Chukchi Plateau Research Articles

Size-fractionated chlorophyll a and primary productivity in the Chukchi Sea and its northern Chukchi Plateau

Investigations on chlorophyll a and primary productivity were carried out in the Chukchi Sea and its northern Chukchi Plateau during the 2 nd Chinese National Arctic Research Expedition in the summer of 2003. The results showed that chlorophyll a concentrations were 0.009–30.390 μg/dm 3 at the surveyed waters; the surface chlorophyll a concentrations were 0.050–4.644 μg/dm 3 and the average value was (0.875±0.981) μg/dm 3 in the surveyed area. In the Chukchi Sea Shelf, chlorophyll a concentrations at the depth from 10 m to bottom were higher than that in the surface water, and the concentrations were lower at the depth below 75 m in the Chukchi Plateau. Chlorophyll a concentrations descended in 3 sequential samplings on Transect R, with average values of (2.564±1.496) μg/dm 3, (1.329±0.882) μg/dm 3 and (0.965±0.623) μg/dm 3, respectively. The potential primary productivity ((2.305± 1.493) mgC/(m 3·h)) in the Chukchi Sea was higher than that ((0.527±0.374) mgC/(m 3·h)) in the Chukchi Plateau. The results of the size-fractionated chlorophyll a and primary productivity showed that microplankton accounted for the majority of the total chlorophyll a (63.13%) and primary productivity (65.16%) at the survey stations. The contributions of the nanoplankton and picoplankton to the total chlorophyll a and primary productivity were roughly the same.

Northward transport of pacific summer water along the Northwind Ridge in the Western Arctic Ocean

The recent sea-ice reduction in the Arctic Ocean is not spatially uniform, but is disproportionally large around the Northwind Ridge and Chukchi Plateau compared to elsewhere in the Canada Basin. In the Northwind Ridge region, Pacific Summer Water (PSW) delivered from the Bering Sea occupies the subsurface layer. The spatial distribution of warm PSW shows a quite similar pattern to the recent ice retreat, suggesting the influence of PSW on the sea-ice reduction. To understand the regionality of the recent ice retreat, we examine the dynamics and timing of the delivery of the PSW into this region. Here, we adopt a two-layer linearized potential vorticity equation to investigate the behavior of Rossby waves in the presence of a topographic discontinuity in the high latitude ocean. The analytical results show a quite different structure from those of mid-latitude basins due to the small value of β. Incident barotropic waves excited by the sea-ice motion with large annual variation can be scattered into both barotropic and baroclinic modes at the discontinuity. Since the scattered baroclinic Rossby wave with annual frequency cannot propagate freely, a strong baroclinic current near the topographic discontinuity is established. The seasonal variation of current near the topographic discontinuity would cause a kind of selective switching system for shelf water transport into the basin. In our simple analytical model, the enhanced northward transport of summer water and reduced northward transport of winter water are well demonstrated. The present study indicates that these basic dynamics imply that a strengthening of the surface forcing during winter in the Canada Basin could cause sea-ice reduction in the Western Arctic through the changes of underlying Pacific Summer Water.

Formation and spreading of Eurasian source oxygen‐rich halocline water into the Canadian Basin in the Arctic Ocean

We identify the source region and spreading pattern of cold, oxygen‐rich water observed in the halocline of the northern Canada Basin using both Joint Western Arctic Climate Studies 2002–2005 and other data. This water originates in the winter mixed‐layer in the Nansen Basin and, because of its convective origin, can be traced by its cold, oxygen‐rich properties together with a signature of low potential vorticity. This water, a component of the cold halocline complex, spreads into the Makarov Basin across the Lomonosov Ridge between 82°N and 86°N, enters the Canada Basin between the Alpha and Mendeleyev ridges, and continues eastward into the Beaufort Gyre north of Chukchi Plateau.

