Sea-air CO₂ flux in the northeastern part of the Black Sea

全文:

Introduction

The global cycle of natural substances includes their transport among various biogeochemical reservoirs and regulating balance and budget of substances in the atmo-, litho- and hydrosphere. One of such natural cycles is the carbon cycle, the most important component of which is carbon dioxide (СО₂) ¹ [1–5].

СО₂ is one of the green gases [1–6] and its entry into the atmosphere and further redistribution in the waters of the World Ocean not only plays a significant role in the formation of the climate on the Earth [1], but also affects the characteristics of waters [1, 6, 7].

The waters of the World Ocean are still its natural stock despite the continuous increase in the level of atmospheric CO₂ (about 0.4% per year) and to date its content achieves more than 420 µatm (https://gml.noaa.gov/ccgg/trends/mlo.html). They absorb up to 25% atmospheric CO₂ from anthropogenic emission, thereby support to reduce CO₂ concentrations in the atmosphere [7]. However, its accumulation in the water column leads to negative consequences for the ecosystems of the World Ocean which is revealed in the disruption of natural balances, in particular carbonate ones, decrease in pH and oxygen concentration and emergence of oxygen deficiency zones. The Ocean’s ability to absorb carbon dioxide from the atmosphere decreases over time [8–10] and waters can even become a source of CO₂ for the atmosphere in some extreme cases [7].

The primary factor determining the influence of CO₂ on the state of marine systems is its flux from the atmosphere which depends, other things being equal, on the ratio of the partial pressure of CO₂ in the near sea surface atmosphere and the equilibrium partial pressure of CO₂ in the sea surface. This ratio determines the direction and values of the CO₂ flux.

An important aspect of the research of the sea–air CO₂ flux and the pCO₂ value in the sea surface is the study of the nature of changes on time scales from seasonal to interannual which is associated with significant spatial and temporal variability of biological and physical processes affecting these characteristics.

Inland seas are characterized by more intense physical and biogeochemical processes compared to open areas of the World Ocean. As a result, their ecosystem is more dynamic on a temporal and spatial scale and any external influence manifests itself more quickly. First of all, such manifestations include changes in the characteristics of the system: oxygen and CO₂ concentrations, pH values, as well as speed and direction of production and destruction processes [10]. Moreover, these ecosystems are characterized by a more pronounced response to changes in CO₂ concentration in the atmosphere which manifests itself primarily in a shift in the carbonate system equilibrium, as well as changes in redox conditions ¹⁾ [5–7, 10].

The research of inland seas makes it possible to study the influence of atmospheric CO₂ on the characteristics of waters and to assess the contribution of regional ecosystems to the total budget of the CO₂ flux of the World Ocean.

The Black Sea is one of such inland seas. The shelf water characteristics of the northern part of the sea are largely determined by freshwater river runoff and atmospheric contribution, of the northeastern part – by the Azov Sea waters, of the deep-water part – by the Rim Current [11]. This sea is characterized by a wide range of changes in salinity and temperature [11], high intensity and seasonal changes in primary production processes [12], high values of alkalinity and total inorganic carbon content [13–15]. All this largely determines the state of the carbonate system of sea waters, the CO₂ content in the sea surface and the formation of the sea–air CO₂ flux.

The factors listed above are influenced by seasonal variability. Accordingly, both CO₂ concentration and CO₂ flux also show intra-annual variability.

It can be assumed that during a cold period, the CO₂ concentration should be primarily determined by an abiotic factor – temperature and vertical transport of CO₂ by deep waters, as well as sea–air metabolic processes. In summer, the predominant factor should be biotic due to the occurrence of biogeochemical processes involving organic matter.

The purpose of this work was to obtain numerical estimates of the sea–air CO₂ flux and to identify its direction and the factors that determine the values of the CO₂ flux in the area of the Crimean coast of the Black Sea during the cold period when the contribution of the abiotic factor predominates.

Previously, the CO₂ flux estimates for this Black Sea ecosystem were carried out based on calculated data [13] or for a local area [14].

Materials and methods

The data obtained during the cruise of R/V Professor Vodyanitsky in December 2022 (the 125th cruise, 02–27.12.2022) were used in this work. According to [11], this period refers to late autumn.

