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11Original Russian Text © N. Yu. Andrulionis, P. O. Zavialov, A. S. Izhitskiy, 2022, published in MORSKOY GIDROFIZICHESKIY ZHURNAL, Vol. 38, Iss. 5 (2022)
Effect of Variations in the Ion-Salt Water Composition on the Accuracy of Salinity Measurements
N. Yu. Andrulionis P. O. Zavialov, A. S. Izhitskiy
P. P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russian Federation
B natalya@ocean.ru
Abstract
Purpose. The work is aimed at assessing the effect of variations in the major ion-salt composition on the accuracy of determining water salinity in the inland seas and other seawater areas. The main goal of the study is to assess the representativeness of the results of the CTD salinity measurements (standard in oceanological practice) for the areas where the ion-salt seawater composition differs from that of the ocean.
Methods and Results. Salinity values of the seawater samples collected in the expeditions in the Black Sea and the Kerch Strait, and also in the Kara and Caspian seas in 2014-2021, were obtained in four different ways: 1) measurements with a CTD-probe based on electrical conductivity (practical salinity); 2) based on the measured density values, calculation by the TE0S-10 equation of state with due regard for the regional correction for the areas under study (absolute salinity); 3) calculation by chlorine content using empirical dependencies for the corresponding water basins; 4) direct calculation based on a sum of components of the major ionic composition (similar to chemical determination in a laboratory).
Conclusions. Differences in the ratios of the main ions in the water chemical compositions of the water areas and basins under study significantly affect the accuracy of salinity determination by standard oceanographic equipment. The variations in the major ionic composition, especially in the surface layer of the sea coastal part, are assumed to be largely influenced by the continental freshwater runoff. The ionic composition variability, having been not taken into account, leads to the errors in the measurements of physical parameters at traditional CTD-probing.
Keywords: determination of salinity, salinity, ionic composition, chemical composition, component composition, density of seawater, seawater, potentiometric titration, Black Sea, Kerch Strait, Caspian Sea, Kara Sea
Acknowledgments: the research was supported by the Russian Science Foundation, grant No. 21-1700191. The authors are grateful to all the participants of the expeditions in 2014-2021, the data obtained were used in the study.
For citation: Andrulionis, N.Yu., Zavialov, P.O. and Izhitskiy, A.S., 2022. Effect of Variations in the Ion-Salt Water Composition on the Accuracy of Salinity Measurements. Physical Oceanography, 29(5), pp. 463-479. doi:10.22449/1573-160X-2022-5-463-479
DOI: 10.22449/1573-160X-2022-5-463-479
© N. Yu. Andrulionis, P. O. Zavialov, A. S. Izhitskiy, 2022
© Physical Oceanography, 2022
1. Introduction
Attempts to determine the seawater salinity have been undertaken since ancient times and have acquired more or less quantitative forms from the 17th century 1. Salinity is defined as the mass of minerals dissolved in 1 kg of seawater. However, there are many such substances, so it is difficult in practice to measure
1 Zolotov, Yu.A., ed., 2002. [Fundamentals of Analytical Chemistry. In 2 Volumes. Volume 1. General Questions. Separation Methods: Textbook for High Schools]. Moscow: Vyschaya Shkola, 351 p. (in Russian).
ISSN 1573-160X PHYSICAL OCEANOGRAPHY VOL. 29 ISS. 5 (2022) 463
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accurately their total content in each seawater sample [1]. By the beginning of the 19th century, it became known that the relative content of the major salt components of seawater in the ocean is constant with a fairly high (but, as it turned out later, not absolute) accuracy (the principle of constant proportions, or Dietmar's law), so it is enough to determine the content of any one element in order to calculate total salinity. The most convenient parameter for measurement was the concentration of chlorides, or chlorinity [2]. Chlorinity was measured using direct titration and then converted to salinity using a simple linear function 2. At present, an improved ratio for oceanic water [3] is used, and its regional variants, for example, for the waters of the Caspian [4, 5] and Black seas [6], are also applied.
