Arctic lithosphere - A review "In 1991 an era of shipborne geophysical investigations of the Arctic began when the first MCS reflection lines and gravity data were collected (Jokat et al., 1992). In subsequent years a significant amount of modern geophysical data, including both seismic reflection and refraction data, were collected from several transects across the entire Eurasia Basin better characterizing the oceanic crust and its sedimentary cover (Coakley and Cochran, 1998; Jokat and Micksch, 2004; Urlaub et al., 2009; Weigelt and Jokat, 2001). Both the Nansen and the Amundsen basins contain significant volumes of sediment, derived mainly from the adjacent Eurasian margin. " [Show abstract] [Hide abstract] ABSTRACT: This article reviews the characteristics of Arctic lithosphere and the principal tectonic events which have shaped it. The current state-of-knowledge associated with the crust, crustal-scale discontinuities, and their ages, as well as knowledge of the lithosphere as a whole from geophysical data, permits the division of Arctic lithosphere into discrete domains.

Arctic continental lithosphere is diverse in age, composition, and structure. It has been affected by at least two periods of thermal overprinting associated with large volumes of magmatism, once in the Permo-Triassic and again in the Aptian. In addition, it was attenuated as the result of at least five phases of rifting (in the late Devonian-early Carboniferous, Permo-Triasic, Jurassic, Early Cretaceous, and Late Cretaceous-Cenozoic). Older phases of consolidation are associated with continental lithosphere and occurred through a series of continent-continent collisions in the Paleozoic. Jurassic and Cretaceous extensional phases are related to the dismembering of Pangea and Eurasia, and were concentrated in the Norway-Greenland region and Canadian-Alaskan Arctic. Large areas of submarine, hyperextended continental (?) lithosphere developed in parts of the Amerasia Basin. After continental breakup and the accretion of new oceanic lithosphere, the Eurasia and Canada basins were formed.

Full-text · Article · Jul 2014 Structure and evolution of the northern Barents–Kara Sea continental margin from integrated analysis of potential fields, bathymetry and sparse seismic data "Transect 1 (Fig. 12 ) extends from eastern Svalbard into the western Nansen Basin (Fig. 2). The top of the oceanic basement is well defined by MCS data (Jokat & Micksch 2004 ). The total sedimentary thickness gradually decreases towards the spreading axis. " [Show abstract] [Hide abstract] ABSTRACT: The northern Barents–Kara Sea continental margin is a poorly investigated area because of a permanent ice cover hampering seismic exploration. The available geological and geophysical data show that the magma-poor margin developed in response to early Cenozoic break-up and subsequent opening of the Arctic Eurasia Basin. In this study, a series of crustal-scale geotransects illustrating the architecture of the continental margin are constructed using sparse seismic reflection profiles and a gravity inversion method incorporating a thermal model of rifting.

The continental side of the northern Barents Sea margin is underlain by Palaeozoic–Early Mesozoic deep sedimentary basins separated from the oceanic side by the marginal uplift. A bathymetry analysis complements low-resolution seismic data to predict the sedimentary depocenters beyond the shelf break. These depocenters are associated with troughs, perpendicular to the shelf edge. The depocenter in front of the St. Anna Trough may contain a sedimentary section more than 4 km thick. The gravity correction for the effect of sedimentary cover was added to the inversion. This correction used an exponential density–depth function. The inversion supports a narrow and steep continent–ocean transition (COT; ca. 100 km). The conjugate Lomonosov Ridge margin is modelled using the same technique. Palaeoreconstructions were made to predict the break-up setting. The northern Barents Sea–Lomonosov Ridge rift system can be described as an initially narrow symmetric rift. A transitional zone of extreme thinning is assumed between the oldest spreading magnetic anomaly and the stretched continental crust.

The free-air gravity anomaly in the western part of the margin can be predicted by the upwelling divergent flow model implying the exhumation of the lower crust and the continental upper mantle within the COT. It is suggested that an episode of shear or oblique extension before breakup is required to explain the observed narrow symmetric conjugate margins in the Eurasia Basin. Full-text · Article · Jan 2011 Gravity crustal models and heat flow measurements for the Eurasia Basin, Arctic Ocean "7 ). However, a threshold value in spreading velocity , as suggested by Jokat and Micksch (2004), cannot be identified in the models' crustal structure although abrupt transitions in mechanisms of melt generation are detectable. " [Show abstract] [Hide abstract] ABSTRACT: The Gakkel Ridge in the Arctic Ocean with its adjacent Nansen and Amundsen Basins is a key region for the study of mantle melting and crustal generation at ultraslow spreading rates. We use free-air gravity anomalies in combination with seismic reflection and wide-angle data to compute 2-D crustal models for the Nansen and Amundsen Basins in the Arctic Ocean.

