Valérie Ballu

Recent tectonic, magmatic, and hydrothermal activity on the East Pacific Rise between 17°S and 19°S: Submersible observations

The objective of the Naudur cruise (December 1993) of the submersible Nautile was to study the interaction among magmatic, tectonic, and hydrothermal processes at a very fast spreading mid-ocean ridge axis. Twenty-three dives were completed, both along and across the axis, in four areas located between 17°10′ and 18°45′S on the East Pacific Rise. Rock, sulfides, water, and biological samples have been collected along each of the segments. Two main types of segments have been distinguished, characterized either by the predominance of present-day volcanic activity or by predominant tectonic activity. Linked to both types of activity, 69 hydrothermal sites have been discovered and sampled. They comprise four types, interpreted as successive evolutionary stages. The first are shimmering water sites which occur immediately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna. The second, in areas dominated by recent volcanic activity, have waters venting directly from lava fissures and more focused discharge areas through black smoker chimneys. The third stage is represented by more mature hydrothermal vents and deposits, along the faults bounding the eastern side of the axial graben in tectonic-dominated areas. The associated fauna is well developed. The fourth stage corresponds to the reactivation of volcanic activity with lava flows, young black smokers, and diffuse venting associated with the faults bounding the axial graben. Fluids collected range from 200° to 340°C and show a wide variability in chemical and gas composition. Within each of the explored areas, evidence of recent volcanic activity has been observed.

Comparing the role of absolute sea-level rise and vertical tectonic motions in coastal flooding, Torres Islands (Vanuatu)

Since the late 1990s, rising sea levels around the Torres Islands (north Vanuatu, southwest Pacific) have caused strong local and international concern. In 2002–2004, a village was displaced due to increasing sea incursions, and in 2005 a United Nations Environment Programme press release referred to the displaced village as perhaps the world’s first climate change “refugees.” We show here that vertical motions of the Torres Islands themselves dominate the apparent sea-level rise observed on the islands. From 1997 to 2009, the absolute sea level rose by 150 + /-20 mm. But GPS data reveal that the islands subsided by 117 + /-30 mm over the same time period, almost doubling the apparent gradual sea-level rise. Moreover, large earthquakes that occurred just before and after this period caused several hundreds of mm of sudden vertical motion, generating larger apparent sea-level changes than those observed during the entire intervening period. Our results show that vertical ground motions must be accounted for when evaluating sea-level change hazards in active tectonic regions. These data are needed to help communities and governments understand environmental changes and make the best decisions for their future.

A coral‐based reconstruction of sea surface salinity at Sabine Bank, Vanuatu from 1842 to 2007 CE

Climate variability associated with the El Nino Southern Oscillation (ENSO) results in large sea-surface temperature (SST) and sea-surface salinity (SSS) anomalies in many regions of the tropical Pacific Ocean. We investigate interannual changes in SSS driven by ENSO in the southwestern Pacific at Sabine Bank, Vanuatu (SBV, 166.04°E, 15.94°S) using monthly variations in coralδ18O from 1842 to 2007 CE. We develop and apply a coral δ18O-SSS transfer function, which is assessed using a calibration-verification exercise (1970-2007 CE). The 165-year reconstructed SSS record contains a prominent trend toward freshening from 1842 to 2007 CE; mean SSS for 1842-1872 CE is 35.46 ± 0.28 psu, which contrasts with a mean value of 34.85 ± 0.31 psu for 1977-2007 CE, with a freshening trend during the latter part of the 20th century that is not unprecedented with respect to the overall record. Variance in the record is concentrated in the interannual (42%) and interdecadal (29%) bands. The SBV-SSS record matches well with a similarly reconstructed SSS time series at Malo Channel, Vanuatu, which is located ∼120 km to the east of SBV. This regional signal is likely driven by ENSO-related changes in the SPCZ and interdecadal changes in surface water advection. The pattern of interdecadal variability at SBV agrees reasonably well with coral records of interdecadal variability from Fiji and Tonga, especially in the pre-1940 portions of the records, further evidence for the regional extent of the salinity signal at Sabine Bank, Vanuatu.

