passive sinking from the upper euphotic zone and in situ growth. Despite the low bloom intensity relative to the marginal ice zone in both of the transects, mean primary productivity (2.6 g C m−2 day−1) in shelf waters corresponding to the northern side of the western transect was as high as in the marginal ice zone (2.1 g C m−2 day−1), and was 4.8 times greater than that in open waters, suggesting that shelf waters are as highly productive as the marginal ice zone. A comparison between the historical productivity data and our data also shows that the most productive regions in the Southern Ocean are shelf waters and the marginal ice zone, with emerging evidence of frontal regions as another major productive site.

Crustal transition between continental and oceanic domains along the North Iberian Margin from wide angle seismic and gravity data

Deep crustal features of the transition between the North Iberian mainland and the Bay of Biscay are constrained from new wide‐angle seismic and gravity data. Velocity‐depth models are derived from far‐offset recordings onshore of two N‐S marine profiles, complemented with new refraction data inland and tested against models that fit the gravity anomalies. Important lateral variations are inferred along the North Iberian margin. In the western transect, a Variscan crust is documented from the mainland across the continental shelf, and a rapid crustal thinning beneath the slope marks the transition to the oceanic domain. The eastern transect shows an outstanding crustal thickening beneath the coastline (Moho depths around 50 km) and a gradual thinning across the shelf. Differences are related to the V‐shape of the Bay of Biscay that causes a W‐E transition along the margin, from a short‐lived oceanic subduction to continental collision during the Tertiary oblique convergence between Europe and Iberia.

At-sea distribution of seabirds and marine mammals around Svalbard, summer 1991

At-sea distribution of seabirds and marine mammals was quantitatively determined during the Arctic EPOS cruise of RV Polarstern, from 21 June till 28 July 1991, during 377 half-an-hour counts. Data were expressed as numbers per count and as density, and daily food intake was calculated using allometric equations from literature. Mean densities for the whole expedition were 29 seabirds per km2 (mainly little auk,Alle alle: 8.7, kittiwake,Rissa tridactyla: 8.2, Brunnich’s guillemot,Uria lomvia: 6.5 and fulmar,Fulmarus glacialis: 3.4), 0.06 pinnipeds, 0.01 cetaceans and 0.002 polar bears. Total food intake by seabirds and marine mammals was 3.9 kg fresh weight/km2 per day, with extreme values of 6.6 in the northern west-to-east transect and 2.5 in the Storfjorden. The major ecological influence were fish eaters (1.7), and more especially Brunnich’s guillemot (1.2). Geographic differences were also detected: food intake by Brunnich’s guillemot represented 62% of total intake in Storfjorden, and by the kittiwake, 45% in the first eastern transect. The first and last transects in the western Barents Sea are described and discussed in more detail. Within different sectors, high concentrations of seabirds were noted, corresponding to hydrological features such as fronts between Atlantic and polar waters, as well as ice edges. Values of seabirds’ density and food intake are higher than in the Greenland Sea, even than at the biologically very active ice edge there. Figures for pinnipeds and cetaceans are similar; numbers of polar bears were higher around Spitsbergen.

Some climatic aspects of biomass productivity of white spruce stem wood

Biomass productivity was determined for white spruce (Piceaglauca (Moench) Voss) in the boreal forests of Alberta, the Northwest Territories, and Manitoba. Comparisons were made between southern and northern locations, between eastern and western transect locations, and between older (200 + years) and younger (110 years) trees. At 13 sampling locations, X-ray densitometric tree ring data were obtained from the base of the stem, breast height, and from five points equidistant along the stem. Markedly higher stem wood biomass productivity was found for the 110-year-old trees than for the 210-year-old trees in Alberta; average ring weights were 3.8 and 1.2 g for the first 100 years of growth in 1 cm thick disks at breast height. These results suggest that climatic warming since the end of the Little Ice Age (ca. 1850) has resulted in higher biomass productivity in the Canadian boreal forest.

