Water Masses and Circulation in the Chain Fracture Zone (Equatorial Atlantic)

Abstract

In this study, we discuss the water masses and their transport in the Chain fracture zone (CFZ), which is a poorly studied part of the Equatorial Atlantic. Our study is based on measurements carried out during the 63rd cruise of R/V “Akademik Ioffe” in 2022. We identified water masses in the CFZ, determined their physical and chemical properties, localized their boundaries and components of the North Atlantic Deep Water (NADW), and calculated the transport of water masses. A four-layer structure of the NADW was identified with two components of middle NADW, which are defined by minimal and maximal oxygen concentrations. The upper boundary of the Antarctic Bottom Water (AABW) corresponds approximately to the isotherm θ = 1.5 °C. The assessed proportion of AABW in the bottom layer at the western entrance to the CFZ is 50%, and not higher than 33% at the eastern exit from the CFZ. For the first time, instrumental observations were carried out at the exit of the CFZ and in its western part. They showed that the AABW flux has an intensity of about 0.02–0.5 Sv depending on the upper boundary of AABW and moves through a passage in the northern wall (at 13° W), and not through the main sill.

1. Introduction

The distribution and transformation of the densest and coldest Antarctic Bottom Water (AABW) from the Weddell Sea, their main source, in the Atlantic Ocean is still poorly studied. Lying in the western Atlantic Ocean at depths over 4000 m, they cannot pass through the Mid-Atlantic Ridge (MAR) in the southern hemisphere. At the same time, the Walvis Ridge limits the spreading of AABW into the eastern Atlantic basin from the south. The first studies [1] showed that AABW propagates through equatorial fracture zones of the MAR from the west and reaches the eastern part of the Atlantic Ocean. It is now established that the Romanche and Chain fracture zones (RFZ and CFZ) (shown on Figure 1) are the main ways of the AABW flux near the equator. Another important source of the AABW flow is the Vema fracture zone at 11° N (Figure 1a). The Mid-Atlantic Ridge is also the obstacle to the penetration of the Lower North Atlantic Deep Water (LNADW), which enters the Eastern Atlantic through the same fracture zones. Then the AABW and LNADW mixture spreads in the Eastern Atlantic, playing a key role in ventilation of its bottom layer.

1.1. Bathymetry

The CFZ is deep, with a length exceeding 1000 km and a maximal depth exceeding 5600 m, and limited on the south and north by relatively high ridges. It is located to the south of the equator and about 200 km southeast of the RFZ, and it connects the Brazilian and Guinean basins. The CFZ ranges from 7 km to 20 km wide. The RFZ and CFZ main sills are found at depths of 4350 and 4050 m, respectively [2].

The CFZ consists of an alternating pattern of rises and deeps that are well-described by the shape of the thalweg (shown in Figure 3b,c). From Ryan et al. [3], the main sill is located at 12.5° W and has a depth of ~3850 m. To the west of the main sill, there is a comparable sill with a depth of about ~4050 m at 12.9° W, which we will call Sill W. The bathymetry in the area of these sills has a bad resolution, since there are no multibeam measurements.

The Northern Wall of the CFZ to the east of 18° W does not have significant gaps below 4000 m, even in the rift valley at 16° W. Only at 13.5° W is there a depth of more than 4100 m [2,3] (hereinafter referred to as the “northern passage”). The South Wall (to the east of 19° W) has significant passages bellow 4000 m in the area of 15–17° W.

High-resolution Global Multi-Resolution Topography bottom relief data [3] were used to construct bottom profiles through fracture zones and morphometry as shown in Figure 3. General Bathymetric Chart of the Oceans (GEBCO-15′) [4] was used for cartograms shown in Figure 1.

