Water characteristics of abyssal and hadal zones in the southern Yap Trench observed with the submersible Jiaolong*

LIU Xuehai , , , LIU Yongzhi , , , , CAO Wei , SUN Chengjun

1 First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China

2 Laboratory for Regional Oceanography and Numerical Modeling, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China

3 Key Laboratory of Marine Science and Numerical Modeling, Ministry of Natural Resources, Qingdao 266061, China

Received Dec. 27, 2018; accepted in principle Feb. 20, 2019; accepted for publication Jun. 10, 2019 © Chinese Society for Oceanology and Limnology, Science Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract Using observations in an applied cruise of the submersible Jiaolong, water characteristics, geostrophic transport, and turbulent mixing in abyssal and hadal zones of the southern Yap Trench were studied. The spatial structures of deep water show that the abyssal water is cold, saline, and oxygen rich. The hadal water has very small changes in potential temperature and potential density, and a little decrease in salinity and obvious decrease in oxygen. The isotherm, isopycnal, and isohaline are depressed in abyss over the central trench. The turbulent mixing is enhanced in the near-bottom zone and the hadal water on the trench slope, especially at the steep slope, the dissipation rate and difference usivity is strong, which weakens the stratifi cation. The geostrophic fl ows move southward in the western region of the trench and northward in the eastern region, indicating cyclonic circulation. In the central region of the trench, the water transport is ～1.74 Sv southward. In the hadal zone, the northward and southward transports are balanced. Our analysis suggests that the abyssal water in the southern Yap Trench is from Lower Circumpolar Water (LCPW) and the hadal water seems to be of the isolated local water rather than LCPW.

Keyword: Yap Trench; abyssal and hadal; the submersible Jiaolong; diapycnal mixing; geostrophic fl ows

1 INTRODUCTION

The Yap Trench is an important segment of the arcuate trench system in the Northwest Pacifi c, linking the waters in Mariana-Caroline basins (Kaneko et al., 1998). The length of the trench is ～700 km with a maximum depth ～8 527 m (Fujioka et al., 2000). Previous studies mainly focused on morphology and tectonics of the trench (Fujioka et al., 2000; Kobayashi, 2004), and so far, the hydrodynamic observation and studies are rare.

Johnson and Toole (1993) and Kawabe and Taira (1998) proposed that there are three types of deep watermass in the Northwest Pacifi c. The bottom salty and oxygen-rich water is referred to as Lower Circumpolar Water (LCPW); the water lying above LCPW is referred to as North Pacifi c Deep Water (NPDW); and the water marked by a minimum salinity above the NPDW is Antarctic Intermediate Water. Usually the water exceeding 3 000 m is defi ned as abyssal water (Macdonald et al., 2009; Hautala, 2018). The abyssal water in Mariana-Caroline regions is from LCPW, with the infl ow of LCPW into the East Mariana Basin (EMB) and a partial outfl ow to the West Mariana Basin (Uehara and Taira, 1990; Johnson and Toole, 1993; Yanagimoto and Kawabe, 2007; Huang et al., 2018). Siedler et al. (2004) found that the channels at the Yap-Mariana Junction and Caroline Ridge were essential for the exchange of deep water such as water fl owing to East Caroline Basin (ECB) from EMB. Using observations by the submersible Jiaolong system, Liu et al. (2018) studied the watermass properties and deep currents in the northern Yap Trench, and suggested that the deep water originates from EMB and ECB as a part of the west propagating LCPW. However, the deep circulation and water properties in the southern Yap Trench remain unrevealed. Besides, the understanding in the hadal zone, which is below 6 000 m, is absent.

Fig.1 Bathymetry and observation stations of the No. 38 Cruise

The water linkage between the southern Yap Trench and the northern part is obstructed by a sill at a depth of ～5 000 m (as shown in Fig.1). Consequently, the properties of abyssal and hadal water in the two parts maybe have some difference erences. This means the local characteristics exists in the southern Yap Trench. Because the southern trench connects the Caroline Basin, the hydrodynamic investigation in the trench is helpful for identifi cation of the water transportation and deep circulation in Mariana-Caroline regions. Additionally, the investigation addressed here is also important for understanding the local biologicalchemical processes and material exchange with adjacent basins and trenches.

