Sources and degradation of organic matter in the surface sediments of the Chukchi Sea:insights from amino acids

In the context of global warming and rapid environment change in the Arctic,the supply of organic matter(OM)has increased significantly and a large amount of OM are buried on the Arctic shelf.Studying the fate of OM in Arctic shelf sediments is crucial to understanding the global carbon sink.As a marginal sea of the Arctic Ocean,the Chukchi Sea is one of the most critical areas where OM is buried.Based on the surface sediment samples collected during the sixth Chinese National Arctic Research Expedition in the summer of 2014 and the SinoRussian joint Arctic Research Expedition in the summer of 2016,this study takes amino acids(AAs)as the primary tool to explore the source and degradation of OM in the surface sediments of the Chukchi Sea.This study shows that total hydrolyzable amino acid(THAA)concentrations(dry weight)are high,with a mean value of(32.7±15.8)μmol/g.Their spatial distribution is related to primary productivity,hydrodynamic conditions,sediment properties and other factors.The source of OM in the surface sediments of the Chukchi Sea is dominated by diatom-dominated marine productivity,with some input from terrestrial sources.Bacteria,as the main source of the D-enantiomer of AA(D-AA),not only have transforming effect on OM,but their cell walls and remnants likewise supply the OM pool.Based on a series of diagenetic indicators,we conclude that the OM in the surface sediments of the Chukchi Sea has undergone extensive degradation[DI(degradation index)=-0.59±0.44],and the degradation degree in the slope is higher than that in the shelf.This study uses AA to explore the sources and degradation of OM in the sediments of the Chukchi Sea,which facilitates our understanding of OM transport and transformation on the Arctic shelf.

Seabed Physical Parameter Research Based on Active-Source OBS Data in the Chukchi Sea Shelf of the Arctic

The acquisition of seabed physical parameters is one of the focuses of marine acoustic researches.However,the activesource ocean bottom seismometer(OBS)detection method in the marine geophysical research is rarely used to acquire seabed physical parameters,and less work is performed in the Arctic.In this study,two active-source OBS data collected from the 9th and 11th Chinese National Arctic Research Expedition(CHINARE)are selected to obtain the physical parameters of seabed sediments.Two kinds of energy spark are used as the active sources,while the cost function inversion method is used based on the arrival time difference between the reflected and direct waves.The thickness and sound velocity of the sediment layers are obtained by inversion,and the empirical formula is used to calculate the physical parameters of the seabed sediment,which are compared with the measured results.The cost function inversion method based on the time difference of arrival of the reflected and direct waves is tested to be effective and feasible in the inversion of seabed parameters from active-source OBS data.The method is further applied to obtain the physical parameters of Chukchi seabed sediments,which provides the idea and reference for the application of marine geophysical activesource OBS detection technology in the inversion of polar seabed physical parameters.

Mesozooplankton distribution and diversity from the Bering Sea shelf to the Chukchi Sea

In recent decades,environmental changes in the Arctic have aroused widespread concern around the world.To better understand ecology issues such as ecosystem dynamics,the Arctic and the subarctic regions were integrated as the“pan-Arctic”region.In this study,mesozooplankton were sampled from the Bering Sea shelf to the northern Chukchi Sea during the 10th Chinese National Arctic Research Expedition in 2019.Based on the species composition and abundance,three geographical communities were identified:the Bering Sea shelf community(BSS),the Bering Strait transitional community(BST),and the Chukchi Sea shelf community(CSS).The BSS was characterized by Bering Sea oceanic species such as Eucalanus bungii;the BST was mainly composed of the pan-Arctic distributed Calanus glacialis,meroplankton of benthos,and neritic species such as Centropages abdominalis;copepods,especially the copepodite of C.glacialis,were predominant in the CSS community.The BSS community structure was strongly affected by the inflow of Bering Shelf Water,while those of BST and CSS were determined by the recruitment of local species.The zooplankton community structure is influenced by both advection and environmental changes such as warming and a prolonged productivity period.Here,it was difficult to distinguish the changes induced by climate change from the effects of the Bering Sea Water.The key to solving this problem is the accumulation of comparable data,which requires continuous monitoring of key species such as C.glacialis and Calanus hyperboreus.