1.2. Hydrology

The first oceanographic measurements in the CFZ were made in its western part in 1903 and 1927, but unfortunately, they did not reach the main part of the fracture zone. Following measurements carried out on the R/V “Chain” in 1961 and generalization of seismic data, a number of transform fractures were discovered in the equatorial Atlantic, including the CFZ itself. Afterwards, the CTZ and RFZ were actively studied in 1991–1994 within the framework of the French ROMANCHE-I, II, III projects. During these studies, measurements at a large number of stations were performed in both fracture zones and current velocity recorders were installed. Moreover, the bathymetry and structure of water masses were studied in sufficient detail [2,5,6]. In 2009, measurements at two oceanographic stations were carried out in the main sill area of the CFZ by the R/V “Akademik Ioffe” [7].

According to previous studies, during the eastward expansion following transformation occurs with the AABW, the density decreases (due to increase in salinity and temperature), the oxygen saturation increases, and the concentration of silicates decreases [6]. The properties of the LNADW core are also transformed—the water here becomes less salty and colder, and the maximal oxygen concentration decreases.

Figure 1. Chain FZ and adjacent areas bathymetry. (a) AABW circulation pattern in the Atlantic: red dots—stations of the AI-63 cruise, arrows—distribution of AABW, with approximate AABW transport in Sverdrups shown in ellipses, (according to [6,7,8]); (b) Enlarged GEBCO-15′ bottom relief of the Chain FZ: red dots—stations of the AI-63 cruise.

Figure 1. Chain FZ and adjacent areas bathymetry. (a) AABW circulation pattern in the Atlantic: red dots—stations of the AI-63 cruise, arrows—distribution of AABW, with approximate AABW transport in Sverdrups shown in ellipses, (according to [6,7,8]); (b) Enlarged GEBCO-15′ bottom relief of the Chain FZ: red dots—stations of the AI-63 cruise.

Changes in the characteristics of the AABW and LNADW occur according to Messias et al. [6] due to intense vertical mixing as well as their interaction with the bottom waters of the eastern basin upon leaving the fracture zone, the origin of which is described in [9]. The amount of AABW and LNADW passing the CFZ and RFZ remains the most controversial issue. According to Morozov et al. [8] and estimates by various authors, the fluxes of these waters variate greatly from 0.15 (0.78) to 1.4 Sv in the RFZ and from 0.11–0.17 to 0.56 Sv in the CFZ [5]. In the Mercier and Speer [5] estimation, the average monthly transport values in the CFZ vary from 0.1 to more than 1 Sv. According to various values from Morozov et al. [8], the transport through the Vema fracture zone is estimated to approach 1 Sv. Thus, the fracture zones play an important role in the deep and bottom water exchange between western and eastern Atlantic, but the water structure, variability, and circulation there still remain poorly understood. The AABW and LNADW are of the greatest interest, because they are limited by the orography. The purpose of this work is to analyze the characteristics of water masses and estimate their transport in the insufficiently studied CFZ of the equatorial Atlantic by our own expedition data in comparison with CMEMS oceanographic reanalysis.

2. Materials and Methods

This study is based on data collected during the 63rd cruise of the R/V “Akademik Ioffe” (hereinafter AI-63). The stations were located on a quasi-latitudinal section along the CFZ. The measurements were performed from 2 November to 8 November 2022 (Figure 1).

The oceanographic measurements were carried out with an SBE19plus V2 profiler CTD continuously from the ocean’s surface to the bottom. Niskin bottles with a volume of 10 L installed on an SBE 32 Carousel sampling system were used for water sampling. Water samples were processed immediately after sampling in an onboard laboratory using standard methods [10]. Concentrations of dissolved oxygen and nutrients (phosphates, silicates) were determined within these chemical analyses.

The water transports during the AI-63 cruise were derived from current measurements performed using an acoustic current meter Nortek Aquadopp 6000 (accuracy ±0.5 cm/c) at stations coinciding with CTD stations. The velocities were then integrated over layers with upper boundaries corresponding to the isotherms θ = 1.5 °C (or θ = 1.8 °C) for the AABW and θ = 2.3 °C for the LNADW. The barotropic tide velocity from the TPXO9 database [11] was subtracted from the measured current velocities; afterwards, the values were interpolated similarly to [5] with closure to the CFZ walls (or other bottom elevations). The layer between the isotherms of θ = 1.5–1.8 °C was considered as a mixing layer of the AABW and LNADW, and the corresponding flux was later included in the calculated transport error.