Warming in Arctic intermediate and deep waters around Chukchi Plateau and its adjacent regions in 1999

Based on the data measured during Arctic scientific expedition of China in 1999, the characteristics of temperature and salinity distributions around the Chukchi Plateau and its ad-jacent regions have been studied. It was found that the intermediate water with temperature higher than 0.5℃ existed in all parts of a 640 km section with a maximum temperature of 0.85℃ indicating a strong signal of the warming in Arctic Intermediate Water in 1999. Two important phenomena are described in this paper. First, the temperature of warm water was horizontally nonuniform. In the area of Chukchi Plateau, the temperature was higher, the layer of warm water was thicker and the depth of the warm water core was shallower than those in the area of con-tinental slope. The horizontal nonuniformity of the temperature distribution of warming water im-plies that the upward heat flux should also be nonuniform, thus exerting different effects on sea ice thickness, ice extent, and air-sea heat exchange. The mechanism to generate higher tem-perature in the plateau region was the bypassing of current around the plateau area caused by the special local topography, which restricted water exchange across the plateau and conserved heat in the water body. Second, the deep water down to 1400 m was also warming with a tem-perature increase of 0.2℃. The warming in deep water reflects the occurrence of complicated heat redistribution processes in the intermediate water, which altered the thermal structure in the upper 1400 m. The warming data embody the obvious impact of global climate change on the Arctic Ocean and further studies are wanted.

Biogeographic Provinces of Total and Methyl Mercury in Zooplankton and Fish from the Beaufort and Chukchi Seas: Results from the SHEBA Drift

Samples of copepods (Calanus hyperboreus) and arctic cod (Boreogadus saida) were collected along the SHEBA (Surface HEat Budget of the Arctic) drift track, which commenced in the Canada Basin (October 1997) and finished in the Mendeleev Basin (October 1998). Here, we report total mercury (HgT) and CH3Hg concentrations in these biological samples and examine concentration variability along the drift track in the context of trophic variation, inputs from land, spring mercury depletion events (MDEs), and oceanographic provinces. We find background concentrations of HgT in C. hyperboreus as low as 0.02 microg/g (dw), with the Canada Basin samples exhibiting approximately 2-fold higher mercury concentrations than those from the Chukchi Plateau and Mendeleev Abyssal Plain. This east-to-west trend in mercury concentration is punctuated by two and possibly three intervals of elevated mercury (HgT, 0.10-0.12 microg/g (dw); CH3Hg, 0.023-0.028 microg/g (dw)) along the drift track. One interval of elevated HgT and CH3-Hg levels occurred during and shortly after melt. %CH3Hg reached a maximum of 60% during this time period, three times higher than any other time during the drift. This transient rise in C. hyperboreus CH3Hg concentration seems to strongly point to mercury accumulated in snow during MDEs. However, the alignment of elevated mercury samples with oceanographic fronts and the observed regional differences between basins suggest that variation of mercury concentration is primarily a consequence of ocean structure. Given that large animals such as whales selectively forage in regions of higher food concentration such as fronts, recent change in the ice climate of the western Arctic Ocean, perhaps mediated by changes in heat storage, may provide the means to change their exposure to mercury thus explaining observed increases in mercury concentrations in western beluga whales during the 1990s.

Near-Inertial Wave Propagation in the Western Arctic

Abstract From October 1997 through October 1998, the Surface Heat Budget of the Arctic (SHEBA) ice camp drifted across the western Arctic Ocean, from the central Canada Basin over the Northwind Ridge and across the Chukchi Cap. During much of this period, the velocity and shear fields in the upper ocean were monitored by Doppler sonar. Near-inertial internal waves are found to be the dominant contributors to the superinertial motion field. Typical rms velocities are 1–2 cm s−1. In this work, the velocity and shear variances associated with upward- and downward-propagating wave groups are quantified. Patterns are detected in these variances that correlate with underlying seafloor depth. These are explored with the objective of assessing the role that these extremely low-energy near-inertial waves play in the larger-scale evolution of the Canada Basin. The specific focus is the energy flux delivered to the slopes and shelves of the basin, available for driving mixing at the ocean boundaries. The energy and shear variances associated with downward-propagating waves are relatively uniform over the entire SHEBA drift, independent of the season and depth of the underlying topography. Variances associated with upward-propagating waves follow a (depth)−1/2 dependence. Over the deep slopes, vertical wavenumber spectra of upward-propagating waves are blue-shifted relative to their downward counterparts, perhaps a result of reflection from a sloping seafloor. To aid in interpretation of the observations, a simple, linear model is used to compare the effects of viscous (volume) versus underice (surface) dissipation for near-inertial waves. The latter is found to be the dominant mechanism. A parallel examination of the topography of the western Arctic shows that much of the continental slope is close to critical for near-inertial wave reflection. The picture that emerges is consistent with “one bounce” rather than trans-Arctic propagation. The dominant surface-generated waves are substantially absorbed in the underice boundary layer following a single roundtrip to the seafloor. However, surface-generated waves can interact strongly with nearby (<300 km) slopes, potentially contributing to dissipation rates of order 10−6–10−7 W m−3 in a zone several hundred meters above the bottom. The waves that survive the reflection process (and are not back-reflected) display a measurable blue shift over the slopes and contribute to the observed dependence of energy on seafloor depth that is seen in these upper-ocean observations.

