Abstract
Abstract. The East Siberian Arctic Shelf (ESAS) is the broadest and shallowest continental shelf in the world. It is characterized by both the highest rate of coastal erosion in the world and a large riverine input of terrigenous dissolved organic matter (DOM). DOM plays a significant role in marine aquatic ecosystems. The chromophoric fraction of DOM (CDOM) directly affects the quantity and spectral quality of available light, thereby impacting both primary production and ultraviolet (UV) exposure in aquatic ecosystems. A multiyear study of CDOM absorption, fluorescence, and spectral characteristics was carried out over the vast ESAS in the summer–fall seasons. The paper describes observations accomplished at 286 stations and 1766 in situ high-resolution optical measurements distributed along the nearshore zone. Spatial and interannual CDOM dynamics over the ESAS were investigated, and driving factors were identified. It was shown that the atmospheric circulation regime is the dominant factor controlling CDOM distribution on the ESAS. This paper explores the possibility of using CDOM and its spectral parameters to identify the different biogeochemical regimes in the surveyed area. The analysis of CDOM spectral characteristics showed that the major part of the Laptev and East Siberian seas shelf is influenced by terrigenous DOM carried in riverine discharge. Western and eastern provinces of the ESAS with distinctly different DOM optical properties were also identified; a transition between the two provinces at around 165–170° E, also consistent with hydrological and hydrochemical data, is shown. In the western ESAS, a region of substantial river impact, the content of aromatic carbon within DOM remains almost constant. In the eastern ESAS, a gradual decrease in aromaticity percentage was observed, indicating contribution of Pacific-origin waters, where allochthonous DOM with predominantly aliphatic character and much smaller absorption capacity predominates. In addition, we found a stable tendency towards reduced concentrations of CDOM and dissolved lignin and an increase in spectral slope and slope ratio values eastward from the Lena River delta; the Lena is the main supplier of DOM to the eastern Arctic shelf. The strong positive correlation (r = 0.97) between dissolved organic carbon (DOC) and CDOM values in the surface shelf waters influenced by terrigenous discharge indicates that it is feasible to estimate DOC content from CDOM fluorescence assessed in situ using a WETStar fluorometer. This approach is reliable over the salinity range of 3 to 24.5. The fact that there is little difference between predicted and observed parameters indicates that the approach is justified. The direct estimation of DOM optical characteristics in the surface ESAS waters provided by this multiyear study will also be useful for validating and calibrating remote sensing data.
Highlights
Current climate change is evident and amplified in the high latitudes of the Northern Hemisphere
The recent studies accomplished within the framework of the International Siberian Shelf Study project (Semiletov and Gustafsson, 2009; Semiletov et al, 2011; Tesi et al, 2014; Bröder et al, 2016; Charkin et al, 2011) demonstrate that coastal erosion is the main source of particulate organic carbon (OC) (POC) to the East Siberian Arctic Shelf (ESAS), the broadest and shallowest shelf in the world
The salinity values varied between 3.1 (2004) and 32.34 (2011), with the lowest values associated with freshwater input from the Lena River and the higher values attributed to the presence of Pacific water
Summary
Current climate change is evident and amplified in the high latitudes of the Northern Hemisphere. The system change is characterized by an increase in the average annual temperature and atmospheric circulation intensity, a reduction in sea ice coverage and thickness, accelerated degradation of permafrost, and an increase in coastal and bottom erosion and river flows (IPCC, 2013). The Arctic region contains an abundance of organic carbon (OC) buried inland and within the sedimentary basin of the Arctic Ocean, which might become a part of the current marine biogeochemical cycle due to thawing of on-land and sub-sea permafrost, increased coastal and bottom erosion, accelerated river discharge, and soil-based carbon losses (Günther et al, 2013; Shakhova et al, 2009, 2017; Semiletov, 1999; Vonk and Gustafsson, 2013; Vonk et al, 2012, 2014). River discharge to shelf waters supplies terrestrial carbon in the form of dissolved OC (DOC) and waters enriched by carbon dioxide (CO2) (Alling et al, 2010; Amon et al, 2012; Anderson et al, 2009, 2011; Semiletov et al, 2011, 2012; Pipko et al, 2010)
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