Mendeleev Ridge Research Articles

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

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

Atlantic water circulation over the Mendeleev Ridge and Chukchi Borderland from thermohaline intrusions and water mass properties

Hydrographic and tracer data from 2002 illustrate Atlantic water pathways and variability in the Mendeleev Ridge and Chukchi Borderland (CBLMR) region of the Arctic Ocean. Thermohaline double diffusive intrusions (zigzags) dominate both the Fram Strait (FSBW) and Barents Sea Branch Waters (BSBW) in the region. We show that details of the zigzags' temperature‐salinity structure partially describe the water masses forming the intrusions. Furthermore, as confirmed by chemical tracers, the zigzags' peaks contain the least altered water, allowing assessment of the temporal history of the Atlantic waters. Whilst the FSBW shows the 1990s warming and then a slight cooling, the BSBW has continuously cooled and freshened over a similar time period. The newest boundary current waters are found west of the Mendeleev Ridge in 2002. Additionally, we show the zigzag structures can fingerprint various water masses, including the boundary current. Using this, tracer data and the advection of the 1990s warming, we conclude the strongly topographically steered boundary current, order 50 km wide and found between the 1500 m and 2500 m isobaths, crosses the Mendeleev Ridge north of 80°N, loops south around the Chukchi Abyssal Plain and north around the Chukchi Rise, with the 1990s warming having reached the northern (but not the southern) Northwind Ridge by 2002. Pacific waters influence the Atlantic layers near the shelf and over the Chukchi Rise. The Northwind Abyssal Plain is comparatively stagnant, being ventilated only slowly from the north. There is no evidence of significant boundary current flow through the Chukchi Gap.

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.

Nematode and macrofaunal diversity in central Arctic Ocean benthos

Deep-sea diversity studies have revealed intriguing patterns on both local and regional scales, but there is insufficient evidence with which to evaluate these trends in the Arctic Ocean basin. We collected data on the diversity of benthic macrofauna and meiofaunal nematodes along two transects from the shelf margin to the North Pole. Contrary to prevailing paradigms, there was no change in diversity with depth between 1000 and 4273 m. There was a trend, however, toward reduced taxonomic richness for both macrofauna and nematodes with increasing latitude. Regional ( β-) diversity differences were not observed for nematodes, but significant contrasts in Bray–Curtis similarity-based community structure of macrofauna were seen between the Eurasian and Amerasian Basins, as well as between the Lomonosov and Mendeleev Ridges. Since fauna within the deep Arctic Ocean appear to represent a single species pool, we suggest that both local ( α-) and β-diversity may be determined by ecological processes in the Arctic, and are not the consequence of historical or evolutionary processes. Furthermore, insights gained from studies of benthic–pelagic coupling, known to play a significant role in determining benthic community structure and function at high latitudes, may also be useful in investigations of Arctic biodiversity. This model may be valuable in designing future studies of biodiversity, and for predicting potential impacts of climate change on diversity patterns.

Freshwater and brine behaviors in the Arctic Ocean deduced from historical data of δ18O and alkalinity (1929–2002 A.D.)

The long‐term data sets of total alkalinity (TA) (1929–2002 A.D.) and δ18O (1966–2002 A.D.) are used to investigate freshwater and brine distributions in the Arctic Ocean. Fractions of sea ice meltwater and other freshwaters (OF) (precipitation, river runoff, and freshwater carried by Pacific water implied as salinity deficit) are calculated on the basis of salinity‐TA and salinity‐δ18O relationships. Rejected brine during sea ice growth resides in surface water in the central Arctic Ocean, while net melting is found along the surface flow of water from the Pacific and Atlantic oceans. Distribution of OF at 10 m water depth suggests that Russian runoff leaves the shelf mainly west of the Mendeleyev Ridge, enters into the deep basin, and exits from the ocean through the western part of Fram Strait. The influence of Mackenzie River water is limited in the region and in depth. Accumulation of freshwater in the Canadian Basin is caused by deep penetration of OF with brine, indicating the transport of freshwater by shelf‐derived water. The major origin of shelf‐derived water entering into the upper halocline layer in the Canadian Basin should be the Chukchi and East Siberian Sea shelves, and the main freshwater sources are the salinity deficit of Pacific water and/or Russian runoff. An increase in OF inventory accompanied by an increase in brine content may suggest an increase of the shelf‐derived water supply into the western Canadian Basin in anticyclonic years.

