Hanna Shoal Research Articles

Abundance, biomass and caloric content of Chukchi Sea bivalves and association with Pacific walrus (Odobenus rosmarus divergens) relative density and distribution in the northeastern Chukchi Sea

The northeastern Chukchi Sea is a shallow subarctic shelf ecosystem that supports a substantial benthic infaunal community of which bivalves are a major component. We assessed the patterns in population abundance, biomass, and caloric content of ten dominant bivalve taxa in relation to the distribution of the upper trophic level consumer Pacific walrus (Odobenus rosmarus divergens). Bivalves were collected over four cruises in the northeastern Chukchi Sea (2009, 2010, 2012, 2013). Our samples were largely dominated by calorie-dense, deposit-feeding species, including Macoma spp., Ennucula tenuis, Nuculana spp. and Yoldia spp. Weight-frequency distributions were strongly right-skewed for most taxa, though some showed evidence of a bimodal distribution. Caloric densities as measured through bomb calorimetry significantly differed among taxa (ANOVA F = 32.57, df = 9, p-value<0.001), and whole animal wet weight was found to be a reliable predictor of whole animal caloric content. Bivalve populations and peak caloric densities were centered on and to the southeast of Hanna Shoal, which coincided with peak Pacific walrus relative density (walruses per km surveyed) from July through October. Significant differences in mean caloric values were found between areas with and without walruses present (student's t-test, t=−2.9088, df = 252.24, p-value = 0.003952), as well as between areas with low and high walrus relative densities in the pooled annual dataset and in each individual month except October (ANOVA, p-value<0.05). The high-calorie deposit feeders that dominate these bivalve communities preferentially consume food sources, such as sea ice algae, that are likely to be affected by shifting sea ice dynamics. As such, continued warming has the potential to alter bivalve communities in the northeastern Chukchi Sea, which may have profound implications for upper trophic levels.

Decadal trends in phytoplankton production in the Pacific Arctic Region from 1950 to 2012

This paper provides a synthesis of available in situ primary production (PP) measurements from the Pacific Arctic Region (PAR), collected between 1950 and 2012. Seasonal integrated primary production (IPP) across the PAR was calculated from 524 profiles, 340 of which were also analyzed to determine the average vertical distribution of PP rates for spring, summer and fall months. The Chirikov Basin and Chukchi Shelf were the most productive areas, with the East Siberian Sea, Chukchi Plateau and Canada Basin the lowest. Decadal-scale changes were indicated in the southern Chukchi Sea, and across Hanna Shoal. In the southern Chukchi Sea in August, IPP increased significantly from 113±35mgCm-2 d-1 in 1959 and 1960 to 833±307mgCm-2 d-1 in the 2000s. Increases in the magnitude of IPP were accompanied by variations in the vertical distribution, the subsurface peak observed in the 1959/60 was not present in the 2000s. The mechanism behind this change was undetermined but could have included changes in stratification, mixing or surface distribution of water masses as well as methodological differences. Over Hanna Shoal, the phytoplankton surface bloom now occurs earlier by several weeks compared to 1993, linked to increases in light due to earlier sea- ice retreat. In 1993 with sea ice still present in the region the surface bloom occurred in August, in 2002 and 2004 this same period was characterized by open water and low surface PP and strong subsurface production. This dataset provides a region-wide quantification of IPP and decadal trends and highlights the need for a cooperative monitoring program to observe the long-term impacts of climate change in the Arctic ecosystem.

