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Sediment redistribution during simulated benthic disturbance and its implications on deep seabed mining
Deep-Sea Research II 48 (2001) 3363–3380
Sediment redistribution during simulated benthic disturbance and its implications on deep seabed mining Rahul Sharma*, B. Nagender Nath, G. Parthiban, S. Jai Sankar National Institute of Oceanography, Dona Paula, Goa 403004, India
Abstract A benthic disturbance experiment was conducted in the Central Indian Basin to evaluate the possible effects of sediment redistribution on marine ecosystem during deep-sea mining. The analyses of disturbance features on the seafloor and particle resettlement indicate that the most intense impact is created within the experiment site, and some of the resuspended sediments that remain in the near-bottom water after the disturbance are transported away from the site, changing the physico-chemical conditions in the area. Resedimentation is observed on either side of the disturbance zone, with an increase in suspended particles by 300% (from 49 to 150 mg/m2/day) during the disturbance and a reduction by 33% (to 95 mg/m2/day) immediately after the experiment. Higher collection of particles as well as an increase in organic carbon in seafloor sediments S–SW of the disturbance zone indicates directional migration of suspended material and redeposition in the adjacent areas. The implications of increase in turbidity and sedimentation rates in the near bottom water column due to sediment re-suspension, as well as lateral migration of sediment, on the benthic ecosystem have been discussed. These parameters provide important inputs in ascertaining longterm effects of deep-sea mining. r 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Discovery of deep-sea minerals, such as manganese nodules, as an alternative source of strategic metals, such as copper, nickel and cobalt (Mero, 1965; Glasby, 1977), has generated considerable interest in mining of these deposits in future (Amann, 1982; Anonymous, 1984; Chung and Tsurusaki, 1994). However, deep-sea mining would create a set of entirely new conditions, which could lead to deleterious effects on the entire marine ecosystem around the mining area (Amos and Roels, 1977; Ozturgut et al., 1978). One of the areas that would be affected more intensely is the seafloor and the water column above it, due to the movement, collection and waste discharge
*Corresponding author. Fax: +91-832-223340. E-mail address: [email protected] (R. Sharma). 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 6 - 7
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by the mining device (Thiel et al., 1997). It is likely to resuspend and redistribute the bottom sediments, leading to changes in the stratigraphic sequence (Gupta, 2000), sedimentological (Valsangkar et al., 1999) and geochemical properties as well as biochemical conditions (Raghukumar et al., 2001). The effects can be varied owing to uni- or multi-directional movement of the particles, resulting in resedimentation over the naturally exposed seafloor, bringing about changes in faunal assemblages (Foell et al., 1992; Shirayama, 1999). The effects of benthic disturbance depend largely on the migration and settlement of particles resuspended during the event. The intensity and direction of sediment migration would depend on strength of the bottom currents, the type of particle assemblages, as well as organic coatings on the sediment particles. In order to evaluate the possible effects of such an event, a small-scale benthic disturbance experiment was conducted in the manganese nodule-bearing area in the Central Indian Basin (Sharma and Nath, 1997). Similar experiments undertaken in the Pacific Ocean (Foell et al., 1990; Trueblood, 1994; Fukushima, 1995; Tkatchenko, 1997) have shown that the impact on the benthic ecosystem could be a variable and would depend on local environmental conditions. This paper describes the Indian Deep-Sea Environment Experiment (INDEX) in the Central Indian Basin and reports the migration trends of suspended sediments in and around the disturbed area. The paper also looks at the implications of sediment redistribution as a result of disturbance that may be created on the seafloor. Several other papers in this issue report the effects on the benthic conditions, as a result of this experiment. The experiment was conducted as a part of the Indian programme on ‘Environmental impact assessment of nodule mining program’, funded by the Dept. of Ocean Development (Govt. of India) and executed by the National Institute of Oceanography, Goa (India).
2. Study area Central Indian Basin is an abyssal area bounded by the 901 East Ridge on the east, Central Indian Ridge to the west, South East Indian Ridge to the south. To the north, it is open towards the Indian peninsula, whereas to the northwest it is bounded by the Chagos–Laccadive Ridge. Terrigenous, siliceous and pelagic brown clay sediments are present in the basin (Rao and Nath, 1988; Nath et al., 1989). A small area (10 10 nautical miles) was surveyed in detail by bathymetric mapping at a spacing of 1 nautical mile in N–S direction and 2 nautical miles in E–W direction, and a test site of 3.5 km 5.4 km, with a disturbance strip of 3 km 200 m in NW–SE direction was selected, which lies between 101010 S and 751590 E and 101030 S, 761020 E and depth ranges between 5300 and 5400 m (Fig. 1). The disturbance zone has even topography, no rock outcrops, low nodule abundance, and uniform benthic conditions (Sharma and Nath, 1997), as per the requirements for operation of the benthic disturber. The total relief around this site is 20–40 m, with low nodule abundance (o2 kg/m2). The site was surveyed by deep-towed sub-bottom profiling (5 kHz) equipment along 3 profiles. The sediment thickness in the area varies from 40 to 90 m, with 4 distinct layers of sediment. The upper two layers are very thin (mean thickness 3.5 and 5 m) and lower two layers are thicker (mean thickness 22 and 35.5 m) (Sharma, 2000). The sediment type is siliceous ooze, mainly
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Fig. 1. Locations of camera profiles, sediment traps, box cores in the experiment site. Depths in meters.
consisting of clay minerals such as smectite, illite, chlorite and kaolinite, as well as authigenic assemblages, such as Fe-rich smectites, phillipsites, other zeolites, and Fe–Mn oxides (Rao and Nath, 1988; Nath et al., 1992; Valsangkar et al., 1999).
3. Benthic disturbance experiment and systems deployed The experiment was conducted during two cruises of R.V. Yuzhmorgeologiya: (i) predisturbance cruise (27 May–8 July 1997); and (ii) disturbance and post-disturbance impact assessment cruise (22 July–5 September 1997). A 6-channel Magnavox MX 200 receiver was used to obtain Global Position System (GPS) fixes. ASMOD (underwater positioning system) was used for underwater positioning of the survey equipment. ASMOD consists of four acoustic transponders that were deployed and calibrated during the pre-disturbance cruise. GPS and ASMOD fixes were integrated to enable maneuvering of underwater survey and sampling equipment. The position data in both GPS and ASMOD were computed on WGS-72 spheroid to enable integration with previous survey data base in the site. GPS was used for the ship’s position, and ASMOD was used for all the operations (such as disturber, deeptow camera and coring) in
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and around the disturbance area. During sediment sampling, precise positioning was obtained with the help of pinger along with the ASMOD data. Deep-sea moorings with sediment traps were deployed close to the bottom (7 m above the seafloor) at 10 locations around the disturbance zone (Fig. 1) to record the time series data on fluxes before, during and after the disturbance. These were retrieved in the post-disturbance cruise after conducting the disturbance experiment and collecting data on the post-disturbance effects. The effects were also studied by deep-towed cameras and box corer samples (Fig. 1) in the pre- as well as post-disturbance phases. 3.1. Benthic disturbance experiment The experiment was conducted with a benthic disturber (Fig. 2), named ‘Deep-Sea Sediment Resuspension System (DSSRS)’, developed by the Sound Ocean System Inc. (SOSI), USA (Brockett and Richards, 1994). The disturber creates an artificial disturbance on the seafloor by fluidising the bottom sediment as it moves on the seafloor, and pumps and discharges the
Fig. 2. Benthic disturber used for the experiment.