Fig. 1 shows the area under study and sampling map. The studied area includes a 12-mile zone of the Crimean coast in the northern part of the Black Sea.

 

Fig. 1. Sampling map

 

Samples from the near sea surface atmosphere were taken at a height of 10 m above sea level. The air intake tube was located in such a way as to avoid the CO₂ influx from the working mechanisms of the vessel, if possible. An LI-7000 by LI‑COR infrared analyzer with a working range of CO₂ concentration of 0–3000 ppm and water vapor of 0–60 mmol/mol was used to determine the volumetric concentration and partial pressure of CO₂ directly. In this case, the measurement error is less than 1% of the measured values [15].

Water samples were taken from the sea surface (1–3 m) using a continuous seawater supply system. Next, the water was transported at a constant speed to an equilibrator with the help of which equilibrium was established with a certain volume of atmospheric air at the temperature of sea water according to the method described in [15]. Air from the equilibrator was pumped at a constant speed through the cell of the LI-7000 by LI‑COR infrared analyzer in which the concentration of CO₂ and water vapor was determined at the cell temperature. The temperature of the cell is determined by a temperature sensor installed inside it and is in equilibrium with the temperature of the atmosphere surrounding the equilibrator. Next, the carbon dioxide concentration was converted to the partial pressure of carbon dioxide:

pCO₂ = x (CO₂) ∙ p_ATM,

where p(CO₂) is partial pressure of carbon dioxide, µatm; x (CO₂) is carbon dioxide concentration, µmol/mol; p_ATM is atmospheric pressure, atm.

The temperature and salinity of the sea surface were measured with an IDRONAUT OCEAN SEVEN 320PlusM WOCE-CTD multiparameter probe, and at shallow water stations (less than 50 m) – with the GAP AK-16 hydrological CTD probe.

Meteorological parameters were measured with recording equipment of the hydrometeorological data collection complex [16]. A sensor for measuring wind speed and direction was installed on a side boom 1.5 m long in the direction of the port side on the foremast, with the north direction chosen according to the vessel’s heading. The sensor is installed at a height of about 8 m from sea level. The data passed quality control with the rejection of unreliable fragments and were reduced to a standard observation height (10 m) [17]. According to the recommendations of the World Meteorological Organization, the measured parameters were averaged over 10 minutes, and further analysis was carried out for the averaged values. Wind gusts are given as instantaneous wind speed values over 5 s [17].

The values of the sea–air flux of carbon dioxide were calculated using the equations and assumptions described in [18] taking into account the wind speed and pCO₂ gradient between the sea surface and the near sea surface atmosphere:

$F_{{\text{CO}}_2}$= k · K₀ · ΔpCO₂,   (1)

where $F_{{\text{CO}}_2}$ is sea–air flux of carbon dioxide, mmol·m^‒2·day^‒1; K₀ is СО₂ solubility, mol·m^–3·atm^–1; ΔpCO₂ is gradient between partial pressure of carbon dioxide in the sea surface and the near sea surface atmosphere, atm; k is gas transport rate, m·day^–1, parameterized as a wind speed function:

k = 0.251·U²·(Sc/660)^–0.5,

where U is wind speed, m·s^–1; Sc is Schmidt number; ratio 0.251 is empirically derived parameter, cm·h^–1 × (m·s^–1)^–2 [19].

In [18], it has been established that the intensity of the carbon dioxide flux is determined by the state of the sea surface (bubbles, roughness) at wind speeds of more than 15 m·s^‒1. Wind speeds of more than 15 m·s^‒1 were not recorded during the 125^th cruise. Thus, only wind speed and рСО₂ gradient were taken into account when assessing fluxes.

Results

In December 2022, the average wind speed was 4.2 ± 3.8 m·s^‒1 with its minimum of 0.7 m·s^‒1 and maximum of 8.2 m·s^‒1. The sea surface temperature varied within 9.6–14.1 ℃ with its average value of 13.04 ± 1.06 ℃.

