Deep Arctic Basin Research Articles

Isotopic constraints on the late Pleistocene glacial water and sediment inputs to the central Arctic ocean

The Quaternary history of the Arctic Ocean reflects pronounced periodic changes in marine and glacial environments. This variability is an important component of the Arctic paleoclimatic system. For more insight, we use isotopic measurements on the detrital (Nd-Sr-Pb) and authigenic (Nd) phases of the Quaternary sediments from the central Arctic Ocean. We analyze core ICE4 from ∼3 km water depth off the Lomonosov Ridge. This core was previously studied for other sedimentary proxies (Dong et al., 2022). The analyzed record is estimated to cover major Late Pleistocene Weichselian/Wisconsinian glaciations occurring over the last ∼100–200 ka. Our results provide new evidence that glacially-derived waters from the glaciated margins reached the bottom of the deep Arctic basins. We infer that these waters were delivered to the deep basins by hyperpycnal flows originating from subglacial outbursts accompanying overflow meltwater and iceberg discharge during glacial/deglacial events. Voluminous outbursts during the older, presumed Early/Middle Weichselian glaciation, ca. 115-35 ka, are traced to the West Siberian margin impacted by the Eurasian Ice Sheet. Similar sedimentary layers are found in longer records from the central Arctic Ocean at older stratigraphic intervals. Sediments constrained to the Last Glacial Maximum have less pronounced glacigenic inputs with a stronger Laurentide Ice Sheet signature. The circulation patterns were likely dependent on the interplay of the timing and extent of the Laurentide and Eurasian ice sheets.

JC2-1 Joint cruise part 1 2021

The Nansen Legacy Joint Cruise 2, part 1 (JC2-1) 12-29 July 2021, continued the investigation of the interannual variability during the late summer season. At the same time will the cruise provide a late summer reference for the seasonal investigation that was separated to late summer and polar night 2019, and winter and spring 2021. The transect represents an environmental gradient going through the northern Barents Sea, and included 7 process stations (P1-P7) lasting 10-39 hrs. Additional CTD stations (NLEG) were taken between the process stations to increase the hydrographic resolution on the transect. The work started at 76°N at the open Atlantic Water dominated station P1, was sea ice covered from station P4 at 79 45.00 °N and included deep water stations at the P7 station at 82°N in the Nansen Basin.
 
 The program included measurements and sampling from the atmosphere, sea ice, ocean and sea floor. Data collected includes several disciplines to map the physical environment, the chemical characterization and biological communities. Key parameters from the Nansen Legacy joint cruises were selected and included in this reduced sampling program, compared to the other joint cruises. An important aim was to characterize the northern Barents Sea to map the interannual variability, but also to see how the proceeding winter and spring conditions has impacted the environment and conditions this year. A third task is to carry out a shelf investigation that can complement the investigations in the deep Arctic Basin starting in late August. Tests and improvements of a modified pelagic trawl (Harstad) were carried out as preparations for the Arctic Basin cruise in late August 2021.

Structure and Circulation of Atlantic Water Masses in the St. Anna Trough in the Kara Sea

The inflow of warm and saline Atlantic water from the North Atlantic to the Western Arctic is provided by two branches, namely, the Fram Strait branch water and the Barents Sea branch water. The pathways of these branches merge at the St. Anna Trough, and then both branches propagate eastward along the continental slope, albeit at different depths. As a result, the local interaction between these branches in the trough affects the properties of the large-scale Atlantic water flow to the Eastern Arctic and the deep Arctic basins. In this study, we report extensively in situ measurements with high spatial coverage (56 hydrological stations organized into 7 transects) in the St. Anna Trough, obtained in August and October 2021. Based on these data, we reconstructed the thermohaline structure and circulation in this area and obtained new insights, which are crucial for the assessment of the interaction and heat balance of water masses in the trough. First, we state that the majority of the Fram Strait branch water is recirculated in the trough within the stable cyclonic gyre, while a smaller fraction returns to the continental slope. The formation of this gyre increases the residence time of the Fram Strait branch water in the trough and decreases the intensity of water and heat exchange between the trough and the continental slope. Second, we describe the dynamic interaction between the northward flow of the Barents Sea branch water and the surface layer. It causes intense transport of warm surface water from the Kara and Barents seas adjacent to the Novaya Zemlya toward the continental slope and its mixing with the Barents Sea branch water along the eastern part of the trough. These processes result in increased surface temperature at the eastern part of the trough, which enhances ice melting at the study area and increases the duration of the ice-free period.

