[Show abstract][Hide abstract] ABSTRACT: Hydrothermal circulation at ultramafic-hosted sites supports a large variety of high-and low-temperature hydrothermal vents and associated ecosystems. The discovery of abundant fossil vesicomyid and thyasirid shell accumulations at the ridge crest, approximately 2.5 km east of the active Rainbow vent field on the Mid-Atlantic Ridge (MAR, 36 degrees 13'N), increased our knowledge regarding the diversity of vent communities at slow spreading ridges. Bivalve molluscs of the family Vesicomyidae were represented by the genus Phreagena. Here we present the first record of this genus in the Atlantic Ocean. This second vesicomyid species known from the MAR, Phreagena sp., was found to be associated with a Thyasira species that is affiliated with T. southwardae (at the Logatchev vent field on the MAR) and with T. vulcolutre (in the Gulf of Cadiz). These two clams have close relationships with seep taxa along the continental margin, and were likely associated with sedimented vent fields. delta O-18 and delta C-13 analyses of the shells suggested that the burrowing bivalve Thyasira could incorporate isotopically light carbon, derived from the oxidation of methane in the sediment, while the signature of Phreagena sp. shells denoted a different carbonate source. C-14 dating of the shells denoted that the hydrothermal activity in the Rainbow area began at least similar to 25.5 kyr BP, which is similar to the model of the hydrothermal vent field distribution that was proposed for the Logatchev hydrothermal site. The results provide new insight regarding the diversity of chemosynthetic fauna on the MAR over geologic time. Ultramafic-hosted, on-axis sedimented vent fields extend the range of habitats for chemosynthetic communities, underlying the need to further explore the geology of these types of environments on slow-spreading ridges and to determine their role in the ecology of deep-sea vent communities.
[Show abstract][Hide abstract] ABSTRACT: The geomagnetic field behaviour during the Cretaceous Normal Superchron (CNS) remains poorly known, in particular on two points: (1) whether the CNS is really a period of constant, or instead of dominant normal polarity, i.e. if any short reversed polarity interval exists within the CNS, as some magnetostratigraphy works have suggested the presence of at least one short event named ISEA; (2) whether the geomagnetic intensity during the CNS is higher, lower, displays stronger variations, or exhibits a behaviour similar to the present-day field, as contradictory results have emerged from studies of individual volcanic samples, sedimentary and volcanic sections, and sea-surface magnetic anomalies. In this paper, we present the first complete high-resolution record of the CNS. This record was collected during cruise Magofond 08 of R/V Le Suroît in the Central Atlantic Ocean, using a deep-tow marine magnetometer towed 500 to 1000 m above the seafloor across anomalies M4-M0, the CNS, anomaly 33r and a part of anomaly 33. The 900 km-long deep-tow profile was acquired at a speed of 1.5 knot during not less than 14 days, along a flowline of the Mid-Atlantic Ridge on the African flank, west of the Canary Islands, at ~26-29°N. Seismic data along the same profile collected during a previous cruise have been used to remove the effect of the basement topography. Comparison with shorter mid-tow (1000-1500 m altitude) profiles collected 300 to 500 km away in the same area shows a good correlation and some variations related to seafloor spreading processes. The record displays strong and uninterrupted variations of the magnetic anomaly before, during and after the CNS (the wavelength of which ultimately depending on the altitude of the measurement above the basement), suggesting similar variations of the geomagnetic intensity during the whole time span. At least five short wavelength anomalies within the CNS are as strong as or stronger than those associated to known reversals along the profile, and could be interpreted as short reversed polarity intervals during the CNS.