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sediment through a 5 m-high stack. The plume created by the resuspended sediment was studied to assess the impact of such a disturbance on the benthic ecosystem. The same disturber was used earlier for three of the deep sea disturbance experiments conducted by NOAA (USA), MMAJ (Japan) and IOM (consortium of Eastern European countries). 3.2. Components of the disturber 3.2.1. Main unit The tow frame, which weighs 3.2 t is hydrodynamically designed to house the subsystems of the disturber and has dimensions of 4.8 m 2.4 m 5.0 m. The frame is connected with a coaxial cable that tows as well as transmits signals and power to the disturber unit. The frame consists of a discharge stack, 5 m high, with a diameter of 30 cm and is bell-shaped at the bottom, where the sediment is sucked up and discharged from the top. There are two pumps on the disturber, one for fluidising the sediment by water jets with 16 nozzles and another to suck the sediment slurry and raise it through the discharge stack. The pumps are controlled from the deck unit. A rosette of 12 bottles of 1.7 l capacity, is attached near the discharge stack to collect samples of water with resuspended sediments at select intervals and used to quantify the resuspension rate. A TV camera is attached to the frame to view the functioning of the disturber, which is monitored from the deck unit. 3.2.2. Depressor and cable This unit weighs about 1 t and is attached to the cable, about 200 m ahead of the disturber, to help dampen the ship motion. A 21-mm coaxial cable is used for deploying, operating and transmitting power and signals to and from the system. 3.2.3. Deck unit This is a PC-based system with custom made software and hardware to monitor and control the operation of the system. Clogging of the pumps can be sensed through the current level from the monitor and corrective measures can be taken. The camera, lights, and rosette can be operated from this unit. Also the video image transmitted by slow scan camera can be seen on the TV monitor. 3.3. Operation of the disturber The disturber is attached to the coaxial cable in front of the frame and has a transponder for acoustic positioning. Once the disturber touches the bottom, its functioning can be monitored from the altitude, roll, pitch and heading displays on the deck unit, where the functioning of the pumps also can be controlled. Plume generation can be seen on the video monitor, and the average sediment pumped can be estimated by taking intermittent water samples in the rosette on the disturber. The position of the disturber was observed real time on the acoustic navigation system (ASMOD).
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3.4. Quantum of disturbance in the study area Salient features of the disturbance are as follows (Sharma et al., 2000): * * * * * *
Total number of tracks=26, Total days of disturbance=9, Disturbance time=42 h 14 min, Estimated sediment resuspension=3555 t (6023 m3), Total dry sediment resuspended=580 t, Total distance covered=88.3 km.
Three other experiments carried out in the Pacific Ocean took a similar approach and conducted between 14 (IOM-BIE) and 49 (NOAA-BIE) tows, discharging between 1281 m3 (IOM-BIE) and 4328 m3 (NOAA-BIE) of sediment, with the JET experiment having in between figures of 19 tows and 2475 m3 of discharge (Trueblood, 1994; Tkatchenko et al., 1997; Fukushima, 1995). The DISCOL experiment used a plough harrow (Foell et al., 1990), which resulted in digging deep into the sediment, rather than creating a sediment plume. Hence, the two different types of disturbances caused during the DISCOL and the other Benthic Impact Experiments are complimentary to one another, as eventually the mining device is expected to dig into as well as resuspend the sediment, while collecting and separating the manganese nodule deposits from the seafloor. Hence, an ideal experiment would be to combine the two disturbance mechanisms.
4. Results 4.1. Seafloor disturbance from photo and video data Observations were made by deep-towed TV and photo system to evaluate the seabed features in and around the disturbance area in order to assess the impact of benthic disturbance on the seafloor. The deep-tow system was operated 4–5 m above the seafloor, during which continuous online observations were made onboard from the video data, and a 35-mm still camera was operated remotely in the areas of interest. The survey profiles were carried out up to 750 m on either side as well as across the disturbance site. Pre-disturbance photographic data show typically undisturbed conditions, with extensive biological activity in the form of benthic organisms, their fecal casts, burrows, tracks and trails (Fig. 3a). In contrast, immediate post-disturbance observations show tracks of disturber as pairs of depressions of the sleds and fluidised sediment surface between them (Fig. 3b). These areas are devoid of any biological activity except for an occasional megabenthic organism. Sediment piled up on the sides of the disturber tracks due to its motion could be observed in areas adjacent to the tracks. The camera profiles parallel to the disturber tracks showed lumps of sediment that had been redeposited from the discharge stack either on the track or besides them during the tow (Fig. 3c). The other areas showed a very fine layer of sediment cover resulting in smooth microrelief (Fig. 3d) and no burrows, fecal casts or animal trails. The areas away from the tow zone do not show any effects of disturbance and have fecal casts, burrows, trails as well as megabenthic organisms.
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Fig. 3. Disturbance features from seabed photos (a) disturber tracks, (b) piles of sediment, (c) resedimentation, (d) undisturbed seafloor.
In order to evaluate the areal extent of the impact, frequency distributions of different disturbance features noted in photographs were plotted with respect to the distance from the center of the disturbance track (Fig. 4). Most of the tracks of the disturber (87%) as well as sediment piles (91%) generated due to the disturber movement are concentrated within 100 m on either side. This is expected because the width of the disturbance site is 200 m. Mostly (57%) the effect of resedimentation is observed within the disturbance site (i.e. 100 m on either side). Many of the (89%) resedimentation features are observed up to a distance of 200 m, and few (11%) up to a distance of 250 m on either side, which means that the sediment has travelled up to 150 m away from the disturbance site. Consequently, most of the undisturbed areas (75%) are concentrated beyond 100 m on either side of the disturbance site and extend up to more than 750 m on either side, which is the limit of the photographic data. Of all these data, the most interesting is the area of influence due to resedimentation, which extends up to 150 m from the edge of the disturbance site. The highest intensity is shown by the locations within disturbance tracks, followed by those with lumps and piles of sediment, the resedimented areas and the undisturbed seafloor representing the least disturbance in the area. The intensity of impact is the highest within the disturbance zone and decreases away from this zone (Fig. 5). Image analyses methods have been used to determine the intensity of disturbance, from the variation in colour intensity of the
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Fig. 4. Frequency distribution of disturbance features.
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Fig. 5. Contour diagram of intensity of disturbance from photographic data: (1) Highest disturbance, (2) moderate disturbance, (3) low disturbance, (4) no disturbance.
sediment in deeptowed photographs (Yamazaki et al., 1997). This photographic and colour intensity data can be used to compare the photos of subsequent observations, during monitoring, to assess the restoration of seafloor features over a period of time. 4.2. Sediment resettlement from flux data Particle fluxes in the near bottom water before, during and after the disturbance, were estimated from time series sediment traps at 7 m height above the seafloor, moored at 10 locations around the disturbance site. Each mooring consisted of one sediment trap (a Russian-made Lotus time series sediment trap with 12 sampling cups). The collection area of the traps is 0.5 m2. In all, 8 moorings could be retrieved after the disturbance. Out of these, 5 moorings (DMS-1, 2, 4, 8 and 9) collected samples during most part of the experiment. The moorings DMS-5 and 6 functioned only partially, collecting particles only during pre-disturbance phase. Hence, the flux data are based on 7 traps. Observations from sediment traps deployed around the disturbance area (Fig. 6), show that the near-bottom particle fluxes in the pre-disturbance phase (B40 days) range from 41 to 55 mg/m2/ day in different traps, with an average of 48 mg/m2/day for the experiment site. During the
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Fig. 6. Relationship between particle fluxes and locations of traps.
disturbance phase (8.5 days), the particle fluxes increased by 3–4 times in different traps, with an average of 150 mg/m2/day. The average fluxes in the post-disturbance phase (5–6 days) dropped to 95 mg/m2/day. This means that the fluxes, which had increased by 300% from the background value during the disturbance phase, reduced to 200% (or by 33%) within 5–6 days of the last disturbance. So it is expected that the remaining particles suspended in the near-bottom water column, would either settle very fast, or the finer particles may remain in suspension for a longer time and get transported to the adjacent areas. Therefore, the silty sized sediments being relatively coarser (and heavier) may settle down soon after the disturbance, whereas the clay sized sediment may remain in suspension for a longer period. Particle fluxes recorded in different cups during the disturbance period were compared (Fig. 7) to analyse the resettlement of particles during this period. Out of the two cups that collected particles during the disturbance period, the first cup contained marginally fewer particles compared to the second cup, at two out of three locations. The rate of sediment pumped being almost the same during the entire 8.5-day period, it appears that some of the sediment from the first part of disturbance remained in suspension, and subsequently settled into the second cup, increasing the collection in the later period. This probably indicates that some of the sediment particles remain in suspension for sometime after the disturbance. The higher collection in the post-disturbance phase, as compared to background values, also indicates the subsequent settling down of some of the resuspended particles. However, the increase is seen in the traps closer to the disturbance zone (within 100 m) compared to one slightly farther away (200 m). Duration of each cup is 5 days, hence it can be presumed that during this period the resuspended particles reach up to 100 m, and may take more time to get transported to more distant areas. In order to evaluate the transportation pattern of the suspended particles, the fluxes were plotted with respect to distance from the center of the disturbance site for all the three phases (Fig. 8). In the pre-disturbance phase, the collection is almost uniform irrespective of the distance, as the traps collect the particles that are settling under natural conditions. During the disturbance phase, the traps closest to the disturbance track (DMS-2 and DMS-9) collected higher amounts of resuspended sediment, compared to others away from the site. However, one trap (DMS-1) had anamolously high sediment flux, which may correspond to the general deep-sea current
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Fig. 7. Comparison of particle fluxes recorded by traps during the disturbance period.
