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5Features of the Oxygen and Sulfide Vertical Distribution in the Black Sea Based on the Expedition Data Obtained by Marine Hydrophysical Institute in 1995-2015
S. I. Kondratev*, A. V. Vidnichuk
Marine Hydrophysical Institute, Russian Academy of Sciences, Sevastopol, Russian Federation
*e-mail: skondratt@mail.ru
Discussed are the features of the oxygen and sulfide vertical distribution resulted from the expedition data obtained by Marine Hydrophysical Institute in the northern part of the Black Sea (within the economic zone of Russia) in 1995-2015. In course of the last 20 years, the upper boundary of the sulfide zone in this area, defined by the sulfide isoline 3 ^M, did not change its position in the scale of relative density and is located on at = 16.10-16.15 kg/m3. Closer to the northwest shelf (in the area of the Sevastopol anticyclone), the hydrogen sulfide isoline 3 ^M is slightly lifted to at = 15.916.0 kg/m3; there is a single case when its position was recorded at at = 15.85 kg/m3. Position of the suboxic zone upper boundary (the threshold oxygen isoline 10 ^M) is characterized by spatial and temporal variability. During four expeditions in 2009-2013, in the region of the northwestern shelf the suboxic zone upper boundary was located, regardless of the season, within at = 15.6-15.7 kg/m3. In November, 2013 in the deep sea part, the threshold oxygen isoline 10 ^M was located on at = 15.7 kg/m3, whereas in November, 2015 it was considerably higher: on at = 15.35 kg/m3. The change of location of the threshold oxygen isoline 10 ^M observed in 1995-2015 provides no possibility to assess unambiguously alteration of the suboxic zone thickness in the central part of the sea. Thus within the depths 1750-2000 m in the Black Sea, no significant increase of the sulfide content in 1995-2015 has been recorded.
Keywords: oxygen, sulfide, suboxic zone, hydrogen sulfide zone, Black Sea.
Acknowledgments: the investigation is carried out within the framework of the state task on the theme № 0827-2018-0003 "Fundamental studies of the oceanologic processes conditioning the marine environment state and evolution under the impact of natural and anthropogenic factors based on the methods of observations and modeling (code "Oceanological processes") and the theme № 08282018-0009 "Physical mechanisms of formation of spatial-temporal structural features of hydrochemi-cal characteristics in different areas of the Black Sea active layer" (in the joint project "Interaction of physical, chemical and biological processes in the World Ocean") state registration № AAAA-A18-118013190197-7 according to the state task № 007-00080-18-00 on 2018 and on the planning period on 2019 and 2020 according to the research plan of FSBSI "A.O. Kovalevsky Institute of Marine Biological Researches, RAS" on 2018 - 2020.
For citation: Kondratev, S.I. and Vidnichuk, A.V., 2018. Features of the Oxygen and Hydrogen Sulfide Vertical Distribution in the Black Sea Based on the Expedition Data Obtained by Marine Hydro-physical Institute in 1995-2015. Physical Oceanography, [e-journal] 25(5), pp. 390-400. doi:10.22449/0233-7584-2018-5-390-400.
DOI: 10.22449/1573-160X-2018-4-390-400
© 2018, S. I. Kondratev, A. V. Vidnichuk © 2018, Physical Oceanography
Introduction
Location of the sulfide zone in the Black Sea was systematically studied by the Soviet and, later, Russian oceanologists in the 1970s-1990s. In the 90s, comprehensive analysis of the obtained results gradually brought to development and application of the relative density at-scale in order to make the data interpretation more logical and simple [1-12].
Since the early 90s, large-scale expeditions in the Black Sea covering the area from the Danube mouth to the Caucasian coast transformed into the studies of individual countries within their economic zones. For Marine Hydrophysical Institute (MHI), the 33rd cruise of the R/V "Professor Kolesnikov" in spring, 1995 became the final many-sided expedition in the Black Sea. Its results were discussed in the preprint [13]. The next complex expedition in the Black Sea (numerous group of the MHI scientists took part in it) was a cruise of the Bulgarian R/V "Akademik" in the region of the Sevastopol anticyclone in May, 2004. The obtained data analyzed the location of the upper and lower boundaries of the suboxic zone (the lower boundary of this zone is the upper boundary of the sulfide zone) above the continental slope [14].
