Deep-Sea Research, Vol. 35, No. 1, pp. 121-131, 1988. 0198-0149/88 $3.00 + 0,00 Printed in Great Britain. © 1988 Pergamon Journals Ltd.

Hydrothermal CH4 between 12°N and 15°N over the Mid-Atlantic Ridge

JEAN Luc CHARLOU,* LEONID DMITRIEV,t HENRI BOUGAULT*

a n d HUBERT DAVID NEEDHAM*

(Received 17 April 1986; in revised form 6 October 1987; accepted 19 October 1987)

Abstract--Hydrothermal effluents enriched in gases such as helium, hydrogen and methane have been found outgassing from mid-ocean ridge zones, crustal rifts, volcanoes and other areas of tectonic activity. Although methane is not conservative in seawater, its analysis on board ship can be used qualitatively or semi-quantitatively to locate hydrothermal fields as a cruise progresses. Between 12 ° and 15°N over the Mid-Atlantic Ridge, CH 4 anomalies (up to 44 nl 1 1) in the water column reveal the presence of plumes originating from hydrothermal discharge. The large amplitude of the CH4 anomaly at one station (Hy-36) integrated over more than 1000 m, reflects a large CH4 input and thus extensive hydrothermal activity in this slow spreading section similar to inputs from fast spreading sites like the EaSt Pacific Rise.

I N T R O D U C T I O N

A COMMON feature of oceanic dissolved methane profiles is a pronounced maximum frequently associated with the pycnocline (50-200 m) (Fig. 1). Concentrations of CH4 measured in samples from the mixed layer are commonly higher than concentrations predicted frorri the solubility of CH4 in seawater and known atmospheric concentrations (LAMONTAGNE et al., 1973; BROOKS and SACKETT, 1973). The CH 4 maximum in the upper seawater column indicates the existence of biological CH4 production at rates much faster than physical removal (i.e. diffusion to the atmosphere) and chemical or biological consumption. Phytoplankton distribution also shows important peaks in the upper oxygenated mixed layers (TRAGANZA et al., 1979). Since no significant amount of C H 4 is generated from photochemical reactions with dissolved organic carbon (WILSON et al., 1970; TRAGANZA et al., 1979), the biogenic production of CH4 in the upper layers must be of considerable importance (SCRANTON and BREWER, 1977; TRANGANZA et al., 1979). From 50% supersaturated concentrations in surface layers, C H 4 contents decrease regularly with depth, to 8-10 nl 1-1 at 1000 m. In deep waters, concentrations are typically about 3 nl 1-1 in the Pacific and 6 nl l -1 in the Atlantic (LAMONTAGNE et al., 1973). However, much higher levels of concentrations are found in anoxic basins (ATKINSON and RICHARDS, 1967; LAMONTAGNE et al., 1973) and hydrothermal sites. In the Red Sea, C H 4 concentrations vary from 50 nl 1-1 in the water column to 150 lal 1-1 in brines (BURKE et al., 1981). Methane concentration in hydrothermal end members at 10°50'N on the East Pacific Rise (EPR) (KIM, 1983), are from 103 to 106 times greater

* IFREMER/Centre de Brest, BP 337, 29273 Brest Cedex, France. t Vernadsky Institute of Geochemistry, USSR Academy of Sciences, Kosigina 19, Moscow 334, U.S.S.R.

121

122 J .L . CHARLOU et al.

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20 4O I I I I I

o~ o ~...o ~

o I

o

0 2O I I

o / o 6

o 3 o

I o ! / / / /

(a) (b)

Fig. 1. (a) Typical CH 4 depth profile in the Atlantic Ocean without detected anomaly. Station Hy-14: 13°20'87"S--14°5'51"W; depth: 3570 m (CHARLOU et al., 1986). (b) Methane depth profile at a control station in the Pacific Ocean. Station Hy-Cy-08 (Cyatherm cruise, 1982): 15°3'04"N -

101°33'72"W; depth: 3700 m.

than normal deep ocean waters. It is such contrasts in CH4 concentrations around hydrothermal discharges on the sea floor that create CH4 anomalies in deep waters.