Fig. 8. Relationship between particle fluxes with respect to distance from disturbance site.
circulation in the area. In the post-disturbance phase, particle fluxes were lower than that during the disturbance period. The other trap closest to the disturbance site (DMS-9) continued to record very high fluxes, but in general the fluxes decreased with distance from the disturbance site. Once again, one trap (DMS-1) showed anamolously high collection during this phase as well, although it was away from the site. Collection of higher quantities of particles may be related to general current direction in the area during the period of disturbance. Observations from current meters deployed at 5100 m depth close to the site indicate the current direction to be S–SW during this period (NIO, 1997). Preferential deposition of sediment in a particular direction (north of tow zone) was observed during NOAA’s Benthic Impact Experiment, as well (Trueblood, 1993). This indicates that although the sediment particles are resuspended very close to the seafloor (B5 m) where the
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intensity of currents is very low, they do follow the prevailing currents at these abyssal depths. This also indicates that the impact of resedimentation will not only be in the disturbed zone, but also will spread to adjacent areas, the intensity of which will depend upon various factors such as the strength of the currents, size of the particles, volume of sediment discharged as well as the altitude of discharge. Consideration of all these parameters will determine the potential impact of sediment redistribution on the seafloor environment. 4.3. Sediment movement deciphered from organic carbon data Sediment sampling was carried out with spade/box cores (50 cm 50 cm 50 cm) at the locations within the disturbance track, adjacent to the disturbance area and at locations away from the disturber track (Fig. 9). Cores were obtained by inserting the acrylic core liners, which were subsampled for the sedimentological, geochemical, mineralogical, biostratigraphic and geotechnical studies. Subsampling was carried out at 2 cm intervals, and organic carbon was determined by wet oxidation method (El Wakeel and Riley, 1957) for sediment cores representing pre- and post-disturbance phases. Analyses of the sediments from both the phases were carried out on the same day to avoid any errors, and precision was checked by repeating the analyses of some of the samples (see Nath et al., 1997 for details). Particulate organic carbon or sedimentary organic matter is an important parameter in benthic ecosystem sustenance. The benthic biomass is directly related to both the quantity and quality of
Fig. 9. Comparison of average organic carbon contents (%) in surface sediments of pre- and post-disturbance phases of different areas.
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the available organic matter (e.g., Tenore, 1977). The organic carbon levels were studied in the sediments representing both the pre- and post-disturbance phases. Within the accuracy levels of bottom transponder based acoustic navigation system, the same location could be sampled in both the phases. Post-disturbance sediments in general showed an increase in organic carbon levels, but varying horizontally (Fig. 9). A pattern of ‘Bioenhancement’ is often encountered at sites with increased organic carbon, both in nearshore (Mauer et al., 1993; Smith, 1987) and deep sea (Rowe et al., 1986). No changes are observed for average organic carbon values in the area to the north of the disturbance site, unlike an increase (by 30%) in the disturbance site, and an even higher increase (45%) in the area to the south of the disturbance site (Fig. 9). It appears that the condition of surface sediment in the area to the north remained stable during the pre- and postdisturbance phases, in comparison to other two areas. This may be due to redeposition of suspended sediments within the site and transportation of organic carbon-rich surface sediments to the south. Further, the distribution of organic carbon in surface sediments plotted for predisturbance phase (Fig. 10) shows that the trend is gradual (from 0.28% to 0.4%) from NW to SE end of the site. The distribution in post-disturbance phase shows a significant increase within the site and also in the reverse direction, i.e. from SE to NW. The values are also significantly high (up to 0.64%) in the SW direction. The organic matter usually resides in finer sized sediment particles (e.g., Trask, 1937). Due to the prevailing currents, the fines may have been removed and deposited in the general direction of the bottom currents. The dominant benthic current direction during June–August is towards S–SW and the spatial distribution map of organic carbon has clearly shown an increase in this direction (Fig. 10), indicating the direction of resedimentation. Microfossil assemblage studies (radiolarians in this case) by Gupta (2000) in the area under consideration has shown evidence of lateral migration, as well as vertical mixing of sediment, within the sediment column in and around the disturbance zone. The interpretations are based on the higher occurrence of Stylatractus universus species (an indicator of older sediment normally observed at deeper horizons in the sediment column) along with the younger fauna Buccinosphaera invaginata in the surface sediments after the disturbance (Gupta, 2000). Hence, biostratigraphic evidence also can provide valuable data on sediment remixing due to artificial disturbance, and help determine the depth of disturbance as well.
5. Discussion and conclusions 5.1. Implications of sediment redistribution on marine ecosystem The artificial disturbance carried out during this experiment has shown signs of intense impact in the areas within the disturbance zone, increased suspended particles in near-bottom water, and changes in geochemical characteristics of the bottom sediments in the proximity of the impact zone. The sediment resuspension and redistribution would have a variety of implications in bringing about changes on different parameters in the benthic environment. Although, the scale of the benthic disturber was much smaller than a commercial mining system, the results show that any device designed to collect nodules would cause a sediment plume close to the seafloor, leading to a sharp increase in density of bottom waters. According to Amos and Roels (1977), for every t of nodules, 2.5–5.5 t of sediment will be resuspended. It is estimated that for mining of 3 106 t of
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Fig. 10. Distribution of organic carbon in surface sediments in pre- and post-disturbance phases. A clear increase in post-disturbance organic carbon contents is noticed in the S–SW direction.
nodules per year (of 8 kg/m2 average abundance), the total volume of sediment to be resuspended will be 504 107 m3, in an area of 600 km2 (Sharma, 1993). 5.2. Effects on benthic fauna This sediment not only will settle in the mining zone, but also be transported to the adjacent areas, changing the physico-chemical conditions on the seafloor, and increasing sedimentation
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rates. The increased turbidity also would lead to clogging of filter-feeding apparatus of the benthic organisms (Pearson, 1975), smothering the less mobile epipelagic organisms on the seafloor (Ozturgut et al., 1978). In the DISCOL experiment, several organisms were coated with sediment even 6 months after the disturbance, the mobile megafauna lacked in diversity and population, and many of the sessile forms were almost non-existent (Foell, 1990). Abyssal benthic communities are known to survive in a fragile ecosystem, maintaining a delicate balance within their environment. More of these organisms (88%) have a close association with sediments, followed by nodules (66%) in the Central Indian Basin (Sharma and Rao, 1992), which makes it particularly critical to take them into consideration for any such activity on the seafloor. These abyssal fauna are quite sensitive to even relatively small alterations in the surrounding environment, as indicated by reduction in meio and macro-fauna in the postdisturbance phase of the experiment (Ingole et al., 1999). On the positive side, there could be mixed effects due to release of high quantity of nutrients from subsurface layers due to mining activity (Raghukumar et al., 2001), which may lead to an increase in their population over a period of time.
5.3. Effects on water column conditions Attention needs to be focused on the method, quantity and levels of discharge of sediment in the water column, so as to avoid major deleterious effects. The sediment as well as the unwanted nodule debris should be discharged as close to the seafloor as possible to minimise the chances of the resuspended material being transported by currents. However, the possibility of lifting some of the debris along with water and nodules, to the surface needs to be considered. The temperature of bottom waters, usually between 11C and 21C, may rise marginally due to conduction and frictional heating, but in any case it is expected to be 15–201C lower than surface water temperature (Yamazaki et al., 1991), leading to increase in primary productivity due to high nutrient levels in these waters. Estimates have shown that the surface discharge will have to be diluted at 1 : 10,000 so as to keep the changes in oxygen induced by discharge is kept to minimum levels (Sousa and Sardessai, 1999).
5.4. Further studies Time-series observations to study the processes of recolonisation and restoration of benthic environment with respect to the benthic conditions would help in planning the deep-sea mining operations in future. The increase in particle fluxes and migration of the sediments can be effectively predicted by developing appropriate models. Such models would have to take into consideration several variables, such as sediment-laden negative buoyant effect, as well as the process of resuspension, in order to be fully applicable for sediment-plume behaviour, due to sediment redistribution (Doi et al., 1999). The potential disturbance to the benthic ecosystem is mainly determined by the grain size of the resuspended material (Rolinski, 1999). Bottom currents also may play a role in plume behaviour. Long-term bottom current measurements in the vicinity of the study area have shown that the current fields vary seasonally (Murty et al., 1999). The
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results of such models will then have to be compared with actual data from sediment traps, photographs and sediment cores to predict accurately the potential impacts of any kind of benthic disturbance in future. Acknowledgements The authors appreciate the support of Russian scientists onboard R.V. Yuzhmorgeologiya, during the benthic disturbance experiment and subsequent data collection. Help rendered by Ms. Sapana Nabar in analysing organic carbon with utmost care is acknowledged. This study was carried out as a part of the ‘Environmental Impact Assessment studies for nodule mining in Central Indian Basin’ funded by the Department of Ocean Development (Government of India). This paper is NIO contribution no no. 3656.
