- Email: [email protected]
An autonomous benthic lander:
Continental Shelf Research 21 (2001) 859–877
An autonomous benthic lander: preliminary observations from the UK BENBO thematic programme Kevin S. Blacka,*, Gary R. Fonesb, Oli C. Peppea, Hilary A. Kennedyc, Ilhem Bentalebd a
Dunstaffnage Marine Laboratory, Oban, Argyll, Scotland, PA34 4AD, UK b Wood Hole Oceanographic Institution, Wood Hole, MA 02453, USA c School of Ocean Sciences, University of Wales, Bangor, Anglesey, Wales, LL59 5EY, UK d Lab. Phyloge´nie, Institut des Sciences de L’Evolution de Montpellier, Pale´ontologie, Pale´obiologie, CC064 UM2, 34095 Montpellier Cedex 5, France Received 5 May 1999; accepted 9 November 1999
Abstract A new, multi-purpose autonomous benthic lander is described, and preliminary experimental data are presented relating to deployments in the Atlantic Frontier (eastern north Atlantic) during the recent UK Thematic Programme ‘BENBO’. The autonomous lander was deployed at two contrasting sites } Site A (mouth of Rockall Trough; 3570 m) and Site B (Hatton-Rockall Bank; 1100 m) } before and following the spring-time surface ocean phytoplankton bloom (May & July, 1998, respectively). Diffusive oxygen uptake and nutrient flux data were obtained using two interchangeable modules } a profiling oxygen micro-electrode unit and a benthic chamber unit. Diffusive O2 uptake across the sediment–water interface and the O2 penetration depths within the sediment were determined from the oxygen micro-profiles. The shallower site, which had previously received phyto-detrital input, had a comparatively large diffusive oxygen uptake within the sediment (1.2 mmol m2 d1) and a maximum penetration depth of only 21 mm. The deeper site had greater oxygen penetration depths ( 80 mm) but a lower diffusive oxygen uptake of 0.6 mmol m2 d1, indicative possibly of little or no phyto-detrital input. Visual observations of retrieved sediment cores support this conclusion, however Site A has also displays generally lower organic content and lower macrobenthic biomass which may contribute to this observation. Nutrient pore water profile data indicated fluxes of nitrate of 0.161 mmol m2 d1 and phosphate 0.0008 mmol m2 d1 into the overlying water. However, the benthic chamber studies showed virtually no change in nutrient
*Corresponding address: School of Environmental and Evolutionary Biology, University of St Andrews, Gatty Marine Laboratory, St Andrews, Fife ky169LB, UK. Tel.: +44-01334-463441; fax: +44-01334-463443. E-mail address: [email protected] (K.S. Black). 0278-4343/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 0 0 ) 0 0 1 1 6 - 3
860
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
concentrations, due probably to the relatively short deployment time used. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Oxygen; Nutrients; Sediments; Benthic lander; North Atlantic; Fluxes
1. Introduction Deep sea sediments form the world’s largest and deepest ecosystem of the order 300 million km2 in extent. To date less than 0.001% of deep ocean sediments have been characterised, in either biological or chemical terms, and thus our knowledge and understanding of the functioning of these depositional environments is at best rudimentary. One of the most dramatic discoveries associated with the deep ocean in recent years has been that of the seasonal deposition of rapidly sinking phyto-detritus or ‘marine snow’ on the deep sea-floor (Lampitt, 1985; Newton et al., 1994). This material, which is usually organically enriched, is utilised by benthic communities (Smith et al., 1998; Drazen et al., 1998; Pfankucke et al., 1999) and fuels the cycling of important biogeochemically active elements (e.g. C, N, S, O2) within the surficial sediment layers. Phytodetritus has been shown to mediate the flux of pore-water solutes (e.g. NO 3 ), and is also important in the cycling of redox metals (e.g. Mn2+; Gehlen et al., 1997). The north-east Atlantic, in particular, is a region of intense seasonality with distinct changes in surface primary production across large geographic distances giving rise to large fluctuations in mid-water and benthic particle flux (Rice et al., 1994). In the past, efforts to study benthic processes in the open ocean in detail have been based upon collection of seabed sediments using a variety of coring devices. Fluxes have subsequently been determined by direct incubation studies on the ship or at land-based laboratories, or have been calculated from pore-water gradients which have been determined by traditional pore-water analysis (e.g. Usui et al., 1998). However, as noted by Sayles et al. (1976) some 20 years ago, retrieval of sediments using conventional means can sometimes give rise to artifacts associated with physical disturbance of the sediments cf. the large decrease in hydrostatic pressure and temperature increase as they are brought to the surface. The latter is especially relevant for very deep ocean sediments (>3000 m; Lampitt et al., 1995) and has been shown recently to disrupt sedimentary oxygen dynamics (Reimers, 1987; Glud et al., 1994a). Technological advances in recent years have more or less circumvented the problems associated with sample retrieval as they permit direct experimentation actually on the seabed. The Woods Hole Interstitial Marine Probe (WHIP), for example, was designed to overcome sampling problems by collecting sedimentary pore water in situ (Sayles, 1992). A relatively new family of scientific instruments called ‘benthic landers’ are being used to deploy sensors in the ocean. Landers are autonomous, unmanned oceanographic research vehicles that free-fall to the sea floor unattached to any cable, and then operate independently according to a series of pre-programmed instructions. At the end of a deployment, ballast weights are released on an acoustic command transmitted from the surface or by a timing device. Landers are being increasingly used to study a
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
861
wide variety of phenomena in the marine environment, including high-resolution near-bed water characterisation (Thomsen et al., 1994), observation of fish behaviour (Armstrong et al., 1992) and time-lapse photography of seabed benthic activity (Billett et al., 1983; Rice et al., 1994). Tengberg et al. (1995) comprehensively reviewed the technical history and operation of 27 lander instruments developed to date. This paper describes a new lander based upon the ELINOR instrument of Glud et al. (1995), and presents some preliminary oxygen and nutrient flux data which were collected at a number of locations in the eastern north Atlantic as part of the recent research programme ‘BENBO’.
1.1. The benthic boundary programme (BENBO) The Atlantic Frontier to the west of Scotland and Ireland is a comparatively pristine marine environment. However, the area is increasingly viewed as potentially rich in hydrocarbon reserves, with prospects for large to medium-sized oil discoveries, and currently licenses have been issued by the UK government (1997 17th Round) to explore just under 25,000 km2 of the Atlantic margin region. Although the region presents greater technical challenges to drilling, deepwater production technology can now operate in the severest metocean conditions and in depths in excess of 1000 m. As part of a wider recognition of the importance of the Atlantic margin to seabird and sea mammal stocks, benthic biodiversity and coastal protection, and respecting recent international regulatory legislation from the IMO (International Maritime Organisation) and OSPAR (the Oslo and Paris Commission), the various exploration companies have formed a co-ordinated, strategic approach to environmental management through the setup of the Atlantic Frontier Environment Network (AFEN). AFEN commissions independent scientific research programmes directed towards describing and understanding the existing environment, in particular the fate and impact of marine snow as well as certain anthropogenic contaminants, identifying key sensitivities, and monitoring and developing protection measures. It programmes such as these which provide a crucially important environmental baseline, and the data can be used to shape industry activities to help ensure that they have at worst no more than a minimal effect on the marine environment. The BENBO study is a multi-disciplinary, inter-institutional marine benthic biogeochemistry programme allied to AFEN, but funded directly by the UK Natural Environment Research Council (NERC). The underlying philosophy of BENBO is identical to that of AFEN, however the scientific content is more experimental in nature than regional and descriptive cf. the AFEN I and II seabed surveys. The principal objective of BENBO is to investigate and quantify the biophysical and biogeochemical processes occurring at the deep ocean bed as a result of the sedimentation of marine snow. This was achieved through the funding of nine projects ranging from organic and inorganic adsorption processes onto mineral surfaces to bacterial consumption of marine snow to diagenesis and storage times-scales at the deep ocean bed (refer to Black, 1999, for further details). Two process cruises were mounted on the UK research vessel RRS Charles Darwin, and data and samples were collected either side of the spring phytoplankton bloom. These cruises were CD111 20 April–15 May, 1998, and CD113 28 June–22 July, 1998.
862
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
2. Methodology 2.1. The benthic lander instrument The BENBO benthic lander, which was designed and constructed in collaboration with a commercial engineering company (KC Denmark), is shown in Figs. 1 and 2. The lander incorporates a number of interchangeable modules. These include (i) an enclosed chamber for determination of total benthic community respiration and solute flux, (ii) a micro-profiling module which can insert up to four oxygen micro-electrodes and two pH micro-electrodes into the sediment to measure pore water concentrations at very high spatial resolution ( 50 mm intervals),
Fig. 1. The BENBO benthic lander instrument. The drawing shows the oxygen chamber module in the lower frame: (F) feet; (C) titanium chamber; (S) hydraulic shovel; (B) ballast tin; (Co) computer housing and control electronics; (Ba) battery; (RM) acoustic/burn-wire release mechanism; (TL) telescopic legs; (P) acoustic pinger; (BS) buoyancy spheres; (SB) satellite beacon. Strobe, flag and pellet line not shown.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
863
Fig. 2. Photograph of the benthic lander with the oxygen micro-profiling module during deployment.