Since the early 1980s, salinity measurements are mainly carried out with CTD-(conductivity, temperature, depth) probes and are based on electrical conductivity, more precisely, on the ratio of the electrical conductivity of seawater to the conductivity of a special reference sample 3 (IAPSO Standard Seawater), which is taken from the surface in a certain area of the Atlantic Ocean [2]. The values of the seawater electrical conductivity at a fixed ion-salt composition are completely determined by salinity, temperature and pressure. The density dependence on temperature, salinity and pressure is determined by EOS-8O equation of state.
In 2010, a new international thermodynamic equation for seawater state TEOS-10 [7] was adopted. It relates the seawater density to its temperature, absolute salinity, and pressure. This equation, at a known density, can be applied to determine salinity very accurately, requiring special densiometric equipment to measure density independently.
The most reliable salinity values can be obtained from direct laboratory chemical concentrations of the major seawater ionic components [8] as the sum of the major ions. For some saline water bodies, such as, for example, the Aral Sea 4 [9-12], the Caspian Sea [13], and other water areas, this method is essentially the only one possible for the correct determination of salinity values. However, the salinity determination by the sum of the major ions is a rather laborious process, which also requires laboratory conditions and equipment.
Based on both field CTD-probing and laboratory studies of the ion-salt composition and density of samples taken during expeditions, the authors of the proposed work set themselves the task to analyze quantitatively the deviations from each other in salinity values obtained by all of the abovementioned methods. The main goal of the study was to assess the representativeness of the results of CTD salinity measurements, standard in oceanological practice, for the areas in which the ionic-salt composition of seawater differs from the "canonical" oceanic one. When writing the article, the materials of the dissertation were used 5.
2 Alekin, O.A. and Lyakhin, Yu.I., 1984. Chemistry of the Ocean. Leningrad: Gidrometeoizdat, 343 p. (in Russian).
3 OSIL, 2020. IAPSO Standard Seawater. [online] Available at: https://osil.com/salinity-measurement-standards/ [Accessed: 09 September 2022].
4 Blinov, L.K., 1956. [Hydrochemistry of the Aral Sea]. Leningrad: Gidrometeoizdat, 232 p. (in Russian).
5 Andrulionis, N.Yu., 2022. [Ion-Salt Composition of Waters of Marine Areas and Inland Water Bodies and Its Influence on Their Hydrophysical Characteristics]. Thesis Cand. Geogr. Sci. Moscow, 140 p. (in Russian).
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2. Materials and methods
Water samples from the Black Sea surface were obtained during coastal expeditions in 2014-2021 (Fig. 1, Table 1).
38 e 38.5 39 39.5 °e 40 36.3 36.4 36.5 °e 36.6 F i g. 1. Location of sampling stations in the Black Sea (2014-2021)
F i g. 2. Location of sampling stations in the Kara Sea (2018)
Water samples from the Kara Sea were obtained during the expedition of R/V Akademik Mstislav Keldysh (cruise No. 73) in 2018. The samples were taken from the surface in five areas of the sea: westwards of the Yamal Peninsula at a distance of about 160 km from the coast, near Beliy Island - about 60 km off the coast, near Shokalsky Island - about 120 km off the coast, as well as between the Taimyr Peninsula (60 km off the coast) and the Arcticheskiy Institut Islands (70 km off the coast) (Fig. 2).
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T a b l e 1
Location coordinates, names of the stations and water sampling dates
Station Station coordinates
Location of sampling Date of sampling
number ° N ° E
1 72.494444 64.170000
Kara Sea September 25-26, 2018 2 3 4 73.776667 73.984722 74.951111 70.476111 74.174167 83.805556
5 75.40 1111 85.222220
Black Sea, from the Feodosiya Bay to the Kerch Strait May 1, 2019 1a 6 24 44.987528 45.012694 45.291056 35.835806 36.209528 36.461444
31 45.183333 36.592972
12 45.071708 36.461732
17 45.103928 36.482090
September 1-8, 2019 20 23 45.119100 45.135783 36.555908 36.623403
24 45.288658 36.457697
28 45.223365 36.535535
31 45.182142 36.589330
6 45.016460 36.215190
16 45.100560 36.468800
23 45.132810 36.623840
24 45.291690 36.460600
July 1, 2020 30 45.193770 36.567890
Kerch Strait 31 45.178270 36.583490
32 45.034790 36.740890
36 45.099130 36.741730
41 45.066560 36.998340
1 45.349800 36.476900
2 45.301800 36.460700
3 45.271700 36.437500
December 15-16, 2021 4 5 45.244200 45.219800 36.421200 36.405700
6 45.229700 36.413600
7 45.178100 36.405900
8 45.166400 36.410700
9 45.059200 36.327143
Black Sea, river Sochi estuary May 27, 2014 1 43.573000 39.722000
3 43.583000 39.699000
Black Sea, the Gelendzhik October 1, 2020 M2 44.498883 38.125930
Bay r4 44.569766 38.033283
Caspian Sea, river Ural estuary April 11-12, 2016, April 14-17, 2017 9 12 17 46.874490 46.784050 46.741570 51.344090 51.577190 51.525490
Water samples from the Caspian Sea surface in the area near the mouth of the Ural (Zhaiyk) River were obtained during coastal expeditions in 2016 and 2017 (Fig. 3).
