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The Origin and Preservation of Suspended Barites near the 90˚E Ridge in the Northeastern Indian Ocean

2023-03-17BUXuejiaoLIUMingDINGYiYANGYadiHUANGMuSUNXiaoxiaFANDejiangandYANGZuosheng

Journal of Ocean University of China 2023年1期

BU Xuejiao, LIU Ming, *, DING Yi, YANG Yadi, HUANG Mu, SUN Xiaoxia, FAN Dejiang, 4), and YANG Zuosheng

The Origin and Preservation of Suspended Barites near the 90˚E Ridge in the Northeastern Indian Ocean

BU Xuejiao1), 2), LIU Ming1), 2), *, DING Yi1), YANG Yadi3), HUANG Mu4), 5), SUN Xiaoxia1), 2), FAN Dejiang1), 2), 4), and YANG Zuosheng1), 2)

1),,266100,2),,,266100,3),261021,4),,266061,5),,266061,

Suspended particulate barite crystals were detected in the water columns at four different stations near the 90˚E ridge in the Indian Ocean. Four distinct morphological types of marine barites were distinguished: euhedral-subhedral crystals, oval or round crystals, rhombic crystals, and irregular crystals. The barite crystals in the study area are typically fine, with a dominant size of 1–3μm. The vertical distribution of barites is significantly affected by the formation and sedimentation processes. Barites begin to appear at a depth of 30m and are formed primarily from the surface to the depth of 2000m with a concentration peak at the depth of 200m, where particles are coarser than those in the other layers. The barites begin to settle and dissolve once formed in the water column, resulting in finer barite particles and lower particle concentrations. The formation of barite crystals is related to biological processes associated with the decomposition of barium-rich skeletons in the microenvironment of decaying organic matter that is affected by the primary productivity and dissolved oxygen content in the water column. The dissolving process of barite crystals showed similar variation with the concentration of dissolved barium in ocean water, and the substitution of strontium for barite in crystals promotes the selective dissolution of barite and exerts an important impact on its morphology. It is approximately 33% of barites in the amount and 22% in the concentration to settle to the bottom of the water column compared to that observed in the main barite formation zone.

marine barite; Indian Ocean; origin and sedimentation process; dissolution and preservation

1 Introduction

Marine barite particles are widely distributed throughout oceans (Sun., 2015). It is well established that barite is the major carrier of barium in suspended matters and the formation, dissolution, sinking, and preservation of the former restricts the biogeochemical cycle of the latter in the marine environment (Hoppema., 2010; Griffith and Paytan, 2012; Singh., 2013). The processes that control the formation of barite in the marine environment are poorly understood (Bishop, 1988; Rushdi., 2000); however, studies on the biogeochemical processes, actions, and mechanisms of suspended barites in ocean water indicated that the majority of marine barite is formed in organic-rich microenvironments during the decay of biogenic debris (Dehairs., 1980; Bishop, 1988; Gingele and Dahmke, 1994; Rushdi., 2000). High barite concentrations in sediments or water column have been found to have a strongly correlation with the highly productivity in upper seawater and are closely related to the high organic carbon contents (Paytan., 1993, 1996; Eagle., 2003; Sternberg, 2007). Therefore, the barite concentration is considered one of the most ideal indicators for reconstructing the primary production in the ocean (Paytan.,1996; Jeandel., 2000; Paytan and Griffith, 2007; Stern- berg., 2008).

Barite generally forms in the mesopelagic zone (100–1000m) within aggregates of decomposing organic detritus (Dehairs., 1990; Stroobants., 1991; Ganesh- ram., 2003; Beek., 2009), below which the oceanwater is typically undersaturated with regard to barites (Monnin., 1999; Rushdi., 2000; Hoppema., 2010). There are a series of changes in the content and particle morphology of barite crystals as they sink throughthousands of meters of the water column; it has been suggested that some barite crystals dissolve before reaching the bottom of the sea, directly affecting the geochemical cycle of barium and the buried flux of barite in sediments (Paytan and Griffith, 2007; Griffith and Paytan, 2012; Sun., 2015). Therefore, it is essential to study the content, morphology, and distribution of barite in the oceanic water column to better understand the biogeochemical process of barite and accurately evaluate the significance of barite as an indicator of oceanic productivity.

