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Thermohaline circulation in the Central Indian Basin (CIB) during austral summer and winter periods of 1997
Deep-Sea Research II 48 (2001) 3327–3342
Thermohaline circulation in the Central Indian Basin (CIB) during austral summer and winter periods of 1997 V. Ramesh Babu*, A. Suryanarayana, V.S.N. Murty Physical Oceanography Division, National Institute of Oceanography, Dona Paula, Goa 403004, India
Abstract As a part of Indian Deep Sea Environment Experiment (INDEX) aimed at assessing the environmental impact of manganese nodule mining in the Central Indian Basin (CIB), a study on baseline physical conditions of water column viz. potential temperature (y), salinity and potential density together with geostrophic circulation regime in the deeper depths of the basin was conducted. The hydrography data used in the present analysis were collected over a wide area of the western part of CIB (711–791E; 91–141S) during austral summer (January 1997) from the Indian research vessel ORV Sagar Kanya, while during the austral winter season (June–July 1997), hydrographical stations were occupied by Russian research vessel RV Yuzhmorgeologia in the central part of CIB (751–771E; 91–111S) where a benthic disturbance on experimental scale was carried out. The spatial variations in the physical parameters decreased below 3500 m, inferring a restricted basin-scale deep circulation. The dynamic topography field at 5000 m relative to 2000 db surface in the central part of CIB, representing the abyssal circulation, was generally characterized by a southwestward weak flow around 101S flanked by cyclonic and anti-cyclonic eddies on its right and left sides, respectively. This flow regime agreed with the earlier one inferred by Warren (J. Mar. Res. 40 (1982) 823) linking the source of deep water in CIB to a saddle overflow across Ninetyeast Ridge from West Australian Basin around 101S. r 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Deep-seabed mineral formations such as polymetallic nodules and crusts are potential resources for the future extraction of various strategic metals, as the land resources gradually become depleted. Seabed mining of polymetallic nodules on commercial basis is, therefore, contemplated, and, in this connection, environmental impact studies (Amann, 1992; Chan and Anderson, 1981;
*Corresponding author. Fax: +91-932-223340/229102. E-mail address: [email protected] (V. Ramesh Babu). 0967-0645/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 1 ) 0 0 0 4 2 - X
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Curtis, 1982; Hirota, 1981; Lavelle and Ozturgut, 1981; Lavelle et al., 1981; Ozturgut et al., 1978; Padan, 1990) on deep-sea mining have assumed greater importance. The exploratory surveys have established the Central Indian Basin (CIB) as a potential area for commercial deep-sea mining of polymetallic nodules (PMN), and so environmental impact assessment (EIA) of the mining activity is needed. An Indian national multi-disciplinary environmental experiment known as Indian Deep-Sea Environment Experiment (INDEX), involving physical, chemical, biological and geological investigations, was undertaken as a part of EIA study of manganese nodule mining in the CIB. A knowledge on physical conditions (e.g., currents, temperature and salinity) is essential, as the dispersion of sediment plumes generated during mining activity is determined by density stratifications and advection fields in the water column. Deployment of current-meter moorings at selected sites representing the basin and the benthic disturbance site together with CTD measurements of temperature and salinity in the CIB mainly forms the data collection activity of the physical oceanographic study in this programme. In this paper, the spatial thermo haline structures based on CTD data collected during southern summer and winter periods of 1997 and the derived geostrophic circulation patterns are discussed. The results of the analysis of current measurements at different mooring sites in the CIB are presented elsewhere (Murty et al., 2001). Though the earlier studies of Warren (1981, 1982) were concerned with deep-water property distributions and circulation in CIB, his results were based only on few stations along a single hydrographic section (B121S) across the CIB whereas under this study, CTD data especially during the southern summer was collected at relatively high spatial resolution to study meso-scale features of circulation. Here we discuss the baseline physical conditions of the CIB and the anticipated effects of benthic disturbance.
2. Data and methodology Baseline data of temperature and salinity depth profiles were obtained with a CTD (Conductivity–Temperature–Depth) system for the austral summer and winter seasons of 1997 in the CIB. Twenty-eight stations spaced at a 11-latitude interval along meridional sections 791E, 771E, 751E and 711E were occupied by the Indian research vessel ORV Sagar Kanya in the austral summer period of 4–22 January 1997. The corresponding winter season CTD data were collected at 10 stations occupied by the Russian Vessel RV Yuzhmorgeologea during 4–30 June 1997 (Cruise No. INDEX 3A) & 27–28 July 1997 (Cruise No. INDEX 3B) covering a smaller area of the CIB in which a benthic disturbance experiment was conducted in August 1997. Details of the baseline environmental data collection on physical, chemical, biological parameters in the CIB prior to the deep-sea mining of the manganese nodules are given in a separate report (NIO Report, 1998). Fig. 1 shows the locations of both southern and summer winter hydrographic stations occupied in the western CIB. A high-resolution CTD system (Sea-bird Inc., USA; Model: SBE 911), consisting underwater and deck units connected through a single conductor cable, was deployed during the ORV Sagar Kanya cruise. The accuracies for temperature, conductivity and pressure sensors of CTD are 70.0021C, 70.0003 Siemens per metre (S m1), and 70.015% of full pressure scale. Further, the water samples were collected for comparing CTD salinity estimates with those obtained by onboard salinometer (Guildline Inc., Canada; model: Autosal 8400A, accuracy: 70.003 psu). A
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Fig. 1. Hydrographic stations for baseline data collection in the CIB under the PMN-EIA programme.
