- Email: [email protected]
A segment-scale survey of the Broken Spur hydrothermal plume
Deep-Sea Research I 46 (1999) 701—714
A segment-scale survey of the Broken Spur hydrothermal plume C.R. German *, M.D. Rudnicki, G.P. Klinkhammer Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK College of Ocean & Atmospheric Sciences, OSU, Corvallis, OR 97331, USA Received 7 August 1997; received in revised form 9 February 1998; accepted 3 April 1998
Abstract We conducted a segment-scale hydrothermal plume survey of the Broken Spur segment, 29°00-20 N, Mid-Atlantic Ridge (MAR). The purpose of the study was to identify the distribution of sources of venting throughout the segment as part of a larger study of hydrothermal fluxes. Evidence from plume particle concentrations (as deduced from in situ nephelometer data) and total dissolvable Mn (TDMn) analyses (from discrete water samples) indicated a restricted source of venting close to the segment centre, coincident with the previously known vent-site. No other pronounced plume signals were observed outside an area bounded by 29°07.5—12.5 N and 43°10—12 W, representing less than 10% of the '300 km of deep water ('2600 m) within the segment. In addition, however, low-level ((2 nmol l\) deepwater TDMn concentrations reveal a pervasive enrichment throughout the segment of *0.15 nmol l\. For the 4;10 m of deepwater within the Broken Spur segment, this corresponds to a standing crop of 6;10 mol of hydrothermal Mn. Future studies of long-term current flow will allow the flux of dissolved Mn out of the segment to be established and will investigate the partitioning of its source, between high temperature and axial diffuse flow. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction When high-temperature hydrothermal fluids issue from the seabed they mix with the overlying water column, and high concentrations of metal-rich sulphide and oxide particles precipitate (e.g. Edmond et al., 1982; Feely et al., 1987; Mottl and
*Corresponding author. Tel.: 0044 1703 596542; fax: 0044 1703 596554; e-mail: [email protected]. 0967-0637/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 8 ) 0 0 0 7 8 - 8
702
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
McConachy, 1990). The resultant buoyant, particle-laden plume rises upward, mixing progressively with the ambient water column until some level of neutral buoyancy is attained (Lupton et al., 1985). The height-of-rise of a hydrothermal plume is typically 100—300 m, during which time the initial vent fluid is diluted by a factor of approximately 10 with ambient seawater (e.g. Speer and Rona, 1989). Because the initial vent fluid may be enriched in certain tracers by factors of as high as 10 relative to ambient seawater, even a dilute neutrally buoyant hydrothermal plume may still exhibit pronounced geochemical enrichments. This has previously been demonstrated for dissolved He (e.g. Lupton and Craig, 1981), TDMn-total dissolvable Mn (e.g. Klinkhammer et al., 1985), dissolved CH (e.g. Charlou et al., 1987) and total suspended matter (e.g. Nelsen et al., 1986). Neutrally buoyant plumes extend much further, laterally, than the hydrothermal vent fields that source them. In previous work, these attributes have been exploited by using hydrothermal plume surveys to prospect for new sites of hydrothermal activity along mid-ocean ridges over length scales of up to 2000 km (see, e.g. review in Baker et al., 1995). On much shorter length scales, plume-process studies have been concentrated around individual vent sites (e.g. Klinkhammer et al., 1986; Kadko et al., 1990; Rudnicki and Elderfield, 1992, 1993; German et al., 1996a, 1998). In this paper, we present a mesoscale approach. One of the major questions established by the international Mid-Ocean Ridge research community has been: ‘‘What are the fluxes imparted from hydrothermal activity to the oceans, on a segment scale?’’ (Elderfield et al., 1996). To begin to address this question, RRS Charles Darwin cruise CD95 (Murton et al., 1995a) set out to investigate combined physical, chemical and biological fluxes in the Broken Spur segment of the MAR, 29°00-20 N. As part of that work we carried out a systematic three-dimensional survey of plume particle distributions and TDMn concentrations throughout the deep water column to investigate the location of high-temperature venting and the dispersal of its products within the segment. The results presented here were collected in parallel with complementary projects that examined the physical oceanography of the segment and the biology associated with the Broken Spur neutrally-buoyant plume (Murton et al., 1997; Herring and Dixon, in press).
2. Geologic setting The Broken Spur hydrothermal field was first located in 1993 from a combination of mid-water transmissometer and nephelometer anomalies (Murton et al., 1994). Subsequent submersible investigations using the DSV Alvin revealed an area of hightemperature hydrothermal venting approximately 100 m in diameter located atop an axial volcanic ridge close to the centre of the Broken Spur segment at 29°10 N (Murton et al., 1995b). This geological setting is very similar to that described for the location of the Snakepit hydrothermal field at 23°N, MAR (Gente et al., 1991). Discrete sources of high-temperature venting at Broken Spur were seemingly hosted by tectonism along the walls of an axial summit caldera (Murton et al., 1995b). Subsequent plume surveys, more spatially restricted than the present work, had provided
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
703
evidence for additional sites of venting close to the known Broken Spur vent site (James et al., 1995a, b; Lukashin et al., 1997), but these have yet to be confirmed by direct submersible observations (BRAVEX Science Team, 1994).
3. Methods and sampling Sampling for this study was carried out using the deep-tow hydrothermal plume instrument BRIDGET (Rudnicki et al., 1995). A limitation of many previous MAR plume surveys has been their reliance upon discrete vertical CTD-profiles for the collection of both physical data and water samples for subsequent shipboard or shore-based geochemical analysis. Three-dimensional along- and across-axis investigations of hydrothermal plume dispersal have been completed previously along various sections of the Pacific spreading centres (e.g. Baker, 1994; Baker et al., 1994; Urabe et al., 1995). In those cases, however, the hydrothermal plume has been demonstrated to rise clear of any axial graben, with dispersion of both dissolved and particulate discharge dictated by prevailing open-ocean currents (e.g. Lupton and Craig, 1981; Klinkhammer and Hudson, 1986; Kadko et al., 1990; Feely et al., 1994). By contrast, hydrothermal plumes rising above the Mid-Atlantic Ridge are typically constrained within the bounding topography of the deep rift-valley and can escape to the surrounding ocean only by advection along-axis toward the segment ends. It is this ‘‘bath tub’’ effect, peculiar to slow-spreading ridges such as the MAR, that our expedition was conceived to exploit (Murton et al., 1995a). A plan view of the BRIDGET lines occupied during our survey is shown in Fig. 1. Initially, a series of three along-axis lines was completed following the deep portion of the rift valley (*3200 m) along the west, centre and east of the segment (Lines B07, B11 and B12). A series of five across-axis surveys were then completed at regular (5 nautical mile) spacings at 29°10 N, 29°05 N, 29°15 N, 29°00 N and 29°20 N (Lines B14—18). Finally, a set of three higher-resolution survey lines were occupied, based upon earlier findings from the on-going survey (B19—B21): two further across-axis lines, at 29°07.5 N and 29°12.5 N, and a final short along-axis line from 29°12 N to 29°06 N. For all deployments, the package was towed at a speed of &1.5 kt whilst hauling and veering (raising and lowering) at a rate of 30 m/min\. BRIDGET was tow-yoed from depths of 2400 m to within 100 m of the seabed, which ranged in depth between 2600 and 3400 m but was typically at 2900—3200 m. In this way more than 300 vertical CTD-nephelometer profiles were collected throughout the deep water column of the Broken Spur segment. For each BRIDGET deployment, 12 watercolumn samples were also taken for shorebased TDMn analysis (cf. Klinkhammer et al., 1985). Samples were typically taken as two 6-point vertical profiles with one sample every 100 m between 2500 and 3000 m at those positions where the across-axis BRIDGET tow-yo lines crossed the outer along-axis lines (B07 and B12, Fig. 1). In addition, in later sampling, TDMn samples were also collected where maximum nephelometer signals were observed (e.g. B14 and B20). In this way, a grid of vertical TDMn profiles were collected from BRIDGET to complement the larger nephelometer data sets. This was important because hydrothermal plume particles might settle
704
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
Fig. 1. Bathymetric contour map of the Broken Spur segment and adjacent non-transform discontinuities (28°50 —29°30 N Mid-Atlantic Ridge) showing the track lines occupied by the hydrothermal deep-tow instrument BRIDGET. Shading indicates depths shallower than 2600 m confirming that the rift valley walls constrain the neutrally buoyant plume (depth 2800—3000 m) between 29°00 N and 29°20 N. (Bathymetric data from Purdy et al., 1990). The site of previously known active venting is also shown at 29°10 N (Murton et al., 1995b), lying beneath the intersection of lines B14 and B20.