47 °N
46.8
ii 12 1 - T i " ~iJ%\---T "
i i • i
Caspian Sea
46.6
51 51.2 51.4 51.6 51.8 °E
F i g. 3. Location of water sampling stations in the Caspian Sea (2016 and 2017)
The water samples were placed in plastic bottles of 1 or 1.5 l in volume, which were pre-rinsed with water from the sample taken, sealed and delivered to the laboratory for further analysis. After determining the total alkalinity and total dissolved inorganic carbon according to the method described in the sources [14], the samples were filtered through a GF/F Whatman 0.7 ^m membrane filter to remove mineral and organic suspension and placed in glass containers of 100-250 ml in volume. To prepare reagent solutions and dilute samples, we used deionized water (conductivity < 0.2 pS/cm), which was obtained using a laboratory deionizer. The reaction of the solution medium during the analysis was monitored using a Metrohm combined pH electrode. The mass of the analyzed sample was measured by weighing on an OHAUS laboratory analytical balance of the first accuracy class with 0.001 g error.
Measurements of the water density of the studied samples were carried out using Anton Paar DMA 5000M precision density meter. The error in measuring the water density 7 was ±10-5 g/cm3. The density of the samples was measured at temperatures from 1 to 29 °C at atmospheric pressure. Before starting the work, the measuring cell was washed with ethyl alcohol at a concentration of 95% and deionized water. For each sample, 3-4 measurements were performed. The mean value was taken as the result. The maximum root-mean-square deviations of sample density amounted to 0.3 kg/m3 for the Black Sea, 0.2 kg/m3 for the Kara Sea, and 0.02 kg/m3 for the Caspian Sea.
Salinity values were determined in several ways. Practical salinity (SP) was measured simultaneously with sampling during the expeditions using CastAway (SonTek, USA), Rinko (JFE Advantech, Japan), and SBE 19plus (Sea-Bird, USA)
6 Guidance Document RD 52.10.743-2010 "Total Alkalinity of Sea Water. Measurement Technique by Titrimetric Method'. Moscow, 20 p. (in Russian); Guidance Document RD 52.10.243-92. "Guide to Chemical Analysis of Sea Waters ". Saint Petersburg, 264 p. (in Russian).
7 Anton Paar GmbH, 2012. Instruction Manual DMA 4100 M, DMA 4500 M DMA 5000 M. Austria: Anton Paar GmbH, 152 p.
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CTD-probes. The salinity calculation using TEOS-10 equation based on density measurements with an Anton Paar DMA 5000M density meter was performed using the MATLAB software with GSW Oceanographic Toolbox 8 package installed, which is recommended by TE0S-10 developers. To determine the seawater salinity using chlorinity values, the equations developed both for ocean water [8] and for the waters of the Black [6], Kara [3], and Caspian [4, 5] seas were used. The value of chlorinity was obtained by the classical titration method (Mohr method), namely, precipitation of halogens with silver nitrate 9 [2]. To determine the salinity of waters as the sum of the major ions (hereinafter, SS), the obtained concentrations of the major composition components were summarized. To determine the concentrations of components of the major ionic composition of the water bodies under study, a Metrohm 905 Titrando (Switzerland) automatic potentiometric titrator, equipped with indicator electrodes, was applied. The apparatus characteristics and the methodological features of its application are described in more detail in our paper [10].