The subsequent MHI expeditionary investigations were of a shortterm occasional character. The main area of researches reduced up to the northwestern shelf (NWS) of the Black Sea [15-17]. In the deep-sea part, the continental slope in the region of the Sevastopol anticyclone was studied rather constantly.
Reunification of Russia and Crimea, and the institute's transfer to the Russian Academy of Sciences started a new stage of expeditionary researches of MHI. In autumn, 2015 a joint expedition of MHI and the Institute of Oceanology (Moscow) was run in the region of the "century" section Cape Chersonese - Bosporus Strait. The expeditionary research in the deep-sea area along the Crimea coast and within the economic zone of Russia was planned for 2016-2017. Elaboration of a strategy for subsequent hydrochemical studies in the Black Sea laid down in MHI requires systematization of separate incomplete results obtained by the institute scientists after 2004. They are planned to be subsequently compared with the data that will be gotten from the future studies of the suboxic and sulfide zones. The present paper deals with this problem.
Materials and methods
The scheme of the MHI hydrochemical stations during the cruises in 20092015 is shown in Fig. 1. Sampling for chemical analysis was done using a 12-bottle cassette of a CTD-probe (produced by Sea-Bird Electronics, Inc.) at the depth of certain isopycnic surfaces. As a rule, the samples for sulfide were taken at the maximum depth and then along the at = 16.30; 16.20; 16.15; 16.10; 16.05; 16.00; 15.90; 15.80; 15.60; 15.40 and 15.20 kg/m3, which covered the suboxic zone completely. At the utmost southwestern station on the section Cape Chersonese -Bosporus Strait, the samples for sulfide were taken at 28 horizons: slightly less than 2000 m, then in every 100 m and, finally, at the at cited above.
Such sampling permitted to define location of the upper boundaries of the intermediate suboxic layer and the sulfide zone, as well as the suboxic layer thickness, resolution is up to Aat = 0.05. It corresponds to the accuracy equal approximately to 5 m, according to the depth scale.
The sulfide content was defined by the iodometric method assuming that iodine consumption on the at = 15.8 kg/m3 is equal to zero and the oxygen concentration - by the Winkler method modified for sampling with low oxygen content according to the method [18]. In both cases, 15 minutes before sampling, the 200 ml volumetric flasks for fixing sulfide and the oxygen bottles with a narrow neck were flushed with argon.
Fig. 1. Scheme of the stations where the H2S and O2 vertical profiles were done: + is R/V "Sapfir", May, 2009; ◊ is R/V "Professor Vodyanitsky", October, 2010; □ is R/V "Professor Vodyanitsky", August, 2011; o is R/V "Professor Vodyanitsky", September, 2013; • is R/V "Maria S. Merian", November, 2013; x is R/V "Professor Vodyanitsky", November, 2015. Solid line marks the section carried out in November, 2013
Discussion of the results
The relatively detailed studies of the R/V "Akademik" in the region of the quasi-stationary Sevastopol anticyclone in 2004 showed that in the areas of the shelf transformation to the continental slope, the sulfide isoline 3 ^M was located on the at = 15.88 kg/m3, whereas in the deep sea region the same sulfide concentrations were observed on the at = 16.05 kg/m3 [14]. Thus, it seems logical to consider individually the vertical profiles of oxygen and sulfide obtained in the region of the Sevastopol anticyclone continental slope in 2009-2013 and those obtained in the central deep-sea part in 2013-2015 and shown in Fig. 2, 3, respectively.