Helium (CRAIG et al., 1975; CRAIG, 1981; JENKINS et al., 1980; LUPTON et al., 1980), manganese (KLINKHAMMER e t al., 1977; BOULEGUE et al., 1980) and CH4 anomalies (K1M, 1983; CHARLOU et al., 1986, 1987) are often correlated over vent discharges. Despite this correlation, there remain two important questions relative to CH4: is it of abiotic or biogenic origin, and is it a non-conservative tracer? Whether biogenic or abiotic in origin, the CH 4 discharged into deep-sea waters through hydrothermal vents can be considered as a tracer of hydrothermal fields. Methane in hydrothermal solutions is generally believed to result from degassing of the mantle or from abiogenic water-rock interaction (WELHAN and CRAIG, 1983; WELHAN et al., 1984), as well as from biogenic production by bacteria (LILLEY e t al., 1983) and thermocatalysis of organic matter in sediments (WELHAN and LUPTON, 1987). It is nevertheless of importance to know its origin. BAROSS et al. (1982) and LILLEY et al. (1979) have shown that bacteria can produce CH4 under high temperature and pressure, but WELHAN and CRAIG (1979) have demonstrated that CH4 produced at 21°N on the EPR is abiogenic.The CH4/He ratio also tends to demonstrate the abiogenic origin of CH4. WELHAN and CRAIG (1979) have shown that this ratio has different values according to a "normal" ocean crust origin or a "hot spot" origin. It is obvious that no such difference in CH4/He ratio could be observed if a biogenic origin was involved. We can thus conclude that the CH4 output as well as the He output is a function of the mantle source characteristics and of the efficiency of hydrothermal activity.

Although He is a conservative tracer in seawater, this is not the case for Mn, CO2, C H 4 and other hydrocarbon gases and nitrogen gases. Any attempt to locate hydrother- mal activity on a ridge segment by CH4 or Mn anomalies in seawater must be normalized to He in an appropriate way in order to correct for the natural depletion of C H 4 o r Mn

Hydrothermal CH4 over the Mid-Atlantic Ridge 123

through time in seawater. Nevertheless, both CH4 and Mn are most important as hydrothermal tracers because of ease of their analysis on board ship compared to He. Manganese (BouLEGtJE et al., 1980; KLINKHAMMER et al., 1985), as well as CH 4 (SCRANTON and BREWER, 1977; KIM, 1983), concentrations in seawater can be determined precisely on board, and thus can be used during a cruise to detect hydrothermal areas (KIM, 1983; KLINKHAMMER et al., 1985; CHARLOU et al., 1986) or for quantification of hydrothermal exchanges, provided post cruise He corrections are made.

Methane in seawater: on board analytical procedures

Methane in seawater can be analysed by a multiple phase equilibration method or a trapping method. The first technique based on successive gas chromatographic analysis of a headspace repeatedly equilibrated with the solution was developed by MCAULIFFE (1971) and has been used by several investigators (ELK1NS, 1980; BULLISTER et al., 1982; KIM, 1983). We chose, however, the alternative trapping method (SwINNERTON et al., 1962; SCRANTON and BREWER, 1977; BAROSS et al., 1982), which allowed us to work on smaller volumes (100-250 ml). Methane was stripped from seawater with He carrier gas, trapped on activated charcoal at -80°C and detected with a flame ionization detector. The limit of detection was 0.5 nl 1-1 CH4 per liter of seawater; the precision was +3% over a 3-150 nl 1 1 range.

The CH4 equipment was set up in a portable clean air-conditioned van, permitting one CH 4 analysis every 15 min for 24 h a day. The first successful on board operation of the CH4 equipment in the van occurred during the cruise of the new research vessel Akade mik Boris Petrov (Vernadsky Institute of Geochemisty, Moscow) from February to March 1985. This cruise was devoted to the study of mantle heterogeneities in the Atlantic (BouGAULT et al., 1985) and to the study of hydrothermal processes at the ridge axis. Seawater sampling to detect hydrothermal areas was based on the dredging strategy for mantle heterogeneities. The correlation that exists between the structure of the ridge axis (i.e. zero age depth variation) and the enriched (i.e. high La/Sm or Nb/Zr ratios in basalts) or depleted (low La/Sm and Nb/Zr ratios) ocean crust (SCHILLING, 1973, 1975; BOUGAULT and TREUIL, 1980) was used to define this dredging strategy. It was based on the structure of the ridge axis and the depleted or enriched character of recovered rocks was controled by ship board Nb/Zr determinations. The same strategy was adopted for hydrocasts for obvious logistic reasons but also because hydrothermal processes are closely dependent on the construction of the ocean crust itself dependent on mantle properties. It was thus useful to use the mantle heterogeneity criteria as a first order guide to the presence or absence of hydrothermal activity along the ridge axis.