Table 1 The observation time and instruments of No. 38 Cruise in the southern Yap Trench

We conducted the investigation in the northern Yap Trench mainly by the submersible Jiaolong in No. 37 of Chinese Ocean Cruises (Liu et al., 2018). As the fi rst deep-sea manned submersible in China (Liu et al., 2010), Jiaolong has made hundreds of deep dives (maximum down to 7 062 m in Mariana Trench in 2012) in the South China Sea, Pacifi c, and Indian Ocean for multidisciplinary explorations (Li, 2017). Jiaolong and its mother ship equip tens of instruments including the conductivity-temperature-depth profi ler (CTD). The information of instruments used in this study is listed in Table 1.

In this paper, using the observations in an applied cruise of the submersible Jiaolong (No. 38 of Chinese Ocean Cruises), we aim to study the water characteristics, geostrophic fl ow and turbulent mixing of abyssal and hadal zones in the southern Yap Trench. The paper is organized as follows. The data and methods used are described in Section 2. Section 3 displays the results and the analysis of water characteristics, geostrophic fl ow and turbulent mixing. A brief discussion and conclusion are presented in Section 4 and Section 5, respectively.

2 DATA AND METHOD

2.1 The study region

The Yap Trench is separated into the northern and southern parts by a sill at 9°N at the depth of around 5 000 m (Fig.1). The southern part forms an important segment of an arc trench system in the Western Pacifi c. The maximum depth reaches 8 527 m in the central basin according to the topography from the General Bathymetric Chart of the Oceans (GEBCO_2014). This topography dataset has a resolution of 30 arc-second interval grid, access from the web (http://www.gebco.net/data_and_products/gridded_bathymetry_data/). Across the trench at around 8°N, the study area is located in the middle of the southern Yap Trench and in the southern margin of the Philippine Sea Plate.

In this paper, only abyssal and hadal water is studied. The range for an abyss is 3 000-6 000 m, and below that, the hadal zone is defi ned.

2.2 Observations and data processing

The No. 38 Cruise was conducted in the southern Yap Trench from June 3 to June 14, 2017, which is an applied oceanic scientifi c expedition of the deep-sea manned submersible Jiaolong with the R/V Xiangyanghong 0 9 as its mother ship, lead by China Ocean Mineral Resources R&D Association and fi nancially supported by the National Key Basic Research Program of China.

As shown in Fig.1b, nine observation sites were deployed across the abyssal trench along slope in a zonal direction overall. Among them, four casts (C01, C02, C03, and C04) were conducted using mainly ship-based CTD and dissolved oxygen sensors, and fi ve dives of Jiaolong (D148, D149, D150, D151, and D152) were observed using a set of CTD and dissolved oxygen sensor to obtain vertical profi les of temperature, salinity, and dissolved oxygen. Table 1 shows the information of the instruments, geographical positions, observation periods, and casting or diving depths. The maximum depth was 4 487-5 952 m for ship-based CTD observation and 4 187-6 685 m for Jiaolong dives, and all dives reached the sea bottom. C01, D148, D149, D150, and D151 were located in the west slope of the trench, C03 was in the central region, and C04, C04, and D152 were at the east slope. Situated in the fl ank of the trench, D148 and C02 had the shallowest bottom and observation depths.

The observation data were reprocessed with SBE (Sea-Bird equipment) Data Processing Software V7.2 to obtain the values of temperature, salinity, density, and concentration of dissolved oxygen in a 1-m interval depth grid by the cubic spline interpolation in vertical after manually eliminating abnormal values. Only the data during lowering process are used in the study. The accuracy of SBE 911 Plus CTD is 0.001°C for temperature and 0.000 3 S/m for conductivity, and that of SBE 16 Plus CTD is 0.005°C for temperature and 0.000 5 S/m for conductivity. The equation of state is Thermodynamic Equation of Seawater 2010 (TEOS- 10), which is implemented with Gibbs Sea Water (GSW) Oceanographic Toolbox (GSW, http://www.teos-10.org/).