Characterization and Stratigraphy of Soil Parent Materials of West‐central Minnesota

AbstractWithin the Alexandria Moraine Complex in west‐central Minnesota there are “islands” of fine textured soils and parent materials embedded within a matrix of coarser textured soils. To study the soils and the stratigraphic relationships of the parent materials, deep borings were made along two transects perpendicular to the Alexandria Moraine. A total of 27 sites were investigated. Particlesize distribution, CaCO3 equivalent, clay‐sized carbonate, and the dominant mineralogy of the &gt;2 mm, 1 to 2 mm very coarse sand (vcos), and &lt;2‐µm fractions were determined. Four glacial materials were identified in this study: New Ulm till, Kandiyohi till, Granite Falls till, and a “mixed” drift of mainly New Ulm and Granite Falls materials. The average clay content of the glacial materials ranges from about 370 g kg−1 in the Kandiyohi to about 130 g kg−1 in the Granite Falls and mixed drifts. The average shale content in the vcos fraction ranges from about 620 g kg−1 in the Kandiyohi till to about 60 g kg−1 in the Granite Falls till. The average CaCO3 equivalent ranges from about 220 g kg−1 in Granite Falls till to about 170 g kg−1 in the mixed drift. Statistically significant differences occur among the glacial materials in the concentration of sand, silt, clay, concentration of shale, carbonate, and crystalline pebbles in the vcos fraction, and the concentration of coarse fragments. A stepwise discriminant analysis was made and the four glacial materials were numerically defined by the first two canonical variables. The stratigraphic relationships of the two areas of Kandiyohi till outcrop studied seem to be quite different. The data from the western part of the county indicate much mixing of New Ulm and Granite Falls tills. Data from the eastern transect indicate the Kandiyohi till is an older till that has been thrust to the surface. It is suggested that the Kandiyohi till is a distinctly different glacial till in that: (i) the soils developed on this till are taxonomically distinct from soils on adjacent tills; (ii) the shale concentrations within the four defined glacial materials in the study region are statistically different; (iii) no other areas of this glacial till have been observed except in the Alexandria Moraine; and (iv) the glacial till outcrops are characteristically narrow linear features.

Radiolarian biogeography in surface sediments of the eastern Indian Ocean

We analyzed recurrent groups of Radiolaria in 74 core top samples from a transect through the eastern Indian Ocean in order to supplement our previous results from the western Indian Ocean (Johnson and Nigrini, 1980). We now identify six distinct recurrent groups and nine radiolarian assemblages in the combined data set of 120 samples; this extended sample coverage has led to several re-interpretations of the oceanographic significance of the radiolarian distribution patterns. Assemblage boundaries closely reflect the presence of major oceanographic fronts and surface currents including the South Equatorial Divergence, Subtropical Gyre, Subtropical Convergence, and Antarctic Convergence. At least four major aspects of the assemblages in the eastern transect are notably different from those in the western transect, leading to a marked east-west asymmetry in faunal distribution patterns across the Indian Ocean. The assemblage formerly associated with strong upwelling near the Arabian Peninsula is present throughout the Bay of Bengal as well, and is interpreted to reflect high salinity and low oxygen in the subsurface waters of the Indian Ocean north of the Equator. A new assemblage has been identified associated with the westward-flowing Pacific water into the eastern Indian Ocean in low latitudes, and may be a potential stratigraphic and paleoclimatic marker for times of low sea level when this westward near-surface flow was shut off (i.e., glacial maxima). An extensive region in the core of the subtropical gyre between 25°S and 35°S is relatively barren of Radiolaria, yet is marked by a characteristic assemblage distributed asymmetrically, perhaps reflecting the lack of a strong boundary current off the west coast of Australia. Assemblage boundaries in the vicinity of the eastward circumpolar flow are not strictly zonal, and may indicate significant deviations from the mean eastward flow as a necessary condition for conservation of potential vorticity when the flow encounters topographic irregularities.