to the seabed whilst dissolved Mn persisted in the overlying water column. It was of interest, therefore, to examine whether TDMn signals might persist within the water column of the Broken Spur segment beyond the limit of the measurable in situ nephelometer anomalies.
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
705
Fig. 2. Two-dimensional cross-sections of nephelometer (optical back-scatter) data through the Broken Spur segment, as recorded from a series of BRIDGET tow-yos (track lines shown in Fig. 1). Note complete absence of suspended plume particle enrichments from all W—E sections except B14 and strong nephelometer signals in N-S section B20 but only weak signals in B22 and B11, none in B12.
706
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
Upon collection, 500 ml unfiltered seawater samples were collected, with rinsing, into acid-cleaned Nalgene LDPE bottles. Each sample was then acidified aboard ship, in a class 100 laminar flow hood, using 1 ml 6 M quartz distilled hydrochloric acid to ensure preservation. Samples were then stored under trace-metal clean conditions for subsequent shore-based analysis. TDMn analyses were performed at Oregon State University using a Dionex chelation ion chromatograph interfaced to a VG PlasmaQuad 2 plus inductively coupled plasma mass spectrometer. In this technique a peristaltic pulls an aliquot of acidified sample from the ICPMS autosampler into a 5 ml sample loop inside the IC. The IC mixes ammonium acetate buffer with the sample stream, raising the pH to 5.4. The mixture is then loaded onto a 0.25 cm column of Chelex resin (Dionex Metpac CC-1). Alkalis wash through the column, and additional buffer elutes the alkaline earths. A VGS 100 flow-injection valve sends the effluent to waste. Next the IC gradient pump rinses the Metpac column with deionized water, then elutes transition elements from the resin with 2 N HNO . The eluent leaves the IC and combines with a 10 ppb solution of In, and the resultant stream passes into the ICPMS nebulizer. The IC separation provides a net preconcentration of 3—4, minimizes ion suppression in the plasma by reducing the total dissolved solids, and increases sensitivity by presenting the sample to the ICPMS over a predictable time window ($2 s). Total analysis time is 11.3 min/sample. The chelation separation and ICPMS analysis is fully automated. For this study a set of seawater standards, acid standards, and matrix blanks were run after every 10 samples. Blank corrections were as high as 50% on low samples, but blank levels varied by (10%. Count rates of In serve as internal standards to minimize the effects of instabilities in the plasma. TDMn concentrations are calculated from Mn/In ratios compared to a standard curve. The average standard deviation calculated on eight randomly selected duplicates was $8%.
4. Results and discussion 4.1. In situ nephelometer data Two-dimensional cross-sections of nephelometer data from the BRIDGET vehicle’s Chelsea Instruments MkIII Nephelometer, as recorded both along-axis and acrossaxis, are shown in Fig. 2. Sections are labelled according to sequential BRIDGET deployments and relate to individual track lines shown in Fig. 1. Because nephelometer data were not recorded during the initial BRIDGET tow-yo along-axis (B07), a repeat line was occupied (B22); these data are reproduced superimposed upon the B07 section in Fig. 2. During the along-axis tows weak nephelometer anomalies, consistent with the presence of a particle-laden hydrothermal plume, were seen along the western and central sections, between 29°10 and 15 N, but no anomalies were detected in the eastern-most line (B12). Of the five initial across-axis lines, nephelometer anomalies were observed only in section B14 at 29°10 N, and no similar hydrothermal plume particle signals were seen in section B16, which passed just north of the secondary plume maximum detected in section B11 at 29°15 N. Nor were any
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
707
plume particle signals revealed by the subsequent, more closely spaced lines run at 29°07.5 N(B19) and at 29°12.5 N(B21), mid-way between plume-influenced line B14 and the adjacent survey lines (B15 and B16). By contrast, pronounced hydrothermal plume particle signals were seen in section B20, run midway between lines B07/22 and B11 (Fig. 1). This survey line followed the strike of the Broken Spur axial volcanic ridge (AVR), from 29°12 N to 29°06 N, but although strong plume signals were seen to the north of the previously identified vent site at 29°10 N (Murton et al., 1995b), no comparable plume particle signals were seen further south. This is in contrast to previous plume studies that identified the presence of neutrally buoyant plume signatures south along the Broken Spur AVR at 29°08—09 N (James et al., 1995a, b; Lukashin et al., 1997). In summary, pronounced plume particle anomalies detected during the present survey were restricted to an area between 29°10—12 N and 43°10—12 W. There was no evidence for any further sources of venting outside of an area ranging between 29°07.5—12.5 N and 43°10—12 W. This does not preclude the presence of further, as yet undiscovered, vent sites within this area (see, e.g. Lukashin et al., 1997) but does establish that all such high-temperature venting should be restricted to a small area at the centre of the segment, which represents no more than &10% of the total deep seafloor ('300 km deeper than 2600 m) of the Broken Spur segment. This is an important result with respect to future segment-scale studies at 29°N, because it identifies that fluxes of high-temperature hydrothermal effluent to the water column can be considered to originate from an effective point-source near the centre of the segment, with no comparable input elsewhere along the segment’s length. The same cannot be assumed to be the case for all segments along the slow-spreading MidAtlantic Ridge (German et al., 1996b). 4.2. Water column total dissolvable Mn (TDMn) analyses The absence of hydrothermal plume particle anomalies, in itself, should not be taken as evidence that no hydrothermal plume impact is present beyond the bounds indicated by the BRIDGET nephelometer data. In prior work, dissolved He and TDMn anomalies associated with hydrothermal plumes have been traced many tens and even hundreds of kilometres from sources of active venting on the East Pacific Rise (e.g. Lupton and Craig, 1981; Klinkhammer and Hudson, 1986). To this end, therefore, we have also investigated the distribution of TDMn anomalies throughout the Broken Spur segment. TDMn data for water samples collected throughout the Broken Spur segment are plotted together versus depth in Fig. 3. All samples with concentrations greater than 2 nmol l\ consistently fall within the narrow depth band of 2800—3000 m. What immediately becomes clear from this analysis of the data set is that the Broken Spur hydrothermal plume exhibits a relatively weak signal. Maximum observed TDMn anomalies during this study were &10 nmol l\, with the majority of values falling in the range 3—6 nmol l\ (Fig. 3). This is consistent with a previous study of the Broken Spur hydrothermal plume by James et al. (1995a), who reported no TDMn anomalies greater than 15 nmol l\. By contrast, previous plume studies from the
708
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
Fig. 3. Vertical profile of all TDMn data for water column samples collected from BRIDGET during RRS Charles Darwin cruise CD95. Black triangles indicate all samples with TDMn concentrations greater than 2 nmol l\. Grey circles indicate samples with TDMn concentrations less than 2 nmol l\. Note the close convergence of samples with high TDMn concentrations between 2800 and 3000 m, consistent with a single coherent plume source.
