ARTICLE IN PRESS Continental Shelf Research 24 (2004) 183–201 In situ measurements of advective solute transport in permeable shelf sands Clare E. Reimers a, *, Hilmar A. Stecher III a , Gary L. Taghon b , Charlotte M. Fuller b , Markus Huettel c,1 , Antje Rusch c,2 , Natacha Ryckelynck a , Christian Wild c a College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin. Bldg., Corvallis, OR 97331, USA b Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, USA c Max Planck Institute for Marine Microbiology, Celsiusstr. 1, Bremen D-28359, Germany Received 14 March 2003; received in revised form 13 June 2003; accepted 16 October 2003 Abstract Solute transport rates within the uppermost 2 cm of a rippled continental shelf sand deposit, with a mean grain size of 400–500 mm and permeabilities of 2.0–2.4 10 11 m 2 , have been measured in situ by detecting the breakthrough of a pulse of iodide after its injection into the bottom water. These tracer experiments were conducted on the USA Middle Atlantic Bight shelf at a water depth of B13 m using a small tethered tripod that carried a close-up video camera, acoustic current meter, motorized 1.5 liter ‘‘syringe’’, and a microprofiling system for positioning and operating a solid- state voltammetric microelectrode. When triggered on shipboard, the syringe delivered a 0.21 M solution of potassium iodide and red dye through five nozzles positioned around and above the buried tip of the voltammetric sensor for 0.65– 5 min. Bottom turbulence rapidly mixed and dispersed the tracer, which then was carried into the bed by interfacial water flows associated with ripple topography. The advective downward transport to the sensor tip was timed by a sequence of repetitive voltammetric scans. The distance-averaged vertical velocity, expressed as the depth of the sensor tip in the sand divided by the time to iodide breakthrough, was found to vary from 6 to 53 cm h 1 and generally to decrease with sediment depth. Because of episodic pumping and dispersion associated with the greatest 5% of wave heights and current speeds recorded, some concentration vs. time responses showed evidence of uneven solute migration. For reasons of mass balance, the advective flow field in the surface layers of permeable beds includes regions of water intrusion, horizontal pore-water flow and upwelling which also may explain some of the observed uneven migration. Pore-water advection was also evident in oxygen profiles measured before and after tracer injection with the voltammetric sensor. These profiles showed irregular distributions and oxygen penetration depths of 4–4.5 cm. Sand cores from the study site subjected to continuous pore fluid pumping showed that oxygen consumption was positively correlated with flow rate. The effect was calculated to be equivalent to increasing the benthic oxygen flux by 0.029 mmol m 2 d 1 for every 1 liter m 2 d 1 flushed through a 4 cm thick oxic zone. Thus, it is concluded that in situ oxygen consumption rates must be highly variable and dependent on the prevalent wave and current conditions. r 2003 Elsevier Ltd. All rights reserved. Keywords: Advection; Voltammetric electrode; Permeable sediments; Sand ripples; Oxygen consumption; Inner shelf *Corresponding author. Fax: +1-541-737-2064. E-mail address: [email protected] (C.E. Reimers). 1 Present address: Department of Oceanography, Florida State University, Tallahassee, FL 32306-4320, USA. 2 Present address: Department of Earth and Planetary Science, Washington University, St. Louis, MO 63130, USA. 0278-4343/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2003.10.005
Transcript
ARTICLE IN PRESS
Continental Shelf Research 24 (2004) 183–201
*Correspondin
E-mail addre1Present addr2Present addr
0278-4343/$ - see
doi:10.1016/j.csr
In situ measurements of advective solute transport inpermeable shelf sands
Clare E. Reimersa,*, Hilmar A. Stecher IIIa, Gary L. Taghonb, Charlotte M. Fullerb,Markus Huettelc,1, Antje Ruschc,2, Natacha Ryckelyncka, Christian Wildc
aCollege of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin. Bldg., Corvallis, OR 97331, USAb Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, USA
cMax Planck Institute for Marine Microbiology, Celsiusstr. 1, Bremen D-28359, Germany
Received 14 March 2003; received in revised form 13 June 2003; accepted 16 October 2003
Abstract
Solute transport rates within the uppermost 2 cm of a rippled continental shelf sand deposit, with a mean grain size of
400–500 mm and permeabilities of 2.0–2.4� 10�11m2, have been measured in situ by detecting the breakthrough of a
pulse of iodide after its injection into the bottom water. These tracer experiments were conducted on the USA Middle
Atlantic Bight shelf at a water depth of B13m using a small tethered tripod that carried a close-up video camera,
acoustic current meter, motorized 1.5 liter ‘‘syringe’’, and a microprofiling system for positioning and operating a solid-
state voltammetric microelectrode. When triggered on shipboard, the syringe delivered a 0.21M solution of potassium
iodide and red dye through five nozzles positioned around and above the buried tip of the voltammetric sensor for 0.65–
5min. Bottom turbulence rapidly mixed and dispersed the tracer, which then was carried into the bed by interfacial
water flows associated with ripple topography. The advective downward transport to the sensor tip was timed by a
sequence of repetitive voltammetric scans. The distance-averaged vertical velocity, expressed as the depth of the sensor
tip in the sand divided by the time to iodide breakthrough, was found to vary from 6 to 53 cmh�1 and generally to
decrease with sediment depth. Because of episodic pumping and dispersion associated with the greatest 5% of wave
heights and current speeds recorded, some concentration vs. time responses showed evidence of uneven solute
migration. For reasons of mass balance, the advective flow field in the surface layers of permeable beds includes regions
of water intrusion, horizontal pore-water flow and upwelling which also may explain some of the observed uneven
migration. Pore-water advection was also evident in oxygen profiles measured before and after tracer injection with the
voltammetric sensor. These profiles showed irregular distributions and oxygen penetration depths of 4–4.5 cm.
Sand cores from the study site subjected to continuous pore fluid pumping showed that oxygen consumption was
positively correlated with flow rate. The effect was calculated to be equivalent to increasing the benthic oxygen flux by
0.029mmolm�2 d�1 for every 1 literm�2 d�1 flushed through a 4 cm thick oxic zone. Thus, it is concluded that in situ
oxygen consumption rates must be highly variable and dependent on the prevalent wave and current conditions.
ess: Department of Oceanography, Florida State University, Tallahassee, FL 32306-4320, USA.
ess: Department of Earth and Planetary Science, Washington University, St. Louis, MO 63130, USA.
front matter r 2003 Elsevier Ltd. All rights reserved.
.2003.10.005
ARTICLE IN PRESS
3Some representations of dispersion describe Dx as an
effective exchange coefficient equal to DD þ D�s ; where DD is
described as the dispersion coefficient. For example, Svensson
and Rahm (1991) separate molecular diffusion from dispersion
and assume DD ¼ 0:05 dv; where d is the particle diameter of the
sediment considered and %v is the magnitude of the velocity
vector.