[Show abstract][Hide abstract] ABSTRACT: Carbonation of ultramafic rocks in geological reservoirs is, in theory, the most efficient way to trap CO2 irreversibly; however, possible feedback effects between carbonation reactions and changes in the reservoir permeability must be considered to realistically assess the efficiency and sustainability of this process. We investigated changes in the hydrodynamic properties of sintered dunite samples by means of percolation experiments, under conditions analogous to that of in situ carbonation. Our results show that carbonation efficiency is controlled by the local renewal of the reactants and the heterogeneity of the pore structure. Preferential flow zones are characterized by the formation of magnetite and of a silica-rich layer at the olivine surfaces, which eventually inhibits olivine dissolution. Conversely, sustainable olivine dissolution together with coprecipitation of magnesite, siderite, and minor Mg-TOT-phyllosilicates, occur in reduced-flow zones. Thus carbonate precipitation only decreases porosity in zones where diffusion-controlled transport is dominant. Consequently, while high flow rates will decrease the carbonation efficiency of the reservoir and low flow rates may reduce the permeability irreversibly close to the injection point, moderate injection rates will ensure a partial carbonation of the rock and maintain the reservoir permeability.
[Show abstract][Hide abstract] ABSTRACT: The Rainbow hydrothermal field at 36°14'N on the Mid-Atlantic Ridge is one of the few known sites hosted in ultramafic basement. The Rainbow Massif is located along the non-transform offset between the AMAR and South AMAR second-order ridge segments, and presents the characteristic dome morphology of oceanic core complexes, although no corrugated surface has been observed so far. One of the objectives of Cruises MOMAR DREAM (July 2007, R/V Pourquoi Pas ?; Aug-Sept 2008, R/V Atalante) was to study the petrological and structural context of the hydrothermal system at the scale of the Rainbow Massif. Our geological sampling complements previous ones achieved during Cruises FLORES (1997) and IRIS (2001), and consisted in dredge hauls, and submersible dives by manned submersible Nautile and ROV Victor. The tectonics of the Rainbow Massif is dominated by a N-S trending fault pattern on the western flank of the massif, and a series of SW-NW ridges on its northeastern side. The active hydrothermal site is located in the area were these two systems crosscut. The most abundant recovered rock type is peridotite (harzburgite and dunite) that presents a variety of serpentinization styles and intensity, and a variety of deformation styles (commonly undeformed, sometimes displaying ductile or brittle foliations). Serpentinites are frequently oxidized. Some peridotite samples have melt impregnation textures. Massive chromitite was recovered in one dredge haul. Variously evolved gabbroic rocks were collected as discrete samples or as centimeter to decimeter-thick dikes in peridotites. Basalts and fresh basaltic glass were also sampled in talus and sediments on the southwestern and northeastern flanks of the massif. Our sampling is consistent with the lithological variability encountered in oceanic core complexes along the Mid-Atlantic Ridge and Southwest Indian Ridge. The stockwork of the hydrothermal system has been sampled on the western side of the present-day hydrothermal field, along N-S trending normal fault scarps, and within the talus underneath. It is made of massive sulfides, strongly altered serpentinites, and breccias containing elements of iron sulfide/oxide impregnated serpentinites. * K. Bukas, V. Cueff Gauchard, L. Durand, F. Gaill, C. Konn, F. Lartaud, N. Le Bris, G. Musset, A. Nunes, J. Renard, V. Riou, A. Tasiemski, P. Torres, I. Vojdani, M. Zbinden
Eos Transactions American Geophysical Union 12/2008; 89(53).