regression line fitted for the pairs of salinometer and CTD salinity estimates at corresponding depths of water sampling and the regression equation was used for correcting CTD salinity measurements. Dynamic heights at the stations were obtained by considering 2000 db pressure surface as a reference level. This reference level falls within the depth range of 1800–2800 m shown by Warren (1982), and the zero velocity surface actually represents the region of minimum
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horizontal motion lying between southward flow of high-salinity waters above and the northward deep flow of low-salinity waters below it. Horizontal distributions of potential temperature, salinity, potential density at various depths and dynamic topography maps were prepared for different pressure surfaces with reference to 2000 db surface. However, in this paper, we show the distribution of dynamic topography fields for the deeper and abyssal parts of the CIB only, as the emphasis here is to evaluate the deep-sea environmental impacts of the nodule mining in the basin. Vertical distributions of physical properties viz., potential temperature, salinity, potential density and geostrophic currents computed relative to an assumed 2000 m level of no notion are presented along 791 and 711E and 101S sections. For the southern winter stations, a Seabird CTD (SEACAT 19) system was used. Seacat had sensors for pressure, temperature and conductivity with respective accuracies of 70.15% full scale, 70.011C and 70.0001 S m1. Though the number of southern winter stations was less than during the summer period, we could collect CTD data as near as 10 m above the seabed at all the stations, whereas near-bottom sampling was possible only at three stations along 791E due to operational problems. Horizontal distribution maps of potential temperature, potential density and dynamic height topography field with reference to 2000 db surface were prepared using winter CTD station data. However, the winter study presented here obviously limits discussion on spatial distributions of potential temperature, salinity and potential density parameters at the depths of 3000, 4000 and 5000 m, the deeper abyssal regions of the CIB.
3. Results 3.1. Distributions of potential temperature, salinity and potential density during southern summer 3.1.1. Along 791E In the upper layers of the water column, the thermocline has a trough shape, with maximum deepening around 111–121S (Fig. 2a), suggesting that thermal flow across this section is characterized by an anti-clockwise circulation. North of 111–121S, the suggested flow is to the west, while to the south an eastward flow is inferred. The southward sloping of isotherms in the intermediate depth range of 800–1500 m indicates a westerly flow across the entire section. The meridional down-sloping of isotherms is least around 2000 m depth as compared to the slopes of isotherms above and below it. In deep waters (>4500 m depth), the orientation of 11C isotherm suggests westward flow concentrated around 101S. Salinity distribution at 791E (Fig. 2b) shows the presence of low-salinity waters (o34.70 psu) occupying the upper 100 m, brought into the study area from the east by the South Equatorial Current (SEC). In intermediate depths around 1000 m depth, fresher waters (o34.14 psu) are encountered prominently in the south between the stations 4 and 6, pointing to the northward spreading of the Antarctic Intermediate Water mass (AIW). Below 3000 m, the salinity is less than 34.72 psu, with reduced gradients similar to those seen in the temperature field. The conspicuous presence of a weak salinity maximum (>34.718 psu) in the depth range of 1500–3000 m is due to southward spreading of the North Indian Deep Water (NIDW). In his discussion on water property distributions along 121S in CIB, Warren (1982) also highlighted the presence of a weak
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Fig. 2. Distribution of (a) potential temperature (1C), (b) salinity (psu) (c) potential density (kg m3) along 791E during southern summer (January 1997).
salinity maximum with a probable northern source in the Arabian Sea wherein a high-salinity regime in the intermediate and deeper depths results from vertical mixing of the Red Sea Water. The distribution of potential density (sy (Fig. 2c) at 791E resembles more or less that of temperature with reduced gradients below 3000 m depth. In very deep waters below 4500 m, high potential density (>27.2 kg m3) waters are confined around 101S on account of relatively cold waters having potential temperatures less than 11C.
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3.1.2. Along 711E Near the western boundary of the CIB (711E), the trough nature of thermocline seen in the east at 791E is absent in the upper 300 m (Fig. 3a). A general southward down-slope of isotherms infers westerly flow in the upper layers across the entire section. Below 2500 m depth the 1.51C isotherm has a reverse slope suggesting a reverse flow towards east between 131 and 141S.
Fig. 3. Distribution of (a) potential temperature (1C), (b) salinity (psu) (c) potential density (kg m3) along 711E during southern summer (January 1997).
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The core salinity of intermediate low-salinity water mass around 1000 m is seen reduced in its value to 34.708 psu (Fig. 3b) at 711E compared to the higher core value of 34.714 psu observed at 791E. This east to west reduction in core salinity minimum of the AIW suggests of the dominance of zonal flow over the meridional flow in the intermediate depths, like in the upper layers. The reduction in core value of the NIDW is from 34.72 psu at 791E to 34.71 psu at 711E. The distribution of potential density ðsy Þ has a profound influence of temperature rather than salinity, as the effect of a sub-surface high-salinity maximum around 250 m is not seen in the density distribution (Fig. 3c). The isopycnals dip to the south in upper layer of 300 m, while at deep waters greater than 2500 m they have the opposite slope, suggesting easterly flow. 3.1.3. Along 101S A striking feature of temperature field at 101S is the nearly horizontal thermocline with relatively strong gradients (Fig. 4a). However, at deeper depths there is a weak down-slope of isotherms. Below 3000 m, the vertical thermal gradients are greatly reduced. It is interesting to notice the presence of a very strong halocline within the upper 20 m at stations 13 and 14 (Fig. 4b) along this section probably due to the surface dilution by heavy precipitation associated with the Inter-tropical Convergence Zone (ITCZ). The austral summer cruise of the ORV Sagar Kanya under PMN-EIA study also was partly devoted to studying the aerosol forcing on regional and global climate as a prelude to the main INDOEX (Indian Ocean Experiment), which actually was conducted during boreal winter of 1999. The studies by Naja et al. (1999) and Ramarao et al. (1999), based on surface atmospheric and oceanic data collected during the pre-INDOEX campaign of boreal winter 1997, show the presence of the ITCZ around 101S. Traces of AIW are seen around 1000 m with a somewhat reduced core salinity in the east. The deeper salinity maximum around 2500 m has a core value of greater than 34.712 psu, which is intermediate between corresponding core values at 791 and 711E. This westward decrease in the core value of the NIDW in the study area infers a general dominance of zonal flow over the meridional flow in deeper depths of the CIB. The potential density ðsy Þ distribution at 101S shows the dominant influence of temperature (Fig. 4c) as seen in similarity between the topographies of thermocline and pycnocline. The isopycnals at deeper depths have a downward slope towards east suggesting that the geostrophic flow is southward across 101S. 3.1.4. At 3000 m The property distributions across various sections as discussed previously have shown a common distinct feature of reduced vertical gradients below 3000 m, which roughly coincides with mean depths of the Ninety-east Ridge and the Central Indian Ridge flanked, respectively, on the eastern and western sides of the CIOB. This indicates a restricted deep-water circulation within very deep regions of the CIOB. A cursory look at property distributions at 3000 m (Fig. 5) draws an inference of northward movement of waters near western boundary of the basin under the influence of a boundary current. The potential temperature distribution (Fig. 5a) varies from a minimum of 1.361C to a maximum of 1.521C towards north in the western boundary region between 711 and 741E. Between 101 and 121S, the minimal zonal variations in the potential temperature ðyÞ infer a weak zonal flow as compared to the strong northward flow of cold waters of the Antarctic origin near the western boundary of the CIOB. This northward boundary flow is
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Fig. 4. Distribution of (a) potential temperature (1C), (b) salinity (psu) (c) potential density (kg m3) along 101S during southern summer (January 1997).