C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201184
1. Introduction
Surface gravity waves and ensuing boundarylayer currents can influence structure, compositionand biogeochemical reactions in coastal and shelfsediments (Komar et al., 1972; Vanderborght et al.,1977; Malan and McLachlan, 1991; Styles andGlenn, 2002). When sediments are permeable anduneven, as is common for the sand beds coveringlarge areas of the continental shelf, these flows trapand resuspend fresh particulate organic matter,winnow other fines, induce interstitial fluid advec-tion, and create oscillatory ripple marks (Websterand Taylor, 1992; Lohse et al., 1996; Huettel et al.,1996; Nelson et al., 1999). Unfortunately, due tothe difficulty of observation under wave action,there are few field studies that report quantitativemeasures of these phenomena or their conse-quences. This has led to a poor understanding ofthe role permeable sediments play in biogeo-chemical cycles and shelf carbon budgets (Roweet al., 1988; Shum and Sundby, 1996; Boudreauet al., 2001).One of the most pioneering studies of sediments
under wave action used the tracer fluorescein tofollow interstitial water flow (Webb and Theodor,1968). Divers injected 2ml samples of seawatermixed with dye into sand ripples at depths of 2.5–10 cm and then timed the rapid upward reemer-gence of the dye. It was emphasized later that theobserved flow pattern could not have been causedby wave effects alone, but must have resulted fromthe interaction of the bottom swell and sedimentripples (Rutgers van der Loeff, 1981). Seawateranions also have made useful tracers for evaluatingrates of solute transport through sediments. Mostnotably, Br� has been used to estimate sedimentirrigation rates resulting from the burrowing andfeeding activities of sediment infauna (Martin andBanta, 1992).On a mathematical basis, mixing by waves and
currents or mixing by benthic animals can betreated the same. For example, in one-dimensionalmodels of biological irrigation rates, it is commonto specify a non-local exchange function, aðxÞ;which has units of inverse time (Emerson et al.,1984; Martin and Sayles, 1987; Marinelli et al.,1998). Boudreau (1997, p. 143) suggests that the
integral of aðxÞ over the depth of exchange can beexpressed as an average exchange velocity (i.e.,pore fluid velocity). Depth distributions of averageexchange velocities resulting from pressure varia-tions associated with horizontal currents interact-ing with sand mounds in a flume were recentlymeasured by Huettel et al. (1996). Mass balancerequires that advection representing a net soluteinflux at a given location is balanced by a changein concentration with time, chemical reaction, oran advective or dispersive efflux. Dispersion is thetype of fluid mixing within sediments that isusually modeled as an effective diffusion processassociated with hydraulic flows (Boudreau, 1997).Empirically (after Lee, 1999), the hydrodynamicdispersion coefficient Dx in the x-direction may berelated to the pore fluid velocity, nx; by
Dx ¼ bx
nx
fþ D�
s ; ð1Þ
where D�s is the sediment molecular diffusion
coefficient in terms of area of sediment per unittime (Berner, 1980), f is the porosity, and bx is thedispersivity, a scale-dependent parameter withunits of length.3
Under a rippled bed subject to irregular waves,flow paths and fluid velocities become timedependent, but wave tank experiments haveconfirmed that fluid intrusion occurs predomi-nantly beneath ripple troughs and flanks, whilepore-water extrusion is focused at ripple crests(Precht and Huettel, 2003). The same flow patternwas predicted by Shum (1992, 1993) using ananalytical model for two-dimensional flow fieldswithin ripples under oscillatory wave motion.Again, as was recognized by Rutgers van derLoeff (1981), the main driving force for suchfiltration systems is the pressure field generated byinteraction between wave-driven bottom currentsand sand ripples.
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C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201 185
The emphasis of this work was primarily tomeasure distance-averaged intrusion velocities inrippled sand deposits on the inner continental shelfof the USA Middle Atlantic Bight (MAB) under insitu conditions to confirm both modeling andexperimental studies. A method was developed inwhich pulses of dissolved iodide were released tothe sediment–water interface and sensed at discretedepths in the sediment with voltammetric electro-des. A video camera system, acoustic Dopplercurrent meter, and sediment cores yielded site-specific information on actively forming ripples,wave and current parameters, sediment properties,and oxidation rates of sediment organic matter asa function of flow. Aspects of the inner continentalshelf of the MAB that divide it from outer shelfregions are its water depth (generallyo30m),higher tidal and wave energy, proximity to fluvialsources, and turbidity. The inner shelf may alsoexhibit higher annual rates of primary productivityand more variable CO2 fluxes to the atmospherecompared to the outer shelf (Boehme et al., 1998;DeGrandpre et al., 2002). The latter is believed tobe in part because very high rates of heterotrophicCO2 metabolism in both the water column andsediment can exceed even the rich production oforganic matter near shore. There are manyproblems associated with applying standard meth-ods for constraining rates of benthic metabolismto shelf sands, however, so we report ratesdetermined by a method tailored to permeablesubstrates. In this way we have begun to set limitsfor the influences of advective pore-water ex-change on rates of organic matter turnover in theshallow regions of the continental shelf.
2. Methods
2.1. Experimental design
‘‘Breakthrough’’ times of pulses of dissolvediodide at discrete depths in the sediment weremeasured. Although we have found ‘‘break-through time’’ defined in the literature as eitherthe time of first arrival of a tracer (Fritsche et al.,2001) or as when a ‘‘chosen isopleth (line of givenconcentration) reaches a given location’’ (Lee,
1999), we define it as when the concentrationmaximum of a pulsed input reaches a givenlocation. By this definition a distance-averagedlinear velocity can be described as the migratingrate of the leading edge of the peak of a pulsedinput, which is expected to decay and spread withdistance because of dispersion (Boudreau, 1997).A tracer was prepared by adding 1 l of deionized
water (colored with 1ml of concentrated red foodcoloring for visualization) to 35.3 g KI to make a0.21M iodide solution equal in density to sea-water. This solution was loaded in a custom-made,motor-driven syringe (Eastern Oceanics) capableof delivering up to 1.5 l through a manifold madeof 0.32 cm outer-diameter (OD), Perfluoroalkoxy(PFA) tubing and stainless steel fittings. Theinjection rate of tracer was adjustable but set at270mlmin�1 (Bone quarter the maximum rate).Tracer was delivered from the PFA tubing to five0.32 cm OD stainless steel tubes and then throughdiffusers (made by drilling several 0.04 cm ODholes in the caps shipped with disposable syringeswhich were affixed to the tips of the stainless steeltubes using silicone adhesive). The tubes anddiffusers were spaced radially 4 cm from a centeredvoltammetric sensor and 60� apart (Fig. 1). Theplane of the diffuser tips was placed from 1 to 4 cmabove the tip of the voltammetric electrodedepending on the anticipated bottom relief andthe final measuring depth of the electrode (0.4–2 cm).The iodide delivery array and voltammetric
sensor (Fig. 1) were positioned relative to theseafloor using a motorized, vertical ball slideassembly (Eastern Oceanics, custom design) witha total coarse-scale travel of 20 cm, and amicroprofiler unit with a total vertical travel of8 cm at 0.25mm resolution while viewed in livecolor video (Deep-Sea Power & Light, Multi-SeaCam 2050). The microprofiler was mounted onthe slide assembly, and the slide assembly wasfixed to a benthic tripod (1.6m pod-to-pod, 1.4mhigh). The tripod also carried an acoustic currentmeter (Nortek, Aquadopp 3D-Vector averagingcurrent meter with pressure and temperaturesensors), the custom-made syringe, the videocamera (positioned to view the sediment–waterinterface at ca. 35�), and three underwater lights
ARTICLE IN PRESS
(A)
(B)
Fig. 1. (A) Artist’s portrayal of the array for emitting tracer
above the sediment–water interface around a centered voltam-
metric electrode with buried tip. The tracer streams flow, pool,
and shift direction under oscillatory wave action. (B) Video
frame grab of tracer release during D16-2.0.