In order to control the accuracy of measurements, similar determinations of the concentrations of major ions and density were also carried out on IAPSO standard seawater samples with a total practical salinity of 34.993 PSU, specially designed for calibrating instruments and verifying salinity measurements. The maximum deviation between the determination of salinity by the sum of ions and the absolute salinity of seawater from [3] was 0.08 g/kg.
The concentration of sodium ions was determined as a difference between the sum of anions and cations in mol equivalents. This method provides good results if all other ions are determined with sufficiently high accuracy 10 [8]. To verify the accuracy of this method in the laboratory of the Testing Center of Moscow State University, control determinations of the sodium ions concentration were performed by atomic emission spectroscopy in accordance with GOST R 57165-2016. The maximum difference between the average calculated value of sodium ions concentration and the averaged measurement data was 0.2 g/kg for a water sample from the Kerch Strait.
The SSW density was determined in the temperature range from 1 to 29 °C and the obtained values were compared with those calculated using TEOS-10 and EOS-80 formulas in order to evaluate the discrepancy in determining the density in two ways and, consequently, the accuracy of the apparatus (Fig. 4). The deviations of the SSW density values calculated using EOS-80 from the values obtained using a density meter averaged 0.2%, and when calculated using TEOS-10, an average of 0.003% from the values given by the density meter was obtained. This once again indicates the preference for using the new TEOS-10 equation of state for hydrophysical studies in the sea water. The density (ot, kg/m3) in Fig. 4 is determined by the formula gt = p • 1000 - 1000, where p is water density, g/cm3.
8 Pawlowicz, R. and McDougall, T.J., 2010. Thermodynamic Equation of Seawater. [online] Available at: https://www.teos-10.org/software.htm [Accessed: 08 September 2022].
9 Morachevsky, Yu.V. and Petrova, E.M., eds., 1965. [Methods for the Analysis of Brines and Salts]. Moscow, Leningrad: Chemistry, 399 p. (in Russian).
10 Reznikov, A.A., Mulikovskaya, E.P. and Sokolov, I.Yu., 1970. [Methods of Analysis of Natural Waters]. Moscow: Nedra, 488 p. (in Russian).
468 PHYSICAL OCEANOGRAPHY VOL. 29 ISS. 5 (2022)
28-1 2726-
cn g
25-
¿4
S 242322-
21l 0
Fig. 4. SSW density obtained in three ways: direct measurement using a density meter (DMA 5000M), calculation by salinity determined from electrical conductivity using EOS-8O, and calculation by salinity using TEOS-IO (a); deviations of the SSW density values calculated by EOS-8O and TEOS-IO, from those obtained using a density meter (DMA 5000M) (b)
3. Results 3.1. The Kara Sea
The nature of the salinity spatial distributions in the Kara Sea in connection with the propagation of desalinated plumes of the Ob, Yenisei, and other rivers was discussed in many works (for example, [15-18]). The salinity of the studied water samples of the Kara Sea ranged from 14 to 31 g/kg. The deviations of salinity values obtained from the electrical conductivity during CTD-probing from the values received as the sum of the major ions (AS), from the chlorinity (ASci), and from TE0S-10 equation based on direct laboratory measurements of density (AST) are given in Table 2.
T a b l e 2
Deviations of the salinity values obtained by electrical conductivity during CTD-soundings, from the values resulted as a sum of the major ions (AS), by chlorine content (ASci) and by the TEOS-IO equation based on direct laboratory density (AST) measurements for the water samples from the Kara Sea
Parameter Station
1 2 3 4 5
AS 3.0 1.4 2.2 0.0 0.8
ASci 2.4 1.2 1.4 1.7 0.2
AST 1.0 0.6 0.2 1.5 1.1
N o t e: the AS, ASci and AST values are presented as a percentage of the sample total salinity (by weight).
From Table 2 it can be seen that the deviations of the salinity values obtained from the electrical conductivity from the values obtained by other methods are from 0 to 3.0% for the studied samples.
The ratios of the major ions in the studied water samples of the Kara Sea differed from the "canonical" oceanic ionic composition, i.e. similar ratios for the SSW (Fig. 5).