TAG, Steinaholl and Rainbow hydrothermal sites, elsewhere along the MAR, have revealed maximum neutrally buoyant plume TDMn anomalies in the range 60—90 nmol l\ (Klinkhammer et al., 1986; German et al., 1994; M. Aballe´a and J. Radford-Knoery, pers. comm., 1997). When viewed in plan (Fig. 4), it is clear that the samples exhibiting the highest TDMn concentrations were predominantly collected from a geographically restricted area toward the western bounds of the central portion of the segment (lines B14, B20 and B22). These are also the sections in which highest hydrothermal plume particle concentrations were observed (Fig. 2). Indeed, at high values, TDMn concentrations correlate well with the nephelometer anomalies. This can be seen in Fig. 5, which displays expanded two-dimensional cross-sections of nephelometer data for BRIDGET tow-yos B14 (upper) and B20 (lower), both of which crossed the known active vent-site at 29°10 N. Superimposed upon the in situ nephelometer signals in Fig. 5 are point analyses of TDMn samples collected from the BRIDGET vehicle’s sampling rosette. These samples exhibit high TDMn concentrations, which tend to coincide with strong
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
709
Fig. 4. Bathymetric contour map of the Broken Spur segment (data from Purdy et al., 1990) showing locations of BRIDGET tows (lines) and locations at which sample bottle TDMn concentrations were, respectively, high (black triangles; TDMn'2 nmol l\) or low (grey circles; TDMn (2 nmol l\).
nephelometer signals, although plume-height TDMn concentrations in section B20 remain significantly higher than background values south beyond the limits of the detectable nephelometer anomaly. To a first approximation, therefore, these data confirm the initial observations drawn from the nephelometer data that all strong plume signals are constrained within a restricted area toward the centre of the segment, between 29°07.5—12.5 N
Fig. 5. Two-dimensional sections of nephelometer data from BRIDGET tow-yos B14 (upper) and B20 (lower) as for Fig. 2. Also shown, are the concentrations of TDMn (nmol l\) at the positions at which the respective sample bottles were fired (white circles).
710 C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
711
Fig. 6. Plot of variations in low-level ((2 nmol l\) TDMn concentrations in deepwater samples from the Broken Spur segment as a function of latitude. Note that minimum TDMn values are notably higher within the topographically constrained central portion of the segment (29°00—20 N) when compared to minimum values from samples collected to the north and to the south.
and 43°10—12 W. The only exceptions to this general case are two deep (3000 m) samples that lie toward the east of the basin, one each from tows B14 and B15 (Fig. 4), which measured 3.3 and 2.7 nmol l\, respectively. Within the lower-concentration ((2 nmol l\) TDMn data, however, a further systematic variation can be observed. In a plot of TDMn against latitude (Fig. 6), it is apparent that samples collected from the centre of the segment (29°00—20 N) all exhibit TDMn concentrations that fall in the range 0.30—2.0 nmol l\. By comparison, samples collected from the segment ends, including samples from above plume height (0—2000 m), exhibit lower minimum TDMn concentrations of 0.15 nmol l\. These latter values coincide with the lowest dissolved Mn concentrations reported from the open N. Atlantic Ocean in a recent IOC baseline survey (0.15—0.25mol l\; Statham et al., 1998). Even the lowest deepwater concentrations measured within the centre of the Broken Spur segment, therefore, exhibit a TDMn enrichment of at least 0.15 nmol l\ with respect to the ambient background. From the known deep volume of the Broken Spur segment (4;10 m for water deeper than 2600 m) we can calculate the minimum standing crop of hydrothermal TDMn represented by these low-level ((2 nmol l\) concentration data: (4;10; 10)(0.15;10\) mol l\"6;10 mol Mn. This is equivalent to the entire dissolved Mn content that could be supplied from 2.4;10 l of high-temperature Broken Spur vent fluids ([Mn]"250 lmol l\, James et al., 1995b) or from an even larger volume of more dilute diffuse-flow effluent. Exactly how the flux of dissolved Mn to the water column might be partitioned between these two sources is beyond the resolution of the present data set. Nevertheless, the presence of a pervasive TDMn enrichment does indicate that the influence of hydrothermal activity from the
712
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
Broken Spur segment may extend significantly beyond what is immediately apparent from the in situ nephelometer data. Studies of both volume flux out of the segment and oxidised (solid phase) Mn removal into the underlying sediments will be required, however, before a meaningful segment-scale hydrothermal flux for TDMn can be constrained.
Acknowledgements We thank Dr B.J. Murton (SOC) for organisation of cruise CD95 and Capt. R. Bourne and the officers and crew of RRS Charles Darwin for their extreme professionalism throughout. We thank S. Riches (UC) and R. Kirk (SOC) for continuing support of the BRIDGET deep-tow instrument and A. Ungerer (OSU) for assistance with TDMn analyses. D. Dixon and P. Herring (SOC) provided informative discussions on plume biology, and the manuscript benefited from the reviews of two anonymous reviewers and M. Mottl (U. Hawaii). This work was supported by the Natural Environment Research Council (UK) under BRIDGE Grants 35 and 18.
References Baker, E.T., 1994. A 6-year time series of hydrothermal plumes over the Cleft segment of the Juan de Fuca Ridge. Journal of Geophysics Research 99, 4889—4904. Baker, E.T., Feely, R.A., Mottl, M.J., Sansone, F.J., Wheat, C.G., Resing, J.A., Lupton, J.E., 1994. Hydrothermal plumes along the East Pacific Rise, 8°40 to 11°50 N: Plume distribution and relationship to the apparent magmatic budget. Earth Planetary Science Letters 128, 1—17. Baker, E.T., German, C.R., Elderfield, H., 1995. Hydrothermal plumes: global distributions and geological inferences. In: Seafloor Hydrothermal Systems: Physical, Chemical, Biological and Geological Interactions, AGU Geophysics Monograph, vol. 91, pp. 47—71. BRAVEX Science Team, 1994. BRAVEX/94: a joint British—Russian expedition to the Broken Spur (29°N) & TAG (26°N) hydrothermal vent sites on the Mid-Atlantic Ridge. BRIDGE Newsletter 7, 6—9. Charlou, J.L., Rona, P., Bougault, H., 1987. Methane Anomalies over TAG Hydrothermal Field on Mid Atlantic Ridge. Journal of Marine Research 45, 461—472. Edmond, J.M., Von Damm, K.L., McDuff, R.E., Measures, C.I., 1982. Chemistry of hot springs on the East Pacific Rise and their effluent dispersal. Nature 297, 187—191. Elderfield, H., Humphris, S. Klinkhammer, G., Schultz, A., 1996. Quantification of fluxes at mid-ocean ridges: design/planning for the segment scale box experiment. InterRidge Mesoscale Workshop Report, 24pp. Feely, R.A., Lewison, M., Massoth, G.J., Robert-Baldo, G., Lavelle, J.W., Byrne, R.H., Von Damm, K.L., Curl Jr, H.C., 1987. Composition and dissolution of Black Smoker Particulates From Active Vents on the Juan de Fuca Ridge. Journal of Geophysics Research 92, 11 347—11 363. Feely, R.A., Massoth, G.J., Trefry, J.H., Baker, E.T., Paulson, A.J., Lebon, G.T., 1994. Composition and sedimentation of hydrothermal plume particles from North Cleft segment, Juan de Fuca Ridge. Journal of Geophysics Research 99, 4985—5006. Gente, P., Auzende, J.M., Karson, J.A., Fouquet, Y., Mevel, C., 1991. An example of a recent accretion on the Mid-Atlantic Ridge: the Snake Pit neovolcanic ridge (MARK area 23°22 N). Tectonophysics 190, 1—29.