C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201186
(Deep-Sea Power & Light, Micro-SeaLite). Inaddition, the tripod carried the microprofiler-controller electronics bottle (Reimers and Glud,2000), an interface bottle for the video, syringe andslide (connected by cable to a surface unit withcontrolling switches and video input/output; East-ern Oceanics), and a transmitter/cable interface(Analytical Instrument Systems, Model DLK-LCT-1) that connected the voltammetric working,counter and reference electrodes through a 30mwaterproof cable to a signal receiver (AIS, Model-LCR-1) and electrochemical analyzer (AIS, DLK-100A) (Luther et al., 1999).
During each seafloor experiment, the tripod wastethered to the Research Vessel (R./V.) Cape
Henlopen while the vessel was on three-pointanchor over the study site (39� 27.01N; 74�
14.27W; 12–13m water depth). Eight experimentswere conducted between July 24 and July 26, 2001after a day of preliminary trials.
2.2. Study site
The study site was located in the inner shelfregion of the USA MAB near the cabledobservatory known as LEO-15 (Glenn et al.,2000). Boundary layer studies have documentedthe seasonal variation of benthic flow conditions inthis region as well as the significance of energeticwave events for initiating and maintaining sedi-ment transport (Styles, 1998). Near bottom waveorbital velocities calculated from long-term re-cords of wave pressure spectra range from 5 cm s�1
to over 70 cm s�1 but rarely exceed 30 cm s�1 insummer months (Styles, 1998). During summerconditions an equilibrium ripple field is the mostcharacteristic bedform with reported ripple heightsof 3–15 cm and wavelengths of 20–100 cm (Tray-kovski et al., 1999; Styles and Glenn, 2002). Therippled sediments are well-sorted, dominantlyquartz, medium to coarse sands with a mediangrain size ranging from 400 to 500 mm and anaverage bulk organic carbon content of 0.02% dryweight.
2.3. Sediment sampling and analyses
Cylindrical cores of surface sediment werecollected by SCUBA divers from both ripple crestsand troughs on July 24, 2001 at approximately11AM. The divers worked along the bottomapproximately 50m east of the site targeted forthe tripod measurements. Different sized coreswere collected for measurements of sedimentpermeability and porosity, percent fines by weight,oxygen consumption rates under varying flowrates, and percent organic C and total N; pigmentsand bacterial activity (the latter are presented inRusch et al., 2003).Cores in 2.65 cm (ID) tubes were connected to a
falling-head permeameter to assess sediment
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C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201 187
permeability (Gray, 1958). Afterwards, the lengthof the sediment column was measured to thenearest 0.1 cm, and the sediment solids were rinsedwith deionized water and dried to constant weight.Porosity was calculated as: 1�[(sample dry weight)/(sample total�wet volume x particle density)]. Aparticle density of 2.65 g cm�3 (quartz) was used.Percent fines by weight were determined from
1 cm depth intervals of 2.65 cm diameter coresbased on the assumption that particle settling-velocity followed Stoke’s Law. After sectioning acore, a small aliquot of wet sediment from eachdepth interval was placed in a test tube, thencovered to a depth of 1.5 cm with filtered (What-man GF/F) seawater. The sample was mixedvigorously, allowed to stand undisturbed for 5 s,then the overlying water and suspended particleswere removed with a pipet. Under these condi-tions, all particles of quartz-density p60 mmremain suspended. The washing steps were re-peated until the water was visibly clear, typically a
Table 1
Column experiment conditions
Column
number
Sediment column
length (cm)
1 6.1
1 6.1
1 6.1
2 9.0
2 9.0
2 9.0
3 7.3
3 7.3
3 7.3
4c 10.0
5 10.5
5 10.5
5 10.5
6 8.0
6 8.0
6 8.0
7 8.5
7 8.5
7 8.5
8c 9.5
aTo convert ml cm�2 h�1 to lm�2 d�1 multiply by the factor 240.bThese are the periods during which in- and out-flowing water wercOrganic substrates were added to the in-flowing waters of colum
substrates was monitored and will be reported in a later publication.
total of five times. All washings were combinedand filtered through a pre-weighed Whatman GF/C filter. The filter was rinsed briefly with deionizedwater then dried to constant weight. Sand-sizedparticles (> 60 mm) remaining in the test tube werealso dried to constant weight.
2.4. Oxygen consumption rates
Oxygen consumption rates per area of seafloorwere determined as a function of flow rate througheight separate cores (3.6 cm diameter, 6.1–10.5 cmlong) at different times after retrieval and labora-tory setup (Table 1). Immediately after recoverythe cores (henceforth called columns) wereclamped and mounted vertically on a stand in awater bath held at 21.470.2�C (close to thetemperature of the bottom water). The watervolume overlying each core was reduced to lessthan 5ml by displacing excess water with pierced
Mean flow rate
(ml cm�2 h�1)aTime since retrieval and
setup under flow (h)b
5.33 3.25–4.75
2.67 5.0–6.5
2.73 20.5–26.0
5.56 3.25–4.75
2.78 5.0–6.5
2.67 20.5–26.0
5.56 3.25–4.75
2.78 5.0–6.5
2.75 20.5–26.0
2.37 21.1–26.0
4.00 3.25–4.75
1.33 5.0–6.5
1.20 21.1–26.5
4.00 3.25–4.75
1.33 5.0–6.5
1.30 21.1–26.5
4.00 3.25–4.75
1.22 5.0–6.5
1.15 21.1–26.5
1.23 21.8–26.5
e monitored for determining oxygen consumption.
ns 4 and 8 during the initial flow periods. The uptake of these
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inert StyrodurTM stoppers. Light was excluded bywrapping all columns with aluminum foil.The pierced rubber stoppers at the inflow of
each column were connected with 50 cm longpieces of 1.02mm ID TygonTM tubing to anaerated reservoir of filtered (0.2 mm) local sea-water. The outflow of each column was connectedwith TygonTM tubing of the same dimensions via aperistaltic pump to 50ml glass syringes thatcollected the water for later analyses. With thisarrangement, seawater that was initially air-saturated was pumped continuously through thesediment columns at pre-set rates ranging from1.20 to 5.56ml cm�2 h�1 (288–1334 lm�2 d�1).Oxygen concentrations in the in- and out-
flowing water were monitored using separateflow-through cells fitted with integrated fiber-opticoxygen microsensors (PreSenss, Sensor type B2,tip diameter o50 mm) that were mounted via LuerLock connectors into the tubing at the upper andlower end of each column. The microsensors wereread after taking into account the time necessaryfor water to percolate the entire column length bya fiber-optic oxygen-meter (Microx-TX, Pre-Senss) that was connected to a standard PC forsignal processing and data storage. Outflow waternever became anoxic, and the sensors werecalibrated before and after each experiment usinga two-point calibration in oxygen-free (addition ofsodium dithionite) and air-saturated seawater (air-bubbled) from the inflow reservoir. Additionally,oxygen concentrations in the reservoir weremeasured using Winkler titration (Grasshoff,1999).