Ba/Ca in Neogloboquadrina pachyderma as an indicator of deglacial meltwater discharge into the western Arctic Ocean

Down‐core samples of the planktonic foraminifer Neogloboquadrina pachyderma sinistral from the Mendeleyev Ridge in the western Arctic Ocean have been analyzed for Ba/Ca and δ18O. The apparent distribution coefficient for N. pachyderma sin. is estimated at DBa = 0.22 ± 0.02. A meltwater event is identified at around 11.8 14C kyr BP and is coincident with elevated Ba/Ca ratios. The barium enrichment is believed to be the result of enhanced weathering and erosion following glaciation. Additionally, barium may have desorbed from shelf sediments as sea level rose. Changes in Ba/Ca correlate with the retreat of the Laurentide Ice Sheet and the evolution of the Mackenzie River drainage basin. Therefore maximum Ba/Ca in Arctic surface waters at 11.8 ka may be indicative of an increase in the export of freshwater from the Arctic to the North Atlantic, potentially contributing to the onset of the Younger Dryas. This work suggests that Ba/Ca in planktonic foraminifera may be a useful indicator of the timing and processes associated with deglaciation.

Contrasting glacial/interglacial regimes in the western Arctic Ocean as exemplified by a sedimentary record from the Mendeleev Ridge

Distinct cyclicity in lithology and microfaunal distribution in sediment cores from the Mendeleev Ridge in the western Arctic Ocean (water depths ca. 1.5 km) reflects contrasting glacial/interglacial sedimentary patterns. We conclude that during major glaciations extremely thick pack ice or ice shelves covered the western Arctic Ocean and its circulation was restricted in comparison with interglacial, modern-type conditions. Glacier collapse events are marked in sediment cores by increased contents of ice-rafted debris, notably by spikes of detrital carbonates and iron oxide grains from the Canadian Arctic Archipelago. Composition of foraminiferal calcite δ 18O and δ 13C also shows strong cyclicity indicating changes in freshwater balance and/or ventilation rates of the Arctic Ocean. Light stable isotopic spikes characterize deglacial events such as the last deglaciation at ca. 12 14C kyr BP. The prolonged period with low δ 18O and δ 13C values and elevated contents of iron oxide grains from the Canadian Archipelago in the lower part of the Mendeleev Ridge record is interpreted to signify the pooling of freshwater in the Amerasia Basin, possibly in relation to an extended glaciation in arctic North America. Unique benthic foraminiferal events provide a means for an independent stratigraphic correlation of sedimentary records from the Mendeleev Ridge and other mid-depth locations throughout the Arctic Ocean such as the Northwind and Lomonosov Ridges. This correlation demonstrates the disparity of existing age models and underscores the need to establish a definitive chronostratigraphy for Arctic Ocean sediments.

Shelf–basin interactions in the Arctic Ocean based on 210Pb and Ra isotope tracer distributions