The fundamental characteristics of current in the Bering Strait and the Chukchi Sea from July to September 2003

The characteristics of current in the Bering Strait and the Chukchi Sea areanalyzed based on the two current data on the mooring stations during the Second National ArcticResearch Expedition of China in 2003. The tidal currents of the principal diurnal and semidiurnalellipses rotate clockwise in the upper layer, except for N_2, S_2, and Q_1 at Sta. ST. In the BeringStrait (Sta. ST), the major semi-axis of tidal current constituent M_2 is 2.9 cm/s in the upperlayer, which is much smaller than that of semi-monthly oscillation (11.8 cm/s); and the mean currentflows northwestward at the amplitude of about 20 cm/s and varies a little with depth. During thecruise, the current has significant semi-monthly oscillation at the two mooring stations. Thespectra analyses of the air pressure gradient and the wind stress show that there are thesemi-monthly oscillations in these two data series. The near-inertial current, approximately 4 cm/s,presents almost the same magnitude of the principal tidal currents in the Bering Strait.

Variations in organic carbon loading of surface sediments from the shelf to the slope of the Chukchi Sea,Arctic Ocean

The content of organic carbon （OC） normalized to the specific surface area （SSA） of sediment is widely used to trace variations in OC loading （OC/SSA）. This study presents observations of OC/SSA of surface sediments collected in the Chukchi Sea, a typical Arctic marginal sea. Shelf sediments exhibit much higher OC/SSA values than slope sediments in the study area. Compared with OC/SSA values reported from the East Siberian Shelf and Mackenzie River, the slope sediments possess lower OC loading. This abrupt decrease in OC/SSA is mostly related to the lower primary production on slope as well as possible oxidization processes. The results of linear regression analysis between OC and SSA indicate a sedimentary source rock for the OC in the Chukchi Sea sediments. Moreover, shelf sediments with low SSA possess a larger rock OC fraction than slope sediments do. The dataset of the present study enables a more thorough understanding of regional OC cycling in the Chukchi Sea.

Application of long-chain alkenones and U_(37)~k values for paleotemperature estimation in the Arctic Chukchi Sea-Bering Sea area

Long-chain alkenones were detected in samples of sea surface sediments from the Chukchi Sea and the Bering Sea areas, the Arctic Pole. The analysis result indicates that C37:3 methylketone is predominate in the long-chain alkenones from the Chukchi and Bering Sea sediments. The abundance of C37 to C39 unsaturated alkenones changes in an order of C37 > C38 > C39. Based on C37 /C38 ratio, the detected organism precursors of the long-chain alkenones are mainly coccolithophrid (Emiliania huxleyi). By the calibration relationship between U37k and U37k indices, the sea surface paleotemperature in these seas is estimated. The estimated values of U37k vary from 4.147℃ to 5.706℃ , with a mean value of 5.092℃.

Sound velocity and related properties of seafloor sediments in the Bering Sea and Chukchi Sea