2.5. Water column measurements
Seawater samples from the study site werecollected once or twice daily within 1m of theseafloor and within 2m of the sea-surface togetherwith CTD measurements using a Seabird 911 PlusCTD-rosette system outfitted with 10 liter GeneralOceanic 1015 bottles. Although these samples wereanalyzed for a suite of parameters, only thebottom water dissolved oxygen data will bereported in this paper because these results areused for calibrating the pore-water oxygen mea-surements determined by voltammetry. These
oxygen analyses were performed on duplicatesamples from each 10 l bottle using an automatedWinkler titration system and procedures devel-oped by Friederich et al. (1991). Six verticalprofiles of upwelling radiance and downwellingirradiance were also measured to within 2m of thebottom between 08:50 and 14:30 during the 4 dayson station with a Biospherical Profiling Reflec-tance Refractometer (PRR-600).
2.6. Voltammetric methods
Amalgamated Au electrodes were used to detectdissolved oxygen and the breakthrough of iodidein sands near the sediment–water boundary. Thesesensors were made according to methods devel-oped by Brendel and Luther (1995) and Lutheret al. (1999) by inserting 100-mm diameter Au wireinto glass pipets pulled from 5-mm OD glasstubing to have 4–6 cm long tapers, with tipdiameters of 0.3–0.5mm. A non-conductive epoxy(West System 105) was used to fill the spacebetween the Au and glass over the entire length(15–25 cm) of the pipet. The sensor tip was sandedagainst a rotating 1-cm2 piece of 400-grit SiCsandpaper to expose the Au wire as a core rimmedwith solid epoxy and glass. Each tip was groundfurther with 1000- and 4000-grit SiC paper, thenfinely polished with a series of diamond-grit pastesand plated with Hg to create hemispheric sensingelements, 100 mm in diameter.In situ voltammetric analyses were carried out
using the Analytical Instrument Systems, Inc.(AIS) DLK-100A electrochemical analyzer withlong-cable transmitter/receiver interfaces, and sin-gle amalgamated Au electrodes in cells with a Ag/AgCl reference and a Pt counter electrode. Theseanalyses were controlled by a microcomputeraboard ship using software provided by themanufacturer (AIS). The analyzer and computerwere run from separate DC power sources andgrounded to the ship’s hull. When using linearsweep voltammetry, O2 gives two waves corre-sponding to the reduction of O2 to H2O2 and ofH2O2 to H2O (Buffle and Tercier-Waeber, 2000).Oxygen concentrations were estimated from thepeak (or half-wave) currents of the first reduction(ca. �0.3 V vs. Ag/AgCl) minus any non-zero
Fig. 2. (A) Representative linear sweep voltammogram mea-
sured in situ with the voltammetric sensor tip positioned at the
sand–water interface. The potential scan was from �0.1 to
�1.8V vs. Ag/AgCl at 1000mV s�1 after 10-s conditioning at
�0.1V. The current peak to the left results from the reduction
of dissolved O2 to peroxide at the electrode surface. Its height,
ip-oxygen; above baseline, b; is proportional to the concentrationof dissolved oxygen under constant environmental conditions.
(B) Square wave voltammogram showing the detection of
dissolved iodide (top curve) and the resultant peak height,
ip-iodide; after baseline subtraction. Values of ip-iodide increase
linearly with iodide concentration. The measure ih is used at
high concentration when peaks are too broad to observe the
leading baseline.
C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201 189
baseline of scans run at 1000mV s�1 between �0.1and �1.8V vs. Ag/AgCl after a 10-s conditioningstep at �0.1V (Fig. 2A). It may be assumed thatpeak currents (ip-oxygen) determined in this way areproportional to oxygen concentration, yielding
½O2x ¼ipx
ipx¼0
½O2BW; ð2Þ
where x represents a depth below the sediment–water interface (x ¼ 0) and [O2]BW is the dissolvedoxygen concentration in the bottom water.
Above the sediment–water interface, the sensi-tivity of voltammetric electrodes of this size wasobserved to be influenced by the hydrodynamicflow such that scans were often irregular andpoorly reproducible. The sediment–water interfaceand ðip-oxygenÞx¼0 were defined by approaching thesediment surface incrementally, observing thesensor by close-up video, and monitoring the formof linear sweep scans. However, this interface wasnot a truly fixed boundary. Instead it was observedto gain and lose up to 2mm during an experimentas sand particles and shell fragments were movedto and fro by the forces of the water motion.Although iodide may also be detected by linear
sweep voltammetry, we used a more sensitivesquare wave technique (0.4mV scan increment,200mV s�1, 15mV pulse height) to detect thearrival of iodide tracer at discrete sand depths(Luther et al., 1998). Scans were programmed torun sequentially between �0.05 and �0.85V (or insome cases �0.45V) with 5 s of deposition timeapplied at �0.05V at the beginning of each scan.Because iodide adsorbs as a surface film on Hg atX�0.1V, well-defined peaks in resultant currentare produced due to the oxidative stripping of theadsorptive Hg complex (Luther et al., 1998). Theheights of these peaks (ip-iodide) were tabulatedafter subtraction of the baseline current asillustrated in Fig. 2B, or in the case of latercontrol experiments simply as the maximum heightof the primary peak above the cathodic baseline(ih). For typical electrodes that are calibrated overan iodide concentration range of 2–25 mM inanoxic seawater at 20�C, ðipÞiodide (Fig. 2B) (orelse the resolved peak area) is the preferredmeasure because it increases as a linear functionof concentration with a calibration slope ofapproximately 1 nA mM�1 (or 1 nC mM�1), andsuch calibrations yield minimum detection limitsof B1 mM, and zero intercepts. However, foriodide concentrations increasing beyond 25 mM,peaks become too broad to define the leadingportion of the baseline. The measure (ih) (Fig. 2B)continues to increase beyond concentrations of300 mM but this trend is non-linear with a non-zerointercept created by the skewing effects of thechloride signal added to the baseline. Iodidecalibrations run in the presence of dissolved O2
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by either method of peak definition exhibitpositive (non-zero) intercepts due to the overlapin the potential ranges that are electroactive for I�
and O2.