[Show abstract][Hide abstract] ABSTRACT: Knowledge of the ancient geomagnetic secular variation is principally restricted to the past few million years. As a consequence, it has been very difficult to assess a connection between the secular variation regime and the magnetic reversal frequency as initially suggested by McFadden et al. (1991). Marine magnetic measurements exhibit coherent short wavelength anomalies (or "tiny wiggles") that are superimposed on the broader polarity-interval anomalies. Recent high-resolution magnetic measurements acquired near the seafloor demonstrated that most tiny wiggles do reflect paleointensity fluctuations recorded by the oceanic crust (e.g. Gee et al., 2000; Pouliquen et al., 2001, Bowles et al., 2003). Although these studies were limited to a few areas and to short periods of time, they show that the sequence of tiny wiggles offers a unique way for constraining the long-term evolution of secular variation. Hence, the purpose of our study was to perform an exhaustive investigation of tiny wiggles over a long period of time in order to study their temporal distribution as a proxy for secular variation. To this end, we performed a careful inspection of sea-surface marine magnetic profiles selected from worldwide databases within the Indian and Pacific Oceans for the period 83-41 Ma. Many tiny wiggles were isolated by comparing stacks of profiles computed within widespread study areas. Modelling of those anomalies confirms that most tiny wiggles are likely ascribed to past fluctuations of the paleointensity rather than undetected short polarity events. We observe that tiny wiggles are ubiquitous and uniformly distributed throughout this long period of time, which may indicate a nearly constant secular variation regime while the magnetic reversal frequency markedly varied from zero during the Cretaceous Normal Superchron (~118-83 Ma) to about 2-3 reversals per Myr at ~40 Ma. These results motivate testing if the pattern of variation continues throughout the Cretaceous Normal Superchron which is characterized by the absence of magnetic reversal during ~35 Myr. To answer these questions, we are acquiring new high-resolution magnetic data across the Cretaceous Quiet Zone on the eastern flank of the Mid-Atlantic ridge (cruises Magofond3 held during Summer 2005 and Magofond3bis planned for Fall 2008).
[Show abstract][Hide abstract] ABSTRACT: The Rainbow hydrothermal field at 36° 14 N on the Mid-Atlantic Ridge is one of the few known sites hosted in ultramafic environment. The active site is located on a dome structure in the non-transform offset between the AMAR and South AMAR second-order ridge segments. One of the objectives of Cruise MOMAR DREAM (Aug-Sept 2008, R/V Atalante and ROV Victor) was a near-bottom detailed and exhaustive mapping of the hydrothermal site and its vicinities using the multibeam echosounder Reson SeaBat 7125 (400 Khz) and the high sensitivity photographic camera OTUS installed on ROV Victor. This first high resolution survey of the Rainbow massif has provided bathymetric maps with a resolution of a centimeter in depth and space for the surveys carried out at the altitude of 10 m (close to Site Rainbow), and some ten centimeters for the surveys at 50 m (a larger area, 4x3 km long). The frequency of the pings is 7 cycles by second for 512 beams with an opening of 150° and a speed of the ROV of 0.3-0.4 m/s. The data have been processed with the CARAIBES software of IFREMER. The ROV is positioned with the Posidonia Ultra Short Baseline system (USBL) and an estimated navigation from the loch and heading of the vehicle. The active hydrothermal site extends along an EW direction on about 200 m. It is localized on one important mound, around 20 m in diameter, which displays the highest chimneys like "Thermitiere". Small chimneys are sparse at the east of this mound, and another inactive mound is located 200m in the northeast. The whole hydrothermal area is located just north of a highly fractured domain made of a series of north-south high- angle normal faults making steps at least 40 meters high. This 400 m wide tectonic area extends to the south on about 600-700 m. The faults give access to the stockwork of the hydrothermal system, which has been sampled. North of the hydrothermal area, a 400 m large landslide cut across the serpentinite environment. At a wider scale, the tectonics of the Rainbow Massif is dominated by a N-S trending fault pattern on its western flank, and a series of SW-NE ridges on its northeastern side. These ridges are bordered by a series of normal fault with facing-slopes to the south-east. Smaller NW-SE oriented fractures seem to affect the whole Rainbow massif. In conclusion, the Rainbow massif is highly tectonized by normal faults oriented along three different directions. No evidence of the large low-angle fault system predicted by the core complex model could be observed. Momardream party : K. Bukas, V. Cueff Gauchard, L. Durand, F. Gaill, C. Konn, F. Lartaud, N. Le Bris, G. Musset, A. Nunes, J. Renard, V. Riou, A. Tasiemski, P. Torres, I. Vojdani, M. Zbinden
Eos Transactions American Geophysical Union 12/2008; 89(53).