Despite the permanent pack-ice cover two geophysical transects cross both entire basins. This means that the complete basin geometry of the world’s slowest spreading system can be analysed in detail for the first time. Applying standard densities for the sediments and oceanic crystalline crust, the gravity models reveal an unexpected heterogeneous mantle with densities of 3.30×103, 3.20×103 and 3.10×103kg/m3 near the Gakkel Ridge. We interpret that the upper mantle heterogeneity mainly results from serpentinisation and thermal effects. The thickness of the oceanic crust is highly variable throughout both transects. Crustal thickness of less than 1km dominates in the oldest parts of both basins, increasing to a maximum value of 6km near the Gakkel Ridge. Along-axis heat flow is highly variable and heat flow amplitudes resemble those observed at fast or intermediate spreading ridges. Unexpectedly, high heat flow along the Amundsen transect exceeds predicted values from global cooling curves by more than 100%.

KeywordsGravity modelling-Heat flow-Crustal structure-Gakkel ridge-Mantle heterogeneity Full-text · Article · Dec 2010 Sedimentary structure of the Nansen and Amundsen basins, Arctic Ocean - Jokat - 2004 - Geophysical Research Letters - Wiley Online Library Sedimentary structure of the Nansen and Amundsen basins, Arctic Ocean Authors Alfred Wegener Institute for Polar Research, Columbusstrasse, Bremerhaven, Germany Geophysical Institute of the University of Karlsruhe, Karlsruhe, Germany Citing literature Abstract [1] The first continuous multichannel seismic profile to investigate the deeper structure of the western Nansen Basin was acquired in 2001 with the German and US icebreakers RV Polarstern and USCGC Healy. The 550-km long profile provides detailed insight into the deeper structure of this part of the Arctic Ocean. The sediments are up to 4.5 km thick close to the North Svalbard margin. The sediments thin continuously towards the north.

The topography of the oceanic basement in the Nansen Basin is very rough, and it crops out north of 84°43'N 22°05'E. Here, it prevents the deposition of thicker sediment units. The rift valley of the Gakkel Ridge, with water depths around 4830 m, was crossed at 85°36'N 16°41'E. The top of the basement along the profile can be fit by a theoretical subsidence curve. In the Amundsen Basin, sediments are only 1.7–2.0 km thick and oceanic basement younger than chron 13 shallows abruptly some 100 km north of the median valley. The contrasting basement structures are related in our interpretation to the differing depositional histories of the two basins, and to asymmetric spreading in Cenozoic times. Standard PDF (239.9 KB) 1.

Introduction [2] The Nansen and Amundsen basins were created by Cenozoic seafloor spreading along the Gakkel Ridge that continues to this day at ultra slow full spreading rates ranging from 1.3 cm/yr in the west to 0.63 cm/yr close to the Laptew Sea shelf. Bordered by Eurasia and Greenland, several researchers hypothesised that the Nansen Basin (NB) received a vast amount of sediments during the last 60 Myr from source areas along the continental shelves. Sparse bathymetric data support this view, by showing that only the highest seamounts of the mid-ocean Gakkel Ridge are free of sediments. However, very little is known about the deeper structure of the Nansen Basin. A small number of short seismic profiles acquired from ice floes east of the Yermak Plateau provided the first seismic information on the sediment thickness over the abyssal plain [ Kristoffersen and Husebye, 1985 ]. A seismic transect between the Morris Jesup Rise and Yermak Plateau imaged an atypical part of the basin [ Jokat et al., 1995b ].

Existing compilations of sediment thickness data are controlled by seismic data in some locations along the margins of the Nansen Basin [ Geissler and Jokat, 2004 ], but otherwise mainly rely on magnetic depth-to-source estimates [ Kovacs and Vogt, 1982 ]. Thus, opinions about the deeper structure of the Nansen Basin are mostly based on speculation. In the Amundsen Basin (AB), a first seismic profile was acquired in 1991 [ Figure 1 ; Jokat et al., 1995a ]. It shows that sedimentary thicknesses along the transect vary between 500 m close to the Gakkel Ridge and 3200 m adjacent to the Lomonosov Ridge. The profile reveals a well-defined succession of layers with respect to structure and seismic velocity. However, it is not clear if the single transect is typical for the AB and how its deeper basement and sedimentary structure compares to the NB. Figure 1.