further inferred from the presence of relatively fresh waters, with salinities less than 34.71 psu, in the southwestern region of the study area (Fig. 5b). The potential density distribution ðsy Þ with incidence of relatively lighter waters in the north and denser waters in the south suggests the eastward turning of boundary current away from the Central Indian Ridge around 101S.
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Fig. 5. Distribution of (a) potential temperature (1C), (b) salinity (psu) (c) potential density (kg m3) at 3000 m during southern summer (January 1997).
3.2. Geostrophic circulation Geostrophic currents are computed across sections 791E, 711E and 101S, and their distributions are presented in Fig. 6. At 791E (Fig. 6a) an anti-clockwise cell is clearly discernible in upper 300 m with moderately strong westerly flow reaching up to 0.7 m s1
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Fig. 6. Geostrophic currents (m s1) across (a) 791E (b) 711E (c) 101S (NFNortherly current; EFEasterly current; SFSoutherly current; WFWesterly current) during southern summer (January 1997).
north of 131S, while south of it the zonal flow is relatively weak but reversed towards east, with maximum speed of 0.3 m s1. It is interesting to note the presence of zero-velocity line which separates the entire upper 2000 m water column into westerly and easterly flow regimes in the north and south, respectively. The closed circulation cell in the upper 300 m at 711E (Fig. 6b) has a reverse flow, with clockwise rotation having weak easterly flow of about 0.1 m s1 north of 121S and strong westerly flow up to 0.4 m s1 to the south. These cells in the upper 300 m of the study area suggest the meandering nature of the SEC with a weak clockwise cell north of the SEC and a strong anticlockwise cell south of the SEC. Between 1000 and 2000 m, a weak westerly flow of less than 0.1 m s1 is evident between 111 and 121S, while the easterly flow of similar order of low intensity
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is encountered between 131 and 141S. However, the boundary between the alternative deeper flow regimes is maintained at about 131S, like that at 791E. The presence of closed circulation cells in upper 300 m in association with the SEC results in setting up the alternative strong northward and southward flow regimes along 101S, reaching up to 1.0 m s1 (Fig. 6c). Below, in the depth range of 1000–2000 m, the southward flow is more dominant. The closed alternative circulation cells in the deeper depths is clearly seen from the dynamic topography field of 1750 db pressure surface obtained with reference to 2000 m level of no motion (Fig. 7). These cells are elongated to the east resulting in a dominant zonal flow rather than a meridional one. A weak clockwise cell is sandwiched around 751E between two prominent anticlockwise cells. 3.3. Distributions of potential temperature, salinity and potential density at deeper depths during southern winter Availability of CTD data very close to bottom during southern winter has facilitated the extension of the analysis of physical properties to the abyssal depths in a smaller central part of the CIB between 751 and 771E and 91 and 111S. The benthic disturbance experiment was carried out in this area in August 1997, just the collection of winter CTD data. Hence these patterns of property distributions and circulation are assumed to represent conditions prevailing at the time of benthic disturbance and as such the effects of benthic disturbance need to be evaluated with reference to near-bottom physical conditions. Fig. 8 presents the distributions of potential temperature, salinity and potential density at near abyssal depths of 3000, 4000 and 5000 m during southern winter. Though the spatial variations
Fig. 7. Dynamic height (dynamic metres) of 1750-db pressure surface above the reference level of assumed no motion at 2000 m during southern summer (January 1997).
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Fig. 8. Distribution of (I) Potential temperature (1C), (II) Salinity (psu) (III) Potential density (kg m3) at deeper depths of (a) 3000 m; (b) 4000 m; (c) 5000 m during southern winter (June–July 1997).
are subtle, the presence of relatively cold waters in the east and southeast suggests an incursion in deeper parts of CIB (Figs. 8Ia–Ic). Salinity distributions at these depths (Figs. 8IIa–IIc) further also show the presence of relatively low-salinity waters in the east. The association of cold, lowsalinity values in the east indicates an overflow of the Antarctic Waters across suspected gaps or saddles of the Ninety-east Ridge. The potential density field is more or less uniform, indicating sluggish motions below the sill-depth. 3.4. Near-bottom circulation during southern winter The dynamic topography of 5000-db surface with reference to 2000 db level of no motion was computed by utilizing the near-bottom CTD data, and the geostrophic circulation pattern is shown in Fig. 9. An approach of downward integration of dynamic depths below an assumed level of no motion as adopted by Warren (1982) is also employed here. The range of dynamic
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Fig. 9. Dynamic depth (dynamic metres) of 5000-db pressure surface below the reference level of assumed no motion at 2000 m during southern winter.
topography depth at the 5000-db surface is only of 0.02 dynamic meters, indicating a weak nearbottom flow regime chiefly characterized by southwesterly flow bounded by clockwise and anticlockwise eddies on its northern and southern sides, respectively. The benthic disturbance site around 101S & 761E is likely influenced by southeastward flow, which is normal to the benthic disturbance tracks oriented from southeast to northwest. This derived abyssal flow pattern normally disperses re-suspended sediment southeast from the disturbance site.