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
713
German, C.R., Briem, J., Chin, C., Danielsen, M., Holland, S., James, R., Jo´nsdottir, A., Ludford, E., Moser, C., O¨lafsson, J., Palmer, M.R., Rudnicki, M.D., 1994. Hydrothermal activity on the Reykjanes Ridge: the Steinaho´ll vent-field at 63°06 N. Earth Planetary Science Letters 121, 647—654. German, C.R., Klinkhammer, G., Rudnicki, M.D., 1996a. The Rainbow hydrothermal plume, 36°15 N, Mid-Atlantic Ridge. Geophysics Research Letters 23, 2979—2982. German, C.R., Parson, L.M., the HEAT scientific team, 1996b. Hydrothermal exploration near the Azores Triple-Junction: tectonic control of venting at slow-spreading ridges? Earth Planetary Science Letters 138, 105—119. German, C.R., Richards, K.J., Rudnicki, M.D., Lam, M.M., Charlou, J.L., FLAME Scientific Party, 1998. Topographic control of a dispersing hydrothermal plume. Earth Planetary Science Letters 156, 267—273. Herring, P.J., Dixon, D.R., 1998. Extensive deep-sea dispersal of post-larval shrimp from a hydrothermal vent. Deep Sea Research, in press. James, R.H., Elderfield, H., Rudnicki, M.D., German, C.R., Palmer, M.R., Chin, C., Greaves, M.J., Gurvich, E., Klinkhammer, G.P., Ludford, E., Mills, R.A., Thomson, J., Williams, A.C., 1995a. Hydrothermal plumes at Broken Spur, 29°N Mid-Atlantic Ridge: Chemical and physical characteristics. In: Hydrothermal Vents and Processes, Geological Society Special Publication 87, 97—110. James, R.H., Elderfield, H., Palmer, M.R., 1995b. The chemistry of hydrothermal fluids from the Broken Spur site, 29°N Mid-Atlantic Ridge. Geochimica Cosmochimica Acta 59, 651—659. Kadko, D.C., Rosenburg, N.D., Lupton, J.E., Collier, R.W., Lilley, M.D., 1990. Chemical reaction rates and entrainment within the endeavour ridge hydrothermal plume. Earth Planetary Science Letters 99, 315—335. Klinkhammer, G., Elderfield, H., Greaves, M., Rona, P., Nelsen, T., 1986. Manganese geochemistry near high-temperature vents in the Mid-Atlantic Ridge Rift Valley. Earth Planetary Science Letters 80, 230—240. Klinkhammer, G., Hudson, A., 1986. Dispersal patterns for hydrothermal plumes in the South Pacific using manganese as a tracer. Earth Planetary Science Letters 79, 241—249. Klinkhammer, G., Rona, P., Greaves, M.J., Elderfield, H., 1985. Hydrothermal manganese plumes over the Mid-Atlantic Ridge rift-valley. Nature 314, 727—731. Lukashin, V.N., Lisitsin, A.P., Ivanov, G.V., Kravtsov, V.A., Rusakov, V.Y., 1997. Investigation of hydrothermal plumes over the rift zone of the Mid-Atlantic Ridge near 29°N. Oceanology 37, 695—704. Lupton, J.E., Craig, H., 1981. A major helium-3 source at 15°S on the East Pacific Rise. Science 214, 13—18. Lupton, J.E., Delaney, J.R., Johnson, H.P., Tivey, M.K., 1985. Entrainment and Vertical Transport of Deep Ocean Water by Buoyant Hydrothermal Plumes. Nature 316, 621—623. Mottl, M.J., McConachy, T.F., 1990. Chemical processes in buoyant hydrothermal plumes on the East Pacific Rise near 21°N. Geochimica Cosmochimica Acta 54, 1911—1927. Murton, B.J., Klinkhammer, G., Becker, K., Briais, A., Edge, D., Hayward, N., Millard, N., Mitchell, I., Rouse, I., Rudnicki, M., Sayanagi, K., Sloan, H., Parson, L., 1994. Direct evidence for the distribution and occurrence of hydrothermal activity between 27°N—30°N on the Mid-Atlantic Ridge. Earth Planetary Science Letters 125, 119—128. Murton, B., German, C., Herring, P., Dixon, D., CD95 scientific party, 1995a. Volcanoes, plumes, minerals and life: FLUXES 1, a segment scale study. BRIDGE Newsletter 9, 14—19. Murton, B.J., Van Dover, C, Southward, E., 1995b. Geological setting and ecology of the Broken Spur hydrothermal vent field: 29°10 N on the Mid-Atlantic Ridge. In: Hydrothermal Vents and Processes. Geological Society Special Publication 97, 33—41. Murton, B., Redbourn, L., German, C., Herring, P., Dixon, D., Doos, K., Coward, A., Rudnicki, M., 1997. Introduction and preliminary results from a segment-scale study of the Broken Spur vent-field. EOS, Transaction of the American Geophysics Union 78, F842. Nelsen, T.A., Klinkhammer, G.P., Trefry, J.H., Trocine, R.P., 1986. Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge. Earth Planetary Science Letters 81, 245—252.
714
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
Purdy, G.M., Sempere´, J.C., Schouten, H., DuBois, D.L., Goldsmith, R., 1990. Bathymetry of the MidAtlantic Ridge, 24°—30°N; a map series. Marine Geophysics Research 12, 247—252. Rudnicki, M.D., Elderfield, H., 1992. Helium, radon and manganese at the TAG and Snakepit hydrothermal vent fields, 26° and 23°N, Mid-Atlantic Ridge. Earth Planetary Science Letters 113, 307—321. Rudnicki, M.D., Elderfield, H., 1993. A chemical model of the buoyant and neutrally buoyant plume above the TAG vent field, 26° N. Mid-Atlantic Ridge. Geochimica Cosmochimica Acta 57, 2939—2957. Rudnicki, M.D., German, C.R., Kirk, R.E., Sinha, M., Elderfield, H., Riches, S., 1995. New instrument platform tested at Mid-Atlantic Ridge. EOS, Transactions of the American Geophysics Union 76, 329—330. Speer, K.G., Rona, P.A., 1989. A model of an Atlantic and Pacific Hydrothermal Plume. Journal of Geophysics Research 94, 6213—6220. Statham, P.J., Yeats, P.A., Landing, W.M., 1998. Manganese in the eastern Atlantic Ocean: processes influencing deep and subsurface water distributions. Marine Chemistry 61, 55—68, 1998. Urabe, T., Baker, E.T., Ishibashi, J., Feely, R.A., Marumo, K., Massoth, G.J., Maruyama, A., Shitashima, K., Okamura, K., Lupton, J.E., Sonoda, A., Yamazaki, T., Aoki, M., Gendron, J., Greene, R., Kaiho, Y., Kisimoto, K., Lebon, G., Matsumoto, T., Nakamura, K., Nishizawa, A., Okano, O., Paradis, G., Roe, K., Shibata, T., Tennat, D., Vance, T., Walker, S.L., Yabuki, T., Ytow, N., 1995. The effect of magmatic activity on hydrothermal venting along the superfast-spreading East Pacific Rise. Science 269, 1092—1095.