Multibeam bathymetric and sediment profiler evidence for ice grounding on the Chukchi Borderland, Arctic Ocean

Multibeam bathymetry and 3.5-kHz sub-bottom profiler data collected from the US icebreaker Healy in 2003 provide convincing evidence for grounded ice on the Chukchi Borderland off the northern Alaskan margin, Arctic Ocean. The data show parallel, glacially induced seafloor scours, or grooves, and intervening ridges that reach widths of 1000 m (rim to rim) and as much as 40 m relief. Following previous authors, we refer to these features as “megascale glacial lineations (MSGLs).” Additional support for ice grounding is apparent from stratigraphic unconformities, interpreted to have been caused by ice-induced erosion. Most likely, the observed sea-floor features represent evidence for massive ice-shelf grounding. The general ESE/WNW direction of the MSGLs, together with sediment, evidently bulldozed off the Chukchi Plateau, that is mapped on the western (Siberian) side of the plateau, suggests ice flow from the Canada Basin side of Chukchi Borderland. Two separate generations of glacially derived MSGLs are identified on the Chukchi Borderland from the Healy geophysical data. The deepest and oldest extensive MSGLs appear to be draped by sediments less than 5 m thick, whereas no sediment drape can be distinguished within the resolution of the sub-bottom profiles on the younger generation.

Atlantic sources of the Arctic Ocean surface and halocline waters

Data obtained during the last 20 years from selected hydrographic stations throughout the Arctic Ocean basins and at the continental slopes and northern parts of the surrounding shelf seas are examined to estimate the contribution of Atlantic water to the upper part of the Arctic Ocean water column, and to follow the circulation of the Atlantic derived halocline waters around the Arctic Ocean basins. A substantial fraction of the Atlantic water entering the Arctic Ocean in the two inflow branches, the Fram Strait branch and the Barents Sea branch, is transformed into less dense waters. The inflow through Fram Strait encounters and melts sea ice. Its upper part becomes less saline, and in winter, ice formation homogenizes this low salinity upper part into a winter mixed layer, which follows the boundary flow eastward. The inflow over the Barents Sea mainly becomes colder, less saline and denser due to cooling and net precipitation, but melting of sea ice also here creates a less dense upper layer, which is subsequently homogenized by haline convection. Both these components enter the Nansen Basin via the St. Anna Trough and flow eastward along the Siberian continental slope. A third component of the Barents Sea inflow, mainly comprising Norwegian Coastal Current water, remains on the shelf, where it absorbs the runoff from the large Siberian rivers and evolves into low salinity shelf water. This part continues as far east as the Laptev Sea before a major inflow to the deep Arctic Ocean basins occurs. Winter convection, which in most of the Nansen Basin extends to the Atlantic layer, then becomes limited to the injected low salinity shelf water, and the more saline upper layers of the Fram Strait and the Barents Sea branches are transformed into halocline waters. The Fram Strait branch supplies the halocline water of the Amundsen, Makarov and part of the Canada basins, while the upper part of the Barents Sea branch, initially confined to the Siberian continental slope, becomes the main source of the lower halocline below the Pacific water in the Canada Basin beyond the Chukchi Cap. Less dense Pacific water close to the North American continent prevents the Atlantic derived upper layers from fl owing through the Canadian Arctic Archipelago and their main exit is Fram Strait. Only some of the Barents Sea branch halocline water passes through Nares Strait and may occasionally renew the deep and bottom waters of Baffin Bay.