F i g. 5. Deviations (in percent by weight) of the content of major composition components in the studied samples from their content in SSW, and the relationship of these deviations with salinity and AS, as well as with the location of a sampling station
A correlation between these deviations and the location of the sampling station is also observed. It is clearly seen that, firstly, deviations of the composition from
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the oceanic (AC) are most clearly manifested in the waters of low salinity, desalinated by continental runoff, and, secondly, these deviations are expressed primarily in an increased content of sulfate ions and a reduced content of chlorine ions.
According to our data, the content of SO4- ions (as well as the hydrocarbonate ion HCO-) in all samples from the Kara Sea was higher than in the SSW. In the samples from stations 1-3 and 5, the ratio SO2-/Cl- (equal to 0.14 for SSW) was more than 0.15, and at station 4 - more than 0.16, that is, the differences from the SSW composition in this indicator exceeded 13%.
As for the relative concentrations of other major ions, their deviations from the SSW composition were less significant, although they were also determinable. Thus, Ca2+ content in samples from stations 3-5 turned out to be 0.1-0.2% higher than in SSW, and in samples from stations 1 and 2, it approximately corresponded to the content in the SSW. K+ content in the samples from stations 1-3 was higher by 0.1% than in the SSW, and at stations 4 and 5 it corresponded to SSW. Na+ content in all samples was lower on average by 0.1% than in SSW, and Mg2+ content almost did not differ from its content in SSW.
Thus, the studies have demonstrated that salinity measurements using CTD-probing in the Kara Sea can lead to errors of up to 3% (several tenths of PSU). The features of the major ionic-salt composition of the Kara Sea are expressed primarily in the content of sulfate ions, an increased concentration of which (in relation to chloride ions) is observed in the areas affected by continental runoff. On the geochemical barrier river - sea, exchange processes occur. They lead to the transformations of the dissolved elements' runoff in the sorbed complex of freshwater terrigenous material, which are described in detail in [19]. It is known from this work that when terrigenous substances penetrate into the marine environment, ion-exchange transformation of the runoff of the dissolved substances occurs. The results of experimental data revealed that the actual Ca2+ input into the ocean with river runoff increases by 8.3-8.7% in the process of desorption of ions from solids, while the input of Na+, K+ h Mg2+, on the contrary, decreases by 14.0-14.6; 22.2-23.3 and 3.0-3.2% of their removal in the composition of river water runoff. The estimates demonstrated that the dissolution of 1 meq Ca2+ from terrigenous material is accompanied by absorption of about 0.72, 0.13, and 0.15 meq Na+, K+ and Mg2 from seawater. The ion exchange processes also affect the increase in the content of dissolved forms of microelements in the runoff, such as Mn2+, Co2+, Ni2+, Cd2+, Tl+, Ba2+ and NH-, and the decrease in the content of Pb2+, Cs+.
3.2. The Black Sea and the Kerch Strait
Since the proportion of freshwater runoff in the Black Sea water balance is much larger than in the ocean as a whole, the average salinity on the Black Sea surface (17.85 PSU) is almost half as much as the salinity of the World Ocean surface waters. The practical salinity, averaged over the entire volume of the Black Sea, is 21.96 PSU, in 0-300 m layer - 20.26 PSU, in the layer deeper than 2000 m - 22.26 PSU [18]. The deviations of the Black Sea water salinity values, determined by various methods, from the salinity values determined using the CTD-probe, are presented in Table 3.