Dissolved methane concentrations in the water column and surface sediments of Hanna Shoal and Barrow Canyon, Northern Chukchi Sea

Current estimates of methane (CH4) flux suggest that Arctic shelves may be a significant source of atmospheric CH4, a potent greenhouse gas. However, little information is known about the CH4 flux from most Arctic shelves, other than the East Siberian Arctic Shelf. We report here dissolved CH4 concentrations in the water column and within surface sediments of the Northern Chukchi Sea. We hypothesized that this area contains high concentrations of CH4 because it receives nutrient rich waters through the Bering Strait, promoting primary production that enhances an organic-rich material flux to the seafloor and eventual microbial methanogenesis in the sediments. In August 2012, as part of the Chukchi Sea Offshore Monitoring in Drilling Area (COMIDA) project, fourteen stations were sampled on Hanna Shoal, a shallow feature on the shelf, and ten stations across the undersea Barrow Canyon. On Hanna Shoal, water column CH4 concentrations ranged from 14 to 74nM, and surface concentrations were up to 15 times supersaturated in CH4 compared to equilibrium with the average atmospheric concentrations (3nM). CH4 concentrations at the sediment-water interface were around 1,500nM, and typically increased with depth in the sediment. At the head of Barrow Canyon, water column CH4 concentrations ranged from 5 to 46nM, with the highest concentrations in the deepest waters that were sampled (118m). Overall, the calculated fluxes to the atmosphere ranged from 1 to 80µmol CH4m−2d−1 for Hanna Shoal and 4 to 17µmol CH4m−2d−1 across the Barrow Canyon stations. Although there was a large range in these fluxes, the average atmospheric flux (20µmol CH4m−2d−1) across Hanna Shoal was 12 times lower than the flux reported from the East Siberian Arctic Shelf in summer. We conclude that while there is a positive flux of CH4 to the atmosphere, this part of the Chukchi Sea is not a significant source of atmospheric CH4 compared to the East Siberian Sea shelf.

Physical control of the distributions of a key Arctic copepod in the Northeast Chukchi Sea

The Chukchi Sea is a highly advective regime dominated by a barotropically driven northward flow modulated by wind driven currents that reach the bottom boundary layer of this shallow environment. A general northward gradient of decreasing temperature and food concentration leads to geographically divergent copepod growth and development rates between north and south. The physics of this system establish the biological connection potential between specific regions. The copepod Calanus glacialis is a key grazer, predator, and food source in Arctic shelf seas. Its summer distribution and abundance have direct effects on much of the food web, from phytoplankton to migrating bowhead whales. In August 2012 and 2013, C. glacialis distributions were quantified over Hanna Shoal in the northeast Chukchi Sea. Here an individual-based model with Lagrangian tracking and copepod life stage development capabilities is used to advect and develop these distributions forward and backward in time to determine the source (production locations) and sink (potential overwintering locations) regions of the transient Hanna Shoal C. glacialis population. Hanna Shoal supplies diapause competent C. glacialis to both the Beaufort Slope and the Chukchi Cap, mainly receives juveniles from the broad slope between Hanna Shoal and Herald Valley and receives second year adults from as far south as the Anadyr Gulf and as near as the broad slope between Hanna Shoal and Herald Valley. The 2013 sink region was shifted west relative to the 2012 region and the 2013 adult source region was shifted north relative to the 2012 adult source region. These connection potentials were not sensitive to precise times and locations of release, but were quite sensitive to depth of release. These patterns demonstrate how interannual differences in the physical conditions well south of Hanna Shoal play a critical role in determining the abundance and distribution of a key food source over Hanna Shoal and in the southern Beaufort Sea.

Circulation of winter water on the Chukchi shelf in early Summer

Using a variety of data sources we investigate the properties and pathways of Pacific-origin winter water as it spreads across the eastern Chukchi shelf in early summer. The focus is on the time period June–July 2011 during which an extensive shipboard hydrographic/velocity survey was undertaken as part of the Impacts of Climate on Ecosystem and Chemistry of the Arctic Pacific Environment (ICESCAPE) program. A revised circulation scheme is constructed revealing that the transport pathways on the Chukchi shelf are more complex than previously thought. Notably, the well known branch progressing northward from the Central Channel bifurcates as it reaches Hanna Shoal, flowing around both sides of the shoal and dividing into smaller filaments that continue towards Barrow Canyon. Mass is conserved in the circulation scheme, with approximately 1 Sverdrup flowing poleward across the Chukchi shelf within these pathways, then exiting Barrow Canyon. The salinity of the winter water varied on the shelf in 2011, with saltier water found in the upstream portion of what is defined as the central pathway. Using sea ice concentration data and atmospheric reanalysis fields, we argue that salinization of the winter water in the central pathway occurred via brine rejection as the parcels progressed north and passed through the Cape Lisburne polynya. This demonstrates that winter water pervading the interior shelf can be transformed by convective overturning north of Bering Strait, presumably stirring up nutrients from the sediments and thereby influencing primary productivity in the region of Hanna Shoal.