The conservative chemical tracer PO₄ * [12] was also calculated to separate waters of Antarctic and North Atlantic origin as follows:

PO₄ * = PO₄ + O₂/175 − 1.95

where PO₄ and O₂ are the measured concentrations of phosphates and dissolved oxygen in µmol/kg. The AABW percentage in a mixture of the two water masses was calculated [12], where 0.73 and 1.95 are the values of PO4 * in the sources of the NADW and AABW, respectively:

f_AABW = [PO₄ * − 0.73]/[1.95 − 0.73] ×100%

Aside from in situ data, values of temperature, salinity, and current velocity from the GLORYS12V1 reanalysis (hereinafter referred to as reanalysis) were used to analyze the water structure and calculate transports. This reanalysis is produced by Copernicus Marine Environment Monitoring Service (CMEMS) [13] using the Nucleus for European Modeling of the Ocean (NEMO). It has a horizontal resolution of 1/12° and 50 depth levels, with a vertical resolution of about 400 m layers corresponding to the abyssal ocean. The distribution of physical parameters at depths 3992 and 4404 m was analyzed, i.e., the layers 3800–4200 m, the LNADW and upper part of AABW, and 4200–4600 m, the lower part of AABW. We used reanalysis, averaged over 25 years, for circulation patterns. For more accurate analysis, reanalysis for November 2022 was also used to calculate water mass transports for the same period of our measurements. Previous studies showed that ocean fracture zones are adequately represented in this reanalysis due to its high spatial resolution [9,14].

Global Biogeochemical Reanalysis CMEMS (hereinafter referred to as bioreanalysis) [15] was used to assess the transport and transformation of waters in the CFZ in terms of dissolved oxygen content. This bioreanalysis contains data on the distributions of dissolved oxygen and nutrients but does not contain data on water dynamics. It was calculated using the biogeochemical model and the NEMO model without data assimilation. The bioreanalysis has a horizontal resolution of 1/4° and 75 levels in depth and was averaged in this study for the period 1994–2004.

3. Results and Discussion

3.1. Water Masses in the Chain Fracture Zone

Based on the analysis of oceanographic data from the AI-63 cruise, the following water masses were defined in the vertical water structure.

The Surface water (SW) is located in the upper quasi-homogenous layer (up to 30–50 m). Its typical features are high temperature (25.5–26.5 °C), high salinity (more than 36), and low content of dissolved oxygen (204–216 µmol/kg) and nutrients (Figure 2,

Table 1. Characteristics of water masses of the Chain fracture zone section by the AI-63 cruise measurements.

Water Mass	Lower Boundary, m	Lower Boundary, γ n	θ °C; at the Lower Boundary	Salinity	Oxygen, µmol/kg	Silicate, µmol/kg	Phosphate, µmol/kg	Oxygen, µmol/kg, Bioreanalysis
Surface + Central (SW + CW)	300	26.9	10	max 35.86–36.2	min 78–216	min 0–0.8	min 0–0.2
Antarctic Intermediate (AAIW)	1000	27.55	4.6	min 34.56–34.80	min 147–159	max 24–33	max 2.0–2.32
Upper Circumpolar (UCPW)	1300	27.73	min 4.47 (in core 4.4)
Upper North Atlantic Deep (UNADW)	1900	27.96	3.6	max 34.99–35.01	190–250	min 14.5–17.0		204–247
Middle NADW1 (MNADW1_maxO2)	2500	28.04	2.7		max >250		min 1.2–1.3	251
Middle NADW2 (MNADW2_minO2)	3200	28.08	2.3		min <250		max 1.3–1.36	247
Lower NADW (LNADW)	4050	28.16	1.5		max >250		min 1.2–1.4	245
Antarctic Bottom (AABW)	bottom	bottom	min 1.08–1.7	min 34.80–34.85	min <250	max >50	max 1.6–1.9	no data

γ ⁿ: Neutral Density.