[Show abstract][Hide abstract] ABSTRACT: Hydrothermal site Rainbow, one of the few known site on an ultramafic basement, is an exceptional target for the multidisciplinary study of hydrothermal phenomena. It is one of the two targets of the MoMAR (Monitoring the Mid Atlantic Ridge) project patronized by InterRidge, and is the focus of an IODP drilling project. What makes this site exceptional is the abundance of natural hydrogen, methane, and iron, an element which plays a major role in active processes, down to the scale of molecules. During Cruise MomarDream (25 Aug. - 15 Sept. 2008), R/V L'Atalante and ROV Victor spent 3 weeks on site Rainbow to carry out detailed investigation of this unique area. The goals of the cruise were, first, to study the role of iron in the geological, hydrological, and biological processes, and second, to identify potential drilling targets. Beyond the requirement of a "zero state" for the repeated observations and in fine the site monitoring in the framework of the MOMAR project, the completion of an exhaustive inventory of the biological populations is needed for the sake of preservation of a fragile environment. Multibeam bathymetry and magnetics have been collected by ROV Victor 50 m above the seafloor on a 4 km × 3 km wide box centered on the site and covering about 25% of the Rainbow Massif. Similarly, multibeam bathymetry, magnetics, and high resolution photographs have been acquired 10 m above the seafloor on a 650 m × 500 m box centered on the site, and on a 300 m × 300 m box centered on a field of dead clams. A nearly full coverage was obtained in these boxes. Direct geological exploration was also carried out and allowed the collection of rock samples, complemented by an intensive dredge program when the ROV was onboard. A large part of the cruise was devoted to biological studies sensu lato, including the collection of fluids dedicated to the study of abiotic organic molecules and metagenomics, the collection of sulfide for microbiological investigations, in situ chemical measurements to characterize the hydrothermal environment, the collection of shrimps and mussels, and the deployment and recollection of colonization modules. As for the initial geological results, the microbathymetry and the rock sampling are presented in two posters (Gente et al.; Ildefonse et al.). A very strong positive magnetic anomaly is clearly associated to the site and likely reflects the strong magnetization of serpentinites impregnated by sulfides. Another strong positive anomaly corresponds to a zone of similar size located 500 m south of Site Rainbow, and may correspond to a fossil site, as suggested by tenuous evidences from rock samples collected in this area. Momardream party : K. Bukas, V. Cueff Gauchard, L. Durand, F. Gaill, C. Konn, F. Lartaud, N. Le Bris, G. Musset, A. Nunes, J. Renard, V. Riou, A. Tasiemski, P. Torres, I. Vojdani, M. Zbinden
Eos Transactions American Geophysical Union 12/2008; 89(53).
[Show abstract][Hide abstract] ABSTRACT: The Rainbow hydrothermal site, at 36°14'N on the Mid-Atlantic Ridge, is one of the few known site hosted in ultramafic basement. The MOMAR DREAM cruise (July 2007, R/V Pourquoi Pas ?) combined biological and geological objectives to study the role of abundant iron in controlling geological, biological and hydrological active processes at all scales. Two Nautile dives and a dredging program were achieved to further constrain the lithology and geological structures on the seafloor at the scale of the massif that hosts Rainbow. This massif is an inside corner high of the non-transform offset between the AMAR and South AMAR second-order ridge segments, and presents the characteristic dome morphology of oceanic core complexes. The abundant sediment cover of the massif precludes continuous geological mapping and completely successful dredging. However, our limited sampling is consistent with the lithological variability encountered in other oceanic core complexes along the Mid-Atlantic Ridge. The Rainbow serpentinite basement was continuously observed to a distance of about 1 km to the south of the hydrothermal site, with serpentinites sampled along N-S trending, fault planes steeply dipping to the West. Serpentinites were also found on the northwestern, northern, and northeastern flanks of the massif. Approximately 800 m the North of the hydrothermal site, the most prominent outcrop, cut by a family of subvertical, ~ E-W faults, is at least partly made of olivine- orthopyroxene bearing gabbro. Basalts and fresh basaltic glass were also recovered in talus and sediments on the Southwest and Northeast flanks of the massif.