2.3 Computation of geostrophic fl ows

The meridional geostrophic fl ows at transects of Section-C and Section-D were computed from observed CTD data using the thermal wind relation:

where v and v0are geostrophic velocities at the depth z and at the reference level z0, respectively; g is the gravitational acceleration; f is the Coriolis parameter; ρ and ρ0is the potential density and characteristic potential density, respectively.

In this paper, the reference level is 3 000 m according to previous abyssal studies and adjacent trenches and basins (Johnson and Toole, 1993; Taira et al., 2005; Huang et al., 2018; Liu et al., 2018). At this weak circulation level, the meridional geostrophic velocity across transects Section-C and Section-D is set to be zero, hence we treated the calculated fl ows relative to the level as the geostrophic fl ows at abyssal and hadal depths. The geostrophic fl ow and volume transport were estimated from the observations by Jiaolong-carried CTD and ship-based CTD, respectively.

2.4 Computation of diapycnal mixing

Turbulent diapycnal mixing were computed using the Thorpe method (Thorpe, 1977) from CTD data. The turbulence dissipation rate ε and the vertical diapycnal difference usivity K are formulated by:

where the ratio of the Ozmidov scale to the Thorpe overturn scale c=0.8 (Dillon, 1982), the mixing eき ciency m=0.2 (Osborn, 1980); N is the buoyancy frequency obtained from CTD data (Klymak et al., 2008); d is the Thorpe scale which is defi ned as the root-mean-square of all displacements between the real depth for a measured density and the depths in a reordered steady density profi le within a complete overturn. The overturn is detected by comparing the original and sorted density profi les. Here, the tests are implemented using the method of a previous study (Yang et al., 2016) to remove the spurious overturns.

The average dissipation rate ε and diapycnal difference usivity K over a depth range H are as follows: where Li, εi, and Kiare the depth range, dissipation rate and diapycnal difference usivity of the i-th overturn within the range H, respectively. Here, H is set to be 300 m. The above turbulent mixing values are computed from 3 000 m to the maximum CTD casting or diving depths of nine stations.

3 RESULT AND ANALYSIS

3.1 Structure of abyssal water observed by shipbased CTD

Figure 2 shows the vertical profi les of temperature and potential temperature θ3(referred to 3 000 dbar), salinity, potential density σ3(potential density referred to 3 000 dbar) and dissolved oxygen in the abyssal zone of the trench between 3 000 m and 6 000 m observed by ship-based instruments.

Fig.2 Vertical profi les below 3 000 m at stations C01-C04 observed with ship-based instruments

The temperature gradually decreases to the minimum values 1.453-1.463°C at around 4 500 m from 1.576-1.595°C, and then continually increases with depth due to adiabatic pressure. The maximum temperature is observed as 1.572°C at 5 716 m of C01, 1.607°C at 5 952 m of C03, and 1.558°C at 5 600 m of C04. Obviously, at the critical depth from the abyss to the hadal zone, the temperature surpasses that at 3 000 m. In a layer of 3 000-3 300 m, the order of the mean temperatures is C01＞C04＞C03＞C02. From 3 500 to 4 300 m, the overall order turns to be C02＞C04＞C03＞C01. This indicates that in the top 1 300 m abyssal water, the temperature on the west trench slope decreases more than that on the east slope. In the ～150 m layer below ～4 500 m where temperature steadily keeps the minimum temperature, C01, on a steeper slope, has the highest temperature, and C03, near the central of the trench, is the coolest. This suggests that the temperature over the central trench is depressed more than that at the fl ank. This is similar to the phenomena in the Challenger Deep of the Mariana Trench (Huang et al., 2018). At lower depths, temperature increases to the almost same in the three stations. On the other hand, the comparison for depths with the same temperature shows that C02 has the shallowest level for isotherm in the upper ～300 m layer but the deepest below the layer. From ～4 500 m to ～5 300 m, the isotherm level is shallower at C01 and deeper at C04.