The average pCO₂ value of the sea surface was 388 ± 9 µatm while pCO₂ of the near sea surface atmosphere varied within a narrower range and the average value was 434 ± 4 µatm. Thus, the pCO₂ gradient between the sea surface and near sea surface atmosphere (ΔpCO₂) was predominantly determined by the pCO₂ variability in the sea surface. The values of ΔрСО₂ varied from ‒32.7 to ‒70.90 µatm with its average of ‒45.64 ± 8.56 µatm. It can be noted that the sea surface was undersaturated with carbon dioxide relative to the atmosphere during the period under study.

Based on the data obtained from equation (1), the CO₂ flux values were calculated.

The CO₂ flux intensity varied over a wide range from ‒0.04 to ‒8.74 mmol·m^‒2·day^‒1, the average value being ‒2.11 ± 1.79 mmol·m^‒2·day^‒1. Negative flux values indicate that the Black Sea waters absorb CO₂ from the atmosphere serving as its stock during the period under study. The calculated flux values are consistent with previously obtained data concerning the waters of the Crimean coast [14] and of the European shelf northwestern part [5].

Spatial variability of CO₂ flux values was characterized by heterogeneity (Fig. 2, а). Local minimum values and maximum flux intensity were observed in the area of the eastern coast of Crimea, as well as in its southern part (Fig. 2, а).

In terms of quality, the CO₂ flux spatial variability coincides with the distribution of temperature, wind speed and ΔрСО₂ in the sea surface (Fig. 2). Minima of temperature and ΔрСО₂ of the sea surface, as well as maximum wind speed were observed in zones of maximum intensity and minimum flux (Fig. 2).

 

Fig. 2. Spatial variability of the sea-air CO₂ flux (a), temperature (b), wind speed (c) and gradient of pCO₂ (d) by data of the 125^th cruise of R/V Professor Vodyanitsky

 

Discussion of results

It is known that the CO₂ flux value depends on wind speed and ΔрСО₂ to the greatest extent [18, 19].

Analysis of our data showed that the CO₂ flux was determined primarily by wind speed in December 2022 (Fig. 3). The correlation ratio (‒0.93, it is statistically significant with probability belief p = 0.99) indicates a strong linear relationship. The relationship is inverse in itself. Flux direction determines ΔрСО₂ between the sea surface and the near sea surface atmosphere. In turn, ΔрСО₂ is determined by the ratio of the partial pressure of CO₂ in the atmosphere and the equilibrium partial pressure of CO₂ in the sea surface.

The pCO₂ value of the sea surface is proportional to the concentration of CO₂ in water. The concentration of CO₂ depends on the biogeochemical factor when the production or removal of CO₂ occur due to the transformation of organic matter and the formation of carbonates, proceeding according to the following equations:

6СО₂ + 6Н₂О ↔ 6H⁺ + 6HCO₃^‒ ↔ С₆Н₁₂О₆ + 6О₂,

Са²⁺ + 2НСО₃^‒ ↔ СаСО₃ + СО₂ + Н₂О .

In addition, the CO₂ content in the sea surface depends on temperature which affects not only the CO₂ solubility, but also the intensity of biological processes, as well as the shift in chemical equilibria in the carbonate system [19]:

CO_2(g) ↔ CO_2(aq) ↔ CO_2(aq) + H₂O ↔ H⁺ + HCO₃^– ↔ 2H⁺ + CO₃^2–.

Changes in CO₂ concentration can also be caused by water dynamics, in particular by the CO₂ influx with waters from underlying layers [20].

Therein, the weak correlation of the СО₂ flux with ΔрСО₂ was unexpected (correlation ratio 0.22, it is statistically significant with probability belief р = 0.95).

Fig. 3. Dependence of CO₂ flux ($F_{{\text{CO}}_2}$) on temperature, ΔрСО₂ and wind speed

 

A decrease in ΔрСО₂ is characterized by a decrease in the flux (Fig. 3). In turn, a decrease in ΔрСО₂ indicates a decrease in the difference between рСО₂ of the sea surface and the near sea surface atmosphere. As рСО₂ of the near sea surface atmosphere showed almost no changes during the period under study (fluctuation range ±1%, average рСО₂ = 434 μatm), the decrease in the difference is due to an increase in рСО₂ and, accordingly, in the СО₂ concentration in the sea surface.

