Dataset on optical characteristics and spectroscopic indices of dissolved organic matter of the Kara, Laptev, and East Siberian seas in August–September 2017

A total of 137 water samples to study optical properties of chromophoric dissolved organic matter were collected in the Artic shelf seas during the 69th cruise on board the R/V Akademik Mstislav Keldysh in August–September 2017. Sampling sites were located in the Kara, Laptev and East Siberian seas and associated to the areas influenced by terrestrial runoff of the Kolyma, Indigirka, Lena, Khatanga, Ob and Yenisei rivers. In this data article, conventional spectroscopic indices and lignin phenol concentrations, calculated on the basis of fluorescence and UV–vis absorption spectra, are presented.


Data
The data presented here come from a complex study of the Arctic shelf seas conducted on board the R/V Akademik Mstislav Keldysh in AugusteSeptember 2017 [1]. Water sampling to study the cromophoric dissolved organic mater (CDOM) optical properties was performed in the Kara, Laptev, and East Siberian seas at the sites shown in Fig. 1. The studied areas included three cross-shelf sections starting from the estuarine regions of the Khatanga, Indigirka and Kolyma rivers, as well as individual sites in the Kara and Laptev seas. Raw fluorescence and absorption spectra are given in Supplementary materials. Table 1 contains conventional spectroscopic indices, namely humification index HIX, index of recent autochthonous contribution BIX, fluorescence index FI, spectral slope S uvb , spectral slope ratio S r and logarithm of lignin phenol concentration calculated on the basis of absorption spectra. The sampling depth and exact sampling date are specified.

Experimental design, materials, and methods
The sampling depths were chosen based on water mass changes identified with the CTD profiles. Water samples were then collected from Niskin bottles of 5 L volume mounted on the CTD/rosette system. The samples were filtered through precombusted at 450 C Whatman GF/F filters with a pore size of about 0.7 mm. The filtrate was collected into the acid-cleaned 30 mL glass vials and stored under dark conditions at 4 C until further analysis.
Absorption spectra A l have been registered at room temperature of 22±2 C with the Solar PB2201 spectrophotometer and 3 or 5 cm quartz cuvettes. The measurements were performed within the spectral range from 220 to 700 nm at 1 nm increments. The blank-corrected absorbance spectra were converted into the Napierian absorption coefficients a l according to the following equation: Specifications Table   Subject  Oceanography  Specific subject area  Optical properties of chromophoric dissolved organic matter  Type of data  Figure  Table  Text files How data were acquired Fluorat-02-Panorama spectrofluorometer (Lumex, Russia) and Solar PB2201 spectrophotometer (Solar, Belarus). Data format Raw Analyzed Parameters for data collection The sampling depths were chosen based on water mass changes identified with the CTD profiles. Water samples were then collected from Niskin bottles and prepared for further analysis. Fluorescence and absorption spectra were registered at room temperature in the stationary laboratory after the cruise. Description of data collection The absorption spectra were measured in a 3-cm or 5-cm quartz-glass cuvette using a dual-beam spectrophotometer against the pure water as a blank. The spectra were measured over the 220e700 nm spectral range in 1 nm increments. The fluorescence spectra were measured in a 1-cm quartzglass cuvette at excitation wavelengths of 254 nm, 310 nm, and 370 nm. Data source location Shelf regions of the Kara, Laptev and East Siberian seas. Data accessibility Data on spectroscopic indices and raw spectra are provided with this article.

Value of the Data
Spectroscopic indices represent efficient tools in tracing dissolved organic matter major sources. Data can be used for the water mass characterization and the study of cross-frontal and vertical mixing in the mixing zones of Arctic rivers. Under the conditions of climate change, a comparative analysis of the data is of great importance for understanding dynamics of supply and transformation of terrestrial dissolved organic matter in the Arctic basin.
where l -the cuvette path length in meters. Absorption spectra within wavelngth ranges 275e295 nm and 350e400 nm and were characterized by the exponential spectral slope coefficient S with respect to the equation suggested by Stedmon et al. [2,3]: and depicted as S uvb and S uva , respectively. The values of S uvb and S uva were determined by linear regression of the log-transformed functions of absorption coefficients a l [4]. The spectral slope ratio S R was calculated as follows Dissolved lignin phenol concentrations TDLP 9 were estimated according to the equations reported for the low-and high-CDOM models by Fichot and co-workers [5]: lnðTDLP 9 Þ ¼ 0:7672a 350 À 0:3987 a 250 < 4 m À 1; (4) lnðTDLP 9 Þ ¼ À2:282lnða 350 Þ À 8:209lnða 275 Þ þ 11:265lnða 295 Þ þ 2:909 a 250 ! 4 m À 1: Fluorescence measurements were performed with a Fluorat-02-Panorama spectrofluorometer (Lumex Instruments) equipped with a Xenon flash lamp as a light source, and a PMT as a detector of luminescence signals. A signal averaging over 20 flashes was applied in order to compensate instability of the flash lamp intensity. The accuracy of excitation and detection wavelength settings was In both cases the ratios of fluorescence intensities are considered for the spectra excited at 370 nm.

Acknowledgments
The study of CDOM in the mixing zones of the Indigirka and Kolyma rivers was supported by the RSF Grant (project 18-77-00053). Investigation of the Kara Sea was performed in the framework of the state assignment of IO RAS (theme 0149-2019-0006). We are grateful to N.V. Lobus for the collection and preparation of water samples, M.V. Flint and the crew of the R/V Akademik Mstislav Keldysh for their contribution during field studies financially supported by RSF Grant (project 14-50-00095).