and (iii) a unit to insert DGT (diffusive gradient in thin films) and DET (diffusive equilibration in thin films) gel probes (Davison et al., 2000) through the sediment–water interface to measure fluxes of trace metals and concentrations of major ions at a spatial resolution of 100 mm to 1.5 mm. The gel probe unit is not described here. The lander comprises two open triangular frames clamped together one on top of the other. One of three self-contained scientific modules (chamber/oxygen micro-profiler/gel profiler) is bolted into the lower instrumentation frame and the upper frame can hold up to nine 17’’ BenthosTM buoyancy spheres which provide the floatation required to bring the lander back to the surface at the end of the deployment. Both frames are made of 2.5 cm2 aluminium piping and are approximately 1.5 m high. Adjustable telescopic feet at the corners of the lower frame allow the
864
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
height of the instrumentation above the seabed to be varied by up to 0.5 m. Circular platforms at the bottom of the feet limit sinking of the frame into unconsolidated muds. Miniature (1.5 cm diameter) core tubes are mounted to these plates to assess how far the feet have sunk into the sediment and the tilt of the lander. The disposable ballast, in the form of steel shot contained in bins or scrap steel circular discs, is suspended within the corners of the instrumentation frame. The number of buoyancy spheres used is adjusted so that the net positive buoyancy on ascent is approximately 75–100 kg. Sufficient ballast is then added to make the net negative buoyancy on descent approximately 50 kg. This configuration gives approximate descent and ascent rates of 50–60 and 60–70 m min1, respectively. A 10 kHz acoustic pinger is attached to the frame to allow the descent and ascent of the lander to be tracked from the ship using a standard echo sounder. Once the bottom experiment is completed the ballast is dropped by triggering an InterOceanTM acoustic release and the lander returns to the surface. A computer-controlled burn-wire provides back-up for the ballast trigger in the event of acoustic release failure. 2.2. Microprocessor and lander electronics All lander operations are controlled by a central computer housed in a 6000 m rated pressure housing. This computer is programmed prior to deployment with the required experimental routine using a custom-designed PC-based software interface. The computer logs all sensor data during the deployment and this data is downloaded to a PC on recovery of the lander. A 12 V DeepSea Power & LightTM lead-acid battery provides power to the main lander systems. However, the acoustic release and various beacons critical to recovering the lander safely are powered independently. 2.3. Chamber module The BENBO chamber is a 30 30 cm open-ended rectangular box manufactured from inert titanium and coated inside with teflon sheet. The corners are rounded to minimise turbulence and reduce stagnation when the water is stirred (Glud et al., 1995), and a hydraulically operated shovel slides beneath the box to recover the sediments prior to lander ascent. The chamber is designed to penetrate into the bottom sediments synchronously as the instrument lands on the seabed (thereby minimising physical disturbance), and after a pre-set time interval the lid is closed by triggering a burn wire, thus enclosing a volume of bottom water. A polycarbonate cruciform blade magnetically coupled to a variable speed pressure compensated oil-filled motor may be used to stir the water within the chamber. A polarographic Clarke-type robust mini-electrode is used to measure oxygen within the chamber. In order to calibrate the sensor against ambient bottom water, water samples are taken in triplicate using a miniature 250 cm3 Niskin bottle-type sampling instrument which is activated when the ballast is released at the end of the deployment. O2 concentration is determined directly on board ship (following Strickland and Parsons, 1972) using a commercial micro-Winkler titration instrument (MetrohmTM 702 SM Titrino). A pH minisensor is co-located with the oxygen sensor, with the reference electrode positioned on the outer casing of the electronics housing. During deployment, up to fifteen 60 cm3 water samples may be withdrawn through teflon tubing from the chamber using a bank of spring-loaded syringes mounted above the chamber. A
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
865
small bi-directional valve in the lid permits ambient benthic water to compensate for the volume of water withdrawn from the chamber during sampling. In each rack, one of the syringes may be used to inject solutes into the chamber rather than withdraw water from the chamber. As the net volume of water in the chamber cannot be established precisely (due to variable lander penetration into sediments), injection of inert substances such as Br have been used to determine enclosed water volume. 2.4. Oxygen micro-profiling module The vertical profile of oxygen in marine sediments has been used for some years as a method of determining the diffusive oxygen uptake, as well as respiration in the upper oxic zone (Glud et al., 1994a). The BENBO micro-profiling module, which is based on the design of Reimers (1987), is designed to measure the vertical profile of sediment oxygen content at very high resolution (typically every 250 mm, but every 25 mm is possible) within the surface layers of oceanic muds. The module consists of a profiling rack into which the electronics housing is fitted vertically. The micro-electrodes (up to four oxygen and two pH) are attached directly to the bottom of the housing, and the whole unit is raised and lowered at a maximum speed of 20 mm min1 by a worm gear driven by a pressure-compensated oil-filled marine motor. An optical sensor on the motor allows the height of the electrodes relative to the zero position to be measured with a resolution of 25 mm. The oxygen micro-electrodes used were Clark-type electrodes with a guard cathode (Revsbech, 1989). The total length of the electrode, which determines the maximum insertion depth, is approximately 80 mm. 2.5. Study area Three contrasting areas of seabed around the Rockall Trough region in the north-east Atlantic, were chosen as the BENBO study sites (Fig. 3), although we report data from two sites only here (sites A and B). The Rockall Trough is a deep-water basin at the foot of the continental slope west of Ireland extending from about 538N to 608N. The trough trends NE–SW and ranges in depth from ca. 2000 m at its northern end to ca. 4000 m at the mouth. The Trough is bound to the west by the Rockall plateau, comprising the shallow (1100–1200 m) Hatton Bank, Rockall Bank and the Hatton–Rockall Basin, and to the north by the Wyville–Thompson ridge. Two seamounts, the Anton–Dohrn and the Rosemary Bank, are found at the northern end of the Trough. The sites comprise carbonate ooze with variable amounts of detrital silt and clay, but have differing sediment properties and differing hydrographic regimes (Table 1). 2.6. In situ measurements 2.6.1. Oxygen profiles In situ oxygen profiles were obtained during three separate deployments at the BENBO sites (Table 2). The procedure used for obtaining oxygen profiles at each of the BENBO sites was as follows. Once the lander had reached the seabed there was a time interval of at least an hour before the profiling was started to allow sediment disturbed during the landing to settle out. After this period the micro-electrodes were inserted gradually into the sediments in 250 mm steps. The
866
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
Fig. 3. Location of three sampling locations (A, B and C) in the BENBO field study area, eastern north Atlantic.
Table 1 Characteristics of the study sites
Latitude Longitude Water depth (m) Bottom water salinity Bottom water temp. (8C) Bottom water O2 (mmol) Median particle size (mm) Wet bulk density (kgm3)a Organic content (%)a Water content (% wet wt.) Carbonate content (%)a Composition a
Site A
Site B
52854.410 N 16854.570 W 1100 34.89 2.15 265 34.73 1640 0.23 60 76.07 Carbonate ooze
57823.310 N 15842.640 W 3570 34.92 6.05 238 12.73 1840 0.66 61 75.20 Carbonate ooze
From Thomson (unpublished data). Measured over the top centimetre.
sensors were allowed to equilibrate at each depth for 10 s before data were logged at a rate of 1 Hz from each of five electrodes over a period of 5 s. Two methods of calibration were undertaken for the oxygen micro-electrodes. In the first method a two-point linear calibration of the micro-electrodes for oxygen concentrations was performed using the constant reading recorded in the anoxic part of the sediment and the
867
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
Table 2 Summary of sampling information for chamber O2 and nutrients, O2 micro-profiling, and pore-water sub-sampling from retrieved mega-cores Deployment
Cruise
Site
Date
Chamber O2 #005 #006
111 111
A B
03/05/98–07/05/98 09/05/98–12/05/98
O2 micro-profiles #004 111 #010 113 #012 113
A B A
01/05/98–02/05/98 02/07/98–03/07/98 06/07/98–07/07/98
Chamber nutrients 54403#03 111 54403#17 111 54407#01 111 57703#17 118
A A B A
01/05/98 03/05/98 10/05/98 09/07/98
Pore waters 54403#13 54404#08 54407#03 54702#08 54703#01 54706#6
A A B B A B
02/05/98 07/05/98 10/05/98 03/07/98 06/07/98 18/07/98
111 111 111 113 113 113
Deployment duration (h)
Latitude
Longitude
Water depth (m)
98 54
52854.240 N 57823.310 N
16854.400 W 15842.640 W
3570 1103
9.50 10.2 9.50
52854.410 N 54824.510 N 52854.320 N
16854.570 W 15844.590 W 16854.350 W
3573 1101 3357
9.30 98 54 129
52854.260 N 52854.240 N 57823.180 N 52855.470 N
16854.420 W 16854.240 W 16842.530 W 16853.010 W
3573 3368 1104 3560
52 52 57 57 52 57
16 16 15 15 16 15
3570 3576 1105 1104 3560 1094
55.01 55.21 25.25 24.25 54.36 24.24
53.31 53.00 44.05 44.31 54.05 41.31
overlying water oxygen concentration (obtained from the bottom water samples collected using the Niskin-type water bottle sampler). The second method was on board ship before and after deployment to ensure the electrodes were registering a linear response and to enable a zero point to be calculated for the profiles that did not reach a constant reading in situ. The electrodes were calibrated using bottom water at the in situ temperature and flushed thoroughly with oxygen and nitrogen, respectively, to create end-member conditions. 2.6.2. Chamber incubations Benthic chamber deployments for measurements of O2 and nutrient fluxes were undertaken during four separate deployments at two of the BENBO sites (Table 2). The electrodes are calibrated before and after deployment using the same technique as for the profiling microelectrodes. To obtain a two-point calibration, bottom water collected by the Niskin bottles and overlying water from the chamber are measured using the micro-titrator. 2.6.3. Nutrient data Measurements of phosphate (PO3 4 ) and nitrate (NO3 ) within the chamber were complemented by analyses made in sediment pore-waters of retrieved sediments during
868
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
each of the BENBO cruises (Table 2). Pore-waters were sampled by sectioning mega-core tubes under a nitrogen atmosphere and at in situ temperatures followed by separation by centrifugation and filtration through GF/F glass-fibre filters. The samples were stored in frozen plastic tubes prior to analysis. Dissolved nitrate and soluble reactive phosphate were determined using a LachatTM Quickhem 8000 flow injection analyser (Lachat Instruments Inc. Milwaukee, WI, USA). 2.7. Calculations 2.7.1. Oxygen The diffusive O2 uptake (Joxy ), O2 penetration depth (L) and the diffusive boundary layer (DBL) thickness (d) were calculated for three deployments from the measured oxygen microprofiles (Fig. 4). The O2 penetration depth at Site B could be determined directly from the profiles as the depth where the signal of the O2 microelectrode reached a constant, low or zero reading.