Atlantic sources of the Arctic Ocean surface and halocline waters

Data obtained during the last 20 years from selected hydrographic stations throughout the Arctic Ocean basins and at the continental slopes and northern parts of the surrounding shelf seas are examined to estimate the contribution of Atlantic water to the upper part of the Arctic Ocean water column, and to follow the circulation of the Atlantic derived halocline waters around the Arctic Ocean basins. A substantial fraction of the Atlantic water entering the Arctic Ocean in the two inflow branches, the Fram Strait branch and the Barents Sea branch, is transformed into less dense waters. The inflow through Fram Strait encounters and melts sea ice. Its upper part becomes less saline, and in winter, ice formation homogenizes this low salinity upper part into a winter mixed layer, which follows the boundary flow eastward. The inflow over the Barents Sea mainly becomes colder, less saline and denser due to cooling and net precipitation, but melting of sea ice also here creates a less dense upper layer, which is subsequently homogenized by haline convection. Both these components enter the Nansen Basin via the St. Anna Trough and flow eastward along the Siberian continental slope. A third component of the Barents Sea inflow, mainly comprising Norwegian Coastal Current water, remains on the shelf, where it absorbs the runoff from the large Siberian rivers and evolves into low salinity shelf water. This part continues as far east as the Laptev Sea before a major inflow to the deep Arctic Ocean basins occurs. Winter convection, which in most of the Nansen Basin extends to the Atlantic layer, then becomes limited to the injected low salinity shelf water, and the more saline upper layers of the Fram Strait and the Barents Sea branches are transformed into halocline waters. The Fram Strait branch supplies the halocline water of the Amundsen, Makarov and part of the Canada basins, while the upper part of the Barents Sea branch, initially confined to the Siberian continental slope, becomes the main source of the lower halocline below the Pacific water in the Canada Basin beyond the Chukchi Cap. Less dense Pacific water close to the North American continent prevents the Atlantic derived upper layers from fl owing through the Canadian Arctic Archipelago and their main exit is Fram Strait. Only some of the Barents Sea branch halocline water passes through Nares Strait and may occasionally renew the deep and bottom waters of Baffin Bay.

The joint roles of Pacific and Atlantic-origin waters in the Canada Basin, 1997–1998

Physical and geochemical data collected weekly during the year-long 2800 km drift of the CCGS des Groseilliers show that Canada Basin waters, and in particular the composition of the halocline, can no longer be viewed as laterally homogeneous and in steady state. The halocline was thinner over the Mendeleyev Abyssal Plain and northern Chukchi Plateau. Here, Pacific-origin upper and middle halocline waters occupied the upper 80 m of the water column and underlying Atlantic-origin lower halocline waters were fresher, colder and much more ventilated than observed in the past. These new observations of a sub-surface oxygen maximum suggest that outflow from the East Siberian Sea now supplies the Canada Basin lower halocline. East of the Northwind Ridge the halocline was thicker and appeared relatively unchanged. Here Pacific-origin upper and middle halocline waters occupied the top 225 m and Atlantic-origin lower halocline waters were identified by an oxygen minimum. The intensity of the Pacific-origin signal, characterized by a nutrient maximum, was strongest over the Chukchi Gap—the passage between the Chukchi Shelf and Plateau—and the Northwind Abyssal Plain and identified two winter-water spreading pathways. Atlantic-origin waters as much as 0.5°C warmer than the historical record were observed over the Chukchi Gap and also over the northern flank of the Chukchi Plateau. These observations signaled that warm-anomaly Fram Strait Branch (FSB) waters, first observed upstream in the Nansen Basin in 1990, had arrived downstream in the Canada Basin eight years later and also indicate two routes whereby FSB waters enter the southern Canada Basin. Although samples were collected throughout one annual cycle, seasonal effects were small and confined to the upper 50 m of the water column. These data show Canada Basin waters are in transition, responding to the effects of upstream change in atmospheric and oceanic circulation.

Annual cycle in abundance, distribution, and size in relation to hydrography of important copepod species in the western Arctic Ocean

