Manganese and methane anomalies in the North Fiji Basin

Manganese and methane anomalies in the North Fiji Basin

Deep-SeaResearch.Vol.37. No. 5. pp. 891-896,1990. Printedin GreatBritain. 0198-0149/90$3.00+ 0.130 © 1990PergamonPressplc Manganese and m e t h a n ...

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Deep-SeaResearch.Vol.37. No. 5. pp. 891-896,1990. Printedin GreatBritain.

0198-0149/90$3.00+ 0.130 © 1990PergamonPressplc

Manganese and m e t h a n e a n o m a l i e s in the N o r t h Fiji Basin PETER N . SEDWICK,* TOSHrrAKA G A M O t a n d GARY M . MCMURTRY*

(Received 5 June 1989; in revised form 22 December 1989; accepted 5 January 1990) Abstract--In an effort to identify back-arc seafloor hydrothermai activity in the North Fiji Basin (NFB), five hydrocasts were made along the NFB spreading center between 15"S and 17"S during January 1987. Near-bottom anomalies in total dissolvable manganese (TDM) and methane were approximately five times background over the NFB triple junction. At four stations over the northwest portion of the spreading center, a mid-depth TDM anomaly of 2-5.5 nmol kg-I was observed between 2 and 2.5 km depth; it has no associated anomaly in methane and extends alongaxis at least 140 km north of the triple junction. A recently discovered, active hydrothermal system on the NFB triple junction is a likely source for these water column anomalies. A single hydrocast made over a small basin northwest of Viti Levu showed elevated near-bottom methane and TDM (approximately twice and four times background, respectively) of either hydrothermai or sedimentary origin.

INTRODUCTION

RECENT geochemical investigations of seafioor hydrothermal activity in back-arc basins and hot spot volcanoes have noted that such systems differ significantly from the better studied mid-ocean ridge hydrothermal systems (HoRtBEet al., 1986; CAMPBELLet al., 1987; CRAIGet al., 1987; EDMONDet al., 1987; KARLet al., 1988). One area of current interest is the North Fiji Basin (NFB) in the southwest Pacific. The NFB is a relatively shallow (3-4 km deep) back-arc basin bounded by the Vitiaz trench, the Vanuatu island arc, the Hunter Fracture Zone and the Fiji Islands. Near the center of the basin is an active spreading ridge that runs north--south for several hundred kilometers at a water depth of 2-3 km. A current tectonic interpretation of the area based on geophysical surveys (AUZEr~D~et al., 1988; KaOENKEet al., in press) suggests that this spreading center terminates in a triple junction located near 17°S, 174"E, to become two active spreading ridges of uncertain extent running roughly NNW and ENE (Fig. 1). The South Pandora Ridge, now thought to be an active spreading center, crosses the northern portion of the basin, shoaling to less than 500 m depth in several areas. These thinly sedimented spreading ridges are likely sites of hydrothermal activity. Few hydrographic data are available for the area. Previous work examined the area around and to the south of the triple junction, during the SEAPSO Leg 3 and Papatua Leg VI expeditions. Pronounced deep-water anomalies in dissolved methane and total *Department of Oceanography and Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI 96822, U.S.A. tOcean Research Institute, University of Tokyo, Tokyo 164, Japan. 891

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Fig. 1. Locationmap of the North Fiji Basin, showingmajor tectonicfeaturesand locationof Moana Wave hydrocaststations. The triple junctionis immediatelynorth of Sta. 4. dissolvable manganese (TDM), well-known indicators of seafioor hydrothermal venting (KI~, 1983; KUNmU~MMEaand HUDSON, 1986), were observed at various depths between 2 and 4 km (Ca~(~ and POa~DA, 1987; AUZENDEet al., 1988). Recent work on the triple junction with the submersible Nautile has identified fossil hydrothermal deposits more than 30 m thick, and high-temperature (285°C) hydrothermal fluids have been sampled there (Auz~NDE et al., 1989). During January 1987 the Hawaii Institute of Geophysics R.V. M o a n a W a v e conducted a SeaMARC II geophysical survey of the NFB, concentrating on areas near and to the north of the triple junction. In an effort to identify active seafloor hydrothermal systems in the area, five deep-water hydrocasts were taken between 15°07'S and 17°07'S. A single hydrocast was made northwest of Viti Levu over a feature thought to be an active pullapart basin (Fig. 1). Hydrocast sites were selected on the basis of the SeaMARC II acoustic imaging (KltOENXEet al., in press). Water samples were analysed for dissolved methane at sea by gas chromatography, after the method of SWI~NERTON and LINNENBOM (1967). TDM was subsequently determined at the Hawaii Institute of Geophysics by flameless atomic absorption spectrophotometry, after preconcentration modified from the method of AI~G~ et aL (1985). RESULTS AND DISCUSSION The vertical concentration profiles of TDM and dissolved methane for Sta. 1 are presented in Fig. 2; the profiles for Stas 2, 3, 4, 6 and 7 are shown in Fig. 3 in south to north