Fig. 4. In situ O2 micro-profiles measured with a spatial resolution of 250 mm at two BENBO study sites in the northeast Atlantic.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
869
However, at Site A where the O2 penetration exceeded 60 mm in the sediment, the depth was estimated by linear extrapolation of the last part of the measured profiles. These estimated penetration depths therefore represent minimum values. The position of the sediment surface was determined from the change in oxygen concentration gradient due to impeded diffusion (Revsbech, 1989; Sweerts et al., 1989). The in situ diffusive O2 uptake (Joxy ; mmol m2 d1) was calculated from the O2 micro-profiles by Fick’s first law of diffusion (1) and the linear concentration gradient measured in the DBL (Crank, 1983; Rasmussen and Jorgensen, 1992) dC ; ð1Þ Joxy ¼ Do dx where Do is the diffusion coefficient of O2 in seawater (Broecker and Peng, 1974) recalculated for in situ temperature with the Stokes–Einstein equation (Li and Gregory, 1974), and x is vertical distance. This method allows calculation of the diffusive O2 uptake without an estimate of the diffusive characteristics within the sediment. Unfortunately the total O2 uptake could not be calculated, as the volume of water in the benthic chamber was not determined. 2.7.2. Nutrients Time-series of PO3 4 and NO3 concentrations from the lander chamber and profiles of sediment pore-water were undertaken at site B (Cruise 111, 10 May, 1998) and are presented here as examples of preliminary results. The trend in concentration through time (decreasing or increasing) in the lander chamber may be used directly to infer the direction and magnitude of the benthic flux of solutes. As a direct comparison, the pore-water gradients of PO3 4 and NO3 at the sediment water interface have also been used to compute a benthic flux. The benthic nutrient flux Jnutr was calculated using @C x ¼ 0; ð2Þ Jnutr ¼ fDs @x where an average porosity was estimated as f ¼ 0:8, Ds is the diffusion coefficient corrected for tortuosity, and ð@C=@xÞjx ¼ 0 is the concentration gradient at the sediment–water interface obtained from the first derivative of fitted model curves at x ¼ 0. 3. Results and discussion 3.1. Oxygen Five in situ O2 profiles obtained by the profiling system at two BENBO sites (A & B) are presented in Fig. 4. Table 3 summarises the diffusive O2 uptake, DBL thickness ðdÞ and O2 penetration depths from each of the lander deployments. To the authors’ knowledge, no O2 micro-profiles at high resolution have been reported from the eastern north Atlantic, only total O2 consumption rates (e.g. Patching et al., 1986; Lampitt et al., 1995). The majority of previous in situ studies of diffusive oxygen uptake and oxygen penetration depths have been conducted in deep ocean sediments from the south and west Atlantic Ocean and the Pacific ocean (e.g. Reimers et al., 1986; Reimers, 1987; Archer et al., 1989; Glud et al., 1994a). The present data therefore
870
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
widen the global coverage of diffusive O2 uptake in deep-sea sediments and provide an important contribution to determining the seasonal and spatial changes in O2 fluxes in Atlantic Frontier region. Fig. 5 presents a detailed O2 profile over only 4 mm from site A and illustrates the change in slope of the O2 gradient at the sediment surface. It clearly shows the linear O2 gradient through the determined diffusive boundary layer (DBL), from which the diffusive O2 uptake, Joxy , is
Table 3 Oxygen concentration in the bottom water (O2), diffusive O2 uptake (Joxy ), O2 penetration depth (L) and diffusive boundary layer (DBL) thickness (d) at the BENBO field sites Deployment Number
Site
Electrode number
Bottom water [O2] (mmol)
Diffusive O2 flux, Joxy (mmol m2 d1)
O2 Penetration depth, L (mm)
DBL thickness, d (mm)
#004 #004 #010 #010 #012
A A B B A
1 6 16 3 3
265 270 234 243 264
1.01 0.25 0.80a 1.56 0.56
78 85 25 21 80
750 750 625 625 750
a
Electrode micro-profile noisy; possible range of 0.4–1.2 mmol m2 day1.
Fig. 5. Detail of an O2 micro-profile, Site A (July, 1998). The sediment surface and upper boundary of the diffusive boundary layer (DBL) are indicated by the line at depth zero and the dashed line, respectively.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
871
calculated. The calculated DBL thickness, d, in this example is 750 mm, and the diffusive O2 uptake is 0.56 mmol m2 d1. Glud et al. (1994b), however, have shown that diffusive O2 uptake values may be underestimated by 25–45% when measured using micro-electrodes, owing to perturbation of the flow field around the sensor. Our data may therefore represent minimum estimates of diffusive O2 uptake. Viewed collectively, the data set indicates that O2 penetrates much farther into the sediments at the deepest site A ( 3500 m) than in sediments of the shallower site B ( 1100 m). A mean O2 penetration depth of ca. 80 mm was measured for Site A versus ca. 20 mm at Site B. The O2 concentration in the overlying bottom water at Site B is only about 10% less than at Site A, and should not, therefore, not have much influence over the depth of O2 penetration (Archer and Devol, 1992). The mean diffusive O2 uptake for Site A was 0.61( 0.38, 1s, n ¼ 3) mmol m2 d1, compared with the higher 1.18 mmol m2 d1 (n ¼ 2) rate for Site B. In addition, the thickness of the DBL was found to be greater at Site A than at Site B, which is consistent with the greater water depth at site A. Both these trends with water depth are not unexpected and are consistent with other Atlantic measurements (Glud et al., 1994a) and also with data from the Californian continental margin sediments (Jahnke et al., 1997). However, as noted by Glud et al. (1994b), the thickness of the DBL, d, is also a function of other variables such as water flow velocity, temperature, pressure and sediment roughness (Boudreau and Guinasso, 1982) and not simply water depth itself. The greater oxygen penetration at Site A may reflect the generally lower sediment organic content (measured over the top 1 cm) at Site A (Table 1). However, around 4–5 mm of phytodetrital ‘fluff’ was obvious in retrieved multi-cores at Site B, whereas only trace amounts were noticeable at Site A, and it is known that episodic inputs of organic material to the sediment– water interface can mediate the oxygen dynamics in the diffusive boundary layer (Smith et al., 1998). Oxic consumption of phytodetrital material will give rise to a greater diffusive flux of O2 which will diminish the depth of oxygen penetration (Cai and Sayles, 1996; Drazen et al., 1998), although some researchers find that this is not the case (e.g. Lohse et al., 1998; Duineveld et al., 1997), especially where the fluff is dominantly refractory in nature. The value of 1.56 mmol m2 d1 at Site B (Table 3) is notably high and approaches the values obtained by Glud et al. (1994a) on the organically enriched Congo river delta. It is possible that this is a result of the micro-electrode piercing a patch of phytodetritus, whilst an adjacent electrode may have missed the fluff altogether (Table 3). Direct and indirect observations have shown that phytodetritus is certainly patchily distributed (e.g. Rice et al., 1994; Tudhope and Scoffin, 1995). However, close inspection of the oxygen micro-profile reveals that it is rather noisy, which makes accurate discrimination of the thickness of the DBL difficult. Consequently, one may calculate a range of values for Joxy from 0.4 to 1.2 mmol m2 d1 with an average value of 0.8 mmol m2 d1. The average value seems low compared to that calculated from the other profile at Site B (Joxy ¼ 1:56 mmol m2 d1) even though the general shapes of the micro-profiles and the depth of oxygen penetration are very similar. This highlights the care that must be taken when interpreting micro-profile data. Given the importance of the thickness of the oxic zone in ocean sediments to organic carbon degradation, the distribution and fate of metals and metal oxides and the structure and distribution of sediment communities (Cai and Sayles, 1996), clearly further work is required to ascertain the exact nature of benthic oxygen dynamics during pre- and post-bloom periods in these environments.
872
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
Fig. 6 shows data from the chamber micro-electrode during a deployment of the benthic chamber module at Site A. The signal decreases after the lid is closed from an initial bottom water concentration value of 266 mmol to a minimum value of 256 mmol, indicating a total oxygen uptake of 0.13 mmol h1. Whilst we are unable to compute a total benthic O2 flux, the observed percentage reduction in oxygen content is within the range reported for other calcareous deep ocean sediments (e.g. Glud et al., 1994a; Smith et al., 1997). The low rate of O2 consumption has implications for the measurement of nutrient fluxes. The potential change in 3 concentration during the 44 h deployment can be estimated from the O2 NO 3 and PO4 concentration assuming Redfield stoichiometry DO2/DNO 3 =138/16=11/DNO3 , DNO3 =1.28 m 3 mol, and similarly DPO4 =11/138=0.08 mmol, which are close to the detection limit of the method. It is interesting to note that diffusive O2 uptake rates measured at the deep Site A compares favourably with measures of total in situ O2 uptake reported by Lampit et al. (1995) from a similar depth to the south of the Rockall Trough (Porcupine Abyssal Plain). This may suggest that oxygen dynamics are governed principally by the diffusion of oxygen/consumption at depth at the seabed rather than by bioirrigation in this region. This may be a function of the generally lower macrobenthic abundance at Site A. Lamont and Hughes (unpublished data) cite organism densities (>250 mm in size) of 3000 m2 for Site A versus 26000 m2 at the shallower Site B. In addition, evidence from x-ray visualisation of sediment fabric at Site A indicates a very low level of bioturbation, whereas biological activity within sediments at site B was notably greater with large (>1 cm diameter) polychaete and echiuran burrow openings in over 75% of box cores, and the surface sediments were replete with anemones, sponges, projecting forams, faecal casts and other biogenic structures. Both Tahey et al. (1994) and Lohse et al. (1998) report a similar situation attributable to reduced macrofaunal activity for a deep water Mediterranean location and an abyssal site on the Goban Spur, respectively. Glud et al. (1994a) also report likewise for
Fig. 6. Oxygen concentration over time measured using a calibrated mini electrode in the lid of the benthic chamber during a deployment at Site A, 3570 m water depth.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
873
one of their stations, although generally their total O2 uptake rates were 1.2–4.2 times the diffusive O2 uptake. 3.2. Nutrients The pore-water profile for both nitrate and phosphate indicates a flux from the sediments into the overlying bottom waters (Fig. 7). The concentration of nitrate decreases from a surface maximum in the upper-most sample analysed (0–0.5 cm) of 34.5 mmol to approximately 2.7 mmol at 17 cm depth. The gradient at the sediment–water interface generates an estimated flux for nitrate of 0.161 mmol m2 d1. The pore-water profile for phosphate displays an increase in concentration down-core also, reflecting the continuing remineralisation of organic matter. The phosphate gradient at the sediment–water interface generates an estimated flux of 0.0008 mmol m2 d1, which is three orders of magnitude lower than that for nitrate, but still
Fig. 7. Nitrate and phosphate data from Site B. (A) Phosphate and nitrate concentrations over time measured in situ using the benthic chamber, (B) nitrate pore-water profile, (C) phosphate pore-water profile measured from collected mega-cores and slicing and centrifugation.
874
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
implying a flux of phosphate into the overlying water column. The computed NO 3 fluxes is close to those calculated and measured by Lohse et al. (1998) in sites on the Goban Spur, north-west European Margin, at similar water depths but slightly south of the BENBO sites. For phosphate, the calculated flux is low compared to the range of 0.007–0.011 mmol m2 d1 measured by Lohse et al. (1998) during shipboard sediment incubations and lander deployments. Nitrate and phosphate show little or no change in concentration in the lander chamber over the period of deployment (54 h). The potential change in nitrate concentration in the water enclosed by the chamber can be estimated from the flux derived from the measured pore water gradient. The flux of 0.161 mmol m2 d1 into the area defined by the lander chamber is equivalent to 33 mmol of nitrate over the 54 h deployment. The net change in nitrate concentration in the overlying water depends on the degree of penetration of the lander chamber into sediment. The overall height of the chamber is 34 cm, and if the depth of penetration into the seabed were 20 cm, the overlying water volume would be 12.6 l and the net change in nitrate 2.6 mmol. However, the depth of penetration was approximately 10 cm, which gives an overlying water volume of 21.6 l and a consequent net change in nitrate of only 1.5 mmol. Thus, it is likely that the lack of any discernible change in nitrate concentration is due to the large volume of water overlying the sediment. Jahnke et al. (1997) report the same problem in chamber experiments off the central Californian slope and rise at a depth of 3700 m, wherein nitrate data are roughly constant in spite of a measurable decrease in chamber oxygen content and a relatively high benthic organic content (ca. 3%). This problem is undoubtedly site specific, and a more organically reactive site, such as the upwelling region off the coast of African studied by Glud et al. (1994a), would require only very short lander deployments. The only means of rectifying this on organic-poor sediments is to leave the lander on the sea-floor for a longer period. The experimental duration of such in situ chamber incubations is evidently a critical factor in determining benthic fluxes, and prior characterisation of the study area under consideration is thus an important factor in benthic solute flux studies.