A yearlong study of the zooplankton biomass and the abundance, vertical distribution, life stage proportions, and body size and condition for five target copepod species ( Calanus glacialis, C. hyperboreus, Metridia longa, Microcalanus pygmaeus, Oithona similis) was conducted from October 1997 to October 1998 in the Western Arctic Ocean. The research was staged from Ice Station SHEBA that drifted from Canadian Basin over the Northwind Ridge and Chukchi Plateau and back over the Basin during this period. Four hydrographic regimes were surveyed during the period of the study. Zooplankton biomass was least over the basin during the fall and winter and greatest over the Chukchi Plateau during summer, with most biomass in the 200–1500 m depth interval except during summer when greatest biomass was present in the upper 200 m. The five copepod species followed two general life history strategies: (1) sustained reproduction with all life stages present throughout the year and constant depth distribution ( M. longa, M. pygmaeus, O. similis) and (2) pulsed reproduction with overlapping cohorts present and ontogenetic redistribution of preferred depths through the year ( C. glacialis, C. hyperboreus). Body size and condition did not demonstrate consistent temporal or regional patterns. Based on population age structure, both C. hyperboreus and C. glacialis were reproducing in the Arctic Ocean. However, extremely low abundances of C. glacialis suggested that this species may not be self-sustaining in the Arctic Ocean. Plankton biomass was consistent with that observed in recent studies and supported an emerging paradigm of a more productive Arctic Ocean than traditionally believed.

Inherent optical properties of the central Arctic surface waters

Measurements of the inherent optical properties were collected during the summer of 1998 above the Chukchi Plateau. These measurements show that attenuation by particles is sometimes very low, but absorption by colored dissolved organic materials (CDOM) is generally high, so the waters of the study site should not be considered to be among the clearest natural waters. The CDOM increases the absorption of shortwave radiation between 350 and 700 nm by over 30% in the top 10 m when compared to the clearest natural waters. Optical properties representative of those measured during phytoplankton blooms show that heating of the top 10 m can increase substantially during blooms. The increased absorption near the sea surface provides more heat for melting the bottom of the ice than previous models have assumed.

Varieties of shallow temperature maximum waters in the Western Canadian Basin of the Arctic Ocean

The properties and spreading pathways of shallow temperature maximum waters (STMs) in the western Canadian Basin are investigated using CTD and mooring data obtained in 1997–98 as part of the SHEBA (Surface Heat Budget of the Arctic Ocean) drift experiment and available historical data. Three distinct varieties of STM are recognized on the basis of salinity range: (1) Surface Mixed Layer Water (SMLW) with S < 30 psu; (2) Eastern Chukchi Summer Water (ECSW) with 31 < S < 32 psu; and (3) Western Chukchi Summer Water (WCSW) with S > 32 psu. These STMs carry sufficient heat within the upper layers of the ocean to significantly affect the rates of ice cover and decay. For example, during the winter of 1997–98 anomalously warm STM (> 0°C) originating from ECSW was observed to spread northwards along the Northwind Ridge and Chukchi Plateau, where the maximum reduction of the ice covers was subsequently observed in late summer, 1998 [Maslanik et al., 1999]. Regional climate variability and ice cover in the western Canadian Basin are thus affected not only by anomalous atmospheric circulation patterns, but also by the circulation of upper ocean water masses.

New gravity data in the Arctic Ocean: Comparison of airborne and ERS gravity

New gravity fields from airborne gravimetry and from ERS‐1 and ‐2 satellite altimetry cover extensive portions of the Arctic Ocean. These two data sets may constitute as much as 60% of the data contributions to the Arctic Gravity Project compilation. Here we evaluate the accuracy and resolution of these data and quantify their impact on the compilation. Both gravity determinations compare favorably with Geological Survey of Canada surface measurements in the Beaufort Sea (airborne, 1.86–2.09 mGal rms; ERS, 2.64–3.11 mGal rms). Comparisons between the airborne and ERS data over the Chukchi Borderlands reveal a 4.38 mGal rms difference over the smoother region of the field and 7.36 mGal rms over the rugose field generated by the shallow ridges and deep troughs. Coherency between the two data sets in the Chukchi region implies a resolution of 19 km. Comparison with Science Ice Expedition submarine measurements over Chukchi Plateau suggests that the ERS field resolves even shorter‐wavelength signal than the airborne data, whereas in the Beaufort Sea the airborne data showed better coherence to ground truth data. Long‐wavelength differences exist between the two data sets, expressed as a 2–3 mGal offset over the Chukchi region. This study highlights the respective strengths of the two data sets. The ERS gravity field has the advantage of ubiquitous coverage of the ocean south of 81.5°N, a denser sampling of the gravity field, and a recovery of signal down to ∼15 km. The airborne data cover a significant portion of the polar hole in the satellite coverage, have lower measurement noise, and recover somewhat higher anomaly amplitudes in the 25–100 km wavelength range.