4. Discussion In addition to understand abyssal circulation in the CIB in context to the benthic disturbance, it is also necessary to observe the circulation patterns in the upper layers as sediment plumes are expected to reach the upper layers during both mining from below and dumping from above from mining ship after separating the manganese nodules. In the CIB, the upper layer circulation is chiefly characterized with westward-flowing SEC. The SEC is more intensive (>0.7 m s1) in the east (at 791E), and weakens (B0.4 m s1) in the west at 711E as parts of flow associated with main SEC are diverted towards both clockwise and anti-clockwise cells situated north and south of the SEC, respectively. Seasonal variations in the wind field over the study area further affect the intensity of the SEC and several studies (Molinari et al., 1990; Heywood et al., 1994; Fieux et al., 1996; Quadfasal et al., 1998) on the SEC variability indicate strong intra-seasonal and interannual fluctuations in the zonal transport. The SEC gradually weakens with depth, and the presence of two anti-clockwise cells interspaced with a clockwise cell between 91 and 141S is the chief constituent of thermohaline circulation in the depth zone of intermediate and deep layers. The zonal elongations of these cells are comparatively higher than north–south elongations, suggesting the influence of the SEC. The inference of the western boundary current around 3000 m in the CIB from the present study confirms result of Warren (1982) who explained the existence of the western boundary
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current in accordance of the deep circulation theory of Stommel and Arons (1960), which requires the necessity of a pole-ward geostrophic return flow in the eastern boundary of the basin away from the western boundary current at the same level. An earlier study by Warren (1974) on deep flows in the Madgascar and Mascarene Basins of the Indian Ocean also showed the presence of western boundary currents. These deep western boundary currents need not always be directed towards north/northeast, but some times they are directed towards southeast/south, not in accordance with the expected theory of deep circulation, if a significant inflow across western neighbor ridge through the fracture zones or gaps or saddles is encountered. For example, a southeasterly boundary current at northeastern flank of the Carlsberg Ridge in the Arabian Sea is a deviation of considerable geostrophic transport through the Owen Fracture Zone (Warren and Johnson, 1992). The lower deep water (depth >4500 m), especially around 101S in the eastern part of the study area (at 791E), is characterized with relatively cold waters ðyo11CÞ and may be linked to an overflow across the saddle of the Ninety-east Ridge from the west Australian Basin. The presence of very cold waters agrees with Warren (1982), who suggested that the overflow water leaves the Ninety-east Ridge in a continuous zonal jet flowing across the CIB. In fact, radiochemical and radiolarian data from cores collected between 101S and 121S around 781E indicate considerable sediment erosion due to this zonal flow (Banakar et al., 1991). The concentrated flow to the west around 101S receives waters from the north as well as it discharges waters towards south. A benthic disturber (Deep-Sea Sediment Re-suspension SystemFDSSRS) was used to disturb the sediment around 101S & 761E in August 1997 just after the southern winter CTD data collection. During the towing of the DSSRS, the slurry of bottom sediment was discharged about 5 m above seabed after fluidizing the sediment, generating a sediment plume. The benthic disturbance operation, conducted for 9 days, resulted in about 6000 m3 of sediment re-suspension over a total track distance of 88 km. The re-suspended sediment settled down during the dispersion of sediment plume and blanketed the sea floor. Post-disturbance analysis has indicated that these effects were mostly close to the tow zone of the disturber (NIO Report, 1997b), but were concentrated in a southwestward direction. Organic carbon content of the sediments increased and sediment-trap data indicted higher fluxes of re-suspended sediment to the southwest. This direction is expected, as it coincides with the mean direction of the benthic current. In fact, thermohaline circulation at 5000 db (Fig. 9) clearly indicates a southwestward current around the disturbance site just prior to the disturbance. Analysis of current meter data obtained over a yearlong period in 1995–1996 near the benthic disturbance site gave monthly mean currents of 0.02 m s–1 in a direction of 2281 for July, and 0.01 m s–1 in a direction of 2011 for August (NIO Report, 1997a). The model of Jankowski et al. (1996) on sediment plume dispersion was adopted for the CIB, assuming the disturber’s speed at 0.6 m s1 and the effective towing of the DSSRS over a distance of about 88 km in 42 h, during which 586 t of sediment mass was discharged at a rate of about 3.9 kg s–1. The disturbed area contains clayey-silt (Valsangkar et al., 1999) sediments with about 70% silt content, resulting to a mean weighted particle size of 23 mm; and this size was used to estimate the particle’s mean settlement velocity, in the order of 1.0 10–4 m s–1. Using these inputs, the model suggests a residence time (e-fold time of reduction of suspended sediment concentration by 66%) of less than one day and a blanket thickness of 0.1 mm within a radial distance less than 0.75 km from disturbance track for weak abyssal currents of 0.01–0.03 m s–1.
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Since the bulk density of sediment mixture is greater than the ambient density of the bottom water (1027.82 kg m3), large-scale re-sedimentation would be expected to occur over a short distance even in the case of relatively moderate currents of about 0.05 m s–1. It is interesting to note the absence of post-disturbance particle fluxes from the data of sediment traps that were deployed 400–500 m away of the disturber tracks, whereas sediment fluxes were recorded during the postdisturbance period at traps 150–300 m from the disturbance tracks (NIO Report, 1997b) Acknowledgements The authors thank Dr. E. Desa, Director, National Institute of Oceanography, Goa (India) for his encouragement to take up this study. They also acknowledge the funding support given by the Dept. of Ocean Development (DOD) Govt. of India, New Delhi, India, for carrying out EIA studies for nodule mining in Central Indian Basin. They appreciate the cooperation given by their colleagues as well as by the ship personnel of ORV Sagar Kanya and RV Yuzhmorgeologea during cruises. This paper is NIO contribution no. 3653.