The Bering Sea shelf and Chukchi Sea shelf are believed to hold enormous oil and gas reserves which have attracted a lot of geophysical surveys. For the interpretation of acoustic geophysical survey results, sediment sound velocity is one of the main parameters. On seven sediment cores collected from the Bering Sea and Chukchi Sea during the 5th Chinese National Arctic Research Expedition, sound velocity measurements were made at 35, 50, 100, 135, 150, 174, 200, and 250 kHz using eight separate pairs of ultrasonic transducers. The measured sound velocities range from 1 425.1 m/s to 1 606.4 m/s and are dispersive with the degrees of dispersion from 2.2% to 4.0% over a frequency range of 35-250 kHz. After the sound velocity measurements, the measurements of selected geotechnical properties and the Scanning Electron Microscopic observation of microstructure were also made on the sediment cores. The results show that the seafioor sediments are composed of silty sand, sandy silt, coarse silt, clayey silt, sand-silt-clay and silty clay. Aggregate and diatom debris is found in the seafloor sediments. Through comparative analysis of microphotographs and geotechnical properties, it is assumed that the large pore spaces between aggregates and the intraparticulate porosity of diatom debris increase the porosity of the seafioor sediments, and affect other geotechnical properties. The correlation analysis of sound velocity and geotechnical properties shows that the correlation of sound velocity with porosity and wet bulk density is extreme significant, while the correlation of sound velocity with clay content, mean grain size and organic content is not significant. The regression equations between porosity, wet bulk density and sound velocity based on best-fit polynomial are given.

Biodiversity and community structural characteristics of macrobenthos in the Chukchi Sea

Field sampling of the macrobenthos from 23 stations in the Chukchi Sea was conducted during the 4th CHI- NARE （Chinese National Arctic Research Expeditions, July-August, 2010）. We identified a total of 140 species of macrozoobenthos belonging to nine phyla, which were dominated by polychaetes （66）, crustaceans （30）, and mollusks （25）, followed by echinoderms （9） and others （ten others, including four cnidarians, one oligochaete, one sipuncula, one priapulida, two bryozoans, and one urochordata）. The dominant species were Aphelochaeta pacifica, Heteromastus filiformis, Nephtys ciliata, Nephtys caeca, Scoletoma fragilis, Golfingia margaritacea, Nuculana pernula, Macoma calcarea, Ennucula tenuis, Macoma inquinata, Musculus discors, Echinarachnius parma, and Ophiura sarsii, so there were more cold-eurythermal boreal immigrants than trulyArctic species （endemics）. The average density and biomass （mean ± SD across all stations） of the total macrozoobenthos were （916 ± 907） ind/m2 and （902.9 ± 1227.7） g/m2 （wet weight）, respectively. Relatively high density and biomass were observed in the samples from the northeastern and southern Chukchi Sea. The spatial variation of benthic communities in the study sea area was relatively large; this spatial heteroge- neity has led to high diversity and a patchy distribution pattern in the community structure. Compared to the 1st CHINARE （July-August, 1999）, this investigation revealed different degrees of decreases in the average taxa numbers and the average density, abundance, and biodiversity in the area over the recent decade, which might be associated with global warming, human activities, and sea ice variations.

Distribution of molluscan remains in the sediment of the Chukchi Sea and its vicinity, the Arctic

The result of an analysis of mollusca remains collected from the Chukchi Sea, Beaufort Sea and Bering Sea in the First Chinese National Arctic Research Expedition, from July to September, 1999 is presented. Seventeen species of mollusca have been identified, which belong to two classes: Bivalvia and Gastropoda. The compositions of the mollusca are very simple. According to the distribution pattern two groups may be distinguished among molluscan species. The Pan-Arctic and circumboreal group comprises Nuculana pernula, N.radiata, Nucula bellotii, Astarte montagui, Seripes groenlandicus, Macoma calcarea, M. moesta alaskana, Liocyrna fluctuosa, Mya pseudoarenaria and Turritella polaris. Three species, Cyclocardia crebricostata, Trichotrois coronata and Argobuccinum oregonense are components of the Pan-Arctic and Pacific boreal group. With regard to feeding habits, detritus feeders dominate. There are 7 species of detritus feeders, i.e., Nuculana pernula, N. radiata, Nucula bellotii, Macoma calcarea, M. moesta alaskana, Macoma sp. and Trichotropis coronata. Detritus feeders are dominant with regard to the numbers of species as well as to the frequency of occurrence. Macoma calcarea is the most abundant species.