2.7. Control experiments
To check whether tracer delivery or channelingalong the electrode could contribute to theadvection of iodide through sands, two types ofcontrol experiment were carried out. In the first,the tripod and all its equipment were positionedover a flat sand bed that was created from grabsamples collected from the study site in November2001. The sand bed was 6 cm deep, containedliving benthic fauna, and was overlain by 9 cm ofseawater (1072�C) in a 34 cm wide� 67 cm longtrough placed on the ship’s deck. The ship’smotion and wind caused some agitation of thewater. Using the same procedures as during in situexperiments, the voltammetric sensorwas lowered until the sediment–water interfacewas detected and then lowered until the tip was at0.8 cm depth. As the diffuser tips of the iodidedelivery array were set 2.5 cm above the electrodetip, their injection height was 1.7 cm above thesand.Without the benefit of bottom water exchange,
the tracer pulse injected for 20 s during this controlexperiment, did not dissipate. Scans for detectingiodide at 0.8 cm depth were run in a continuoussequence for 4.75 h (or six times the length of thelongest in situ experiment). Because no iodide wasdetected over this time, the voltammetric sensorwas then raised to the overlying water and used tomeasure three consecutive depth profiles of dis-solved iodide separated by horizontal distances ofseveral centimeters and in steps of 0.5–2mm. Thethird profile was completed 6.25 h after the tracerinjection.The second type of control experiment was
completed in our shore-based laboratory at19.770.5�C. A voltammetric sensor was intro-duced with a micromanipulator into a bed of sandfrom the LEO-15 study site, contained within a1 liter plastic jar. The arrival and then rate ofchange in iodide peak current were measured atone depth, 2.0 cm, when the sand bed was overlain
by stagnant seawater. Tracer was added to theoverlying water with a pipet with very littleimposed mixing. We later estimated a dispersioncoefficient for iodide from the Einstein–Smolu-chowski relation
Dx ¼L2
2tD
ð3Þ
that relates the displacement of a concentrationpulse to elapsed time (Boudreau, 1997). In thisapplication, L was equated to the sedimentthickness (or depth of the electrode tip), and tD
to the time-lag before breakthrough that isapproximated graphically as the zero concentra-tion intercept of the linear portion of a concentra-tion–time curve (Meier et al., 1988, 1991). Theexpectation was that if no pore-water advectionwas introduced by our methods, derived values ofDx should approximate the molecular diffusioncoefficient for iodide in sediments (i.e., D�
s ofEq. (1)) at the experimental temperatures. Grossdeviation from diffusion behavior in either type ofcontrol experiment would be evidence of experi-mental artifacts.
3. Results
3.1. Seafloor properties and diffusion coefficients
During the 4 days of this study the percentsurface photosynthetically available radiation(PAR) reaching 10m depth (or 2–3m abovebottom) averaged 0.370.1% (n ¼ 6). In this dimlight, sand ripples were observed by video anddivers to cover the seafloor and to be formingactively. Porosity, permeability and fine fractiondata for both ripple crests and troughs arepresented in Table 2. The surface intervals of sandfrom troughs were enriched in fine particlescompared to deeper intervals and crests. However,the porosity and permeability determinations ofwhole cores (lengths: 6.8–8.8 cm) from either crestsor troughs fell within a narrow range.Whole sediment diffusion coefficients for solutes
may be estimated by dividing molecular diffusioncoefficients by a tortuosity factor equal to theporosity times the sediment formation factor, F
ARTICLE IN PRESS
Table 2
Sediment properties
Sample location
in ripple field
Depth interval (cm) N Porosity Permeability (m2) Weight % fines
(o60mm)
Crest 0–6.8 1 0.38 2.4� 10�11
Trough 0–8.0 1 0.37 2.0� 10�11
Trough 0–8.8 1 0.37 2.3� 10�11
Crest 0–1 3 0.06370.043
Crest 1–2 3 0.03770.003
Crest 2–3 3 0.04170.008
Crest 3–4 3 0.04470.006
Crest 4–5 3 0.05270.007
Trough 0–1 3 0.31070.073
Trough 1–2 3 0.11270.050
Trough 2–3 3 0.06270.018
Trough 3–4 3 0.05670.008
Trough 4–5 3 0.06770.014
C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201 191
(Berner, 1980):
D�s ¼
D
fF: ð4Þ
We have made in situ measurements of electricalresistivity Rx in these sediments and calculatedformation factors as F ¼ Rx=Rbottom water (McDuffand Ellis, 1979; Reimers et al., 2001). Throughoutthe uppermost centimeters of the sediment columnF ¼ 323:5: Estimates for the molecular diffusioncoefficient of I� in seawater between 10�C and20�C are from 1.4 to 1.8� 10�5 cm2 s�1 (Li andGregory, 1974; Boudreau, 1997). Accordingly,during the experiments in this study D�
s for iodideis estimated to have equaled approximately 1.1–1.6� 10�5 cm2 s�1.
3.2. Control experiments
No iodide was detected at 0.8 cm depth after4.75 h of voltammetric monitoring during the firstcontrol experiment run with the complete tripodand iodide delivery system placed over an enclosedsand bed. Iodide concentration profiles measuredwith the same sensor over the next hour and a half(Fig. 3A), however, indicated tracer concentra-tions at 0.8 cm that were near the sensor’sdetection limit and a concentration gradient
throughout the first centimeter of sand that isconsistent with diffusive transport. The minimumtime-lag for tracer arrival assuming only diffusivetransport and D�
s ¼ 1:1� 10�5 cm2 s�1 shouldhave been 8 h according to Eq. (3). A larger D�
s
caused by a smaller than estimated tortuosityeffect or a small degree of dispersion at thesediment–water interface could explain the slightlyshorter time delay. Advective transport caused bytracer injection was not indicated.Similarly, the laboratory control experiment
showed a pattern of tracer transport that may bepredicted assuming vertical molecular diffusion.Sensor scans at 2.0 cm were monitored longenough to observe steadily increasing iodideconcentrations (Fig. 3B). The tD value of thisexperiment (22.1 h) predicts a sediment dispersioncoefficient (Dx) of 2.5� 10�5 cm2 s�1. For ourpurposes this value is close enough to the predictedmolecular diffusion coefficient to assert thatintroduction of a microelectrode into the sanddoes not provide a path that promotes tracerpenetration.
3.3. In situ experiments
The conditions that distinguish the eight in situexperiments from the controls are the presence of
ARTICLE IN PRESS
0 10 20 30 40
Time after Tracer Injection (hr)
0
10
20
30
40
50
60
70
80
90
100
110
120
Iodi
de P
eak
Hei
ght (
nA)
tD
L= 2.0 cm
(B)
0 5 10 15 20Iodide (µM)
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
Dep
th in
San
d (c
m)
P1P2P3
(A)
Fig. 3. (A) Iodide profiles from the shipboard control experi-
ment measured 4.75–6.25 h after tracer release from the diffuser
array. The deepest point of each profile represents the last point
at which an iodide peak could be clearly separated from the
signal produced by dissolved O2. Horizontal error bars
represent71 standard deviation of replicate measurements
(usually 3) at each electrode position. (B) The results of a
second laboratory control experiment presented as a break-
through curve for tracer diffusion into sand from the study site,
L ¼ 2:0 cm.