An increase in the CO₂ concentration in the sea surface at its low temperatures (about 13 ℃) can be caused either by an increase in the CO₂ solubility with a decrease in temperature, or by the dynamics of water ensuring the CO₂ influx from underlying water layers, as well as by the decomposition of organic matter formed during the autumn blooming [12, 21].

The correlation of the СО₂ flux with the sea surface temperature was moderate enough (correlation ratio 0.47, it is statistically significant with probability belief р = 0.99). The sea–air intensity of the CO₂ flux decreased with increasing temperature (Fig. 3). However, since the intensity of the flux is also affected by ΔрСО₂ in addition to the wind speed, in this case it is advisable to consider the absolute values (to modulo) of the flux which determine its intensity. Thus, it should be noted that an increase in temperature leads to a decrease in ΔрСО₂ and, accordingly, a decrease in CO₂ flux during the cold season.

Therefore, we can conclude that in December 2022, the predominant contribution to the intensity of the flux is made by the wind speed while the direction of the CO₂ flux is determined by the difference in pCO₂ between the sea surface and the near sea surface atmosphere.

Conclusions

The waters of the northeastern part of the Black Sea serve as a stock of atmospheric CO₂ during the cold season.

According to the direct measurements of pCO₂ in the sea surface and
in the near sea surface atmosphere, the values of the CO₂ flux in December 2022 varied widely from ‒0.048 to ‒8.74 mmol·m^‒2·day^‒1, the average value being ‒2.11 ± 1.79 mmol·m^‒2·day^‒1. At the same time, no pronounced features of spatial variability were identified. Local minima of flux values were observed in the eastern and southern regions of the Crimean peninsula.

In terms of quality, the CO₂ flux spatial variability coincided with the distribution of temperature, wind speed and ΔрСО₂.

When analyzing the correlation of the CO₂ flux with temperature, wind speed and ΔpCO₂, the strongest relationship was found with wind speed (‒0.93), while the weakest one was with ΔpCO₂ (0.22). When the wind speed increases, an increase in the intensity of the CO₂ flux is observed, while the direction of the CO₂ flux is determined by ΔрСО₂ and, accordingly, by the value of рСО₂ and the CO₂ concentration in the sea surface.

The measurements were carried out at the Center for Collective Use
R/V Professor Vodyanitsky of A.O. Kovalevsky Institute of Biology of the Southern Seas of RAS.

 

¹ Raven, J., Caldeira, K., Elderfield, H., Hoegh-Guldberg, O., Liss, P., Riebesell, U., Shepherd, J., Turley, C. and Watson, A., 2005. Ocean Acidification due to Increasing Atmospheric Carbon Dioxide. London: The Royal Society, 57 p.

作者简介

Natalia Orekhova

Marine Hydrophysical Institute of RAS

Email: natalia.orekhova@mhi-ras.ru
ORCID iD: 0000-0002-1387-970X
SPIN 代码: 9050-4772
Scopus 作者 ID: 35784884700
Researcher ID: I-1755-2017

Leading Research Associate, Head of Marine Biogeochemistry Department, Ph.D. (Geogr.)

俄罗斯联邦, 2 Kapitanskaya St., Sevastopol, 299011

Eugene Medvedev

Marine Hydrophysical Institute of RAS

编辑信件的主要联系方式.
Email: eugenemedvedev@mhi-ras.ru
ORCID iD: 0000-0003-0624-5319
SPIN 代码: 6332-4572

Junior Research Associate

俄罗斯联邦, 2 Kapitanskaya St., Sevastopol, 299011

Igor Mukoseev

Marine Hydrophysical Institute of RAS

Email: natalia.orekhova@mhi-ras.ru

Senior Engineer

俄罗斯联邦, 2 Kapitanskaya St., Sevastopol, 299011

Anton Garmashov

Marine Hydrophysical Institute of RAS

Email: ant.gar@mail.ru
SPIN 代码: 8941-9305
Scopus 作者 ID: 54924806400
Researcher ID: P-4155-2017

Senior Research Associate, Ph.D. (Geogr.)

俄罗斯联邦, 2 Kapitanskaya St., Sevastopol, 299011

参考

补充文件