References Amann, N., 1982. Technological trends in ocean mining. Philosophical Transactions of the Royal Society of London A 307, 377–403. Amos, A.F., Roels, O.A., 1977. Environmental aspects of maganese nodule mining. Marine Pollution International Bulletin 1, 160–162. Anonymous, 1984. Analysis of Exploration and Mining Technology for Mn Nodules, Seabed Mineral Series, Vol. 2. UN Ocean Economic and Technology Branch, Graham and Trotman, London, pp. 57–68. Brockett, T., Richards, C.Z., 1994. Deep mining simulator for environmental impact studies. Sea Technology (August), 77–81. Chung, J.S., Tsurusaki, K., 1994. Advance in deep-ocean mining systems research. Proceedings of the Fourth International Offshore and Polar Engineering Conference, Osaka, Japan, pp. 18–31. Doi, T., Nakata, K., Kubota, M., Aoki, S., 1999. Environmental study on the deep-sea mining of manganese nodules in the northeastern Tropical PacificFmodeling the sediment-laden negative buoyant flow. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 163–169. El Wakeel, Riley, J.P., 1957. Determination of organic carbon in the marine muds. Journal du Conseil International pour L’exploration de La Mer 22, 180–183. Foell, E.J., 1990. DISCOL: a long-term, large-scale, disturbance-recolonization experiment in the abyssal eastern tropical South Pacific Ocean. Proceedings Offshore Technology Conference, Houston, USA, OTC 6328, pp. 497–503. Foell, E.J., Schriever, G., Bluhm, H., Borowski, C., Bussau, C., Thiel, H., 1992. Disturbance and recolonization experiment in the abyssal South Pacific Ocean (DISCOL): an update. Proceedings Offshore Technology Conference, Houston, USA, OTC 6805, pp. 25–33. Fukushima, T., 1995. Overview ‘‘Japan Deep-Sea Impact Experiment=JET’’. Proceedings of the ISOPE Ocean Mining Symposium, Tsukuba, Japan, pp. 47–53. Glasby, G.P., 1977. Marine Manganese Deposits. Elsevier, Amsterdam, 523 pp. Gupta, S.M., 2001. Biostratigraphic analysis of the top layer of sediment cores from reference and test sites of the INDEX area. Marine Georesources and Geotechnology Journal 18 (3), 259–262. Ingole, B.S., Ansari, Z.A., Matondkar, G.P., Rodrigues, N., 1999. Immediate response of meio and macrobenthos to disturbance caused by a benthic disturber. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 191–197. Mauer, D., Robertson, G., Gerlinger, T., 1993. Long-term temporal and spatial fluctuation of soft bottom infaunal invertebrates from the San Pedro Shelf, California. Internationale revere du geramten hydrobiologie 78, 535–555. Mero, J.L., 1965. The Mineral Resources of the Sea. Elsevier, Amsterdam, 312 pp.
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Murty, V.S.N., Babu, V.R., Suryanarayana, A., Beena, B.S., 1999. Current structure and kinetic energy of the abyssal waters in the Central Indian Ocean Basin. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 216–223. Nath, B.N., Bau, M., Rao, B.R., Rao, Ch.M., 1997. Trace and rare-earth elemental variations in Arabian Sea sediments through a transect across the oxygen minimum zone. Geochimica et Cosmochimica Acta 61, 2375–2388. Nath, B.N., Rao, V.P., Becker, .K.P., 1989. Geochemical evidence of terrigenous influence upto 81S in the Central Indian Basin. Marine Geology 87, 301–313. Nath, B.N., Roelandts, I., Sudhakar, M., Plueger, W.L., 1992. Rare earth elements in Central Indian Basin sediments related to their lithology. Geophysical Research Letters 19, 1197–1200. NIO, 1997. Report on baseline studies on ocean currents in the Central Indian Basin prior to benthic disturbance. NIO/ SP-14/97, National Institute of Oceanography, Goa, India, 40 pp. Ozturgut, E., Anderson, G.C., Burns, R.E., Lavelle, J.W., Swift, S.A., 1978. Deep ocean mining of manganese nodules in North Pacific Ocean. Pre-mining environmental conditions and anticipated mining effects. NOAA-TM-ERL MESA-33, National Oceanic and Atmospheric Administration, USA, p. 138. Pearson, J.S., 1975. Ocean Floor Mining. Noyes Data Coporation., New Jersey, USA, 201 pp. Raghukumar, C., Loka Bharathi, P.A., Ansari, Z.A., Nair, S., Ingole, B., Sheelu, G., Mohandass, C., Nath, B.N., Rodrigues, N., 2001. Bacterial standing stock, meiofauna and sediment nutrient characteristics: indicators of benthic disturbance in the Central Indian Basin. Deep-Sea Research II 48, 3381–3399. Rao, V.P., Nath, B.N., 1988. Nature, distribution and origin of clay minerals in grain size fractions of sediments from manganese nodule field, Central Indian Ocean Basin. Indian Journal of Marine Sciences 17, 202–207. Rolinski, S., 1999. Large scale impact of manganese nodule mining in the Pacific numerical simulations of the dispersion of tailings. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 156–162. Rowe, G.T., Smith, S., Palkowski, P., Whitledge, T., Theroux, R., Phoel, W., Ducklow, H., 1986. Do continental shelves export organic matter. Nature 324, 559–561. Sharma, R., 1993. Quantitative estimation of seafloor features from photographs and their application to nodule mining. Marine Georesources and Geotechnology 11, 311–331. Sharma, R., 2000. Assessment of impact on seafloor features in INDEX area. Marine Georesources and Geotechnology 18 (3), 237–250. Sharma, R., Nath, B.N., 1997. Benthic disturbance and monitoring experiment in the Central Indian Ocean Basin. Proceedings of the Second ISOPE Ocean Mining Symposium, Seoul, Korea, pp. 146–153. Sharma, R., Nath, B.N., Valsangkar, A.B., Parthiban, G., Sivakholundu, K.M., Walker, G., 2000. Benthic disturbance and impact experiment in Central Indian Ocean Basin. Marine Georesources and Geotechnology 18 (3), 209–222. Sharma, R., Rao, A.S., 1992. Geological factors associated with megabenthic activity in the Central Indian Basin. Deep-Sea Research 39, 705–713. Shirayama, Y., 1999. Biological results of the JET project: an overview. Proceedings of the third ISOPE Ocean Mining Symposium, Goa, India, pp. 185–190. Smith Jr., K.L., 1987. Food energy and demand: a discrepancy between particulate organic carbon flux and sediment community oxygen consumption in deep ocean. Limnology and Oceanography 32, 201–220. Sousa, S.N., Sardessai, S.D., 1999. Baseline studies and evaluation of effects of surface discharge of deep-sea miningINDEX area. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 145–151. Tenore, K.R., 1977. Growth of Capitella capitata on various levels of detritus derived from different sources. Limnology and Oceanography 22, 936–941. Thiel, H., Angel, M. V., Foell, E.J., Rice, A.L., Schriever, G., 1997. Environmental risks from large-scale ecological research in the deep sea: a desk study. Marine Science and Technology, office of official publication of the European communities, vol. XIV, pp. 209. Tkatchenko, G.G., Radziejewska, T., Stoyanova, V., Modlitba, I., Parizek, A., 1997. Benthic Impact experiment in the IOM pioneer area: Testing for effects of deep seabed disturbance. Proceedings of International Seminar on Deep Seabed Mining Technology, Beijing, China, pp. C55–C68. Trueblood, D.D., 1993. US cruise report for BIE II cruise. NOAA Techical Memorandum No. OCRS 4, National Oceanic and Atmospheric Administration, USA, 51 pp.
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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. Yamazaki, T. Barnett, B.G., Suzuki, T., 1997. Optical determination of the JET deep sea sediment disturbance. Proceedings of International Symposium on Environmental Studies for Deep-Sea Mining, Tokyo, Japan, pp. 153–168. Yamazaki, T., Tsurusaki, K., Handa, K., 1991. Discharge from manganese nodule mining system. Proceedings of the First International Offshore and Polar Engineering Conference, Edinburg, UK, Vol. 1, pp. 440–446.