). The Central water (CW) is located below the euphotic zone in the subsurface layer. In this layer, the accumulation of nutrients occurs alongside the consumption of dissolved oxygen. This water mass is featured by low content of dissolved oxygen (78–100 µmol/kg) and high content of nutrients. This water mass forms in the Canary upwelling area and spreads southward in the oxygen minimum layer, while the dissolved oxygen content increases, reaching 90 µmol/kg in the Mid-Atlantic Ridge region, and up to 135 µmol/kg in the western Atlantic [16]. The lower boundary of these waters corresponds to the maximal vertical gradient of salinity, coinciding with the oxygen minimum layer and the isotherm θ = 10 °C at depths of about 300 m (Table 1).

The CW is followed by intermediate waters, which mainly consist of the Antarctic Intermediate Water (AAIW). In the CFZ, this water features minimal salinity (up to 34.49), a low content of dissolved oxygen (147–159 µmol/kg), as well as high concentrations of silicates (up to 33 µmol/kg) and phosphates (up to 2.32 µmol/kg). According to the thermohaline and chemical characteristics, the core of the AAIW is located at depths of 700–900 m, and its lower boundary corresponds to the isotherm θ = 4.6 °C.

The Upper Circumpolar Water (UCPW) is located below the AAIW at depths of 950–1300 m. The UCPW is featured by minimal values of potential temperature (θ = 4.4 °C). The presence of UCPW in the Equatorial Atlantic was discussed in some previous studies [16,17]. Due to the large vertical intervals between water samplings, the UCPW could not be reliably distinguished from the AAIW in terms of chemical parameters.

A complex of the North Atlantic Deep Water (NADW) is located below the intermediate waters at depths of 1300–4000 m. These waters are featured by maximal salinity, high dissolved oxygen content, and low nutrient concentrations. In the scientific community there is still no consensus on the quantity of NADW components. The NADW is commonly divided into at least three components of different origin: The Upper (UNADW), Middle (MNADW), and Lower (LNADW) [1,17,18,19]. However, according to Liu and Tanhua [20], the NADW in the tropical Atlantic consists of two layers only—the Upper and Lower NADW. In the tropical and South Atlantic Ocean, it is necessary to distinguish a fourth layer of NADW featured by minimal oxygen concentration [21,22,23], and we agree with this opinion.

Based on our data, the NADW in the CFZ also consists of four layers: the Upper NADW, the Middle NADW (which, in turn, consists of 2 layers), and the Lower NADW.

The Upper NADW by Demidov et al. [9] is a mixture of the Labrador Sea Water (LSW) and Iceland–Shetland Overflow Water (ISOW), which is featured by maximal salinity and minimal silicates concentration at depths of 1300–1900 m. The lower boundary of the UNADW corresponds to the isotherm θ = 3.6 °C.

The origin of the Middle NADW is most controversial. According to the “classical” classification by Wüst [1] as well as Demidov et al. [24], this water is formed in the Labrador Sea and features maximal oxygen concentrations at depths of 2000–2500 m. On the other hand, the works of Andrié et al. [19,25] stated that the middle layer of the NADW originates from the ISOW, which is afterwards transformed and mixed with the LSW. It is featured by lower oxygen concentrations compared to the overlying UNADW and the underlying LNADW [1]. The MNADW is located at depths of 2000–3300 m and is featured by maximal oxygen concentrations [17,18].

