DISTRIBUTION OF SUSPENDED PARTICULATE MATTER IN THE BARENTS SEA IN LATE WINTER 2019

Arctic summer and winter sea-ice extent is continuously declining as a result of climate change, affecting the hydrography and biogeochemical cycles on the seasonally ice-free Eurasian Shelves. The prolongation of the open-water season causes higher sediment resuspension and coastal erosion due to larger wind fetch and wave heights. This impacts the optical properties of the water column and hence biological productivity in this region. During “Transarktika-2019” leg 1 in late winter 2019, a comprehensive dataset of filtered water samples and optical data was collected throughout the central and northern Barents Sea. Combining suspended particulate matter concentrations obtained from water samples and optical data revealed a pronounced bottom nepheloid layer on the Barents Sea shelf even under ice-covered conditions. Moreover, the data indicate that the Franz Viktoria Trough could be a major pathway for sediment transport into the Eurasian Basin. Therefore, to link changes in sediment distribution and its impact on the ecosystem under a warming climate, further studies of sediment dynamics are required, particularly during winter.


INTRODUCTION
The summer and winter sea-ice extent in the northern Barents Sea has been declining over recent years [1,2]. As a result, the open water period prolongs and is expected to lead to higher sediment resuspension, particularly in the shallow parts of the shelf sea, and to increased costal erosion due to larger wind fetch and wave heights [3,4]. The suspended particulate matter (SPM) within the water column plays a critical role for the ecosystem by contributing to the amount of nutrient supply (e.g. [5,6]) and by influencing the amount of sunlight penetrating through the water column, through absorption and scattering [7]. In order to assess possible impacts on the ecosystem by future changes in SPM distribution and its dynamics, it is necessary to fully evaluate and describe presentday sedimentological conditions. Thus far, SPM investigations on the Siberian Arctic shelves were carried out in late summer / early autumn when sea ice and weather conditions are most favorable (e.g. [8]). In the surface water of the Barents Sea, SPM concentrations are mainly driven by the varying intensity of biological productivity reaching maximum values during spring bloom [8,9]. In the eastern Barents Sea, a pronounced layer of high SPM concentrations extends from the Kara Sea towards the northern Barents Sea   shelf break in summertime [10]. This bottom nepheloid layer is sustained by sediment input of rivers, Ob and Yenisei, in the south and by cascading dense shelf water carrying resuspended material [9]. However, in the Franz Viktoria Trough region only low SPM concentrations are observed in the water column during early autumn [8]. Similar investigations on the central and eastern Laptev Sea shelf also showed two pronounced layers of high SPM concentrations at the surface and at the bottom during summer. There, its occurrence and extent are mainly linked to prevailing atmospheric conditions and the turbid outflow of the Lena river [11,12]. The presence of bottom nepheloid layers (BNL) are crucial for benthic organisms as they feed on the suspended material [13]. Furthermore, most of the cross-shelf sediment transport [11] as well as export of organic carbon off the shelf takes place within the BNL [14,15]. In order to assess the distribution of suspended particles in a year-round perspective reliable in situ data during wintertime is needed. In this study we present combined results from SPM measurements and optical backscatter data obtained during "Transarktika-2019" leg 1 [16], in late winter 2019 (Fig. 1).