Table 3
Deviations of the salinity values obtained by electrical conductivity during CTD-soundings from the values resulted as a sum of the main ions (AS), by chlorine content (AS. ]) and by the TEOS-IO equation based on direct laboratory density measurements ( VS / i for the water samples from the Black Sea and the Kerch Strait
Parameter Station number Mean value RMS deviation
J la 2 3 4 5 6 7 S 9 12 16 17 20 23 24 28 30 31 32 36 41 M2 T4
_1. Kerch Strait, May, 2019_
AS - 3.02 - 3.02 - 3.05 - 3.02 - - 2.69 _____ 2.98 0.20
ASci - 0.17 - 0.L7 - 0.84 - 0.17 - - 0.20 _____ 0.20 0.22
AST - 1.12 - 1.12 - 1.14 - 1.12 - - 1.55 ----- ¡.48 0.70
hd _2. Kerch Strait, September, 2019_
ffi AS - - __________ 2,48 3.23 3.46 3,09 3.25 _ 3.84 _____ 3.20 0.38
3 AS® - -- -- -- -- -- - 1.27 2.21 1.64 2.12 1.03 - 2.09 ----- 1.60 0.52
^ AST - -- -- -- -- -- - 1.29 1.52 1.35 1.53 1.76 - 1.48 ----- 1.44 0.18
¡> _3. Kerch Strait, July, 2020_
t-1 AS----- - 2.85 - 2.87 - - 2.39 1.94 - 2.70 1.97 2.88 2.12 2.22 - - 2.44 0.37
O ASci - - - - - - 0.67 - - - - 0.41 - - 0.47 0.39 - 0.40 -0.69 0.65 0.25 0.33 - - 0.32 0.3 S
Q AST ------ 1.48 _ _ _ _ 1.14 - - 1.70 1.20 - 1.60 1.50 2.37 1.09 1.27 - - 1.48 0.37
4. Kerch Strait, December, 2021
% AS ZIO - 2/70 220 230 230 Z50 2A0 Z60 2A0 2A7 - - - - - - - - - - - - - 2^47 (US
8 ASci 0.80 - -0.33 0.98 0.43 1.30 0.68 0.58 0.41 -0.01 0.54 _______ _ _____ 0.54 0.47
AST 1.61 - 3.26 1.67 2.31 1.70 2.07 2.41 2.48 1.90 2.16 - -- -- -- - _____ 2.16 0.50
^ 3. Estuaiy of river Sochi, May, 2014
^ AS 2A9 I I 4~50 I I I I I I I I I I I I I I I I I I I I 150 LOT
Kj ASci 1.39 _ _ 1.43 _____ _________ _ _____ l.4i 0.02
AST 3.13 - - 1.23 - -- -- - -- -- -- -- - - -- -- 2.18 0.95
^ _Gelendzhik Bay, September, 2020_
AS — — — ______ _________ _ ___ 3.57 3.00 3.29 0.29
^ ASci - - - ______ _________ _ ___ 3.31 3.11 3.21 0.10
AST - - - - -- -- - - -- -- -- -- - - -- 1.69 1.40 1.54 0.14
ryi N o t e: the AS, ASci and AST values are presented as a percentage of the sample total salinity (by weight).
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i 8 * * • • s X
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F i g. 6. Salinity values of the water samples from the Kerch Strait (2019-2020) obtained in different ways: summing of principal ions (SS), recalculating the electrical conductivity (SP), using the TEOS-10 equation (SA is the absolute salinity which takes into account the regional correction (SAS) to SP, and SPT is the practical salinity of a sample calculated by density [7]) and by chlorine content using the ratio (S) from [6]
The salinity values of water samples from the Kerch Strait, obtained by various methods in 2019-2020, are presented in Fig. 6.
Similar studies were carried out for water samples taken near the mouth of the Sochi River in May 2014, during the passage from the Feodosiya Bay to the Kerch Strait in May 2019, and also from the Gelendzhik Bay in October 2020.
The highest deviations in all areas were noted for AS, i.e. the difference between the CTD-probing data and the sum of ions (up to 3.5% or 0.6 g/kg, near the mouth of the Sochi River during the spring flood). The smallest AS values (2.44% or 0.5 g/kg) correspond to the July measurements in the Kerch Strait. The salinity calculations by chlorinity and by TEOS-10 equation based on density measurements provide somewhat better agreement with CTD measurements, however, they also show significant discrepancies (up to 2% or more). In general, the determination of SAS using TEOS-10 equation gave the results that are the closest to SS, which is especially evident in the samples from the Kerch Strait.