First of all, Fig. 2 demonstrates a good coincidence of configurations of the sulfide content data resulted from different surveys. That, in fact, permits to analyze all the data. The results obtained in May, 2009 do not fall into the pattern as they show that the sulfide concentrations within the interval at = 16.016.4 kg/m3 in the region of the continental slope at the NWS edge were slightly higher than those observed in all the other regions. This fact confirms the conclusion [14] that the sulfide isolines at the beginning of the continental slope go up to the higher at, but it requires more scrupulous studies. In particular, it seems reasonable to include several stations where the depths correspond to the at location near 16.1-16.2 kg/m3 (approximately at 150-170 m) in the subsequent investigations of the vertical structures of dissolved oxygen and sulfide. The sampling interval should be Aat = 0.05 kg/m3 up to the at = 15.7 kg/m3.
Fig. 3 provides a more detailed pattern of the oxygen and sulfide vertical distribution between the at = 15.0-16.3 kg/m3 where the suboxic zone is located; its upper boundary coincides with the oxygen isoline 10 ^M (in some papers -20 ^M), and the lower one - with the sulfide isoline 3 ^M (in some papers -5 ^M).
£ 13
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v- oxygen, May 2009
+ -oxygen, Oct 2010
□ - oxygen, Aug 2011 X- oxygen, Nov 2013 +4f f * 0»
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К -sulfide, Aug 2011
-sulfide, Nov 2013 :it® » ci (i atT»m
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0 100 200 300 400
Oxygen and sulfide concentrations, jiM
V- oxygen, Sept 2013 □ - oxygen, Nov 2013 4- - oxygen, Nov 2015
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Д- sulfide, Sept 2013 О -sulfide, Nov 2013 О-sulfide, Nov 2015
100 200 300 400 Oxygen and sulfide concentrations, цМ
b
Fig. 2. Vertical profiles of oxygen and sulfide in the region of the Sevastopol anticyclone in 20092013 (a; and in the deep part of the Black Sea in 2013-2015 (b)
a b
Fig. 3. Vertical profiles of oxygen and sulfide in the region of the Sevastopol anticyclone in 20092011 (a) and in the deep part of the Black Sea in 2013-2015 (b)
Fig. 3, a shows that in August, 2011, at the most western station, located at the beginning of the continental slope on the depth ~ 120 m (see Fig. 1), an almost zero oxygen content was recorded at the at = 15.2 kg/m3. This corresponds to the highest location of the suboxic zone upper boundary revealed in course of the studies.
Except for the already above-mentioned increased sulfide concentrations observed in May, 2009, Fig. 3 also shows that the oxygen concentrations exceeding 10 ^M were recorded above the shelf edge at the at = 15.7 kg/m3 only in November, 2013, whereas in the deep-sea part such values were observed
a
regularly. As for the at > 15.8 kg/m3, the oxygen content at them was lower than 10 ^M in all the performed surveys.
The suboxic zone configuration and dimensions can be defined more accurately by the oxygen and sulfide vertical distributions at single sections. Thus, as for the Sevastopol anticyclone region, distribution of these elements on the sections perpendicular to the shelf edge in 2009-2011 is represented in Fig. 4. In May, 2009, on two sections, the sulfide isoline 3 ^M was located above the at = = 16.0 kg/m3 rising (in the maximum case) up to at = 15.85 kg/m3 (Fig. 4, a). These data coincide with the results of the studies carried out from board the R/V "Akademik" in May, 2004 [14].
c d
Fig. 4. Vertical distribution of the oxygen and sulfide concentrations, ^M, on the Sevastopol anticyclone sections in May, 2009 (the meridian 32° E (a), the meridian 32.5° E (b)), August, 2011 (c) and October, 2010 (d); ◊ and • denote the horizons of sampling
Most likely, the above noted sulfide isoline elevation (in the density scale) represents a seasonal phenomenon since in the very same region in summer, 2011 the sulfide isoline 3 ^M was located below the at = 16.0 kg/m3, and in autumn, 2010 - just a little lower than the at = 16.1 kg/m3. In other words, during transition from spring to autumn, the gradual deepening of the sulfide isoline 3 ^M is observed. However, one cannot exclude influence of water dynamics (for example, various phases of the Sevastopol anticyclone development [19-21]) on location of the upper boundary of the sulfide zone in this region.