Download Powerpoint slide Location map of profiles 99176, 20010100, 20010300 and 20010460 (red lines) as well as the existing seismic lines acquired in 1991 (black lines). Profile 20010100 starts close to the North Svalbard margin at 81°53'N 29°12'E. The line was terminated at 85°43'N 15°33'E at the northern shoulder of the mid-ocean Gakkel Ridge. The blue lines represent the magnetic ages [after Vogt et al., 1979 ]. Line 99176 was acquired with a 2700 m streamer, the lines in 2001 with a 500 m cable (48-channels, 6.25 m group spacing). In both cases a 24 l airgun array was used as seismic source. All data were standard processed into a CDP distance of 25 m.

Additionally 30 sonobuoys were deployed for accurate mapping of the velocity structure of the sediments and upper crust ( Figures 2 and 3 ). They are marked by black dots. Bathymetry is taken from the IBCAO grid [ Jakobsson et al., 2000 ]. The water depths are contoured in 500 m intervals. The free-air anomaly is taken from the Arctic Gravity Grid ( ). [3] The first continuous multi-channel seismic profile across the entire NB north of Svalbard was acquired in moderate ice conditions in 2001 by the research ice breakers RV Polarstern and USCGC Healy. The ships sailed in convoy, thus allowing continuous seismic profiling. In addition, a seismic profile across the AB was acquired in more difficult ice conditions almost along 70°E ( Figure 1 ). Both lines crossed the magnetic flow lines almost perpendicular to their strike direction and, thus, the general structure should be representative for the western and central Eurasia Basin. 2.

The Deeper Structure of the Western Nansen Basin [4] The NB transect consists of two profiles, 20010100 and 99176, acquired in 2001 and 1999, respectively ( Figure 2a ). The seismic velocities along the NB transect are calculated from measurements at 10 sonobuoys, ( Figures 1 , 2a, and 2b ) and are used to depth convert the entire section. Figure 2. Download Powerpoint slide Line drawing of seismic lines 99176 and 20010100. The profile 20010100 is almost 550 km long and covers the entire Nansen Basin north of Kvitoya. The deployment positions of the sonobuoys are marked and their generalized velocity depth functions are displayed ( add. material 1 and 2 ). For calculating the sediment corrected subsidence curve an age model based on aeromagnetic data is used [ Karasik, 1974 ; Vogt et al., 1979 ].

The subsidence curve is corrected for the sediment load after Allen and Allen [1990] . For the NB a mean density between 2.47 and 2.52 gcm−3, a porosity at the sea floor of 0.75 and a compaction coefficient of 0.5 was used. The subsidence curve is a least-squares fit for all values. A sensitivity test provides a ±150 m variation of this curve when using extreme values. [5] The water depth gradually increases from 3800 to 4050 m along line 20010100 until after 83°07'N 26°32'E ( Figure 2 , CDP6000) and then remains constant for 230 km. This plain is disrupted at 84°18'N 22°39'E (CDP12420) by a large seamount that shallows to a water depth of 3710 m.

The monotonous sediment pile is dammed at 84°43'N 22°05'E ( Figure 2 , CDP15000) by an increasing number of basement highs. Oceanic basement of the ridge crops out continuously north of CDP15000. The flank of the Gakkel Ridge shows some pronounced basement highs around 85°13'N 20°14'E ( Figure 2 , CDP18000). The floor of the median valley is at a water depth of 4830 m (85°36'N 16°41'E) and contains only a thin veneer of sediments over modest topography that may represent axial volcanoes. [6] The southernmost line of the transect, 99176, crosses the Svalbard margin at 30°E ( Figures 1 and 2 ) and shows a prograding wedge of glacial sediments created by repeated retreats and advances of glaciers/ice streams ( Geissler and Jokat, 2004 ), most likely Plio/Pleistocene in age.