C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201192
bedforms and oscillating flow. Table 3 charac-terizes these experiments according to the sedimentdepth (cm) monitored for tracer arrival, locationrelative to bedforms, duration of monitoring, andduration of tracer release. Each experiment is alsodesignated by a tripod deployment code (e.g.,D11). During tripod deployment D13, two sepa-
rate tracer experiments were conducted. The firstwas at 0.9 cm; the second was at 0.6 cm andinitiated 50min after the first. Fig. 4 illustrates theexperimental operations overlain on the record ofcurrent speed and wave-generated pressure varia-tions for experiment D13-0.9. This data formatreveals that peak current speeds and wave heightsoften coincided. Average current and wave condi-tions for all eight in situ experiments were notsignificantly different, but the temperature of thebottom water decreased over the 3 days ofoperations (Table 4). This shift was caused bylocalized upwelling and the transport of offshorewaters shoreward.The average wave period was 7 s (range 6–7
between experiments). By comparing ripples ob-served in video records to objects of known scale(such as the diffuser array), we estimated mostripples to have heights of B3 cm (Table 3).Traykovski et al. (1999) describe the predominantripple pattern at LEO-15 as two-dimensional waveripples with a mean ripple steepness (height/wavelength=0.15). This would suggest ripplewavelengths B20 cm during this study. Using therelationships of Airy wave theory (Komar et al.,1972) to make some approximate calculations ofthe bottom water orbital motion under wavesduring our observations, an average orbitaldiameter do ¼ 17 cm is computed. This measurealso is expected to approximately equal the ripplelength (Komar et al., 1972). Using the Grant andMadsen (1982) model of bedform-generatedroughness in oscillatory flow, average surfacewave conditions during all deployments were morethan sufficient to erode sand. The percentage of allwaves recorded that generated wave shear velo-cities in excess of the threshold shear velocity forsand movement ranged from 50% for D11 to 87%for D12.Tracer penetration was detected in every in situ
experiment except D21-2.0, the only one samplingat a ripple crest. Vertical oxygen profiles andiodide breakthrough records are presented inFig. 5. Oxygen concentrations were measured atincreasing depths into the sediment until reachingthe depth chosen for tracer detection. Then aftertracer detection and dissipation, the oxygen profilewas resumed in some cases (Fig. 5). Experiment
ARTICLE IN PRESS
Table 3
Conditions of in situ experiments
Experiment
designation
Depth of sensor
tip in sand (cm)
Location/microtopography/flow and experimental observations Duration of
scanning (mm:ss)
Tracer pulse
(mm:ss)
D11-0.4 0.4 Area with irregular relief, 1–2 cm in height; sand transport
D15-1.0 1.0 Trough between ripples, shell debris concentrated in trough;
experiment was aborted after tripod lifted due to tension on
tether
10:58 02:00
D14-1.5 1.5 Trough between ripples; oscillating sand transport especially at
ripple crests
19:08 05:00
D16-2.0 2.0 Base of ripple flank, B8 cm away from crest; ripple height
B4 cm and building
47:32 03:13
D21-2.0 2.0 Ripple crest; ripple height B4 cm; oscillating sand transport at
ripple crest, produced episodic scouring and infilling of grains
around sensor; no tracer arrival was detected
24:24 02:49
7:46:30 7:51:30 7:56:30
Time
0
0.1
0.2
0.3
0.4
0.5
Cur
rent
spe
ed (
m s
-1)
11
11.5
12
Pre
ssur
e 1
mab
(db
ar)tracer release iodide detection at 0.9 cm
Fig. 4. A wave and current record from 1m above the seafloor during the time interval voltammetric scans were run in a repetitive
sequence during experiment D13-0.9. The time intervals of tracer release and detection at 0.9 cm are indicated, as are the greatest 5% of
all current velocities (D) and wave heights (}) measured during this deployment.
C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201 193
D15-1.0 had to be aborted soon after the tracerwas detected because the tripod lifted momentarilyfrom the bottom due to tension on the tether.
Experiment D21-2.0 was also stopped (after24min 24 s) because of worsening sea conditionsand a high degree of sediment movement and
ARTICLE IN PRESS
Table 4
Physical conditions during tracer experiments
Experiment Horizontal flow
speed at 1mab
(m s�1)
Water column
height at 1mab
(m)
Surface wave
height (m)
Water temp.
(�C)
D12-0.6 0.1370.08 11.570.2 0.3470.17 18.070.4
D11-0.4 0.1070.05 11.870.1 0.2170.11 20.470.4
D13-0.6 0.1670.09 11.970.2 0.3370.16 16.670.0
D13-0.9 0.1570.09 11.770.1 0.2970.16 17.770.3
D15-1.0 0.1270.07 12.070.1 0.2770.13 18.070.8
D14-1.5 0.1370.08 12.670.1 0.3070.18 17.470.4
D16-2.0 0.1270.07 11.670.1 0.2570.13 14.770.2
D21-2.0 0.0970.05 12.970.1 0.2370.11 15.671.2
Note: Except for wave height, each value represents the average of current meter readings every 1 s over the time interval when scanning
for iodide (Table 3). Individual wave heights were derived from the difference of alternating maximum (crest) and minimum (trough)
pressures and averaged over the scanning time.
C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201194
associated scouring at the ripple crest (Table 3).We did not see (through the close-up video)indications of tripod movement or scouringaround the sensors during all other experiments.However as indicated earlier, sand grains and shellfragments at the very surface of the sediment wereoften in oscillatory motion.Tracer advection rates were calculated as
vertical velocities by dividing the sediment depthof tracer detection by the time delay. The latterwas calculated as the difference between the tracerbreakthrough time and the mid-point of the tracerrelease pulse (Fig. 5). Although the conditionsduring each experiment were in detail unique, theuniformity of water depth, wave characteristics,and sediment type allows intercomparison bysediment depth (Fig. 6). Error bars in Fig. 6represent the ranges in computed velocitiesthat result if delay times are calculated as break-through time minus either the start or endpoint of the tracer pulse. Since in D11-0.4, tracerarrival was detected before the end of theinjection pulse, no upper limit is shown for thevelocity at 0.4 cm (Fig. 6). The range for eachexperiment tightens with increasing sedimentdepth and as the ratio of breakthroughtime to the duration of the release pulse increases.The derived velocities generally decrease withincreasing depth in the sand, and an exponentialcurve is fit through the data to emphasize thistrend.