F i g. 7. Deviations in the content of major ions in the compositions of the studied water samples taken in the Kerch Strait (December, 2021) from their content in SSW, and the relationship of these deviations with SS (a) and AS (b)
Fig. 7 shows the graphs of the content of the major composition components in the samples of the Kerch Strait waters of different salinity, taken in December 2021, and the related deviations AS. It can be seen that the maximum deviations of the ionic-salt composition from the oceanic one were noted at station 2, the waters in its area were the most desalinated among all stations by the Sea of Azov waters. At the same station, the errors of CTD measurements of salinity AS were also the greatest (with regard to salinity values calculated from the sum of salts), reaching 2.7% here. On the contrary, in the water at station 9 characterized by the highest salinity and, consequently, the smallest proportion of freshwater runoff, the differences in the ionic composition from the SSW composition were the smallest. As for the analyzed samples of the Kara Sea, the differences in the ion-salt composition from the ocean in the areas of continental runoff effect are also manifested in the Kerch Strait, primarily in an increase in the sulfate-chloride ratio, and also (to a lesser extent) in a reduced Na+ content and an increased K+ and HCO- content. The processes of the dissolved elements runoff transformation under the effect of exchange processes in the sorbed complex of freshwater terrigenous material at the geochemical river - sea barrier are described in [19].
3.3. The Caspian Sea
The Caspian Sea is an inland water body not connected with the World Ocean; therefore, the ratios of the major ions in its waters are very different from their ratios in the SSW. In addition, the ionic composition of the sea waters is not the same in different areas due to the strong effect of river runoff [13]. In this regard, the correct salinity measurement in the Caspian Sea presents significant difficulties.
Fig. 8 demonstrates the deviations of the major components of the ion-salt composition of the Caspian Sea water at the mouth of the Ural (Zhaiyk) River on the SSW composition along with the corresponding salinity values calculated as
474 PHYSICAL OCEANOGRAPHY VOL. 29 ISS. 5 (2022)
the sum of ions, and in Fig. 9 - salinity values of water samples from the estuarine seashore area of the Ural River of the Caspian Sea, obtained by various methods, including those calculated using previously published special regional formulas for determining the salinity of the Caspian Sea waters by chlorinity (S) [4] and density (Sp) [5]. The latter formula has the following form:
(p - p0) / S = 0.924 ± 0.00015,
where p is the density of the Caspian Sea water sample; p0 is distilled water density.
F i g. 8. Deviations in the content of major ions in the composition of the studied water samples taken in the Caspian Sea with different salinity from their content in SSW in 2016, and the relationship of these deviations with SS (a) and AS (b)
F i g. 9. Salinity of the water samples from the estuary region of the river Ural in the Caspian Sea (2016-2017) obtained by different methods
It should be noted that the relative content of chloride ions in the studied samples of the Caspian Sea waters is on average 15% lower, and sulfate ions content - by the same amount higher than in the SSW. The significant chemical features of these waters should also include a 4% lower content of Na+ (relative to SSW) and an increased content of other cations. The content of calcium ions exceeded their content in SSW by an average of 2%.
The results of the major salt composition study also indicated its significant interannual variability. Thus, the content of sodium ions in the samples of 2016 was lower than in the SSW by 3%, and in the samples of 2017 - by 8%. The content of potassium ions was lower than in SSW, on average by 0.2% in 2016 and by 0.1% in 2017. And the content of magnesium ions, on the contrary, was higher than in the SSW by 0.1% in 2016 and 2% in 2017.
In [4], a deviation of about 1.4 g/kg (13%) was reported in the salinity values calculated from the electrical conductivity from the values calculated from the density in the southern part of the Caspian Sea with a total water salinity of 1012 g/kg. Similar deviations in the Caspian Sea northern part for the estuary seashore of the Ural River amounted to 0.2-1.1 g/kg (3-27%) (Fig. 9). In Table 4, the deviations of the salinity values obtained from the electrical conductivity from the values received by other methods for the waters of the studied samples are given.
As can be seen from Table 4 and Fig. 9, for the waters of the Caspian Sea, the values of practical salinity determined by standard CTD-probing in almost all cases turn out to be strongly underestimated in relation to the results of independent determinations by other methods. If the sum of salts is taken as the "reference" value, then this underestimation reaches 1.6 g/kg, or 52% (!), with a practical salinity of the sample of about 2.9 PSU.