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order. Featureless water column profiles of TDM and dissolved methane obtained by AUZENDE et al. (1988) and CRAm and POREDA (1987) in the NFB indicate background concentrations of approximately 1 nmol kg -] and 5/~cc kg -1, respectively, below 2 km depth. These are the same as our lowest deep-water values and are considered as background deep-water concentrations in the following discussion. Hydrocast 1 was made over a small, nodal depression northwest of Viti Levu, just south of the Fiji Fracture Zone. Seismic reflection surveys indicate a moderate sediment cover of approximately 100 m. Slightly elevated near-bottom concentrations of TDM (almost four times background) and dissolved methane (approximately twice background) were observed (Fig. 2). These may be weak hydrothermal anomalies, although at this site a sedimentary source of methane and manganese is also possible. Hydrocasts 4, 3, 2, 7 and 6 represent an approximately 220 km on-axis transect of the NFB. Hydrocast 4 was taken just south of the triple junction, and the stations for hydrocasts 3, 2, 7 and 6 extend from near the triple junction out along the northwest arm of

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Vertical water column profiles of dissolved methane and total dissolvable manganese at hydrocast Sta. 1 (16%8.5'$, 176"01.6'E, water depth 2250 m).

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Fig. 3. Vertical water column profiles of dissolved methane and total dissolvable manganese at hydrocast Stas 2 (16"I1.4'S, 173"35.8'E, water 0epth 3450 m), 3 (16°23.3'S, 173"35.0'E, water depth 3500 m), 4 (17"06.6'S, 173"52.6'E, water depth 2200 m), 6 (15"0"/.0'S, 173"16.0'E, water depth 4100 m) and 7 (15"49.7'S, 173"22.6'E, water depth 3400 m).

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the NFB spreading axis (Fig. 1). The profiles of TDM and methane obtained in hydrocast 4 display small, pronounced, near-bottom anomalies of around five times background. A sedimentary source of methane and manganese is unlikely in this area because seismic reflection profiles show only very light sediment cover on the spreading axis, whereas thick off-axis sediments flanking the ridge are below 3 km depth (L. KROENKE, personal communication, 1988). The Sta. 4 anomalies are most likely hydrothermal in origin, and they are of similar magnitude to water column anomalies observed within 10 km of the East Pacific Rise and Mid Atlantic Ridge vents (KIM, 1983; KLINKHAMMERet al., 1986). An unexpected feature in the vertical TDM concentration profiles of hydrocast Stas 2, 3, 6 and 7 is a small, mid-depth anomaly, approximately 2-5.5 nmol kg -1, with no corresponding anomaly in dissolved methane. Our sampling pattern defines only the lower portion of this manganese anomaly, because it was not identified at sea (background methane concentrations were observed at these depths). Maximum T D M values were measured between 2 and 2.5 km depth, and the similarity of the profiles in this depth range suggests a layer of manganese-enriched water extending at least 140 km along-axis. Given the geologic setting of the ridge and the horizontal extent of this anomaly, reducing sediments or resuspended hydrothermal sediments from shallow portions of the ridge (such as the triple junction) are unlikely sources. The anomaly is most likely a hydrothermal emission. Comparison of the sigma-t values for the deepest sample at Sta. 4 (at = 27.68) with those corresponding to the maximum TDM anomaly at Stas 2, 3 and 6 (at = 27.69) suggests that these anomalies have a common source. Slightly elevated near-bottom TDM concentrations were observed at Stas 2 and 3 (approximately 3 nmol kg -1 near 3200 m depth); methane also increases toward the seafloor at Stas 2, 3 and 7. These may be small hydrothermal anomalies, although sedimentary input cannot be discounted at this depth. Elevated methane concentrations measured in the upper water column at all stations are typical of the open ocean. The elevated concentrations of TDM measured above 500 m depth at Stas 1 and 2 (approximately three times deep-water background) and notably at Sta. 6 (approximately seven times deep-water background) are unusual. They may reflect coastal input from Fiji (at Sta. 1) or hydrothermal input from shallow areas of the South Pandora Ridge (at Stas 2 and 6), where fresh scoriaceous rocks were recovered during the cruise. Recent submersible studies of the dome forming the center of the triple junction (Fig. 1) revealed extensive hydrothermal sulfide deposits and active high-temperature venting. Chloride-depleted, 285°C fluids were sampled from anhydrite chimneys at 1900 m depth, indicative of the vapor phase of a boiling hydrothermal system, and temperature anomalies on the seafloor in this area suggest the discharge of a dense brine phase (AUZENDE et al., 1989; J.-M. AUZENDE, personal communication, 1989). This area is a probable source for the chemical anomalies observed at Sta. 4, and the mid-depth TDM anomalies of Stas 2, 3, 6 and 7. The absence of a methane anomaly at Stas 2, 3, 6 and 7 implies that either the plume is of sufficient age such that hydrothermal methane has been consumed, or that the anomaly arises from the injection of a gas-poor, metal-rich hydrothermal brine. If the latter case is true, then this may be the first documentation of the long-range transport of hydrothermal brine effluent. This work underscores the importance of measuring both dissolved methane and TDM, preferably while at sea, in characterizing hydrothermal emissions.

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Acknowledgements--The authors thank the captain and crew of the R.V. Moana Wave, and chief scientists Loren Kroenke (University of Hawaii) and Richard Price (Latrobe University). Ken Farley (Scripps Institution of Oceanography), Hitoshi Sakai (University of Tokyo) and Bronte Tilbrook (University of Hawaii) are acknowledged for their assistance. This work was supported by a grant from the U.S. Department of State, Aid to Developing Countries Program, under contract no. LAC5724-C-55-4086-00 with the Office of Marine Science and Technology. Hawaii Institute of Geophysics Contribution no. 2257.

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