1985–1999 gravity field variations across the Asal Rift: insights on vertical movements and mass transfer

Repeated gravity measurements across the Asal Rift zone reveal that the central part of the northern rift might be subject to mass input in addition to vertical movements. Precise relative gravity measurements were made in 1985, 1988 and 1999. Temporal variations in the gravity field over the 11- and 14-yr periods reveal a probable relative gravity increase on the order of 30 μGal (300 nm/s2) in the central zone of the rift (active zone since the Ardoukoba rifting episode) with respect to both sides of the rift. The gravity data are compared with vertical deformation data collected by the French National Geographical Institute (IGN) in 1984 and 2000 along 50 km of the across-axis leveling profile. The comparison shows that vertical movements alone cannot explain the observed gravity variations and that, unlike in many other volcano studies, our gravity and elevation changes cannot be modeled by a single process such as subsidence, inflation or localized mass injection.

Hydrography and flow in the Lucky Strike segment of the Mid-Atlantic Ridge

The Lucky Strike segment between 37 and 38N on the Mid-Atlantic Ridge is the focus of the international MoMAR program to monitor seafloor-spreading processes. During the GRAVILUCK cruise in August 2006, hydrographic, velocity and light-scattering data were collected in the rift valley at Lucky Strike in order to investigate the regional dynamics and to provide background information for the monitoring effort. The survey observations reveal a remarkably simple dynamical setting dominated by persistent northward flow transporting approximate to 0.2 Sv of water along the rift valley. Approximately half of this transport must upwell within a deep basin that occupies the northern half of the segment. In the comparatively shallow segment center, the along-valley transport takes place in two parallel, hydraulically controlled overflows on both sides of an active volcano that rises from the rift-valley floor. Within the better sampled of these overflows instantaneous velocities recorded during the Survey were northward more than 95% of the time and occasionally exceeded 20 cm s(-1). Similar to other laterally confined overflows in the deep ocean, the cross-sill density gradients are characterized by a lower layer with streamwise decreasing densities and an upper layer where the densities increase along the path of the flow. This vertical density-gradient dipole is the signature of the buoyancy flux associated with high levels of diapycnal mixing near the sill. Overall, the hydrography and dynamics in the rift valley of the Lucky Strike segment are highly reminiscent of many ridge-flank canyons in the western South Atlantic, where mean along-axial advection of density is balanced by vigorous diapycnal mixing. There is circumstantial evidence from historic hydrographic data Suggesting that northward flow below approximate to 1800 m in the rift valley in the MoMAR region is persistent on time scales of years to decades and that it extends more than 200 km to the south...

Infragravity waves across the oceans

Ocean infragravity (IG) waves are low-frequency waves generated along shorelines by incident seas and swell and with heights of the order of 1 cm in the open ocean. Despite these small amplitudes, they can be of much importance for ice shelf break up and errors in measurements of sea level by future satellite altimeters. A combination of numerical model results and in situ data is used to show that bottom pressure signals in the infragravity frequency band can be dominated by bursts of energy that travel across ocean basins, and can last for several days. Two particularly strong events recorded in 2008 are studied, one in the North-Pacific and the other in the North-Atlantic. It is shown that infragravity waves can travel across whole oceans basins with the signal recorded on the western shores often dominated by IG waves coming from the opposite shore of that same ocean basin.

The sources of deep ocean infragravity waves observed in the North Atlantic Ocean

Infragravity waves are long-period (25–250 s) ocean surface gravity waves generated in coastal zones through wave-wave interactions or oscillation of the breaking point. Most of the infragravity wave energy is trapped or dissipated near coastlines, but a small percentage escapes into the open oceans. The source of deep ocean infragravity waves is debated, specifically whether they come mostly from regions with strong source waves or from sites with particular morphologies/orientations. We correlate measurements of infragravity waves in the deep North Atlantic Ocean with infragravity wave generation parameters throughout the Atlantic Ocean to find the dominant sources of deep ocean infragravity wave energy in the North Atlantic Ocean. The deep ocean infragravity wave data are from a 5 year deployment of absolute pressure gauges west of the Azores islands (37°N, 35°W) and shorter data sets from seafloor tsunami gauges (DART buoys). Two main sources are identified: one off of the west coast of southern Europe and northern Africa (25°N–40°N) in northern hemisphere winter and the other off the west coast of equatorial Africa (the Gulf of Guinea) in southern hemisphere winter. These regions have relatively weak source waves and weak infragravity wave propagation paths to the main measurement site, indicating that that the site morphology/orientation dominates the creation of deep ocean infragravity waves. Both regions have also been identified as potential sources of global seismological noise, suggesting that the same mechanisms may be behind the generation of deep ocean infragravity waves and global seismological noise in the frequency band from 0.001 to 0.04 Hz.