The northeastern Indian Ocean is one of the most important sedimentary units in the world because of its special geographical position and timely response to global en- vironmental changes since the Cenozoic era (Hovan and Rea,1992; Klootwijk., 1992; Burton and Vance, 2000;Fang., 2002; Wei., 2007). The largest abyssal fan in the world (the Bengal abyssal fan) and an important ridge (the 90˚E Ridge) both developed in the northeastern Indian Ocean. The sediments making up the Bengal Fan arederived primarily from terrigenous detritus from the denu- dation of the Qinghai-Tibet Plateau and are dominated byturbidity current deposits and hemipelagic deposits (Stow., 1990; Fagel., 1994). The monsoons in this region cause changes in the surface currents; in winter they run counterclockwise while in summer they move clockwise(Wyrtkik, 1973; Qiao., 2014). The northern movements of the Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW) that occur in the middle and bot- tom layers are the most important deep water masses affect- ing this region (Gorsline, 1984; Alexander., 2009). The90˚E Ridge is 4000km long and extends from 5˚N to 31˚S along the longitude line, with the central Indian Ocean Basin and Wharton Basin located to the west and east, respectively (Fig.1). Previous studies have demonstrated that the unique structural units and towering terrains inhibit the direct influence of turbidity currents and the pelagic sedimentary sequences occur in this region (Wei., 2007). The primary productivity of surface water in this area is high due to the influence of upwelling and the local climate (Fagel., 1994; Liu, 2002). However, there have been no studies of suspended barite particles in the water in this area. This study, therefore, examined the morpho- logy, particle size, and spatial distribution of suspended marine barite in the waters from the surface to the depth ofapproximately 5000m at four stations near the 90˚E Ridge in the Indian Ocean using scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). The origin, settling, and preservation of barite particles near the 90˚E Ridge are discussed systematically, with the aim of improving the understanding of biogeochemical pro- cesses related to marine barite.

Fig.1 Geographical setting and hydrography of the study area and the location of sampling sites. The blue arrows show the currents of Antarctic Bottom Water (AABW) and Deep Western Boundary Current (DWBC).

2 Materials and Methods

Water samples were collected at four stations located on both sides of the 90˚E ridge in northeastern Indian Ocean by using Niskin bottles coupled to a conductivity-tempera-ture-depth (CTD) instrument (Sea-Bird 911plus) throughout the water column during the DY52-1 cruise of thefrom December 2018 to January 2019. Watersamples were collected from 9 to 13 different water depths at each station, and a total of 45 samples were obtained. Spe- cific sampling information is given in Table 1. Temperature, salinity, and chlorophyll-concentrations were measured simultaneously by using the CTD instrument.

The suspended particulate matter was concentrated by filtering the water samples (3–5L) through a double acetate fiber microporous filter membrane (47mm in diameter, micropore diameter of 0.45μm) in the shipboard che- mistry laboratory. The filter membranes were rinsed 3 to 5 times with deionized water to remove salts, dried in the oven at 40℃, stored in tin foil, and refrigerated at 4℃until analysis.

The filter membrane obtained was dried in an oven at 40℃ for 24h again, and an approximately 3mm×5mmpiece was cut from the center of the filter and glued onto the sample platform of the SEM. The sample was gold-coat-ed under high vacuum to conduct electricity and then placedin a sample chamber for crystal morphology, surface struc- ture, and chemical composition analysesSEM/EDS in the laboratory.

Barite crystals were identified by using a backscattered electron detector combined with composition analysis based on the EDS technology. The particle sizes were mea- sured by using the length measuring tool of the SEM, and images of each barite particle were acquired to document its morphology. Point determination and surface scanning were performed to determine the characteristics of the che- mical composition of the barite crystals based on their shapes.

Table 1 The sampling information

The SEM used in this study was a TESCAN VEGA3 SBH, and the working regimen for the SEM/EDS analysis was as follows: an accelerating voltage of 25kV, emissionscurrent of 100μA, and working distance of 10mm. The X- ray spectra collection time was greater than 60s until the counts/second (c/s) were stabilized.

3 Results

3.1 Morphology of the Barite Crystals

Barite crystals are rhombic, biconical and typically existas plate-like, granular, and fibrous aggregates. The morpho- logies of barites observed in the study area are complex and various, and can be classified into four evident morphological types: euhedral-subhedral, oval or round, rhombic crystals, and irregular. For each type of barite crystal, the complete crystal morphology has been observed in some particles, while many crystals possessed dissolution features at their edges or as cavities in their central portions.

