Céline Rommevaux-Jestin

The Southwest Indian Ridge between 49°15′E and 57°E: focused accretion and magma redistribution

Abstract Bathymetric, gravity, magnetic and backscattering strength data have been used to characterise the segmentation of an 800 km long portion of the ultraslow-spreading Southwest Indian Ridge (SWIR, full rate 14 mm/yr) between 49°15′E and 57°E. This analysis reveals that the segmentation defined by along-axis variations of depth and by occurrence of axial offsets does not systematically correspond to the segmentation determined by the along-axis variations of backscattering strength, mantle Bouguer anomaly (MBA) and amplitude of the central magnetic anomalies (CMA). At axial discontinuities with offsets larger than 15 km, thin crust and reduced volcanic production are suggested by the occurrence of MBA highs, almost non-existent CMA and 50% lower backscattering strength relative to the segment centres. By contrast, smaller non-transform discontinuities, with offsets smaller than 15 km, correspond to very weak variations or to no variation of the MBA, the CMA or the reflectivity of the seafloor, suggesting that there is little or no variation of volcanic production and crustal thickness associated with those small discontinuities. These small axial discontinuities bound low-relief bathymetric segments (500–700 m), corresponding to weak or no MBA lows (amplitude 1000 m), corresponding to large MBA lows (amplitude >30 mGal). We suggest that the magma supply to these low-relief segments is controlled by near-surface processes such as melt migration and/or crustal plumbing from adjacent high-relief segments. Pronounced MBA lows at high-relief segments are thought to correspond to spreading cells where magma supply is focused in the mantle. These spreading cells are spaced by about 100 km along the SWIR axis. We suggest that the spacing of spreading cells on slow-spreading ridges is primarily controlled by the spreading rate with larger spacing between spreading cells on ultraslow-spreading ridges than on slow-spreading ridges.

Melt supply variations to a magma‐poor ultra‐slow spreading ridge (Southwest Indian Ridge 61° to 69°E)

The Southwest Indian Ridge (SWIR) to the east of the Melville Fracture zone receives anomalously low volumes of melt on average. However, a small number of ridge segments appear to receive more melt than this regional average. We use off-axis bathymetry, gravity, and magnetic data to show that this melt distribution pattern, quite distinct from what is observed at the Mid-Atlantic Ridge (MAR), has been a characteristic of the easternmost SWIR for at least the past 10 myr. We also show that segments of the easternmost SWIR are substantially shorter lived than most segments of the MAR. Melt distribution in our SWIR study area is therefore both more focused and more variable in time than at the MAR. We tentatively propose a mechanism by which strong and transient melt-focusing events could be initiated by a localized increase in the volume of melt supplied by the melting mantle to the base of the axial lithosphere, causing thermal thinning of this lithosphere and along-axis melt migration. These two processes may combine to effectively focus larger volumes of melt toward the center of future thick crust segments. Rapid melt extraction by dikes that feed large volcanic constructions on the seafloor, followed by tectonic disruption of these volcanic constructions by deep-reaching faults, may then cool the axial lithosphere back to its original thickness and end the melt-focusing events. The easternmost SWIR is also characterized by a common departure from isostatic compensation of seafloor topography and by a pronounced asymmetry of crustal thickness and seafloor relief between the two ridge flanks. At the faster spreading MAR, similar characteristics are found near the ends of ridge segments. We propose that spreading at the ultra-slow SWIR during periods when the melt supply is low (i.e., most of the time for the easternmost SWIR) is dominated by large offset asymmetric normal faulting, with significant flexural uplift of the footwalls. Faults face either north or south, and changes in fault polarity are frequent, both along axis and along flow lines (i.e., with time). Producing large faults and maintaining high uncompensated reliefs require the axial lithosphere to be thick, a predictable characteristic for this ultra-slow ridge, which has an anomalously low regionally averaged melt supply.