Observed features of temperature, salinity and current in central Chukchi Sea during the summer of 2012

During the summer of 2012, the fifth CHINARE Arctic Expedition was carried out, and a submersible mooring system was deployed in M5 station located at （69°30.155＇N,169°00.654＇W） and recovered 50d later. A set of temperature, salinity and current profile records was acquired. The characteristics of these observations are analyzed in this paper. Some main results are achieved as below. （1） Temperature generally decreases while salinity generally increases with increasing depth. The average values of all records are 2.98℃ and 32.21 psu. （2） Salinity and temperature are well negatively correlated, and the correlation coefficient between them is -0.84. However, they did not always vary synchronously. Their co-variation featured different characters during different significant periods. （3） The average velocity for the whole water column is 141 mm/s with directional angle of 347.1°. The statistical distribution curve of velocity record number gets narrower with increasing depth. More than 85% of the recorded velocities are northward, and the mean magnitudes of dominated northward velocities are 100-150 mm/s. （4） Rotary spectrum analysis shows that motions with low frequency take a majority of energy in all layers. The most significant energy peaks for all layers are around 0.012 cph （about 3.5 d period）, while the tidal motion in mooring area is nonsignificant. （5） Velocities in all layers feature similar and synchronous temporal variations, except for the slight decrease in magnitude and leftward twist from top to bottom. The directions of velocity correspond well to those of Surface wind. The average northward volume transport per square meter is 0.1-0.2 m3/s under southerly wind, but about -0.2 m3/s during northerly wind burst.

Abundance and distribution of meiofauna in the Chukchi Sea

The metazoan meiofauna in the Chukchi Sea were collected from seven shallow water stations （depths rang- ing 46 to 52 m） and five deep sea stations （depths ranging between 393 and 2 300 m） during the 4th Chinese National Arctic Research Expedition in 2010. The results showed that abundance of meiofauna was higher in shallow water sediments （average of 2445 ind./（10 cm2）） than in deep sea sediments （407.06 ind./（10 cm2））. A UNIANOVA test for difference between the two different regions was highly significant （F=10h 15, p〈0.Ol）. Nematodes were numerically dominant, representing （96.6±4.6）% of the total meiofaunal abundance at the shallow water stations and （98.90±1.42）% at deep sea stations. The number of higher taxonomic groups and abundance of meiofauna were higher at Stas CC1, CC4, and R06 near the Bering Strait and the continent, than at the rest of the shallow water and deep sea stations. The primary factors causing the differences were concentrations of nutrients P and Si of bottom seawater （R=0.831, p〈0.003）, followed by depth （R=-0.655, p〈0.05） and sand fractions of sediments （R=0.632, p 〈0.05）. The numbers of meiofauna on the 65 lam and 32 llm sieves were significantly higher than those on the rest of the screens. Differences in numbers of meiofauna retained on screens with different mesh openings were highly significant among all sampling stations （F=31.60, p〈0.01）. The highest numbers of individuals on screens with 32 μm mesh openings were found at deep sea stations. The number of meiofauna in the top 6-1, 1-2, and 2-4 cm segments constituted 84.4% of the total and was significantly higher than those in the bottom 4-6 and 6-10 cm segments （F=15, p〈0.01）.

Ocean current observation and spectrum analysis in central Chukchi Sea during the summer of 2008

During the summer of 2008, the third CHINARE Arctic Expedition was carried out on board of Xuelong Icebreaker in the central Chukchi Sea. A submersible mooring system was deployed and recovered at Station CN-01 （71.40.024＇N, 167.58.910＇W） with 33 days of the current profile records, and continuous observation of temperature and salinity data were collected. This mooring station locates in the blank of similar observation area and it is the first time for our Chinese to finish this kind of long-term mooring work in this area. This mooring system finished integrated hydrological observations with long-term continuous record of the whole profile velocity for the first time. Based on time series analysis of temperature, salinity, velocity and flow direction, we get the following main results. （1） During the observation period, the mean surface current velocity is 70.2 cm/s eastward, and velocity reaches its maximum in average at 3 m level with magnitude 90.0 cm/s, direction 206.. （2） In 9-30 m layers, the semidiurnal period variation is the most obvious, the flow direction is quite stable, and the flow is synchronous and consistent vertically. （3） Besides the semidiurnal period variation, the main variation in the upper layer is in 11-d period, with variations in period 5.5, 5.5, and 3.7 d, which reflect the influences of sea surface wind change and maintenance. （4） Near the bottom the temperature change is correlated and synchronized with the conductivity.