1) Euhedral-subhedral barite crystals

The outline of this type of barite crystal was clear, and the crystal surface was smooth with obvious edges and cor- ners. However, many crystals were observed to possess dissolution features at their edges or on their faces. A subhedral barite crystal with a rhombic column shape, clear edge, and smooth crystal surface is shown in Fig.2a; however, a dissolution phenomenon appears at the local edges. Fig.2b presents a euhedral barite with clear edges and no obvious dissolution features.

Fig.2 Barite crystals with subhedral (a) and euhedral (b) mor- phologies.

2) Ovoid or rounded barite crystals

Two crystal morphologies (intact and dissolved) were also observed in this type of barite crystal. Barite crystals with intact morphologies are predominantly ovoid, elongated-ovoid, or rounded (Figs.3a, b), and have been reported briefly in other studies as suspended particulates in the Atlantic and Pacific oceans (Dehairs., 1980; Bertram and Cowen, 1997; Sun, 2015). Dissolved barites of this type showed dissolution etch pits on different portions of the crystal edges, which occasionally affected the crystal surface or appeared as a thinned etched pattern on particle faces (Figs.3c, d).

3) Rhombic barite crystals

This type of barite exhibits hexagonal, rhomboid, or octagonal shapes with clear crystal edges and smooth crystal surfaces. Several such crystals also have dissolution features on their surfaces (Figs.4a, b).

Fig.3 Barite crystals. (a)–(b), intact and ovoid or round; (c)–(d), oval or round with dissolution features.

4) Irregular barite crystals

Barite crystals of this type possess clear crystal edges, almost without dissolution features (Fig.5a). Several crystals are arrow-like in shape, with an ‘arrowhead’ at one end of the crystal that may have been formeddissolution (Fig.5b). Intensive dissolution may also pass through the entire particle so that it almost altered the original crystal profile (Figs.5c, d). Most barite crystals with strong dis- solution features contained strontium as part of their che- mical composition.

3.2 Size of Suspended Barite Crystal Particles

A total of 211 suspended barite crystals from the water column at four stations throughout the study area were ana- lyzed. The particle sizes of barite crystals were typically fine and ranged from 0.5μm to 7μm, with a dominant size of 1–3μm (Fig.6a). As presented in Fig.6b, the particle sizes of barite crystals from different stations were similar; how- ever, the distribution of barite sizes varied with regard to the groupings of different sizes.

Fig.4 Rhombic barite crystals.

Fig.5 Irregular barite crystals. (a), irregular polygonal; (b), arrowhead-shaped; (c)–(d), strongly dissolved.

The barite crystals in the water column from the surface to the depth of 200m were relatively fine, most of which are less than 3μm in length. Barite particles at depths of 200–2000m were coarser, and their sizes peaked at the depth of 200m or 300m. The particle size of barites then typically decreased as the water depth increased. However, large barite particles also appeared in the bottom of water column at stations CTD08 and CTD13, at depths of appro- ximately 4500–4700m.

The frequency distribution for different intervals of ba- rite particle sizes shows that the peak typically occurred within the ranges of 1–2μm and 2–3μm; while it was with- in the ranges of 0–1μm and 2–4μm at station CTD06. Ap- proximately 60%–70% of the barite particles were within these dominant size ranges.

3.3 Spatial Distribution of Barite Crystal Concentrations

The concentration of barite crystals in the water column can reflect their spatial distribution. Therefore, the number density of barite particles was calculated based on the proportion of the observed filter membrane area to the whole filter membrane area and the volume of filtered water to reflect the concentration of barite particles in the study area. The number density of barite crystals is expressed as follows:

whereis the concentration of barite,is the number of barite crystals observed under the SEM,is the volume of filtered water,1is the statistical area of the filter membrane observed under the microscope, and2is the area of the filter membrane covered with suspended particulate matter.

The barite particle concentration calculated and the tem- perature, salinity, and chlorophyll-concentration measuredby CTD instrument at each station are given in Fig.7. The variation trends of the hydrological parameters were consistent at all stations within the study area. The halocline occurred between 200 and 400m with a maximum salinity of approximately 35.4, while the thermocline appeared at 200–500m, characterized by high surface water temperatures and relatively low water depths. The chlorophyll-content peaked near the depth of 80–100m with a maximum value of 1.5mgL−1.

