A free-vehicle benthic chamber instrument for sea floor studies

Deep-Sea Research, Vol. 36, No. 4, pp. 625-637, 1989.

0198-0149/89 $3.00 +0.00 © 1989 Pergamon Press plc.

Printed in Great Britain.

INSTRUMENTS AND METHODS A free-vehicle benthic chamber instrument for sea floor studies R. A. JAHNKE* and M. B. CHRISTIANSENt (Received 12 July 1988; in revised form 18 November 1988; accepted 23 November 1988) Abstract--A new free-vehicle instrument has been developed that is capable of performing controlled benthic chamber experiments on the sea floor at water depths up to 6 km. The instrument emplaces a single 30 x 30 cm titanium chamber in the sediments with little sediment disturbance. At pre-set equally spaced time intervals, a total of 18 individual samples or chemical tracers may be withdrawn from or injected into the chamber waters. The volume of water removed or added in each operation is balanced by bottom water which enters or leaves the chamber through a long, open tube. At the termination of the experiment a hydraulic system closes a scoop beneath the chamber capturing the sediments in the chamber for further study. The instrument is simple to deploy, has been extremely reliable in the field, and has yielded geochemically consistent results.

INTRODUCTION THE sea floor is an i m p o r t a n t location o f organic m a t t e r d e g r a d a t i o n in the d e e p o c e a n (JAHNKE and JACKSON, '1987; TSUNOGAI and NORIKI, 1987). H o w e v e r , inaccessibility by divers, limited submersible availability, chemical and physiological sampling artifacts due to potential changes in t e m p e r a t u r e and pressure, and the c o n c e n t r a t i o n of m a n y i m p o r t a n t processes very near the s e d i m e n t - w a t e r interface have i m p e d e d o u r study of this e n v i r o n m e n t . R e c e n t l y , technological advances in two areas have significantly i m p r o v e d o u r ability to study chemical cycling at the sea floor. First, it is n o w possible to obtain p o r e water data with 1-2 m m vertical resolution near the sediment surface in situ via microelectrodes (REIMERS, 1987; ARCHER and EMERSON, in p r e p a r a t i o n ) and the W h o l e C o r e S q u e e z e r (BENDER et al., 1987). S e c o n d , benthic flux c h a m b e r experiments, initially p i o n e e r e d by SMITH et al. (1979), have b e c o m e m o r e c o m m o n with the recent d e v e l o p m e n t o f several n e w instruments (BERELSON and HAMMOND, 1986; DEVOL, 1987; BERELSON et al., 1987). T h e following is a description o f a new instrument capable of p e r f o r m i n g a variety of benthic c h a m b e r experiments at the d e e p sea floor. T h e m a j o r attributes of this instrument are that it: (1) is a free vehicle capable o f o p e r a t i n g in water depths as great as 6 k m , (2) contains a 30 x 30 c m experimental c h a m b e r , (3) causes little sediment *Skidaway Institute of Oceanography, P.O. Box 13687, Savannah, GA 31416, U.S.A. tScripps Institution of Oceanography, A-020, La Jolla, CA 92093, U.S.A. 625

626

R . m . JAHNKEand M. B. CHRISTIANSEN

disturbance during chamber emplacement, (4) can perform a total of 18 sampling or tracer injection operations, (5) recovers the sediments in the chamber, (6) stirs the chamber waters at a calibrated rate, and (7) is simple enough to be maintained and deployed by a single technician. While many of these characteristics are similar to those of previously developed instruments, significant differences exist. Following the description of the vehicle, we present evidence that verifies the operation of the instrument and examples of recent results. The unique capabilities and aspects of this device will be discussed in the final section. INSTRUMENT

DESCRIPTION

The overall tubular frame of the Benthic Experimental Chamber Instrument (BECI) (Fig. 1) is a smaller version of the frame originally designed by K. L. Smith, Jr for his Free Vehicle Grab Respirometer (SMITHet al., 1976) and is similar to that used by C. E. Reimers for her in situ oxygen microelectrode profiler (REIMERS, 1987). It consists of an

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Fig. 1.

lass floats (1 of 8) Main ba!~st release Secondary ballast release (1 of 3} Electronicspressure case Sampling racks Syringes Expendable ballast Chamber lid and stirr~g mechanism Chamber& chamber scoop

Schematic drawing of the benthic experimental chamber instrument. Note that one of the legs of the triangular frame has been omitted from the figure for clarity.