Based on our measurements, the MNADW in the CFZ should be divided into two layers. MNADW1_maxO₂ is the upper layer at depths of 1900–2500 m, which is featured by maximal oxygen (up to 257 µmol/kg) and minimal phosphates concentrations (below 1.4 µmol/kg), and originates from the LSW. MNADW2_minO₂ is the lower “old” layer with minimal oxygen (below 250 µmol/kg) and maximal phosphates concentrations (up to 1.4 µmol/kg) at depths of 2400–3200 m. Moreover, the data from the CFZ clearly shows an increase in dissolved oxygen content in the entire MNADW2_minO₂ layer from east to west, which is in good agreement with previous studies of the origin of this water [24]. According to Demidov et al. [24], MNADW2_minO₂ is the “old” NADW that spreads from the western Atlantic to the eastern basin through the MAR, mixes with the waters of the eastern basin with lower oxygen content and higher concentrations of phosphates and nitrates, and then recirculates into the Western Atlantic through the MAR. Then it wedges into the “fresh” NADW, thereby dividing the single deep oxygen maximum, a distinctive feature of NADW, into MNADW1-maxO2, MNADW2—min O2 and LNADW—max O2.

Figure 2. Distribution of physical and chemical parameters along the Chain FZ based on AI-63 data: (a) potential temperature, °C; (b) salinity; (c) eastern component of the current velocity, cm/s; (d) dissolved oxygen, µmol/kg; (e) phosphates, µmol/kg; (f) silicates, µmol/kg. The gray lines show the boundaries of water masses. Black points—location of measurements.

Figure 2. Distribution of physical and chemical parameters along the Chain FZ based on AI-63 data: (a) potential temperature, °C; (b) salinity; (c) eastern component of the current velocity, cm/s; (d) dissolved oxygen, µmol/kg; (e) phosphates, µmol/kg; (f) silicates, µmol/kg. The gray lines show the boundaries of water masses. Black points—location of measurements.

In our study, maps of the spatial dissolved oxygen distribution were created for the layers corresponding to MNADW1_maxO₂, MNADW2_minO₂, and LNADW based on in situ data and the bioreanalysis. According to the bioreanalysis in the MNADW1_maxO₂ layer at a depth of 1945 m, i.e., equivalent to the layer between 1800 and 2150 m with maximal oxygen concentrations, the main flux of waters with maximal oxygen concentrations propagates from west to east through the RFZ. However, in situ data shows that this flux also moves from west to east, but through the CFZ. The dissolved oxygen concentrations from the bioreanalysis are lower than that from in situ data.

According to the bioreanalysis, the MNADW2_minO₂ layer at depth 2865 m is equivalent to the layer between 2700 and 3050 m with minimal oxygen concentrations; the values of dissolved oxygen content from the bioreanalysis are close to in situ data. Withal, the main flux of MNADW2_minO₂ by the bioreanalysis is directed not along the CFZ but from south to north, thus transporting water over the Mid-Atlantic Ridge. Afterwards, this flux moves generally westward through the RFZ. According to in situ data, the flux of MNADW2_minO₂ is directed westward through the CFZ, which contradicts the bioreanalysis. The lower boundary of MNADW2_minO₂ corresponds to the isotherm θ = 2.3 °C.

The Lower NADW is the densest component of the NADW. These waters originate from the Denmark Strait [1,21,25]. In the CFZ, the LNADW is well observed thanks to a deep oxygen maximum (up to 258 µmol/kg) at depths between 3200 and 4050 m. However, such maximal values are most expressed in the western part of the CFZ. In its eastern part (approximately at 13° W), the oxygen maximum becomes less expressed and the LNADW layer is thinner.

We compared data from the bioreanalysis at a depth of 3700 m, i.e., the layer with oxygen maximum between 3500 and 3900 m against in situ measurements. Through this comparison, absolute oxygen concentrations from the bioreanalysis are underestimated. Moreover, the main feature of the LNADW—maximal oxygen concentration values—is not reproduced by the bioreanalysis.

The eastward flux of these waters passes to the south of the CFZ and enters the CFZ via gaps in the southern wall at approximately 15.5–17° W. The LNADW further propagates eastward along the CFZ and leaves it northward at approximately 13° W. However, by the bottom topography data, the FZ continues eastward for another 0.5°. Based on both the bioreanalysis and in situ data there is an eddy, which propagates these waters further eastward. Moreover, in the layer between 3900 and 4300 m, this eddy is observed only using in situ data.