The distribution and characteristics of suspended particulate matter in the Chukchi Sea

Samples taken from the Chukchi Sea （CS） during the 4th Chinese National Arctic Research Expedition, 2010, were analyzed to determine the content and composition of suspended particulate matter （SPM） to improve our understanding of the distribution, sources and control factors of the SPM there. The results show that the SPM in the water column is highest in the middle and near the bottom in the south and central-north CS, followed by that off the Alaskan coast and in Barrow Canyon. The SPM content is lowest in the central CS. Scanning electron microscope （SEM） analysis shows that the SPM in the south and central-north CS is composed mainly of diatoms, but the dominant species in those two areas are different. The SPM off the Alaskan coast and in Barrow Canyon is composed mainly of terrigenous material with few bio-skeletal clasts. The distribution of temperature and salinity and the correlation between diatom species in SPM indicate that the diatom dominant SPM in the south CS is from the Pacific Ocean via the Bering Strait in summer. The diatom dominant SPM in the central-north CS is also from Pacific water, which reaches the CS in winter. The SPM in the middle and near the bottom of the water column off the Alaskan coast and in Barrow Canyon is from Alaskan coastal water and terrigenous material transported by rivers in Alaska.

Diatom distribution of surface sediment in the Bering Sea and Chukchi Sea

Diatoms from surface sediment samples in the Bering and Chukchi seas were analyzed to reveal the distribution patterns and their relationship with the ocean environment. A low abundance was found to the north of the Arctic Sea ice Minimum （ASIMin）, indicating that diatom growth is strongly inhibited by perennial sea ice. Between the ASIMin and the Arctic Sea Ice Maximum （ASIMax） which experiences seasonal sea ice, the sea ice related diatoms （mainly Fragilariopsis oceanica and Fragi- lariopsis cylindrus） were dominant, thereby confirming that sea ice is an important influencing factor. The Chaetoceros resting spores were more abundant in the Chukchi Sea, which corresponds well with the active phytoplankton distribution in the water column, and is possibly attributed to the stronger hydrodynamic conditions present in the Bering Sea. The abundances of Chaeto- ceros resting spores were the lowest on the northeast Bering Shelf, possibly because of lower water depth, stronger coastal cur- rents, river influx, coarser particle sizes and stronger winds and bottom currents. The Arctic Diatom Group （dominated by Bac- terosira bathyomphala, Thalassiosira antarctica v. borealis and Thalassiosira antarctica resting spores） was more abundant in the Bering Basin and the areas central of and to the north of Chukchi Plateau, while the Coastal Benthic Diatoms （including Paralia sulcata and Delphineis surirella） were mainly found on the northeast Bering Shelf and nearby Cape Lisburne in the Chukchi Sea. Thalassiosira nordenskioeldii was found to be the most abundant around the Bering Strait, while Neodenticula seminae was only found in the Bering Sea and mainly distributed in the Bering Basin, indicating its close correlation with the Pacific waters

Oxygen isotopic composition and its application to the study of tracing oceanographical process in Bering Sea and Chukchi Sea