Free-vehicle benthic chamber instrument

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upper section which contains large glass flotation spheres (1) which nominally each provide 22.7 kg of positive buoyancy when fully submerged in seawater. The flotation package is secured on top of a triangular tubular frame (Fig. 1). When assembled, the instrument is approximately 3 m tall and 2 m wide. Adjacent to each of the feet are thin-walled, open-ended cylinders (7) in which expendable ballast is suspended from the secondary jaw releases located directly above them (3). Each of the secondary releases is connected by a steel cable to the primary release at the top of the triangular frame (2). An upward force of > 5 kg on the lever arm of the main release is sufficient to maintain the release in its closed position during a deployment. The force on the arm is supplied by a short steel cable attached in series to two nichrome burn-wire segments, a dissolvable Mg link, a turn buckle and the tripod frame. One of the burn-wires is activated by the main timer circuitry while the other is connected to a completely independent back-up timer located in a separate pressure case with its own battery power supply. The Mg link consists of a Mg rod (approx. 5 cm long) with short (1.5 cm) segments of steel tubing pressed onto the rod at both ends. The electrochemical potential difference between the Fe and the Mg causes the Mg to dissolve at a nearly constant rate when submerged in seawater. Thus, the period of time required to dissolve through the Mg segment can be controlled by altering the diameter of the rod. Because these releases are linked in series, breakage of any one of them releases the ballast. The redundancy of the mechanism provides reasonable assurance for the return of the instrument. The experiment is performed in a single chamber (Fig. 1: 9; Fig. 2) that is located in the center of the tripod. Although the use of a single chamber provides for no redundancy of operation and prohibits study of small-scale variability, several considerations have led to the adoption of this design. First, because one of our principal uses of

Fig. 2. Side and front views of the chamber assembly. Numbered features are the titanium chamber (9), chamber lid (10), gasket in the upper edge of the chamber againstwhichthe lid seals (11), chamber water stirring mechanism (12), scoop for recovering the sediments within the chamber at the end of the experiment (13), and the hydrauliccylinder (1 of 2) which closes the scoop (14).

628

R.A. JAHNKEand M. B. CHRISTIANSEN

this instrument is to determine elemental benthic exchange rates, we desired a large chamber to minimize the effects of small-scale variability. We also considered it important to recover the sediments in the chamber so that chamber volume can be determined directly, tracer penetration into the sediments and pore waters can be measured, and sediment disturbance during chamber emplacement can be assessed. Because a large force is required to free a large sediment box core from the sea floor in a timely fashion, we are limited to a single chamber without implementing a more complicated recovery system. The chamber assembly that we are using (Fig. 2) was originally designed and constructed for the M A N O P Lander as part of the Manganese Nodule Program (WEISS et al., 1977; KIRSTEN and JAHNKE, 1985). The chamber consists of a 30 × 30 cm box constructed from thin-walled (0.159 cm) titanium that is open at the top and bottom. Hinged along one of the top edges is a lid constructed from polyvinylidene fluoride, a trace-metal clean plastic of very low gas permeability, with a backing of PVC and aluminum for strength. This lid seals against a butyl rubber gasket which is glued to the upper edge of the box. Lid closure is actuated by a hydraulic ram mounted between the chamber lid and frame (not visible in the figure) which, when pressurized, maintains a force of >75 kg, insuring that an air- and water-tight seal is achieved. Attached to the inside of the chamber lid is a stirring mechanism that consists of four 1.27 cm diameter Delrin rods extending outward (10 cm) from a central hub. This assembly is magnetically coupled radially through the chamber lid to a stepper motor whose speed can be controlled such that the stirrer can be driven from 1 to 10 rpm. The turbulence created by this system has been discussed in detail by BUCHrtOLTZ-TEN BRINK et al. (submitted). Hinged on the sides of the box is a scoop which, when rotated to its vertical position, captures the sediments in the chambers. Closure of the scoop is driven by two hydraulic cylinders mounted on the the sides of the chamber which, when pressurized, pull the scoop closed with a force of up to 273 kg. The entire chamber assembly is positioned vertically such that the lowest edge of the chamber is well below the feet and ballast weights but slightly above the lowest point of the vertical guide poles that extend below the feet. This configuration, combined with a descent velocity of 35 m min -1, insures that the core is taken squarely (i.e. that the instrument is not drifting laterally to a significant extent during the coring operation) and that the chamber is in place before the bow wave associated with the feet and ballast disturbs the sediment surface. We will discuss potential sediment disturbance in greater detail in the Verification section. Hydraulic pressure for operating the lid and scoop is provided by a spring-driven assembly. Prior to deployment, a hydraulic car jack is employed to compress the spring and draw hydraulic fluid into the cylinder. The main valve is then closed and the jack is removed. The valve is mounted onto sampling assemblies (discussed in the next paragraph). At a preset time, the sampling assembly is triggered and the valve is opened, pressurizing the hydraulic system and closing the lid or scoop. Even after the scoop or lid is fully closed, the hydraulic system remains pressurized, insuring that the lid or scoop does not open accidently during vehicle recovery. The sampling system consists of inexpensive spring-driven syringes attached to the relatively simple release assembly (Fig. 3). Basically, the mechanism consists of a plate with vertical slots in which an angle bracket slides. A spring is attached to the back of the plate (not shown in Fig. 3) oriented to pull the elevator upward if a sample is to be