Sediment redistribution during simulated benthic disturbance and its implications on deep seabed mining Rahul Sharma*, B. Nagender Nath, G. Parthiban, S. Jai Sankar National Institute of Oceanography, Dona Paula, Goa 403004, India
Abstract A benthic disturbance experiment was conducted in the Central Indian Basin to evaluate the possible effects of sediment redistribution on marine ecosystem during deep-sea mining. The analyses of disturbance features on the seafloor and particle resettlement indicate that the most intense impact is created within the experiment site, and some of the resuspended sediments that remain in the near-bottom water after the disturbance are transported away from the site, changing the physico-chemical conditions in the area. Resedimentation is observed on either side of the disturbance zone, with an increase in suspended particles by 300% (from 49 to 150 mg/m2/day) during the disturbance and a reduction by 33% (to 95 mg/m2/day) immediately after the experiment. Higher collection of particles as well as an increase in organic carbon in seafloor sediments S–SW of the disturbance zone indicates directional migration of suspended material and redeposition in the adjacent areas. The implications of increase in turbidity and sedimentation rates in the near bottom water column due to sediment re-suspension, as well as lateral migration of sediment, on the benthic ecosystem have been discussed. These parameters provide important inputs in ascertaining longterm effects of deep-sea mining. r 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Discovery of deep-sea minerals, such as manganese nodules, as an alternative source of strategic metals, such as copper, nickel and cobalt (Mero, 1965; Glasby, 1977), has generated considerable interest in mining of these deposits in future (Amann, 1982; Anonymous, 1984; Chung and Tsurusaki, 1994). However, deep-sea mining would create a set of entirely new conditions, which could lead to deleterious effects on the entire marine ecosystem around the mining area (Amos and Roels, 1977; Ozturgut et al., 1978). One of the areas that would be affected more intensely is the seafloor and the water column above it, due to the movement, collection and waste discharge
*Corresponding author. Fax: +91-832-223340. E-mail address: [email protected] (R. Sharma). 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 6 - 7
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by the mining device (Thiel et al., 1997). It is likely to resuspend and redistribute the bottom sediments, leading to changes in the stratigraphic sequence (Gupta, 2000), sedimentological (Valsangkar et al., 1999) and geochemical properties as well as biochemical conditions (Raghukumar et al., 2001). The effects can be varied owing to uni- or multi-directional movement of the particles, resulting in resedimentation over the naturally exposed seafloor, bringing about changes in faunal assemblages (Foell et al., 1992; Shirayama, 1999). The effects of benthic disturbance depend largely on the migration and settlement of particles resuspended during the event. The intensity and direction of sediment migration would depend on strength of the bottom currents, the type of particle assemblages, as well as organic coatings on the sediment particles. In order to evaluate the possible effects of such an event, a small-scale benthic disturbance experiment was conducted in the manganese nodule-bearing area in the Central Indian Basin (Sharma and Nath, 1997). Similar experiments undertaken in the Pacific Ocean (Foell et al., 1990; Trueblood, 1994; Fukushima, 1995; Tkatchenko, 1997) have shown that the impact on the benthic ecosystem could be a variable and would depend on local environmental conditions. This paper describes the Indian Deep-Sea Environment Experiment (INDEX) in the Central Indian Basin and reports the migration trends of suspended sediments in and around the disturbed area. The paper also looks at the implications of sediment redistribution as a result of disturbance that may be created on the seafloor. Several other papers in this issue report the effects on the benthic conditions, as a result of this experiment. The experiment was conducted as a part of the Indian programme on ‘Environmental impact assessment of nodule mining program’, funded by the Dept. of Ocean Development (Govt. of India) and executed by the National Institute of Oceanography, Goa (India).
2. Study area Central Indian Basin is an abyssal area bounded by the 901 East Ridge on the east, Central Indian Ridge to the west, South East Indian Ridge to the south. To the north, it is open towards the Indian peninsula, whereas to the northwest it is bounded by the Chagos–Laccadive Ridge. Terrigenous, siliceous and pelagic brown clay sediments are present in the basin (Rao and Nath, 1988; Nath et al., 1989). A small area (10 10 nautical miles) was surveyed in detail by bathymetric mapping at a spacing of 1 nautical mile in N–S direction and 2 nautical miles in E–W direction, and a test site of 3.5 km 5.4 km, with a disturbance strip of 3 km 200 m in NW–SE direction was selected, which lies between 101010 S and 751590 E and 101030 S, 761020 E and depth ranges between 5300 and 5400 m (Fig. 1). The disturbance zone has even topography, no rock outcrops, low nodule abundance, and uniform benthic conditions (Sharma and Nath, 1997), as per the requirements for operation of the benthic disturber. The total relief around this site is 20–40 m, with low nodule abundance (o2 kg/m2). The site was surveyed by deep-towed sub-bottom profiling (5 kHz) equipment along 3 profiles. The sediment thickness in the area varies from 40 to 90 m, with 4 distinct layers of sediment. The upper two layers are very thin (mean thickness 3.5 and 5 m) and lower two layers are thicker (mean thickness 22 and 35.5 m) (Sharma, 2000). The sediment type is siliceous ooze, mainly
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Fig. 1. Locations of camera profiles, sediment traps, box cores in the experiment site. Depths in meters.
consisting of clay minerals such as smectite, illite, chlorite and kaolinite, as well as authigenic assemblages, such as Fe-rich smectites, phillipsites, other zeolites, and Fe–Mn oxides (Rao and Nath, 1988; Nath et al., 1992; Valsangkar et al., 1999).
3. Benthic disturbance experiment and systems deployed The experiment was conducted during two cruises of R.V. Yuzhmorgeologiya: (i) predisturbance cruise (27 May–8 July 1997); and (ii) disturbance and post-disturbance impact assessment cruise (22 July–5 September 1997). A 6-channel Magnavox MX 200 receiver was used to obtain Global Position System (GPS) fixes. ASMOD (underwater positioning system) was used for underwater positioning of the survey equipment. ASMOD consists of four acoustic transponders that were deployed and calibrated during the pre-disturbance cruise. GPS and ASMOD fixes were integrated to enable maneuvering of underwater survey and sampling equipment. The position data in both GPS and ASMOD were computed on WGS-72 spheroid to enable integration with previous survey data base in the site. GPS was used for the ship’s position, and ASMOD was used for all the operations (such as disturber, deeptow camera and coring) in
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and around the disturbance area. During sediment sampling, precise positioning was obtained with the help of pinger along with the ASMOD data. Deep-sea moorings with sediment traps were deployed close to the bottom (7 m above the seafloor) at 10 locations around the disturbance zone (Fig. 1) to record the time series data on fluxes before, during and after the disturbance. These were retrieved in the post-disturbance cruise after conducting the disturbance experiment and collecting data on the post-disturbance effects. The effects were also studied by deep-towed cameras and box corer samples (Fig. 1) in the pre- as well as post-disturbance phases. 3.1. Benthic disturbance experiment The experiment was conducted with a benthic disturber (Fig. 2), named ‘Deep-Sea Sediment Resuspension System (DSSRS)’, developed by the Sound Ocean System Inc. (SOSI), USA (Brockett and Richards, 1994). The disturber creates an artificial disturbance on the seafloor by fluidising the bottom sediment as it moves on the seafloor, and pumps and discharges the
Fig. 2. Benthic disturber used for the experiment.
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sediment through a 5 m-high stack. The plume created by the resuspended sediment was studied to assess the impact of such a disturbance on the benthic ecosystem. The same disturber was used earlier for three of the deep sea disturbance experiments conducted by NOAA (USA), MMAJ (Japan) and IOM (consortium of Eastern European countries). 3.2. Components of the disturber 3.2.1. Main unit The tow frame, which weighs 3.2 t is hydrodynamically designed to house the subsystems of the disturber and has dimensions of 4.8 m 2.4 m 5.0 m. The frame is connected with a coaxial cable that tows as well as transmits signals and power to the disturber unit. The frame consists of a discharge stack, 5 m high, with a diameter of 30 cm and is bell-shaped at the bottom, where the sediment is sucked up and discharged from the top. There are two pumps on the disturber, one for fluidising the sediment by water jets with 16 nozzles and another to suck the sediment slurry and raise it through the discharge stack. The pumps are controlled from the deck unit. A rosette of 12 bottles of 1.7 l capacity, is attached near the discharge stack to collect samples of water with resuspended sediments at select intervals and used to quantify the resuspension rate. A TV camera is attached to the frame to view the functioning of the disturber, which is monitored from the deck unit. 3.2.2. Depressor and cable This unit weighs about 1 t and is attached to the cable, about 200 m ahead of the disturber, to help dampen the ship motion. A 21-mm coaxial cable is used for deploying, operating and transmitting power and signals to and from the system. 3.2.3. Deck unit This is a PC-based system with custom made software and hardware to monitor and control the operation of the system. Clogging of the pumps can be sensed through the current level from the monitor and corrective measures can be taken. The camera, lights, and rosette can be operated from this unit. Also the video image transmitted by slow scan camera can be seen on the TV monitor. 3.3. Operation of the disturber The disturber is attached to the coaxial cable in front of the frame and has a transponder for acoustic positioning. Once the disturber touches the bottom, its functioning can be monitored from the altitude, roll, pitch and heading displays on the deck unit, where the functioning of the pumps also can be controlled. Plume generation can be seen on the video monitor, and the average sediment pumped can be estimated by taking intermittent water samples in the rosette on the disturber. The position of the disturber was observed real time on the acoustic navigation system (ASMOD).