4. Summary The Atlantic Margin is a new deep water frontier as yet undisturbed and unpolluted by man. Most oil industry activity in the region remains in the exploration phase, although projects in two fields are now in the production phase. The region is the focus of substantial collaboration between oil companies, public bodies and regulatory authorities to assess the impact of current and future exploration on the marine environment. The work described here contributes directly to providing quantitative environmental information about conditions at the seabed prior to anthropogenic disturbance, in particular regarding the oxygen demand of the system. The comparison of our O2 uptake data with data from the literature is encouraging, as the values for similar deep-water environments are of a similar magnitude to those at the BENBO sites. The data also highlight specific logistical problems, such as the length of time necessary for chamber incubations for nutrient flux studies, thereby helping to direct future similar studies. Although we have reported only preliminary findings from two experimental locations, our data together with other recent studies seem to support the idea that water depth, organic input, and benthic assemblages play a key role in benthic oxygen uptake in deep ocean sediments.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
875
Acknowledgements The authors would like to thank the Masters and crew members of the RRS Charles Darwin during the data collection phases of BENBO. Hilary Kennedy acknowledges the assistance and facilities provided by Ivan Ezzi and Ken Jones to perform the nutrient analyses. The authors also gratefully acknowledge the loan of various items of equipment by Dr. Brian Bett. This work forms part of the BENBO Thematic Research Programme of the Natural Environment ResearchCouncil, and was supported through grants to Hilary Kennedy GST/02/1754 and William Davison GST/02/1746. This is Publication Number 05 of the Thematic Research Programme BENBO.
References Archer, D., Emerson, S., Smith, C.R., 1989. Direct measurement of the diffusive sub-layer at the deep sea floor using micro-electrodes. Nature 340, 623–626. Archer, D., Devol, A., 1992. Benthic oxygen fluxes on the Washington shelf and slope: a comparison of in situ microelectrode and chamber flux measurements. Limnology and Oceanography 37 (3), 614–629. Armstrong, J.D., Bagley, P.M., Priede, I.G., 1992. Tracking deep-sea using ingestible transmitters and an autonomous seafloor instrument package. In: Priede, I.G., Swift, S.M. (Eds.), Wildlife Telemetry. Ellis Horwood, Chichester, pp. 376–386. Billett, D.S.M., Lampitt, R.S., Rice, A.L., Mantoura, R.C.F., 1983. Seasonal sedimentation of phytoplankton to the deep sea benthos. Nature 302, 520–522. Black, K.S., 1999. NERC News. Winter 1999 Issue, pp. 6–7. Boudreau, B.P., Guinasso Jr., N.L., 1982. The influence of a diffusive sub-layer on accretion, dissolution, and diagenesis at the sea floor. In: Fanning, K.A., Manheim, F.T. (Eds.), The Dynamic Environment at the Ocean Floor. Lexinton, Massachusetts, pp. 115–145. Broecker, W.S., Peng, T.H., 1974. Gas exchange rates between sea and air. Tellus 26, 21–35. Cai, W.-J., Sayles, F.L., 1996. Oxygen penetration depths and fluxes in marine sediments. Marine Chemistry 52, 123–131. Crank, J., 1983. The Mathematics of Diffusion. Calrendon Press, Oxford, 372pp. Davison, W., Fones, G.R., Harper, M., Teasdale, P., Zhang, H., 2000. Dialysis, DET and DGT: in situ diffusional techniques for studying water, sediments and soils, 495–569. In: Buffle, J., Horvai, G. (Eds.), In situ Monitoring of Aquatic Systems: Chemical Analysis and Speciation. John Wiley & Sons Ltd, New York. Drazen, J.C., Baldwin, R.J., Smith, K.L., 1998. Sediment community response to a temporally varying food supply at an abyssal station in the NE Pacific. Deep Sea Research 45 (4–5), 893–913. Duineveld, G.C.A., Lavaleye, M.S.S., Berhuis, E.M., deWilde, P.A.W.J., van der Weele, J., Kok, A., Batten, S.D., deLeeuw, J.W., 1997. Patterns of benthic fauna and benthic respiration on the Celtic continental margin in relation to the distribution of phytodetritus. International Revue der Gesamten Hydrobiologie 82 (3), 395–424. Gehlen, M., Rabouille, C., Ezat, U., Guidi-Guilvard, L.D., 1997. Drastic changes in deep-sea sediment pore-water composition induced by episodic input of organic matter. Limnology and Oceanography 42, 980–986. Glud, R.N., Gundersen, J.K., Jorgensen, B.B., Revsbech, N.P., Schulz, H.D., 1994a. Diffusive and total oxygen uptake of deep-sea sediments in the eastern South Atlantic Ocean: in situ and laboratory measurements. Deep-Sea Research 41, 1767–1788. Glud, R.N., Gundersen, J.K., Revsbech, N.P., Jorgensen, B.B., 1994b. Effects on the benthic diffusive boundary layer imposed by microelectrodes. Limnology and Oceanography 39, 462–467. Glud, R.N., Gundersen, J.K., Revsbech, N.P., Jorgensen, B.B., Huttel, M., 1995. Calibration and performance of the stirred flux chamber from the benthic lander Elinor. Deep-Sea Research 42, 1029–1042.
876
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
Jahnke, R.A., Craven, D.B., McCorkle, D.C., Reimers, C.E., 1997. CaCO3 dissolution in California continental margin sediments: the influence of organic matter remineralization. Geochimica Cosmochimica Acta 61, 3587–3604. Lampitt, R.S., 1985. Evidence for the seasonal deposition of detritus to the deep-sea floor and its subsequent resuspension. Deep Sea Research 32, 885–897. Lampitt, R.S., Raine, R.C.T., Billett, D.S.M., Rice, A.L., 1995. Material supply to the European continental slope: a budget based on oxygen demand and organic supply. Continental Shelf Research 42 (11/12), 1865–1897. Li, Y.H., Gregory, S., 1974. Diffusion of ions in deep-sea sediments. Geochimica et Cosmochimica Acta 38, 703–714. Lohse, L., Helder, W., Epping, E.H.G., Balzer, W., 1998. Recycling of organic matter along a shelf slope transect across the N W European Continental Margin (Goban Spur). Progress in Oceanography 42, 77–110. Newton, P.P., Lampitt, R.S., Jickells, T.D., King, P., Boutle, C., 1994. Temporal and spatial variability of biogenic particle fluxes during the JGOFS north-east Atlantic process studies at 478N, 208W. Deep Sea Research 41, 1617–1642. Patching, J.W., Raine, R.C.T., Barnett, P.R.O., Watson, J., 1986. Abyssal benthic oxygen consumption in the northeastern Atlantic: measurements using the suspended core technique. Oceanologica Acta 9 (1), 1–7. Pfannkuche, O., Boetius, A., Lochte, K., Lundgreen, U., Tiel, H., 1999. Responses of deep-sea benthos to sedimentation patterns in the north-east Atlantic in 1992. Deep-Sea Research 46 (4), 573–596. Rasmussen, H., Jorgensen, B.B., 1992. Micro-electrode studies of seasonal oxygen uptake in a coastal sediment: role of molecular diffusion. Marine Ecology Progress Series 81, 289–303. Reimers, C.E., 1987. An in situ micro-profiling instrument for measuring interfacial pore water gradients: methods and oxygen profiles from the North Pacific Ocean. Deep Sea Research 34, 2019–2035. Reimers, C.E., Fischer, K.M., Merewether, R., Smith, K.L., Jahnke, R.A., 1986. Oxygen micro-profiles measured in situ in deep ocean sediments. Nature 320, 741–744. Revsbech, N.P., 1989. Diffusion characteristics of microbial communities determined by use of oxygen microsensors. Journal of Microbial Methods 9, 111–122. Rice, A.L., Thurston, M.H., Bett, B.J., 1994. The IOSDL DEEPSEAS Programme: photographic evidence for the presence and absence of a seasonal input of phyto-detritus at contrasting abyssal sites in the north-eastern Atlantic Ocean. Deep Sea Research 41, 1305–1320. Sayles, F.L., Mangelsdorf, R.C., Wilson, T.R.S., Hume, D.N., 1976. A sampler for the in situ collection of marine sedimentary pore waters. Deep Sea Research 23, 259–264. Sayles, F.L., 1992. Biogeochemical processes on the seafloor Oceanus 35 (1), 68–75. Smith, K.L., Gatts, R.C., Baldwin, R.J., Beaulieu, S.E., Uhlman, A.H., Horn, R.C., Reimers, C.E., 1997. An autonomous, bottom-transecting vehicle for making long time-series measurements of sediment community oxygen consumption to abyssal depths. Limnology and Oceanography 42, 1601–1612. Smith, K.L., Baldwin, R.J., Glatts, R.C., Kaufman, R.S., Fisher, E.C., 1998. Detrital aggregates on the sea floor: chemical composition and aerobic decomposition rates at a time-series in the abyssal NE Pacific. Deep-Sea Research 45 (4–5), 843–880. Strickland, J.P.H., Parsons, T.R., 1972. A practical handbook of seawater analysis. Bulletin of the Fisheries Research Board of Canada 167, 711. Sweerts, J.P.R.A., St Louis, V., Cappenberg, T.E., 1989. Oxygen concentration profiles and exchange in sediment cores with circulated overlying water. Freshwater Biology 21, 552–556. Tahey, T.M., Duineveld, G.C.A., Berghuis, E.M., Helder, W., 1994. Relation between sediment-water fluxes of oxygen and silicate and faunal abundance at continental shelf, slope and deep water stations in the NW Mediterranean. Marine Ecology Progress Series 104 (1–2), 119–130. Tengberg, A., de Bovee, F., Hall, P., Berelson, W., Chadwick, D., Ciceri, G., Crassous, P., Devol, A., Emerson, S., Gage, J., Glud, R., Graziottini, F., Gunderson, J., Hammond, D., Helder, W., Hinga, K., Holby, O., Jahnke, R., Khripounoff, A., Lieberman, A., Lieberman, S., Nuppenau, V., Pfannkuche, O., Riemers, C., Rowe, G., Sahami, A., Sayles, F., Schurter, M., Smallman, D., Wehrli, B., de Wilde, P., 1995. Benthic chamber and profiling landers in oceanography: a review of design, technical solutions and functioning. Progress in Oceanography 35, 253–294.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
877
Thomsen, L., Graf, G., Martens, V., Steen, E., 1994. An instrument for sampling water from the benthic boundary layer. Continental Shelf Research 14, 871–882. Tudhope, A.W., Scoffin, T.P., 1995. Processes of sedimentation in Gollum Channel, in Porcupine Seabight } submersible observations and sediment analyses. Transactions of the Royal Society of Edinburgh (Earth Sciences) 86 (1), 49–55. Usui, T., Koike, I., Ogura, N., 1998. Vertical profiles of nitrous oxide and dissolved oxygen in marine sediments. Marine Chemistry 59, 253–270.