Free-vehicle benthic chamber instrument

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Fig. 3. Samplingsystem. Numbered features are the elevator used to raise or lower the syringe pistons (15), the trip-pin that travels between the plates holding the syringes and releases the elevator latching mechanism (16), the housing in which is located the DC motor used to drive the trip-pin (17), a glass ampule for gas and dissolved organics samples (18), and a valve used to control the hydraulic system (19). withdrawn or downward if a spike is to be injected. Prior to deployment, the elevator and syringe plungers are locked in their appropriate starting positions with a latching mechanism located on the back of the plate. Two of these plate assemblies with five independent syringes each are mounted back to back on a rack (Fig. 3). A threaded rod, which is rotated by a D C motor, is used to drive a small pin between the plates that sequentially releases each syringe. Sampling is initiated by a 10 s pulse from the central timer electronics to the D C motor. This pulse causes the motor to begin turning which, in turn, closes a second switch located in the motor housing. After 10 s, the signal from the timer is terminated but the motor will continue to run until the second switch is opened. This second switch is mechanically linked to the central shaft and trip pin such that it opens after the trip pin travels exactly 2.54 cm, one sampling position. This insures that the sampling motor stops at the correct position for each sample and minimizes the electronic control necessary to operate the system. When the sample has been fully recovered, the elevator is locked into position, preventing sample exchange during the remainder of the experiment or during vehicle recovery. The spring is still pulling with approximately 5 kg of force at full travel of the elevator. Because this system is activated by a single pulse, it can be operated with any simple timer and requires no controlling microprocessor and software. To use this system for the injection of isotopic or chemical tracers, the direction of the spring tension is simply reversed. Thus, injections can be performed at any point during the experiment using any number of syringes. As presently configured, one sample motor is used to release 10 syringes (two, 5-syringe plates, one on each side of the central shaft and trip pin) and two motor assemblies are used simultaneously. Thus, 20 timed operations are performed on each experiment. The valves that control the lid and scoop hydraulics are controlled by two of these positions. The remaining 18 positions are used for samples and injections.