3.4. Column experiments
The flow rate of seawater through the columnexperiments was set to vary from 1.20 to5.56ml cm�2 h�1 or 288–1334 lm�2 d�1 (Table 1).Given an average sediment porosity of 0.37 (Table2), these pumping rates would propagate a fluidfront down column at 3.2–15 cmh�1, in keepingwith the in situ velocities between 0.4 and 2.0 cmdepth (Fig. 6) and probably a few centimetersdeeper.The oxygen consumption rates for each sedi-
ment column at known pumping rates werecalculated from ([O2]inflow�[O2]outflow) times thevolume of water (pore space) in each column timesthe flow rate. These values were then normalizedto a conservative flushing depth of 4 cm (assumedbased on the observed depth of pore-water oxygenpenetration, Fig. 5) to yield oxygen fluxes per areaof seafloor (Fig. 7). Since the column oxygenmeasurements were recorded first with six freshlyretrieved cores at two flow rates, then repeated thenext day at the slower flow rate (plus two columns;Table 1), the results in Fig. 7 are separated intotwo groups of measurements. After 20.5 h orgreater of incubation, the oxygen consumptionrates were all higher than initial values at similarflow rates and more reproducible between col-umns. The relatively large scatter in the initial datamay be due to sediment heterogeneity, or differentdensities of micro-, meio- and macrofauna in the
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C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201 195
columns.4 Assuming a flushing depth of 4 cm,these first sediment column experiments indicatethat the oxygen flux to the sediment decreases onthe order of 0.029mmolm�2 d�1 when the rate offlushing decreases by 1 lm�2 d�1. Deeper flushing(e.g., if rates were normalized to 6 cm instead of4 cm) would increase the estimated rates and theslope of the flow rate vs. flux relationshipproportionally. We have measured sediment oxy-gen penetration depths with both voltammetricand amperometric microelectrodes at this siteranging from 0.4 to >6 cm at different timesbetween April 2000 and July 2001. These observa-tions indicate the flushing depth deepens as waveconditions and associated sand ripples build.
4. Discussion
4.1. Patterns of advective transport
Streamlines and rates of wave-induced advectivetransport within a rippled sand bed have beenpredicted with two-dimensional steady state trans-port models, compared to diffusive transport, andused to argue for high organic matter processingrates in continental shelf sediments (Shum, 1992,1993; Shum and Sundby, 1996). These patternshave also been followed in flumes and in fieldstudies of rippled intertidal sandflats with dyetracers and natural solutes (Huettel et al., 1996,1998; Ziebis et al., 1996; Huettel and Webster,2001; Precht and Huettel, 2003). Although depen-dent on the prevalent wave conditions, the rippleprofile, and sediment properties, net advectivetransport is expected to be downward into thesands of ripple troughs and flanks, and to lead outof the bed near crests. While following thesecourses the trajectories of discrete pore-waterparcels are predicted to vary much more widely.The measurements of iodide transport from this
study are in remarkable agreement with thephysical models and provide the first estimates of
4Macrofaunal counts were: column 1, 1 small bivalve;
none; column 5, 1 amphiurid; column 6, 2 small polychaetes;
column 7, 2 small amphiurids; and column 8, none.
the magnitude of advective flows within shelfsediments under fairly low-energy sea conditions.The one experiment at a ripple crest (D21-2.0),although only maintained for 24.4min, showed notracer arrival, consistent with the crest being azone of fluid extrusion. Interfacial advectivevelocities at ripple troughs and flanks wereobserved to be into the sand and to vary fromapproximately 53 to 6 cmh�1 (Fig. 6, mid-pointvelocities, 0.4–2.0 cm, respectively). These veloci-ties are approximately an order of magnitudegreater than rates of penetration observed in theflume studies of Huettel et al. (1996). However,these flume studies were conducted under steadyhorizontal flow velocities selected to equal10 cm s�1 at 10 cm above the bed (compared tothe non-steady oscillating in situ flow), and theflume sands were finer (median grain size 250–300 mm compared to 400–500 mm).In this study, distance-averaged intrusion velo-
cities generally decreased with sediment depth, andthe depth of advective penetration can be inferredto have been approximately 4–4.5 cm (where porefluids became anoxic; Fig. 5). Interestingly, theflume studies of Huettel et al. (1996) showedsimilar penetration depths, and their verticalintrusion velocities (derived by time lapse photo-graphy of tracer fronts) also decreased rapidlywith sediment depth.The records of near-bottom current velocities
and pressure fluctuations (Table 4; Fig. 4) fromthis study suggest that forces from waves andcurrents commonly peak every few minutes whenthe largest 5% of waves and current velocitiescoincide. In turn, the iodide tracer breakthroughrecords show evidence of non-steady interstitialadvection. Breakthrough records that exhibitedabrupt peaks that were shorter in duration thanthe corresponding tracer release pulse (e.g., D11-0.4, D14-1.5), or that were bimodal (D12-0.6),suggest bursts of intrusive flow followed bydispersive dissipation. Relatively long periods oftracer detection, such as was observed during D16-2.0, suggest an absence of vigorous flushing afteran initial tracer intrusion event.Further evidence of in-bed temporal changes
being driven by the changeable flow is found inthe voltammetric electrode profiles of dissolved
ARTICLE IN PRESS
0 40 80 120 160 200Oxygen (µM)
5
4
3
2
1
0D
epth
in S
and
(cm
)
0 2 4 6 8Elapsed Scan Time (min)
0
100
200
300
400
500
Pea
k H
eigh
t (nA
)
D110.4 cm
0 40 80 120 160
Oxygen (µM)
5
4
3
2
1
0
Dep
th in
San
d (c
m)
0 2 4 6 8 10 12Elapsed Scan Time (min)
0
20
40
60
80
100
120
140P
eak
Hei
ght (
nA)
D120.6 cm
0 2 4 6 8Elapsed Scan Time (min)
0
10
20
30
40
Pea
k H
eigh
t (n
A)
D130.6 cm
0 2 4 6 8 10 12Elapsed Scan Time (min)
0
4
8
12
16
20
Pea
k H
eigh
t (nA
)
D130.9 cm0 40 80 120 160
Oxygen (µM)
5
4
3
2
1
0
De
pth
in S
an
d (
cm)
C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201196
ARTICLE IN PRESS
Fig. 5. Oxygen profiles (left) and tracer breakthrough records (right) measured in situ with voltammetric electrodes. The results for
each experiment are presented in order of increasing sediment depth for tracer detection. The error bars for each oxygen measurement
represent the standard deviation about the mean of 3 or 4 scans with (~) representing measurements before the tracer release and (U)representing measurements after. Profiles were extended deep enough to penetrate anoxic sediments during only two deployments.
Within the breakthrough plots the bold horizontal bar corresponds to the time interval of the tracer release pulse. The arrow points to
the designated ‘‘breakthrough’’ time. The horizontal scales vary according to the duration of scanning (Table 3).
0 40 80 120 1605
4
3
2
1
0D
epth
in S
and
(cm
)
0 2 4 6 8 10 12 14Elapsed Scan Time (min)
0
10
20
30
Pea
k H
eigh
t (n
A)
D151.0 cm
-tripod lifted
0 40 80 120 1605
4
3
2
1
0
Dep
th in
San
d (
cm)
0 4 8 12 16 20Elapsed Scan Time (min)
0
10
20
30
40
50
60
70
Pea
k H
eigh
t (n
A)
D141.5 cm
0 10 20 30 40 50Elapsed Scan Time (min)
0
4
8
12
16
20
Pea
k H
eigh
t (n
A)
D162.0 cm
0 40 80 120 160Oxygen (µM)
5
4
3
2
1
0
Dep
th in
San
d (
cm)
Oxygen (µM)
Oxygen (µM)
Fig. 5 (continued).