North of the margin, the basement is poorly imaged for a distance of almost 130 km with both experimental set-ups ( Figure 2 ; 99176-CDP 6000; 20010100-CDP1-3000). In our interpretation, the noisy character of the seismic section is due to more coarse material that was deposited close to the Svalbard margin. In addition, recent slumping processes, e.g., at approximately 4.2 km depth between CDP1500 and 10000, dominate the deposition and therefore mask signals from the acoustic basement. The seismic signal characteristics change radically north of CDP 6000 ( Figure 2 ). Here, we suggest that finer material was deposited by turbidites and possibly also by current controlled processes. 3. The Deeper Structure of the Central Amundsen Basin [7] A total of 15 sonobuoys/RefTek stations were deployed in order to calculate the sediment thickness and yield velocities that are used to depth convert the seismic section ( Figure 3 ).

The Gakkel Ridge median valley is buried by sediments up to 1000 m thick ( Figure 3 , CDP1-1000). Oceanic basement of the ridge crops out at the seafloor for almost 100 km to the north (CDP1000-5000). After this, the top of the basement drops from 4.4 km to 6.5 km depth over a 15 km distance ( Figure 3 , CDP5000-5600). The mean thickness of sediments on the profile is around 1.7 km. Thicknesses greater than 2 km are only locally reached in the central part of the basin (CDP5600-2 km; CDP10000-2.2 km; CDP13500-2.8 km). The seismic units have a better signal-to-noise ratio than in the southern part of the NB, and they dip slightly southwards towards the Gakkel Ridge.

A very similar structure was found in the AB along a line some 200–350 km to the west [ Figure 1 ; Jokat et al., 1995a ]. Thus, this seems to be a typical basin structure for the AB. Towards the north, the basement shallows ( Figure 3 ; CDP14000-17500) in the direction of the Lomonosov Ridge ( Figure 1 ). Sonobuoy 2119 reveals velocities between 2.8 and 3.0 km/s for the top of the acoustic basement, which is not typical for 55 My old oceanic crust. Thus, the acoustic basement might still consist of metamorphosed or highly altered continental sedimentary rocks. This part of the line is located within a negative gravity anomaly that trends sub-parallel to Lomonosov Ridge. We interpret this zone to represent the continent-ocean transition between the ridge and the basin. Figure 3. Download Powerpoint slide Line drawing of profile 20010300 across the Amundsen Basin at 70°E ( add. material 3 ).

The same age model and parameters for calculating the sediment load as in Nansen Basin have been applied to determine the subsidence curve. Since most of the chrons along this line coincide with an abnormally shallow basement, the sediment thicknesses in the vicinity have been used to give an unbiased value for the influence of the sediment load. The seismic velocities were picked in close conjunction with the seismic reflection units. The hatched area close to Lomonosov Ridge marks a zone, which we interpret as the continent-ocean transition into the AB. 4. Differences in Structure of Both Basins [8] Water depths in the NB (4050 m) are significantly shallower than in the AB (4400 m). This difference has been reported by almost all researchers working in the Eurasia Basin and is explained by the minor amount of sediments received by AB during its evolution. The sedimentary structure in the oldest parts of the basins differs significantly. The sediment thickness in the NB is more than double that in the AB. In the case of the NB, the Siberian and Barents shelves are a vast source area to support a large sediment flux. The NB is dominated by sedimentary processes downslope and perpendicular to the slope of the shelf.

In contrast, only the Lomonosov Ridge acted for some time as a close sediment source area to the AB, as can be seen from the lower, southward dipping sedimentary units. According to the stratigraphic model of Jokat et al. [1995a] , the ridge subsided below sea level, and subaerial erosion of the Lomonosov Ridge ceased around 50 Myr. Since then, sediments reaching the AB had their sources on Greenland and the Laptew Sea and parts of the East Siberian Sea. However, since our profile over AB is rather distant from these sources, it received only the fine turbidtic material from these source areas. We explain the lowermost, southward dipping sedimentary layers in the AB, that are not observed in the NB, as a result of slower sediment deposition, and that the main transport directions were more along the strike than perpendicular to the basin axis. Lomonosov Ridge did not provide enough material just after break-up to significantly depress the oceanic crust close to its foot. Thus, apart from the rough oceanic basement due to the slow spreading, the AB was uniformly filled with sediments.