Seawater samples were collected in 1.21 Niskin bottles mounted on a General Oceanics Rosette (24 bottles) fitted with a Neil Brown CTD. The sites to be sampled were previously surveyed by a SeaBeam multi-channel echo-sounding bathymetric system. This program led us to sample a very deep rift valley (Hy-36 at almost 5000 m) at 12°24'4"N which is a typically depleted ocean crust (low Nb/Zr ratio in basalts) and two other sites, Hy-38 at 13°46'9"N and Hy-39 at 14°15'0"N on the so-called "14°N zero age anomaly" (NEEDHAM, 1981). The latter is a topographic high associated with an enriched oceanic crust (high Nb/Zr ratio in basalts) (Fig. 2) (BOUGAULT et al., 1986; Dosso and BOUGAULT, 1986). Since no sample was available, we could not verify the presence of CH4 anomalies in the 200 m above the seafloor, particularly the Hy-36 and Hy-38 profiles. Data are presented in Tables 1-3.

124 J .L . CHARLOU et al.

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Fig. 2. (a,b) Sample localities for this study along MAR ridge axis. Hy-36 is located between T.F.Z. and U.F.Z. fractures zones; Hy-38 and Hy-39 are located between 15 ° F.Z. and T.F.Z. fractures zones. Station Hy-36: 12°24'N-44°05'W; depth: 4800 m. Station Hy-38: 13°47'N - 44°59'W; depth: 3900 m. Station Hy-39: 14°05'N-45°01'W; depth: 3000 m. Hy-39 is located on

the zero age topographic high.

Table 1. In situ temperature, salinity and methane values vs depth for Hy-36 profile

Depth Temp. Salinity SiO2 C H 4

(m) (°C) (g kg ') (p.M kg ') (nil ')

10 9.8 56 100 9.6 53 500 24.8 17

1000 33.6 10 2000 28.4 2159 3.117 35.323 30.3 10 2320 2.910 33.316 35.7 16 2493 2.799 35.314 39.3 15 2636 2.671 35.314 41.6 12 2823 2.547 35.313 46.0 2996 2.504 35.315 47.1 15 3287 2.368 35.310 25 3541 2.280 35.306 53.6 42 3835 2.163 35.299 56.8 33 3917 2.124 35.296 56.8 24 3999 2.121 35.296 56.8 25 4086 2.121 35.296 56.8 24

Hydrothermal CH 4 over the Mid-Atlantic Ridge 125

Table 2. In situ temperature, salinity, silica and methane values vs depth for Hy-38 profile

Depth Temp. Salinity SiO2 CH4 (m) (°C) (g kg 1) (p.M kg -1) (nl l 1)

12 24.322 36.783 9.6 54 95 23.746 36.992 18.0 36

297 11.338 35.682 27.1 11 800 5.946 35.074 32.8 8

1998 3.220 35.326 2195 2.923 35.314 37.6 17 2297 2.833 35.314 40.2 15 2401 2.780 35.313 39.4 17 2590 2.668 35.312 42.6 17 2690 2.652 35.313 43.7 15 2791 2.608 35.312 45.6 10 2902 2.548 35.309 46.4 10 3097 2.484 35.307 50.4 10 3222 2.460 35.305 48.6 10 3276 2.456 35.307 50.2 13

Table 3. In situ temperature, salinity, silica and methane values vs depth for Hy-39 profile

Depth Temp. Salinity SiO2 CH4 (m) (°C) (g kg -1) (laM kg -~) (n i l ~)

10 100 300 800

1500 4.277 1897 3.423 1998 3.248 2080 3.123 2175 2.989 2264 2.894 2344 2.847 2442 2.818 2534 2.720 2622 2.686 2715 2.568

35.333 35.238 35.326 35.322 35.320 35.318 35.319 35.316 35.314 35.308

9.2 42 9.2 45

14.0 37 32.8 9 26.4 9 27.8 11 29.9 10 31.9 12

35.9 17 37.3 20 40.5 20 41.4 15 41.8 17 41.8 29

RESULTS

Station Hy-36 The CTD rosette was positioned in the middle of the rift valley (Fig. 3). The CH 4

profile (Fig. 4) shows a small anomaly (15 nl 1-1) at about 2500 m and a much larger one ranging from 3000 to 4000 m, with a maximum concentration of 44 nl 1 -~ at 3600 m. The CH4 concentration maximum was 1000 m above the seafloor, similar to that found in the Marianas Trough (KIM, 1983), whereas it is usually located only 200-300 m above the ridge axis on the East Pacific. Both the small anomaly at 2500 m and the large one located between 3000 and 4000 m are correlated with a temperature anomaly (0.050°C) (Fig. 5).