The potential temperature decreases continually with depth (Fig.2a). At the reference level, θ3is 1.572-1.599°C. The minimum is 1.243°C, appearing in the lowest layer of abyss at C03. The cooling is obvious in the top 1 500 m and very small in the next ～1 000 m. Below 5 500 m, temperature is steady. Similar to the real temperature, potential temperature varies difference erently at four stations. The profi les show that from 3 000 m to 4 500 m, potential temperature decreases less at a fl ank station C02 and more at a central station C03, which is same as the aforementioned for real temperature. In the top 300 m layer, C02 is colder than other stations. However, C01 is the coolest in the next 1 000 m where the slope is steep. Below 4 500 m, the potential temperature at C03 varies less than that at the fl ank stations. The pattern of the isotherm level for potential temperature is the same to that for the real temperature.

The profi les of salinity present a similar pattern to those of potential temperature (Fig.2a & b). From the reference level to the maximum depth, salinity continually increases to 34.682 at more than ～5 500 m from 34.655-34.657. There is an obviously increase in the top ～1 600 m and a slight rise for deeper water. In addition, C02 is more saline than those of other stations at 3 000-3 400 m but evolves into less saline in the next ～1 000 m layer, in which C01 is more saline. This means that the isohaline at C02 is shallower in the fi rst 400 m and deeper in the layer from 400 m to 1 400 m than other stations, and in this layer C01 has shallower isohaline. Generally, the central station C03 is less saline at the isobar levels in abyss.

Potential density has the similar patterns to those of salinity and potential temperature both in vertical and in horizontal. At 3 000 m, the density σ3is 41.441-41.447 kg/m3, and then increases to 41.510 kg/m3at ～4 700 m. Below the depth, the increase is very small (0.003 kg/m3totally). At the isobar levels in the upper 300 m, C02 is denser than other stations. Between 3 600 m and 4 300 m, the density is highest at C01 and least at C02. Below 4 700 m, C04 has higher densities and C03 has lower. From an isopycnal perspective, C02 is shallower for a density less than 41.459 kg/m3, but its isopycnal is deeper for the potential density exceeding 41.468 kg/m3. Overall, C3 has the relatively deep isopycnal. This means that the isopycnal of abyssal water is depressed in the central region of the trench.

The content of dissolved oxygen increases with depth. In the upper 1 600 m, the continuous increase is apparent (from ～2.90 mL/L to ～3.41 mL/L). It is indicated from Fig.2a-d that the saline, cold, and dense water is rich in oxygen. Therefore, the pattern for profi les of oxygen is consistent with those of salinity, potential density, and potential temperature (oxygen-rich corresponding to cold). As shown in Fig.2d, C02 has more oxygen than other stations between 3 000-3 400 m but less content in the next ～1 000 m layer, in which C01 is relatively rich in oxygen. In addition, below ～4 800 m, the oxygen content decreases obviously with the depth increase at the stations.

In summary, the conventional CTD observation displays the abyssal water characteristics for difference erent regions across the trench, and the spatial patterns are consistent for temperature, salinity, density, and oxygen content. Their spatial structures are directly related to geostrophic fl ow and turbulent mixing, which are investigated in following sections.

3.2 Water characteristics in abyssal and hadal zones observed by Jiaolong

Figure 3 presents the profi les of potential temperature θ3, salinity, potential density σ3and dissolved oxygen at abyssal and hadal depths in the trench observed by Jiaolong instruments.