Ringed seal (Pusa hispida) seasonal movements, diving, and haul-out behavior in the Beaufort, Chukchi, and Bering Seas (2011-2017).

Continued Arctic warming and sea‐ice loss will have important implications for the conservation of ringed seals, a highly ice‐dependent species. A better understanding of their spatial ecology will help characterize emerging ecological trends and inform management decisions. We deployed satellite transmitters on ringed seals in the summers of 2011, 2014, and 2016 near Utqiaġvik (formerly Barrow), Alaska, to monitor their movements, diving, and haul‐out behavior. We present analyses of tracking and dive data provided by 17 seals that were tracked until at least January of the following year. Seals mostly ranged north of Utqiaġvik in the Beaufort and Chukchi Seas during summer before moving into the southern Chukchi and Bering Seas during winter. In all seasons, ringed seals occupied a diversity of habitats and spatial distributions, from near shore and localized, to far offshore and wide‐ranging in drifting sea ice. Continental shelf waters were occupied for >96% of tracking days, during which repetitive diving (suggestive of foraging) primarily to the seafloor was the most frequent activity. From mid‐summer to early fall, 12 seals made ~1‐week forays off‐shelf to the deep Arctic Basin, most reaching the retreating pack‐ice, where they spent most of their time hauled out. Diel activity patterns suggested greater allocation of foraging efforts to midday hours. Haul‐out patterns were complementary, occurring mostly at night until April‐May when midday hours were preferred. Ringed seals captured in 2011—concurrent with an unusual mortality event that affected all ice‐seal species—differed morphologically and behaviorally from seals captured in other years. Speculations about the physiology of molting and its role in energetics, habitat use, and behavior are discussed; along with possible evidence of purported ringed seal ecotypes.

What Feeds the Benthos in the Arctic Basins? Assembling a Carbon Budget for the Deep Arctic Ocean

Half of the Arctic Ocean is deep sea (> 1000 m), and this area is currently transitioning from being permanently ice-covered to being seasonally ice-free. Despite these drastic changes, it remains unclear how organisms are distributed in the deep Arctic basins, and particularly what feeds them. Here, we summarize data on auto- and heterotrophic organisms in the benthic, pelagic, and sympagic realm of the Arctic Ocean basins from the past three decades and put together an organic carbon budget for this region. Based on the budget, we investigate whether our current understanding of primary and secondary production and vertical carbon flux are balanced by the current estimates of the carbon demand by deep-sea benthos. At first glance, our budget identifies a mismatch between the carbon supply by primary production (3-46 g C m-2 yr-1), the carbon demand of organisms living in the pelagic (7-17 g C m-2) and the benthic realm (< 5 g C m-2 yr-1) versus the low vertical carbon export (at 200 m: 0.1-1.5 g C m-2 yr-1, at 3000-4000 m: 0.01-0.73 g C m-2 yr-1). To close the budget, we suggest that episodic events of large, fast sinking ice algae aggregates, export of dead zooplankton, as well as large food falls need to be quantified and included. This work emphasizes the clear need for a better understanding of the quantity, phenology, and the regionality of carbon supply and demand in the deep Arctic basins, which will allow us to evaluate how the ecosystem may change in the future.

Dissolved organic matter in pore water of Arctic Ocean sediments: Environmental influence on molecular composition

Marine organic matter (OM) sinks from surface water to the seafloor via the biological pump. Benthic communities, which use this sedimented OM as an energy and carbon source, produce dissolved OM (DOM) in the process of degradation, enriching the sediment pore water with fresh DOM compounds. In the oligotrophic deep Arctic basin, particle flux is low but highly seasonal. We hypothesized that the molecular signal of freshly deposited, primary produced OM would be detectable in surface sediment pore water, which should differ in DOM composition from bottom water and deeper sediment pore water. The study focused on (i) the molecular composition of the DOM in sediment pore water of the deep Eurasian Arctic basins, (ii) the signal of marine vs. terrigenous DOM represented by different compounds preserved in the pore water and (iii) the relationship between Arctic Ocean ice cover and DOM composition. Composition based on mass spectrometric information, obtained via 15T Fourier transform ion cyclotron resonance mass spectrometry, was correlated with environmental parameters with partial least square analysis. The fresh marine detrital OM signal from surface water was limited to pore water from <5cm sediment depth. The productive ice margin stations showed a higher abundance of peptide, unsaturated aliphatic and saturated fatty acid molecular formulae, indicative of recent phytodetritus deposition, than the multiyear ice-covered stations, which had a stronger aromatic signal. The study contributes to the understanding of the coupling between Arctic Ocean productivity and its depositional regime, and how it may be altered in response to sea ice retreat and increasing river runoff.

