Hans-Hermann Gennerich

Hydrothermal venting at pressure-temperature conditions above the critical point of seawater, 5°S on the Mid-Atlantic Ridge

Hydrothermal circulation within oceanic crust depends on pressure ( P ) and temperature ( T ); the critical point (CP) of seawater at 298 bar and 407 °C represents the threshold between subcritical and supercritical conditions. Here we present data from the first hydrothermal system in which the sampled fluids fall on and above the CP. The vent system discovered at 5°S on the Mid-Atlantic Ridge is characterized by multiple fluid emanations at variable temperatures in water depths of ~3000 m. Vigorous vapor phase bubbling, stable emanation of superhot fluid at 407 °C, and decreased salinity indicate phase separation at conditions above the CP at one site. At another site the measured maximum T of 464 °C during a 20 s interval is by far the hottest fluid ever measured at the seafloor and falls into the vapor-phase supercritical region of seawater. Besides these two separate fields with ongoing phase separation and extremely hot fluids, a third vent field emanates non-phase-separated fluids at 349 °C and is used as a reference site. Fluid chemistry shows that supercritical fluids evolve differently than subcritical fluids, making this vent system a unique natural laboratory to investigate processes at high P - T conditions. The stability of the high temperature and fluid geochemistry measured in 2005 and 2006 after the assumed seismic trigger event in 2002 supports this as an exceptional site along the Mid-Atlantic Ridge.

Collapse and reconstruction of Monowai submarine volcano, Kermadec arc, 1998–2004

Monowai submarine volcano is one of the three most historically active volcanoes of the Kermadec arc. Repeat multibeam surveys of Monowai Cone from September 1998 and September 2004 and T wave data recorded by the Reseau Sismique Polynesien network for the same period document the collapse and subsequent regrowth of the cone within this 6-a period. Grid differencing of the two bathymetric data sets, acquired 6 a apart, reveals that a landslide ∼2230 m long occurred between the surveys, within which a postcollapse cone and talus ridge (∼0.023 km3 in volume) subsequently formed. The volume of this collapse, minus postcollapse construction, is ∼0.085 km3. We interpret an unusual, strong-amplitude T wave event on 24 May 2002 as recording “hot landsliding”, where the 100- to 160-m-thick collapse has “unroofed” the uppermost parts of the vent conduit, with the subsequent explosive interaction, and cooling, of hot magma and volcaniclastic rubble with ambient seawater. This interpretation is consistent with the lack of emergent events, sharp onset, and large amplitude of the 24 May 2002 T waves. The subsequent >2500 T wave events, between November 2002 and September 2004, occurred in swarms with emerging and waning activity and with typical explosive volcanic acoustic signatures, which are interpreted as recording the regrowth of an ∼90-m-high cone back to a near-1998 elevation, at an average rate of 47 m a−1. This study provides (1) a lower bound for frequency-magnitude relationships of landsliding for submarine arc volcanoes and (2) estimates of 0.013 km3 a−1 of submarine cone growth during eruptive cycles.

Bacterial sulfur cycling shapes microbial communities in surface sediments of an ultramafic hydrothermal vent field.

The ultramafic-hosted Logatchev hydrothermal field (LHF) is characterized by vent fluids, which are enriched in dissolved hydrogen and methane compared with fluids from basalt-hosted systems. Thick sediment layers in LHF are partly covered by characteristic white mats. In this study, these sediments were investigated in order to determine biogeochemical processes and key organisms relevant for primary production. Temperature profiling at two mat-covered sites showed a conductive heating of the sediments. Elemental sulfur was detected in the overlying mat and metal-sulfides in the upper sediment layer. Microprofiles revealed an intensive hydrogen sulfide flux from deeper sediment layers. Fluorescence in situ hybridization showed that filamentous and vibrioid, Arcobacter-related Epsilonproteobacteria dominated the overlying mats. This is in contrast to sulfidic sediments in basalt-hosted fields where mats of similar appearance are composed of large sulfur-oxidizing Gammaproteobacteria. Epsilonproteobacteria (7-21%) and Deltaproteobacteria (20-21%) were highly abundant in the surface sediment layer. The physiology of the closest cultivated relatives, revealed by comparative 16S rRNA sequence analysis, was characterized by the capability to metabolize sulfur components. High sulfate reduction rates as well as sulfide depleted in (34)S further confirmed the importance of the biogeochemical sulfur cycle. In contrast, methane was found to be of minor relevance for microbial life in mat-covered surface sediments. Our data indicate that in conductively heated surface sediments microbial sulfur cycling is the driving force for bacterial biomass production although ultramafic-hosted systems are characterized by fluids with high levels of dissolved methane and hydrogen.