Fig.6 Vertical distribution in the water column (a) and the frequency distribution (b) of barite crystal particle size.

Fig.7 Variation in barite particle concentration and hydrological parameters.

The vertical distribution pattern of barite crystal concentration at each station in the water column exhibited its own distinct characteristics while adhering to a consistent law. Because of the shallow water depth, approximately 1800 m, at the CTD06 station, which is located at the Osborn Knoll Ridge, a total of 25 barite crystals were found, and most of the crystals were distributed within the water column at 100–500m and 1500m. The concentrations of barite at the other three stations exhibited similar distribution charac- teristics, with low concentrations in the surface water from the depth of 30m. The barite particles were abundant in the water column at the depth of 200–300m, with the maxi- mum value at 200m. The concentration of barites then decreased rapidly, followed by an increase at approximately 1000–2000m. The concentration then decreased again with a downward oscillation in the water column below 2000m. However, higher concentrations of barites were also de- tected close to the bottom of the water column at approximately 4500m and at 4700m at stations CTD08 and CTD13, respectively.

In terms of horizontal distribution, there was no significant difference in barite concentration between the stations located on both sides of the 90˚E Ridge. The particle size, concentration, morphology and vertical distribution of barite crystals at most of the stations were consistent, ex- cept the station CTD06. The barite crystals exhibited cha- racteristics of rapid enrichment in the water column from the surface to depth of approximately 200–300m, follow- ed by gradual enrichment in the range of 1000–2000m, and remained in settlement and dissolution states after theyformed below 2000m. The vertical distribution patterns of barite at the station CTD08 and CTD13, on two sides of the 90˚E Ridge, are similar. However, at CTD12 the distribution pattern of oceanic barite represents a typical one in water column.

4 Discussion

4.1 Vertical Distribution Pattern of Barite Crystals and Its Relationship with Key Environmental Interfaces

Because the water is shallow at station CTD06, the distribution pattern of barite there did not represent the main characteristics of barites in the study area. However, the patterns of barite distribution at stations CTD08, CTD12, and CTD13 are consistent; and they better reflect the development and evolution characteristics of barites in the water column of the study area. Fig.8 presents a statistical assessment of the characteristics of barite distribution at these three stations. In this study, there is a little controversy on the depth of the water samples collected, especial- ly in deep water, according to the depth of the study stations to adjust the sample water depth. When calculating the average concentration of barite, if there is only one sampling layer researched at one station, we took this data as the average to reflect the real distribution characteristics of barite concentrations.

Barite crystals begin to appear at a depth of 30m, but their particle sizes are small and the quantity and concentration are low. This indicates that barite is formed in near- surface water; however, the amount of barite was low and the particles were fine. The concentration of chlorophyll-was high at the depth range of 85–100m and peaked in this zone, accompanied by a gradual increase in the barite crystal concentration and particle size, which reached a peak value of the entire vertical profile in the water layer at approximately 200m. According to data from studies of the oxygen minimum zone (OMZ) in the Bay of Bengal (Paulmier and Ruiz, 2009; Wang., 2018), the OMZ in our study area was roughly distributed at depths of 200–500m. The formation and development of barite was clear- ly affected by the presence of the OMZ, which resulted in a decrease in the amount and concentration of barite (Fig.8). However, the mean particle sizes of the barites reached their maximum values at a depth of 500m due to their post- formation settlement. Yet, as the dissolved oxygen (DO) contents in the water recovered, the amount and concentration of barites increased until a depth of 2000m whilethe particle size continued to decrease. This suggests that new barites are likely formed in the water below 500m up to a depth of 2000m. The amount and concentration of barite then decreased at water depths below 2000m; however, their particle sizes tended to remain stable. The particle size and concentration of barite did peak again at approximately 4500–4700m.

Fig.8 Vertical distribution patterns of barite crystals and their relationships with key environmental interfaces. (a)–(d), the meanconcentration of Chl-a, mean barite particle size, total number of barite particles, and mean concentration of barites detected at stations CTD08, CTD12, and CTD13, respectively. (e)–(h), dissolved barium (DBa) concentrations in the Northwest Indian Ocean (NIO; Jeandel et al., 1996), Southern Ocean (SO; Pyle et al., 2018), Pacific Ocean (PO; Sugiyama et al., 1984) and South China Sea (SCS; Cao et al., 2016), respectively. Shaded areas represent the oxygen minimum zone, the red dash line represents the carbonate compensation depth (a–d), and different symbols represent different research stations (f–g).