Above 5 500 m, four elements have the similar variation with depth. Potential temperatures decrease monotonously and the others increase continually. This means the deeper water is more saline, colder, denser, and oxygen-richer. The vertical changes are marked between 3 000-4 500 m, which is consistent with the observed by ship-based CTD. From ～1.60°C, θ3decreases to 1.25°C at ～5 300 m at D151 and ～5 000 m at D152. Below the depths, the value decreases only 0.007°C, indicating that potential temperature varies very little in hadal water and lower abyss. From 34.664 at 3 000 m, salinity increases to the maximum value 34.685 at ～5 300 m. Below the depth it keeps steady even though there is a very little drop at hadal depths. σ3increases to ～41.515 kg/m3at ～5 000 m from ～41.448 kg/m3, and keeps homogeneous in the hadal zone. From ～3.5 mL/L, dissolved oxygen increases to over 3.95 mL/L (with a maximum ～4.0 mL/L at the depth of ～4 800 m) below ～4 600 m, and then decrease obviously below ～5 400 m with a minimum of 3.5 mL/L.

Fig.3 Vertical profi les below 3 000 m along Section-D observed by Jiaolong-carried instruments

Additionally, although there is a difference erence in the oxygen content between the observed by casting and that by Jiaolong using difference erent sensors, their distribution patterns are identical. Two kinds of observations show that oxygen increases with depth, and the cold, saline, and dense water is relatively rich in oxygen. This has been observed in adjacent trenches and basins (Johnson and Toole, 1993; Kaneko et al., 1998; Kato and Kawabe, 2009; Huang et al., 2018; Liu et al., 2018). In particular, using Jiaolong-carried instruments, we observed that the oxygen is poorer in the hadal zone than that in the abyssal layer lying above the zone. This is probably related to the shortage of oxygen supplement from LCPW. Besides, the higher biomass in the near-bottom and hadal zones (Ichino et al., 2015) needs more consumption of oxygen. The observation demonstrates spatial variability. The contours of potential temperature, salinity, and potential density in Section-D are depressed over the trench between 3 000 m and ～4 200 m, in which the isotherm, isopycnal and isohaline at the western fl ank are shallower than those over the central trench. This was observed by shipbased CTD. Among the layers of ～4 500-5 500 m in the trench, the isolines are tilting and lift from west to east. Below 5 500 m, the difference erence is not obvious across the trench, so the potential temperature, salinity and potential density are uniformly distributed in the hadal zone.

In summary, the observation of submersible Jiaolong presents similar spatial pattern with ship-based CTD, reveals that the isolines of potential temperature, salinity and potential density are depressed over the central trench. In hadal zone, these elements are homogeneous, but the oxygen content decreases.

Fig.4 The θ- S scatter diagram with σ3 contours overlain

3.3 Characteristics in θ- S diagram and abyssal watermass

Figure 4a & b present the θ- S scatter diagrams below the reference level observed by ship-based and Jiaolong-carried CTDs. The diagrams show that the abyssal and hadal water becomes colder and more saline with the increasing of depth, with the exception of hadal and near-hadal depths. In Fig.4a for shipbased CTD, both of potential temperature and salinity have the very small rate of change when θ3drops to ～1.245°C below ～5 500 m (as shown in Fig.2a & b). This is also observed by Jiaolong as indicated in Figs.4b, 3a & 3b. Moreover, Fig.4b shows that in hadal zone, potential temperature keeps homogeneous and salinity decreases a little.

The relation of potential temperature versus salinity with potential density contours overlain shows the spatial difference erence between stations zonally across the trench. For the fact that a lower potential temperature corresponds to a higher salinity for abyssal water, the difference erence of the relation at difference erent stations is not distinct in the diagrams. Even so, some small difference erence enlarges the spatial disparity in isopycnal. For the ship-based observation, at the same θ3between ～1.59°C and ～1.47°C, the salinity at C04 is highest, resulting in denser water and shallower isopycnal. In general, the central station C03 has a relatively low salinity at a same temperature, consequently lowering the density and deepening the isopycnal. For observations of Jiaolong (Fig.4b), at the same depth, the water at D148 is relatively cold and saline, and D150 has higher temperature and lower salinity. This indicates that above 4 200 m, the isopycnal is shallower at the fl ank than in the central trench.

