Manganese and methane in hydrothermal plumes along the East Pacific Rise, 8°40′ to 11°50′N

Abstract

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 measurements. 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 endmember 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 abundant bacterial Darticles. The high CH, concentradons are clearly associated with the volcanic eruption, but the origin of the CH, is uncle&.

Development and application of a gas chromatography method for simultaneously measuring H2 and CH4 in hydrothermal plume samples

“…All the profiles displayed peak concentrations around 2600 m, about 150 m above seafloor, which is comparable to the previous results reported by Wang et al (2012). Compared with previous reported data from venting systems at other spreading ridges, it is interested to be noticed that the H 2 and CH 4 concentrations of the Dragon Flag hydrothermal filed are intermediate in the global series (Lilley et al 1995;Mottl et al 1995;Bougault et al 1998;Bennett et al 2013). CH 4 : Mn ratios at plume height can be used as an indication of "end-member" vent type (Charlou et al 1991;Mottl et al 1995;German et al 2010b).…”

“…Compared with previous reported data from venting systems at other spreading ridges, it is interested to be noticed that the H 2 and CH 4 concentrations of the Dragon Flag hydrothermal filed are intermediate in the global series (Lilley et al 1995;Mottl et al 1995;Bougault et al 1998;Bennett et al 2013). CH 4 : Mn ratios at plume height can be used as an indication of "end-member" vent type (Charlou et al 1991;Mottl et al 1995;German et al 2010b). Combining the CH 4 data in this article and our Mn data (not published), the CH 4 : Mn ratios in Dragon Flag hydrothermal plume were < 1.…”

“…However, it is hard to analyze H 2 and CH 4 simultaneously by only one type of GC because of the chromatographic differences between H 2 and CH 4 . Generally, CH 4 is analyzed by a GC equipped with a flame ionization detector (FID), which only responds to organic gases, or a thermal conductivity detector (TCD) in combination with a headspace or a purge-trap technique to enrich CH 4 (Mottl et al 1995;Charlou et al 1998;Radford-Knoery et al 1998;Connelly et al 2007;Love et al 2008); H 2 is usually measured by a GC equipped with a pulsed discharge helium ionization detector (PDHID) or reduction gas detector, which cannot analyze CH 4 , combined with headspace technique, (Kelley et al 1998;Kawagucci et al 2010;Marbler et al 2010). A GC with a TCD can determine H 2 and CH 4 concentrations simultaneously at high concentration levels, for example, for hydrothermal vent fluid samples, but not at the low concentrations found in hydrothermal plumes or seawater, about several nM dozens of nM.…”

Understanding the Compositional Variability of the Major Components of Hydrothermal Plumes in the Okinawa Trough

“…However, the collected venting fluids ( max = 328 ∘ C) from the Yonaguni Knoll IV hydrothermal field in the southern Okinawa Trough had a wide range of chemical compositions (e.g., Cl − 376-635 mmol kg −1 ; Ca 2+ 14.3-26.1 mmol kg −1 ; Mn 2+ 0.70-1.25 mmol kg −1 ), which is considered as evidence for subseafloor phase separation [14]. Furthermore, Mn 2+ has also been studied extensively (e.g., [15][16][17]), and the distribution of Mn 2+ in the seawater column is strongly affected by external sources and sinks, hydrothermal processes, and redox condition (e.g., [17][18][19][20][21][22][23][24]). …”

Methane along the western Mexican margin

“…While in some areas this maximum may be the result of horizontal advection from methane-rich source regions (Scranton and Farrington 1977;Brooks 1979;Sansone et al 1999) or from coastal upwelling (Rehder et al 2002), the majority of researchers have attributed this supersaturation to in situ methane production (e.g., Scranton and Brewer 1977;Conrad and Seiler 1988;Holmes et al 2000). Supersaturation of methane in deep ocean waters can occur as a result of hydrothermal discharge (e.g., de Angelis et al 1993;Mottl et al 1995;Cowen et al 2002), release of methane-rich pore waters (e.g., Reeburgh 1976;Scranton and Farrington 1977), or seepage from hydrates (Suess et al 1999) or other gas reserves (Brooks 1979).…”