The Antarctic bottom water (AABW) is located in the bottom layer of the CFZ. It features minimal potential temperature (below 1.5 °C), reduced salinity (less than 34.85) compared to the overlying LNADW, minimal oxygen (below 245 µmol/kg), and a maximal concentration of nutrients, primarily silicates (above 50 µmol/kg). All these parameters reach their extreme values in the bottom layer. The upper boundary of the AABW corresponds better to the isotherm θ = 1.5 °C, which is significantly lower than most previously published estimates (1.9–2.0 °C). An oxygen maximum layer in the core of the LNADW is located at depths of θ = 1.9–2.0 °C. Moreover, the isotherm θ = 1.5 °C corresponds well to the Si/P = 33 value, which is also used to determine the LNADW/AABW boundary [24]. We suggest considering the layer between the isotherms θ = 1.5 °C and θ = 1.8 °C as a mixture zone of the LNADW and AABW. Moreover, the properties of AABW change significantly during its passage through the CFZ. Thus, at the western entrance of the CFZ the potential temperature and silicate concentration in the bottom layer were 0.35 °C and above 115 µmol/kg, respectively, is about a 50% share of pure AABW. On the other hand, at the eastern end of the CFZ, the potential temperature of the bottom layer increased till 0.9–1.0 °C and the silicate concentration dropped until 45 µmol/kg, i.e., about to a 33% share of pure AABW.

According to the bioreanalysis, the AABW does not propagate in the CFZ at a depth of 4400 m, which corresponds to the layer with minimal oxygen at 4200–4600 m.

By a common opinion, the AABW is strongly transformed in the eastern Atlantic. Therefore, numerous researchers even suggest separate designations for this transformed water (see the review in [9] for more details). Thus, waters of Antarctic origin to the east of the Mid-Atlantic Ridge are called Eastern Basin Waters or Lower Deep Water by Morozov et al. [8]. The term “Northeast Atlantic Bottom water” (NEABW) was introduced for waters passing through the Mid-Atlantic Ridge fracture zones into the Cabo Verde Basin and extending below the NADW [9,19]. We similarly suggest defining the waters transported via the RFZ and CFZ as “East Equatorial Atlantic Bottom Waters” (EEABW).

3.2. Water Transport in the Chain Fracture Zone

Our study is the first to analyze in situ current measurements in the layer corresponding to the AABW along the entire CFZ. In Figure 3, to the west of 15° W, the AABW flux was directed westward, not eastward. We assume that the main flux of the AABW enters the CFZ from the south, as shown in Figure 3, after which the flux bifurcates inside the CFZ and spreads both eastward and westward, which is confirmed by our in situ data.

According to the reanalysis, the main AABW flux in the western part of the CFZ is also paradoxically directed westward. The AABW enters the CFZ through a passage in the southern wall at 15.5–17° W. This westward flux is 0.2–0.5 Sv according to our in situ data and 0.6 Sv according to the reanalysis (Figure 3).

At the station of 15° W, using two AABW boundaries (θ= 1.8 and 1.5 °C), transport in the AABW layer (from 4100 till bottom) generally moves in the westward direction. At the same time, if we consider the vertical profile of current velocities at this station, we can see that in the deepest AABW (below 5000 m), the main flow in the AABW layer was directed to the east (Figure 2c). The recorders at 13.5° W from the work of [5] show long periods of predominant flows to the east with episodic flows to the west. Perhaps this is the variability mentioned during the expedition where we fixed the westward AABW flux. East of 14° W, the general AABW transport flows to the east. This is evidenced by the positive transport values according to expeditionary measurements and to the reanalysis of November 2022.