Diversity and distribution of DNA sequences with affinity to ammonia-oxidizing bacteria of the beta subdivision of the class Proteobacteria in the Arctic Ocean.

The spatial distribution and diversity of ammonia-oxidizing bacteria of the beta subdivision of the class Proteobacteria (hereinafter referred to as ammonia oxidizers) in the Arctic Ocean were determined. The presence of ammonia oxidizers was detected by PCR amplification of 16S rRNA genes using a primer set specific for this group of organisms (nitA and nitB, which amplifies a 1.1-kb fragment between positions 137 and 1234, corresponding to Escherichia coli 16S rDNA numbering). We analyzed 246 samples collected from the upper water column (5 to 235 m) during March and April 1995, September and October 1996, and September 1997. Ammonia oxidizers were detected in 25% of the samples from 5 m, 80% of the samples from 55 m, 88% of the samples from 133 m, and 50% of the samples from 235 m. Analysis of nitA-nitB PCR product by nested PCR-denaturing gradient gel electrophoresis (DGGE) showed that all positive samples contained the same major band (band A), indicating the presence of a dominant, ubiquitous ammonia oxidizer in the Arctic Ocean basin. Twenty-two percent of the samples contained additional major bands. These samples were restricted to the Chukchi Sea shelf break, the Chukchi cap, and the Canada basin; areas likely influenced by Pacific inflow. The nucleotide sequence of the 1.1-kb nitA-nitB PCR product from a sample that contained only band A grouped with sequences designated group 1 marine Nitrosospira-like sequences. PCR-DGGE analysis of 122 clones from four libraries revealed that 67 to 71% of the inserts contained sequences with the same mobility as band A. Nucleotide sequences (1.1 kb) of another distinct group of clones, found only in 1995 samples (25%), fell into the group 5 marine Nitrosomonas-like sequences. Our results suggest that the Arctic Ocean beta-proteobacterial ammonia oxidizers have low diversity and are dominated by marine Nitrosospira-like organisms. Diversity appears to be higher in Western Arctic Ocean regions influenced by inflow from the Pacific Ocean through the Bering and Chukchi seas.

Circulation features in the central Arctic Ocean revealed by nuclear fuel reprocessing tracers from Scientific Ice Expeditions 1995 and 1996

Measurements of the tracer radionuclides 129I and 137Cs were conducted on seawater samples collected during the Scientific Ice Expedition cruises to the Arctic Ocean of the U.S. Navy nuclear submarines, USS Cavalla and USS Pogy in 1995 and 1996, respectively. These radionuclides are derived mainly from discharges from the Sellafield (England, United Kingdom) and La Hague (France) nuclear fuel reprocessing plants and are subsequently transported with Atlantic water into the Arctic Ocean through Fram Strait and the Barents Sea. Iodine 129 results from halocline waters (water depths of 59 and 134 m) collected virtually synoptically throughout the central Arctic Ocean during the USS Cavalla cruise clearly show the front between Atlantic origin water having high 129I levels (>100×107 atoms L−1) and Pacific origin water labeled mainly by fallout (<5×107 atoms L−1). This front is aligned with the Mendeleyev Ridge and is displaced toward the Canada Basin with increasing water depth. Iodine 129 levels decrease on isohaline surfaces from stations over the continental slope and Mendeleyev Ridge to stations in the interior of the Makarov Basin. These results are consistent with circulation mechanisms in which the interior of the Makarov Basin is ventilated by lateral mixing from topographically steered boundary currents. Low 129I levels (<2×107 atoms L−1) measured in Atlantic and intermediate water in the northern Canada Basin and over the Alpha Ridge during the USS Pogy cruise indicate that ventilation rates are extremely low in this region. The much higher 129I levels (>30×107 atoms L−1) measured in central regions of the Canada Basin indicate that the interior is more efficiently ventilated than previously thought, possibly by separation from boundary currents flowing over the continental margin north of the Chukchi Plateau. The 129I and 137Cs data were interpreted using a transit time model that provided estimates of 6–7 years (±0.5 years) for the passage of halocline and Atlantic Water from the Norwegian Coastal Current (60°N) to the continental slope of the Makarov Basin and a lower limit of 8 years for transport to interior regions of the Makarov and Amundsen Basins.