630

R.A. JAHNrO~and M. B. CHRISTIANSEN

Many types of syringes or other sample storage closures can be adapted to this system depending on the needs of the analyses to be performed. Presently, we use standard plastic medical syringes for nutrient and carbonate samples (60 ml), all-plastic polypropylene syringes for trace metal samples (60 ml), and glass ampules for oxygen samples (11 ml). These can be mounted in any order on the sampling racks. The glass ampules are filled with deionized water prior to deployment. Laboratory tests have demonstrated that <0.1% of the deionized water remains in the ampules after sampling. A very small (<0.3 ~tM) correction to the oxygen results is made to account for this contamination. Although all syringes are latched in their filled positions to minimize exchange during vehicle recovery, the oxygen samples are additionally secured by a one-way check valve connected in front of the glass ampule. The sampling syringe assemblies are connected to the chamber lid using small-bore (1.6 mm), thick-walled (1.9 mm) Tygon tubing. The volume of chamber water removed during each sampling operation is replaced by bottom water that enters the chamber through a 0.32 cm i.d., 30 cm long Tygon tube. This results in a small (usually <0.5%) but significant dilution of the chamber water. All results from this instrument are corrected to the concentrations which would be present if there were no dilution. Several attributes of this design should be noted. The use of a motor-driven latching mechanism requires three electrical connections to be made through the pressure case, regardless of the number of sampling operations. Thus, the present design could easily be expanded if greater sampling was required. Also, because this design utilizes a reuseable latching mechanism, all 20 sampling positions can be cocked and made ready for deployment in less than 30 min. The sampling operations are controlled by a simple timing circuit housed in the central pressure case (4 in Fig. 1). Also in this central pressure housing is the electronic circuitry that controls the stepper motor which drives the stirrer. Two settings are made on the timer board prior to deployment, a delay period (0-10 h) and a sampling interval (010 h). Once the power is supplied to the circuitry, the timer waits the preset delay period before initiating the sampling intervals. After each interval period has passed, the timer circuitry sends out a 10 s pulse to the sample motor that initiates the sampling process as described above. After all 20 timer operations are performed, the timer waits an additional hour and then energizes the primary burn-wire release. Once the ballast is released, the vehicle has a positive buoyancy of approximately 91 kg that is sufficient to break the chamber core free of the sea floor and causes it to rise to the sea surface at a rate of approximately 40 m min-~. A small pressure case housing a simple one event timer and independent batteries is mounted on the frame and connected to the second burn-wire release. The release is generally set for several hours after the planned termination of the experiment and is intended to provide a back-up recoyery system should the main circuits fail. One of the important aspects of this system is that it is simple to deploy, construct, and maintain. A single technician can sample and add fixatives (acid, oxygen reagents, etc.) to the 18 time-series samples in approximately 20 min. Because a reusable latching mechanism is used, the entire sampling mechanism can be cocked and prepared for deployment in 30 rain. Installing the expendable weights and preparing the hydraulic system, chamber lid and scoops, and burn-wire ballast release mechanisms requires an additional 4 h. A single technician therefore, can completely prepare the instrument for

Free-vehicle benthic chamber instrument

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re-deployment within 6 h of recovery. Although difficult to estimate, the total construction cost of the instrument is less than $25,000. VERIFICATION

OF INSTRU ME NT

PERFORMANCE

Valid benthic flux experiments require that the chamber be emplaced without significantly disturbing the sediment surface. Recently, a deep-sea video camera system was mounted in a Soutar corer to study sediment disturbance during the coring process. These studies revealed that sediment disturbance when it occurred, was caused primarily by two phenomena. First, ship motion causes the corer to surge while descending (even in 5000 m of water) at a lowering speed of 40 m min-1. If the corer contacts the bottom while j ~ s descending slowly, the coring box does not penetrate the sediments rapidly enough-t~'prevent the bow wave from reaching the sediment surface. In the extreme case this phenomena may actually cause the corer to be lifted back off the bottom after the initial contact, causing great disturbance. The second source of disturbance is the bow wave associated with the horizontal portions of the frame which may contact the sediment surface prior to the coring box. The design of the BECI tripod eliminates these potential sources of disturbance. First, being a free vehicle, it is not affected by sea state or ship motion and it makes a clean single penetration of the bottom at a descent speed of approximately 35 m min-1. Secondly, as can be seen in Fig. 1, there are no horizontal or large surface-area parts extending down near the chamber opening. Thus, by the time the bow wave from the vehicle frame and weights contacts the sediment surface, the chamber has already penetrated the sediment surface and is protecting the sediments where the experiment will be performed. The effectiveness of this sampling system has been tested in two ways. First, we have used a deep-sea video camera to observe directly the landing of the tripod on the sea floor. These observations revealed that although surface sediments were resuspended upon impact outside of the chamber, inside, there was no visible disturbance. Second, core disturbance was assessed by comparing the fine-scale sediment distribution of ATP from the BECI chamber samples to those measured on Soutar box cores (Fig. 4) from Santa Monica Basin, California. The weather was extremely calm during all coring operations (seas < 30 cm) and the sediment interfaces were judged to be intact, free of the possible disturbance discussed above. Clarity of the overlying water, lack of variation in the flocculent surface bacterial layer, and no observable wall effects were the main criteria for judging the quality of the cores. Santa Monica Basin sediments exhibit extremely high porosities (>97%) near the sediment surface and are thus easily disturbed. Note that the depth scale in Fig. 4 is from 0 to 2 cm. Thus, a loss of even 2 mm of surface sediment would be noticeable. The agreement between the BECI and Soutar corer samples is very good, suggesting that the BECI system does indeed embed the chamber with minimal disturbance. For comparison, similar tests with the original MANOP Lander reveal significant differences suggesting the loss of 3--6 mm of surface sediments in certain areas of the chambers (Fig. 4). Stirring of the chamber water is important for obtaining valid experimental results. Besides mixing the chamber water so that a representative sample can be withdrawn, stiffing also controls the hydrodynamic state and hence diffusive sublayer thickness very