In the eastern part of the CFZ, the AABW flux extends northward through a “northern passage”. From our in situ data, the flux of AABW with θ < 1.5 °C (and even θ < 1.8 °C) does not pass directly through the main sill of the CFZ at 12.4° W [7], and waters with θ < 1.5 °C do not even spread through Sill W (Figure 1) at 12.9° W. This confirms the suggestion described in Mercier et al. [2], that the main flux of AABW propagates through the “northern passage” and the transport of this flux is up to 0.1 Sv. From our in situ data, the transport of water with θ < 1.5 °C through the “northern passage” is directed southward, and the northward transport was only 0.02 Sv. During the AI-63 cruise, in situ measurements in the “northern passage” were carried out for the first time. However, the transport of water with θ < 1.8 °C reached 0.5 Sv (0.09 Sv by reanalysis). Based on these measurements, bottom potential temperature close to 1 °C was recorded. The northward AABW transport through this passage was also confirmed by lower bottom temperatures (θ < 1.5 °C) at the stations of the ROMANCHE expedition, located 80 km north of the CFZ.

According to the reanalysis, the CFZ is impassable for the densest part of the AABW at the depth of 4405 m, i.e., the layer 4200–4600 m, not only at the main sill, but also in the area of 14° W. However, at the depth of 3992 m, which corresponds to the layer of 3800–4200 m, the transport is observed throughout the entire CFZ. In general, the reanalysis reproduces the directions of the AABW transport in the CFZ in a satisfactory manner.

Based on current recorders data analysis, the AABW transport through the CFZ and RFZ is 0.56 Sv and 0.66 Sv, respectively [5]. But it should be noted that, in Mercier and Speer [5], transport estimations assumed the isotherm θ = 1.9 °C is the upper boundary of the AABW, which, as we discussed earlier, is not optimal. The upper boundary of the AABW corresponds better to the isotherm θ = 1.5 °C. Thus, if we adjust the AABW boundaries (and assume the AABW boundary to be θ = 1.5°), the total transport through the CFZ and RFZ from [5] would decrease to 0.27 and 0.4 Sv, respectively. It should also be noted that in the CFZ, the recorders were set at 13.5° W, i.e., before the main sill, through which waters with θ < 1.9 °C do not pass, and the transport through the “northern passage” is 0.1 Sv [2]. We propose that bottom waters on the eastern slope of the MAR in the area between the two fracture zones come mainly from the CFZ rather than from the RFZ.

Figure 3. The AABW transport and CFZ morphometry. (a) The AABW transport through the Chain FZ and Romanche FZ. Red diamonds—AI-63 stations with the AABW transport values in Sverdrups (positive values show eastward flow); black numbers—AI-63 data; blue numbers—GLORYS12V1 reanalysis 11.2022; black arrows—reanalysis averaged over 25 years (m/s); blue arrows—generalization of reanalysis; purple arrows—the assumed penetration is not shown by the reanalysis; black rectangles—areas with no reanalysis values at the depth of 4405 m (4200–4600 m layer). The AABW entrance and “northern passage” areas are marked red. (b) Profiles of the thalweg and Northern Wall of the Chain FZ with isotherms. (c) Profiles of the thalweg and Southern Wall of the Chain FZ with isotherms θ = 1.5 and 1.8 °C (AABW boundary).

Figure 3. The AABW transport and CFZ morphometry. (a) The AABW transport through the Chain FZ and Romanche FZ. Red diamonds—AI-63 stations with the AABW transport values in Sverdrups (positive values show eastward flow); black numbers—AI-63 data; blue numbers—GLORYS12V1 reanalysis 11.2022; black arrows—reanalysis averaged over 25 years (m/s); blue arrows—generalization of reanalysis; purple arrows—the assumed penetration is not shown by the reanalysis; black rectangles—areas with no reanalysis values at the depth of 4405 m (4200–4600 m layer). The AABW entrance and “northern passage” areas are marked red. (b) Profiles of the thalweg and Northern Wall of the Chain FZ with isotherms. (c) Profiles of the thalweg and Southern Wall of the Chain FZ with isotherms θ = 1.5 and 1.8 °C (AABW boundary).

