Particle Ux in the Rainbow Hydrothermal Vent

Journalof Marine Research, 59, 633–656, 2001 Particle uxinthe Rainbow hydrothermal vent eld (Mid-AtlanticRidge): Dynamics, mineral and biological composition byAlexis Khripounoff 1,Annick Vangriesheim 1,PhilippeCrassous 1, MichelSegonzac 1,Ana Colac¸o 2,DanielDesbruye `res 1 andRoxane Barthelemy 3 ABSTRACT Inorder to provideinformation about the export and the distribution of hydrothermalparticulate materialto the surrounding deep ocean, four moorings were deployed in the vicinity of the hydrothermalRainbow vent eld(Mid-Atlantic Ridge, 36° 14 9N,2250m depth).The rstmooring wasa sedimenttrap with a currentmeter deployed at 2 mfroma chimneyof the Rainbow vent eld and1.5 m abovethe bottom(a.b.) for 16 days. It representedthe reference for the initial composition ofparticles produced by the vent. The total mean mass particle ux(6.9 g m 22 d21 )wasdistinctly higherthan the uxmeasured at the shallower hydrothermal vents on the MAR segment.This particulate uxshoweda hightemporal variation at thescale of afewdaysand was characterized by ahighconcentration of sulphur(17.2%) and copper (3.5%) and a verylow concentrationof organic carbon(0.14%). Several hundred bivalve larvae belonging to thehydrothermalmytilid Bathymodio- lusazoricus werecollected in this trap at the beginning of the experiment. The density of larvae decreasedstrongly at theend, indicating a patchinessdistribution or adiscontinuousreproduction of thisspecies. The other three moorings, including sediment traps, current-meters and thermistor chains,were deployed for 304 days at differentdistances and altitudes from the Rainbow vent eld. Themean speed of the current in the rift valley was low (6 cm s 21 )andwas oriented toward the north.The total mean particle mass uxmeasuredwith the vesedimenttraps varied little, from 10.6 to 25.0 mg m m22 d21 ,anddisplayed temporal variations which are typical of deep-sea environ- mentswith seasonal changes in the overlying production. However, in the trap at 500 m fromthe vents150 m a.b.,the presence of the hydrothermal plume can be observed: the sulphur, iron and copperconcentrations of particleswere signi cantly higher compared to the particles sampled in the pelagicreference trap. The plume composition was about 50% hydrothermal particles and 50% pelagicparticles and its upper limit reached 300 m a.b.at this distance. In thetraps at 1000 m from thevents, the elemental composition of particleswas similar to thepelagic particles and we assume thatthese traps were not in the plume during the experiment. The zooplankton obtained in the long-termtrap samples revealed high density variations in relationto thedistance from the vent site. Thenutrient enrichment around the hydrothermal area and the abundance of free living bacteria explainthese variations in zooplankton density. 1.IFREMER Centre deBrest, De´partement DRO/EP,BP 70, 29280 Plouzane ´,France. email: alexis.khripounoff@ifremer.fr 2.IMAR, Laborato ´rioMaritimo da Guia,Faculdade da Ciencias deLisboa,Estrada doGuincho,2750 Cascais, Portugal. 3.Universite ´deProvence, Laboratoire de Biologie Animale, 3place VictorHugo, 13331 Marseille Cedex3, France. 633 634 Journalof MarineResearch [59, 4 1.Introduction Themost spectacular manifestations of hydrothermalactivity are the high-temperature vent elds,which create particle-rich plumes rising hundreds of metersabove the sea oor. Theproduction of this particulate material and its dispersal to the deep ocean mostly dependson theemission of vent uidsthat are precipitated as neparticlescarried by the neutrallybuoyant hydrothermal plume. The hydrothermal plumes, transported by thedeep currents,can be detectedup to100 km from theemitting vent (Dymond and Roth, 1988; German andSparks, 1993). The study of plumesis nowan importantpart of hydrothermal researchbecause of the in uence of vent uidson the ocean chemical balance (see Eldereld and Schultz, 1996). Hydrothermal activity also supports the production of biomasssuch as planktonic larval stages or juvenilesof vent species. These are carried by thenear-bottom currents and/ orthebuoyant hydrothermal plume and are then transported severalhundred meters above the bottom (Herring and Dixon, 1998; Kim et al., 1994; Kim andMullineaux, 1998; Mullineaux and France, 1995; Mullineaux et al., 1995).Through theseprocesses, hydrothermal plumes participate in the dispersal of ventspecies along the mid-oceanridges and constitute a possiblefood source for thesurrounding deep-sea fauna. Theaim of this work is to assess the in uence of the hydrothermal plume in the surroundingocean. The Rainbow hydrothermal eld(36° 15 9N)was chosenbecause it producesthe strongest plume of itskind known on theMid-Atlantic Ridge (German et al., 1996;Fouquet et al., 1997),making it an ideal natural laboratory to study plume processes. Toprovideinformation about production and exportof hydrothermalmaterial by ventsand plumesto the deep ocean, four moorings with sediment traps, current meters and thermistorchains were deployedat differentdistances from theRainbow site for periods varyingfrom 16daysto 10 months. 