An autonomous benthic lander: preliminary observations from the UK BENBO thematic programme Kevin S. Blacka,*, Gary R. Fonesb, Oli C. Peppea, Hilary A. Kennedyc, Ilhem Bentalebd a
Dunstaffnage Marine Laboratory, Oban, Argyll, Scotland, PA34 4AD, UK b Wood Hole Oceanographic Institution, Wood Hole, MA 02453, USA c School of Ocean Sciences, University of Wales, Bangor, Anglesey, Wales, LL59 5EY, UK d Lab. Phyloge´nie, Institut des Sciences de L’Evolution de Montpellier, Pale´ontologie, Pale´obiologie, CC064 UM2, 34095 Montpellier Cedex 5, France Received 5 May 1999; accepted 9 November 1999
Abstract A new, multi-purpose autonomous benthic lander is described, and preliminary experimental data are presented relating to deployments in the Atlantic Frontier (eastern north Atlantic) during the recent UK Thematic Programme ‘BENBO’. The autonomous lander was deployed at two contrasting sites } Site A (mouth of Rockall Trough; 3570 m) and Site B (Hatton-Rockall Bank; 1100 m) } before and following the spring-time surface ocean phytoplankton bloom (May & July, 1998, respectively). Diffusive oxygen uptake and nutrient flux data were obtained using two interchangeable modules } a profiling oxygen micro-electrode unit and a benthic chamber unit. Diffusive O2 uptake across the sediment–water interface and the O2 penetration depths within the sediment were determined from the oxygen micro-profiles. The shallower site, which had previously received phyto-detrital input, had a comparatively large diffusive oxygen uptake within the sediment (1.2 mmol m2 d1) and a maximum penetration depth of only 21 mm. The deeper site had greater oxygen penetration depths ( 80 mm) but a lower diffusive oxygen uptake of 0.6 mmol m2 d1, indicative possibly of little or no phyto-detrital input. Visual observations of retrieved sediment cores support this conclusion, however Site A has also displays generally lower organic content and lower macrobenthic biomass which may contribute to this observation. Nutrient pore water profile data indicated fluxes of nitrate of 0.161 mmol m2 d1 and phosphate 0.0008 mmol m2 d1 into the overlying water. However, the benthic chamber studies showed virtually no change in nutrient
*Corresponding address: School of Environmental and Evolutionary Biology, University of St Andrews, Gatty Marine Laboratory, St Andrews, Fife ky169LB, UK. Tel.: +44-01334-463441; fax: +44-01334-463443. E-mail address: [email protected] (K.S. Black). 0278-4343/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 0 0 ) 0 0 1 1 6 - 3
860
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
concentrations, due probably to the relatively short deployment time used. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Oxygen; Nutrients; Sediments; Benthic lander; North Atlantic; Fluxes
1. Introduction Deep sea sediments form the world’s largest and deepest ecosystem of the order 300 million km2 in extent. To date less than 0.001% of deep ocean sediments have been characterised, in either biological or chemical terms, and thus our knowledge and understanding of the functioning of these depositional environments is at best rudimentary. One of the most dramatic discoveries associated with the deep ocean in recent years has been that of the seasonal deposition of rapidly sinking phyto-detritus or ‘marine snow’ on the deep sea-floor (Lampitt, 1985; Newton et al., 1994). This material, which is usually organically enriched, is utilised by benthic communities (Smith et al., 1998; Drazen et al., 1998; Pfankucke et al., 1999) and fuels the cycling of important biogeochemically active elements (e.g. C, N, S, O2) within the surficial sediment layers. Phytodetritus has been shown to mediate the flux of pore-water solutes (e.g. NO 3 ), and is also important in the cycling of redox metals (e.g. Mn2+; Gehlen et al., 1997). The north-east Atlantic, in particular, is a region of intense seasonality with distinct changes in surface primary production across large geographic distances giving rise to large fluctuations in mid-water and benthic particle flux (Rice et al., 1994). In the past, efforts to study benthic processes in the open ocean in detail have been based upon collection of seabed sediments using a variety of coring devices. Fluxes have subsequently been determined by direct incubation studies on the ship or at land-based laboratories, or have been calculated from pore-water gradients which have been determined by traditional pore-water analysis (e.g. Usui et al., 1998). However, as noted by Sayles et al. (1976) some 20 years ago, retrieval of sediments using conventional means can sometimes give rise to artifacts associated with physical disturbance of the sediments cf. the large decrease in hydrostatic pressure and temperature increase as they are brought to the surface. The latter is especially relevant for very deep ocean sediments (>3000 m; Lampitt et al., 1995) and has been shown recently to disrupt sedimentary oxygen dynamics (Reimers, 1987; Glud et al., 1994a). Technological advances in recent years have more or less circumvented the problems associated with sample retrieval as they permit direct experimentation actually on the seabed. The Woods Hole Interstitial Marine Probe (WHIP), for example, was designed to overcome sampling problems by collecting sedimentary pore water in situ (Sayles, 1992). A relatively new family of scientific instruments called ‘benthic landers’ are being used to deploy sensors in the ocean. Landers are autonomous, unmanned oceanographic research vehicles that free-fall to the sea floor unattached to any cable, and then operate independently according to a series of pre-programmed instructions. At the end of a deployment, ballast weights are released on an acoustic command transmitted from the surface or by a timing device. Landers are being increasingly used to study a
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
861
wide variety of phenomena in the marine environment, including high-resolution near-bed water characterisation (Thomsen et al., 1994), observation of fish behaviour (Armstrong et al., 1992) and time-lapse photography of seabed benthic activity (Billett et al., 1983; Rice et al., 1994). Tengberg et al. (1995) comprehensively reviewed the technical history and operation of 27 lander instruments developed to date. This paper describes a new lander based upon the ELINOR instrument of Glud et al. (1995), and presents some preliminary oxygen and nutrient flux data which were collected at a number of locations in the eastern north Atlantic as part of the recent research programme ‘BENBO’.
1.1. The benthic boundary programme (BENBO) The Atlantic Frontier to the west of Scotland and Ireland is a comparatively pristine marine environment. However, the area is increasingly viewed as potentially rich in hydrocarbon reserves, with prospects for large to medium-sized oil discoveries, and currently licenses have been issued by the UK government (1997 17th Round) to explore just under 25,000 km2 of the Atlantic margin region. Although the region presents greater technical challenges to drilling, deepwater production technology can now operate in the severest metocean conditions and in depths in excess of 1000 m. As part of a wider recognition of the importance of the Atlantic margin to seabird and sea mammal stocks, benthic biodiversity and coastal protection, and respecting recent international regulatory legislation from the IMO (International Maritime Organisation) and OSPAR (the Oslo and Paris Commission), the various exploration companies have formed a co-ordinated, strategic approach to environmental management through the setup of the Atlantic Frontier Environment Network (AFEN). AFEN commissions independent scientific research programmes directed towards describing and understanding the existing environment, in particular the fate and impact of marine snow as well as certain anthropogenic contaminants, identifying key sensitivities, and monitoring and developing protection measures. It programmes such as these which provide a crucially important environmental baseline, and the data can be used to shape industry activities to help ensure that they have at worst no more than a minimal effect on the marine environment. The BENBO study is a multi-disciplinary, inter-institutional marine benthic biogeochemistry programme allied to AFEN, but funded directly by the UK Natural Environment Research Council (NERC). The underlying philosophy of BENBO is identical to that of AFEN, however the scientific content is more experimental in nature than regional and descriptive cf. the AFEN I and II seabed surveys. The principal objective of BENBO is to investigate and quantify the biophysical and biogeochemical processes occurring at the deep ocean bed as a result of the sedimentation of marine snow. This was achieved through the funding of nine projects ranging from organic and inorganic adsorption processes onto mineral surfaces to bacterial consumption of marine snow to diagenesis and storage times-scales at the deep ocean bed (refer to Black, 1999, for further details). Two process cruises were mounted on the UK research vessel RRS Charles Darwin, and data and samples were collected either side of the spring phytoplankton bloom. These cruises were CD111 20 April–15 May, 1998, and CD113 28 June–22 July, 1998.
862
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
2. Methodology 2.1. The benthic lander instrument The BENBO benthic lander, which was designed and constructed in collaboration with a commercial engineering company (KC Denmark), is shown in Figs. 1 and 2. The lander incorporates a number of interchangeable modules. These include (i) an enclosed chamber for determination of total benthic community respiration and solute flux, (ii) a micro-profiling module which can insert up to four oxygen micro-electrodes and two pH micro-electrodes into the sediment to measure pore water concentrations at very high spatial resolution ( 50 mm intervals),
Fig. 1. The BENBO benthic lander instrument. The drawing shows the oxygen chamber module in the lower frame: (F) feet; (C) titanium chamber; (S) hydraulic shovel; (B) ballast tin; (Co) computer housing and control electronics; (Ba) battery; (RM) acoustic/burn-wire release mechanism; (TL) telescopic legs; (P) acoustic pinger; (BS) buoyancy spheres; (SB) satellite beacon. Strobe, flag and pellet line not shown.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
863
Fig. 2. Photograph of the benthic lander with the oxygen micro-profiling module during deployment.