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3.4. Quantum of disturbance in the study area Salient features of the disturbance are as follows (Sharma et al., 2000): * * * * * *
Total number of tracks=26, Total days of disturbance=9, Disturbance time=42 h 14 min, Estimated sediment resuspension=3555 t (6023 m3), Total dry sediment resuspended=580 t, Total distance covered=88.3 km.
Three other experiments carried out in the Pacific Ocean took a similar approach and conducted between 14 (IOM-BIE) and 49 (NOAA-BIE) tows, discharging between 1281 m3 (IOM-BIE) and 4328 m3 (NOAA-BIE) of sediment, with the JET experiment having in between figures of 19 tows and 2475 m3 of discharge (Trueblood, 1994; Tkatchenko et al., 1997; Fukushima, 1995). The DISCOL experiment used a plough harrow (Foell et al., 1990), which resulted in digging deep into the sediment, rather than creating a sediment plume. Hence, the two different types of disturbances caused during the DISCOL and the other Benthic Impact Experiments are complimentary to one another, as eventually the mining device is expected to dig into as well as resuspend the sediment, while collecting and separating the manganese nodule deposits from the seafloor. Hence, an ideal experiment would be to combine the two disturbance mechanisms.
4. Results 4.1. Seafloor disturbance from photo and video data Observations were made by deep-towed TV and photo system to evaluate the seabed features in and around the disturbance area in order to assess the impact of benthic disturbance on the seafloor. The deep-tow system was operated 4–5 m above the seafloor, during which continuous online observations were made onboard from the video data, and a 35-mm still camera was operated remotely in the areas of interest. The survey profiles were carried out up to 750 m on either side as well as across the disturbance site. Pre-disturbance photographic data show typically undisturbed conditions, with extensive biological activity in the form of benthic organisms, their fecal casts, burrows, tracks and trails (Fig. 3a). In contrast, immediate post-disturbance observations show tracks of disturber as pairs of depressions of the sleds and fluidised sediment surface between them (Fig. 3b). These areas are devoid of any biological activity except for an occasional megabenthic organism. Sediment piled up on the sides of the disturber tracks due to its motion could be observed in areas adjacent to the tracks. The camera profiles parallel to the disturber tracks showed lumps of sediment that had been redeposited from the discharge stack either on the track or besides them during the tow (Fig. 3c). The other areas showed a very fine layer of sediment cover resulting in smooth microrelief (Fig. 3d) and no burrows, fecal casts or animal trails. The areas away from the tow zone do not show any effects of disturbance and have fecal casts, burrows, trails as well as megabenthic organisms.
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Fig. 3. Disturbance features from seabed photos (a) disturber tracks, (b) piles of sediment, (c) resedimentation, (d) undisturbed seafloor.
In order to evaluate the areal extent of the impact, frequency distributions of different disturbance features noted in photographs were plotted with respect to the distance from the center of the disturbance track (Fig. 4). Most of the tracks of the disturber (87%) as well as sediment piles (91%) generated due to the disturber movement are concentrated within 100 m on either side. This is expected because the width of the disturbance site is 200 m. Mostly (57%) the effect of resedimentation is observed within the disturbance site (i.e. 100 m on either side). Many of the (89%) resedimentation features are observed up to a distance of 200 m, and few (11%) up to a distance of 250 m on either side, which means that the sediment has travelled up to 150 m away from the disturbance site. Consequently, most of the undisturbed areas (75%) are concentrated beyond 100 m on either side of the disturbance site and extend up to more than 750 m on either side, which is the limit of the photographic data. Of all these data, the most interesting is the area of influence due to resedimentation, which extends up to 150 m from the edge of the disturbance site. The highest intensity is shown by the locations within disturbance tracks, followed by those with lumps and piles of sediment, the resedimented areas and the undisturbed seafloor representing the least disturbance in the area. The intensity of impact is the highest within the disturbance zone and decreases away from this zone (Fig. 5). Image analyses methods have been used to determine the intensity of disturbance, from the variation in colour intensity of the
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Fig. 4. Frequency distribution of disturbance features.
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Fig. 5. Contour diagram of intensity of disturbance from photographic data: (1) Highest disturbance, (2) moderate disturbance, (3) low disturbance, (4) no disturbance.
sediment in deeptowed photographs (Yamazaki et al., 1997). This photographic and colour intensity data can be used to compare the photos of subsequent observations, during monitoring, to assess the restoration of seafloor features over a period of time. 4.2. Sediment resettlement from flux data Particle fluxes in the near bottom water before, during and after the disturbance, were estimated from time series sediment traps at 7 m height above the seafloor, moored at 10 locations around the disturbance site. Each mooring consisted of one sediment trap (a Russian-made Lotus time series sediment trap with 12 sampling cups). The collection area of the traps is 0.5 m2. In all, 8 moorings could be retrieved after the disturbance. Out of these, 5 moorings (DMS-1, 2, 4, 8 and 9) collected samples during most part of the experiment. The moorings DMS-5 and 6 functioned only partially, collecting particles only during pre-disturbance phase. Hence, the flux data are based on 7 traps. Observations from sediment traps deployed around the disturbance area (Fig. 6), show that the near-bottom particle fluxes in the pre-disturbance phase (B40 days) range from 41 to 55 mg/m2/ day in different traps, with an average of 48 mg/m2/day for the experiment site. During the
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Fig. 6. Relationship between particle fluxes and locations of traps.
disturbance phase (8.5 days), the particle fluxes increased by 3–4 times in different traps, with an average of 150 mg/m2/day. The average fluxes in the post-disturbance phase (5–6 days) dropped to 95 mg/m2/day. This means that the fluxes, which had increased by 300% from the background value during the disturbance phase, reduced to 200% (or by 33%) within 5–6 days of the last disturbance. So it is expected that the remaining particles suspended in the near-bottom water column, would either settle very fast, or the finer particles may remain in suspension for a longer time and get transported to the adjacent areas. Therefore, the silty sized sediments being relatively coarser (and heavier) may settle down soon after the disturbance, whereas the clay sized sediment may remain in suspension for a longer period. Particle fluxes recorded in different cups during the disturbance period were compared (Fig. 7) to analyse the resettlement of particles during this period. Out of the two cups that collected particles during the disturbance period, the first cup contained marginally fewer particles compared to the second cup, at two out of three locations. The rate of sediment pumped being almost the same during the entire 8.5-day period, it appears that some of the sediment from the first part of disturbance remained in suspension, and subsequently settled into the second cup, increasing the collection in the later period. This probably indicates that some of the sediment particles remain in suspension for sometime after the disturbance. The higher collection in the post-disturbance phase, as compared to background values, also indicates the subsequent settling down of some of the resuspended particles. However, the increase is seen in the traps closer to the disturbance zone (within 100 m) compared to one slightly farther away (200 m). Duration of each cup is 5 days, hence it can be presumed that during this period the resuspended particles reach up to 100 m, and may take more time to get transported to more distant areas. In order to evaluate the transportation pattern of the suspended particles, the fluxes were plotted with respect to distance from the center of the disturbance site for all the three phases (Fig. 8). In the pre-disturbance phase, the collection is almost uniform irrespective of the distance, as the traps collect the particles that are settling under natural conditions. During the disturbance phase, the traps closest to the disturbance track (DMS-2 and DMS-9) collected higher amounts of resuspended sediment, compared to others away from the site. However, one trap (DMS-1) had anamolously high sediment flux, which may correspond to the general deep-sea current
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Fig. 7. Comparison of particle fluxes recorded by traps during the disturbance period.