2.Materials and methods a.Study area (Fig. 1a) TheRainbow site (36° 14 9N, 33°549W,2250m depth)was detectedon the Azores Mid-AtlanticRidge segment (AMAR) in1996 (German et al., 1996)and observed by submersiblein 1997 (Fouquet et al., 1997,2001; German et al., 2001).It is situated on the ankof araisedblock of ultramac basementin the discontinuity between two segments of theridge and produces a strongand persistent hydrothermal plume. It is oneof themost activehydrothermal vent eldsdiscovered in the North Atlantic with at least ten major groupsof extremelyactive black smokers without visible fauna. Some diffusive vents are present,located 25– 100 m from blacksmokers with small mussel beds and shrimp swarms (Desbruye`res et al., 2001). b. Sampling i.Mooring S (Short-termmooring with small sediment traps). OntheRainbow hydrother- malvent eld(Fig. 1a), one triplicate sediment trap frame (trapS) was deployed 2001] Khripounoffet al.:Hydrothermal particulate material 635 Figure1. (a) Bathymetric map showing the location of theRainbow eldwith the positions of the differentmoorings. (b) Scheme of thelong-termmoorings. (36°13.8N, 33° 54.1W) during the Marvel cruise (August 1997) by thesubmersible Nautile for 16daysat 2 minthesouthwest of anactive vent eld(2275 m depth).The description ofthe deployment method is detailed in Khripounoff and Alberic (1991). The top- collectingsurface of traps was situated1.5 m abovethe bottom (a.b.), while an Aanderaa current-meterwith a recordinginterval of 5 minuteswas positioned2 mabovethe trap. Thesettling particles were collectedin 3 epoxy breglasstraps of cylindrical-conic shape with5 collectionbottles each and assembled (S trap)on a singleframe (Khripounoffand Alberic,1991). Only 4 collectorswere usedwith a samplingperiod of 4 dayseach. Each sedimenttrap had a samplingaperture of 0.07 m 2 andwas coveredwith a honeycomb 636 Journalof MarineResearch [59, 4 bafe of1 cmdiameter and 10 cmdeepcells at the top. Prior todeployment, the sampling bottlesof twotraps were lledwith lteredseawater containing sodium borate buffered formalinto give a nalconcentration of 3% in order to prevent in situ microbial decomposition.The bottles of the third trap were lledwith a buffercomposed of 5M NaCl,10% DMSO and0.001% bromophenol blue to prevent in situ DNA degradation (Comtet et al., 2000).After recovery,the samples were storedin the dark at 4° C pending analyses. ii.Moorings L (Long-termmoorings with large traps). Threemoorings (Fig. 1b) with sedimenttraps, current meters and thermistor chains were deployedon theRainbow eld duringthe Marvel cruise (August 1997) and recovered during the Flame 2 cruise(June 1998).The settling particles were collectedover 304 days using cone-shaped traps (PPS5 model,Technicap ) with22 collection bottles. These traps had a samplingaperture of 1 m2 andwere coveredwith a honeycombbaf e of1cmdiameter and 10 cmdeepcells at thetop. As inthe S mooring,the sampling bottles were lledwith lteredseawater and sodiumborate buffered formalin to givea nalconcentration of 3%.The sampling periods were 14dayseach except for thelast collector (10 days). Theaim of the rst mooring(H500) was tosample the rising hydrothermal plume. Its positionwas selectedaccording to theCTD resultsobtained in this area by German et al. (1998).It was deployed ;500m tothe north of the Rainbow vents at 2300 m depth (M500)with two traps (150 m and300 m a.b.),two current-meters and a thermistorchain inbetween.Unfortunately, the trap at 150m a.b.did not work properly between February andApril (7 collectors). The location (34° 14.0N, 33° 54.0W) and depth are those obtained onthevessel at the mooring launching (Fig. 1). Due to a possibledrift during the mooring descent,its exact location on the bottom and its depth are given at 6100m. Thesecond mooring(M1000), with the same con guration as the rst,was usedto studythe neutrally buoyanthydrothermal plume. It was deployed ;1000m (36°14.2N, 33° 53.6W) to the north-northeastof thevents at 2250m depth.The last mooring (M Pelagic),with only one trapat 200m a.b.and one current-meter, was positioned(36° 13.3N, 33° 52.8W) at 1950 m depth,away from theaxis at ;2kmtothesoutheast of thevents. It was intendedto sample thepelagic uxonlyas areference(Fig. 1). On each of thethree moorings, an Aanderaa current-meterwas installed10 maboveeach trap. The current-meters were equippedwith anarrowrange temperature sensor (resolution: 0.006° C) anda pressuresensor. The current andits

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