In the NB, higher sedimentation rates and sediment loading close to the margin depressed the oceanic crust more. However, this cannot explain why the oceanic basement in AB is almost at the same depth level between chrons 24 and 18 ( Figure 3 ). [9] In NB, basement shallows constantly from 7 km close to Svalbard to 3 km at the southern Gakkel Ridge flank. The pronounced relief of basement is mainly due to the consistently slow spreading rates of less than 1.1 cm/yr half rate since chron 20 [ Vogt et al., 1979 ]. In the AB, the basement topography is also very rough. In contrast to the NB, there is a sharp decrease in basement depth between chrons 12 and 18 ( Figure 3 ). Towards the north, the depth of the basement varies around a depth of approximately 6 km. Only locally, at CDP13500, it deepens to 7 km. These differences might be due to the basin's different sediment load.

To investigate this possibility, we applied the relationship between the depth of subsided oceanic basement and age [ Parsons and Sclater,1977 ] in combination with the sediment correction of Allen and Allen [1990] . While the curve in the NB fits reasonably well the general trend of the acoustic basement, the AB displays some deviation from the theoretical curve ( Figure 3 , CDP3000-5000). One of the reasons for the different shape of the basement topography might be that since chron 21 spreading rates in the Eurasia Basin decreased by a factor of two [from 1.1 cm/yr to 0.59 cm/yr; Vogt et al., 1979 ] and have remained low since chron 13. The transition to ultra-slow spreading rates most likely is responsible for the rough basement topography. [10] The reason for this anomalously fast shallowing compared to the NB might be a slight spreading asymmetry. According to Vogt et al.

[1979] the asymmetry is already visible for chrons 5 and 6, and becomes greater on older anomalies, such that spreading half rates in the AB were consistently 10–20% lower than in the NB. However, these estimates are not very accurate, since they are based on the slopes of their age-distance curves [ Vogt et al., 1979 ]. Karasik [1974] and Vogt et al. [1979] computed average spreading half-rates of 0.5, 0.32, 0.30, 0.59 and 1.1 cm/yr for the time intervals bounded by chrons 1, 5, 6, 13, 20 and 24, and are unable to distinguish between gradual and stepwise changes in spreading rate. To test a possible relationship between the asymmetry and the different basement topography in basement depth, the newly released Arctic Gravity grid ( ) as well as the existing sparse seismic data are inspected. In the free-air anomaly map ( Figure 1 ), a rift-parallel positive anomaly marks the area of shallow basement along both flanks.

No distinct differences in the width of the anomalies are visible for both basins. [11] A seismic profile across the AB more in the west also indicates a steep shallowing of the oceanic basement approximately 100 km north of Gakkel Ridge [ Jokat et al., 1995a ]. The westernmost seismic transect across the basin between the Morris Jesup Rise and Yermak Plateau does not show a similar structure to either NB or AB, and the depth of the basement follows a standard subsidence curve on both flanks [ Jokat et al., 1995b ]. However, the oceanic basement on this transect is younger than chron 13. [12] We speculate that our observations can be explained by the difference in spreading rates due to asymmetry, according to which a threshold value/range for the topographic style of the ridge changed abruptly on one flank (AB) and not on the other (NB). Recent geophysical experiments along Gakkel Ridge show such an along-strike change in tectono-magmatic style with spreading rate [ Jokat et al., 2003 ].

A singularity in the magmatic, magnetic and bathymetric expression of the Gakkel Ridge at 3°30'E [ Michael et al., 2003 ] appears to indicate the point where mechanisms of melt generation and extraction make an abrupt transition from ones more characteristic of intermediate/ultra-slow spreading to those more typical of ultra-slow spreading. Some kind of threshold is crossed as half spreading rates decrease from 0.69 mm/yr to 0.64 mm/yr or 5% [ Jokat et al., 2003 ]. We speculate that the same threshold value was crossed much earlier in the AB than in the NB, since the later spread faster. This effect of the asymmetric spreading is not visible for the older parts of either basin, since spreading rates are above this threshold. 5. Conclusions [13] The seismic profiles presented in this study provide the first complete view of the deeper structure of the Nansen and Amundsen basins in the western part of the Eurasia Basin. Along both lines, the basement topography was almost continuously imaged. Wide-angle data acquired in parallel with the seismic reflection data provide constraints on the seismic velocities in the sedimentary column.

Sediment thickness along line 20010100 (550-km long) varies between 4.5 km at 81°53'N close to the Svalbard margin and vanishes towards Gakkel Ridge. Seismic velocities range between 1.8 and 3.8 km/s in the sedimentary section. Less sediments were found in the central Amundsen Basin, with seismic velocities only up to 3 km/s. [14] The top of oceanic basement in the Nansen Basin is in good agreement with a theoretical subsidence curve corrected for sedimentary loading. This is also mostly true for the Amundsen Basin. Anomalously shallow oceanic basement in the Amundsen Basin around chron 13 appears over a short distance. The two existing seismic transects suggest that this might be a characteristic feature for the entire basin and not a local observation. The shape of the basement topography younger than chron 18 may be related to the crossing of a threshold value in spreading rate on one flank of an asymmetrically spreading system.