The distribution of barites in the water column suggests that they are formed primarily at water depths from the sur- face to 2000m. However, their formation is clearly divided into two stages: rapid formation in the water column at 0–500m resulting in a rapid increase in barite concentration and particle size, and gradual formation in the water column at 500–2000m. At the same time, however, baritesare also dissolved; this resulted in an increase in barite con- centrations and a reduction in their particle sizes. The formation of barite is closely related to the primary producti- vity of the ocean (represented here by chlorophyll-concentration) and restricted by the presence of OMZs in the ocean. Barite crystals may gradually settle after formation in the water column, accompanied by a continuous decrease in the particle size. Barite crystals may not form in the water below 2000m, and the barites in deep water prima- rily come from the sedimentation of the upper water particles. Therefore, the amount and concentration of barites continue to decrease while the particle size tends to remain stable. The particle size and concentration of barites in the water at approximately 4500–4700m increased primarily because of the accumulation in this layer of barites settled from the upper water. Carbonate minerals may dissolve in deep water, at depths of more than 4800m, that’s below thecarbonate compensation depth, in the Indian Ocean (Campbell., 2018). The particle size and quantity of barite particles also had a similar decrease, especially for the mean particle size, but the average concentration did not change significantly. This layer has only been studied at Station CTD12, so its changes can be analyzed from the data of CTD12. Obviously, the particle size of barite reduces signi- ficantly (Fig.6), but the concentration donot significantly change compared with the overlying layer (Fig.7), which indicates that barite may also be dissolved at this stage, but the concentration does not decrease.

4.2 Formation of Marine Barite in the Indian Ocean

Marine barites are not saturated in ocean water, and thereis no consensus regarding their origins. However, there aretwo main views regarding the formation mechanism of ma- rine barite in water (Sun, 2011). The first is the celestite model, which holds that the formation of barite crystals is related to biological processes of barium-rich skeletons in decaying organic matter. For example, radiolarians, fibrous cyanobacteria, and bryozoans can enrich barium in their skeletons. Although the barium contents in their carbonate skeletons are low, these organisms are present in the oceansin high densities. This can remove barium from in the surface layer of the ocean. After the death of these organisms, their bones decompose in the water; and barium is bound totransparent exopolymer particles, cell wall-associated poly- saccharides, or extracellular polymeric substances from bacterial biofilm in microenvironments formed by the cell walls and shell material of phytoplankton (Martinez-Ruiz., 2018) before reacting with sulfate derived largely from seawater to form barite (Gooday and Nott, 1982; Bernstein., 1992; Dymond and Collier, 1996; Bertram and Cowen,1997; Bernstein., 1998; Pyle,2018). The secondviewpoint, the organic aggregate model, suggests that marine organisms with carbonate skeletons rich in elemental barium will gradually decompose during the sedimentation process after their deaths. As part of this, the carbonate dissolution will release barium into deep seawater, where it would combine with sulfate from rotten organic matters, leading finally to the crystallization and precipitation of ba- rite crystals. This suggests that biogenic barite, in particular, is formed during the process of organic matter migrating from surface water to the bottom of the ocean in conjunction with the decomposition of organic matters. Therefore, the amount of barite increases toward the deep water and reaches its maximum value at the bottom of the ocean (Dy- mond, 1992; Francois, 1995; Klump., 2000, 2001; Esser and Volpe, 2002).

In the study area, sunlight was sufficient, the chlorophyll-content of surface water was high (Fig.7), and there weremany phytoplankton in the surface water (Liu, 2002). There were also numerous algal particles among the suspended particles, as SEM analysis showed. The maximum barite crystal concentration is just below the layer with the ma- ximum concentration of chlorophyll-(Fig.8). Therefore, we inferred that a large quantity of plankton with enriched barium in their bones lived in the surface water, and when they died and decomposed with other organic matters, the barium is released and the barite crystals are formed. There- fore, the formation of marine barite crystals in the study area follows the celestite model.

