Water Column Of Arctic Ocean Research Articles

Strong Margin Influence on the Arctic Ocean Barium Cycle Revealed by Pan-Arctic Synthesis.

Early studies revealed relationships between barium (Ba), particulate organic carbon and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. But, what controls the distribution of barium (Ba) in the oceans? Here, we investigated the Arctic Ocean Ba cycle through a one‐of‐a‐kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba ratio (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ∼50% of the dBa inventory (upper 500 m of the Arctic water column) was supplied by nonconservative inputs. Calculated areal dBa fluxes are up to 10 μmol m−2 day−1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. The Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean‐derived waters and Baffin Bay‐derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a sedimentary remineralization and terrigenous sources (e.g., submarine groundwater discharge or fluvial particles) are contributors.

Methane and nitrous oxide distributions across the North American Arctic Ocean during summer, 2015

AbstractWe collected Arctic Ocean water column samples for methane (CH4) and nitrous oxide (N2O) analysis on three separate cruises in the summer and fall of 2015, covering a ∼10,000 km transect from the Bering Sea to Baffin Bay. This provided a three‐dimensional view of CH4 and N2O distributions across contrasting hydrographic environments, from the oligotrophic waters of the deep Canada Basin and Baffin Bay, to the productive shelves of the Bering and Chukchi Seas. Percent saturation relative to atmospheric equilibrium ranged from 30 to 800% for CH4 and 75 to 145% for N2O, with the highest concentrations of both gases occurring in the northern Chukchi Sea. Nitrogen cycling in the shelf sediments of the Bering and Chukchi Seas likely constituted the major source of N2O to the water column, and the resulting high N2O concentrations were transported across the Arctic Ocean in eastward‐flowing water masses. Methane concentrations were more spatially heterogeneous, reflecting a variety of localized inputs, including likely sources from sedimentary methanogenesis and sea ice processes. Unlike N2O, CH4 was rapidly consumed through microbial oxidation in the water column, as shown by the 13C enrichment of CH4 with decreasing concentrations. For both CH4 and N2O, sea‐air fluxes were close to neutral, indicating that our sampling region was neither a major source nor sink of these gases. Our results provide insight into the factors controlling the distribution of CH4 and N2O in the North American Arctic Ocean, and an important baseline data set against which future changes can be assessed.

Observed increases in Bering Strait oceanic fluxes from the Pacific to the Arctic from 2001 to 2011 and their impacts on the Arctic Ocean water column

Mooring data indicate the Bering Strait throughflow increases ∼50% from 2001 (∼0.7 Sv) to 2011 (∼1.1 Sv), driving heat and freshwater flux increases. Increase in the Pacific‐Arctic pressure‐head explains two‐thirds of the change, the rest being attributable to weaker local winds. The 2011 heat flux (∼5 × 1020J) approaches the previous record high (2007) due to transport increases and warmer lower layer (LL) temperatures, despite surface temperature (SST) cooling. In the last decade, warmer LL waters arrive earlier (1.6 ± 1.1 days/yr), though winds and SST are typical for recent decades. Maximum summer salinities, likely set in the Bering Sea, remain remarkably constant (∼33.1 psu) over the decade, elucidating the stable salinity of the western Arctic cold halocline. Despite this, freshwater flux variability (strongly driven by transport) exceeds variability in other Arctic freshwater sources. Remote data (winds, SST) prove insufficient for quantifying variability, indicating interannual change can still only be assessed by in situ year‐round measurements.

Analysis and modeling of hydrothermal plume data acquired from the 85°E segment of the Gakkel Ridge

We use data from a CTD plume‐mapping campaign conducted during the Arctic Gakkel Vents (AGAVE) expedition in 2007 to constrain the nature of hydrothermal processes on the Gakkel Ridge at 85°E. Thermal and redox potential (Eh) anomalies were detected in two discrete depth intervals: 2400–2800 m (Interval 1) and 3000–3800 m (Interval 2). The spatial and temporal patterns of the signals indicate that the Interval 1 anomalies were most likely generated by a single large, high‐temperature (T > 100°C) vent field located on the fault terraces that form the NE axial valley wall. In contrast, the Interval 2 anomalies appear to have been generated by up to 7 spatially distinct vent fields associated with constructional volcanic features on the floor of the axial valley, many of which may be sites of diffuse, low‐temperature (T < 10°C) discharge. Numerical simulations of turbulent plumes rising in a weakly stratified Arctic Ocean water column indicate that the high‐temperature field on the axial valley wall has a thermal power of ∼1.8 GW, similar to the Trans‐Atlantic Geotraverse and Rainbow fields in the Atlantic Ocean, whereas the sites on the axial valley floor have values ranging from 5 to 110 MW.