Previous magnetic studies have shown that the spreading in Amundsen Basin is 10–20% slower than in the Nansen Basin. This may have caused the different basement topography between chrons 1 and 18 in the slower spreading Amundsen Basin compared to the Nansen Basin. [15] The presented seismic profiles provide constraints for a reliable sediment thickness estimation in the Eurasia Basin. Any depth-to-source algorithm for magnetic data can be calibrated against our results. Acknowledgments [16] We thank the captains and crews of RV Polarstern and the USCGC Healy for their excellent support during the expedition. This analysis was funded by the Deutsche Forschungsgemeinschaft. Ancillary Arctic Ocean - Marine Biodiversity Wiki Marine Biodiversity Wiki Local environment Definition and basic facts The Arctic Ocean consists of a deep Ocean Basin, the broad shelves of the Barents, Kara, Laptev, East Siberian, Chukchi and Beaufort Seas, the White Sea, the Lincoln Sea and the narrow shelf off Canadian Arctic Archipelago and northern Greenland [1] .

Its spatial extent is constrained by the Fram Strait, the western limit of the Barents Sea, the Bering Strait and the Canadian Archipelago (Fig. 1). Figure 1: Arctic Ocean and its constituent seas. Red dotted line indicates the limits of the Arctic Ocean. Main basins of the deep Arctic Ocean basin: NB – Nansen Basin, AB – Amundsen Basin, MB – Makarov Basin, CB – Canada Basin. The bathymetric map is based on IBCAO model v.2. [2] Arctic Ocean and its constituent seas make around 3 % of the total ocean area and only about 1 % of the volume. The Arctic Ocean is the shallowest (mean depth 1361 m) and has significantly larger continental shelves than other oceans. The deep central Arctic Ocean Basin consists of four abyssal plains separated by submarine ridges. The abyssal plains make 12 %, while ridges make 16 % of the total area of the Arctic Ocean [1] . Lomonosov Ridge divide the Arctic Basin into two major sub-basins: Eurasian Basin and Amerasian Basin.

Gakkel Ridge subdivides Eurasian Basin into Amundsen and Nansen Basins, while the Alpha-Mendelejev Ridge subdivides Amerasian Basin into Canada and Makarov Basins (Fig. 1). Continental shelves encompass broad shelves of Eurasia and narrow shelves off North America and northern Greenland and altogether make up as much as 53 % of the total area of the Arctic Ocean. The Barents Sea is the largest of the epicontinental seas. The Laptev and East Siberian Seas are the shallowest (mean depths – 48 and 58 m, respectively). The Lincoln Sea and the adjacent area of the Northern Canadian Arctic Archipelago are the deepest portions of the Arcic Ocean shelve seas (mean depths – 257 and 338 m, respectively). Arctic Ocean is nearly landlocked, the only connections to the other oceans are: 1) Bering Strait (depth 45 m), 2) Canadian Archipelago (depth 220 m) and 3) Fram Strait (depth 2600 m).

Stratification and circulation Figure 2: Stratification of the Arctic Ocean water masses and seawater exchange with Pacific and Atlantic Oceans. (modified from MacDonald and Bewers [3] ) The stratification in the Arctic Ocean is maintained by the fresh water dynamics. The major freshwater sources include: rivers (Arctic Ocean receives around 11 % of the global river runoff discharge), precipitation and ice melt. The low- salinity water from Pacific Ocean inflows via Bering Strait. The low-saline, cold surface waters occupy the upper ~ 50 m and form PML (Polar Mixed Layer) (Fig. 2). Polar halocline (a layer of cold water with a steep salinity gradients) forms below PML and limits the exchange between surface and deep ocean water masses. Deeper water masses are formed by transformed Atlantic waters. The dominant features of the surface circulation are the clockwise Beaufort Gyre (over the Canadian Basin) and the Transpolar Drift (flowing from the Siberian coast towards the Fram Strait) (Fig. 3). The cold and low salinity Arctic surface waters are exported to the North Atlantic Ocean by East Greenland Current and through the Canadian Archipelago.

