Miho Asada

Preliminary analysis of the Knipovich Ridge segmentation: influence of focused magmatism and ridge obliquity on an ultraslow spreading system

Abstract Bathymetry, gravity and deep-tow sonar image data are used to define the segmentation of a 400 km long portion of the ultraslow-spreading Knipovich Ridge in the Norwegian–Greenland Sea, Northeast Atlantic Ocean. Discrete volcanic centers marked by large volcanic constructions and accompanying short wavelength mantle Bouguer anomaly (MBA) lows generally resemble those of the Gakkel Ridge and the easternmost Southwest Indian Ridge. These magmatically robust segment centers are regularly spaced about 85–100 km apart along the ridge, and are characterized by accumulated hummocky terrain, high relief, off-axis seamount chains and significant MBA lows. We suggest that these eruptive centers correspond to areas of enhanced magma flux, and that their spacing reflects the geometry of underlying mantle upwelling cells. The large-scale thermal structure of the mantle primarily controls discrete and focused magmatism, and the relatively wide spacing of these segments may reflect cool mantle beneath the ridge. Segment centers along the southern Knipovich Ridge are characterized by lower relief and smaller MBA anomalies than along the northern section of the ridge. This suggests that ridge obliquity is a secondary control on ridge construction on the Knipovich Ridge, as the obliquity changes from 35° to 49° from north to south, respectively, while spreading rate and axial depth remain approximately constant. The increased obliquity may contribute to decreased effective spreading rates, lower upwelling magma velocity and melt formation, and limited horizontal dike propagation near the surface. We also identify small, magmatically weaker segments with low relief, little or no MBA anomaly, and no off-axis expression. We suggest that these segments are either fed by lateral melt migration from adjacent magmatically stronger segments or represent smaller, discrete mantle upwelling centers with short-lived melt supply.

Hydrothermal plumes imaged by high‐resolution side‐scan sonar on a cruising AUV, Urashima

An autonomous underwater vehicle (AUV) is an ideal platform that enables a closer approach to the seafloor than other platforms. The AUV provides remarkable advancement not only in imaging small-scale bathymetry with high resolution but also in imaging the spatial distribution of scattering signals within a water column. Acoustic echoes can be recorded continuously by AUVs, potentially providing information related to hydrothermal plumes within a water column. In May 2007, such an AUV, Urashima, of the Japan Agency for Marine–Earth Science and Technology, captured fantastic acoustic images within the water column above an active hydrothermal field, the Iheya-North field, off Okinawa. Numerous filamentous and hyperbolic echoes were recorded on the side-scan sonar images. According to the temperature and geochemical anomaly, the scattering signals within the water column were hydrothermal plumes upwelling from the vent area. This proof of concept has implications for long-range exploration for vent field with AUVs.

Hydrothermal activity on the ultra-slow spreading southern Knipovich Ridge

We report first evidence for hydrothermal activity from the southern Knipovich Ridge, an ultra-slow spreading ridge segment in the Norwegian-Greenland Sea. Evidence comes from optical backscatter anomalies collected during a systematic side-scan sonar survey of the ridge axis, augmented by the identification of biogeochemical tracers in the overlying water column that are diagnostic of hydrothermal plume discharge (Mn, CH4, ATP). Analysis of coregistered geologic and oceanographic data reveals that the signals we have identified are consistent with a single high-temperature hydrothermal source, located distant from any of the axial volcanic centers that define second-order segmentation along this oblique ridge system. Rather, our data indicate a hydrothermal source associated with highly tectonized seafloor that may be indicative of serpentinizing ultramafic outcrops. Consistent with this hypothesis, the hydrothermal plume signals we have detected exhibit a high methane to manganese ratio of 2–3:1. This is higher than that typical of volcanically hosted vent sites and provides further evidence that the source of the plume signals reported here is most probably a high-temperature hydrothermal field that experiences some ultramafic influence (compare to Rainbow and Logachev sites, Mid-Atlantic Ridge). While such sites have previously been invoked to be common on the SW Indian Ridge, this may be the first such site to be located along the Arctic ultra-slow spreading ridge system.

High‐resolution magnetic signature of active hydrothermal systems in the back‐arc spreading region of the southern Mariana Trough

High-resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back-arc spreading region of the southern Mariana Trough using Shinkai 6500, a deep-sea manned submersible. A new 3-D forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization. The results revealed that magnetic-anomaly-derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region. The distribution of magnetic-anomaly-derived absolute magnetization suggests that all five andesite-hosted hydrothermal fields are associated with a lack of magnetization, as is generally observed at basalt-hosted hydrothermal sites. Furthermore, both the Pika and Urashima sites were found to have their own distinct low-magnetization zones, which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension. The spatial extent of the resulting low magnetization is approximately 10 times wider at off-axis sites than at on-axis sites, possibly reflecting larger accumulations of nonmagnetic sulfides, stockwork zones, and/or alteration zones at the off-axis sites.