A segment-scale survey of the Broken Spur hydrothermal plume C.R. German *, M.D. Rudnicki, G.P. Klinkhammer Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK College of Ocean & Atmospheric Sciences, OSU, Corvallis, OR 97331, USA Received 7 August 1997; received in revised form 9 February 1998; accepted 3 April 1998
Abstract We conducted a segment-scale hydrothermal plume survey of the Broken Spur segment, 29°00-20 N, Mid-Atlantic Ridge (MAR). The purpose of the study was to identify the distribution of sources of venting throughout the segment as part of a larger study of hydrothermal fluxes. Evidence from plume particle concentrations (as deduced from in situ nephelometer data) and total dissolvable Mn (TDMn) analyses (from discrete water samples) indicated a restricted source of venting close to the segment centre, coincident with the previously known vent-site. No other pronounced plume signals were observed outside an area bounded by 29°07.5—12.5 N and 43°10—12 W, representing less than 10% of the '300 km of deep water ('2600 m) within the segment. In addition, however, low-level ((2 nmol l\) deepwater TDMn concentrations reveal a pervasive enrichment throughout the segment of *0.15 nmol l\. For the 4;10 m of deepwater within the Broken Spur segment, this corresponds to a standing crop of 6;10 mol of hydrothermal Mn. Future studies of long-term current flow will allow the flux of dissolved Mn out of the segment to be established and will investigate the partitioning of its source, between high temperature and axial diffuse flow. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction When high-temperature hydrothermal fluids issue from the seabed they mix with the overlying water column, and high concentrations of metal-rich sulphide and oxide particles precipitate (e.g. Edmond et al., 1982; Feely et al., 1987; Mottl and
*Corresponding author. Tel.: 0044 1703 596542; fax: 0044 1703 596554; e-mail: [email protected]. 0967-0637/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 8 ) 0 0 0 7 8 - 8
702
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
McConachy, 1990). The resultant buoyant, particle-laden plume rises upward, mixing progressively with the ambient water column until some level of neutral buoyancy is attained (Lupton et al., 1985). The height-of-rise of a hydrothermal plume is typically 100—300 m, during which time the initial vent fluid is diluted by a factor of approximately 10 with ambient seawater (e.g. Speer and Rona, 1989). Because the initial vent fluid may be enriched in certain tracers by factors of as high as 10 relative to ambient seawater, even a dilute neutrally buoyant hydrothermal plume may still exhibit pronounced geochemical enrichments. This has previously been demonstrated for dissolved He (e.g. Lupton and Craig, 1981), TDMn-total dissolvable Mn (e.g. Klinkhammer et al., 1985), dissolved CH (e.g. Charlou et al., 1987) and total suspended matter (e.g. Nelsen et al., 1986). Neutrally buoyant plumes extend much further, laterally, than the hydrothermal vent fields that source them. In previous work, these attributes have been exploited by using hydrothermal plume surveys to prospect for new sites of hydrothermal activity along mid-ocean ridges over length scales of up to 2000 km (see, e.g. review in Baker et al., 1995). On much shorter length scales, plume-process studies have been concentrated around individual vent sites (e.g. Klinkhammer et al., 1986; Kadko et al., 1990; Rudnicki and Elderfield, 1992, 1993; German et al., 1996a, 1998). In this paper, we present a mesoscale approach. One of the major questions established by the international Mid-Ocean Ridge research community has been: ‘‘What are the fluxes imparted from hydrothermal activity to the oceans, on a segment scale?’’ (Elderfield et al., 1996). To begin to address this question, RRS Charles Darwin cruise CD95 (Murton et al., 1995a) set out to investigate combined physical, chemical and biological fluxes in the Broken Spur segment of the MAR, 29°00-20 N. As part of that work we carried out a systematic three-dimensional survey of plume particle distributions and TDMn concentrations throughout the deep water column to investigate the location of high-temperature venting and the dispersal of its products within the segment. The results presented here were collected in parallel with complementary projects that examined the physical oceanography of the segment and the biology associated with the Broken Spur neutrally-buoyant plume (Murton et al., 1997; Herring and Dixon, in press).
2. Geologic setting The Broken Spur hydrothermal field was first located in 1993 from a combination of mid-water transmissometer and nephelometer anomalies (Murton et al., 1994). Subsequent submersible investigations using the DSV Alvin revealed an area of hightemperature hydrothermal venting approximately 100 m in diameter located atop an axial volcanic ridge close to the centre of the Broken Spur segment at 29°10 N (Murton et al., 1995b). This geological setting is very similar to that described for the location of the Snakepit hydrothermal field at 23°N, MAR (Gente et al., 1991). Discrete sources of high-temperature venting at Broken Spur were seemingly hosted by tectonism along the walls of an axial summit caldera (Murton et al., 1995b). Subsequent plume surveys, more spatially restricted than the present work, had provided
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
703
evidence for additional sites of venting close to the known Broken Spur vent site (James et al., 1995a, b; Lukashin et al., 1997), but these have yet to be confirmed by direct submersible observations (BRAVEX Science Team, 1994).
3. Methods and sampling Sampling for this study was carried out using the deep-tow hydrothermal plume instrument BRIDGET (Rudnicki et al., 1995). A limitation of many previous MAR plume surveys has been their reliance upon discrete vertical CTD-profiles for the collection of both physical data and water samples for subsequent shipboard or shore-based geochemical analysis. Three-dimensional along- and across-axis investigations of hydrothermal plume dispersal have been completed previously along various sections of the Pacific spreading centres (e.g. Baker, 1994; Baker et al., 1994; Urabe et al., 1995). In those cases, however, the hydrothermal plume has been demonstrated to rise clear of any axial graben, with dispersion of both dissolved and particulate discharge dictated by prevailing open-ocean currents (e.g. Lupton and Craig, 1981; Klinkhammer and Hudson, 1986; Kadko et al., 1990; Feely et al., 1994). By contrast, hydrothermal plumes rising above the Mid-Atlantic Ridge are typically constrained within the bounding topography of the deep rift-valley and can escape to the surrounding ocean only by advection along-axis toward the segment ends. It is this ‘‘bath tub’’ effect, peculiar to slow-spreading ridges such as the MAR, that our expedition was conceived to exploit (Murton et al., 1995a). A plan view of the BRIDGET lines occupied during our survey is shown in Fig. 1. Initially, a series of three along-axis lines was completed following the deep portion of the rift valley (*3200 m) along the west, centre and east of the segment (Lines B07, B11 and B12). A series of five across-axis surveys were then completed at regular (5 nautical mile) spacings at 29°10 N, 29°05 N, 29°15 N, 29°00 N and 29°20 N (Lines B14—18). Finally, a set of three higher-resolution survey lines were occupied, based upon earlier findings from the on-going survey (B19—B21): two further across-axis lines, at 29°07.5 N and 29°12.5 N, and a final short along-axis line from 29°12 N to 29°06 N. For all deployments, the package was towed at a speed of &1.5 kt whilst hauling and veering (raising and lowering) at a rate of 30 m/min\. BRIDGET was tow-yoed from depths of 2400 m to within 100 m of the seabed, which ranged in depth between 2600 and 3400 m but was typically at 2900—3200 m. In this way more than 300 vertical CTD-nephelometer profiles were collected throughout the deep water column of the Broken Spur segment. For each BRIDGET deployment, 12 watercolumn samples were also taken for shorebased TDMn analysis (cf. Klinkhammer et al., 1985). Samples were typically taken as two 6-point vertical profiles with one sample every 100 m between 2500 and 3000 m at those positions where the across-axis BRIDGET tow-yo lines crossed the outer along-axis lines (B07 and B12, Fig. 1). In addition, in later sampling, TDMn samples were also collected where maximum nephelometer signals were observed (e.g. B14 and B20). In this way, a grid of vertical TDMn profiles were collected from BRIDGET to complement the larger nephelometer data sets. This was important because hydrothermal plume particles might settle
704
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
Fig. 1. Bathymetric contour map of the Broken Spur segment and adjacent non-transform discontinuities (28°50 —29°30 N Mid-Atlantic Ridge) showing the track lines occupied by the hydrothermal deep-tow instrument BRIDGET. Shading indicates depths shallower than 2600 m confirming that the rift valley walls constrain the neutrally buoyant plume (depth 2800—3000 m) between 29°00 N and 29°20 N. (Bathymetric data from Purdy et al., 1990). The site of previously known active venting is also shown at 29°10 N (Murton et al., 1995b), lying beneath the intersection of lines B14 and B20.