Seasonal spatial patterns in seabird and marine mammal distribution in the eastern Chukchi and western Beaufort seas: Identifying biologically important pelagic areas

The Chukchi and Beaufort seas are undergoing rapid climate change and increased human activity. Conservation efforts for upper trophic level predators such as seabirds and marine mammals require information on species’ distributions and identification of important marine areas. Here we describe broad-scale distributions of seabirds and marine mammals. We examined spatial patterns of relative abundance of seabirds and marine mammals in the eastern Chukchi and western Beaufort seas during summer (15 June–31 August) and fall (1 September–20 November) from 2007 to 2012. We summarized 49,206km of shipboard surveys for seabirds and 183,157km of aerial surveys for marine mammals into a grid of 40-km×40-km cells. We used Getis-Ord Gi∗ hotspot analysis to test for cells with higher relative abundance than expected when compared to all cells within the study area. We identified cells representing single species and taxonomic group hotspots, cells representing hotspots for multiple species, and cells representing hotspots for both seabirds and marine mammals. The locations of hotspots varied among species but often were located near underwater canyons or over continental shelf features and slopes. Hotspots for seabirds, walrus, and gray whales occurred primarily in the Chukchi Sea. Hotspots for bowhead whales and other pinnipeds (i.e., seals) occurred near Barrow Canyon and along the Beaufort Sea shelf and slope. Hotspots for belugas occurred in both the Chukchi and Beaufort seas. There were three hotspots shared by both seabirds and marine mammals in summer: off Wainwright in the eastern Chukchi Sea, south of Hanna Shoal, and at the mouth of Barrow Canyon. In fall, the only identified shared hotspot occurred at the mouth of Barrow Canyon. Shared hotspots are characterized by strong fronts caused by upwelling and currents, and these areas can have high densities of euphausiids in summer and fall. Due to the high relative abundance of animals and diversity of taxa, these sites are clearly important areas of congregation for seabirds and marine mammals that should be prioritized in the development of management and conservation plans.

Summertime circulation in the eastern Chukchi Sea

The transport of Pacific-origin water across the eastern Chukchi Sea during summer is studied using shipboard hydrographic and Acoustic Doppler Current Profiler data from multiple surveys. The study identifies two major transport pathways in this region. The well-known Alaskan Coastal Current (ACC) flows poleward along the Alaska coast, while a more recently discovered and slower current flows northward through the Central Channel between Herald and Hanna Shoals. The two currents separate past Point Hope and appear to merge again in Barrow Canyon via previously unresolved pathways flowing west to east across the Chukchi Sea. The collective flow that transports Pacific water from Bering Strait to Barrow Canyon is termed the Bering to Barrow Current System (BBCS). The coastal branch of the BBCS, which encompasses the ACC, is a weakly baroclinic flow with a maximum speed in excess of 80cm/s inside Barrow Canyon. The Central Channel branch of the BBCS is considerably weaker, with a maximum flow speed under 20cm/s. A portion of the Pacific water that flows towards Herald Canyon to the west may also contribute to the BBCS, the amount, but the amount is within the margin of error of our data. The difference in the advective time scales of the various branches allows water masses of different seasonal characteristics to be transported simultaneously by the BBCS. Summer water masses occupy most of the southern and central Chukchi Sea while winter water masses occupy Hanna Shoal and shelfbreak regions in the northern Chukchi Sea during summer. The total BBCS volume transport in summer is measured at four different locations in the eastern Chukchi Sea and found to be generally consistent. The overall mean transport is 0.86Sv, compared to the summer Bering Strait transport of 1.08Sv. This study suggests that the BBCS is the main transport pathway delivering Pacific water masses into the western Arctic Basin.