C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201 197
ARTICLE IN PRESS
0 0.5 1 1.5 2 2.5
Depth in Sand (cm)
0
20
40
60
Vel
ocity
(cm
hr-1
)
Velocity = 47.7e-1.12x
R2 = 0.74
Fig. 6. Distance-averaged advection rates vs. depth in the sand
in zones of bottom water intrusion. Symbols indicate rates
calculated from delay times measured from the mid-point of the
tracer release pulse to the breakthrough time. Ranges represent
rates based on times estimated from either the beginning or end
C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201198
oxygen. Oxygen concentrations measured at dis-crete depths on time scales of several minutes (3–4scans, then averaged to give points within profiles),or as sets of scans at the same location before andafter tracer experiments, often exhibited highvariance (Fig. 5). These examples have highervariance than the analytical precision reflected inbottom water values (Fig. 5) and can be explainedif each single scan ‘‘sampled’’ a unique parcel ofpore fluid while it and neighboring parcels were inmotion under a rippled bed. The irregular shapesof the vertical oxygen profiles are also signaturesof complex, pressure-driven transport (Ziebis et al.,1996; Lohse et al., 1996). Since streamlines mustbend and run horizontally to connect zones ofintrusion and extrusion (Huettel et al., 1996), thesubsurface gradient reversal, seen for example inthe oxygen profile from D13, indicates compli-cated pore-water exchange.
4.2. Oxygen fluxes to sands under waves
The finite length of each tracer release and theprobability that only some parcels of the tracerpulse entered the bed in the vicinity of thevoltammetric sensor were reasons to calculate a
range of pore-water velocities for each in situexperiment in this study (Fig. 6). However, it canbe concluded without doubt that bottom waterintrusion does occur between ripples and that theflow rates of 288–1334 lm�2 d�1 (3.2–15 cmh�1, asapplied to the sediment columns on shipboard) arerealistic. As oxygenated bottom water is forcedinto permeable sediments by the oscillatorypressure field, suspended organic particles will becarried along, trapped and degraded (the biocata-lytic filter effect, Rusch et al., 2001). Bacterial andalgal cell abundances and particulate organiccarbon and nitrogen contents in additional corestaken during this study showed near-surfaceenrichments but also high between-column
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C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201 199
variability (Rusch et al., 2003). This spatialvariability, as well as differences in meio- andmacrofauna, and the complex coupling betweenorganic substrate lability, oxygen supply, andbenthic respiration are presumed to explain thescatter in the oxygen consumption rates observedat individual flow rates in the column experimentsbeginning just 3.25 h after core retrieval (Fig. 7open symbols).Although the oxygen consumption rates for the
column experiments were not uniform initially,each decreased by similar amounts when flow rateswere lowered. We interpret this result to indicatethat rates of aerobic organic matter oxidation inpermeable sediments are flow-dependent, probablybecause higher rates of flow cause greater disper-sion and greater penetration of dissolved oxygeninto microenvironments within a heterogeneoussand column. It is also suggested that oxygenconsumption must vary considerably in space andtime depending on the conditions controlling thedepth (and thus volume) of sediment affected byflow. Chemical oxidation rates (e.g., by sulfideoxidation) were not a factor in our columnexperiments because the sands were kept oxicunder flow. When the sand columns had incubatedfor > 20:5 h under steady flow, oxygen consump-tion rates rose to more consistent values. This maybe due to an increase in the metabolic rate ofsediment-attached bacteria (perhaps stimulated bydecaying biota trapped in cores), or to thedevelopment of mutualism between microorgan-isms. Whatever the explanation, the importance ofbiological variables in regulating oxygen consump-tion in the presence of pore-water flow needsfurther investigation.The results presented in Figs. 6 and 7 when
considered together suggest benthic respirationrates at LEO-15 range between 10 and 40mmolO2m
�2d�1 under modest summer waves. Suchrates bracket an earlier estimated mean (12.8mmolO2m
�2d�1) for sandy regions of the continentalshelf south of New England based on whole coreincubations without flow (Rowe et al., 1988).However, neither the earlier core experiments norour column experiments included light effects onnet oxygen fluxes. Benthic microalgal primaryproduction rates of oxygen have been observed to
nearly cancel dark oxygen consumption on thesoutheastern USA continental shelf in summer(Jahnke et al., 2000). The light levels at the seafloorof the southeastern USA shelf are nearly alwayssignificantly higher in intensity and percent surfaceirradiance (even at water depths of 30m; Nelsonet al., 1999) than what we have measured at LEO-15, but longer-term and more wide-spread lightrecords are needed to assess the role of light forbenthic metabolism across the MAB. We can addanecdotally that we have measured steadilydecreasing bottom water O2 levels (and risingtotal-CO2 levels) during several multi-day periodsof summer water-column stratification at LEO-15,and local hypoxia occurs in years when theseconditions persist. Thus, most available evidencesuggests a dominance of benthic respiration overbenthic primary production in the New Jerseycoastal region in contrast to the southeastern USAshelf. We conclude that benthic oxygen consump-tion is significantly enhanced under waves by pore-water flow-through ripples, but without moremeasurements under varying hydrodynamic con-ditions we cannot predict the full impact of suchsand filtering processes on the MAB shelf ecosys-tem. More measurements of the kind pioneered inthis study should also be undertaken with light anddark benthic chamber measurements in otherregions with different types of permeable sedi-ments. By such efforts, oceanographers may cometo describe the effects of wave-induced advectivetransport on fluxes of solutes and the processing oforganic detritus in the coastal ocean.
Acknowledgements
This study would not have been successfulwithout the assistance of several generous, patientand hard-working people. Dr. George Luther III(U. Delaware) helped us to master the voltam-metric techniques and loaned us critical electrodepolishing equipment from his laboratory for thiswork. Dr. Don Nuzzio (AIS) also helped us totrouble shoot the underwater voltammetric equip-ment and provided backup circuit boards. DavidLovalvo (Eastern Oceanics) developed parts of thetripod specifically for these experiments and
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C.E. Reimers et al. / Continental Shelf Research 24 (2004) 183–201200
helped design the diffuser array. A team of diversfrom Rutgers University collected the cores. Thetechnicians, crew and captain of the R./V. Cape
Henlopen kept the ship on three-point anchor andassisted with the deployments. Dr. Robert Keyassisted with the deployments and water sampling.Drs. Stefan Forster and Bernard Boudreau pro-vided helpful reviews of the initial manuscript.This work was supported by a grant from the
US National Science Foundation to CR, GT andCF. Support for the participation of MH, AR andCW was provided by the Max Planck Institute forMarine Microbiology, Germany.
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