to the seabed whilst dissolved Mn persisted in the overlying water column. It was of interest, therefore, to examine whether TDMn signals might persist within the water column of the Broken Spur segment beyond the limit of the measurable in situ nephelometer anomalies.
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
705
Fig. 2. Two-dimensional cross-sections of nephelometer (optical back-scatter) data through the Broken Spur segment, as recorded from a series of BRIDGET tow-yos (track lines shown in Fig. 1). Note complete absence of suspended plume particle enrichments from all W—E sections except B14 and strong nephelometer signals in N-S section B20 but only weak signals in B22 and B11, none in B12.
706
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
Upon collection, 500 ml unfiltered seawater samples were collected, with rinsing, into acid-cleaned Nalgene LDPE bottles. Each sample was then acidified aboard ship, in a class 100 laminar flow hood, using 1 ml 6 M quartz distilled hydrochloric acid to ensure preservation. Samples were then stored under trace-metal clean conditions for subsequent shore-based analysis. TDMn analyses were performed at Oregon State University using a Dionex chelation ion chromatograph interfaced to a VG PlasmaQuad 2 plus inductively coupled plasma mass spectrometer. In this technique a peristaltic pulls an aliquot of acidified sample from the ICPMS autosampler into a 5 ml sample loop inside the IC. The IC mixes ammonium acetate buffer with the sample stream, raising the pH to 5.4. The mixture is then loaded onto a 0.25 cm column of Chelex resin (Dionex Metpac CC-1). Alkalis wash through the column, and additional buffer elutes the alkaline earths. A VGS 100 flow-injection valve sends the effluent to waste. Next the IC gradient pump rinses the Metpac column with deionized water, then elutes transition elements from the resin with 2 N HNO . The eluent leaves the IC and combines with a 10 ppb solution of In, and the resultant stream passes into the ICPMS nebulizer. The IC separation provides a net preconcentration of 3—4, minimizes ion suppression in the plasma by reducing the total dissolved solids, and increases sensitivity by presenting the sample to the ICPMS over a predictable time window ($2 s). Total analysis time is 11.3 min/sample. The chelation separation and ICPMS analysis is fully automated. For this study a set of seawater standards, acid standards, and matrix blanks were run after every 10 samples. Blank corrections were as high as 50% on low samples, but blank levels varied by (10%. Count rates of In serve as internal standards to minimize the effects of instabilities in the plasma. TDMn concentrations are calculated from Mn/In ratios compared to a standard curve. The average standard deviation calculated on eight randomly selected duplicates was $8%.
4. Results and discussion 4.1. In situ nephelometer data Two-dimensional cross-sections of nephelometer data from the BRIDGET vehicle’s Chelsea Instruments MkIII Nephelometer, as recorded both along-axis and acrossaxis, are shown in Fig. 2. Sections are labelled according to sequential BRIDGET deployments and relate to individual track lines shown in Fig. 1. Because nephelometer data were not recorded during the initial BRIDGET tow-yo along-axis (B07), a repeat line was occupied (B22); these data are reproduced superimposed upon the B07 section in Fig. 2. During the along-axis tows weak nephelometer anomalies, consistent with the presence of a particle-laden hydrothermal plume, were seen along the western and central sections, between 29°10 and 15 N, but no anomalies were detected in the eastern-most line (B12). Of the five initial across-axis lines, nephelometer anomalies were observed only in section B14 at 29°10 N, and no similar hydrothermal plume particle signals were seen in section B16, which passed just north of the secondary plume maximum detected in section B11 at 29°15 N. Nor were any
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
707
plume particle signals revealed by the subsequent, more closely spaced lines run at 29°07.5 N(B19) and at 29°12.5 N(B21), mid-way between plume-influenced line B14 and the adjacent survey lines (B15 and B16). By contrast, pronounced hydrothermal plume particle signals were seen in section B20, run midway between lines B07/22 and B11 (Fig. 1). This survey line followed the strike of the Broken Spur axial volcanic ridge (AVR), from 29°12 N to 29°06 N, but although strong plume signals were seen to the north of the previously identified vent site at 29°10 N (Murton et al., 1995b), no comparable plume particle signals were seen further south. This is in contrast to previous plume studies that identified the presence of neutrally buoyant plume signatures south along the Broken Spur AVR at 29°08—09 N (James et al., 1995a, b; Lukashin et al., 1997). In summary, pronounced plume particle anomalies detected during the present survey were restricted to an area between 29°10—12 N and 43°10—12 W. There was no evidence for any further sources of venting outside of an area ranging between 29°07.5—12.5 N and 43°10—12 W. This does not preclude the presence of further, as yet undiscovered, vent sites within this area (see, e.g. Lukashin et al., 1997) but does establish that all such high-temperature venting should be restricted to a small area at the centre of the segment, which represents no more than &10% of the total deep seafloor ('300 km deeper than 2600 m) of the Broken Spur segment. This is an important result with respect to future segment-scale studies at 29°N, because it identifies that fluxes of high-temperature hydrothermal effluent to the water column can be considered to originate from an effective point-source near the centre of the segment, with no comparable input elsewhere along the segment’s length. The same cannot be assumed to be the case for all segments along the slow-spreading MidAtlantic Ridge (German et al., 1996b). 4.2. Water column total dissolvable Mn (TDMn) analyses The absence of hydrothermal plume particle anomalies, in itself, should not be taken as evidence that no hydrothermal plume impact is present beyond the bounds indicated by the BRIDGET nephelometer data. In prior work, dissolved He and TDMn anomalies associated with hydrothermal plumes have been traced many tens and even hundreds of kilometres from sources of active venting on the East Pacific Rise (e.g. Lupton and Craig, 1981; Klinkhammer and Hudson, 1986). To this end, therefore, we have also investigated the distribution of TDMn anomalies throughout the Broken Spur segment. TDMn data for water samples collected throughout the Broken Spur segment are plotted together versus depth in Fig. 3. All samples with concentrations greater than 2 nmol l\ consistently fall within the narrow depth band of 2800—3000 m. What immediately becomes clear from this analysis of the data set is that the Broken Spur hydrothermal plume exhibits a relatively weak signal. Maximum observed TDMn anomalies during this study were &10 nmol l\, with the majority of values falling in the range 3—6 nmol l\ (Fig. 3). This is consistent with a previous study of the Broken Spur hydrothermal plume by James et al. (1995a), who reported no TDMn anomalies greater than 15 nmol l\. By contrast, previous plume studies from the
708
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
Fig. 3. Vertical profile of all TDMn data for water column samples collected from BRIDGET during RRS Charles Darwin cruise CD95. Black triangles indicate all samples with TDMn concentrations greater than 2 nmol l\. Grey circles indicate samples with TDMn concentrations less than 2 nmol l\. Note the close convergence of samples with high TDMn concentrations between 2800 and 3000 m, consistent with a single coherent plume source.