Biodegradation of Oil and Dispersed Oil by Arctic Marine Microorganisms

As oil exploration and shipping routes expand in Arctic regions, the potential for oil to enter these cold water environments increases. Therefore it becomes important to understand how the indigenous Arctic microbial community will respond to oil. Arctic seawater was incubated with Alaska North Slope (ANS) crude oil to detect the presence of known petroleum degrading microorganisms and to assess how the baseline microbial community changes with inputs of ANS and ANS dispersed with Corexit 9500. Surface seawater was collected from the Chukchi Sea in near shore (Barrow, AK) and offshore (Hanna Shoal Study Area) environments to provide an indigenous consortium of microorganisms. Incubations were conducted at the temperature of the ocean at the time of collection (-1°C and 4°C), with minimal nutrient addition and sampled at 5, 10 and 28 days. To determine the baseline microbial community, seawater was collected from varying water depths within the Hanna Shoal Study Area. All seawater samples were filtered (0.2 μm) and frozen (–80°C) for DNA extraction. Pyrosequencing of 16S rRNA genes provided taxonomic information to identify microbes that are present in near shore and offshore marine environments and microbes that grow in response to oil or chemically dispersed oil. The indigenous Arctic marine microbial community was found to shift in response to the presence of oil and dispersant, providing an indication of the identity of oil degraders.

Phytoplankton assemblage structure in and around a massive under-ice bloom in the Chukchi Sea

Standard and imaging flow cytometry were used to examine the composition of phytoplankton assemblages in and around a massive under-ice bloom in the Chukchi Sea in 2011. In the core of this bloom, roughly 100km northwest of Hanna Shoal, diatoms represented roughly 87% of the water column carbon-specific biomass of phytoplankton, while nanophytoplankton contributed ~9%. Picoeukaryotes were also observed in this bloom, as were phycoerythrin-containing cells consistent with Synechococcus spp., but picophytoplankton, dinoflagellates, and prymnesiophytes each represented only ~1% of the bloom׳s phytoplankton biomass. More broadly along this part of the Chukchi shelf, nanophytoplankton typically comprised a larger fraction of phytoplankton biomass in the water column, 22% on average but up to 82% at certain locations. Dinoflagellates and prymnesiophytes contributed at most 2% of water column biomass at any location and were most abundant in the deeper slope stations northeast of Hanna Shoal, east of the bloom. Picophytoplankton were most abundant in these deeper slope stations as well, and also in recently ice-free areas to the south around Hanna Shoal. These cell-derived estimates of phytoplankton carbon biomass, which were computed from imaging and standard cytometric observations of phytoplankton cell sizes and from published carbon:volume relationships, agree well with independent measurements of particulate organic carbon concentration from traditional biochemical assays.

Distribution, abundance, biomass and diversity of benthic infauna in the Northeast Chukchi Sea, Alaska: Relation to environmental variables and marine mammals