In this paper, the 18 O distribution of surface water from the central sea areas of the Bering Sea and the Chukchi Sea was studied. The δ 18 O value of surface water from the Bering Sea is averagely -0.5‰; the δ 18 O contents of the Chukchi Sea are distributionally lower in northeast and higher in southwest; the δ 18 O value at the margin of Canadian Basin is -2.8‰, and averagely -0.8‰ in the southern area of the Chukchi Sea. The δ 18 O vertical distribution in some deep water stations from the Chukchi Sea and the Bering Sea is also studied. In the southern margin of Canadian Basin, the δ 18 O value is -2‰ -3‰ for surface layer and rises to 0 at 100 m depth layer. In the Bering Sea, the δ 18 O is about -0.5‰ for surface layer and increases to 0 at the depth of 300 m. The NO tracer can reflect obviously three water masses vertically distributed in the central Bering Sea: the upper Bering water mass, the middle Bering water mass and the deep Pacific water mass. The distributive ranges of NO and temperature for the various water masses are T<7℃, NO>780 μmol/dm 3 and T≥7℃, NO>650 μmol/dm 3 for upper Bering water mass, T<4℃, 550<NO<780 μmol/dm 3 for middle Bering water mass, and T<4℃, 330<NO<550 μmol/dm 3 for deep Pacific water mass. It is found from δ 18 O-S relation diagram and δ 18 O vertical profiles that the δ 18 O is about +0.3‰ from halocline layer till sea bottom. Its isotopic characteristics are the same as the Atlantic water, showing that the sea water comes from the north Atlantic. The freshwater end member of the Chukchi Sea in the survey period is also explored.

Distribution of general aerobic heterotrophic bacteria in sediment core taken from the Canadian basin and the Chukchi Sea

The occurrence percentage and abundance of General Aerobic Hetero- trophic Bacteria （GAB） were determined by using the method of MPN for 182 sub- samples from 10 sediment cores taken from the Canadian basin and the Chukchi Sea at two different culturing temperatures. The results showed that the general occurrence percentage of GAB was quite high, average abundanees of GAB at cultured temperatares of 4℃ and 25℃ were 4.46 ×10^7 and 5.47×10^7 cells·g^-1（wt）, respectively. The highest abundance of GAB occurred at 20 -22 cm section in the sediment. GAB abundances changed among the section of sediments, but there is a trend ： the a-bundances at the middle or lower sections were lower than those at upper section. Cultivation at 25℃ could improve the occurrence percentage and abundances of GAB, which suggests that the increasing of temperature may change the living circum-stances of GAB. The differences of GAB among the latitudes areas indicated that occurrence percentage and abundances of GAB in middle latitude areas were higher than those in the higher or lower latitude areas, and were more obvious at 4℃ than those at 25℃. The GAB abundances in sediment under the shallower water seemed to be low- er than those in sediments under the deeper water and this status was more obvious at 25℃ than that at 4℃.

Outflow of Pacific water from the Chukchi Sea to the Arctic Ocean

Pacific water exits the Chukchi Sea shelf through Barrow Canyon in the east and Herald Canyon in the west, forming an eastward-directed shelfbreak boundary current that flows into the Beaufort Sea. Here we summarize the transformation that the Pacific water undergoes in the two canyons, and describe the characteristics and variability of the resulting shelfbreak jet, using recently collected summertime hydrographic data and a year-long mooting data set. In both canyons the northward-flowing Pacific winter water switches from the western to the eastern flank of the canyon, interacting with the northward-flowing summer water. In Barrow canyon the vorticity structure of the current is altered, while in Herald canyon a new water mass mode is created. In both instances hydraulic effects are believed to be partly responsible for the observed changes. The shelfl）reak jet that forms from the canyon outflows has distinct seasonal configurations, from a bottom-intensified flow carrying cold, dense Pacific water in spring, to a surface-intensified current advecting warm, buoyant water in summer. The current also varies significantly on short timescales, from less than a day to a week. In fall and winter much of this mesoscale variability is driven by storm events, whose easterly winds reverse the current and cause upwelling. Different types of eddies are spawned from the current, which are characterized here using hydrographic and satellite data.