Measurements of the naturally occurring radionuclides, 210Po, 210Pb, 228Ra and 226Ra were conducted on water and sediment samples collected during the Arctic Ocean Section (1994) and Beaufort Sea (1995) expeditions and are used to assess circulation and shelf/slope scavenging in the Western Arctic Ocean. 228Ra concentrations are higher over the continental shelves of the Chukchi and Beaufort Seas compared to interior waters, consistent with a shelf source for 228Ra. However, 228Ra/ 226Ra ratios for Pacific-origin water have a lower end member value (ca. 1.0) compared to literature values of 3.6 for end member 228Ra/ 226Ra ratios for Atlantic-origin water passing across the shelf of the Laptev Sea. The shift during the 1990s in the position of the axis of the Trans-Polar Drift and the consequent change in the pathways for the flow of freshwater from the Russian continental shelves is reflected in a shift of elevated 228Ra concentrations in surface water from the Lomonosov Ridge to the Mendeleyev Ridge. 210Pb water column distributions in the Eurasian Basin are similar to those measured in the North Atlantic and indicate scavenging residence times of 15–20 yr. 210Pb concentrations are lower in intermediate waters of the Makarov and Canada Basins, because of enhanced scavenging at the continental margin. The lowest 210Pb concentrations (<0.2 dpm/100 l) were in the halocline and are a result of scavenging over the continental shelves. Low 210Pb levels in upper halocline waters of Pacific origin extend hundreds of kilometers into the interior of the Makarov and Canada Basins and reflect scavenging over the Chukchi and Beaufort Sea shelves. Low 210Pb levels in lower halocline waters of Atlantic origin reflect both scavenging over the Barents and Kara Sea shelves and removal during boundary current transport along the continental margin in the Makarov and Canada Basins. Focusing factors calculated from comparisons of 210Pb sediment inventories with 210Pb water column deficits confirmed the net scavenging of 210Pb at the continental margins.

SHE Analysis for Biozonation of Benthic Foraminiferal Assemblages from Western Arctic Ocean

Other| June 01, 2002 SHE Analysis for Biozonation of Benthic Foraminiferal Assemblages from Western Arctic Ocean LISA E. OSTERMAN; LISA E. OSTERMAN 1U.S. Geological Survey, MS 926A, Reston, VA 20192 Search for other works by this author on: GSW Google Scholar MARTIN A. BUZAS; MARTIN A. BUZAS 2Smithsonian Institution, Washington, D.C., 20560-0121 Search for other works by this author on: GSW Google Scholar LEE-ANN C. HAYEK LEE-ANN C. HAYEK 3Smithsonian Institution, Washington, D.C., 20560-0136 Search for other works by this author on: GSW Google Scholar Author and Article Information LISA E. OSTERMAN 1U.S. Geological Survey, MS 926A, Reston, VA 20192 MARTIN A. BUZAS 2Smithsonian Institution, Washington, D.C., 20560-0121 LEE-ANN C. HAYEK 3Smithsonian Institution, Washington, D.C., 20560-0136 Publisher: SEPM Society for Sedimentary Geology Accepted: 29 Nov 2001 First Online: 03 Mar 2017 Online ISSN: 1938-5323 Print ISSN: 0883-1351 Society for Sedimentary Geology PALAIOS (2002) 17 (3): 297–303. https://doi.org/10.1669/0883-1351(2002)017<0297:SAFBOB>2.0.CO;2 Article history Accepted: 29 Nov 2001 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation LISA E. OSTERMAN, MARTIN A. BUZAS, LEE-ANN C. HAYEK; SHE Analysis for Biozonation of Benthic Foraminiferal Assemblages from Western Arctic Ocean. PALAIOS 2002;; 17 (3): 297–303. doi: https://doi.org/10.1669/0883-1351(2002)017<0297:SAFBOB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract Benthic foraminiferal species abundance in samples from three Mendeleyev Ridge box cores were analyzed by cluster analysis and the newer method of SHE analysis. Previously, the latter technique only has been used on foraminiferal data from depth transects of modern surface sediment samples. Unlike most methods, which initially compare all possible pairs of samples, the SHE procedure results in a linear pattern if a sequence of samples are from the same statistical distribution. A change in slope indicates a statistical change in community structure and/or a change in species composition.The research reported herein is the first application of SHE for the purpose of identifying biozones in sediment core samples for the purpose of stratigraphic correlation. Both cluster analysis and the SHE method provided zonation within cores. However, the cluster method often produced clusters that were difficult to identify and also contained a mixture of samples without stratigraphic continuity. In contrast, SHE resulted in easily identifiable biozones and ensured temporal continuity within them. In general, the cluster analysis produced more zones than the SHE analysis. About 87% of the cluster zones and 64% of the SHE zones were correlated across more than one core. The average age range for correlated biozone boundaries among the three cores, based on radiocarbon dates, was 821 years using cluster analysis and 296 years using SHE. The sequential nature of the analysis, ease in choosing boundaries, and correlation of these boundaries across cores makes SHE the preferred technique. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Wind‐driven transport pathways for Eurasian Arctic river discharge

