Pergamon Geochimica et Cosmochimica Acta, Vol. 59, No. 20, pp. 4147-4165, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016.7037/95 $9.50 + .OO 0016-7037( 95)00245-6 Manganese and methane in hydrothermal plumes along the East Pacific Rise, So40 ’ to 1150 ‘N MICHAEL J. MOTTL, ’ FRANCIS J. SANSONE, ’ C. GEOFFREY WHEAT, 2 JOSEPH A. RESING, ’ EDWARD T. BAKER,’ and JOHN E. LUPTO~ ’ Department of Oceanography, University of Hawaii, Honolulu, HI 96822, USA ’ Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, USA ’ NOAA/Pacific Marine Environmental Laboratory,7600 Sand Point Way NE, Seattle, WA 98115-0070, USA 4 NOAA/Pacific Marine Environmental Laboratory,Hatfield Marine Science Center, Newport, OR 97365, USA (Received July 29, 1994; accepted in revised zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK form July 8, 1995) Abstract-In November, 1991, we surveyed the water column for hydrothermal plumes along 350 km of the East Pacific Rise axis from 8’40 ’ to 1 lo50 ’ N, using a combination of physical and chemical measure- ments. Our survey included the two major ridge segments north and south of the Clipperton Transform Fault at about lO”lO’N, both limbs of the overlapping spreading centers (OSC’s) at 9”03 ‘N and 1 lo45 ‘N, and a 30&m section of the next ridge segment to the south. We found vigorous plumes along most of this ridge axis, in keeping with its magmatically robust cross-section, axial summit caldera, and shallow, magma-related seismic reflector. These plumes were detectable by both physical (temperature and light attenuation) and chemical (dissolved Mn and CR) measurements, although the chemical measurements were more sensitive. The least active sections were the southern third of the northern segment from lo”20 to 52’N and the OSCs, especially the OSC at ll”45’N. Plumes there had weak Mn and CH4 signals and were barely detectable by physical methods. These axial sections were the only ones surveyed that lie deeper than 2600 m and appear to be magma starved. The most active sections on the northern segment gave stronger signals for Mn and temperature than for CH, and light attenuation, whereas the opposite was true on the southern segment, which was the site of a volcanic eruption at 9”45-52’N only seven months prior to our cruise. On the northern segment the four physical and chemical plume tracers correlated positively and linearly with one another, suggesting that the segment was fed by relatively uniform end- member fluids with a mean C&Nn molar ratio of 0.075. The southernmost section surveyed, from 8”42’ to 9’08 ‘N, closely resembled the northern segment. The rest of the southern segment fell into three sections with different CHJMn ratios: 9”39 to 53’N with CHJMn as high as 10, 9’08 to 39’N with CHJMn of 0.51, and 9”53 ’ to lO”07’N with CH,/Mn of 0.85. The section with the highest CHJMn was the site of the volcanic eruption, which produced high-temperature, low-salinity, gas-rich vent fluids carrying abun- dant bacterial Darticles. The high CH, concentradons are clearly associated with the volcanic eruption, but the origin of the CH, is uncle&. 1. INTRODUCTION Formation of oceanic crust along the global mid-ocean ridge system is typically accompanied by hydrothermal circulation of seawater through the newly formed crust. The resulting heat loss and chemical exchange are large and have an im- portant effect on crustal structure and composition. Evaluating this effect as a function of spreading rate requires detailed knowledge of the distribution and intensity of hydrothermal venting in space and time along the mid-ocean ridge axis. Seafloor vents are small features that are difficult to detect, but they give rise to plumes of warm water that mix with ocean bottomwater and rise buoyantly through the water col- umn. Because these plumes reach density equilibrium and spread laterally, they can integrate the output from a large number of individual vents. The hydrothermal plumes repre- sent a larger target than the vents themselves and they are detectable both physically and chemically. For these reasons, detecting and measuring these plumes is the most efficient way to determine the distribution and relative intensity of hy- drothermal venting along the mid-ocean ridge axis. The plumes also carry information about the physical and chem- ical characteristics of the hydrothermal systems that feed them. On the TROUGHS (Tropical Ridge Observations of Underwater Geochemical Hydrothermal Signals) cruise in November, 1991, we surveyed the water column along 350 km of the East Pacific Rise (EPR) axis using a com- bination of physical and chemical measurements that were applied to both hydrocasts (thirty-seven, including three background casts) and tow-yos (fourteen along axis and four across, along a total track length of 540 km). The survey required only 18 days and covered two major ridge segments, a southern segment from the overlapping spreading center (OSC) at 9” 03 ‘N to the Clipperton Transform Fault (CTF) at lO”lO’N, and a northern segment from the CTF at lO” 18’N to the OSC at 1 lo45 ‘N (Fig. 1) . We also surveyed both limbs of both OSC’s and a short section of the next segment south, to 8”42’N. Preliminary results of the survey, including data on temperature, light attenuation, ‘He, methane, manganese, and aluminum, were reported by Lupton et al. ( 1993 ) . Details of the physical measurements and analysis of sus- pended particles were given by Baker et al. (1994) and Feely et al. (1994), respectively. Here we present ship- board analyses for dissolved Mn and CH, and compare 4147