Sound propagation from the Canadian Basin to the Chukchi shelf during Summer 2015

During the summer of 2015 a pilot experiment for the Canadian Basin Acoustic Propagation Experiment (CANAPE) was conducted between deep Arctic basin and shallow Chukchi shelf. A vertical line array (VLA) was deployed on the shelf (72.336 N, 157.449 W) at a depth of 161 m from July 26 to August 13, 2015. Sound sources were deployed from R/V Sikuliaq at locations ranging from 131 to 375 km from the VLA. M-Sequences centered at 250 Hz (bandwidth of 62.5 Hz) were transmitted from each location. Discrete shipboard CTD data were conducted at each transmission location and continuous CTD data were recorded along the VLA. Water column data suggests a sound channel located vertically between Pacific and Atlantic waters near the depth of Arctic halocline layer. They also show upward shoaling of lower halocline waters onto the shelf. This upwelling over the continental slope moves the sound channel from offshore to onshore near the bottom. The axis of this sound duct is 100 m below the surface and is about 100 m thick. Received acoustic signals on the shelf increase rapidly in intensity at time scales that range from minutes to days. In this paper we show the acoustic variations are due to the variable sound speed profile. Signal intensity, noise, and temporal variability for geotime scale of minutes to hours are reported. Numerical simulations are conducted to investigate the associated acoustic effects in data. [Work supported by ONR 322OA.]

The future of Arctic benthos: Expansion, invasion, and biodiversity

One of the logical predictions for a future Arctic characterized by warmer waters and reduced sea-ice is that new taxa will expand or invade Arctic seafloor habitats. Specific predictions regarding where this will occur and which taxa are most likely to become established or excluded are lacking, however. We synthesize recent studies and conduct new analyses in the context of climate forecasts and a paleontological perspective to make concrete predictions as to relevant mechanisms, regions, and functional traits contributing to future biodiversity changes. Historically, a warmer Arctic is more readily invaded or transited by boreal taxa than it is during cold periods. Oceanography of an ice-free Arctic Ocean, combined with life-history traits of invading taxa and availability of suitable habitat, determine expansion success. It is difficult to generalize as to which taxonomic groups or locations are likely to experience expansion, however, since species-specific, and perhaps population-specific autecologies, will determine success or failure. Several examples of expansion into the Arctic have been noted, and along with the results from the relatively few Arctic biological time-series suggest inflow shelves (Barents and Chukchi Seas), as well as West Greenland and the western Kara Sea, are most likely locations for expansion. Apparent temperature thresholds were identified for characteristic Arctic and boreal benthic fauna suggesting strong potential for range constrictions of Arctic, and expansions of boreal, fauna in the near future. Increasing human activities in the region could speed introductions of boreal fauna and reduce the value of a planktonic dispersal stage. Finally, shelf regions are likely to experience a greater impact, and also one with greater potential consequences, than the deep Arctic basin. Future research strategies should focus on monitoring as well as compiling basic physiological and life-history information of Arctic and boreal taxa, and integrate that with projections of human activities and likely ecosystem consequences to facilitate development of management strategies now and in the future.

A near-uniform fluctuation of ocean bottom pressure and sea level across the deep ocean basins of the Arctic Ocean and the Nordic Seas

Across the Arctic Ocean and the Nordic Seas, a basin-wide mode of ocean bottom pressure and sea level fluctuation is identified using satellite and in situ observations in conjunction with a global ocean circulation model and its adjoint. The variation extends across the interconnected deep ocean basins of these semi-enclosed Arctic seas, collectively called the Arctic Mediterranean, with spatially near-uniform amplitude and phase. The basin-wide fluctuation is barotropic and dominates the region’s large-scale variability from sub-monthly to interannual timescales. The fluctuation results from bifurcating coastally trapped waves generated by winds along the continental slopes of the Arctic Mediterranean and its neighboring seas, including the North Atlantic Ocean. The winds drive Ekman transport across the large bathymetric gradients, forcing mass divergence between the shallow coastal area and the deep ocean basins and creating ocean bottom pressure anomalies of opposite signs in the two regions. The anomalies rapidly propagate away as barotropic coastally trapped waves with the coast and continental slope as respective boundaries. The waves subsequently bifurcate at the shallow straits connecting the Arctic Mediterranean with the rest of the globe. The straits transmit the shallow anomalies but not the deep variations, thereby inhibiting the anomalies’ mutual cancelation by geographically separating the two. Anomalies that enter the deep Arctic basins equilibrate uniformly across the domain characterized by a homogeneous depth-integrated planetary potential vorticity distribution. The potential vorticity’s steep gradient that borders the basins shields the region from neighboring shallow variations, giving rise to the observed spatially confined fluctuation. Compensating anomalies outside the Arctic adjust similarly across the rest of the globe but are comparatively negligible in amplitude because of the global ocean’s larger area relative to that of the deep Arctic Mediterranean. The study, from a technical perspective, illustrates the utility of a model’s adjoint in identifying causal mechanisms underlying a complex system.

