Variability of the thermohaline structure and transport of Atlantic water in the Arctic Ocean based on NABOS (Nansen and Amundsen Basins Observing System) hydrography data

Conductivity–temperature–depth (CTD) transects across continental slope of the Eurasian Basin and the St. Anna Trough performed during NABOS (Nansen and Amundsen Basins Observing System) project in 2002–2015 and a transect from the 1996 Polarstern expedition are used to describe the temperature and salinity characteristics and volume flow rates (volume transports) of the current carrying the Atlantic water (AW) in the Arctic Ocean. The variability of the AW on its pathway along the slope of the Eurasian Basin is investigated. A dynamic Fram Strait branch of the Atlantic water (FSBW) is identified in all transects, including two transects in the Makarov Basin (along 159∘ E), while the cold waters on the eastern transects along 126, 142, and 159∘ E, which can be associated with the influence of the Barents Sea branch of the Atlantic water (BSBW), were observed in the depth range below 800 m and had a negligible effect on the spatial structure of isopycnic surfaces. The geostrophic volume transport of AW decreases farther away from the areas of the AW inflow to the Eurasian Basin, decreasing by 1 order of magnitude in the Makarov Basin at 159∘ E, implying that the major part of the AW entering the Arctic Ocean circulates cyclonically within the Nansen and Amundsen basins. There is an absolute maximum of θmax (AW core temperature) in 2006–2008 time series and a maximum in 2013, but only at 103∘ E. Salinity S(θmax) (AW core salinity) time series display a trend of an increase in AW salinity over time, which can be referred to as an AW salinization in the early 2000s. The maxima of θmax and S(θmax) in 2006 and 2013 are accompanied by the volume transport maxima. The time average geostrophic volume transports of AW are 0.5 Sv in the longitude range 31–92∘ E, 0.8 Sv in the St. Anna Trough, and 1.1 Sv in the longitude range 94–107∘ E.

Seaward Dipping Reflectors Sequences (SDRs) mapping in the field of the Mendeleev Rise

<p>The report focuses on the strata of the Mendeleev Rise and adjacent Podvodnikov Basin, Makarov Basin, Toll Basin, and North Chukchi Basin together with Lomonosov Ridge and Chukchi Plateau. Eleven 2-D seismic profiles with a total length of 7540 km were interpreted. The uplifts within the study area are represented by asymmetric raised blocks of the crust with strongly rugged by half-graben structures. We found semi-continuous, from moderate to bright high-amplitude gently dipping reflectors similar to SDRs inside some half-grabens. The SDRs complexes distribute only in half-grabens. A few wedges with several kilometers thick can be distinguished here. The lower boundary of SDRs does not clearly trace. The relationship with underlying complexes is uncertain.  SDRs top is bright enough and interpreted as an angular unconformity, that is progressively onlapped by overlying sediments. Top of SDRs probably coincides with rift-postrift boundary age of 110-100 Ma.  We traced the distribution and direction of SDRs and made a map. SDRs dip from the central axis of Mendeleev Ridge in opposite directions – toward to Toll and Podvodnikov basins. In the central part of Podvodnikov and Toll basins are recognized small raised blocks of continental crust to which anti-directional SDRs converge. The nature of this rises can be explained by tectonic uplift. Thus, SDRs complexes dip symmetrically in two directions from the Mendeleev Rise. Two-directional SDRs also occur in conjugate Podvodnikov and Toll basins. They dip from the Mendeleev Rise and from the Lomonosov Terrace and the Chukchi Plateau, respectively. The SDRs occur on the hyperextension continental crust complex and accompany magmatism on volcanic passive margin (VPM). We propose that the Mendeleev Rise was formed as two-directional VPM, and the Lomonosov Terrace and the Chukchi Plateau also was formed as one-directional VPM. The Mendeleev Rise was formed simultaneously with Podvodnikov, Toll and North Chukchi basins ca. 125-100 Ma because of extensional tectonics. We also assume that the Makarov Basin (with obvious half-graben structures) could form simultaneously with the Nautilus Basin. This work was supported by RFBR grants (18-05-70011 and 18-05-00495).</p>