A different pattern of ridge segmentation and mantle Bouguer gravity anomalies along the ultra-slow spreading Southwest Indian Ridge (15°30′E to 25°E)

Abstract The results of a recent bathymetric and geophysical investigation of a ∼650 km-long portion of the very slowly opening (16 mm/yr full rate) Southwest Indian Ridge (SWIR) between 15°30′E and 25°E are presented. Bathymetry and mantle Bouguer gravity anomalies (MBA), caused by variations in crustal thickness and/or crustal and upper mantle densities, show different characteristics from those observed at faster spreading centers like the Mid-Atlantic Ridge (MAR) (20–30 mm/yr full rate). With the exception of the Du Toit Transform, none of the ridge-axis discontinuities have offsets greater than 10 km and few of the discontinuities have clearly defined off-axis traces. The MBA patterns associated with individual segments are much more complex than the simple circular bulls eyes lows reported along the MAR. While the short wavelength ridge segment length is comparable to that of the MAR, there is little correlation with MBA amplitude and segment length and axial relief. Furthermore, an eastward propagating magma source and an ∼84 km-long zone of oblique spreading appears to define a fundamental boundary along the SWIR between two 250–300 km-long sections characterized by distinctly different axial morphology and gravity signatures. We interpret these results to indicate a long-wavelength segmentation pattern of the underlying upwelling mantle. Melt separates from the upwelling mantle at the base of the lithosphere and is channeled to the surface along dikes. Fissure eruptions within the rift valley build linear ridges defining a short-wavelength spatial pattern of ridge segmentation that is not directly related to the segmentation pattern of the upwelling mantle. Our results and interpretation are quite different than that predicted by extending current models of the faster spreading MAR to these ultra-slow spreading rates.

Magmato-tectonic cyclicity at the ultra-slow spreading Southwest Indian Ridge: Evidence from variations of axial volcanic ridge morphology and abyssal hills pattern

On-axis deep tow side scan sonar data are used together with off-axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra-slow spreading Southwest Indian Ridge. Differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments. We interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development, including periods of volcanic construction and periods of tectonic dismemberment. Using off-axis bathymetric data, we identify numerous abyssal hills with a homogeneous size for each segment. These abyssal hills all display an asymmetric shape, with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away. We suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment, i.e., a magmato-tectonic cycle. We observe that large abyssal hills are in ridge sections of thicker crust, whereas smaller abyssal hills are in ridge sections of thinner crust. This suggests that the magma supply controls the size of abyssal hills. The abyssal hills in ridge sections of thinner crust are regularly spaced, indicating that the magmato-tectonic cycle is a pseudoperiodic process that lasts ~0.4 m.y., about 4 to 6 times shorter than in ridge sections of thicker crust. We suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long-lived segments. By contrast, when magma supply strongly decreases and becomes highly discontinuous, regular abyssal hills patterns are no longer observed.

Mantle Bouguer Anomaly Along an Ultra Slow-Spreading Ridge: Implications for Accretionary Processes and Comparison with Results from Central Mid-Atlantic Ridge

A three-dimensional analysis of gravity andbathymetry data has been achieved along the Southwest Indian Ridge (SWIR)between the Rodriguez Triple Junction (RTJ) and the Atlantis II transform,in order to define the morphological and geophysical expression ofsecond-order segmentation along an ultra slow-spreading ridge(spreading rate of 8 mm/yr), and to compare it with awell-studied section along a slow-spreading ridge (spreadingrate of 12.5 mm/yr): the Mid-Atlantic Ridge (MAR) between 28°and 31°30′ N.Between the Atlantis II transform and theRTJ, the SWIR axis exhibits a deep axial valley with an ∼30°oblique trend relative to the north–south spreading direction. Onlythree transform faults offset the axis, so the obliquity has to beaccommodated by the second-order segmentation. Alongslow-spreading ridges such as the MAR, second-order segmentshave been defined as linear features perpendicular to the spreadingdirection, with a shallow axial valley floor at the segment midpoint,deepening to the segment ends, and are associated with Mantle BouguerAnomaly (MBA) lows. Along the SWIR, our gravity study reveals the presenceof circular MBA lows, but they are spaced further apart than expected. Thesegravity lows are systematically centred over narrow bathymetric highs, andinterpreted as the centres of spreading cells. However, along some obliquesections of the axis, the valley floor displays small topographicundulations, which can be interpreted as small accretionary segments frommorphological analysis, but as large discontinuity domains from thegeophysical data. Therefore, both bathymetry and MBA variations have to beused to define the second-order segmentation of an ultraslow-spreading ridge. This segmentation appears to be characterisedby short segments and large oblique discontinuity domains. Analysis of alongaxis bathymetric and gravimetric profiles exhibits three different sectionsthat can be related to the thermal structure of the lithosphere beneath theSWIR axis.The comparison between characteristics of segmentationalong the SWIR and the MAR reveals two major differences: first, the poorcorrelation between MBA and bathymetry variations and second, the largerspacing and amplitude of MBA lows along the SWIR compared to the MAR. Theseobservations seem to be correlated with the spreading rate and the thermalstructure of the ridge. Therefore, the gravity signature of the segmentationand thus the accretionary processes appear to be very different: there areno distinct MBA lows on fast-spreading ridges, adjacent ones on slowspreading ridges and finally separate ones on ultra slow-spreadingridges. The main result of this study is to point out that 2nd ordersegmentation of an ultra slow-spreading ridge is characterised bywide discontinuity domains with very short accretionary segments, suggestingvery focused mantle upwelling, with a limited magma supply through a cold,thick lithosphere. We also emphasise the stronger influence of themechanical lithosphere on accretionary processes along an ultra slow-spreading ridge.