Examination of Volcanic Activity: AUV and Submersible Observations of Fine-Scale Lava Flow Distributions Along the Southern Mariana Trough Spreading Axis

A high-resolution acoustic investigation using the AUV Urashima has revealed detailed volcanic and tectonic features along the neo-volcanic zone of the intermediate-rate spreading Southern Mariana Trough, where the high magma flux forms fast-spreading type axial high morphology. Side-scan sonar imagery suggests that the survey area mainly consists of two types of terrain: high-backscattering lumpy terrain occupies the majority of the neo-volcanic zone, and low-backscattering terrain is scattered over the entire area to form various bathymetric features. Visual observations by the submersible Shinkai 6500 show that the former corresponds to bulbous pillow lava and the latter to jumbled or wrinkled sheet lavas. The estimated proportion of sheet lava with respect to study area is approximately 10 %. Pillow lavas are flatly distributed and do not form the pillow mounds that are common in the slow-spreading Mid-Atlantic Ridge. Furthermore, we did not observe any pillars, collapse features, or axial summit troughs, all of which are frequently reported in the fast-spreading East Pacific Rise.

Brief Report of Side-Scan Sonar Observations Around the Yokoniwa NTO Massif

We conducted the side-scan sonar observations of the Yokoniwa Rise, a non-transform offset massif at the southern Central Indian Ridge using the autonomous underwater vehicle AUV-r2D4 fitted with a 100 kHz sidescan sonar system. We identified two terrain types with high backscattering signals; one terrain type exhibited typical volcanic features, while the other appeared to correspond to peridotite outcrops. The orientation of linear features identified in the survey area was highly variable, but appeared to be affected by local bathymetry. The standard deviations of the orientation and average length of these linear features were larger and smaller, respectively, than those of similar features observed along the East Pacific Rise. These observations showed that the linear features on the sonar image were likely to be flow channels or areas of the seafloor that had experienced gravitational collapse. A few small chimney-like structures were also detected.

Brief Report of Side-Scan Sonar Imagery Observations of the Archaean, Pika, and Urashima Hydrothermal Sites

A high-resolution acoustic investigation using AUV-Urashima reveals the geological features of three off-axis hydrothermal sites at the Southern Mariana Trough. They are developed ~5 km in distance from backarc spreading axis. The Archaean site is developed at the foot of axial horst and forms 60 m-high mound. Hydrothermal chimneys in the site are arrayed along the ridge of the mound. Our acoustic observations detect small structures along the ridge which could be interpreted as the chimneys. To the south of the Archaean mound, the seafloor is characterized by rough and elongated fabrics approximately trending in NE–SW direction, which coincides with the strike of background seafloor slope. Visual observation indicates that the area consists of lava tube slightly covered by sediment. The Pika and the Urashima sites are developed on top and at foot of a ~1,800 m-high off-axis knoll. Unprocessed sidescan sonar imagery above the Pika and the Urashima sites shows anomalous backscattering signatures in water column. A series of hills with convex shape develops on the southwestern slope of the off-axis knoll. It shows unique facies that is rough surface with high-backscattering intensity on the sonar imagery. Tube lavas are recognized in corresponding seafloor by visual observation.

Discovery and characterization of a new hydrothermal vent based on magnetic and acoustic surveys

A new hydrothermal vent site in the southern Mariana Trough has been discovered using acoustic and magnetic surveys. Two stage surveys by the autonomous underwater vehicle (AUV) Urashima (Japan Agency for Marine-Earth Science and Technology, JAMSTEC) and the manned submersible Shinkai 6500 (JAMSTEC) were conducted in 2009 and 2010, respectively. In the first stage, we detected a clear magnetization low that extends from a previously known hydrothermal vent site, the Pika site, by using a vector magnetometer attached to the AUV. The acoustic signals suggest the presence of hydrothermal plumes emanating from the seafloor within the water column of the 120kHz side scan sonogram; 10 m scale chimney like structures in the 400-kHz multibeam bathymetry data were also detected in the area of low magnetization. These observations strongly suggest the presence of an unseen hydrothermal vent. The subsequent submersible dive discovered a new vent site, which was named the Urashima site [1]. The calculated absolute magnetization using the submersible-attached magnetometer succeeded in estimating the extent of hydrothermally altered zone for both the Pika and Urashima sites. It should be emphasized that the search for a hydrothermal vent site was a direct consequence of the geophysical surveys undertaken in the first stage. Our method can bridge the gap between conventional hydrothermal plume surveys (resolution on a scale of 103 m) and visual/photographic surveys (resolution on a scale of 100 m) and can precisely delineate the hydrothermally altered zone.