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NIO Report, 1998. Report on baseline environmental conditions on the physical, biological and chemical oceanographic parameters in the CIOB prior to the deep-sea mining of manganese nodules. NIO Report No.NIO/SP-16/98, May 1998, Goa (India), 58pp. Ozturgut, E., Anderson, G.C., Burns, R.E., Levelle, J.W., Swift, S.A., 1978. Deep ocean mining of manganese nodules in the north Pacific: pre-mining environmental conditions and anticipated mining effects. Report No. ERL-MESA33, NOAA, Boulder (USA), 119pp. Padan, J.W., 1990. Commercial recovery of deep seabed manganese nodules: twenty years of accomplishments. Marine Mining 9, 87–103. Quadfasel, D., Friscche, A., Cresswell, G., 1996. The circulation in the source area of the South Equatorial Current in the eastern Indian Ocean. Journal of Geophysical Research 101(C5), 12 483–12 488. Ramarao, E.P., Ramesh Babu, V., Rao, L.V.G., 1999. Heat content variability in the tropical Indian Ocean during second pre-INDOEX campaign (boreal winter 1996–1997). Current Science 76, 1001–1004. Stommel, H., Arons, A.B., 1960. On the abyssal circulation of the world oceanFII: an idealized model of the circulation pattern and amplitude in oceanic basin. Deep-Sea Research 6, 217–233. Valsangkar, A.B., Ambre, N.V., Rodrigues, M., 1999. Sedimentological impacts of INDEX experiment. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 131–137. Warren, B.A., 1974. Deep flow in the Madgascar and Mascarene basins. Deep-Sea Research 21, 1–21. Warren, B.A., 1981. Trans-Indian hydrographic section at Lat. 181S: property distributions and circulation in the south Indian Ocean. Deep-Sea Research 28A, 759–788. Warren, B.A., 1982. The deep water of the Central Indian Basin. Journal of Marine Research 40, 823–860. Warren, B.A., Johnson, G.C., 1992. Deep currents in the Arabian Sea in 1987. Marine Geology 104, 279–288.
Thermohaline circulation in the Central Indian Basin (CIB) during austral summer and winter periods of 1997 V. Ramesh Babu*, A. Suryanarayana, V.S.N. Murty Physical Oceanography Division, National Institute of Oceanography, Dona Paula, Goa 403004, India
Abstract As a part of Indian Deep Sea Environment Experiment (INDEX) aimed at assessing the environmental impact of manganese nodule mining in the Central Indian Basin (CIB), a study on baseline physical conditions of water column viz. potential temperature (y), salinity and potential density together with geostrophic circulation regime in the deeper depths of the basin was conducted. The hydrography data used in the present analysis were collected over a wide area of the western part of CIB (711–791E; 91–141S) during austral summer (January 1997) from the Indian research vessel ORV Sagar Kanya, while during the austral winter season (June–July 1997), hydrographical stations were occupied by Russian research vessel RV Yuzhmorgeologia in the central part of CIB (751–771E; 91–111S) where a benthic disturbance on experimental scale was carried out. The spatial variations in the physical parameters decreased below 3500 m, inferring a restricted basin-scale deep circulation. The dynamic topography field at 5000 m relative to 2000 db surface in the central part of CIB, representing the abyssal circulation, was generally characterized by a southwestward weak flow around 101S flanked by cyclonic and anti-cyclonic eddies on its right and left sides, respectively. This flow regime agreed with the earlier one inferred by Warren (J. Mar. Res. 40 (1982) 823) linking the source of deep water in CIB to a saddle overflow across Ninetyeast Ridge from West Australian Basin around 101S. r 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Deep-seabed mineral formations such as polymetallic nodules and crusts are potential resources for the future extraction of various strategic metals, as the land resources gradually become depleted. Seabed mining of polymetallic nodules on commercial basis is, therefore, contemplated, and, in this connection, environmental impact studies (Amann, 1992; Chan and Anderson, 1981;
*Corresponding author. Fax: +91-932-223340/229102. E-mail address: [email protected] (V. Ramesh Babu). 0967-0645/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 1 ) 0 0 0 4 2 - X
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Curtis, 1982; Hirota, 1981; Lavelle and Ozturgut, 1981; Lavelle et al., 1981; Ozturgut et al., 1978; Padan, 1990) on deep-sea mining have assumed greater importance. The exploratory surveys have established the Central Indian Basin (CIB) as a potential area for commercial deep-sea mining of polymetallic nodules (PMN), and so environmental impact assessment (EIA) of the mining activity is needed. An Indian national multi-disciplinary environmental experiment known as Indian Deep-Sea Environment Experiment (INDEX), involving physical, chemical, biological and geological investigations, was undertaken as a part of EIA study of manganese nodule mining in the CIB. A knowledge on physical conditions (e.g., currents, temperature and salinity) is essential, as the dispersion of sediment plumes generated during mining activity is determined by density stratifications and advection fields in the water column. Deployment of current-meter moorings at selected sites representing the basin and the benthic disturbance site together with CTD measurements of temperature and salinity in the CIB mainly forms the data collection activity of the physical oceanographic study in this programme. In this paper, the spatial thermo haline structures based on CTD data collected during southern summer and winter periods of 1997 and the derived geostrophic circulation patterns are discussed. The results of the analysis of current measurements at different mooring sites in the CIB are presented elsewhere (Murty et al., 2001). Though the earlier studies of Warren (1981, 1982) were concerned with deep-water property distributions and circulation in CIB, his results were based only on few stations along a single hydrographic section (B121S) across the CIB whereas under this study, CTD data especially during the southern summer was collected at relatively high spatial resolution to study meso-scale features of circulation. Here we discuss the baseline physical conditions of the CIB and the anticipated effects of benthic disturbance.
2. Data and methodology Baseline data of temperature and salinity depth profiles were obtained with a CTD (Conductivity–Temperature–Depth) system for the austral summer and winter seasons of 1997 in the CIB. Twenty-eight stations spaced at a 11-latitude interval along meridional sections 791E, 771E, 751E and 711E were occupied by the Indian research vessel ORV Sagar Kanya in the austral summer period of 4–22 January 1997. The corresponding winter season CTD data were collected at 10 stations occupied by the Russian Vessel RV Yuzhmorgeologea during 4–30 June 1997 (Cruise No. INDEX 3A) & 27–28 July 1997 (Cruise No. INDEX 3B) covering a smaller area of the CIB in which a benthic disturbance experiment was conducted in August 1997. Details of the baseline environmental data collection on physical, chemical, biological parameters in the CIB prior to the deep-sea mining of the manganese nodules are given in a separate report (NIO Report, 1998). Fig. 1 shows the locations of both southern and summer winter hydrographic stations occupied in the western CIB. A high-resolution CTD system (Sea-bird Inc., USA; Model: SBE 911), consisting underwater and deck units connected through a single conductor cable, was deployed during the ORV Sagar Kanya cruise. The accuracies for temperature, conductivity and pressure sensors of CTD are 70.0021C, 70.0003 Siemens per metre (S m1), and 70.015% of full pressure scale. Further, the water samples were collected for comparing CTD salinity estimates with those obtained by onboard salinometer (Guildline Inc., Canada; model: Autosal 8400A, accuracy: 70.003 psu). A
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Fig. 1. Hydrographic stations for baseline data collection in the CIB under the PMN-EIA programme.