T a b l e 4
Deviations of the salinity values obtained by electrical conductivity during
CTD-soundings from the values resulted as a sum of the major salt components ions
(AS), by chlorine content (ASci) and by the TEOS-10 equation based on direct
laboratory density
measurements (AST) and by density by means of the regional ratio (ASp) [5] for the water samples from the Caspian Sea
Station number Mean value
9 12 17
Parameter
RMS deviation
April, 2016
AS ASci AST ASp
12.20 7.30 24.91 9.85
9.10 11.40 21.48 5.81
3.60 4.30
18.78
2.61
8.33 7.67 21.7 6.09
2.05 1.68 1.45 1.71
May, 2017
AS ASci AST ASp
52.20 30.00 64.93 26.95
32.70
20.90
43.91 16.33
-1.00 -7.70 8.73 -10.78
27.97 31.36 39.19 25.26
12.69 13.49 13.39 12.39
N o t e: the AS, ASci and AST values are presented as a percentage of the sample total salinity (by weight).
4. Discussions
In the ion-salt composition of the seawater of all the considered water areas, there are clearly recorded differences from the oceanic composition. They are expressed primarily in a shift in the sulfate-chloride ratio towards its increase, i.e. in an increase in the relative content of sulfate ions (by 0.4-0.8% for the Kara Sea, 1.0-1.8% for the Black Sea and the Kerch Strait, 14-16% for the Caspian Sea) and a similar decrease in the relative content of chloride ions. In these areas, there is also a decrease in the relative content of sodium ions (0.1-0.2% for the Kara Sea,
0.1.0.5% for the Black Sea and the Kerch Strait, 1-5% for the Caspian Sea) due to an increase in the content of other cations, as well as bicarbonate ions. These deviations are inversely related to salinity, i.e. they become more pronounced, the larger the proportion in the sample belongs to freshwater continental runoff.
The deviations of the ion-salt composition from the SSW composition lead to the fact that salinity values according to the results of CTD-probing are systematically underestimated. For the studied samples, this underestimation reached up to 3% (or about 1 g/kg) in the Kara Sea, up to 3.5% (or about 0.6 g/kg) in the Black Sea, and up to 52% (or about 1.6 g/kg) in the Caspian Sea. For the Black and Caspian seas, the errors in salinity values according to CTD data are generally greater, the greater the deviation of the ionic composition (expressed, for example, in the sulfate-chloride ratio) from the SSW composition, as expected. However, for the samples from the Kara Sea, such a regularity could not be found. Thus, the performed studies show that the errors in salinity determinations associated with variations in the ion-salt composition in water areas affected by continental runoff are significant and must be taken into account in oceanological practice.
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About the authors:
Natalia Yu. Andrulionis, Junior Research Associate, Laboratory of Ocean-Land Waters and Anthropogenic Processes, P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences (36 Nakhimovsky Ave, Moscow, 117997, Russian Federation), ORCID ID: 0000-0001-9141-1945, Web of Science ResearcherlD: AGP-4038-2022, Scopus Author ID: 57209575290, natalya@ocean.ru
Petr O. Zavialov, Head of the Laboratory of Ocean-Land Waters and Anthropogenic Processes, Deputy Director, P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences (36 Nakhimovsky Ave, Moscow, 117997, Russian Federation), Dr.Sci. (Geogr.), Corresponding Member of RAS, ORCID ID: 0000-0002-3712-8302, Scopus Author ID: 6603611237, ResearcherlD: E-7026-2014, peter@ocean.ru
Aleksandr S. Izhitskiy, Senior Research Associate, Laboratory of Ocean-Land Waters and Anthropogenic Processes, P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences (36 Nakhimovsky Ave, Moscow, 117997, Russian Federation), ORCID ID: 0000-0001-6156-6460, Web of Science ResearcherlD: E-6914-2014, Scopus Author ID: 55941565100, izh@ocean.ru
Contribution of the co-authors:
Natalia Yu. Andrulionis - initiated the goals and objectives of the research work, as well as the development of methods for studying the ion-salt composition and salinity, participated in field studies and sampling, performed all laboratory analyzes, analyzed the data obtained, prepared graphic material and the main text of the paper
Petr O. Zavialov - participated in setting the goals and objectives, field research and sampling, development of approaches to the analysis of laboratory material, writing and editing the text of the paper
Aleksandr S. Izhitskiy - participated in field studies and sampling, as well as in the preparation of the graphic material of the paper
The authors have read and approved the final manuscript.
The authors declare that they have no conflict of interest.