Distributions of temperature, salinity, and barium in near‐surface waters (depth ≤ 50 m) of the Laptev Sea and adjacent areas of the Arctic Ocean are presented for the summers of 1993, 1995, and 1996. The tracer data indicate that while fluvial discharge was largely confined to the shelf region of the Laptev Sea in the summer of 1993, surface waters containing a significant fluvial component extended beyond the shelf break and over the slope and basin areas north of the Laptev Sea in the summers of 1995 and 1996. These distributions of fluvial discharge are consistent with local winds and suggest two principal pathways by which river waters can enter the central Arctic basins from the Laptev Sea. When southerly to southeasterly wind conditions prevail, river waters are transported northward beyond the shelf break and over the slope and adjacent basin areas. These waters can then enter the interior Arctic Ocean via upper layer flow in the vicinity of the Lomonosov Ridge. Under other wind conditions, river waters are steered primarily along the inner Laptev shelf and into the East Siberian Sea as part of the predominantly eastward coastal current system. These waters then appear to cross the shelf and enter the interior Arctic Ocean via upper layer flow aligned roughly along the Mendeleyev Ridge. The extent to which either pathway is favored in a given year is largely determined by local wind patterns during the summer months, when fluvial discharge is greatest and shelf waters are at the lowest salinity of their annual cycle.

River runoff, sea ice meltwater, and Pacific water distribution and mean residence times in the Arctic Ocean

Hydrographic and tracer data collected during ARK IV/3 (FS Polarstern in 1987), ARCTIC91 (IB Oden), and AOS94 (CCGS Louis S. St‐Laurent) expeditions reveal the evolution of the near‐surface waters in the Arctic Ocean during the late 1980s and early 1990s. Salinity, nutrients, dissolved oxygen, and δ18O data are used to quantify the components of Arctic freshwater: river runoff, sea ice meltwater, and Pacific water. The calculated river runoff fractions suggest that in 1994 a large portion of water from the Pechora, Ob, Yenisey, Kotuy, and Lena Rivers did not flow off the shelf closest to their river deltas, but remained on the shelf and traveled via cyclonic circulation into the Laptev and East Siberian Seas. River runoff flowed off the shelf at the Lomonosov Ridge and most left the shelf at the Mendeleyev Ridge. ARCTIC91 and AOS94 Pacific water fraction estimates of Upper Halocline Water, the traditionally defined core of the Pacific water mass, document a decrease in extent compared to historical data. The front between Atlantic water and Pacific water shifted from the Lomonosov Ridge location in 1991 to the Mendeleyev Ridge in 1994. The relative age structure of the upper waters is described by using the 3H‐3He age. The mean 3H‐3He age measured in the halocline within the salinity surface of 33.1±0.3 is 4.3±1.7 years and that for the 34.2±0.2 salinity surface is 9.6±4.6 years. Lateral variations in the relative age structure within the halocline and Atlantic water support the well‐known cyclonic boundary current circulation.

Recent Environmental Changes in ihe Arctic: A Review

Numerous recent observations indicate that the Arctic is undergoing a significant change. In the last decade, the hydrography of the Arctic Ocean has shifted, and the atmospheric circulation has undergone a change from the lower stratosphere to the surface. Typically the eastern Arctic Ocean, on the European side of the Lomonosov Ridge, is dominated by water of Atlantic origin. A cold halocline of varying thickness overlies the warmer Atlantic water and isolates it from the sea ice and surface mixed layer. The western Arctic Ocean, on the North American side of the Lomonosov Ridge, is characterized by an added layer of water from the Pacific immediately below the surface mixed layer. Data collected during several cruises from 1991 to 1995 indicate that in the 1990s the boundary between these eastern and western halocline types shifted from a position roughly parallel to the Lomonosov Ridge to near alignment with the Alpha and Mendeleyev Ridges. The Atlantic Water temperature has also increased, and the cold halocline has become thinner. The change has resulted in increased surface salinity in the Makarov Basin. Recent results suggest that the change also includes decreased surface salinity and greater summer ice melt in the Beaufort Sea. Atmospheric pressure fields and ice drift data show that the whole patterns of atmospheric pressure and ice drift for the early 1990s were shifted counterclockwise 40°-60° from earlier patterns. The shift in atmospheric circulation seems related to the Arctic Oscillation in the Northern Hemisphere atmospheric pressure pattern. The changes in the ocean circulation, ice drift, air temperatures, and permafrost can be explained as responses to the Arctic Oscillation, as can changes in air temperatures over the Russian Arctic.