and (iii) a unit to insert DGT (diffusive gradient in thin films) and DET (diffusive equilibration in thin films) gel probes (Davison et al., 2000) through the sediment–water interface to measure fluxes of trace metals and concentrations of major ions at a spatial resolution of 100 mm to 1.5 mm. The gel probe unit is not described here. The lander comprises two open triangular frames clamped together one on top of the other. One of three self-contained scientific modules (chamber/oxygen micro-profiler/gel profiler) is bolted into the lower instrumentation frame and the upper frame can hold up to nine 17’’ BenthosTM buoyancy spheres which provide the floatation required to bring the lander back to the surface at the end of the deployment. Both frames are made of 2.5 cm2 aluminium piping and are approximately 1.5 m high. Adjustable telescopic feet at the corners of the lower frame allow the
864
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
height of the instrumentation above the seabed to be varied by up to 0.5 m. Circular platforms at the bottom of the feet limit sinking of the frame into unconsolidated muds. Miniature (1.5 cm diameter) core tubes are mounted to these plates to assess how far the feet have sunk into the sediment and the tilt of the lander. The disposable ballast, in the form of steel shot contained in bins or scrap steel circular discs, is suspended within the corners of the instrumentation frame. The number of buoyancy spheres used is adjusted so that the net positive buoyancy on ascent is approximately 75–100 kg. Sufficient ballast is then added to make the net negative buoyancy on descent approximately 50 kg. This configuration gives approximate descent and ascent rates of 50–60 and 60–70 m min1, respectively. A 10 kHz acoustic pinger is attached to the frame to allow the descent and ascent of the lander to be tracked from the ship using a standard echo sounder. Once the bottom experiment is completed the ballast is dropped by triggering an InterOceanTM acoustic release and the lander returns to the surface. A computer-controlled burn-wire provides back-up for the ballast trigger in the event of acoustic release failure. 2.2. Microprocessor and lander electronics All lander operations are controlled by a central computer housed in a 6000 m rated pressure housing. This computer is programmed prior to deployment with the required experimental routine using a custom-designed PC-based software interface. The computer logs all sensor data during the deployment and this data is downloaded to a PC on recovery of the lander. A 12 V DeepSea Power & LightTM lead-acid battery provides power to the main lander systems. However, the acoustic release and various beacons critical to recovering the lander safely are powered independently. 2.3. Chamber module The BENBO chamber is a 30 30 cm open-ended rectangular box manufactured from inert titanium and coated inside with teflon sheet. The corners are rounded to minimise turbulence and reduce stagnation when the water is stirred (Glud et al., 1995), and a hydraulically operated shovel slides beneath the box to recover the sediments prior to lander ascent. The chamber is designed to penetrate into the bottom sediments synchronously as the instrument lands on the seabed (thereby minimising physical disturbance), and after a pre-set time interval the lid is closed by triggering a burn wire, thus enclosing a volume of bottom water. A polycarbonate cruciform blade magnetically coupled to a variable speed pressure compensated oil-filled motor may be used to stir the water within the chamber. A polarographic Clarke-type robust mini-electrode is used to measure oxygen within the chamber. In order to calibrate the sensor against ambient bottom water, water samples are taken in triplicate using a miniature 250 cm3 Niskin bottle-type sampling instrument which is activated when the ballast is released at the end of the deployment. O2 concentration is determined directly on board ship (following Strickland and Parsons, 1972) using a commercial micro-Winkler titration instrument (MetrohmTM 702 SM Titrino). A pH minisensor is co-located with the oxygen sensor, with the reference electrode positioned on the outer casing of the electronics housing. During deployment, up to fifteen 60 cm3 water samples may be withdrawn through teflon tubing from the chamber using a bank of spring-loaded syringes mounted above the chamber. A
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
865
small bi-directional valve in the lid permits ambient benthic water to compensate for the volume of water withdrawn from the chamber during sampling. In each rack, one of the syringes may be used to inject solutes into the chamber rather than withdraw water from the chamber. As the net volume of water in the chamber cannot be established precisely (due to variable lander penetration into sediments), injection of inert substances such as Br have been used to determine enclosed water volume. 2.4. Oxygen micro-profiling module The vertical profile of oxygen in marine sediments has been used for some years as a method of determining the diffusive oxygen uptake, as well as respiration in the upper oxic zone (Glud et al., 1994a). The BENBO micro-profiling module, which is based on the design of Reimers (1987), is designed to measure the vertical profile of sediment oxygen content at very high resolution (typically every 250 mm, but every 25 mm is possible) within the surface layers of oceanic muds. The module consists of a profiling rack into which the electronics housing is fitted vertically. The micro-electrodes (up to four oxygen and two pH) are attached directly to the bottom of the housing, and the whole unit is raised and lowered at a maximum speed of 20 mm min1 by a worm gear driven by a pressure-compensated oil-filled marine motor. An optical sensor on the motor allows the height of the electrodes relative to the zero position to be measured with a resolution of 25 mm. The oxygen micro-electrodes used were Clark-type electrodes with a guard cathode (Revsbech, 1989). The total length of the electrode, which determines the maximum insertion depth, is approximately 80 mm. 2.5. Study area Three contrasting areas of seabed around the Rockall Trough region in the north-east Atlantic, were chosen as the BENBO study sites (Fig. 3), although we report data from two sites only here (sites A and B). The Rockall Trough is a deep-water basin at the foot of the continental slope west of Ireland extending from about 538N to 608N. The trough trends NE–SW and ranges in depth from ca. 2000 m at its northern end to ca. 4000 m at the mouth. The Trough is bound to the west by the Rockall plateau, comprising the shallow (1100–1200 m) Hatton Bank, Rockall Bank and the Hatton–Rockall Basin, and to the north by the Wyville–Thompson ridge. Two seamounts, the Anton–Dohrn and the Rosemary Bank, are found at the northern end of the Trough. The sites comprise carbonate ooze with variable amounts of detrital silt and clay, but have differing sediment properties and differing hydrographic regimes (Table 1). 2.6. In situ measurements 2.6.1. Oxygen profiles In situ oxygen profiles were obtained during three separate deployments at the BENBO sites (Table 2). The procedure used for obtaining oxygen profiles at each of the BENBO sites was as follows. Once the lander had reached the seabed there was a time interval of at least an hour before the profiling was started to allow sediment disturbed during the landing to settle out. After this period the micro-electrodes were inserted gradually into the sediments in 250 mm steps. The
866
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
Fig. 3. Location of three sampling locations (A, B and C) in the BENBO field study area, eastern north Atlantic.
Table 1 Characteristics of the study sites
Latitude Longitude Water depth (m) Bottom water salinity Bottom water temp. (8C) Bottom water O2 (mmol) Median particle size (mm) Wet bulk density (kgm3)a Organic content (%)a Water content (% wet wt.) Carbonate content (%)a Composition a
Site A
Site B
52854.410 N 16854.570 W 1100 34.89 2.15 265 34.73 1640 0.23 60 76.07 Carbonate ooze
57823.310 N 15842.640 W 3570 34.92 6.05 238 12.73 1840 0.66 61 75.20 Carbonate ooze
From Thomson (unpublished data). Measured over the top centimetre.
sensors were allowed to equilibrate at each depth for 10 s before data were logged at a rate of 1 Hz from each of five electrodes over a period of 5 s. Two methods of calibration were undertaken for the oxygen micro-electrodes. In the first method a two-point linear calibration of the micro-electrodes for oxygen concentrations was performed using the constant reading recorded in the anoxic part of the sediment and the
867
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
Table 2 Summary of sampling information for chamber O2 and nutrients, O2 micro-profiling, and pore-water sub-sampling from retrieved mega-cores Deployment
Cruise
Site
Date
Chamber O2 #005 #006
111 111
A B
03/05/98–07/05/98 09/05/98–12/05/98
O2 micro-profiles #004 111 #010 113 #012 113
A B A
01/05/98–02/05/98 02/07/98–03/07/98 06/07/98–07/07/98
Chamber nutrients 54403#03 111 54403#17 111 54407#01 111 57703#17 118
A A B A
01/05/98 03/05/98 10/05/98 09/07/98
Pore waters 54403#13 54404#08 54407#03 54702#08 54703#01 54706#6
A A B B A B
02/05/98 07/05/98 10/05/98 03/07/98 06/07/98 18/07/98
111 111 111 113 113 113
Deployment duration (h)
Latitude
Longitude
Water depth (m)
98 54
52854.240 N 57823.310 N
16854.400 W 15842.640 W
3570 1103
9.50 10.2 9.50
52854.410 N 54824.510 N 52854.320 N
16854.570 W 15844.590 W 16854.350 W
3573 1101 3357
9.30 98 54 129
52854.260 N 52854.240 N 57823.180 N 52855.470 N
16854.420 W 16854.240 W 16842.530 W 16853.010 W
3573 3368 1104 3560
52 52 57 57 52 57
16 16 15 15 16 15
3570 3576 1105 1104 3560 1094
55.01 55.21 25.25 24.25 54.36 24.24
53.31 53.00 44.05 44.31 54.05 41.31
overlying water oxygen concentration (obtained from the bottom water samples collected using the Niskin-type water bottle sampler). The second method was on board ship before and after deployment to ensure the electrodes were registering a linear response and to enable a zero point to be calculated for the profiles that did not reach a constant reading in situ. The electrodes were calibrated using bottom water at the in situ temperature and flushed thoroughly with oxygen and nitrogen, respectively, to create end-member conditions. 2.6.2. Chamber incubations Benthic chamber deployments for measurements of O2 and nutrient fluxes were undertaken during four separate deployments at two of the BENBO sites (Table 2). The electrodes are calibrated before and after deployment using the same technique as for the profiling microelectrodes. To obtain a two-point calibration, bottom water collected by the Niskin bottles and overlying water from the chamber are measured using the micro-titrator. 2.6.3. Nutrient data Measurements of phosphate (PO3 4 ) and nitrate (NO3 ) within the chamber were complemented by analyses made in sediment pore-waters of retrieved sediments during
868
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
each of the BENBO cruises (Table 2). Pore-waters were sampled by sectioning mega-core tubes under a nitrogen atmosphere and at in situ temperatures followed by separation by centrifugation and filtration through GF/F glass-fibre filters. The samples were stored in frozen plastic tubes prior to analysis. Dissolved nitrate and soluble reactive phosphate were determined using a LachatTM Quickhem 8000 flow injection analyser (Lachat Instruments Inc. Milwaukee, WI, USA). 2.7. Calculations 2.7.1. Oxygen The diffusive O2 uptake (Joxy ), O2 penetration depth (L) and the diffusive boundary layer (DBL) thickness (d) were calculated for three deployments from the measured oxygen microprofiles (Fig. 4). The O2 penetration depth at Site B could be determined directly from the profiles as the depth where the signal of the O2 microelectrode reached a constant, low or zero reading.