632

R . A . JAHNKEand M. B. CHRISTIANSEN

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near the sediment surface. If reaction rates in the sediment are very rapid, supply or removal of solutes by diffusion across the diffusive subl.ayer may limit the benthic flux. Possible examples of flux-limited diagenetic systems are the dissolution of CaCO3 below the lysocline (BoUDREAUand GUINASSO,1982), trace metal adsorption onto surface sediments (SANTSCHI e t al., 1983, 1984) and oxygen fluxes under highly productive nearshore and lagoonal er, vironments (JORGENSENand REVSBECK,1985). The stirring mechanism in the BECI chamber has been calibrated and described in detail elsewhere (BucHHOLTz-TENBRINKet al., in press). The calibration tests reveal that this system, when running at speeds of 3-9 rpm above a smooth bottom, produces friction velocities ranging from 0.1 to 0.7 cm s-t that are equivalent to a mean diffusive

Free-vehicle benthic chamber instrument

633

sublayer thickness of 500-180 lxm. All results presented here were obtained with the stirrer rotating at 5 rpm. Because of the shape of the chamber and stirring rods, variation in diffusive sublayer thickness between the center of the chamber and walls is generally less than a factor of 2. The sealing of the lid at the start of the experiment has been verified by 22Na tracer studies. In the experiments performed, tracer activities were observed to decrease at a rate of <0.1% h-1. Without correcting for diffusive losses into the sediments, 93% of the injected tracer could be accounted for in a 60-h deployment. Including an estimate for the 22Na in the pore waters it appears that we can account for >98% of the tracer injected. Thus, it appears that the major mechanical operations required for a successful benthic chamber experiment are met with this instrument design. RECENT RESULTS

Examples of results obtained on recent deployments in a deep, nearshore basin in the California Borderland and on the continental rise adjacent to Point Sur, California are displayed in Figs 5 and 6. The geochemical and oceanographic implications of these .-results will be discussed in greater detail elsewhere. In this paper, we will focus on the aspects of the results by which the accuracy and precision of the data can be assessed. Overall, it appears that within each data set consistent trends are observed that permit benthic flux estimates to be made. It is important to note that at most locations the BECI results are in good agreement with bottom water values (triangles on figures) which were recovered with a Niskin bottle mounted on a Soutar box corer. Because these results are from different casts and, hence, slightly different locations and water depths, small differences are to be expected. However, due to the reasonable agreement observed between the Niskin and BECI samples, it appears that this instrument is capable of recovering good quality samples, adequate for our needs. It is also important to note that the samples are not poisoned, filtered or preserved in any manner and one might expect microbial alterations to occur after sampling has taken place. However, the first sample withdrawn has the longest storage time in the syringes and hence the greatest probability of being significantly altered. Reasonably good agreement between the early BECI and Niskin samples suggests that changes during sample storage in the syringes are also small relative to the changes observed during the experiments. Besides the general observations discussed above, several specific features should be noted. Bottom water 02 concentrations in Santa Monica Basin are approximately 5-6 Ixmol kg-~ (G. JACKSON, personal communication) providing an extremely stringent test of the BECI sampling system. Not only must the system prevent atmospheric contamination during vehicle recovery and sample processing, but all traces of atmospheric 02 must be flushed from the chamber and sample tubing prior to the start of the experiment as well. The low values measured and good agreement with bottom water concentrations strongly suggest that near contamination-free 02 sampling and (more importantly) chamber experiments have been accomplished. Another important observation is the curvature in the time-series results, most obvious in experiments 2 and 3. Benthic fluxes estimated from linear regressions of the data would differ by > 50% from those based on data fits that account for the observed curvature. BENDERet al. (in press), in their analysis of MANOP Lander benthic chamber data, have developed a model that predicts the response of pore water and chamber