In summer 2009 and 2010, as part of Chukchi Sea Offshore Monitoring in Drilling Area – Chemical and Benthos (COMIDA CAB) program, we performed a quantitative assessment of the biomass, abundance, and community structure of benthic infaunal populations of the Northeastern Chukchi Sea. This analysis documented a benthic species inventory of 361 taxa collected from 142 individual van Veen grab samples (0.1m−2) at 52 stations. Infaunal abundance was dominated by Polychaeta, Mollusca, and Crustacea. Large concentrations of bivalves (up to 1235m−2; 920.2gwwm−2) were collected south of Hanna Shoal where flow from two water masses converge and deposit labile carbon to the seafloor, as indicated by low surface sediment C:N ratios. Amphipods (up to 1640m−2; 26.0gwwm−2), and polychaetes (up to 4665m−2; 114.7gwwm−2) were documented from multiple stations west of and within Barrow Canyon. This high productivity was most likely due to the “canyon effect”, where marine and coastal detrital carbon supplies are channeled by the canyon structure, enhancing carbon deposition and flux, which supports rich benthic communities within the canyon and surrounding areas. To examine the relationships between infaunal distributions of all collected taxa with the physical environment, we used a Biota and Environment matching (BIO–ENV) routine. A combination of water depth, bottom-water temperature and salinity, surface sediment total organic nitrogen (TON) and sediment C:N molar ratios correlated closest with infaunal abundance distribution (ρ=0.54), indicating that multiple factors influence the success of benthic communities. BIO–ENV routines produced similar correlation results when performed on targeted walrus prey items (bivalves (ρ=0.50), polychaetes (ρ=0.53), but gray whale prey items (amphipods) were not strongly correlated to any combination of physical environmental factors (ρ=0.24). Distributions of primary prey items for gray whales (amphipods) and walruses (bivalves, gastropods and polychaetes) were compared with gray whale and walrus distribution as described by sightings from the 2009 and 2010 aerial survey component of COMIDA. In general, concentrations of walruses and their prey occurred in a swath located south of Hanna Shoal and on the shoal itself although the large differences in sea-ice distribution between the two study years showed that walrus distributions were closely linked to sea-ice location. Other areas within Barrow Canyon and the shelf west of the canyon showed high concentrations of benthic amphipods that were coincident with gray whale sightings as quantified by COMIDA aerial surveys. Overall, data collected on this project indicate that the Northeast Chukchi Sea supports a highly productive and diverse benthic ecosystem that is of significant importance to higher trophic level megafauna.

Marine mammal acoustic detections in the northeastern Chukchi Sea, September 2007–July 2011

Several cetacean and pinniped species use the northeastern Chukchi Sea as seasonal or year-round habitat. This area has experienced pronounced reduction in the extent of summer sea ice over the last decade, as well as increased anthropogenic activity, particularly in the form of oil and gas exploration. The effects of these changes on marine mammal species are presently unknown. Autonomous passive acoustic recorders were deployed over a wide area of the northeastern Chukchi Sea off the coast of Alaska from Cape Lisburne to Barrow, at distances from 8km to 200km from shore: up to 44 each summer and up to 8 each winter. Acoustic data were acquired at 16kHz continuously during summer and on a duty cycle of 40 or 48min within each 4-h period during winter. Recordings were analyzed manually and using automated detection and classification systems to identify calls.Bowhead (Balaena mysticetus) and beluga (Delphinapterus leucas) whale calls were detected primarily from April through June and from September to December during their migrations between the Bering and Beaufort seas. Summer detections were rare and usually concentrated off Wainwright and Barrow, Alaska. Gray (Eschrichtius robustus) whale calls were detected between July and October, their occurrence decreasing with increasing distance from shore. Fin (Balaenoptera physalus), killer (Orcinus orca), minke (Balaenoptera acutorostrata), and humpback (Megaptera novaeangliae) whales were detected sporadically in summer and early fall. Walrus (Odobenus rosmarus) was the most commonly detected species between June and October, primarily occupying the southern edge of Hanna Shoal and haul-outs near coastal recording stations off Wainwright and Point Lay. Ringed (Pusa hispida) and bearded (Erignathus barbatus) seals occur year-round in the Chukchi Sea. Ringed seal acoustic detections occurred throughout the year but detection numbers were low, likely due to low vocalization rates. Bearded seal acoustic detections peaked in April and May during their breeding season, with much lower detection numbers in July and August, likely as a result of reduced calling rates after breeding season. Ribbon seals (Histriophoca fasciata) were only detected in the fall as they migrated south through the study area toward the Bering Sea. These results suggest a regular presence of marine mammals in the Chukchi Sea year-round, with species-dependent seasonal and spatial density variations.