On three sections the upper boundary of the suboxic zone (oxygen isoline 10 ^M) was located approximately at the at = 15.6 kg/m3 (Fig. 4, a). Fig. 4, a, b shows increase of the oxygen content in the northern part of the sections above the continental slope. This feature of the oxygen distribution was revealed due to application of the oxygen sensor, which determined oxygen concentration with the resolution 0.5 m. But after 2009 it was not used that resulted in rougher and less reliable distributions.
In November, 2015 the "century" section Cape Chersonese - Bosporus Strait in the central part of the Black Sea was investigated. The parameters of its suboxic zone can be compared with the long-term data [6] (Fig. 5).
Years
^ & & J? J3 JP
43,6° 43,7" 43,8° 43,9° 44,0° 44,1° 44,2° 44,3° N
a b
Fig. 5. Vertical distribution of the oxygen and sulfide concentrations, ^M, on the "century" section Cape Chersonese - Bosporus Strait in 2015 (a) and the one based on the multi-year data [6] (b)
Fig. 5 shows that in 2015 the lower boundary of the suboxic zone, i.e. the sulfide isoline 3 ^M, was within the at = 16.10-16.15 kg/m3 (the distance between them, according to the depth scale, is about 5 m). The same position of 3 (iM sulfide isoline was observed on the long-term data given in [6] (Fig. 5). According to the scientific literature data, the approximately same position of the sulfide isoline 3 ^M was observed to the east and west off the "century" section Cape Chersonese - Bosporus Strait: in the northwest - at at = 16.12-16.15 kg/m3, in the southeast - at at = 16.10-16.20 kg/m3 [22-24]. Such long-term and spatial stability of location of the suboxic zone lower boundary is explained by sulfide oxidation due to the oxygen-containing Marmara Sea water inflow through the Bosporus Strait, whereas the vertical oxygen flow provides oxidation only of ~ 5 % of sulfide rising from the sea depth [10].
As for location of the suboxic zone upper boundary, it is characterized by temporal variability. This is confirmed both by the long-term data (Fig. 5) and the results of the MHI observations in 2009-2015. So, in 2009-2011 in the Sevastopol anticyclone region, the oxygen isoline 10 ^M was located on the at = 15.6015.65 kg/m3 (Fig. 4), and in November, 2015 on the "century" section Cape Chersonese - Bosporus Strait it was much higher - on the at = 15.35-15,40 kg/m3 (Fig. 5).
The most detailed (the oxygen sensor with the vertical resolution 0.5 m was used) and extensive (the whole northern part of the Black Sea - in Fig. 1 this section is shown by the solid line) investigation of the suboxic zone upper and lower boundaries was got in the 33rd cruise of the R/V "Maria S. Merian" in November, 2013 (Fig. 6, a).
32" 33" 34° 35° 36° E 32° 33" 34" 35" 36" E
a b
Fig. 6. Location of the upper and lower boundaries of the suboxic zone (a) and the cold intermediate layer (b) on the section from the northwestern shelf (on the left) up to the northeastern one (on the right) in November, 2013
The sulfide isoline 3 ^M, as well on for the previously considered sections, was located within a very narrow interval between the at = 16.12-16.15 kg/m3. The oxygen isoline 10 ^M is characterized by its higher location in the section western part - at at = 15.75 kg/m3 than in the section eastern part - at at = 15.85 kg/m3. Most likely, this is due to more intensive oxygen consumption for oxidation of suspended organic matter. Its high content in the NWS waters is provided by the freshwater runoff, which ensures the phytoplankton growth and subsequent death processes.
The vertical temperature distribution in the at scale was analyzed for the section across the northern Black Sea (Fig. 6, b). It exhibits presence of two CIL cores in the western and eastern parts with the minimum temperature 7.83 °C and absence of such a core (i. e. CIL weakening) in the central part at the longitude 34 E.
These features of CIL (the thicker the layer and lower the temperature, the higher the oxygen content in it) did not affect location of the oxygen isoline 10 ^M, but became quite well pronounced in position of the oxygen isolines within the 1550 ^M range: these isolines were elevated at the longitude 34° E as compared to the data of the nearest stations. In other words, weakening of CIL and, consequently, decrease in supply of the underlying layers with oxygen resulted in diminution of the oxygen content in them.