634

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635

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636

R . A . JAHNKE and M. B. CHRIST1ANSEN

water concentrations of 02, NO q and Si(OH)4 during a chamber experiment. They have used the model of JAHNKE et al. (1982) to represent organic matter oxidation by 02 and NO q and assumed that Si dissolution was represented by first-order kinetics. Their results predict that chamber water concentrations should change non-linearly and that this effect should be most pronounced when the chamber volume is small and the observed concentration changes are large. However, they further report that even when benthic chamber water concentrations change by only 10--20% during an experiment, failure to account for this curvature in the time-series results lead to errors of 20-30% in benthic flux estimates. The source of the curvature is the response of the pore water profile and sediment reaction rates to changing chamber water concentrations. These model predictions depend on the mathematical representation of the important transport and diagenetic processes. It is difficult at present, therefore, to determine if the influence of changing chamber water concentrations can account for all of the observed curvature. Thus, until the validity of the models is firmly demonstrated, it is desirable to re cover as many time-series samples as possible so that the curvature in the results can be evaluated directly by data-fitting procedures. The overall accuracy of the results may be assessed by comparison to fluxes estimated from pore water diffusion calculations at locations where diffusive transport dominates benthic exchange. Such conditions are found in Santa Monica Basin where low bottom water oxygen concentrations limit the abundance of macrobenthic organisms. Under these conditions, benthic silicate fluxes based on near-surface pore water measurements are within 10% of those measured with benthic chambers. CONCLUSIONS

A free-vehicle benthic chamber instrument has been designed, fabricated and tested in the ocean. Although several other instruments of this type presently exist, they differ significantly in their capabilities. The major differences between these instruments are summarized in Table 1. As described in the.text, the BECI device only deploys a single chamber. It is also important to note that this device does not utilize electrodes. On the other hand, this device is a free vehicle and thus independent of ship motion, recovers discrete samples so that fluxes of many elements may be estimated, recovers many samples, permitting curvature in the concentration-time relationships to be resolved, and recovers the sediments in the chambers to permit tracer, pore water and sediment studies. Also, this device has a stirring system which has been demonstrated to yield a

Table 1. Comparison of capabilities of existing benthic chamber instruments SMITH (1976) No. of chambers Oxygen electrodes No. of discrete samples Recovers sediments Free vehicle Implants c h a m b e r prior to frame contacting sediments

Device DEVOL (1987)

BERELSON(1986)

BECI

4 Yes Yes Yes

2 No 8 Yes No

3 No 4 No Yes

1 No 18 Yes Yes

No

No

No

Yes

Free-vehicle benthic chamber instrument

637

d i f f u s i v e s u b l a y e r t h i c k n e s s t h a t is t h o u g h t t o r o u g h l y m a t c h t h a t o f t h e d e e p s e a . P e r h a p s m o s t i m p o r t a n t is t h e f a c t t h a t t h i s i n s t r u m e n t i m p l a n t s t h e c h a m b e r b e f o r e t h e f r a m e contacts the bottom. This configuration appears to minimize disturbance of the surface s e d i m e n t s . F i n a l l y , this i n s t r u m e n t is s i m p l e t o a s s e m b l e , d e p l o y a n d s a m p l e a n d m a y b e routinely operated by a single technician.

Acknowledgements--We gratefully acknowledge R. F. Weiss, O. Kirsten, A. Krause, F. Mansir and T. Lillie, who contributed design ideas to the MANOP Lander that were subsequently incorporated into this instrument, the Captain and crew of the R.V. New Horizon for their help during vehicle launch and recovery operations, A. Boyette for artwork and L. Land for typing the manuscript. This work was supported by NSF grant OCE 86-13968. REFERENCES BENDER M. L., W. MARTIN,J. HESS, F. SAYLES,L. BALL and C. LAMBERT(1987) A whole-core squeezer for interstitial pore-water sampling. Limnology and Oceanography, 32, 1214-1225. BENDER i . L., R. A. JAHNKE, R. F. WEISS, W. MARTIN, n . T. HEGGIE, J. ORCHARDOand T. SOWERS (in press) Organic carbon oxidation and benthic nitrogen and silica dynamics in San Ciemente Basin.

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