regression line fitted for the pairs of salinometer and CTD salinity estimates at corresponding depths of water sampling and the regression equation was used for correcting CTD salinity measurements. Dynamic heights at the stations were obtained by considering 2000 db pressure surface as a reference level. This reference level falls within the depth range of 1800–2800 m shown by Warren (1982), and the zero velocity surface actually represents the region of minimum
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horizontal motion lying between southward flow of high-salinity waters above and the northward deep flow of low-salinity waters below it. Horizontal distributions of potential temperature, salinity, potential density at various depths and dynamic topography maps were prepared for different pressure surfaces with reference to 2000 db surface. However, in this paper, we show the distribution of dynamic topography fields for the deeper and abyssal parts of the CIB only, as the emphasis here is to evaluate the deep-sea environmental impacts of the nodule mining in the basin. Vertical distributions of physical properties viz., potential temperature, salinity, potential density and geostrophic currents computed relative to an assumed 2000 m level of no notion are presented along 791 and 711E and 101S sections. For the southern winter stations, a Seabird CTD (SEACAT 19) system was used. Seacat had sensors for pressure, temperature and conductivity with respective accuracies of 70.15% full scale, 70.011C and 70.0001 S m1. Though the number of southern winter stations was less than during the summer period, we could collect CTD data as near as 10 m above the seabed at all the stations, whereas near-bottom sampling was possible only at three stations along 791E due to operational problems. Horizontal distribution maps of potential temperature, potential density and dynamic height topography field with reference to 2000 db surface were prepared using winter CTD station data. However, the winter study presented here obviously limits discussion on spatial distributions of potential temperature, salinity and potential density parameters at the depths of 3000, 4000 and 5000 m, the deeper abyssal regions of the CIB.
3. Results 3.1. Distributions of potential temperature, salinity and potential density during southern summer 3.1.1. Along 791E In the upper layers of the water column, the thermocline has a trough shape, with maximum deepening around 111–121S (Fig. 2a), suggesting that thermal flow across this section is characterized by an anti-clockwise circulation. North of 111–121S, the suggested flow is to the west, while to the south an eastward flow is inferred. The southward sloping of isotherms in the intermediate depth range of 800–1500 m indicates a westerly flow across the entire section. The meridional down-sloping of isotherms is least around 2000 m depth as compared to the slopes of isotherms above and below it. In deep waters (>4500 m depth), the orientation of 11C isotherm suggests westward flow concentrated around 101S. Salinity distribution at 791E (Fig. 2b) shows the presence of low-salinity waters (o34.70 psu) occupying the upper 100 m, brought into the study area from the east by the South Equatorial Current (SEC). In intermediate depths around 1000 m depth, fresher waters (o34.14 psu) are encountered prominently in the south between the stations 4 and 6, pointing to the northward spreading of the Antarctic Intermediate Water mass (AIW). Below 3000 m, the salinity is less than 34.72 psu, with reduced gradients similar to those seen in the temperature field. The conspicuous presence of a weak salinity maximum (>34.718 psu) in the depth range of 1500–3000 m is due to southward spreading of the North Indian Deep Water (NIDW). In his discussion on water property distributions along 121S in CIB, Warren (1982) also highlighted the presence of a weak
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Fig. 2. Distribution of (a) potential temperature (1C), (b) salinity (psu) (c) potential density (kg m3) along 791E during southern summer (January 1997).
salinity maximum with a probable northern source in the Arabian Sea wherein a high-salinity regime in the intermediate and deeper depths results from vertical mixing of the Red Sea Water. The distribution of potential density (sy (Fig. 2c) at 791E resembles more or less that of temperature with reduced gradients below 3000 m depth. In very deep waters below 4500 m, high potential density (>27.2 kg m3) waters are confined around 101S on account of relatively cold waters having potential temperatures less than 11C.
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3.1.2. Along 711E Near the western boundary of the CIB (711E), the trough nature of thermocline seen in the east at 791E is absent in the upper 300 m (Fig. 3a). A general southward down-slope of isotherms infers westerly flow in the upper layers across the entire section. Below 2500 m depth the 1.51C isotherm has a reverse slope suggesting a reverse flow towards east between 131 and 141S.
Fig. 3. Distribution of (a) potential temperature (1C), (b) salinity (psu) (c) potential density (kg m3) along 711E during southern summer (January 1997).
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The core salinity of intermediate low-salinity water mass around 1000 m is seen reduced in its value to 34.708 psu (Fig. 3b) at 711E compared to the higher core value of 34.714 psu observed at 791E. This east to west reduction in core salinity minimum of the AIW suggests of the dominance of zonal flow over the meridional flow in the intermediate depths, like in the upper layers. The reduction in core value of the NIDW is from 34.72 psu at 791E to 34.71 psu at 711E. The distribution of potential density ðsy Þ has a profound influence of temperature rather than salinity, as the effect of a sub-surface high-salinity maximum around 250 m is not seen in the density distribution (Fig. 3c). The isopycnals dip to the south in upper layer of 300 m, while at deep waters greater than 2500 m they have the opposite slope, suggesting easterly flow. 3.1.3. Along 101S A striking feature of temperature field at 101S is the nearly horizontal thermocline with relatively strong gradients (Fig. 4a). However, at deeper depths there is a weak down-slope of isotherms. Below 3000 m, the vertical thermal gradients are greatly reduced. It is interesting to notice the presence of a very strong halocline within the upper 20 m at stations 13 and 14 (Fig. 4b) along this section probably due to the surface dilution by heavy precipitation associated with the Inter-tropical Convergence Zone (ITCZ). The austral summer cruise of the ORV Sagar Kanya under PMN-EIA study also was partly devoted to studying the aerosol forcing on regional and global climate as a prelude to the main INDOEX (Indian Ocean Experiment), which actually was conducted during boreal winter of 1999. The studies by Naja et al. (1999) and Ramarao et al. (1999), based on surface atmospheric and oceanic data collected during the pre-INDOEX campaign of boreal winter 1997, show the presence of the ITCZ around 101S. Traces of AIW are seen around 1000 m with a somewhat reduced core salinity in the east. The deeper salinity maximum around 2500 m has a core value of greater than 34.712 psu, which is intermediate between corresponding core values at 791 and 711E. This westward decrease in the core value of the NIDW in the study area infers a general dominance of zonal flow over the meridional flow in deeper depths of the CIB. The potential density ðsy Þ distribution at 101S shows the dominant influence of temperature (Fig. 4c) as seen in similarity between the topographies of thermocline and pycnocline. The isopycnals at deeper depths have a downward slope towards east suggesting that the geostrophic flow is southward across 101S. 3.1.4. At 3000 m The property distributions across various sections as discussed previously have shown a common distinct feature of reduced vertical gradients below 3000 m, which roughly coincides with mean depths of the Ninety-east Ridge and the Central Indian Ridge flanked, respectively, on the eastern and western sides of the CIOB. This indicates a restricted deep-water circulation within very deep regions of the CIOB. A cursory look at property distributions at 3000 m (Fig. 5) draws an inference of northward movement of waters near western boundary of the basin under the influence of a boundary current. The potential temperature distribution (Fig. 5a) varies from a minimum of 1.361C to a maximum of 1.521C towards north in the western boundary region between 711 and 741E. Between 101 and 121S, the minimal zonal variations in the potential temperature ðyÞ infer a weak zonal flow as compared to the strong northward flow of cold waters of the Antarctic origin near the western boundary of the CIOB. This northward boundary flow is
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Fig. 4. Distribution of (a) potential temperature (1C), (b) salinity (psu) (c) potential density (kg m3) along 101S during southern summer (January 1997).