Fig. 4. In situ O2 micro-profiles measured with a spatial resolution of 250 mm at two BENBO study sites in the northeast Atlantic.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
869
However, at Site A where the O2 penetration exceeded 60 mm in the sediment, the depth was estimated by linear extrapolation of the last part of the measured profiles. These estimated penetration depths therefore represent minimum values. The position of the sediment surface was determined from the change in oxygen concentration gradient due to impeded diffusion (Revsbech, 1989; Sweerts et al., 1989). The in situ diffusive O2 uptake (Joxy ; mmol m2 d1) was calculated from the O2 micro-profiles by Fick’s first law of diffusion (1) and the linear concentration gradient measured in the DBL (Crank, 1983; Rasmussen and Jorgensen, 1992) dC ; ð1Þ Joxy ¼ Do dx where Do is the diffusion coefficient of O2 in seawater (Broecker and Peng, 1974) recalculated for in situ temperature with the Stokes–Einstein equation (Li and Gregory, 1974), and x is vertical distance. This method allows calculation of the diffusive O2 uptake without an estimate of the diffusive characteristics within the sediment. Unfortunately the total O2 uptake could not be calculated, as the volume of water in the benthic chamber was not determined. 2.7.2. Nutrients Time-series of PO3 4 and NO3 concentrations from the lander chamber and profiles of sediment pore-water were undertaken at site B (Cruise 111, 10 May, 1998) and are presented here as examples of preliminary results. The trend in concentration through time (decreasing or increasing) in the lander chamber may be used directly to infer the direction and magnitude of the benthic flux of solutes. As a direct comparison, the pore-water gradients of PO3 4 and NO3 at the sediment water interface have also been used to compute a benthic flux. The benthic nutrient flux Jnutr was calculated using @C x ¼ 0; ð2Þ Jnutr ¼ fDs @x where an average porosity was estimated as f ¼ 0:8, Ds is the diffusion coefficient corrected for tortuosity, and ð@C=@xÞjx ¼ 0 is the concentration gradient at the sediment–water interface obtained from the first derivative of fitted model curves at x ¼ 0. 3. Results and discussion 3.1. Oxygen Five in situ O2 profiles obtained by the profiling system at two BENBO sites (A & B) are presented in Fig. 4. Table 3 summarises the diffusive O2 uptake, DBL thickness ðdÞ and O2 penetration depths from each of the lander deployments. To the authors’ knowledge, no O2 micro-profiles at high resolution have been reported from the eastern north Atlantic, only total O2 consumption rates (e.g. Patching et al., 1986; Lampitt et al., 1995). The majority of previous in situ studies of diffusive oxygen uptake and oxygen penetration depths have been conducted in deep ocean sediments from the south and west Atlantic Ocean and the Pacific ocean (e.g. Reimers et al., 1986; Reimers, 1987; Archer et al., 1989; Glud et al., 1994a). The present data therefore
870
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
widen the global coverage of diffusive O2 uptake in deep-sea sediments and provide an important contribution to determining the seasonal and spatial changes in O2 fluxes in Atlantic Frontier region. Fig. 5 presents a detailed O2 profile over only 4 mm from site A and illustrates the change in slope of the O2 gradient at the sediment surface. It clearly shows the linear O2 gradient through the determined diffusive boundary layer (DBL), from which the diffusive O2 uptake, Joxy , is
Table 3 Oxygen concentration in the bottom water (O2), diffusive O2 uptake (Joxy ), O2 penetration depth (L) and diffusive boundary layer (DBL) thickness (d) at the BENBO field sites Deployment Number
Site
Electrode number
Bottom water [O2] (mmol)
Diffusive O2 flux, Joxy (mmol m2 d1)
O2 Penetration depth, L (mm)
DBL thickness, d (mm)
#004 #004 #010 #010 #012
A A B B A
1 6 16 3 3
265 270 234 243 264
1.01 0.25 0.80a 1.56 0.56
78 85 25 21 80
750 750 625 625 750
a
Electrode micro-profile noisy; possible range of 0.4–1.2 mmol m2 day1.
Fig. 5. Detail of an O2 micro-profile, Site A (July, 1998). The sediment surface and upper boundary of the diffusive boundary layer (DBL) are indicated by the line at depth zero and the dashed line, respectively.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
871
calculated. The calculated DBL thickness, d, in this example is 750 mm, and the diffusive O2 uptake is 0.56 mmol m2 d1. Glud et al. (1994b), however, have shown that diffusive O2 uptake values may be underestimated by 25–45% when measured using micro-electrodes, owing to perturbation of the flow field around the sensor. Our data may therefore represent minimum estimates of diffusive O2 uptake. Viewed collectively, the data set indicates that O2 penetrates much farther into the sediments at the deepest site A ( 3500 m) than in sediments of the shallower site B ( 1100 m). A mean O2 penetration depth of ca. 80 mm was measured for Site A versus ca. 20 mm at Site B. The O2 concentration in the overlying bottom water at Site B is only about 10% less than at Site A, and should not, therefore, not have much influence over the depth of O2 penetration (Archer and Devol, 1992). The mean diffusive O2 uptake for Site A was 0.61( 0.38, 1s, n ¼ 3) mmol m2 d1, compared with the higher 1.18 mmol m2 d1 (n ¼ 2) rate for Site B. In addition, the thickness of the DBL was found to be greater at Site A than at Site B, which is consistent with the greater water depth at site A. Both these trends with water depth are not unexpected and are consistent with other Atlantic measurements (Glud et al., 1994a) and also with data from the Californian continental margin sediments (Jahnke et al., 1997). However, as noted by Glud et al. (1994b), the thickness of the DBL, d, is also a function of other variables such as water flow velocity, temperature, pressure and sediment roughness (Boudreau and Guinasso, 1982) and not simply water depth itself. The greater oxygen penetration at Site A may reflect the generally lower sediment organic content (measured over the top 1 cm) at Site A (Table 1). However, around 4–5 mm of phytodetrital ‘fluff’ was obvious in retrieved multi-cores at Site B, whereas only trace amounts were noticeable at Site A, and it is known that episodic inputs of organic material to the sediment– water interface can mediate the oxygen dynamics in the diffusive boundary layer (Smith et al., 1998). Oxic consumption of phytodetrital material will give rise to a greater diffusive flux of O2 which will diminish the depth of oxygen penetration (Cai and Sayles, 1996; Drazen et al., 1998), although some researchers find that this is not the case (e.g. Lohse et al., 1998; Duineveld et al., 1997), especially where the fluff is dominantly refractory in nature. The value of 1.56 mmol m2 d1 at Site B (Table 3) is notably high and approaches the values obtained by Glud et al. (1994a) on the organically enriched Congo river delta. It is possible that this is a result of the micro-electrode piercing a patch of phytodetritus, whilst an adjacent electrode may have missed the fluff altogether (Table 3). Direct and indirect observations have shown that phytodetritus is certainly patchily distributed (e.g. Rice et al., 1994; Tudhope and Scoffin, 1995). However, close inspection of the oxygen micro-profile reveals that it is rather noisy, which makes accurate discrimination of the thickness of the DBL difficult. Consequently, one may calculate a range of values for Joxy from 0.4 to 1.2 mmol m2 d1 with an average value of 0.8 mmol m2 d1. The average value seems low compared to that calculated from the other profile at Site B (Joxy ¼ 1:56 mmol m2 d1) even though the general shapes of the micro-profiles and the depth of oxygen penetration are very similar. This highlights the care that must be taken when interpreting micro-profile data. Given the importance of the thickness of the oxic zone in ocean sediments to organic carbon degradation, the distribution and fate of metals and metal oxides and the structure and distribution of sediment communities (Cai and Sayles, 1996), clearly further work is required to ascertain the exact nature of benthic oxygen dynamics during pre- and post-bloom periods in these environments.
872
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
Fig. 6 shows data from the chamber micro-electrode during a deployment of the benthic chamber module at Site A. The signal decreases after the lid is closed from an initial bottom water concentration value of 266 mmol to a minimum value of 256 mmol, indicating a total oxygen uptake of 0.13 mmol h1. Whilst we are unable to compute a total benthic O2 flux, the observed percentage reduction in oxygen content is within the range reported for other calcareous deep ocean sediments (e.g. Glud et al., 1994a; Smith et al., 1997). The low rate of O2 consumption has implications for the measurement of nutrient fluxes. The potential change in 3 concentration during the 44 h deployment can be estimated from the O2 NO 3 and PO4 concentration assuming Redfield stoichiometry DO2/DNO 3 =138/16=11/DNO3 , DNO3 =1.28 m 3 mol, and similarly DPO4 =11/138=0.08 mmol, which are close to the detection limit of the method. It is interesting to note that diffusive O2 uptake rates measured at the deep Site A compares favourably with measures of total in situ O2 uptake reported by Lampit et al. (1995) from a similar depth to the south of the Rockall Trough (Porcupine Abyssal Plain). This may suggest that oxygen dynamics are governed principally by the diffusion of oxygen/consumption at depth at the seabed rather than by bioirrigation in this region. This may be a function of the generally lower macrobenthic abundance at Site A. Lamont and Hughes (unpublished data) cite organism densities (>250 mm in size) of 3000 m2 for Site A versus 26000 m2 at the shallower Site B. In addition, evidence from x-ray visualisation of sediment fabric at Site A indicates a very low level of bioturbation, whereas biological activity within sediments at site B was notably greater with large (>1 cm diameter) polychaete and echiuran burrow openings in over 75% of box cores, and the surface sediments were replete with anemones, sponges, projecting forams, faecal casts and other biogenic structures. Both Tahey et al. (1994) and Lohse et al. (1998) report a similar situation attributable to reduced macrofaunal activity for a deep water Mediterranean location and an abyssal site on the Goban Spur, respectively. Glud et al. (1994a) also report likewise for
Fig. 6. Oxygen concentration over time measured using a calibrated mini electrode in the lid of the benthic chamber during a deployment at Site A, 3570 m water depth.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
873
one of their stations, although generally their total O2 uptake rates were 1.2–4.2 times the diffusive O2 uptake. 3.2. Nutrients The pore-water profile for both nitrate and phosphate indicates a flux from the sediments into the overlying bottom waters (Fig. 7). The concentration of nitrate decreases from a surface maximum in the upper-most sample analysed (0–0.5 cm) of 34.5 mmol to approximately 2.7 mmol at 17 cm depth. The gradient at the sediment–water interface generates an estimated flux for nitrate of 0.161 mmol m2 d1. The pore-water profile for phosphate displays an increase in concentration down-core also, reflecting the continuing remineralisation of organic matter. The phosphate gradient at the sediment–water interface generates an estimated flux of 0.0008 mmol m2 d1, which is three orders of magnitude lower than that for nitrate, but still
Fig. 7. Nitrate and phosphate data from Site B. (A) Phosphate and nitrate concentrations over time measured in situ using the benthic chamber, (B) nitrate pore-water profile, (C) phosphate pore-water profile measured from collected mega-cores and slicing and centrifugation.
874
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
implying a flux of phosphate into the overlying water column. The computed NO 3 fluxes is close to those calculated and measured by Lohse et al. (1998) in sites on the Goban Spur, north-west European Margin, at similar water depths but slightly south of the BENBO sites. For phosphate, the calculated flux is low compared to the range of 0.007–0.011 mmol m2 d1 measured by Lohse et al. (1998) during shipboard sediment incubations and lander deployments. Nitrate and phosphate show little or no change in concentration in the lander chamber over the period of deployment (54 h). The potential change in nitrate concentration in the water enclosed by the chamber can be estimated from the flux derived from the measured pore water gradient. The flux of 0.161 mmol m2 d1 into the area defined by the lander chamber is equivalent to 33 mmol of nitrate over the 54 h deployment. The net change in nitrate concentration in the overlying water depends on the degree of penetration of the lander chamber into sediment. The overall height of the chamber is 34 cm, and if the depth of penetration into the seabed were 20 cm, the overlying water volume would be 12.6 l and the net change in nitrate 2.6 mmol. However, the depth of penetration was approximately 10 cm, which gives an overlying water volume of 21.6 l and a consequent net change in nitrate of only 1.5 mmol. Thus, it is likely that the lack of any discernible change in nitrate concentration is due to the large volume of water overlying the sediment. Jahnke et al. (1997) report the same problem in chamber experiments off the central Californian slope and rise at a depth of 3700 m, wherein nitrate data are roughly constant in spite of a measurable decrease in chamber oxygen content and a relatively high benthic organic content (ca. 3%). This problem is undoubtedly site specific, and a more organically reactive site, such as the upwelling region off the coast of African studied by Glud et al. (1994a), would require only very short lander deployments. The only means of rectifying this on organic-poor sediments is to leave the lander on the sea-floor for a longer period. The experimental duration of such in situ chamber incubations is evidently a critical factor in determining benthic fluxes, and prior characterisation of the study area under consideration is thus an important factor in benthic solute flux studies.