New perspectives on paleoglaciology

Paleoglaciology deals with glaciation cycles of the Quaternary Ice Age. It combines the dynamics of present-day ice sheets deduced from glaciology with the history of former ice sheets deduced from glacial geology. Cosmogenic dating now makes a detailed chronology of a cycle possible. Radiocarbon dating has provided a detailed chronology for Termination of the last cycle, but cannot date how the cycle began. Here we identify the central challenge in applying paleoglaciology to Quaternary glaciation cycles in general, and initiation of these cycles in particular. The challenge is the role of thickening sea ice in the Arctic Ocean, possibly becoming thick ice shelves floating over deep Arctic basins and high marine ice sheets grounded on shallow Arctic continental shelves. In that case, an Arctic Ice Sheet existed that was larger and less stable than the Antarctic Ice Sheet is now or in the past. Two approaches to this problem are presented as Part 1 and Part 2. In Part 1, the height of these former ice sheets is linked primarily to the strength of ice-bed coupling, which is deduced from glacial geology. It provides “snapshots” of ice sheets that give ice elevations independent of the past history and largely independent of ice-surface conditions, temperatures and the mass balance in particular. In Part 2, heights of former ice sheets depend on both their past history, particularly initial conditions, and changing surface conditions during the glaciation cycle. This provides a “motion picture” of ice sheets during the full cycle. The challenge is to establish a “mind meld” of these two approaches. They allow new perspectives on paleoglaciology.

High epibenthic foraminiferal δ13C in the Recent deep Arctic Ocean: Implications for ventilation and brine release during stadials

Low planktic and benthic δ18O and δ13C values in sediments from the Nordic seas of cold stadials of the last glaciation have been attributed to brines, formed similar to modern ones in the Arctic Ocean. To expand on the carbon isotopes of this hypothesis, I investigated benthic δ13C from the modern Arctic Ocean. I show that mean δ13C values of live epibenthic foraminifera from the deep Arctic basins are higher than mean δ13C values of upper slope epibenthic foraminifera. This agrees with mean high δ13C values of dissolved inorganic carbon (DIC) in Arctic Bottom Water (ABW), which are higher than mean δ13CDIC values from shallower water masses of mainly Atlantic origin. However, adjustments for oceanic 13C Suess depletion raise subsurface and intermediate water δ13CDIC values over ABW δ13CDIC ones. Accordingly, during preindustrial Holocene times, the δ13CDIC of ABW was as high or even higher than today but lower than the δ13CDIC of younger subsurface and intermediate water. If brine‐enriched water significantly ventilated ABW, brines should have had high δ13CDIC values. Analogously, high‐δ13CDIC brines may have been formed in the Nordic seas during warm interstadials. During cold stadials, when most of the Arctic Ocean was perennially sea ice covered, a cessation of high‐δ13CDIC brine rejection may have lowered δ13CDIC values of ABW, and ultimately the δ13CDIC in Nordic seas intermediate and deep water. So in contrast to the idea of enhanced brine formation during cold stadials, the results of this investigation imply that a cessation of brine rejection would be more likely.