~=,~~HY 36 1

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Fig. 4. Methane profiles at the three Hy-36, Hy-38 and Hy-39 stations. A first CH 4 anomaly (20 nl 1 -~) is seen around 2400 m depth. The large amplitude of the CH4 anomaly between 3000 and 4000 m depth on Hy-36, reflects an extensive hydrothermal activity (1 nmol

CH4 = 2.24 × 10 5 ml STP CH4).

Hydrothermal CH 4 over the Mid-Atlantic Ridge 127

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, CH 4

/ ~ . . . . . . S°r

Hy - 39 4"01 I

20 2.5 30 ToC I I I I I I

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~(CH 4 ) - 3 4 nl/I.

My -36

Fig. 5. Comparative profiles of temperature (°C) and methane (nl I ~) vs depth (km) obtained on the reference station (Hy-14) and the three stations Hy-36, Hy-38, Hy-39 of interest. Temperature trace was obtained during the down cast of the Nell Brown CTD. Note the correlation of CH4 and temperature anomalies. The vertical dotted line represents the typical background of the CH4 profile in deep Atlantic waters (8 nl 11). The temperature anomaly maximum was 0.05°C, on Hy-36 profile, corresponding to a CH4 anomaly of 44 nl 1 1. The depths of the Hy-14, Hy-36, Hy-38, Hy-39 (not represented on the figure) are, respectively: 3570, 4800,

3900 and 3000 m.

Station Hy-38

This station is located at the axis of the rift valley (Fig. 3), the inner floor being at 3900 m depth. A small CH4 anomaly (17 n 1 1) was found at the same depth (about 2500 m) as for Hy-36. No significant deeper CH4 anomaly was observed, but we have no sample between the bottom (3900 m) and 3300 m. The small CH4 anomaly at 2500 m was also correlated with a small temperature anomaly (about 0.020°C) (Fig. 5).

Station Hy-39

The valley of this zero age topographic high is at 3000 m depth (Fig. 3). The small C H 4

anomaly (19 nl 1-1) at about 2500 m, already reported for Hy-36 and Hy-38, was also present here (Fig. 4). Immediately below this anomaly, there was an increase in CH4 concentration up to 28 nl 1-1 at 2800 m (200 m above the seafloor). A corresponding temperature anomaly (up to 0.030°C) also was found at this site (Fig. 5).

128 J.L. CHARLOU et al.

DISCUSSION

The hydrothermal activity on the Mid-Atlantic Ridge (MAR) has already been well documented from temperature anomalies (RONA et al. , 1975; LOWELL and RONA, 1976), manganese deposits (ScoTr et al. , 1974), He concentrations in seawater (JENKINS et al. , 1972, 1980) and more recently from Mn concentrations in seawater (KL1NKHAMMER et al. , 1985, 1986). Black smoker hydrothermal venting was discovered in July 1985 at the TAG hydrothermal field in the rift of the MAR near 26°N (CrtARLOU et al. , 1987); subsequent to this discovery at the TAG site (26°N), black smokers were found 310 km to the south (23°N) at 3600 m at the axis of the rift valley (Leg 106 SCIENXlFIC DRILLING PARTY, 1986). Methane anomalies associated to temperature anomalies (Fig. 5) found over a new section of MAR, between 12 ° and 15°N confirm the existence of hydrothermal venting on slow spreading ridges. The first large CH4 anomaly (44 nl 1-1) in seawater at 12°24'4"N (Hy-36) correlated with temperature anomaly (=0.05°C) reflects a hydrothermal activity similar to results presented in previous reports. The occurrence of a small CH 4 anomaly at about 2500 m at the three sites between 12°24'4"N and 14°05'N with the same amplitude over more than two degrees in latitude, probably reflects a regional feature. This observation does not favor a hydrothermal origin, in that it corresponds poorly to our present picture of hydrothermal sites. If hydrothermal, the regional character could reflect a very large input and thus very extensive hydrothermal activity.