3.4 Geostrophic fl ows and water transport

Figure 5a shows the meridional geostrophic fl ows at Section-C estimated from temperature and salinity observed by ship-based CTD. From C01 to C04, the fl ows basically travel southward except for the upper ～1 800 m layer (C01-C03) in which there is weak northward fl ow (＜1 cm/s). Between ～4 000 m and ～5 000 m, the southward fl ows are relatively strong (in 2-3 cm/s). From C04 to C02, the east fl ank of the trench shows northward fl ows with a maximum velocity of 3.3 cm/s.

Figure 5b presents the fl ows along Section-D estimated from observations of Jiaolong. At the west fl ank, the fl ows travel southward in the whole abyssal column. In the bottom layer at D149 on the steep slope, the velocity is the highest (with a maximum of 13.5 cm/s). From a west station D150 to D152 on the east slope, the southward fl ows gradually become to be in an opposite direction. Above ～3 700 m, the east of ～137.6°E shows a weak northward fl ow. Between ～3 700-4 700 m, the fl ows become northward at ～137.8°E. Below the layer, the fl ows are basically southward in the west of ～137.8°E and northward in the east with a maximum speed of 2.5 cm/s at depths close to the hadal zone.

Fig.5 Meridional geostrophic fl ows relative to the reference level in abyss crossing transects

Fig.6 Meridional geostrophic transports across transects estimated between 3 000 m and abyssal depths

Both contours show that the meridional geostrophic fl ows are southward in the west and northward in the east. This opposing pattern suggests the cyclonic circulation in abyss. With consideration of the spatial difference erence between D148-D149 and C01, the strong fl ows occurred on the west slope according to Jiaolong’s observations, while this pattern was not presented in Section-C. Jiaolong can observe in deeper water than the conventional method to obtain hadal fl ows, but it has no observation in the central area, resulting in a certain discrepancy in estimated fl ows between two methods. Nevertheless, they show a similar pattern for the fl ows.

With the results shown in Fig.5, we estimated the geostrophic transports from the reference level to abyssal depths at Section-C and Section-D (Fig.6a). Along section-C, the net volume in northward keeps increasing with depth and reaches the maximum of ～0.26 Sv at ～4 000 m. The volume reaches zero at ～4 500 m and increases to ～0.8 Sv (the positive means southward) at the maximum observation depth ～5 950 m. At Section-D, the net transport keeps close to zero in the fi rst 500 m and then continually increases to ～1.74 Sv at 6 000 m. In hadal zone, the transport reaches the maximum ～1.77 Sv at ～6 500 m and then decreases slightly to ～1.75 Sv at the maximum diving depth of 6 685 m.

Fig.7 Profi les of average dissipation rate ε over a depth range H=300 m

Fig.8 Profi les of average diapycnal difference usivity K over a depth range H=300 m

For the difference erence in width and locations between two sections, their variations in the net transport are similar but somewhat difference erent. We computed the transport along two close transects: D149-D152 for Jiaolong and C01-D152 for ship-based CTD (Fig.6b). The panel shows two identical profi les of transport volumes. In the central part across the trench, the net transport is close to zero at the layer above 3 500 m, and below the level, it keeps increasing with depth. In the hadal zone, the southward transport is balanced by the northward.

In summary, the meridional geostrophic fl ows in abyssal and hadal zones are southward in the western part of the southern Yap Trench and opposite in the eastern. In the main region of the trench, the net transport of abyssal water is southward. The net transport of the hadal water decreases to zero.

3.5 Diapycnal mixing

The estimated dissipation rate and eddy difference usivity of abyssal water at C01-C04 and D148-D152 are shown in Figs.7 & 8. The magnitude of dissipation rate and difference usivity is ε～O (10-8-10-6W/kg) and K～O (10-3-10-1m2/s), respectively.