Geochemical constraints on the diversity and activity of H2-oxidizing microorganisms in diffuse hydrothermal fluids from a basalt- and an ultramafic-hosted vent

Mixing processes of reduced hydrothermal fluids with oxygenated seawater and fluid-rock reactions contribute to the chemical signatures of diffuse venting and likely determine the geochemical constraints on microbial life. We examined the influence of fluid chemistry on microbial diversity and activity by sampling diffuse fluids emanating through mussel beds at two contrasting hydrothermal vents. The H(2) concentration was very low at the basalt-hosted Clueless site, and mixing models suggest O(2) availability throughout much of the habitat. In contrast, effluents from the ultramafic-hosted Quest site were considerably enriched in H(2) , while O(2) is likely limited to the mussel layer. Only two different hydrogenase genes were identified in clone libraries from the H(2) -poor Clueless fluids, but these fluids exhibited the highest H(2) uptake rates in H(2) -spiked incubations (oxic conditions, at 18 °C). In contrast, a phylogenetically diverse H(2) -oxidizing potential was associated with distinct thermal conditions in the H(2) -rich Quest fluids, but under oxic conditions, H(2) uptake rates were extremely low. Significant stimulation of CO(2) fixation rates by H(2) addition was solely illustrated in Quest incubations (P-value <0.02), but only in conjunction with anoxic conditions (at 18 °C). We conclude that the factors contributing toward differences in the diversity and activity of H(2) oxidizers at these sites include H(2) and O(2) availability.

In situ measurements of hydrogen sulfide, oxygen, and temperature in diffuse fluids of an ultramafic‐hosted hydrothermal vent field (Logatchev, 14°45′N, Mid‐Atlantic Ridge): Implications for chemosymbiotic bathymodiolin mussels

The Logatchev hydrothermal vent field (14°45′N, Mid-Atlantic Ridge) is located in a ridge segment characterized by mantle-derived ultramafic outcrops. Compared to basalt-hosted vents, Logatchev high-temperature fluids are relatively low in sulfide indicating that the diffuse, low-temperature fluids of this vent field may not contain sufficient sulfide concentrations to support a chemosymbiotic invertebrate community. However, the high abundances of bathymodiolin mussels with bacterial symbionts related to free-living sulfur-oxidizing bacteria suggested that bioavailable sulfide is present at Logatchev. To clarify, if diffuse fluids above mussel beds of Bathymodiolus puteoserpentis provide the reductants and oxidants needed by their symbionts for aerobic sulfide oxidation, in situ microsensor measurements of dissolved hydrogen sulfide and oxygen were combined with simultaneous temperature measurements. High temporal fluctuations of all three parameters were measured above the mussel beds. H2S and O2 coexisted with mean concentrations between 9 and 31 μM (H2S) and 216 and 228 μM (O2). Temperature maxima (≤7.4°C) were generally concurrent with H2S maxima (≤156 μM) and O2 minima (≥142 μM). Long-term measurements for 250 days using temperature as a proxy for oxygen and sulfide concentrations indicated that the mussels were neither oxygen limited nor sulfide limited. Our in situ measurements at Logatchev indicate that sulfide may also be bioavailable in diffuse fluids from other ultramafic-hosted vents along slow and ultraslow spreading ridges.

Deciphering the ocean bottom pressure variation in the Logatchev hydrothermal field at the eastern flank of the Mid‐Atlantic Ridge

Ocean bottom pressure data from the Logatchev hydrothermal field (LHF) are presented and analyzed. The data were collected with two ocean bottom pressure meters (OBPs), constructed at the University of Bremen, that are capable of recording signals with frequencies up to 0.25 Hz. Over the long-term, a nearly 2.5 kPa (25 cm water column equivalent) pressure variation over 3.7 years is observed, which is consistent with uplift followed by subsidence, but cannot unequivocally be discerned from instrumental drift. Medium-term pressure variations are compared with satellite surface topography, satellite gravity, ocean modeling, and in situ data from an OBP 700 km away. It is shown that fluctuations in the oceanic mass distribution dominate the variations in this frequency range and that oceanic modeling and data from a 700 km distant OBP are positively correlated with the LHF bottom pressure time series. The short-term variations are dominated by microseisms originating from sea surface waves and pressure waves from earthquakes as can be shown by comparison with weather buoy and teleseismic data.

A new concept for an ocean bottom pressure meter capable of precision long‐term monitoring in marine geodesy and oceanography

Long-term vertical seafloor displacements and geostrophic changes in the water column height could be easily monitored if pressure meters were less susceptible to drift. Currently, these signals, which have typical amplitudes from decimeters to less than 1 mm/yr, cannot be differentiated from instrumental drift. In this paper, we introduce and outline a new constructional concept for an ocean bottom pressure meter that aims for unequivocal detection and monitoring of long-term trends. The concept is based on a differential pressure sensor that measures the pressure difference between the environment and a reference pressure within a sealed volume. This sealed volume conserves the instantaneous pressure at the moment of its closure at the monitoring location in a temperature-compensated manner. Furthermore, the approach enables easy in situ calibration of the differential pressure gauge by simply opening the reference pressure chamber to the environment and checking the zero point offset.