The changing Arctic Ocean

The vast frozen surface of the Arctic Ocean, often referred to as the polar ice cap, separates the atmosphere from the underlying ocean, reflects incoming radiation from the sun back out to space, and provides a unique habitat for creatures ranging in size from microbes to 1500 pound (680 kg) polar bears. While the amount of Arctic sea ice has waxed and waned with the seasons for all of recorded human history, a large fraction would always persist throughout the year. This old, multiyear sea ice reached thicknesses of several meters and resisted melting, even in the polar summer (Eicken et al., 1995). As temperatures have increased in recent decades, much of this old ice has disappeared and been replaced by thinner first-year ice that melts earlier in spring (Maslanik et al., 2011). Earlier melt has allowed the surface ocean to absorb more solar radiation, delaying the onset of freeze-up in the fall. As a result, a positive feedback has developed in which rising Arctic Ocean temperatures have been accompanied by a markedly thinner and less extensive sea ice cover and an ever-lengthening open water season, which allows the ocean to absorb more radiation, further increasing the temperature (Perovich, 2011). Estimates suggest that the volume of sea ice in the Arctic today is only 20% of that present just a few decades ago (Laxon et al., 2013). Scientists are no longer debating if the Arctic Ocean will eventually become ice-free in summer—they are debating when, with estimates ranging from 20 to 50 years from now (Wang and Overland, 2009). For some, the loss of Arctic sea ice is a unique opportunity. Improved access to ice-free Arctic waters has sparked interest in the development of a viable Arctic commercial fishery. It has also fuelled interest from energy companies wishing to explore the shallow continental shelves for fossil hydrocarbons. Already, a more predictably ice-free Northwest Passage (Smith and Stephenson, 2013), a northern gateway between the Atlantic and Pacific oceans, has resulted in increased commercial ship traffic in Arctic waters. As all of these activities ramp up, it is essential that strategies are developed and implemented to minimize environmental degradation (Pew, 2013). For others, the loss of Arctic sea ice looks to be a disaster in the making, particularly for indigenous populations that rely on the ocean for their food. Access to the interior ice pack for hunting is hampered as shore-fast ice diminishes and pack ice retreats further from shore. Subsistence whaling is becoming more difficult as the sea ice historically used as a reliable hunting platform disappears (Struzik, 2012). Newly ice-free waters are encouraging the northward migration of non-native animals, such as killer whales, that compete with native species and indigenous humans for food (Higdon, Hauser and Ferguson, 2012). And coastal populations are rapidly losing valuable land to erosion (Solomon, 2005; Kittel et al., 2011) caused by intense wave action from frequent and stronger storms (Hakkinen, Proshutinsky and Ashik, 2008) with more intense storm surges (Vermaire et al., 2013) in increasingly ice-free waters. Yet the consequences of Arctic sea ice loss extend far beyond the subsistence and commercial activities of humans there will be profound ecological implications as well. Some of these are already apparent, such as habitat reduction for large mammals like ringed seals and polar bears that require stable sea ice in spring/ summer for reproduction and feeding (Stirling and Derocher, 2012). But most consequences are still playing out and our ability to either understand or predict them is limited. This is particularly true for the smallest Arctic inhabitants on which the rest of the marine ecosystem relies.

Dissolved aluminium and the silicon cycle in the Arctic Ocean

Concentrations of dissolved (0.2 µm filtered) aluminium (Al) have been determined for the first time in the Eurasian part of the Arctic Ocean over the entire water column during expedition ARK XXII/2 aboard R.V. Polarstern (2007). An unprecedented number of 666 samples was analysed for 44 stations along 5 ocean transects. Dissolved Al in surface layer water (SLW) was very low, close to 1 nM, with lowest SLW concentrations towards the Canadian part of the Arctic Ocean and higher values adjacent to and in the shelf seas. The low SLW concentrations indicate no or little influence from aeolian dust input. Dissolved Al showed a nutrient-type increase with depth up to 28 nM, but large differences existed between the different deep Arctic basins. The differences in concentrations of Al between water masses and basins could largely be related to the different origins of the water masses. In the SLW and intermediate water layers, Atlantic and Pacific inflows were of importance. Deep shelf convection appeared to influence the Al distribution in the deep Eurasian Basin. The Al distribution of the deep Makarov Basin provides evidence for Eurasian Basin water inflow into the deep Makarov Basin. A strong correlation between Al and Silicon (Si) was observed in all basins. This correlation and the nutrient-like profile indicate a strong biological influence on the cycling and distribution of Al. The biological influence can be direct by the incorporation of Al in biogenic silica, indirect by preferential scavenging of Al onto biogenic siliceous particles, or by a combination of both processes. From the slope of the overall Al–Si relationship in the intermediate water layer (AIDW; ~ 200–2000 m depth), an Al/Si ratio of 2.2 atoms Al per 1000 atoms Si was derived. This ratio is consistent with the range of previously reported Al/Si uptake ratio in biogenic opal frustules of diatoms. In the deepest waters (>2000 m depth) a steeper slope of the Al–Si relationship of 7.4 to 13 atoms Al per 1000 atoms Si likely results from entrainment of cold shelf water into the deep basins, carrying the signal of dissolution of terrigenous particles with a much higher Al:Si ratio of crustal abundance. Only a small enrichment with such crustal Al and Si component may readily account for the higher Al:Si slope in the deepest waters.