TOBI sidescan sonar imagery of the very slow-spreading Southwest Indian Ridge: evidence for along-axis magma distribution

New deep tow sidescan sonar data from the Southwest Indian Ridge reveal complex volcanic/tectonic interrelationships in the axial zone of this ultra-slow spreading ridge. While some constructional volcanic features resemble examples documented at the slow-spreading Mid-Atlantic Ridge, such as axial volcanic ridges, hummocky and smooth lava flows, their distribution and dimensions differ markedly. The largest axial volcanic ridges occur at segment centres, but fresh-looking volcanic constructions also occur at segment ends and in the deep basins marking the non-transform discontinuities. The orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge. Minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes. This observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes, with associated fractures opening in response to magma pressure, may control local emplacements of axial volcanic ridges at obliquely spreading ridges. This non-systematic pattern of ridge characteristics suggests an along-axis variation between focused and distributed magmatic supply, a model which is supported by our interpretation of low-amplitude mantle Bouguer anomalies calculated for the area. We propose that a change of the axial segmentation pattern, from two segments to the present-day three segments, may have introduced additional instability into the crustal accretion process.

Microbial colonization of basaltic glasses in hydrothermal organic-rich sediments at Guaymas Basin.

Oceanic basalts host diverse microbial communities with various metabolisms involved in C, N, S, and Fe biogeochemical cycles which may contribute to mineral and glass alteration processes at, and below the seafloor. In order to study the microbial colonization on basaltic glasses and their potential biotic/abiotic weathering products, two colonization modules called AISICS (“Autonomous in situ Instrumented Colonization System”) were deployed in hydrothermal deep-sea sediments at the Guaymas Basin for 8 days and 22 days. Each AISICS module contained 18 colonizers (including sterile controls) filled with basaltic glasses of contrasting composition. Chemical analyses of ambient fluids sampled through the colonizers showed a greater contribution of hydrothermal fluids (maximum temperature 57.6°C) for the module deployed during the longer time period. For each colonizer, the phylogenetic diversity and metabolic function of bacterial and archaeal communities were explored using a molecular approach by cloning and sequencing. Results showed large microbial diversity in all colonizers. The bacterial distribution was primarily linked to the deployment duration, as well as the depth for the short deployment time module. Some 16s rRNA sequences formed a new cluster of Epsilonproteobacteria. Within the Archaea the retrieved diversity could not be linked to either duration, depth or substrata. However, mcrA gene sequences belonging to the ANME-1 mcrA-guaymas cluster were found sometimes associated with their putative sulfate-reducers syntrophs depending on the colonizers. Although no specific glass alteration texture was identified, nano-crystals of barite and pyrite were observed in close association with organic matter, suggesting a possible biological mediation. This study gives new insights into the colonization steps of volcanic rock substrates and the capability of microbial communities to exploit new environmental conditions.

Structural Iron (II) of Basaltic Glass as an Energy Source for Zetaproteobacteria in an Abyssal Plain Environment, Off the Mid Atlantic Ridge