Hydrographic variability over the northeastern Chukchi Sea shelf in summer-fall 2008–2010

We used shipboard CTD, mooring, meteorological, glider, and high-frequency radar data to examine spatial and temporal (seasonal and interannual) variations in the circulation and water properties over the central portion of the northeastern Chukchi Sea shelf from August–October of 2008, 2009, and 2010. Seasonally, warm and moderately saline Bering Sea Water (BSW) replaces cool, dilute surface meltwaters and cold, saline, sub-surface, winter-formed waters. BSW advection thus affects shelf stratification and the heat budget with oceanic heat flux convergence supplanting solar heating as the dominant shelf heat source by September. BSW spreads eastward from the Chukchi's Central Channel, so that water property and stratification transitions proceed from west to east across the study region. Models predict a mean clockwise flow around Hanna Shoal (which lies to the north of the study area) suggesting winter-formed waters from northeast of the Shoal are advected southwestward into a portion of the study area. The observations, though limited, support this notion. We hypothesize that the convergence of BSW from the west and winter waters from the northeast leads, in some years, to large horizontal variations in water properties, stratification, and ocean heat flux convergence over spatial scales of ~50–100km.Interannual variations in summer/fall shelf water properties appear linked to processes occurring in the Bering and/or southern Chukchi Sea, and the regional winds (which affect the local circulation). Although there were large interannual differences in shelf-wide ice retreat patterns from May–July, these differences were not reflected in late summer water properties.

The offshore northeastern Chukchi Sea, Alaska: A complex high-latitude ecosystem

We conducted an interdisciplinary ecological study in and near 3 nearby proposed exploratory oil and gas prospects in the offshore northeastern Chukchi Sea during the open-water seasons of 2008–2010. This region exhibits a classical pelagic–benthic dichotomy of food-web structure in ecological function. The Klondike study area borders the eastern edge of the Central Channel and functions as a pelagic-dominated ecosystem, whereas the Burger study area lies south of Hanna Shoal and functions as a benthic-dominated ecosystem. The Statoil study area, which is located north of Klondike and northwest of Burger, has both pelagic and benthic attributes, although it is more like Burger than like Klondike. Klondike has lower benthic density and biomass, a higher biomass of oceanic zooplankton, and more fishes and planktivorous seabirds than does Burger, which has benthic communities with high density and biomass, primarily neritic zooplankton, and higher densities of benthic-feeding marine mammals than Klondike; Statoil has characteristics of both ecosystems. Patterns of sea-ice retreat vary interannually; in some years, much of the northeastern Chukchi is ice-free by mid-May, leading to pelagic and ice-edge phytoplankton blooms, whereas heavy ice cover in other years leads to substantial within-ice production. The characteristics of this region during the open-water season are not consistent among years, in that Bering Sea Water impinges onto all study areas only in some years, resulting in interannual variation in the distribution and abundance of zooplankton, planktivorous seabirds, and pelagic-feeding seals. These interannual variations alter several aspects of this pelagic–benthic dichotomy, and some aspects of this region suggest unusual structure (e.g., replacement of benthic-feeding fishes in some areas by predatory invertebrates, a lack of benthic-feeding seaducks).

Walrus areas of use in the Chukchi Sea during sparse sea ice cover

The Pacific walrus Odobenus rosmarus divergens feeds on benthic invertebrates on the con- tinental shelf of the Chukchi and Bering Seas and rests on sea ice between foraging trips. With climate warming, ice-free periods in the Chukchi Sea have increased and are projected to increase further in fre- quency and duration. We radio-tracked walruses to estimate areas of walrus foraging and occupancy in the Chukchi Sea from June to November of 2008 to 2011, years when sea ice was sparse over the conti- nental shelf in comparison to historical records. The earlier and more extensive sea ice retreat in June to September, and delayed freeze-up of sea ice in Octo- ber to November, created conditions for walruses to arrive earlier and stay later in the Chukchi Sea than in the past. The lack of sea ice over the continental shelf from September to October caused walruses to forage in nearshore areas instead of offshore areas as in the past. Walruses did not frequent the deep waters of the Arctic Basin when sea ice retreated off the shelf. Walruses foraged in most areas they occu- pied, and areas of concentrated foraging generally corresponded to regions of high benthic biomass, such as in the northeastern (Hanna Shoal) and south- western Chukchi Sea. A notable exception was the occurrence of concentrated foraging in a nearshore area of northwestern Alaska that is apparently de - pauperate in walrus prey. With increasing sea ice loss, it is likely that walruses will increase their use of coastal haul-outs and nearshore foraging areas, with consequences to the population that are yet to be understood.