Fig. 8. Relationship between particle fluxes with respect to distance from disturbance site.
circulation in the area. In the post-disturbance phase, particle fluxes were lower than that during the disturbance period. The other trap closest to the disturbance site (DMS-9) continued to record very high fluxes, but in general the fluxes decreased with distance from the disturbance site. Once again, one trap (DMS-1) showed anamolously high collection during this phase as well, although it was away from the site. Collection of higher quantities of particles may be related to general current direction in the area during the period of disturbance. Observations from current meters deployed at 5100 m depth close to the site indicate the current direction to be S–SW during this period (NIO, 1997). Preferential deposition of sediment in a particular direction (north of tow zone) was observed during NOAA’s Benthic Impact Experiment, as well (Trueblood, 1993). This indicates that although the sediment particles are resuspended very close to the seafloor (B5 m) where the
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intensity of currents is very low, they do follow the prevailing currents at these abyssal depths. This also indicates that the impact of resedimentation will not only be in the disturbed zone, but also will spread to adjacent areas, the intensity of which will depend upon various factors such as the strength of the currents, size of the particles, volume of sediment discharged as well as the altitude of discharge. Consideration of all these parameters will determine the potential impact of sediment redistribution on the seafloor environment. 4.3. Sediment movement deciphered from organic carbon data Sediment sampling was carried out with spade/box cores (50 cm 50 cm 50 cm) at the locations within the disturbance track, adjacent to the disturbance area and at locations away from the disturber track (Fig. 9). Cores were obtained by inserting the acrylic core liners, which were subsampled for the sedimentological, geochemical, mineralogical, biostratigraphic and geotechnical studies. Subsampling was carried out at 2 cm intervals, and organic carbon was determined by wet oxidation method (El Wakeel and Riley, 1957) for sediment cores representing pre- and post-disturbance phases. Analyses of the sediments from both the phases were carried out on the same day to avoid any errors, and precision was checked by repeating the analyses of some of the samples (see Nath et al., 1997 for details). Particulate organic carbon or sedimentary organic matter is an important parameter in benthic ecosystem sustenance. The benthic biomass is directly related to both the quantity and quality of
Fig. 9. Comparison of average organic carbon contents (%) in surface sediments of pre- and post-disturbance phases of different areas.
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the available organic matter (e.g., Tenore, 1977). The organic carbon levels were studied in the sediments representing both the pre- and post-disturbance phases. Within the accuracy levels of bottom transponder based acoustic navigation system, the same location could be sampled in both the phases. Post-disturbance sediments in general showed an increase in organic carbon levels, but varying horizontally (Fig. 9). A pattern of ‘Bioenhancement’ is often encountered at sites with increased organic carbon, both in nearshore (Mauer et al., 1993; Smith, 1987) and deep sea (Rowe et al., 1986). No changes are observed for average organic carbon values in the area to the north of the disturbance site, unlike an increase (by 30%) in the disturbance site, and an even higher increase (45%) in the area to the south of the disturbance site (Fig. 9). It appears that the condition of surface sediment in the area to the north remained stable during the pre- and postdisturbance phases, in comparison to other two areas. This may be due to redeposition of suspended sediments within the site and transportation of organic carbon-rich surface sediments to the south. Further, the distribution of organic carbon in surface sediments plotted for predisturbance phase (Fig. 10) shows that the trend is gradual (from 0.28% to 0.4%) from NW to SE end of the site. The distribution in post-disturbance phase shows a significant increase within the site and also in the reverse direction, i.e. from SE to NW. The values are also significantly high (up to 0.64%) in the SW direction. The organic matter usually resides in finer sized sediment particles (e.g., Trask, 1937). Due to the prevailing currents, the fines may have been removed and deposited in the general direction of the bottom currents. The dominant benthic current direction during June–August is towards S–SW and the spatial distribution map of organic carbon has clearly shown an increase in this direction (Fig. 10), indicating the direction of resedimentation. Microfossil assemblage studies (radiolarians in this case) by Gupta (2000) in the area under consideration has shown evidence of lateral migration, as well as vertical mixing of sediment, within the sediment column in and around the disturbance zone. The interpretations are based on the higher occurrence of Stylatractus universus species (an indicator of older sediment normally observed at deeper horizons in the sediment column) along with the younger fauna Buccinosphaera invaginata in the surface sediments after the disturbance (Gupta, 2000). Hence, biostratigraphic evidence also can provide valuable data on sediment remixing due to artificial disturbance, and help determine the depth of disturbance as well.
5. Discussion and conclusions 5.1. Implications of sediment redistribution on marine ecosystem The artificial disturbance carried out during this experiment has shown signs of intense impact in the areas within the disturbance zone, increased suspended particles in near-bottom water, and changes in geochemical characteristics of the bottom sediments in the proximity of the impact zone. The sediment resuspension and redistribution would have a variety of implications in bringing about changes on different parameters in the benthic environment. Although, the scale of the benthic disturber was much smaller than a commercial mining system, the results show that any device designed to collect nodules would cause a sediment plume close to the seafloor, leading to a sharp increase in density of bottom waters. According to Amos and Roels (1977), for every t of nodules, 2.5–5.5 t of sediment will be resuspended. It is estimated that for mining of 3 106 t of
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Fig. 10. Distribution of organic carbon in surface sediments in pre- and post-disturbance phases. A clear increase in post-disturbance organic carbon contents is noticed in the S–SW direction.
nodules per year (of 8 kg/m2 average abundance), the total volume of sediment to be resuspended will be 504 107 m3, in an area of 600 km2 (Sharma, 1993). 5.2. Effects on benthic fauna This sediment not only will settle in the mining zone, but also be transported to the adjacent areas, changing the physico-chemical conditions on the seafloor, and increasing sedimentation
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rates. The increased turbidity also would lead to clogging of filter-feeding apparatus of the benthic organisms (Pearson, 1975), smothering the less mobile epipelagic organisms on the seafloor (Ozturgut et al., 1978). In the DISCOL experiment, several organisms were coated with sediment even 6 months after the disturbance, the mobile megafauna lacked in diversity and population, and many of the sessile forms were almost non-existent (Foell, 1990). Abyssal benthic communities are known to survive in a fragile ecosystem, maintaining a delicate balance within their environment. More of these organisms (88%) have a close association with sediments, followed by nodules (66%) in the Central Indian Basin (Sharma and Rao, 1992), which makes it particularly critical to take them into consideration for any such activity on the seafloor. These abyssal fauna are quite sensitive to even relatively small alterations in the surrounding environment, as indicated by reduction in meio and macro-fauna in the postdisturbance phase of the experiment (Ingole et al., 1999). On the positive side, there could be mixed effects due to release of high quantity of nutrients from subsurface layers due to mining activity (Raghukumar et al., 2001), which may lead to an increase in their population over a period of time.
5.3. Effects on water column conditions Attention needs to be focused on the method, quantity and levels of discharge of sediment in the water column, so as to avoid major deleterious effects. The sediment as well as the unwanted nodule debris should be discharged as close to the seafloor as possible to minimise the chances of the resuspended material being transported by currents. However, the possibility of lifting some of the debris along with water and nodules, to the surface needs to be considered. The temperature of bottom waters, usually between 11C and 21C, may rise marginally due to conduction and frictional heating, but in any case it is expected to be 15–201C lower than surface water temperature (Yamazaki et al., 1991), leading to increase in primary productivity due to high nutrient levels in these waters. Estimates have shown that the surface discharge will have to be diluted at 1 : 10,000 so as to keep the changes in oxygen induced by discharge is kept to minimum levels (Sousa and Sardessai, 1999).
5.4. Further studies Time-series observations to study the processes of recolonisation and restoration of benthic environment with respect to the benthic conditions would help in planning the deep-sea mining operations in future. The increase in particle fluxes and migration of the sediments can be effectively predicted by developing appropriate models. Such models would have to take into consideration several variables, such as sediment-laden negative buoyant effect, as well as the process of resuspension, in order to be fully applicable for sediment-plume behaviour, due to sediment redistribution (Doi et al., 1999). The potential disturbance to the benthic ecosystem is mainly determined by the grain size of the resuspended material (Rolinski, 1999). Bottom currents also may play a role in plume behaviour. Long-term bottom current measurements in the vicinity of the study area have shown that the current fields vary seasonally (Murty et al., 1999). The
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results of such models will then have to be compared with actual data from sediment traps, photographs and sediment cores to predict accurately the potential impacts of any kind of benthic disturbance in future. Acknowledgements The authors appreciate the support of Russian scientists onboard R.V. Yuzhmorgeologiya, during the benthic disturbance experiment and subsequent data collection. Help rendered by Ms. Sapana Nabar in analysing organic carbon with utmost care is acknowledged. This study was carried out as a part of the ‘Environmental Impact Assessment studies for nodule mining in Central Indian Basin’ funded by the Department of Ocean Development (Government of India). This paper is NIO contribution no no. 3656.