As shown in Figs.7 and 8, barite crystals begin to appear at approximately 30m in the water column; however, their concentration is low in the shallow water above 200 m, which indicates that they began to form in the euphotic layer of the water (Paytan and Griffith, 2007; Sun., 2015). The high temperature of seawater in this layer also makes it more suitable for the plankton growth, and se- veral dead organisms also begin decomposing in this area. All of these features and processes promote the development of barite. The concentration and size of the barite crystals also increased significantly at the water depth of 200m, indicating that a large number of barite crystals were formed in this zone–exactly the depth at which plankton died and began to decay and sink. We also found, by SEM, that many barite crystals were wrapped in or attached to biological debris or organic films. Additionally, the barite crystals observed in this layer were relatively thick and the crystal morphologies were relatively intact. Therefore, barite crystals are mainly formed in the water from the surface to the depth of 200m.

However, the DO concentration of seawater decreases below 200m, and OMZs are located at depths of approximately 200–500m in the study area (Paulmier and Ruiz, 2009; Wang., 2018). This was not conducive to the de- composition of organic organisms and the release of ba- rium, thus limited the formation of barite in water. As a result, the concentration of barite crystals decreased sharp- ly in the water layers at 300m and 500m (Figs.7 and 8). At depths ranging from 500m to 2000m, influenced by the oxygen-rich AAIW, DO increased again (Alexander., 2009; Wang., 2018). Therefore, residual organic matters began decomposing again, leading to a subsequent in- crease in the concentration of barite crystals.

Barite particles in the water column below 2000m are primarily derived from the settling of the upper water particles, accompanied by the continuous dissolution. In the near-bottom water, sedimentary processes as well as the partition of the bottom current from the AABW (Alexander., 2009; Wang., 2018) bring more barite particles from the upper water, thus resulting in the increase in both the concentration and particle size of barite crystals.

4.3 Dissolution and Preservation of Barite Crystals

Barite in marine sediments can be used as an important indicator of the primary productivity (Dymond., 1992; Paytan and Griffith, 2007), so it is of great significance to understand how marine barite is dissolved and preserved after its formation in the water column. The ocean is typically unsaturated in terms of pure marine barite (Monnin., 1999), and previous studies have demonstrated that its crystals are subject to dissolution after they are formed in ocean water. It is therefore a common for marine barite to possess a dissolution morphology in addition for some of it to have an intact morphology (Putnis., 1995; Bosbach., 1998; Kai., 1999; Wang., 1999).

In the open ocean, the concentration of dissolved barium(DBa) is low in the surface water and increases with depth (Wolgemut and Broecker, 1970; Bacon and Edmond, 1972; Bernat., 1972; Chan., 1976). The concentration of DBa in seawater is controlled predominantly by the so- lubility of barite (Church and Wolgemuth, 1972; Dehairs., 1980; Sugiyama., 1984). The elemental barium is either taken up by suspended substances in the surface water, or combined with sulfate to form particulate barite.

Some of these substances are destroyed as they settle to the bottom of the ocean, and barium is finally released into the deep water again (Sugiyama., 1984). There is no database on the DBa concentrations in the water of our study area yet; however, the DBa concentrations in the Pacific Ocean, the Southern Ocean, the Indian Ocean and the South China Sea all gradually increase from the surface to the deep water and become stable at depths below 2000m (Fig.8). The coordinated changes in the DBa concentrations in global ocean water confirm that barite is pri- marily formed in the water column above 2000m, and thatthe dissolution begins as it settles after formation. This can explain the increase in the DBa concentrations in water above 2000m. As there is no new generation of barite in the water column below 2000m, the amount and concentration of barite crystals gradually decrease and the dissolution-compensation equilibrium tends to be stable.

The morphology of barite always changes, indicating its crystal shape is destroyed due to the dissolving process in the water column. Etch pits were observed around the edges or on the surface of the barite crystals unevenly, which may make crystals irregular in shape (Fig.5). Barite particles with dissolution features around the edges of the crystals were widely distributed within the study area (Figs.9a, b, c) and observed at all depths of all stations. Additionally, they were more likely to be found in deeper water. However, a notable dissolution characteristic is that the etch pits were not homogeneously distributed on the crystal sur- faces, and the intensive dissolution occurred in the middle or center of the crystal surfaces rather than just along theiredges. The inner etch pits were detected at the center of the particle surfaces or in inner cavities that occurred at the core of the crystals (Figs.9d, e). The proportion of barite particles with strong dissolution features is low; however, an X-ray energy spectrum study incorporating the use of SEM showed that they all had high strontium contents (Figs.9e).