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

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

Late Pleistocene and Holocene meltwater events in the western Arctic Ocean

Accelerator mass spectrometer 14 C dated stable isotope data from Neogloboquadrina pachyerma in cores raised from the Mendeleyev Ridge and slope provide evidence for significant influx of meltwater to the western Arctic Ocean during the early part of marine oxygen isotope stage 1 (OIS 1) and during several intervals within OIS 3. The strongest OIS 3 meltwater event occurred before ca. 45 ka (conventional radiocarbon age) and was probably related to the deglaciation at the beginning of OIS 3. Major meltwater input to the western Arctic Ocean during the last deglaciation coincides closely with the maximum rate of global sea-level rise as determined from the Barbados sea-level record, demonstrating a strong link between the global record and changes in the central Arctic Ocean. OIS 2, which includes the last glacial maximum, is very condensed or absent in the cores. Abundance and δ 13 C values for N. pachyderma in the middle part of OIS 3 are similar to modern values, indicating high productivity and seasonal ice-free areas along the Arctic margin at that time. These records indicate that the Arctic Ocean was a source of heat and moisture to the northern polar atmosphere during parts of OIS 3.

Hydrography of the upper Arctic Ocean measured from the nuclear submarine U.S.S. Pargo

In 1993, the USS Pargo made the first civilian oceanographic submarine cruise in the Arctic Ocean. The hydrographic measurements provide a unique synoptic description of the upper Arctic Ocean. The results indicate that the influence of Atlantic Water has increased relative to existing climatologies. The front between waters with an Atlantic versus Pacific character has shifted from over the Lomonosov Ridge to roughly over the Alpha and Mendeleyev Ridges. Warm cores of Atlantic Water are observed over the Lomonosov and Mendeleyev Ridges. These data indicate a fundamental change in the circulation of the Arctic Ocean beginning in the early 1990s.

129I and 137Cs tracer measurements in the Arctic Ocean

Levels of 137Cs and 129I measured in 1993 in the Barents and Kara Seas and western Arctic Ocean were in excess of fallout levels, owing mainly to inputs of Atlantic-origin water contaminated by discharges from the Sellafield (U.K.) and La Hague (France) nuclear reprocessing plants. A pronounced concentration gradient in 129I extended across the Arctic Ocean, with levels in surface mixed layer water decreasing from 100–200×107 at/l in the Barents and Kara Seas to 80×107at/l in the Makarov Basin and then to <3×107at/l in the Canada Basin. Elevated levels of 129I and 137Cs were measured in halocline and Atlantic layer water in the Makarov and Canada Basins, reflecting the labelling of these water masses by discharges from the European reprocessing plants. In contrast, surface mixed layer and upper halocline water layers in the Canada Basin were characterized by relatively low 129I and 137Cs signals associated with Pacific-origin water contaminated mainly by nuclear weapons fallout. A “front” between Atlantic-origin and Pacific-origin water located over the Mendeleyev Ridge was delineated by changes of an order of magnitude in the 129I and 137Cs concentrations. The 129I and 137Cs input functions for Atlantic water entering the Arctic Ocean were estimated from fallout and Chernobyl inputs and reconstructed discharge records for Sellafield and La Hague using transfer factors for a reference location at 60°N in the Norwegian Coastal Current. 129I/137Cs tracer measurements were then used in a simple mixing/advection model to estimate “transit times” to sampling locations in the Arctic Ocean. By 1993, the dynamic range of the model was sufficient to establish transit times of 1–2 yr from the Norwegian Coastal Current to the southern Barents Sea, 2–4yr to the western Kara Sea and 5–6yr to the deep waters (>330m water depth) of the Novaya Zemlya Trough. Although the dynamic range of the model in 1993 was sufficient only to establish a lower limit of 6yr for transit to the Makarov and Canada Basins, the continuing increase in 129I releases from La Hague will have expanded the transit time range of the 129I/137Cs tracer model to 10 yr by 1997.