[Show abstract][Hide abstract] ABSTRACT: During the SERPENTINE cruise (feb. 25 to apr. 5, 2007) on board the new RV Pourquoi Pas?, we used the VICTOR 6000 ROV to conduct a multidiciplinary (geology, fluid studies, biology) exploration of the Ashadze 1 and 2 (12deg58N) and Logatchev 1 and 2 (14deg43N and 14deg45N) hydrothermal fields. ROV observations and sampling were complemented by a night program of dredging, water sampling and underway geophysics. The Ashadze and Logatchev fields are located respectively on the west and east flank of the Mid-Atlantic Ridge, on outcrops of mantle-derived peridotites and interspersed gabbroic rocks. We present our principal findings in terms of geology, focusing on the geological context of hydrothermal vents. Ashadze comprises two active vent fields located at different levels on the western wall of the axial valley near 13°N : Azhadze-1 at 4000 m depth, and Ashadze-2 at 3300 m. We also sampled extinct sulfide chimneys near the base of the axial valley wall at 4530 m depth. The top of the wall, at 2300 m, corresponds to the termination of a large fossil corrugated surface. The axial valley at this latitude is strongly asymmetric, with higher relief to the west. This asymmetry is reversed immediately to the south, where the axial magnetic anomaly appears offset by a few kilometers to the west. The active and extinct Ashadze vents are roughly aligned to the north of this minor axial discontinuity. We find similarities between this general context, and the setting of the two Logatchev active vent fields: Logatchev-1 on the east axial valley wall near 14deg45N, and the smaller Logatchev-2 in a seemingly off-axis position near 15deg43N. Both fields lie to the north of a small offset axial discontinuity, and in an inward position relative to fossil corrugated surfaces. Based on seafloor morphology, dive observations, and rock sampling, we develop a model whereby ultramafic-hosted hydrothermal venting in the 13-15N region of the MAR involves both large active normal faults, and an inside corner-type position relative to a small offset of the volcanic axis. We discuss the relevance of this model for the TAG hydrothermal field at 26N, and for the Rainbow field at 36deg10N.
[Show abstract][Hide abstract] ABSTRACT: Geomagnetic field intensity variations deduced from magnetostratigraphic data are mainly restricted to the past few million years. As a consequence, many important questions, such as the long-term evolution of the geomagnetic field intensity and the temporal distribution of excursions, remain unsolved yet. The possibility to recover these fluctuations over a long time interval from marine magnetic anomalies is therefore of particular interest. Within the period documented by these anomalies, the Cretaceous Normal Superchron (CNS) presents a major interest in geomagnetism as little is known on the characteristics of the geomagnetic field during this event, except that it apparently did not reverse for about 35 Myr. Cruise Magofond 3 of R/V Suroit (July-August 2005) was dedicated to the CNS, with a target area on the Cretaceous Quiet Zone off Western Africa, on the eastern flank of the Mid-Atlantic Ridge. Both sea-surface and deep tow magnetic anomaly profiles have been collected. Magnetic observatory data from M'Bour (Senegal) and Guimar (Canary Islands) have been regularly transmitted to the ship in order to check for any external magnetic field disturbance which may have affected the data. In addition, seismic reflection data were acquired to insure that the observed anomalies are not caused by the basement topography and, eventually, to estimate and correct such an effect. Altogether, these data brings new constraints on the variability of the geomagnetic field during the superchron. As a preliminary result, they show the occurrence of several consistent short-wavelength magnetic anomalies which may be linked either to short reversed polarity intervals or to excursions. In particular, the ISEA reversed polarity subchron at the beginning of the CNS seems to be present in most profiles. Other observed anomalies may also depict subchrons and would therefore challenge the concept of a non-reversing geodynamo during the exceptionally long CNS.