To explore the capability of basaltic glass to support the growth of chemosynthetic microorganisms, complementary in situ and in vitro colonization experiments were performed. Microbial colonizers containing synthetic tholeitic basaltic glasses, either enriched in reduced or oxidized iron, were deployed off-axis from the Mid Atlantic Ridge on surface sediments of the abyssal plain (35°N; 29°W). In situ microbial colonization was assessed by sequencing of the 16S rRNA gene and basaltic glass alteration was characterized using Scanning Electron Microscopy, micro-X-ray Absorption Near Edge Structure at the Fe-K-edge and Raman microspectroscopy. The colonized surface of the reduced basaltic glass was covered by a rind of alteration made of iron-oxides trapped in a palagonite-like structure with thicknesses up to 150 μm. The relative abundance of the associated microbial community was dominated (39% of all reads) by a single operational taxonomic unit (OTU) that shared 92% identity with the iron-oxidizer Mariprofundus ferrooxydans PV-1. Conversely, the oxidized basaltic glass showed the absence of iron-oxides enriched surface deposits and correspondingly there was a lack of known iron-oxidizing bacteria in the inventoried diversity. In vitro, a similar reduced basaltic glass was incubated in artificial seawater with a pure culture of the iron-oxidizing M. ferrooxydans DIS-1 for 2 weeks, without any additional nutrients or minerals. Confocal Laser Scanning Microscopy revealed that the glass surface was covered by twisted stalks characteristic of this iron-oxidizing Zetaproteobacteria. This result supported findings of the in situ experiments indicating that the Fe(II) present in the basalt was the energy source for the growth of representatives of Zetaproteobacteria in both the abyssal plain and the in vitro experiment. In accordance, the surface alteration rind observed on the reduced basaltic glass incubated in situ could at least partly result from their activity.

Propagation of the Southwest Indian Ridge at the Rodrigues Triple Junction

The analysis of multibeam bathymetric data of the Southwest Indian Ridge(SWIR) domain between the triple junction traces from 68° E to theRodrigues Triple Junction (RTJ; 70° E) reveals the evolution of thisridge since magnetic anomaly 4 (8 Ma). Image processing has been used toshow that the horizontal component of strain due to a network of normal stepfaults increases dramatically between 69°30′ E and the RTJ. Thisarea close to the RTJ is characterized by a deep graben at the foot of thetriple junction trace on the African plate and by a narrow fault-boundedridge that joins an offset of the trace on the Antarctic plate. In thatarea, spreading is primarily amagmatic and dominated by tectonic extensionprocesses. To the west of 69°30′ E, some lobate bathymetricfeatures atop of a large topographic high suggest volcanic constructions.Between 68°10′ E and 69°25′ E the southern flank of theSWIR domain is wider than the northern one and is characterized by a series of 7 en echelon bathymetric highs similar in size,shape and orientation to the one centred at 69°30′E near the present-day triple junction. Their en echelon organization along the triple junction trace on the Antarctic plate and the typical lack of conjugated parts on the northern flank show that these bathymetric highs have been shifted to the south by successive northward relocalisations of the SWIR rifting zone. This evolution results in the asymmetric spreading of the SWIR in the survey area. The off-axis bathymetric highs connect to the offsets of the triple junction trace on the Antarctic plate when the Southeast Indian Ridges lightly lengthenstoward the northwest and the triple junction is relocated to the north. We propose that the SWIR lengthens toward the northeast with two propagation modes: 1) a continuous and progressive propagation with distributed deformation in preexisting crust of the Central Indian Ridge, 2) a discontinuous propagation with focusing of the deformation in a rift zone when the triple junction migrates rapidly to the north. The modes of propagation of the SWIR are related to different localisation and distribution of strain which are in turn controlled by changes of the triple junction configurations due to propagation, recession or a symmetric spreading on the Central and Southeast Indian Ridges.

Biogeochemical insights into microbe-mineral-fluid interactions in hydrothermal chimneys using enrichment culture

Active hydrothermal chimneys host diverse microbial communities exhibiting various metabolisms including those involved in various biogeochemical cycles. To investigate microbe–mineral–fluid interactions in hydrothermal chimney and the driver of microbial diversity, a cultural approach using a gas-lift bioreactor was chosen. An enrichment culture was performed using crushed active chimney sample as inoculum and diluted hydrothermal fluid from the same vent as culture medium. Daily sampling provided time-series access to active microbial diversity and medium composition. Active archaeal and bacterial communities consisted mainly of sulfur, sulfate and iron reducers and hydrogen oxidizers with the detection of Thermococcus, Archaeoglobus, Geoglobus, Sulfurimonas and Thermotoga sequences. The simultaneous presence of active Geoglobus sp. and Archaeoglobus sp. argues against competition for available carbon sources and electron donors between sulfate and iron reducers at high temperature. This approach allowed the cultivation of microbial populations that were under-represented in the initial environmental sample. The microbial communities are heterogeneously distributed within the gas-lift bioreactor; it is unlikely that bulk mineralogy or fluid chemistry is the drivers of microbial community structure. Instead, we propose that micro-environmental niche characteristics, created by the interaction between the mineral grains and the fluid chemistry, are the main drivers of microbial diversity in natural systems.