4. Summary The Atlantic Margin is a new deep water frontier as yet undisturbed and unpolluted by man. Most oil industry activity in the region remains in the exploration phase, although projects in two fields are now in the production phase. The region is the focus of substantial collaboration between oil companies, public bodies and regulatory authorities to assess the impact of current and future exploration on the marine environment. The work described here contributes directly to providing quantitative environmental information about conditions at the seabed prior to anthropogenic disturbance, in particular regarding the oxygen demand of the system. The comparison of our O2 uptake data with data from the literature is encouraging, as the values for similar deep-water environments are of a similar magnitude to those at the BENBO sites. The data also highlight specific logistical problems, such as the length of time necessary for chamber incubations for nutrient flux studies, thereby helping to direct future similar studies. Although we have reported only preliminary findings from two experimental locations, our data together with other recent studies seem to support the idea that water depth, organic input, and benthic assemblages play a key role in benthic oxygen uptake in deep ocean sediments.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
875
Acknowledgements The authors would like to thank the Masters and crew members of the RRS Charles Darwin during the data collection phases of BENBO. Hilary Kennedy acknowledges the assistance and facilities provided by Ivan Ezzi and Ken Jones to perform the nutrient analyses. The authors also gratefully acknowledge the loan of various items of equipment by Dr. Brian Bett. This work forms part of the BENBO Thematic Research Programme of the Natural Environment ResearchCouncil, and was supported through grants to Hilary Kennedy GST/02/1754 and William Davison GST/02/1746. This is Publication Number 05 of the Thematic Research Programme BENBO.
References Archer, D., Emerson, S., Smith, C.R., 1989. Direct measurement of the diffusive sub-layer at the deep sea floor using micro-electrodes. Nature 340, 623–626. Archer, D., Devol, A., 1992. Benthic oxygen fluxes on the Washington shelf and slope: a comparison of in situ microelectrode and chamber flux measurements. Limnology and Oceanography 37 (3), 614–629. Armstrong, J.D., Bagley, P.M., Priede, I.G., 1992. Tracking deep-sea using ingestible transmitters and an autonomous seafloor instrument package. In: Priede, I.G., Swift, S.M. (Eds.), Wildlife Telemetry. Ellis Horwood, Chichester, pp. 376–386. Billett, D.S.M., Lampitt, R.S., Rice, A.L., Mantoura, R.C.F., 1983. Seasonal sedimentation of phytoplankton to the deep sea benthos. Nature 302, 520–522. Black, K.S., 1999. NERC News. Winter 1999 Issue, pp. 6–7. Boudreau, B.P., Guinasso Jr., N.L., 1982. The influence of a diffusive sub-layer on accretion, dissolution, and diagenesis at the sea floor. In: Fanning, K.A., Manheim, F.T. (Eds.), The Dynamic Environment at the Ocean Floor. Lexinton, Massachusetts, pp. 115–145. Broecker, W.S., Peng, T.H., 1974. Gas exchange rates between sea and air. Tellus 26, 21–35. Cai, W.-J., Sayles, F.L., 1996. Oxygen penetration depths and fluxes in marine sediments. Marine Chemistry 52, 123–131. Crank, J., 1983. The Mathematics of Diffusion. Calrendon Press, Oxford, 372pp. Davison, W., Fones, G.R., Harper, M., Teasdale, P., Zhang, H., 2000. Dialysis, DET and DGT: in situ diffusional techniques for studying water, sediments and soils, 495–569. In: Buffle, J., Horvai, G. (Eds.), In situ Monitoring of Aquatic Systems: Chemical Analysis and Speciation. John Wiley & Sons Ltd, New York. Drazen, J.C., Baldwin, R.J., Smith, K.L., 1998. Sediment community response to a temporally varying food supply at an abyssal station in the NE Pacific. Deep Sea Research 45 (4–5), 893–913. Duineveld, G.C.A., Lavaleye, M.S.S., Berhuis, E.M., deWilde, P.A.W.J., van der Weele, J., Kok, A., Batten, S.D., deLeeuw, J.W., 1997. Patterns of benthic fauna and benthic respiration on the Celtic continental margin in relation to the distribution of phytodetritus. International Revue der Gesamten Hydrobiologie 82 (3), 395–424. Gehlen, M., Rabouille, C., Ezat, U., Guidi-Guilvard, L.D., 1997. Drastic changes in deep-sea sediment pore-water composition induced by episodic input of organic matter. Limnology and Oceanography 42, 980–986. Glud, R.N., Gundersen, J.K., Jorgensen, B.B., Revsbech, N.P., Schulz, H.D., 1994a. Diffusive and total oxygen uptake of deep-sea sediments in the eastern South Atlantic Ocean: in situ and laboratory measurements. Deep-Sea Research 41, 1767–1788. Glud, R.N., Gundersen, J.K., Revsbech, N.P., Jorgensen, B.B., 1994b. Effects on the benthic diffusive boundary layer imposed by microelectrodes. Limnology and Oceanography 39, 462–467. Glud, R.N., Gundersen, J.K., Revsbech, N.P., Jorgensen, B.B., Huttel, M., 1995. Calibration and performance of the stirred flux chamber from the benthic lander Elinor. Deep-Sea Research 42, 1029–1042.
876
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
Jahnke, R.A., Craven, D.B., McCorkle, D.C., Reimers, C.E., 1997. CaCO3 dissolution in California continental margin sediments: the influence of organic matter remineralization. Geochimica Cosmochimica Acta 61, 3587–3604. Lampitt, R.S., 1985. Evidence for the seasonal deposition of detritus to the deep-sea floor and its subsequent resuspension. Deep Sea Research 32, 885–897. Lampitt, R.S., Raine, R.C.T., Billett, D.S.M., Rice, A.L., 1995. Material supply to the European continental slope: a budget based on oxygen demand and organic supply. Continental Shelf Research 42 (11/12), 1865–1897. Li, Y.H., Gregory, S., 1974. Diffusion of ions in deep-sea sediments. Geochimica et Cosmochimica Acta 38, 703–714. Lohse, L., Helder, W., Epping, E.H.G., Balzer, W., 1998. Recycling of organic matter along a shelf slope transect across the N W European Continental Margin (Goban Spur). Progress in Oceanography 42, 77–110. Newton, P.P., Lampitt, R.S., Jickells, T.D., King, P., Boutle, C., 1994. Temporal and spatial variability of biogenic particle fluxes during the JGOFS north-east Atlantic process studies at 478N, 208W. Deep Sea Research 41, 1617–1642. Patching, J.W., Raine, R.C.T., Barnett, P.R.O., Watson, J., 1986. Abyssal benthic oxygen consumption in the northeastern Atlantic: measurements using the suspended core technique. Oceanologica Acta 9 (1), 1–7. Pfannkuche, O., Boetius, A., Lochte, K., Lundgreen, U., Tiel, H., 1999. Responses of deep-sea benthos to sedimentation patterns in the north-east Atlantic in 1992. Deep-Sea Research 46 (4), 573–596. Rasmussen, H., Jorgensen, B.B., 1992. Micro-electrode studies of seasonal oxygen uptake in a coastal sediment: role of molecular diffusion. Marine Ecology Progress Series 81, 289–303. Reimers, C.E., 1987. An in situ micro-profiling instrument for measuring interfacial pore water gradients: methods and oxygen profiles from the North Pacific Ocean. Deep Sea Research 34, 2019–2035. Reimers, C.E., Fischer, K.M., Merewether, R., Smith, K.L., Jahnke, R.A., 1986. Oxygen micro-profiles measured in situ in deep ocean sediments. Nature 320, 741–744. Revsbech, N.P., 1989. Diffusion characteristics of microbial communities determined by use of oxygen microsensors. Journal of Microbial Methods 9, 111–122. Rice, A.L., Thurston, M.H., Bett, B.J., 1994. The IOSDL DEEPSEAS Programme: photographic evidence for the presence and absence of a seasonal input of phyto-detritus at contrasting abyssal sites in the north-eastern Atlantic Ocean. Deep Sea Research 41, 1305–1320. Sayles, F.L., Mangelsdorf, R.C., Wilson, T.R.S., Hume, D.N., 1976. A sampler for the in situ collection of marine sedimentary pore waters. Deep Sea Research 23, 259–264. Sayles, F.L., 1992. Biogeochemical processes on the seafloor Oceanus 35 (1), 68–75. Smith, K.L., Gatts, R.C., Baldwin, R.J., Beaulieu, S.E., Uhlman, A.H., Horn, R.C., Reimers, C.E., 1997. An autonomous, bottom-transecting vehicle for making long time-series measurements of sediment community oxygen consumption to abyssal depths. Limnology and Oceanography 42, 1601–1612. Smith, K.L., Baldwin, R.J., Glatts, R.C., Kaufman, R.S., Fisher, E.C., 1998. Detrital aggregates on the sea floor: chemical composition and aerobic decomposition rates at a time-series in the abyssal NE Pacific. Deep-Sea Research 45 (4–5), 843–880. Strickland, J.P.H., Parsons, T.R., 1972. A practical handbook of seawater analysis. Bulletin of the Fisheries Research Board of Canada 167, 711. Sweerts, J.P.R.A., St Louis, V., Cappenberg, T.E., 1989. Oxygen concentration profiles and exchange in sediment cores with circulated overlying water. Freshwater Biology 21, 552–556. Tahey, T.M., Duineveld, G.C.A., Berghuis, E.M., Helder, W., 1994. Relation between sediment-water fluxes of oxygen and silicate and faunal abundance at continental shelf, slope and deep water stations in the NW Mediterranean. Marine Ecology Progress Series 104 (1–2), 119–130. Tengberg, A., de Bovee, F., Hall, P., Berelson, W., Chadwick, D., Ciceri, G., Crassous, P., Devol, A., Emerson, S., Gage, J., Glud, R., Graziottini, F., Gunderson, J., Hammond, D., Helder, W., Hinga, K., Holby, O., Jahnke, R., Khripounoff, A., Lieberman, A., Lieberman, S., Nuppenau, V., Pfannkuche, O., Riemers, C., Rowe, G., Sahami, A., Sayles, F., Schurter, M., Smallman, D., Wehrli, B., de Wilde, P., 1995. Benthic chamber and profiling landers in oceanography: a review of design, technical solutions and functioning. Progress in Oceanography 35, 253–294.
K.S. Black et al. / Continental Shelf Research 21 (2001) 859–877
877
Thomsen, L., Graf, G., Martens, V., Steen, E., 1994. An instrument for sampling water from the benthic boundary layer. Continental Shelf Research 14, 871–882. Tudhope, A.W., Scoffin, T.P., 1995. Processes of sedimentation in Gollum Channel, in Porcupine Seabight } submersible observations and sediment analyses. Transactions of the Royal Society of Edinburgh (Earth Sciences) 86 (1), 49–55. Usui, T., Koike, I., Ogura, N., 1998. Vertical profiles of nitrous oxide and dissolved oxygen in marine sediments. Marine Chemistry 59, 253–270.