At the stations of ship-based observation, the average dissipation rate is almost constant in the upper ～1 400 m layer. Below 4 100 m, the average dissipation increases with depth. At the deepest water, the rate reaches ～7.8× 10-8W/kg. C03 has the highest dissipation rate above 4 700 m except for ～4 100-4 400 m. The diapycnal difference usivity presents the similar pattern to the dissipation rate. The average difference usivity increases a little in the fi rst ～700 m. After a slight decrease, the difference usivity strengthens continually from 3 700 m to 5 900 m and reaches the maximum of ～1.1× 10-2m2/s. Generally, the water on the steep slope and the deeper abyssal water in the central trench show relatively strong mixing.

At stations of Jiaolong dives, the magnitude of dissipation rate and difference usivity is same as those of C01-C04. Generally, the mean values are higher in deeper water. Above 5 200 m, both the dissipation rate and difference usivity are relatively low at D151 where the topography is fl at, while those at locations on the steep slope are quite high and weaken the stratifi cation (shown in Fig.3). This corresponds to the depressions of isolines of potential temperature, salinity, and potential density over the central trench as shown in Fig.3. Compared with the results at stations C01-C04, we found that the mixing is active in the hadal water on the steep slope, especially at the near-bottom zone where the geostrophic fl ows are strong and the gradient of potential density is weak.

In summary, the dissipation rate and difference usivity keep almost constant in the upper layer and then increase with depth. The turbulent mixing is strong on the steep slope and hadal zone where the strong geostrophic fl ows and small gradient of potential density exist.

4 DISCUSSION

4.1 Comparison of water properties in the southern Yap Trench with the northern

The observations reveal that in the southern Yap Trench, the water obviously becomes colder, denser, more saline more oxygen with the depth increase between 3 000 m and ～4 500 m, and below this layer, the rate of change is very small but the oxygen content decreases obviously. The vertical patterns are consistent to the observed in the northern Yap Trench (Liu et al., 2018). This indicates that two parts of Yap Trench have the same or similar properties of watermass. In addition, the isolines of temperature, salinity, and density are depressed over the central trench, and in hadal zone. This phenomenon was observed in the Challenger Deep of the Mariana Trench (Huang et al., 2018). However, difference erent spatial patterns were shown in the north of the trench by Liu et al. (2018), where the isopycnal continually becomes deeper from the western side of the trench to the eastern side. These difference erences lead to the difference erent patterns for geostrophic transport. In the southern trench, the abyssal circulation is cyclonic with the southward fl ows in the western area and the northward in the eastern, and the net transport is southward. While in the northern trench, the route of LCPW is northward. Additionally, the circulation in the Challenger Deep is cyclonic (Taira et al., 2004; Huang et al., 2018). Johnson (1998) reported that the cyclonic sense of circulation is often evident in trenches of the Pacifi c.

Comparison shows the similar pattern of diapycnal mixing in the south and north of the Yap Trench. The magnitude of dissipation rate and difference usivity in the southern trench is same to that in the north (Liu et al., 2018). In both areas, the dissipation changes little at the depth range of 3 000-4 500 m, in the deeper layer, the diapycnal mixing increases with depth, and the enhancement of mixing weakens water stratifi cation in the deeper abyssal and hadal zone. Besides, the mixing is strong along the slopes in the two areas of the trench. The strong mixing seems to be related to the geostrophic fl ows and internal tide dissipation analyzed in the north trench by Liu et al. (2018). Additionally, our fi ndings agree largely with the measurements by Huang et al. (2018) in the Challenger Deep.