Foraminifera in surface sediments of the Bering and Chukchi Seas and their sedimentary environment

Based on a quantitative analysis of foraminifera in 39 surface samples of the Bering andChukchi Seas, the nearly absence of planktonic foraminifera in the surface sediments can be related to the low surface primary productivity and strong carbonate dissolution in the study area. It has been revealed that the surface primary productivity, and carbonate dissolution and properties of water masses related to the water depth mainly control the distribution of benthic foraminifera. The shelf of the Chukchi Sea is dominated by the Elphidium spp. assemblage and Nonionella robusta assemblage with low foraminiferal abundance and diversity, which is controlled by the coastal water mass of the Arctic Ocean. The slope of the Bering Sea is dominated by the Uvigerina peregrina - Globobulimina affinis assemblage with abundant N. robusta, and relatively high foraminiferal abundance and diversity, which is controlled by the intermediate and deep water masses of the Pacific Ocean. However, the Bering Sea has relatively shallow carbonate lysocline and compensation depth (CCD) , at about 2 000 and 3 800 m, respectively. In addition, there exists Stetsonia arctica in the surface sediments of the upper slope in the Bering Sea, which is a typical deep-sea benthic foraminiferal species of the slope in the Arctic Ocean. This indicates that the deep water of the two seas beside the Bering Strait had ever exchanged.

Zooplankton diel vertical migration and influence of upwelling on the biomass in the Chukchi Sea during summer

The diel vertical migration （DVM） of zooplankton and the influence of upwelling on zooplankton biomass were examined using water column data of current velocity and mean volume backscattering strength （MVBS） collected by moored acoustic Doppler current profilers （ADCPs） deployed in the southeastern Chukchi Sea during the 5th Chinese National Arctic Research Expedition （CHINARE） in summer 2012, combined with the satellite observational data such as sea surface temperature （SST）, wind, and chlorophyll a （Chl a）. Hourly acoustic data were continuously collected for 49-d in the mooring site. Spectral analysis indicated that there were different migrating patterns of zooplankton, even though precisely classifying the zooplankton taxa was not available. The prevailing 24-h cycle corresponded to the normal DVM with zooplankton swimming upwards at sunrise and returning to deep waters at sunset. There was a clear DVM in the upper 17 m of the water column during the period with distinct day-night cycles, and no active DVM throughout the water column when the sun above the horizon （polar day）, suggesting that light intensity was the trigger for DVM. Also there was a second migrating pattern with 12-h cycle. The upwelling event occurring in the northwest of Alaskan coastal area had important influence on zooplankton biomass at the mooring site. During the upwelling, the SST close to the mooring site dropped significantly from maximal 6.35℃to minimal 1.31℃ within five days. Simultaneously, there was a rapid increase in the MVBS and Chl a level, suggesting the aggregation of zooplankton related to upwelling.

Low ^(210)Pb in the upper thermocline in the Canadian Basin: scavenge process over the Chukchi Sea

210pb was measured during the 3rd Chinese National Arctic Research Expedition cruise to investigate its spa- tial pattern in the western Arctic Ocean, as well as its relation with the thermocline in the Canadian Basin. The specific activities varied from 0.04 to 2.72, 〈0.013 to 4.37, and 0.1 to 4.85 Bq/m3 for dissolved, particu- late, and bulk 210pb, respectively, corresponding to respective averages of 0.65, 0.43, and 1.08 Bq/m3. In the Canadian Basin, the minimum 210pb activities occurred in the thermocline, which was characterized by low temperature of-1.52℃ and salinity of 33.1. Combining the spatial distribution of 210pb and hydrographical characteristics in the western Arctic Ocean, this scenario was ascribed to the effective scavenging of 210pb when the Pacific water flowed across the Chukchi Shelf. Quantitatively, this interpretation was supported by both the shorter residence times and higher scavenging efficiencies （SE） of dissolved 210pb over the Chukchi Shelf. The highest SE values were observed in the Herald Shoal and bottom waters over the slope.