further inferred from the presence of relatively fresh waters, with salinities less than 34.71 psu, in the southwestern region of the study area (Fig. 5b). The potential density distribution ðsy Þ with incidence of relatively lighter waters in the north and denser waters in the south suggests the eastward turning of boundary current away from the Central Indian Ridge around 101S.
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Fig. 5. Distribution of (a) potential temperature (1C), (b) salinity (psu) (c) potential density (kg m3) at 3000 m during southern summer (January 1997).
3.2. Geostrophic circulation Geostrophic currents are computed across sections 791E, 711E and 101S, and their distributions are presented in Fig. 6. At 791E (Fig. 6a) an anti-clockwise cell is clearly discernible in upper 300 m with moderately strong westerly flow reaching up to 0.7 m s1
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Fig. 6. Geostrophic currents (m s1) across (a) 791E (b) 711E (c) 101S (NFNortherly current; EFEasterly current; SFSoutherly current; WFWesterly current) during southern summer (January 1997).
north of 131S, while south of it the zonal flow is relatively weak but reversed towards east, with maximum speed of 0.3 m s1. It is interesting to note the presence of zero-velocity line which separates the entire upper 2000 m water column into westerly and easterly flow regimes in the north and south, respectively. The closed circulation cell in the upper 300 m at 711E (Fig. 6b) has a reverse flow, with clockwise rotation having weak easterly flow of about 0.1 m s1 north of 121S and strong westerly flow up to 0.4 m s1 to the south. These cells in the upper 300 m of the study area suggest the meandering nature of the SEC with a weak clockwise cell north of the SEC and a strong anticlockwise cell south of the SEC. Between 1000 and 2000 m, a weak westerly flow of less than 0.1 m s1 is evident between 111 and 121S, while the easterly flow of similar order of low intensity
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is encountered between 131 and 141S. However, the boundary between the alternative deeper flow regimes is maintained at about 131S, like that at 791E. The presence of closed circulation cells in upper 300 m in association with the SEC results in setting up the alternative strong northward and southward flow regimes along 101S, reaching up to 1.0 m s1 (Fig. 6c). Below, in the depth range of 1000–2000 m, the southward flow is more dominant. The closed alternative circulation cells in the deeper depths is clearly seen from the dynamic topography field of 1750 db pressure surface obtained with reference to 2000 m level of no motion (Fig. 7). These cells are elongated to the east resulting in a dominant zonal flow rather than a meridional one. A weak clockwise cell is sandwiched around 751E between two prominent anticlockwise cells. 3.3. Distributions of potential temperature, salinity and potential density at deeper depths during southern winter Availability of CTD data very close to bottom during southern winter has facilitated the extension of the analysis of physical properties to the abyssal depths in a smaller central part of the CIB between 751 and 771E and 91 and 111S. The benthic disturbance experiment was carried out in this area in August 1997, just the collection of winter CTD data. Hence these patterns of property distributions and circulation are assumed to represent conditions prevailing at the time of benthic disturbance and as such the effects of benthic disturbance need to be evaluated with reference to near-bottom physical conditions. Fig. 8 presents the distributions of potential temperature, salinity and potential density at near abyssal depths of 3000, 4000 and 5000 m during southern winter. Though the spatial variations
Fig. 7. Dynamic height (dynamic metres) of 1750-db pressure surface above the reference level of assumed no motion at 2000 m during southern summer (January 1997).
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Fig. 8. Distribution of (I) Potential temperature (1C), (II) Salinity (psu) (III) Potential density (kg m3) at deeper depths of (a) 3000 m; (b) 4000 m; (c) 5000 m during southern winter (June–July 1997).
are subtle, the presence of relatively cold waters in the east and southeast suggests an incursion in deeper parts of CIB (Figs. 8Ia–Ic). Salinity distributions at these depths (Figs. 8IIa–IIc) further also show the presence of relatively low-salinity waters in the east. The association of cold, lowsalinity values in the east indicates an overflow of the Antarctic Waters across suspected gaps or saddles of the Ninety-east Ridge. The potential density field is more or less uniform, indicating sluggish motions below the sill-depth. 3.4. Near-bottom circulation during southern winter The dynamic topography of 5000-db surface with reference to 2000 db level of no motion was computed by utilizing the near-bottom CTD data, and the geostrophic circulation pattern is shown in Fig. 9. An approach of downward integration of dynamic depths below an assumed level of no motion as adopted by Warren (1982) is also employed here. The range of dynamic
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Fig. 9. Dynamic depth (dynamic metres) of 5000-db pressure surface below the reference level of assumed no motion at 2000 m during southern winter.
topography depth at the 5000-db surface is only of 0.02 dynamic meters, indicating a weak nearbottom flow regime chiefly characterized by southwesterly flow bounded by clockwise and anticlockwise eddies on its northern and southern sides, respectively. The benthic disturbance site around 101S & 761E is likely influenced by southeastward flow, which is normal to the benthic disturbance tracks oriented from southeast to northwest. This derived abyssal flow pattern normally disperses re-suspended sediment southeast from the disturbance site.