The Recent Decline of the Arctic Summer Sea-Ice Cover in the Context of Internal Climate Variability

By means of a 21-year simulation of a coupled regional pan-Arctic atmosphere-ocean-ice model for the 1980's and 1990's and comparison of the model results with SSM/I satellite-derived sea-ice concentrations, the patterns of maximum amplitude of interannual variability of the Arctic summer sea-ice cover are revealed. They are shown to concentrate beyond an area enclosed by an isopleth of barotropic planetary potential vorticity that marks the edge of the cyclonic rim current around the deep inner Arctic basin. It is argued that the propagation of the interannual variability signal farther into the inner Arctic basin is hindered by the dynamic isolation of upper Arctic Ocean and the high summer cloudiness usually appearing in the central Arctic. The thinning of the Arctic sea-ice cover in recent years is likely to be jointly responsible for its exceptionally strong decrease in summer 2007 when sea-ice decline was favored by anomalously high atmospheric pressure over the western Arctic Ocean, which can be regarded as a typical feature for years with low sea-ice extent. In addition, unusually low cloud cover appeared in summer 2007, which led to substantial warming of the upper ocean. It is hypothesized that the coincidence of several favorable factors for low sea-ice extent is responsible for this extreme event. Owing to the important role of internal climate variability in the recent decline of sea ice, a temporal return to previous conditions or stabilization at the current level can not be excluded just as further decline.

Seismic profiling across the Mendeleev Ridge at 82°N: evidence of continental crust

SUMMARY In year 2000, a ship-based expedition carried out a ca. 500-km-long geophysical profile (‘Arctic-2000’) across the Mendeleev Ridge at 82°N, from the Podvodnikov Basin to the Mendeleev Basin. A crustal-scale refraction experiment was combined with shallow reflection and complementary gravity measurements. Bottom samples were also collected. The reflection survey provided data on the depth to the seafloor and the thickness of the sedimentary cover, the latter being divisible in some areas into three layers (I, II and III) and reaching a maximum thickness of 3.5 km in the Podvodnikov Basin. The composition of the underlying bedrock was investigated by the refraction survey, with the uppermost unit (layer IV) having a velocity Vp of 5.0–5.4 km s−1 and a thickness of up to 4 km, being greatest below the ridge. A sharp increase in velocity marks the boundary to the underlying, layer V rocks (Vp= 5.9–6.5 km s−1), which are inferred to be crystalline basement, with a general thickness of 1–3 km, reaching 4 km below the axis of the ridge. Layer VI has a velocity Vp ranging downwards from 6.7 to 7.3 km s−1 over a thickness of 19–20 km below the arch of the ridge; this decreases to 5–16 km below the western slope towards the Podvodnikov Basin and 7–14 km beneath the eastern slope towards the Mendeleev Basin. These velocities may correspond to the composition of basic granulites. The lowermost unit above the Moho (layer VII), with a Vp of 7.4–7.8 km s−1, is thought to be of mixed crust–mantle composition, perhaps the result of underpating; it has a maximum thickness of 7 km beneath the ridge and thins rapidly to east and west. The base of the crustal section is taken at the boundary with a Vp= 7.9–8.0 km s−1, which defines the Moho. The overall thickness of the crust along the ‘Arctic-2000’ profile varies from a maximum of 32 km below the ridge to 13 km below the Mendeleev Basin and 20 km below the Podvodnikov Basin. Based on bottom sampling by piston coring and dredging, the lithologies of layer IV have been inferred to be dominated by carbonate and terrigenous sedimentary rocks, with some igneous intercalations. This evidence, taken together with the identification of the immediately underlying layer V with a Vp velocity of 5.9–6.5 km s−1 suggests that the Mendeleev Ridge may be composed of continental material that has been substantially altered during the development of the deep Arctic Basin and associated magmatism. The gentle gradient southward across the Kucherov Terrace to the continental shelf suggests that it is an extension of the Eurasian margin and can be compared with other margins with highly attenuated continental crust.