4.2 The origin of the abyssal and hadal water in the Southern Yap Trench

Previous studies proved that the deep water in trenches and basins of Mariana-Caroline regions including the northern Yap Trench is from LCPW (Siedler et al., 2004; Huang et al., 2018; Liu et al., 2018). The same characteristics of the abyssal watermass is cold, saline, and rich in oxygen, indicating that the abyssal water in the southern Yap Trench is linked to adjacent water by deep fl ows, which is of LCPW. Although in the past, no observation was conducted in the abyss of the southern Yap Trench, Kaneko et al. (1998) found a broad isopycnal depression and suggested that a part of the westward fl ow at the Mariana Basin enters the Caroline Basin via the Yap Trench based on the observation of watermass at about 7°N and other sections in the west North Pacifi c. Studies by Kawabe et al. (2003) and Siedler et al. (2004) also proved the connection between the Yap Trench and two basins where the deep water below NPDW is of LCPW.

On the other hand, the fl ow of LCPW is blocked by sills in trenches and basins, and the downwelling water is diき cult to reach the depths below 6 000 m. Furthermore, the salinity and oxygen content decrease in hadal water (seen in Figs.2d, 3c-d & 4b), so they are lower in the observed hadal zone than those of LCPW. This suggests that the hadal water in the Southern Yap Trench maybe is the isolated local water rather than LCPW. Nevertheless, based on this local observation and very few previous studies, we cannot make sure whether the hadal water in the trench is of LCPW or isolated local water.

5 CONCLUSION

Based on the observations of ship-based CTD and the manned submersible Jiaolong, we analyzed the water characteristics, geostrophic transport and turbulent mixing in abyssal and hadal zones of the southern Yap Trench. The conventional CTD observation presented the results for the depth less than 6 000 m at stations covering the trench slope and the central area, while the submersible conducted observation from the sea surface to the sea bed below 6 000 m along the slope of the hadal trench. Two kinds of observation obtained the consistent and mutually complementary results.

The similar spatial patterns for temperature, salinity, density and oxygen content are shown. The abyssal water is cold, saline and oxygen rich. In hadal zone, the potential temperature and potential density keep almost homogeneous, with a little decrease in salinity and a distinct reduction in the content of oxygen. Results show that the isolines of potential temperature, salinity and potential density are depressed over the central trench in abyss. At the steep slope where the stratifi cation is weakened, the dissipation rate and difference usivity are relatively large. The diapycnal mixing is enhanced in the near-bottom zone and also active in the hadal water.

In abyssal and hadal zones of the southern Yap Trench, the estimated meridional geostrophic fl ows are southward in the western part and northward in the eastern. In the main region of the trench, the water transport is ～1.74 Sv and southward in the range of 3 000-6 000 m. Below 6 000 m, the southward transport is balanced by the northward. This suggests existence of cyclonic circulation.

The analysis of properties of watermass indicates that the abyssal water in the southern Yap Trench is of LCPW, and suggests that the hadal water seems to be of the isolated local water rather than LCPW. However, the existed studies are still not enough to verify the attribution of the watermass. Moreover, the connections and water exchanges of the southern Yap Trench to adjacent trenches and basins such as the northern Yap Trench, Caroline Basin, and Mariana Basin are to be studied with more observations, in order to reveal the origins and fl ow paths of the abyssal and hadal water in the trench.

6 DATA AVAILABILITY STATEMENT

The data that support the fi ndings of this study, described in Section 2.2, are available from the National Deep Sea Center, Ministry of Natural Resources of China but restrictions apply to the availability of the data, which were used under license for the current study, and so are not publicly available. However, data are available from the fi rst author of this paper upon reasonable request and with permission of the National Deep Sea Center, Ministry of Natural Resources of China.

7 ACKNOWLEDGMENT

We thank the crews and scientists aboard R/V Xiangyanghong 09 and the submersible Jiaolong for data acquisition and sample collections in the No. 38 of Chinese Ocean Cruises. We acknowledge the National Deep Sea Center, Ministry of Natural Resources of China for providing observation data of Jiaolong. We thank Dr. YANG Jichao, Dr. WANG Chunsheng, and Dr. LU Bo for their cooperation and helps in data sharing. We also thank the anonymous reviewers for providing valuable suggestions.

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