Seafloor gravity evidence for hydrothermal alteration of the sediments in Middle Valley, Juan de Fuca Ridge

Abstract Gravity data collected at two different levels, sea-surface and seafloor, are compared and interpreted to characterize the effect of hydrothermal circulation on the sediment density in Middle Valley, a deeply sedimented spreading center on the Juan de Fuca Ridge. The sea-surface gravity data constrain density variations at depth beneath the seafloor, whereas sea-bottom measurements are more sensitive to shallow sources. At least two different types of hydrothermal signatures in the sediments can be distinguished from the gravity data: short-wavelength anomalies associated with sulfide deposits and broader anomalies associated with areas of lithified sediments. In Middle Valley, three distinct gravity anomalies were identified. (a) An anomaly over a sulfide mound, south of Bent Hill, shows that the sulfide body extends to depths of 120 to 180 m and has been fed by several near-surface conduits. (b) An anomaly at the base of the fault bordering the valley to the east is interpreted as a regional anomaly combined with the local effect of lithified sediments and possibly sulfide deposits. (c) An anomaly paralleling an intra-valley fault, that limits the deepest part of the graben, is interpreted as indicating lithification of the upper sediment layer. A high heat flow anomaly is located 1 to 2 km east of this fault, suggesting that sediment lithification occurred in a wide band above the fault and eastward to the current high heat flow area, due to the progressive migration of the hot fluid circulation.

No significant steady state surface creep along the North Anatolian Fault offshore Istanbul: Results of 6 months of seafloor acoustic ranging

The submarine Istanbul-Silivri fault segment, within 15km of Istanbul, is the only portion of the North Anatolian Fault that has not ruptured in the last 250years. We report first results of a seafloor acoustic ranging experiment to quantify current horizontal deformation along this segment and assess whether the segment is creeping aseismically or accumulating stress to be released in a future event. Ten transponders were installed to monitor length variations along 15 baselines. A joint least squares inversion for across-fault baseline changes, accounting for sound speed drift at each transponder, precludes fault displacement rates larger than a few millimeters per year during the 6month observation period. Forward modeling shows that the data better fit a locked state or a very moderate surface creep-less than 6mm/yr compared to a far-field slip rate of over 20mm/yr-suggesting that the fault segment is currently accumulating stress.

THE DENSITY STRUCTURE ASSOCIATED WITH OCEANIC CRUSTAL RIFTING AT THE HESS DEEP : A SEAFLOOR AND SEA-SURFACE GRAVITY STUDY

Abstract Seafloor and sea-surface gravity measurements provide new constraints on the lithospheric structure of the Hess Deep at the triple junction between the Pacific, Cocos and Nazca plates. As the Cocos-Nazca ridge propagates westward, breaking into young oceanic crust created at the East Pacific Rise, rocks attributed to the base of the crust or to the upper mantle are exposed on the seafloor at Hess Deep, the tip of the propagator. Seafloor gravity data were collected along two north-south profiles: the western profile runs through the deepest part of the Hess Deep valley, and the eastern profile runs over the tip of the Cocos-Nazca neovolcanic ridge. Shipboard gravity track lines provide coverage of the rift valley from the northern to the southern rim. Bouguer gravity anomalies are negative over the deepest part of the basin and positive north of it at the intra-rift ridge where gabbros and hartzburgites have been drilled. Seafloor and sea-surface gravity data are modeled together in a 2D stochastic inversion and a 3D ideal body inversion. When interpreted in the light of geologic data, gravity results favor an asymmetric emplacement mode for the deep-origin rocks, the alteration of the mantle material playing a key role in the gravity signal. In addition, new magma is emplaced from the east forming a neovolcanic ridge over strongly serpentinized peridotite; the density distribution shows that positive buoyancy cannot be the driving force for magma ascent through the upper crust.