Our AABW transport estimates at nearly the same point where the above-mentioned recorders were installed are 0.32 Sv (if θ < 1.5 °C is assumed as the upper boundary) and 0.53 Sv (if the boundary is θ < 1.8 °C), which is close to the calculations of [5].

Thus, if the boundaries of the AABW are adjusted, the total transport in the exits of the RFZ and CFZ is less than 0.5 Sv (0.66 Sv at the recorders location), which is significantly less than the transport through the Vema fracture zone (0.9 Sv [14]), and their total transport to the eastern Atlantic is significantly lower than previous assessments by Morozov et al. [8].

The Lower NADW transport through the CFZ was also estimated. The water transport through the CFZ and RFZ in the layer with potential temperatures between 1.9 and 2.1° was 0.22 and 0.14 Sv, respectively [5]. If the LNADW transport is assessed within the boundaries of potential temperature between 1.5 and 2.1 °C, it increases up to 0.51 and 0.41 Sv, respectively. However, the upper boundary of the LNADW, as shown above, corresponds to θ = 2.3 °C, but in Mercier and Speer [5], the layer with θ > 2.1 °C was not considered.

4. Conclusions

The water structure of the Chain fracture zone (CFZ) in the equatorial Atlantic was studied using recent expedition data, GLORYS12v1 reanalysis, and CMEMS biogeochemical reanalysis. The main conclusions are as follows:

A 4-layer structure of the North Atlantic Deep Water (NADW) was revealed. The middle NADW layer with an oxygen maximum (MNADW1_maxO₂) becomes thinner towards the east by 13° W, with a gradual decrease in dissolved oxygen content. The MNADW2_minO₂ layer with minimal oxygen, on the contrary, spreads westward, and the dissolved oxygen concentrations in this layer increase in the same direction.

According to the position of maximal vertical gradients of the thermohaline and chemical characteristics, the isotherm θ = 1.5 °C is the optimal proxy for the upper boundary of the Antarctic Bottom Water (AABW). The proportion of the AABW at the western entrance to the CFZ is 50%, and at the eastern exit, it does not exceed 33%.

For the first time, the current velocities and directions in the western part of the CFZ have been measured. In this part of the fracture (west of 16.7° W), both instrumental measurements and reanalysis show a westward transport of AABW (up to 0.54 Sv and 0.58 Sv, respectively). The main eastward movement of waters in this area was south of the CFZ, entering inside the FZ through a passage in the “southern wall” at 15.5–17° W, and to the west of it, a counterflow was observed.

The AABW leaves the CFZ through a passage in the “northern wall” at 13° W without passing through the main sill at 12.4° W. Measurements, carried out for the first time in this passage, showed that the main flux through the passage was about 0.02–0.5 Sv to the north. This passage is unique because no other sources of AABW for the eastern Atlantic have been found in other parts of the CFZ.

The isotherm θ = 1.5° C corresponds better to the upper boundary of the AABW. Using this boundary, the recalculated transport values in the CFZ and RFZ from Mercier and Speer [5] are 0.27 Sv and 0.4 Sv, respectively, which is close to our in situ measurements. Our AABW transport estimates in the CFZ are 0.32 Sv at 13.5° W. The total transport through the CFZ and RFZ is probably significantly lower than the transport through the Vema fracture zone.

In general, the reanalysis reproduces adequately the directions of AABW transport in the CFZ, but there is no end-to-end transport in the lower part of AABW (4200–4600 m). Simultaneously, the bioreanalysis underestimates the dissolved oxygen content in the Lower NADW (LNADW) significantly and does not reproduce its maximal values. Moreover, there was no data for the AABW layer (4289 m).

Moving along the CFZ and beyond the “northern passage”, the AABW loses one of its distinctive features, namely its minimal oxygen content, due to mixing with the overlying LNADW. For this reason, we propose to introduce the name “East Equatorial Atlantic Bottom Waters” (EEADW) for the near-bottom waters of the Guinea basin and adjacent basins.