The depth and shape of the large CH4 anomaly of Hy-36 (3600 m) differ notably from those observed on the East Pacific Rise (EPR); this is probably due to the contrasting morphologies of the ridge axes of the Atlantic and Pacific. If the small anomalies discussed above have a hydrodynamic origin, their location in depth also could be the result of a hydrodynamic effect peculiar to the morphology of the MAR. The location in depth and the shape of the large CH4 anomaly of Hy-36 as well as the lowest anomaly of Hy-39 are compatible with the Mn profiles shown by KLINKHAMMER et al. (1985).

A quantitative comparison of CH4 anomalies between the EPR (fast spreading) and the MAR (slow spreading) is difficult because of the different hydrodynamic regimes generated by the different morphologies of the ridge axis. Over the EPR the existence of submarine hydrothermal convection systems has been confirmed by the discovery of hydrothermal plumes over the Galapagos spreading center (WEISS, 1977), 13°N (BOULE- GUE et al. , 1980), 21°N (LuPTON et al. , 1980), 15°S (LuPxON and CRAIG, 1981), Juan de Fuca Ridge (NORMARK e t al. , 1982; BAKER et al. , 1985) and Guaymas Basin (CAMPBELL and G1ESKES, 1984). Evidence for the existence of hydrothermal plumes at these sites has been obtained by physical measurements of water temperature anomalies, levels of suspended particulate matter, and by analysis of geochemical tracers in the seawater column. The amplitudes of CH4 anomalies found to date vary between the different studied sites: 105 nl 1-1 at Vulcan Sta. 3 and 250 nl 1-1 at Vulcan Sta. 6 at 20°S EPR zone, 60 nl 1-1 on the 21°N EPR zone at Pluto Sta.13, above the vent field (KIM, 1983). During the Cyatherm cruise, in 1982, we measured 70 nl I -I above a vent field located at 12°50N on the EPR axis and 250 nl 1-1 in the same zone during the Hydrofast cruise in 1986 (BOUGAULT and HYDROFAST TEAM, 1987; CRANE et al. , 1987). Over the MAR, the large amplitude of the CH4 anomaly at Hy-36 (12°24N) integrated over more than 1000 m, reflects a large CH4 output and thus extensive hydrothermal activity at 12°N on the MAR. This demonstrates--if such is necessary (e.g. Red Sea)--that intense hydrother- mal activity at spreading centers is not restricted to fast spreading ridges.

Hydrothermal CH 4 over the Mid-Atlantic Ridge 129

It has been suggested (for the EPR) that hydrothermal activity should generally occur on topographic highs located between fracture zones (FRANCHEXEAV and BALLARD, 1983). Topographic highs in the Atlantic (slow spreading) are larger than those of the EPR (fast spreading) by an order of magnitude. Nevertheless Hy-38 and Hy-39 located on the 14°N topographic high (Fig. 2) show smaller CH4 anomalies than those of Hy-36 where there is no topographic high. This observation suggests that hydrothermal activity is not necessarily correlated with zero age bathymetry. We think that hydrothermal cells in zero age crust occur within complex series of tectonic magmatic cycles (GENTE et al., 1986); their frequency is probably higher for fast spreading rates than for low spreading rates and thus reflect, on average, more extensive hydrothermal activity on the EPR than on the MAR. However the construction of a topographic high is probably not required for one of these tectonic magmatic cycles and thus is not necessarily a preferred location for hydrothermal activity.

Acknowledgements--We are grateful to the captain and crew of the R.V. Akademik Boris Petrov who helped make this scientific cruise a success. A special acknowledgement is dedicated to those who permitted us to set up the IFREMER van on board for seawater analysis (including CH4). We particularly want to thank G. Udinsev, A. Sobolev, N. Susheskaya and S. Cilandev for their stimulating discussion and important contributions during the cruise. Academician Barsukov (Vernadsky Institute of Geochemistry) and Y. Sillard (IFREMER) are thanked for encouraging a cooperation that permitted the French participation on board the R.V. Akademik Boris Petrov. We thank S. Scott and K. Juniper for critical review of this manuscript, and J. P. Maz6 for drawing the figures. Contribution no. 100 du Centre de Brest de I 'IFREMER.

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