TAG, Steinaholl and Rainbow hydrothermal sites, elsewhere along the MAR, have revealed maximum neutrally buoyant plume TDMn anomalies in the range 60—90 nmol l\ (Klinkhammer et al., 1986; German et al., 1994; M. Aballe´a and J. Radford-Knoery, pers. comm., 1997). When viewed in plan (Fig. 4), it is clear that the samples exhibiting the highest TDMn concentrations were predominantly collected from a geographically restricted area toward the western bounds of the central portion of the segment (lines B14, B20 and B22). These are also the sections in which highest hydrothermal plume particle concentrations were observed (Fig. 2). Indeed, at high values, TDMn concentrations correlate well with the nephelometer anomalies. This can be seen in Fig. 5, which displays expanded two-dimensional cross-sections of nephelometer data for BRIDGET tow-yos B14 (upper) and B20 (lower), both of which crossed the known active vent-site at 29°10 N. Superimposed upon the in situ nephelometer signals in Fig. 5 are point analyses of TDMn samples collected from the BRIDGET vehicle’s sampling rosette. These samples exhibit high TDMn concentrations, which tend to coincide with strong
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
709
Fig. 4. Bathymetric contour map of the Broken Spur segment (data from Purdy et al., 1990) showing locations of BRIDGET tows (lines) and locations at which sample bottle TDMn concentrations were, respectively, high (black triangles; TDMn'2 nmol l\) or low (grey circles; TDMn (2 nmol l\).
nephelometer signals, although plume-height TDMn concentrations in section B20 remain significantly higher than background values south beyond the limits of the detectable nephelometer anomaly. To a first approximation, therefore, these data confirm the initial observations drawn from the nephelometer data that all strong plume signals are constrained within a restricted area toward the centre of the segment, between 29°07.5—12.5 N
Fig. 5. Two-dimensional sections of nephelometer data from BRIDGET tow-yos B14 (upper) and B20 (lower) as for Fig. 2. Also shown, are the concentrations of TDMn (nmol l\) at the positions at which the respective sample bottles were fired (white circles).
710 C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
711
Fig. 6. Plot of variations in low-level ((2 nmol l\) TDMn concentrations in deepwater samples from the Broken Spur segment as a function of latitude. Note that minimum TDMn values are notably higher within the topographically constrained central portion of the segment (29°00—20 N) when compared to minimum values from samples collected to the north and to the south.
and 43°10—12 W. The only exceptions to this general case are two deep (3000 m) samples that lie toward the east of the basin, one each from tows B14 and B15 (Fig. 4), which measured 3.3 and 2.7 nmol l\, respectively. Within the lower-concentration ((2 nmol l\) TDMn data, however, a further systematic variation can be observed. In a plot of TDMn against latitude (Fig. 6), it is apparent that samples collected from the centre of the segment (29°00—20 N) all exhibit TDMn concentrations that fall in the range 0.30—2.0 nmol l\. By comparison, samples collected from the segment ends, including samples from above plume height (0—2000 m), exhibit lower minimum TDMn concentrations of 0.15 nmol l\. These latter values coincide with the lowest dissolved Mn concentrations reported from the open N. Atlantic Ocean in a recent IOC baseline survey (0.15—0.25mol l\; Statham et al., 1998). Even the lowest deepwater concentrations measured within the centre of the Broken Spur segment, therefore, exhibit a TDMn enrichment of at least 0.15 nmol l\ with respect to the ambient background. From the known deep volume of the Broken Spur segment (4;10 m for water deeper than 2600 m) we can calculate the minimum standing crop of hydrothermal TDMn represented by these low-level ((2 nmol l\) concentration data: (4;10; 10)(0.15;10\) mol l\"6;10 mol Mn. This is equivalent to the entire dissolved Mn content that could be supplied from 2.4;10 l of high-temperature Broken Spur vent fluids ([Mn]"250 lmol l\, James et al., 1995b) or from an even larger volume of more dilute diffuse-flow effluent. Exactly how the flux of dissolved Mn to the water column might be partitioned between these two sources is beyond the resolution of the present data set. Nevertheless, the presence of a pervasive TDMn enrichment does indicate that the influence of hydrothermal activity from the
712
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
Broken Spur segment may extend significantly beyond what is immediately apparent from the in situ nephelometer data. Studies of both volume flux out of the segment and oxidised (solid phase) Mn removal into the underlying sediments will be required, however, before a meaningful segment-scale hydrothermal flux for TDMn can be constrained.
Acknowledgements We thank Dr B.J. Murton (SOC) for organisation of cruise CD95 and Capt. R. Bourne and the officers and crew of RRS Charles Darwin for their extreme professionalism throughout. We thank S. Riches (UC) and R. Kirk (SOC) for continuing support of the BRIDGET deep-tow instrument and A. Ungerer (OSU) for assistance with TDMn analyses. D. Dixon and P. Herring (SOC) provided informative discussions on plume biology, and the manuscript benefited from the reviews of two anonymous reviewers and M. Mottl (U. Hawaii). This work was supported by the Natural Environment Research Council (UK) under BRIDGE Grants 35 and 18.
References Baker, E.T., 1994. A 6-year time series of hydrothermal plumes over the Cleft segment of the Juan de Fuca Ridge. Journal of Geophysics Research 99, 4889—4904. Baker, E.T., Feely, R.A., Mottl, M.J., Sansone, F.J., Wheat, C.G., Resing, J.A., Lupton, J.E., 1994. Hydrothermal plumes along the East Pacific Rise, 8°40 to 11°50 N: Plume distribution and relationship to the apparent magmatic budget. Earth Planetary Science Letters 128, 1—17. Baker, E.T., German, C.R., Elderfield, H., 1995. Hydrothermal plumes: global distributions and geological inferences. In: Seafloor Hydrothermal Systems: Physical, Chemical, Biological and Geological Interactions, AGU Geophysics Monograph, vol. 91, pp. 47—71. BRAVEX Science Team, 1994. BRAVEX/94: a joint British—Russian expedition to the Broken Spur (29°N) & TAG (26°N) hydrothermal vent sites on the Mid-Atlantic Ridge. BRIDGE Newsletter 7, 6—9. Charlou, J.L., Rona, P., Bougault, H., 1987. Methane Anomalies over TAG Hydrothermal Field on Mid Atlantic Ridge. Journal of Marine Research 45, 461—472. Edmond, J.M., Von Damm, K.L., McDuff, R.E., Measures, C.I., 1982. Chemistry of hot springs on the East Pacific Rise and their effluent dispersal. Nature 297, 187—191. Elderfield, H., Humphris, S. Klinkhammer, G., Schultz, A., 1996. Quantification of fluxes at mid-ocean ridges: design/planning for the segment scale box experiment. InterRidge Mesoscale Workshop Report, 24pp. Feely, R.A., Lewison, M., Massoth, G.J., Robert-Baldo, G., Lavelle, J.W., Byrne, R.H., Von Damm, K.L., Curl Jr, H.C., 1987. Composition and dissolution of Black Smoker Particulates From Active Vents on the Juan de Fuca Ridge. Journal of Geophysics Research 92, 11 347—11 363. Feely, R.A., Massoth, G.J., Trefry, J.H., Baker, E.T., Paulson, A.J., Lebon, G.T., 1994. Composition and sedimentation of hydrothermal plume particles from North Cleft segment, Juan de Fuca Ridge. Journal of Geophysics Research 99, 4985—5006. Gente, P., Auzende, J.M., Karson, J.A., Fouquet, Y., Mevel, C., 1991. An example of a recent accretion on the Mid-Atlantic Ridge: the Snake Pit neovolcanic ridge (MARK area 23°22 N). Tectonophysics 190, 1—29.