Warm and high salinity Atlantic water inflow the Arctic Ocean via the Fram Strait and the Barents Sea. Figure 3: Surface circulation and sea ice extent in the Arctic Ocean. A graphic produced by Philippe Rekacewicz, (UNEP/GRID-Arendal) and available at (Last visited March 11, 2009) Sea ice The polar halocline isolates surface water masses and sea-ice from warm deep water and thus acts as a determining factor for the existence of all-year sea ice cover. The ocean’s interior is all year covered with perennial (multi-year) pack ice, while seasonal (first-year) ice-cover is formed on the marginal seas from October to June. The spatial extent of sea ice varies from 14-15 mln km2 in March to 6-7 mln km2 in September (Fig. 3). The inflows of warm Atlantic waters usually keeps the southern part of the Barents sea ice-free year round. The average thickness of sea ice in the Arctic Ocean is about 3 m. Land-fast ice (up to 2 m in thickness) grows seaward from the shore and usually extends to the 20 m isobath.

Two forms of open water areas can occur in the winter ice-cover: 1) flaw lead systems – discontinuities between the land-fast ice and off-shore sea-ice or 2) polynyas – where ice is carried away by winds or currents or melted by the local convection of warm deep water masses. Primary production Primary productivity in the Arctic Ocean is relatively low, but can vary considerably between regions [4] . The productivity of the central Arctic Basin is among the lowest recorded in the world seas. The primary production is limited by the presence of multiyear ice- cover and very short growing season. The Siberian shelf seas also exhibit low productivity that is mostly nutrient-limited: the Siberian rivers are phosphorous-poor and the multi-year ice covers the shelf break, thus hindering the wind-driven upwelling of nutrient-rich deep waters at the shelf edge. In contrast, Barents Sea (that is kept ice-free by the inflows of warm Atlantic waters) and Chukchi Sea (that is supplied with nutrient-rich Bering Sea waters) can have primary production several times higher that the Arctic Ocean average (Table 1). Table 1.

Average annual primary production (av PP [gC m-2 y-1]) and total areal primary production (tot PP [106 t C]) in the Arctic Ocean and its constituent seas. After several authors compiled by Sakshaug [4] area >329 Specific biodiversity issues Arctic Ocean fauna is relatively young and comprise few endemics, it is mostly composed of species of Pacific or Atlantic affinity [5] .

The origin and evolution of the arctic biota was summarized by Dunton [5] : north polar sea originated as a large northern embayment of the North Pacific in the Mesozoic, in late Cretaceous large connections to subtropical Gulf of Mexico and to tropical Tethys sea were developed (that could result in some subtropical taxa migrations and may explain the origin of some Arctic species with phylogenetic affinities with warm-water taxa) about the end of Cretaceous the deep-water connection (and the exchange of bathyal and abyssal fauna) between Arctic Ocean and North Pacific was closed by continental plate movements, deep-water seaways between Arctic Ocean and North Atlantic (and the migration of Atlantic taxa) occurred by the end of Eocene, that coincided with a large cooling of the northern high latitude areas (temperature decreased to below 10˚C) successive cooling (temperatures dropped to below 5˚C in late Miocene) and the exchange with the North Atlantic led to the development of cool-temperate arctic biota of the Atlantic character in late Pliocene the Bering Land Bridge was breached and the shallow water passage between Pacific Ocean and Arctic Ocean opened – the arctic fauna was strongly supplemented by migrants from Pacific

 Ocean perennial ice cover developed in Pleistocene, when the Arctic was subjected to numerous glacial and interglacial intervals; during the maximum of the last major glaciation (18 ky ago) the Arctic shelves were either covered with permanent ice sheets or emergent and so the shallow water fauna was nearly completely eradicated.

Arctic shallow seas are now considered to remain in a phase of colonization (after the last deglaciation, i.e. for the last 6-14 ky) [5] . Figure 4: Numbers of invertebrate species recorded in the Arctic deep basin and in the eurasian Arctic marginal seas according to Sirenko [6] There are only few records of the fauna of the deep Arctic basin, and they are limited to a low number of samples. All the up-to date records show very low biomass (from 5 to 32 mg C m-2) and species diversity (from 1 to 11 species per 0.02 m2) of macrozoobenthos dwelling in arctic deep basins [7] . The fauna of the Siberian shallow shelf seas is also relatively impoverished as it is confronted with the low primary productivity and physical stress produced by massive riverine inflows. The productive waters of the Barents Sea host much higher numbers of species that other arctic regions [8] (Fig. 4).