The presence of several barite crystals in seawater may be related to the dissolution system of barite and celestite coexisting in a certain proportion in the marine environment, and a considerable amount of strontium and barium exists in solid solution. This may alter the saturation state of barite in seawater to a value that can be as high as 30% (Rushdi, 2000). Sun(2015) proposed that the dif- ferential dissolution of barite crystals in the eastern equatorial Pacific Ocean is a consequence of the heterogeneous distribution of strontium in barite crystals, exerting an important effect on the morphology of the latter. The elemental strontium in barite plays a unique role in its dissolution; the solubility of marine barite containing strontium is 1013times higher than that of pure barite (Putnis., 1995; Prieto., 1997). Previous studies have demonstrated that the composition of crystals may change from the core to the edge in a complete series of celestite and barite solidsolution systems in the process of forming the (Ba, Sr)SO4crystal. The barite crystals tends to be rich in strontium in their center and are almost pure barite at the edge (Prieto., 1993). The selective dissolution of barite particles is caused by the non-homogeneous participation of strontium during the barite formation, and the Sr-enriched portion dissolves first and the barium-enriched portion is preserved. Therefore, the inner etch pits or cavities in the barite crystals were likely caused by the selective dissolution of crystal nuclei that is enriched in elemental strontium.

Fig.9 Morphology of dissolved barite crystals (a)–(d) and the chemical composition of strongly dissolved barites determined by the X-ray energy spectrum (e).

The barite particles in the study area with a dominant size of 1–3μm remained in the water for a long period of time, until they eventually settled on the bottom of the ocean after formation in the upper water. According to the distribution characteristics of barite in the water column at sta- tions CTD08, CTD12, and CTD13, approximately 33% of the total barite and 22% of the barite concentration settled to the bottom water of the stable area below a depth of 4000m, compared to the former in the main barite formation area in water at depths of 0–500m (Fig.8). A large proportion of barite in the underlying water will be deposited and buried in seafloor sediment; this is an important indicator of the primary productivity of water in this area.

5 Conclusions

Marine barite crystals were observed as suspended particles in the water columns at four stations near the 90˚E Ridge of the Indian Ocean. The crystals came in four morphological types: euhedral-subhedral, oval or round, rhombic, and irregular. The barite crystals in the study area weretypically fine and ranged in size from 0.5–7μm, with a do- minant particle size of 1–3μm.

The horizontal distribution of barite crystals was not cru- cially influenced by the presence of the 90˚E Ridge; however, its vertical distribution was significantly affected by its formation and sedimentation process. The surface to a depth of 2000m is a formation zone, with explosive formation occurring at approximately 200m; at this depth, barite particles are coarse. Areas below 2000m are sedimentation and dissolution zones, where barite particles are fine and their concentration decreases downward. Barite is formed primarily based on the celestite model, with the decomposition of barium-rich carbonate skeletons of organisms in the decaying microenvironment. The process is affected by hydrological conditions especially primary pro- ductivity and DO content in the water column.

Barite did not form below 2000m; however, continuousdissolution occurred during the settlement process, exhibi- ting a consistent relationship with the concentration of dissolved barium in ocean water. The dissolution of barites caused a decrease in their particle size and a significant change in their morphology. The substitution and non-uni- form distribution of strontium in barite crystals promoted the dissolution of barite, leading to the selective dissolution of the nucleus of the barite crystal and exerting an important impact on its morphology. Once formed, barite took a long time to settle to the ocean floor; approximately 33% of the total and a concentration of 22% achieved this compared to the levels of barite present in the main formation zone.

Acknowledgements

The authors would like to thank the captain and the crew of thefor field sampling assistance and Prof. Tan Jinshan of Qingdao University for assistance in SEM observations. This study was supported by the COMRAMajor Project (No. DY135-S1-01-09), and the Opening Foundation of Key Laboratory of Submarine Geosciences and Prospecting Techniques, Ocean University of China (No. SGPT-2019OF-02).

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(November 8, 2021; revised March 1, 2022; accepted March 28, 2022)

© Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2023

Corresponding author. E-mail: mingliuouc@163.com

(Edited by Chen Wenwen)