References Amann, N., 1982. Technological trends in ocean mining. Philosophical Transactions of the Royal Society of London A 307, 377–403. Amos, A.F., Roels, O.A., 1977. Environmental aspects of maganese nodule mining. Marine Pollution International Bulletin 1, 160–162. Anonymous, 1984. Analysis of Exploration and Mining Technology for Mn Nodules, Seabed Mineral Series, Vol. 2. UN Ocean Economic and Technology Branch, Graham and Trotman, London, pp. 57–68. Brockett, T., Richards, C.Z., 1994. Deep mining simulator for environmental impact studies. Sea Technology (August), 77–81. Chung, J.S., Tsurusaki, K., 1994. Advance in deep-ocean mining systems research. Proceedings of the Fourth International Offshore and Polar Engineering Conference, Osaka, Japan, pp. 18–31. Doi, T., Nakata, K., Kubota, M., Aoki, S., 1999. Environmental study on the deep-sea mining of manganese nodules in the northeastern Tropical PacificFmodeling the sediment-laden negative buoyant flow. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 163–169. El Wakeel, Riley, J.P., 1957. Determination of organic carbon in the marine muds. Journal du Conseil International pour L’exploration de La Mer 22, 180–183. Foell, E.J., 1990. DISCOL: a long-term, large-scale, disturbance-recolonization experiment in the abyssal eastern tropical South Pacific Ocean. Proceedings Offshore Technology Conference, Houston, USA, OTC 6328, pp. 497–503. Foell, E.J., Schriever, G., Bluhm, H., Borowski, C., Bussau, C., Thiel, H., 1992. Disturbance and recolonization experiment in the abyssal South Pacific Ocean (DISCOL): an update. Proceedings Offshore Technology Conference, Houston, USA, OTC 6805, pp. 25–33. Fukushima, T., 1995. Overview ‘‘Japan Deep-Sea Impact Experiment=JET’’. Proceedings of the ISOPE Ocean Mining Symposium, Tsukuba, Japan, pp. 47–53. Glasby, G.P., 1977. Marine Manganese Deposits. Elsevier, Amsterdam, 523 pp. Gupta, S.M., 2001. Biostratigraphic analysis of the top layer of sediment cores from reference and test sites of the INDEX area. Marine Georesources and Geotechnology Journal 18 (3), 259–262. Ingole, B.S., Ansari, Z.A., Matondkar, G.P., Rodrigues, N., 1999. Immediate response of meio and macrobenthos to disturbance caused by a benthic disturber. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 191–197. Mauer, D., Robertson, G., Gerlinger, T., 1993. Long-term temporal and spatial fluctuation of soft bottom infaunal invertebrates from the San Pedro Shelf, California. Internationale revere du geramten hydrobiologie 78, 535–555. Mero, J.L., 1965. The Mineral Resources of the Sea. Elsevier, Amsterdam, 312 pp.
R. Sharma et al. / Deep-Sea Research II 48 (2001) 3363–3380
3379
Murty, V.S.N., Babu, V.R., Suryanarayana, A., Beena, B.S., 1999. Current structure and kinetic energy of the abyssal waters in the Central Indian Ocean Basin. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 216–223. Nath, B.N., Bau, M., Rao, B.R., Rao, Ch.M., 1997. Trace and rare-earth elemental variations in Arabian Sea sediments through a transect across the oxygen minimum zone. Geochimica et Cosmochimica Acta 61, 2375–2388. Nath, B.N., Rao, V.P., Becker, .K.P., 1989. Geochemical evidence of terrigenous influence upto 81S in the Central Indian Basin. Marine Geology 87, 301–313. Nath, B.N., Roelandts, I., Sudhakar, M., Plueger, W.L., 1992. Rare earth elements in Central Indian Basin sediments related to their lithology. Geophysical Research Letters 19, 1197–1200. NIO, 1997. Report on baseline studies on ocean currents in the Central Indian Basin prior to benthic disturbance. NIO/ SP-14/97, National Institute of Oceanography, Goa, India, 40 pp. Ozturgut, E., Anderson, G.C., Burns, R.E., Lavelle, J.W., Swift, S.A., 1978. Deep ocean mining of manganese nodules in North Pacific Ocean. Pre-mining environmental conditions and anticipated mining effects. NOAA-TM-ERL MESA-33, National Oceanic and Atmospheric Administration, USA, p. 138. Pearson, J.S., 1975. Ocean Floor Mining. Noyes Data Coporation., New Jersey, USA, 201 pp. Raghukumar, C., Loka Bharathi, P.A., Ansari, Z.A., Nair, S., Ingole, B., Sheelu, G., Mohandass, C., Nath, B.N., Rodrigues, N., 2001. Bacterial standing stock, meiofauna and sediment nutrient characteristics: indicators of benthic disturbance in the Central Indian Basin. Deep-Sea Research II 48, 3381–3399. Rao, V.P., Nath, B.N., 1988. Nature, distribution and origin of clay minerals in grain size fractions of sediments from manganese nodule field, Central Indian Ocean Basin. Indian Journal of Marine Sciences 17, 202–207. Rolinski, S., 1999. Large scale impact of manganese nodule mining in the Pacific numerical simulations of the dispersion of tailings. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 156–162. Rowe, G.T., Smith, S., Palkowski, P., Whitledge, T., Theroux, R., Phoel, W., Ducklow, H., 1986. Do continental shelves export organic matter. Nature 324, 559–561. Sharma, R., 1993. Quantitative estimation of seafloor features from photographs and their application to nodule mining. Marine Georesources and Geotechnology 11, 311–331. Sharma, R., 2000. Assessment of impact on seafloor features in INDEX area. Marine Georesources and Geotechnology 18 (3), 237–250. Sharma, R., Nath, B.N., 1997. Benthic disturbance and monitoring experiment in the Central Indian Ocean Basin. Proceedings of the Second ISOPE Ocean Mining Symposium, Seoul, Korea, pp. 146–153. Sharma, R., Nath, B.N., Valsangkar, A.B., Parthiban, G., Sivakholundu, K.M., Walker, G., 2000. Benthic disturbance and impact experiment in Central Indian Ocean Basin. Marine Georesources and Geotechnology 18 (3), 209–222. Sharma, R., Rao, A.S., 1992. Geological factors associated with megabenthic activity in the Central Indian Basin. Deep-Sea Research 39, 705–713. Shirayama, Y., 1999. Biological results of the JET project: an overview. Proceedings of the third ISOPE Ocean Mining Symposium, Goa, India, pp. 185–190. Smith Jr., K.L., 1987. Food energy and demand: a discrepancy between particulate organic carbon flux and sediment community oxygen consumption in deep ocean. Limnology and Oceanography 32, 201–220. Sousa, S.N., Sardessai, S.D., 1999. Baseline studies and evaluation of effects of surface discharge of deep-sea miningINDEX area. Proceedings of the Third ISOPE Ocean Mining Symposium, Goa, India, pp. 145–151. Tenore, K.R., 1977. Growth of Capitella capitata on various levels of detritus derived from different sources. Limnology and Oceanography 22, 936–941. Thiel, H., Angel, M. V., Foell, E.J., Rice, A.L., Schriever, G., 1997. Environmental risks from large-scale ecological research in the deep sea: a desk study. Marine Science and Technology, office of official publication of the European communities, vol. XIV, pp. 209. Tkatchenko, G.G., Radziejewska, T., Stoyanova, V., Modlitba, I., Parizek, A., 1997. Benthic Impact experiment in the IOM pioneer area: Testing for effects of deep seabed disturbance. Proceedings of International Seminar on Deep Seabed Mining Technology, Beijing, China, pp. C55–C68. Trueblood, D.D., 1993. US cruise report for BIE II cruise. NOAA Techical Memorandum No. OCRS 4, National Oceanic and Atmospheric Administration, USA, 51 pp.
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R. Sharma et al. / Deep-Sea Research II 48 (2001) 3363–3380
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. Yamazaki, T. Barnett, B.G., Suzuki, T., 1997. Optical determination of the JET deep sea sediment disturbance. Proceedings of International Symposium on Environmental Studies for Deep-Sea Mining, Tokyo, Japan, pp. 153–168. Yamazaki, T., Tsurusaki, K., Handa, K., 1991. Discharge from manganese nodule mining system. Proceedings of the First International Offshore and Polar Engineering Conference, Edinburg, UK, Vol. 1, pp. 440–446.