METHODS
To investigate horizontal and vertical distribution of SPM in the Barents Sea, 179 oceanographic stations were carried out during the expedition "Transarktika" leg 1 [19] aboard the RV "Akademik Tryoshnikov" between March and May 2019 (Fig. 1). 152 water samples of 1 -2 L each, were collected from different water depths at 32 stations. While the ship was anchored to an ice floe (late March -beginning of May), a 2 m x 2 m hole at the side of the ship provided safe and reliable measurements with the shipboard CTD / Rosette. The water samples were filtered through pre-weighed MILLIPORE filters, with a diameter of 47 mm and a pore size of 0.45 µm, and subsequently dried at 50 °C onboard the ship. In the last processing step, the dried filters were weighed at GEOMAR in Kiel, Germany, to obtain SPM concentrations, taking into account the mean elutable portion of 0.2 mg/L of each filter. In order to obtain high resolution vertical profiles of SPM concentrations, a Seapoint Turbidity Meter was connected to a Seabird SBE 19 CTD at 159 stations. The turbidity meter emits light of 880 nm wavelength with a sampling frequency of 10 Hz. It detects light scattered by suspended particles in water and generates an output voltage proportional to the suspended particles. The detector receives light scattered at angles between 15 and 150 degrees and has an accuracy of ± 2 % [20]. The output is given in Nephelometric Turbidity Unit (NTU), a calibration unit based on formazine as a reference suspension. All SPM measurements obtained by filtration and weighing were correlated with corresponding turbidity meter measurements to obtain SPMturb mass concentrations, thereby taking into account the effects of different mineralogy, varying particle size and darkness on the response of the turbidity meter [21].
A good correlation was obtained between the weighed SPM concentrations and optical measurements (Fig. 3). The standard error of the estimate (s est ) is 0.41. The linear relationship between SPMfilt and optical backscatter intensity can be expressed as: SPMturb = 9.49 . BI + 0.376. Yet, the reflected variability between optical and weighed data shows the strong influence of different sediment compositions. In particular, the clay mineral distribution in surface sediments within the FVT region is highly variable, depending on whether the source area is located around Spitzbergen (Svalbard) or Franz Joseph Land [22].  A similar vertical SPM distribution compared to the central Barents Sea is observed in the southern Franz Viktoria Trough under ice-covered conditions (Fig. 5A2 -C2). However, the range of SPMturb and SPMfilt concentration is noticeably lower in the upper water column, only varying between 0.4 -0.6 mg/L. The BNL is comparably thinner with a thickness of approximately 30 m but with slightly higher SPMturb concentrations of up to 4.0 mg/L close to the seafloor. The upper boundary appears to be more pronounced than in the central Barents Sea. This sharp transition zone may further explain the observed differences of SPMfilt and SPMturb concentrations at 300 m water depth (Fig. 4A2, C2). In contrast to the central Barents Sea and the southern FVT, turbidity and SPMturb concentrations close to the shelf break are significantly reduced in the entire water column, ranging between 0.4 -1 mg/L (Fig. 4A3 -C3). A well-defined BNL is absent, although concentrations slightly increase at depths closer to the seafloor.

Vertical Distribution of SPM
Horizontal distribution of SPM A south -north transect from the northern tip of Kola Peninsula to the shelf break between Spitzbergen (Svalbard) and Franz Joseph Land along 39° E is studied (Fig. 5) to investigate the horizontal distribution of SPM. The BNL in the southern and central Barents Sea is distinct, noticeably varying in thickness and clearly influenced by ocean bathymetry. Highest SPMturb concentrations are observed at the northern flank of Skolpen Bank and along the Central Bank. Furthermore, elevated SPMturb concentrations are observed in valleys along the Central Bank and Great Bank. Towards the north, the bottom nepheloid layer decreases in thickness and in concentration. North of the ice-edge at 78° N, the entire water column becomes less turbid and the BNL decreases in its intensity and becomes almost absent at the shelf break.  Рис. 6. Концентрации взвешенного вещества SPMturb вдоль разреза W -E между 79° с.ш. и 80° с.ш. Серая пунктирная линия представляет собой грубую оценку сплоченности морского льда вдоль разреза. Батиметрия IBCAO V3.0 [17]).

A west -east transect across Franz Viktoria Trough (FVT) towards Franz Joseph
Land reveals a distinct BNL in the deepest parts of the trough between 40°E and 42°E with a sharp upper boundary and maximum SPMturb concentrations of up to 4 mg/L (Fig. 6). Yet, east of Ushakov Bank, SPMturb concentrations are noticeably lower, varying between 0.25 mg/L and 2 mg/L, and a BNL is absent. Across Ushakov Bank at 45°E high SPMturb concentrations are observed throughout the entire water column. During the sampling at this station a large polynya was observed east of Franz Joseph Land which also covered the area around Ushakov Bank (Fig. 1). This recurring polynya is a common feature during the winter and is referred to the Franz Joseph Land Polynya [23].