Barium as a tracer of Arctic halocline and river waters

Dissolved barium (Ba) concentrations are reported for water samples collected during six oceanographic cruises to the Arctic in 1993 and during the 1994 Arctic Ocean Section. Upper Arctic (<200 m) values ranged widely (19–168 nmol Ba 1−1) in a manner geographically consistent with identified sources and sinks. Although surface waters in the eastern part of the Bering Strait and the southern Chukchi Sea were highly enriched in Ba (reflecting the influence of the Yukon River), surface waters over most of the Chukchi Sea were generally depleted in Ba due to the intense seasonal biological activity which occurs in the area. The highest Ba concentrations (>-75 nmol Ba 1−1) observed in the surface mixed layer of the Arctic interior (i.e. beyond the 200 m isobath) occurred in the Canada Basin. As a result of the biological removal of Ba from surface waters of the Chukchi Sea and the tendency of the Bering inflow to enter the Arctic interior at depths below the relatively fresher mixed layer, we hypothesize that the Mackenzie River is the dominant source of the high Ba observed in the surface waters of the Canada Basin.Features characterized by local salinity minima and Ba maxima in surface waters down to depths of 30–50 m were observed over the Arlis Plateau and Mendeleyev Ridge in both 1993 and 1994, and over the Makarov Basin and Lomonosov Ridge in 1994. The physical and chemical properties of these waters suggest that they have been significantly influenced by fluvial discharge. It could not be determined from available data whether these features arose from offshore transport of Eurasian river discharge from the Russian Arctic seas, meanders of the Mackenzie River-influenced Beaufort Gyre, or a branch of Bering inflow that crossed the Chukchi shelf and entrained river water from North American and/or Eurasian sources.In both 1993 and 1994 Ba concentrations in the upper halocline layer (as defined by a core salinity of 33·1) ranged between 42 and 77 nmol Ba 1−1, with highest values observed in the Canada Basin. The Ba maximum and the related nutrient maxima and oxygen minimum that characterize the upper halocline in the Canada Basin suggest sources in the Chukchi and Beaufort seas, and that the mechanism by which these waters are formed is linked with the influx of water through the Bering Strait. The Ba distributions for 1993 and 1994 delineate the lateral extent of upper halocline waters with Canada Basin character and show them to be chemically distinct from upper halocline waters in the Eurasian Basin. The front separating these water types ran roughly along the Mendeleyev and Alpha ridges, consistent with reports that the boundary between Atlantic and Pacific water mass assemblies in the Arctic has recently been displaced from its historical position over the Lomonosov Ridge. Lateral gradients in both Ba and the NO parameter were observed within the lower halocline layer (defined by an NO minimum and core salinity of 34·2–34·4) along the 1994 Arctic Ocean Section, indicating that lower halocline waters receive additional inputs as they transit to the Canadian Basin from their region of formation in the Eurasian sector of the Arctic.

Physical and geochemical properties across the Atlantic/Pacific water mass front in the southern Canadian Basin