Scavenging of thorium isotopes in the Canada Basin of the Arctic Ocean

Investigations on the distribution of thorium isotopes in the water column have been utilized as powerful tracers to investigate the geochemical pathways of particle-reactive material in the ocean. We measured Th ( 228Th, 230Th, 232Th and 234Th) isotopes in samples collected along a horizontal profile from Barrow, AK to the Canada Basin as well as two vertical profiles in the Canada Basin. The suspended particle concentrations were also measured to investigate the role of suspended particulate matter (SPM) in Th scavenging. The SPM inventory is more than an order of magnitude higher than that reported for the Alpha Ridge ice station, indicating that the SPM inventories vary over an order of magnitude in the deep Arctic Basins. The conditional partition coefficient of Th isotopes between suspended particulate matter and dissolved phase generally appears to be higher than those in non-polar regions, and this is attributed to more reactive suspended particulate matter. The residence times of total 230Th are significantly lower than those calculated for the Alpha Ridge station, while higher than those found in the Eurasian Basin and Beaufort Sea in the Canadian Basin. The calculated residence time in all the deep basins of the Arctic Ocean varied between 13±1 and 45±1 years. A reversible scavenging model applied to the distribution of Th isotopes ( 230Th– 228Th in the deep waters and 234Th– 228Th in the upper 100 m) indicates that the forward removal rate constant is higher than the value reported for the Alpha Ridge while the reverse rate constant is comparable to the Alpha Ridge station suggesting faster removal onto particulate matter and slower desorption of Th from the particulate matter.

Different benthic size-compartments and their relationship to sediment chemistry in the deep Eurasian Arctic Ocean

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 199:31-41 (2000) - doi:10.3354/meps199031 Different benthic size-compartments and their relationship to sediment chemistry in the deep Eurasian Arctic Ocean Ingrid Kröncke1,*, Ann Vanreusel2, Magda Vincx2, Jutta Wollenburg3, Andreas Mackensen3, Gerd Liebezeit4, Brigitte Behrends4 1Senckenberg Institute, Department for Marine Research, Schleusenstr. 39a, 26382 Wilhelmshaven, Germany 2University of Gent, Zoology Institute, Marine Biology Section, K.L. Ledeganckstraat 35, 9000 Gent, Belgium 3Alfred-Wegener-Institute for Polar and Marine Research, POB 120161, 27515 Bremerhaven, Germany 4Terramare Research Centre, Schleusenstr. 1, 26382 Wilhelmshaven, Germany *E-mail: ingrid.kroencke@sam.terramare.de ABSTRACT: During the Arctic Expedition ARK VIII/3 (August to September 1991) with RV ŒPolarstern¹, the macrofauna, meiofauna, foraminifera, bacteria were sampled and sediment chemistry was determined at 13 stations along a transect from the Barents Sea slope across the deep Arctic Eurasian Basins towards the Lomonosov Ridge. Water depths ranged from 258 to 4427 m. In general, higher values for all benthic compartments as well as total organic carbon (TOC) and total hydrolysable amino acids (THAA) were recorded for the Barents Sea slope than for the deeper stations in the basins and the ridge slopes. The only significant correlation found was between macrofaunal abundance and depth. Bacterial and all faunal abundances as well as bacterial and macrofaunal biomass decreased significantly with increasing latitude. Although correlations between food items such as TOC and THAA and the fauna were weak, significant relationships between the bacterial and faunal size-classes reflected a distinct food chain typical of oligotrophic systems. The smallest compartments‹bacteria, meiofauna and foraminifera‹were more abundant than the macrofauna in the central Arctic Ocean. Macrofauna biomass dominated the biomass on the Barent Sea shelf and slope and on the Lomonosov Ridge, but bacterial biomass was equally or even more important on the Gakkel Ridge and in the deep basins. The results reveal the Eurasian Basin as one of the most oligotrophic regions in the World Ocean. Although primary production is low, recent foraminiferal investigations have revealed that benthic communities in the central Arctic Ocean are driven by the sedimentation of fresh organic material. Lateral transport of organic material from the Siberian shelf may provide additional food. The various benthic compartments compete either for fresh organic matter or for refractory material that is transferred to higher levels of the food chain by bacterial mineralisation. KEY WORDS: Arctic Ocean · Deep-sea · Bacteria · Foraminifera · Meiofauna · Macrofauna · TOC · Amino acids · Chloroplastic pigments · C-flux Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 199. Publication date: June 26, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Transport of 137Cs And 239,240Pu with Ice-rafted Debris in the Arctic Ocean