Assessment of variability of the thermohaline structure and transport of Atlantic water in the Arctic Ocean based on NABOS CTD data

Data of CTD transects across continental slope of the Eurasian Basin and the St. Anna Trough performed during NABOS (Nansen and Amundsen Basins Observing System) project in 2003–2015 are used to assess transport and propagation features of the Atlantic Water (AW) in the Arctic Ocean. Estimates of θ-S indices and volume flow rate of the current carrying the AW in the Eurasian Basin were obtained. The assessments were based on the analysis of CTD data including 33 sections in the Eurasian Basin, 4 transects in the St. Anna Trough and 2 transects in the Makarov Basin; additionally a CTD transect of the PolarStern-1996 expedition (PS-96) was considered. Using spatial distributions of temperature, salinity, and density on the transects and applying θ-S analysis, the variability of thermohaline pattern on the AW pathway along the slope of Eurasian Basin was investigated. The Fram Strait branch of the Atlantic Water (FSBW) was satisfactorily identified on all transects, including two transects in the Makarov Basin (along 159° E), while the сold waters, which can be associated with the influence of the Barents Sea branch of the Atlantic water (BSBW), on the transects along 126° E, 142° E and 159° E, were observed in the depth range below 800 m and had a negligible effect on the spatial structure of isopycnic surfaces. Special attention was paid to the variability of the volume flow rate of the AW propagating along the continental slope of the Eurasian Basin. The geostrophic volume flow rate was calculated using the dynamic method. An interpretation of the spatial and temporal variability of hydrological parameters characterizing the flow of the AW in the Eurasian Basin is presented. The geostrophic volume flow rate decreases significantly farther away from the areas of the AW inflow to the Eurasian Basin. Thus, the geostrophic estimate of the volume rate for the AW flow in the Makarov Basin at 159° E was found to be more than an order of magnitude smaller than the estimates of the volume flow rate in the Eurasian Basin, implying that the major part of the AW entering the Arctic Ocean circulates cyclonically within the Nansen and Amundsen Basins. There is an absolute maximum of θmax (AW core temperature) in 2006–2008 time series and a maximum in 2013, but only at 103° E. Salinity S(θmax) (AW core salinity) time series display an increase of the AW salinity in 2006–2008 and 2013 (at 103° E) that can be referred to as a AW salinization in the early 2000-ies. The maxima of θmax and S(θmax) in 2006–2008 and 2013 were accompanied by the volume flow rate highs. Additionally the time average volume rates were calculated for the FSBW flow (in the longitude range 31–92° E), for the BSBW flow in the St. Anna Trough and for a combined FSBW and BSBW flow in longitude range 94–107° E. A detailed discussion of the results is presented.

Bathymetry and oceanic flow structure at two deep passages crossing the Lomonosov Ridge

The Lomonosov Ridge represents a major topographical feature in the Arctic Ocean which has a large effect on the water circulation and the distribution of water properties. This study presents detailed bathymetric survey data along with hydrographic data at two deep passages across the ridge: a southern passage (80–81∘ N), where the ridge crest meets the Siberian continental slope, and a northern passage around 84.5∘ N. The southern channel is characterized by smooth and flat bathymetry around 1600–1700 m with a sill depth slightly shallower than 1700 m. A hydrographic section across the channel reveals an eastward flow with Amundsen Basin properties in the southern part and a westward flow of Makarov Basin properties in the northern part. The northern passage includes an approximately 72 km long and 33 km wide trough which forms an intra-basin in the Lomonosov Ridge morphology (the Oden Trough). The eastern side of the Oden Trough is enclosed by a narrow and steep ridge rising 500–600 m above a generally 1600 m deep trough bottom. The deepest passage (the sill) is 1470 m deep and located on this ridge. Hydrographic data show irregular temperature and salinity profiles indicating that water exchange occurs as midwater intrusions bringing water properties from each side of the ridge in well-defined but irregular layers. There is also morphological evidence that some rather energetic flows may occur in the vicinity of the sill. A well expressed deepening near the sill may be the result of seabed erosion by bottom currents.