Temperature, salinity, nutrients, oxygen, and halocarbon data collected in the Arctic Ocean reveal a frontal structure previously unrecognized in the hydrography of the Canadian Basin. Samples were collected on a 1300‐km section extending from the Beaufort Sea in the Canada Basin to the East Siberian Sea in the Makarov Basin. These data, collected in 1993 aboard the CCGS Henry Larsen, reveal a lateral boundary between water masses of Atlantic and Pacific origin. The term “water mass assembly” is introduced to describe the basic arrangement or vertical stacking of water masses found in the Arctic Ocean, recognizing that water mass components within each assembly may differ from basin to basin. Using historical data, two primary water mass assemblies are defined, each consisting of three layers: an upper layer, an Atlantic layer, and a deep layer. These two assemblies are marked by important differences. One assembly, here defined as the Western Arctic (WA) assembly, is characterized by an upper layer of relatively fresh, high‐nutrient water of Pacific origin; below this, by an Atlantic layer with a core temperature generally below 0.5°C; and, finally, by a deep layer of higher salinities and colder temperatures (about −0.5°C) than found in the overlying Atlantic layer. The second assembly, here defined as Eastern Arctic (EA) assembly, is characterized by the absence of Pacific water in the upper layer; below this, by an Atlantic layer core as warm as 2° to 3°C; and by a colder (about −0.9°C) deep layer. Because the presence or absence of Pacific origin water is a key characteristic distinguishing the two assemblies, we will refer to the water mass boundary between the two assemblies as the Atlantic/Pacific front. Earlier research indicated that water masses in the Arctic Ocean were separated by a front above the Lomonosov Ridge into the Canadian and Eurasian basins. Although all Larsen‐93 stations from the Canada Basin (A1–D1) display classic WA assembly characteristics, the Makarov Basin station (E1) shows EA assembly characteristics in the upper and Atlantic layers and a WA assembly deep layer. This suggests a relocation in the position of the Atlantic/Pacific boundary away from the Lomonosov Ridge. Further, Larsen‐93 data show the transition region between the Atlantic and deep layers is fresher in the Makarov Basin than corresponding water in either the Canada or Eurasian basins, implying a source of cold, low‐salinity water, perhaps from the Laptev and East Siberian shelves. The front separating these two assemblies lies above the Mendeleyev Ridge and is marked by large lateral gradients in all measured properties. In particular, the penetration of anthropogenic halocarbons is 2 to 3 times deeper in the Makarov Basin than in the Canada Basin, implying enhanced rates of ventilation. This suggests that direct exchange between the Canadian and Eurasian basins has occurred recently near the perimeter and that physical and chemical properties, including contaminants, may have been transported by boundary currents more quickly from one basin to the other.

Expanded record of Quaternary oceanographic change: Amerasian Arctic Ocean

Four sediment cores collected from the Northwind and Mendeleyev ridges, Arctic Ocean, from 1089 m to 1909 m water depth, provide an oceanographic record extending back into the Matuyama reversed polarity chron. Benthic foraminiferal analyses show four prominent assemblage zones: Bolivina arctica , Cassidulina teretis , Bulimina aculeata , and Oridorsalis tener from the upper Matuyama reversed polarity chronozone through the Brunhes normal polarity chronozone. These assemblage zones represent depth-dependent benthic foraminiferal biofacies changes associated with oceanographic events that occurred in the Amerasian basin at ∼780 and 300 ka, and indicate oceanographic influence from the North Atlantic. Recognition of these benthic assemblage zones in Arctic cores from the Alpha Ridge indicates that the benthic foraminiferal zonations in intermediate to deep water (>1000 m) Arctic cores may be more useful than preexisting lithostratigraphic zonations and should provide important information pertaining to the Quaternary paleoceanographic evolution of the Arctic Ocean.

Evidence for warming of Atlantic water in the Southern Canadian Basin of the Arctic Ocean: Results from the Larsen‐93 Expedition

Potential temperature (θ) and salinity (S) data obtained along the perimeter of the southern Canadian Basin north of the East Siberian Sea in 1993 aboard the CCGS Henry Larsen show higher temperatures in waters of Atlantic origin than in available climatological data for the Canadian Basin. In particular, a front is observed near the Mendeleyev Ridge which separates the cooler Atlantic waters of the Canada Basin from the warmer Atlantic waters observed in the Makarov Basin. The front is further characterized by a change in the θ/S slope of Arctic thermocline water, and by thermohaline intrusions (θ and S reversals) within the Atlantic layer. The idea that this warm variety of Atlantic water has come recently from the Eurasian Basin is supported by its higher level of the tracer CFC‐11.