Ice rafting is the dominant mechanism responsible for the transport of fine-grained sediments from coastal zones to the deep Arctic Basin. Therefore, the drift of ice-rafted debris (IRD) could be a significant transport mechanism from the shelf to the deep basin for radionuclides originating from nuclear fuel cycle activities and released to coastal Arctic regions of the former Soviet Union. In this study, 28 samples of IRD collected from the Arctic ice pack during expeditions in 1989-95 were analyzed for 137Cs by gamma spectrometry and for 239Pu and 240Pu by thermal ionization mass spectrometry. 137Cs concentrations in the IRD ranged from less than 0.2 to 78 Bq/kg (dry weight basis). The two samples with the highest 137Cs concentrations were collected in the vicinity of Franz Josef Land, and their backward trajectories suggest origins in the Kara Sea. Among the lowest 137Cs values are seven measured on sediments entrained on the North American shelf in 1989 and 1995, and sampled on the shelf less than six months later. Concentrations of 239Pu + 240Pu ranged from about 0.02 to 1.8 Bq/kg. The two highest values came from samples collected in the central Canada Basin and near Spitsbergen; calculated backward trajectories suggest at least 14 years of circulation in the Canada Basin in the former case, and an origin near Severnaya Zemlya (at the Kara Sea/Laptev Sea boundary) in the latter case. While most of the IRD samples showed 240Pu/239Pu ratios near the mean global fallout value of 0.185, five of the samples had lower ratios, in the 0.119 to 0.166 range, indicative of mixtures of Pu from fallout and from the reprocessing of weapons-grade Pu. The backward trajectories of these five samples suggest origins in the Kara Sea or near Severnaya Zemlya.

Late Quaternary paleoceanography of the Eurasian Basin, Arctic Ocean

We reconstructed late Quaternary deep (3000–4100 m) and intermediate depth (1000–2500 m) paleoceanographic history of the Eurasian Basin, Arctic Ocean from ostracode assemblages in cores from the Lomonosov Ridge, Gakkel Ridge, Yermak Plateau, Morris Jesup Rise, and Amundsen and Makarov Basins obtained during the 1991 Polarstern cruise. Modern assemblages on ridges and plateaus between 1000 and 1500 m are characterized by abundant, relatively species‐rich benthic ostracode assemblages, in part, reflecting the influence of high organic productivity and inflowing Atlantic water. In contrast, deep Arctic Eurasian basin assemblages have low abundance and low diversity and are dominated by Krithe and Cytheropteron reflecting faunal exchange with the Greenland Sea via the Fram Strait. Major faunal changes occurred in the Arctic during the last glacial/interglacial transition and the Holocene. Low‐abundance, low‐diversity assemblages from the Lomonosov and Gakkel Ridges in the Eurasian Basin from the last glacial period have modern analogs in cold, low‐salinity, low‐nutrient Greenland Sea deep water; glacial assemblages from the deep Nansen and Amundsen Basins have modern analogs in the deep Canada Basin. During Termination 1 at intermediate depths, diversity and abundance increased coincident with increased biogenic sediment, reflecting increased organic productivity, reduced sea‐ice, and enhanced inflowing North Atlantic water. During deglaciation deep Nansen Basin assemblages were similar to those living today in the deep Greenland Sea, perhaps reflecting deepwater exchange via the Fram Strait. In the central Arctic, early Holocene faunas indicate weaker North Atlantic water inflow at middepths immediately following Termination 1, about 8500–7000 year B.P., followed by a period of strong Canada Basin water overflow across the Lomonosov Ridge into the Morris Jesup Rise area and central Arctic Ocean. Modern perennial sea‐ice cover evolved over the last 4000–5000 years. Late Quaternary faunal changes reflect benthic habitat changes most likely caused by changes in the import of cold, deepwater of Greenland Sea origin and warmer and middepth Atlantic water to the Eurasian Basin through the Fram Strait, and export of Arctic Ocean deepwater.Appendix Tables Al‐A4 are available on microfiche. Orderfrom the American Geophysical Union, 2000 Florida Avenue,N.W., Washington, DC 20009. Document P94‐002; $2.50.Payment must accompany order.

Distribution of 230Th and 231Pa in the water column in relation to the ventilation of the deep Arctic basins

Distribution of 230Th and 231Pa in the water column was measured in the Eurasian basins of the Arctic Ocean, i.e. the Makarov, Amundsen and Nansen basins. The profiles obtained were compared to one previously published from the Alpha Ridge (Bacon et al., Earth and Planetary Science Letters, 95 (1989) 15–22. The Amundsen and Nansen basins are similar in their 23OTh and 231Pa distribution, whereas in the Makarov Basin and at the Alpha Ridge these radionuclides are comparatively enriched in the deep basin. This may be due to low particle flux in conjunction with low water mass renewal rates. The 230Th concentration in water depth < 1000 m in the Makarov Basin is relatively low compared with the Alpha Ridge, which may be caused by an exchange of water masses between the Amundsen and Makarov basins. In the Eurasian Basin Deep Water, 230Th profiles can be explained both by a scavenging model and by a mixing-scavenging model. Enhanced particle load caused by resuspension may explain the increased scavenging in the Eurasian Basin Bottom Water. The differences in scavenging between the Norwegian-Greenland Sea and the Eurasian basins at present are not large enough to cause significant fractionation between 230Th and 231Pa.