Three‐dimensional Bayesian passive acoustic tracking of walruses in the Chukchi Sea.

A Bayesian travel‐time inversion method was developed for 3‐D localization and tracking using multipath arrival times of knock‐type sounds produced by walruses. The method accounts for data errors and uncertainties in environmental and geometric parameters. Data were collected on three seafloor sound recorders, arranged in an equilateral triangle with approximately 500‐m sides, deployed in 30 m of water in the Chukchi Sea near the Hanna Shoal, west of Barrow, Alaska. Pacific Walrus (Odobenus rosmarus divergens) calls were recorded by these recorders for 2.5 months in the summer of 2009. A regularized, linearized inversion algorithm was used to estimate 3‐D tracks and track uncertainties for these calling walruses. Regularization incorporates prior information (expected values and uncertainty estimates) for the recorder locations and acoustic environment (water depth and sound speed) as well as preferred track characteristics. Inverting for the smoothest (simplest) track consistent with the acoustic data and prior information mitigates the risk of over‐interpreting track structure due to data errors and environmental and geometric uncertainties. Hence, this approach provides more plausible sequences of source positions than a non‐regularized solution. Tracks for walrus dives from the 2009 dataset will be presented.

210Pb as a tracer of shelf–basin transport and sediment focusing in the Chukchi Sea

Abstract Activities of dissolved, particulate, and sedimentary 210Pb were measured in the shelf-slope region of the Chukchi Sea. Samples were collected as part of the Shelf–Basin Interactions (SBI) Phase II process study (6 May–15 June, 2002) along three shelf–basin transects identified as West Hanna Shoal, East Hanna Shoal, and Barrow Canyon. Distributions of 210Pb and suspended particulate matter indicate efficient removal of 210Pb over the shelf by particle scavenging. Low 210Pb activities measured throughout the halocline of the Canada Basin are attributed to shelf scavenging and subsequent advective transport into the interior basin. Additionally, 210Pb inventories were used to construct a water-column-sediment budget of 210Pb and determine regions of particle export and deposition on the continental shelf and slope. Sediment focusing calculated with this 210Pb budget was observed throughout the shelf-slope region, particularly in shallow (∼100 m) shelf waters at Barrow Canyon. Despite elevated concentrations of suspended particulate matter in Barrow Canyon, the 210Pb budget does not indicate that sediment transport occurred from the West and East Hanna Shoals into Barrow Canyon.

Export fluxes of biogenic matter in the presence and absence of seasonal sea ice cover in the Chukchi Sea

Drifting sediment traps were deployed at 9 stations in May-June (ice-covered conditions) and July-August (ice-free conditions) 2004 in the Chukchi Sea to investigate the variability in export fluxes of biogenic matter in the presence and absence of sea ice cover. Measurements of chlorophyll- a (Chl- a), particulate organic carbon (POC), particulate nitrogen (PN), phytoplankton, zooplankton fecal pellets, and the stable carbon isotope composition ( δ 13C) of the sinking material were performed along Barrow Canyon (BC) and a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. POC export fluxes were similarly high in the presence (378±106 mg C m −2 d −1) and in the absence of ice cover (442±203 mg C m −2 d −1) at the BC stations, while fluxes were significantly higher in the absence (129±98 mg C m −2 d −1) than in the presence of ice cover (44±29 mg C m −2 d −1) at the EHS stations. The C/N ratios and δ 13C values of sinking organic particles indicated that POC export fluxes on the Chukchi continental shelf were mostly composed of freshly produced labile material, except at the EHS stations under ice cover where the exported matter was mostly composed of refractory material probably advected into the EHS region. Chl- a fluxes were higher under ice cover than in ice-free water, however, relatively low daily loss rates of Chl- a and similar phytoplankton carbon fluxes in ice-covered and ice-free water suggest the retention of phytoplankton in the upper water column. An increase in fecal pellet carbon fluxes in ice-free water reflected higher grazing pressure in the absence of ice cover. Elevated daily loss rates of POC at the BC stations confirmed other indications that Barrow Canyon is an important area of carbon export to the basin and/or benthos. These results support the conclusion that there are large spatial and temporal variations in export fluxes of biogenic matter on the Chukchi continental shelf, although export fluxes may be similar in the presence and in the absence of ice cover in highly productive regions.