The local hot-spots of both productivity and biodiversity in the Arctic include flaw lead systems, polynyas, areas of oceanographic fronts where mixing between cold polar and temperate waters occur and marginal ice zone [9] . Threats Figure 5: The decrease of Arctic Ocean multiyear sea ice: minimum sea-ice extent in 1982 and 2007, and projections of the future successive decrease of sea-ice extent within this century (according to the Arctic Climate Impact Assessment ). The graphic was produced by Hugo Ahlenius (UNEP/GRID-Arendal) and is available at . (Last visited March 11, 2009). The main threats to biodiversity in this region are generated by the potential effects of the global warming. Land-surface air temperatures increased with an average rate of 0.09˚C/decade in the Arctic form 1900 to 2003 that is greater than overall trend documented for the northern hemisphere (0.06˚C/decade) [10] . The warming is accompanied by the decrease of both spatial extent and thickness of the Arctic sea-ice cover (Fig. 5).

The possible effects of the climate warming on the marine ecosystems of the Arctic Ocean were summarized by Loeng et al [9] and include: poleward shift in species distributions – both southern limits of distribution of coldwater species and northern limits of warm water temperate species will move northward, increase of primary productivity (driven by the removal of light limitation after the disappearance of sea ice cover) and possible change of timing of the blooms – that can affect the pelago-benthic coupling processes, the dramatic decrease of the sea-ice extent will affect the sea ice ecosystems . The decline of the sea-ice extent will improve the accessibility of the Arctic Ocean to shipping and exploration of the natural resources (extensive fishing and exploration of large oil and gas reserves). That will certainly largely increase the anthropogenic pressure on the relatively pristine ecosystems of the Arctic Oceans. References ↑ 1.0 1.1 Jakobsson, M., 2002, Hypsometry and volume of the Arctic Ocean and its constituent's seas, Geochemistry Geophysics Geosystems, v. 3, no. 2. ↑ Jakobsson, M., R. Macnab, L. Mayer, R. Anderson, M. Edwards, J.

Hatzky, H. W. Schenke, and P. Johnson (2008), An improved bathymetric portrayal of the Arctic Ocean: Implications for ocean modeling and geological, geophysical and oceanographic analyses, Geophysical Research Letters, DOI: doi: 10.1029/2008gl033520. ↑ MacDonald, R.W., Bewers, J.M., 1996, Contaminants in the arctic marine environment: priorities for protection. ICES J Mar Sci 53: 537-563 ↑ 4.0 4.1 Sakshaug, E., 2003, Primary and Secondary Production in the Arctic Seas. in: Stein, R., MacDonald, R.W., The organic carbon cycle in the Arctic Ocean. Springer, pp 57-81 ↑ 5.0 5.1 5.2 Dunton, K., 1992, Arctic biogeography: the paradox of the marine benthic fauna and flora. TREE 7, 183-189.

↑ Sirenko, B.I., 2001, List of species of free-living invertebrates of eurasian arctic seas and adjacent deep waters. Explorations of the fauna of the seas 51(59) St Petersburg, 1-129. ↑ Klages, M., Boetius A., Christensen, J.P., Deubel, H., Piepenburg, D., Schewe, I., Soltwedel, T., 2003, The benthos of Arctic Seas and its role for the organic carbon cycle at the seafloor. in: Stein, R., MacDonald, R.W., The organic carbon cycle in the Arctic Ocean. Springer, pp 139-167. ↑ Sirenko, B.I., 2001, List of species of free-living invertebrates of eurasian arctic seas and adjacent deep waters. Explorations of the fauna of the seas 51(59) St Petersburg, 1-129.

↑ 9.0 9.1 Loeng H, Brander K, Carmack E, Denisenko S and others (2005) Marine systems. In: Symon C, Arris L, Heal B (eds) Arctic climate impact assessment, ACIA. Cambridge University Press, Cambridge, p 453–538. available at ↑ McBean, G, Alekseev, G., Chen, D., Forland, E. and others (2005) Arctic Climate: Past and Present. In: Symon C, Arris L, Heal B (eds) Arctic climate impact assessment, ACIA. Cambridge University Press, Cambridge, p 21- 60. available at See also

Question: The Nansen Basin contains the lowest part of which ocean?
Answer: Arctic ocean