DISCUSSION
The vertical and horizontal distribution of SPM showed a pronounced BNL in icefree and under ice-covered conditions. A general decrease in thickness of the BNL as well as in SPM concentrations was observed from south to north. The formation of a BNL is generally related to local resuspension of particles by waves, currents and internal waves [24,25]. In terms of the Barents Sea, inflowing Atlantic Water masses from the Norwegian Sea, spreading eastwards across the shelf, and cold Arctic Water, play a key role in the oceanographic setting (Fig. 2). Due to the absence of sea ice, strong winter storms can lead to intensified surface and bottom currents in the southern Barents Sea [26,27]. Therefore, high SPM concentrations observed along the flanks of Skolpen Bank and across the Central Bank were possibly caused by strong eastward flows of modified Atlantic Water. Compared to SPM data obtained in September 2016 by [8], near bottom SPM concentrations from this study obtained during winter are notably higher. This may also be attributed to the general higher intensity of storms during winter [26] causing stronger currents and hence higher resuspension in the central Barents Sea. Further north, the Barents Sea is seasonally ice covered which generally leads to reduced current speeds. Hence, less resuspension could explain the lower SPM concentrations observed in the northern region. However, local increases in SPM as observed on Ushakov Bank might be explained by wind-induced resuspension in a polynya (Fig. 6) or very intense vertical convection over the bank [16]. Although it remains unclear, due to a lack of current data, whether the particulate matter was locally resuspended or advected laterally from shallower regions further north.
Connecting the Barents Sea shelf and the deep Arctic Ocean, the FVT could be a key pathway for exporting shelf-derived material. According to [28] and [29] Atlanticderived waters enter the Barents Sea through the western part of FVT, while Cold Bottom Waters and southern Barents Sea Atlantic-derived waters are exiting towards the north in the eastern part of the FVT. Therefore, the absence of the BNL at the slope can be explained by the station's positions being located mostly in the western FVT where less turbid water masses from the Arctic Ocean dominate. Assuming a near-bottom northward flow in the eastern FVT and relating it to the well pronounced BNL in the southern FVT, leads to the conclusion that shelf material can be exported through the FVT within the BNL during winter. Due to the absence of a BNL in the FVT in late summer / early autumn [8] it can further by implied that most of the bottom sediment transport occurs during wintertime. Yet, to further support this hypothesis, year-round current and SPM measurements are needed.

CONCLUSION
Sedimentological investigations were conducted in the Barents Sea to gain new insights into the distribution of suspended particulate matter during late winter. A distinct bottom nepheloid layer was observed during the sampling period across the entire Barents Sea shelf with a decreasing intensity trend from the south towards the north. The highest SPM concentrations of up to 4 mg/L were observed in bathymetric troughs (e.g. Franz Viktoria Trough) and at the flanks of bathymetric elevations (e.g. Skolpen Bank).
Relating this study to SPM investigations carried out mainly in autumn which observed only very low SPM concentrations below 1 mg/L leads to the conclusion that a distinct BNL is formed in late autumn or winter. Hence, sediment transport on the Barents Sea shelf may be most pronounced during winter. The Franz Viktoria Trough, a deep valley with water depths of more than 400 m could act as an important transport conduit for shelf material into the Eurasian Basin. In order to quantify sediment transport within the BNL and to gain more certainties about the fate of the suspended material in this region, more detailed investigations of SPM combined with current measurements at the continental slope are needed.
Competing interests. The authors declare that they have no competing interests. Funding. This research was supported by the Russian-German research projects: "CATS: The Changing Arctic Transpolar System" funded by the BMBF (03F0776) and by the Russian Foundation for Basic Research (RFBR) grant 18-05-60083.