Atlantic sources of the Arctic Ocean surface and halocline waters

Data obtained during the last 20 years from selected hydrographic stations throughout the Arctic Ocean basins and at the continental slopes and northern parts of the surrounding shelf seas are examined to estimate the contribution of Atlantic water to the upper part of the Arctic Ocean water column, and to follow the circulation of the Atlantic derived halocline waters around the Arctic Ocean basins. A substantial fraction of the Atlantic water entering the Arctic Ocean in the two inflow branches, the Fram Strait branch and the Barents Sea branch, is transformed into less dense waters. The inflow through Fram Strait encounters and melts sea ice. Its upper part becomes less saline, and in winter, ice formation homogenizes this low salinity upper part into a winter mixed layer, which follows the boundary flow eastward. The inflow over the Barents Sea mainly becomes colder, less saline and denser due to cooling and net precipitation, but melting of sea ice also here creates a less dense upper layer, which is subsequently homogenized by haline convection. Both these components enter the Nansen Basin via the St. Anna Trough and flow eastward along the Siberian continental slope. A third component of the Barents Sea inflow, mainly comprising Norwegian Coastal Current water, remains on the shelf, where it absorbs the runoff from the large Siberian rivers and evolves into low salinity shelf water. This part continues as far east as the Laptev Sea before a major inflow to the deep Arctic Ocean basins occurs. Winter convection, which in most of the Nansen Basin extends to the Atlantic layer, then becomes limited to the injected low salinity shelf water, and the more saline upper layers of the Fram Strait and the Barents Sea branches are transformed into halocline waters. The Fram Strait branch supplies the halocline water of the Amundsen, Makarov and part of the Canada basins, while the upper part of the Barents Sea branch, initially confined to the Siberian continental slope, becomes the main source of the lower halocline below the Pacific water in the Canada Basin beyond the Chukchi Cap. Less dense Pacific water close to the North American continent prevents the Atlantic derived upper layers from fl owing through the Canadian Arctic Archipelago and their main exit is Fram Strait. Only some of the Barents Sea branch halocline water passes through Nares Strait and may occasionally renew the deep and bottom waters of Baffin Bay.

Atlantic sources of the Arctic Ocean surface and halocline waters

Data obtained during the last 20 years from selected hydrographic stations throughout the Arctic Ocean basins and at the continental slopes and northern parts of the surrounding shelf seas are examined to estimate the contribution of Atlantic water to the upper part of the Arctic Ocean water column, and to follow the circulation of the Atlantic derived halocline waters around the Arctic Ocean basins. A substantial fraction of the Atlantic water entering the Arctic Ocean in the two inflow branches, the Fram Strait branch and the Barents Sea branch, is transformed into less dense waters. The inflow through Fram Strait encounters and melts sea ice. Its upper part becomes less saline, and in winter, ice formation homogenizes this low salinity upper part into a winter mixed layer, which follows the boundary flow eastward. The inflow over the Barents Sea mainly becomes colder, less saline and denser due to cooling and net precipitation, but melting of sea ice also here creates a less dense upper layer, which is subsequently homogenized by haline convection. Both these components enter the Nansen Basin via the St. Anna Trough and flow eastward along the Siberian continental slope. A third component of the Barents Sea inflow, mainly comprising Norwegian Coastal Current water, remains on the shelf, where it absorbs the runoff from the large Siberian rivers and evolves into low salinity shelf water. This part continues as far east as the Laptev Sea before a major inflow to the deep Arctic Ocean basins occurs. Winter convection, which in most of the Nansen Basin extends to the Atlantic layer, then becomes limited to the injected low salinity shelf water, and the more saline upper layers of the Fram Strait and the Barents Sea branches are transformed into halocline waters. The Fram Strait branch supplies the halocline water of the Amundsen, Makarov and part of the Canada basins, while the upper part of the Barents Sea branch, initially confined to the Siberian continental slope, becomes the main source of the lower halocline below the Pacific water in the Canada Basin beyond the Chukchi Cap. Less dense Pacific water close to the North American continent prevents the Atlantic derived upper layers from fl owing through the Canadian Arctic Archipelago and their main exit is Fram Strait. Only some of the Barents Sea branch halocline water passes through Nares Strait and may occasionally renew the deep and bottom waters of Baffin Bay.

Formation and evolution of the surface mixed layer and halocline of the Arctic Ocean

Fresh water from summer ice melt and the total freshwater content of the Arctic Ocean water column above the thermocline are estimated from vertical profiles of temperature and salinity observed on the I/B Oden 1991 cruise. The seasonal ice melt ranges from 0.5 m to slightly above 1 m and is moderately uniform over the observation area. Regions of lower melting are seen over the Morris Jesup Plateau. The freshwater content is calculated relative to the salinity just above the thermocline north of the Barents Sea. The freshwater content increases toward the interior of the Arctic Ocean, showing that fresh water is advected from other regions into the observation area. Regions of different freshwater content are separated by fronts over the Nansen‐Gakkel Ridge, over the Lomonosov Ridge, and in the western Eurasian Basin between waters derived from the Eurasian and Canadian Basins. Denser water, homogenized north of the Barents Sea, is recognized by a temperature minimum layer. The absence of the temperature minimum near the Nansen‐Gakkel Ridge indicates that heat is transferred from the Atlantic Layer over a longer time than the shortest route would allow. This observation can be explained if the layer circulates together with the Atlantic Layer, i.e., toward the east, and returns above the Nansen‐Gakkel Ridge and along the Amundsen Basin. North of the Laptev Sea, this water formed north of the Barents Sea becomes covered by low‐salinity shelf water. The increased freshwater content limits the winter convection, so it no longer reaches the thermocline and an intermediate halocline is formed. The halocline in the Eurasian Basin consists of water originating from winter convection in the Arctic Ocean north of the Barents Sea, which then circulates around the basin. Such a formation mechanism also explains the observed distribution of low NO water. The strong density increase limits vertical exchange, and the vertical diffusion coefficient in the halocline is small (∼1 × 10−6 m2 s−1). The increased temperature of the halocline shows that the heat lost upward by the Atlantic Layer, mainly by double‐diffusive convection, is trapped below the mixed layer.