4. Discussion In addition to understand abyssal circulation in the CIB in context to the benthic disturbance, it is also necessary to observe the circulation patterns in the upper layers as sediment plumes are expected to reach the upper layers during both mining from below and dumping from above from mining ship after separating the manganese nodules. In the CIB, the upper layer circulation is chiefly characterized with westward-flowing SEC. The SEC is more intensive (>0.7 m s1) in the east (at 791E), and weakens (B0.4 m s1) in the west at 711E as parts of flow associated with main SEC are diverted towards both clockwise and anti-clockwise cells situated north and south of the SEC, respectively. Seasonal variations in the wind field over the study area further affect the intensity of the SEC and several studies (Molinari et al., 1990; Heywood et al., 1994; Fieux et al., 1996; Quadfasal et al., 1998) on the SEC variability indicate strong intra-seasonal and interannual fluctuations in the zonal transport. The SEC gradually weakens with depth, and the presence of two anti-clockwise cells interspaced with a clockwise cell between 91 and 141S is the chief constituent of thermohaline circulation in the depth zone of intermediate and deep layers. The zonal elongations of these cells are comparatively higher than north–south elongations, suggesting the influence of the SEC. The inference of the western boundary current around 3000 m in the CIB from the present study confirms result of Warren (1982) who explained the existence of the western boundary
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current in accordance of the deep circulation theory of Stommel and Arons (1960), which requires the necessity of a pole-ward geostrophic return flow in the eastern boundary of the basin away from the western boundary current at the same level. An earlier study by Warren (1974) on deep flows in the Madgascar and Mascarene Basins of the Indian Ocean also showed the presence of western boundary currents. These deep western boundary currents need not always be directed towards north/northeast, but some times they are directed towards southeast/south, not in accordance with the expected theory of deep circulation, if a significant inflow across western neighbor ridge through the fracture zones or gaps or saddles is encountered. For example, a southeasterly boundary current at northeastern flank of the Carlsberg Ridge in the Arabian Sea is a deviation of considerable geostrophic transport through the Owen Fracture Zone (Warren and Johnson, 1992). The lower deep water (depth >4500 m), especially around 101S in the eastern part of the study area (at 791E), is characterized with relatively cold waters ðyo11CÞ and may be linked to an overflow across the saddle of the Ninety-east Ridge from the west Australian Basin. The presence of very cold waters agrees with Warren (1982), who suggested that the overflow water leaves the Ninety-east Ridge in a continuous zonal jet flowing across the CIB. In fact, radiochemical and radiolarian data from cores collected between 101S and 121S around 781E indicate considerable sediment erosion due to this zonal flow (Banakar et al., 1991). The concentrated flow to the west around 101S receives waters from the north as well as it discharges waters towards south. A benthic disturber (Deep-Sea Sediment Re-suspension SystemFDSSRS) was used to disturb the sediment around 101S & 761E in August 1997 just after the southern winter CTD data collection. During the towing of the DSSRS, the slurry of bottom sediment was discharged about 5 m above seabed after fluidizing the sediment, generating a sediment plume. The benthic disturbance operation, conducted for 9 days, resulted in about 6000 m3 of sediment re-suspension over a total track distance of 88 km. The re-suspended sediment settled down during the dispersion of sediment plume and blanketed the sea floor. Post-disturbance analysis has indicated that these effects were mostly close to the tow zone of the disturber (NIO Report, 1997b), but were concentrated in a southwestward direction. Organic carbon content of the sediments increased and sediment-trap data indicted higher fluxes of re-suspended sediment to the southwest. This direction is expected, as it coincides with the mean direction of the benthic current. In fact, thermohaline circulation at 5000 db (Fig. 9) clearly indicates a southwestward current around the disturbance site just prior to the disturbance. Analysis of current meter data obtained over a yearlong period in 1995–1996 near the benthic disturbance site gave monthly mean currents of 0.02 m s–1 in a direction of 2281 for July, and 0.01 m s–1 in a direction of 2011 for August (NIO Report, 1997a). The model of Jankowski et al. (1996) on sediment plume dispersion was adopted for the CIB, assuming the disturber’s speed at 0.6 m s1 and the effective towing of the DSSRS over a distance of about 88 km in 42 h, during which 586 t of sediment mass was discharged at a rate of about 3.9 kg s–1. The disturbed area contains clayey-silt (Valsangkar et al., 1999) sediments with about 70% silt content, resulting to a mean weighted particle size of 23 mm; and this size was used to estimate the particle’s mean settlement velocity, in the order of 1.0 10–4 m s–1. Using these inputs, the model suggests a residence time (e-fold time of reduction of suspended sediment concentration by 66%) of less than one day and a blanket thickness of 0.1 mm within a radial distance less than 0.75 km from disturbance track for weak abyssal currents of 0.01–0.03 m s–1.
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Since the bulk density of sediment mixture is greater than the ambient density of the bottom water (1027.82 kg m3), large-scale re-sedimentation would be expected to occur over a short distance even in the case of relatively moderate currents of about 0.05 m s–1. It is interesting to note the absence of post-disturbance particle fluxes from the data of sediment traps that were deployed 400–500 m away of the disturber tracks, whereas sediment fluxes were recorded during the postdisturbance period at traps 150–300 m from the disturbance tracks (NIO Report, 1997b) Acknowledgements The authors thank Dr. E. Desa, Director, National Institute of Oceanography, Goa (India) for his encouragement to take up this study. They also acknowledge the funding support given by the Dept. of Ocean Development (DOD) Govt. of India, New Delhi, India, for carrying out EIA studies for nodule mining in Central Indian Basin. They appreciate the cooperation given by their colleagues as well as by the ship personnel of ORV Sagar Kanya and RV Yuzhmorgeologea during cruises. This paper is NIO contribution no. 3653.
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