Export fluxes of particulate organic carbon in the Chukchi Sea: A comparative study using 234Th/ 238U disequilibria and drifting sediment traps

Large-volume sampling of 234Th and drifting sediment trap deployments were conducted as part of the 2004 Western Arctic Shelf–Basin Interactions (SBI) spring (May 15–June 23) and summer (July 17–August 26) process cruises in the Chukchi Sea. Measurements of 234Th and particulate organic carbon (POC) export fluxes were obtained at five stations during the spring cruise and four stations during the summer cruise along Barrow Canyon (BC) and along a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. 234Th and POC fluxes obtained with in situ pumps and drifting sediment traps agreed to within a factor of 2 for 70% of the measurements. POC export fluxes measured with in situ pumps at 50 m along BC were similar in spring and summer (average = 14.0 ± 8.0 mmol C m − 2 day − 1 and 16.5 ± 6.5 mmol C m − 2 day − 1 , respectively), but increased from spring to summer at the EHS transect (average = 1.9 ± 1.1 mmol C m − 2 day − 1 and 19.5 ± 3.3 mmol C m − 2 day − 1 , respectively). POC fluxes measured with sediment traps at 50 m along BC were also similar in both seasons (31.3 ± 9.3 mmol C m − 2 day − 1 and 29.1 ± 14.2 mmol C m − 2 day − 1 , respectively), but were approximately twice as high as POC fluxes measured with in situ pumps. Sediment trap POC fluxes measured along the EHS transect also increased from spring to summer (3.0 ± 1.9 mmol C m − 2 day − 1 and 13.0 ± 6.4 mmol C m − 2 day − 1 , respectively), and these fluxes were similar to the POC fluxes obtained with in situ pumps. Discrepancies in POC export fluxes measured using in situ pumps and sediment traps may be reasonably explained by differences in the estimated POC/ 234Th ratios that arise from differences between the techniques, such as time-scale of measurement and size and composition of the collected particles. Despite this variability, in situ pump and sediment trap-derived POC fluxes were only significantly different at a highly productive station in BC during the spring.

The effect of possible Taylor columns on the summer ice retreat in the Chukchi Sea

The Chukchi Sea topography consists of a broad, flat, 50‐m‐deep plain, with two prominent shoals, Herald and Hanna, which have horizontal dimensions of about 100 km and rise to depths of about 30 m. Herald Shoal in particular is a prominent, isolated seamount which, because of the warm water flux in summer through Bering Strait, has a warm 0.1 m s−1 flow incident upon it. Examination of active and passive microwave imagery of the region for 1992–1994 shows that as the general ice cover recedes, the ice remains preferentially over Herald Shoal for 3–4 weeks after the surrounding ice has melted. A scale analysis suggests that the cause of the ice persistence is the formation of a Taylor column over the shoal, which traps cold water and ice above it. A similar trapping of ice occurs over Hanna Shoal and on its eastern slope, but the response is complicated by a very different topography and the northward flow down Barrow Canyon.

Growth Mechanisms of "Katie's Floeberg"

The growth mechanism of the grounded ice feature on Hanna's Shoal, located at approximately 162 deg W, 72 deg N in the Chukchi Sea, has been analyzed using Landsat I and II satellite imagery. The primary mechanism of growth was deduced to be the formation of large wedges of ice against the upstream edge of the feature. These wedges of piled ice form in three stages and may or may not become permanently affixed to the core of the feature. Remains of these growth patterns can sometimes be seen in the interior of the feature on the satellite imagery.