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
713
German, C.R., Briem, J., Chin, C., Danielsen, M., Holland, S., James, R., Jo´nsdottir, A., Ludford, E., Moser, C., O¨lafsson, J., Palmer, M.R., Rudnicki, M.D., 1994. Hydrothermal activity on the Reykjanes Ridge: the Steinaho´ll vent-field at 63°06 N. Earth Planetary Science Letters 121, 647—654. German, C.R., Klinkhammer, G., Rudnicki, M.D., 1996a. The Rainbow hydrothermal plume, 36°15 N, Mid-Atlantic Ridge. Geophysics Research Letters 23, 2979—2982. German, C.R., Parson, L.M., the HEAT scientific team, 1996b. Hydrothermal exploration near the Azores Triple-Junction: tectonic control of venting at slow-spreading ridges? Earth Planetary Science Letters 138, 105—119. German, C.R., Richards, K.J., Rudnicki, M.D., Lam, M.M., Charlou, J.L., FLAME Scientific Party, 1998. Topographic control of a dispersing hydrothermal plume. Earth Planetary Science Letters 156, 267—273. Herring, P.J., Dixon, D.R., 1998. Extensive deep-sea dispersal of post-larval shrimp from a hydrothermal vent. Deep Sea Research, in press. James, R.H., Elderfield, H., Rudnicki, M.D., German, C.R., Palmer, M.R., Chin, C., Greaves, M.J., Gurvich, E., Klinkhammer, G.P., Ludford, E., Mills, R.A., Thomson, J., Williams, A.C., 1995a. Hydrothermal plumes at Broken Spur, 29°N Mid-Atlantic Ridge: Chemical and physical characteristics. In: Hydrothermal Vents and Processes, Geological Society Special Publication 87, 97—110. James, R.H., Elderfield, H., Palmer, M.R., 1995b. The chemistry of hydrothermal fluids from the Broken Spur site, 29°N Mid-Atlantic Ridge. Geochimica Cosmochimica Acta 59, 651—659. Kadko, D.C., Rosenburg, N.D., Lupton, J.E., Collier, R.W., Lilley, M.D., 1990. Chemical reaction rates and entrainment within the endeavour ridge hydrothermal plume. Earth Planetary Science Letters 99, 315—335. Klinkhammer, G., Elderfield, H., Greaves, M., Rona, P., Nelsen, T., 1986. Manganese geochemistry near high-temperature vents in the Mid-Atlantic Ridge Rift Valley. Earth Planetary Science Letters 80, 230—240. Klinkhammer, G., Hudson, A., 1986. Dispersal patterns for hydrothermal plumes in the South Pacific using manganese as a tracer. Earth Planetary Science Letters 79, 241—249. Klinkhammer, G., Rona, P., Greaves, M.J., Elderfield, H., 1985. Hydrothermal manganese plumes over the Mid-Atlantic Ridge rift-valley. Nature 314, 727—731. Lukashin, V.N., Lisitsin, A.P., Ivanov, G.V., Kravtsov, V.A., Rusakov, V.Y., 1997. Investigation of hydrothermal plumes over the rift zone of the Mid-Atlantic Ridge near 29°N. Oceanology 37, 695—704. Lupton, J.E., Craig, H., 1981. A major helium-3 source at 15°S on the East Pacific Rise. Science 214, 13—18. Lupton, J.E., Delaney, J.R., Johnson, H.P., Tivey, M.K., 1985. Entrainment and Vertical Transport of Deep Ocean Water by Buoyant Hydrothermal Plumes. Nature 316, 621—623. Mottl, M.J., McConachy, T.F., 1990. Chemical processes in buoyant hydrothermal plumes on the East Pacific Rise near 21°N. Geochimica Cosmochimica Acta 54, 1911—1927. Murton, B.J., Klinkhammer, G., Becker, K., Briais, A., Edge, D., Hayward, N., Millard, N., Mitchell, I., Rouse, I., Rudnicki, M., Sayanagi, K., Sloan, H., Parson, L., 1994. Direct evidence for the distribution and occurrence of hydrothermal activity between 27°N—30°N on the Mid-Atlantic Ridge. Earth Planetary Science Letters 125, 119—128. Murton, B., German, C., Herring, P., Dixon, D., CD95 scientific party, 1995a. Volcanoes, plumes, minerals and life: FLUXES 1, a segment scale study. BRIDGE Newsletter 9, 14—19. Murton, B.J., Van Dover, C, Southward, E., 1995b. Geological setting and ecology of the Broken Spur hydrothermal vent field: 29°10 N on the Mid-Atlantic Ridge. In: Hydrothermal Vents and Processes. Geological Society Special Publication 97, 33—41. Murton, B., Redbourn, L., German, C., Herring, P., Dixon, D., Doos, K., Coward, A., Rudnicki, M., 1997. Introduction and preliminary results from a segment-scale study of the Broken Spur vent-field. EOS, Transaction of the American Geophysics Union 78, F842. Nelsen, T.A., Klinkhammer, G.P., Trefry, J.H., Trocine, R.P., 1986. Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge. Earth Planetary Science Letters 81, 245—252.
714
C.R. German et al. / Deep-Sea Research I 46 (1999) 701—714
Purdy, G.M., Sempere´, J.C., Schouten, H., DuBois, D.L., Goldsmith, R., 1990. Bathymetry of the MidAtlantic Ridge, 24°—30°N; a map series. Marine Geophysics Research 12, 247—252. Rudnicki, M.D., Elderfield, H., 1992. Helium, radon and manganese at the TAG and Snakepit hydrothermal vent fields, 26° and 23°N, Mid-Atlantic Ridge. Earth Planetary Science Letters 113, 307—321. Rudnicki, M.D., Elderfield, H., 1993. A chemical model of the buoyant and neutrally buoyant plume above the TAG vent field, 26° N. Mid-Atlantic Ridge. Geochimica Cosmochimica Acta 57, 2939—2957. Rudnicki, M.D., German, C.R., Kirk, R.E., Sinha, M., Elderfield, H., Riches, S., 1995. New instrument platform tested at Mid-Atlantic Ridge. EOS, Transactions of the American Geophysics Union 76, 329—330. Speer, K.G., Rona, P.A., 1989. A model of an Atlantic and Pacific Hydrothermal Plume. Journal of Geophysics Research 94, 6213—6220. Statham, P.J., Yeats, P.A., Landing, W.M., 1998. Manganese in the eastern Atlantic Ocean: processes influencing deep and subsurface water distributions. Marine Chemistry 61, 55—68, 1998. Urabe, T., Baker, E.T., Ishibashi, J., Feely, R.A., Marumo, K., Massoth, G.J., Maruyama, A., Shitashima, K., Okamura, K., Lupton, J.E., Sonoda, A., Yamazaki, T., Aoki, M., Gendron, J., Greene, R., Kaiho, Y., Kisimoto, K., Lebon, G., Matsumoto, T., Nakamura, K., Nishizawa, A., Okano, O., Paradis, G., Roe, K., Shibata, T., Tennat, D., Vance, T., Walker, S.L., Yabuki, T., Ytow, N., 1995. The effect of magmatic activity on hydrothermal venting along the superfast-spreading East Pacific Rise. Science 269, 1092—1095.