The values of the sulfide maximum concentrations in the Black Sea revealed in different expeditions should be considered as an individual problem. Table and Fig. 7 represent the data obtained in 2010-2015. It follows from these data that at the depths exceeding 1700 m, where practically no gradient in the vertical distributions of salinity, temperature and sulfide concentrations is observed [25], the concentration values should constitute about 380 ^M. This exceeds
significantly the values observed in the 1960-1970s, but does not differ from the values typical of the sulfide concentration at the depth about 2000 m in the 1980-1990s [10]. Thus, the drawn conclusion implies that in course of previous 20 years no significant increase of the sulfide content in the Black Sea near-bottom waters was recorded.
Maximum sulfide concentrations revealed in the MHI expeditions in 2010-2016
Period
Depth, m Relative density, kg/m3 Concentration, ^M
October, 2010 1743
August, 2011 1250
September, 2013 1750
November, 2013 2136
November, 2015 1801
17.219
17.205
17.220
17.206 17.216
378 354
379 382 384
Fig. 7. Vertical distribution of sulfide in 2009-2016 on the depths below 400 m according to the scales of relative density (the lower graph) and depth (the upper graph)
Conclusions
In course of the past 20 years, the sulfide zone upper boundary, defined by the sulfide isoline 3 ^M, has not changed its location in the northern part of the Black Sea and is still at the at = 16.10-16.15 kg/m3. Closer to the NWS, in the Sevastopol anticyclone region the sulfide isoline 3 ^M rises to at = 15.9-16.0 kg/m3; in one case its position was recorded at at = 15.85 kg/m3.
Location of the suboxic zone upper boundary defined by the oxygen isoline 10 ^M is characterized by spatial and temporal variability. In the NWS region where four expeditions were carried out in 2009-2013, the oxygen isoline 10 ^M was located within the at = 15.6-15.7 kg/m3, regardless of the season. In the deep-sea region, in November, 2013 the oxygen isoline 10 ^M was located on the PHYSICAL OCEANOGRAPHY VOL. 25 ISS. 5 (2018) 397
at = 15.7 kg/m3, whereas in November, 2015 it was much higher - on the at = = 15.35 kg/m3. Variation of position the oxygen isoline 10 ^M observed during 1995-2015 provides no grounds for assessing unambiguously alteration of the suboxic zone thickness in the central part of the sea.
During last 20 years no drastic increase of the sulfide content took place in the Black Sea waters within the depths 1750-2000 m.
It seems reasonable to continue investigations of the suboxic zone and, for this purpose, to carry out a few stations on the continental shelf edge. At that the at = = 16.1-16.2 kg/m3 should be located in the bottom layer. The sea depth in the points of such stations should be about 150-170 m. Rather flat continental slope to the south off the Cape Tarkhankut or to the south off the Kerch Strait is assumed to be a convenient region for such an experiment.
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About the authors:
Sergey I. Kondratev - Senior Research Associate, Department of the Sea Biogeochemistry, FSB SI MHI (2 Kapitanskaya Str., Sevastopol, 299011, Russian Federation), Ph.D. (Chem. Sci.), ORCID ID: 0000-0002-2049-7750, skondratt@mail.ru
Anna V. Vidnichuk - Senior Engineer, Post-graduate, Department of the Sea Biogeochemistry, FSBSI MHI (2 Kapitanskaya Str., Sevastopol, 299011, Russian Federation), anna_vidnichuk@mhi-ras.ru
Contribution of the co-authors:
Sergey I. Kondratev - statement of the investigation tasks and problems, analysis of the scientific literature data, data analysis and interpretation, discussion of the investigation results, preparation of the paper text
Anna V. Vidnichuk - analysis of scientific literature on the problem under study, data accumulation and systematization, construction of the graphs and figures, discussion of the investigation results
All the authors have read and approved the final manuscript.
The authors declare that they have no conflict of interests.