Upper ocean velocities in the Beaufort Gyre

A 23 month time series of upper ocean velocities in the Beaufort Gyre was obtained from an autonomous drifting buoy equipped with an Acoustic Doppler Current Profiler. The buoy was deployed in the Beaufort Sea and drifted over the Canada Basin, Chukchi Shelf, and Chukchi Plateau. Time‐evolving spectral analysis was used to examine velocity variability in three frequency bands: an eddy band (2–5 day periods), a diurnal band, and a semidiurnal band. The eddy band and the semidiurnal band dominated the horizontal kinetic energy. Although eddy energy was clearly confined to the Beaufort Sea and Canada basin, eddies were found further to the south and west than in previous observations. The semidiurnal band was dominated by near‐inertial motions with amplitude that varied seasonally (largest in late summer and smallest in late winter). Diurnal tides were weak (<1 cm s−1) except over a shallow portion of the Chukchi Shelf.

Nutrients, organic carbon and organic nitrogen in the upper water column of the Arctic Ocean: implications for the sources of dissolved organic carbon

Particulate and dissolved organic carbon and nitrogen were analyzed for shelf, slope and basin samples collected during the 1994 Arctic Ocean Section. Concentrations of organic carbon and nitrogen were highest in the surface water and decreased dramatically below 100 m. Over the shelf 40–50% of the organic material was present in the particulate phase, while in the slope and basin waters >-90% of the organic material was present in the dissolved phase. The halocline layer over the Chukchi plateau contained high concentrations of inorganic nutrients but no significant elevation of either organic carbon or organic nitrogen. Highest concentrations of total organic carbon were found in the surface water of the Makarov and Amundsen Basins between 80 and 90° N. The C:N ratio of the dissolved organic material in the upper 100 m ranged from 9 to 25, with highest ratios along the Makarov flank of the Lomonosov Ridge. We suggest that a major portion of the carbon-rich organic material in surface waters is derived from Eurasian shelf riverine inputs, which tend to flow in perimeter currents along the slope and ridges. Our preliminary dissolved organic carbon (DOC) budget suggests that the three major sources of DOC in the central Arctic are in situ production (56%), river run-off (25%), and Pacific water (19%).

Origin of ice‐rafted debris: Pleistocene paleoceanography in the western Arctic Ocean

The composition of Pleistocene ice‐rafted debris (IRD) >250 µm was analyzed quantitatively by grain counting in five sediment cores from the western central Arctic Ocean and compared with the composition of till clasts from NW Canada in order to determine the dropstone origin and to reconstruct the Pleistocene ice driftways and surface currents. The IRD composition alternates repeatedly between carbonate‐ and quartz‐dominated assemblages, along with metamorphic and igneous rocks, clastic rocks, and some chert. The highest quartz content is found on the Alpha Ridge, while carbonate percentages are highest on the Northwind Ridge (NWR) and the Chukchi Cap. The source for the carbonates is the area around Banks and Victoria Islands and parts of northern Canada. Quartz most likely originated from the central Queen Elizabeth Islands. IRD on the southeastern Alpha Ridge is dominated by mafic crystalline rocks from northern Ellesmere Island and northern Greenland. At least six major glacial intervals are identified within the last 1 million years, during which icebergs drifted toward the west in the Beaufort Sea, straight northward in the central Arctic Ocean, and northeastward on the SE Alpha Ridge.

Biochronology and geochronology of Late Neogene Central Arctic deep-sea cores

Microfaunal and foraminifera1 oxygen isotope data from the Chukchi Plateau, the Alpha-Mendeleev Rise and the Lomonosov Ridge deep-sea cores (Table 1) representing the last 4.5-5 million years, yield evidence of significant paleoceanographic and paleoclimatic changes. Uranium series isotopes and magnetic stratigraphy provide the time frame for these Late Neogene sequences (Herman & Osmond 1984). The similarity in faunal and floral patterns as well as in lithology allows cor- .relation of cores over widely separated regions. Several datum levels were recognized. They include the Globigerina egelida acme zone between -1.8 and -1.6Ma, the Globigerina quinqueloba acme zone 0.7 Ma and -0.34-0.24 Ma, the Stetsonia horuarhi acme zone between -1.66 and - 1 Ma and the Orthopithonella acme zone between -1.66 and -0.73 Ma (Fig. 1).