Magmatism, serpentinization and life: Insights through drilling the Atlantis Massif (IODP Expedition 357)

IODP Expedition 357 used two seabed drills to core 17 shallow holes at 9 sites across Atlantis Massif ocean core complex (Mid-Atlantic Ridge 30°N). The goals of this expedition were to investigate serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration in one borehole. The cores show highly heterogeneous rock types and alteration associated with changes in bulk rock chemistry that reflect multiple phases of magmatism, fluid-rock interaction and mass transfer within the detachment fault zone. Recovered ultramafic rocks are dominated by pervasively serpentinized harzburgite with intervals of serpentinized dunite and minor pyroxenite veins; gabbroic rocks occur as melt impregnations and veins. Dolerite intrusions and basaltic rocks represent the latest magmatic activity. The proportion of mafic rocks is volumetrically less than the amount of mafic rocks recovered previously by drilling the central dome of Atlantis Massif at IODP Site U1309. This suggests a different mode of melt accumulation in the mantle peridotites at the ridge-transform intersection and/or a tectonic transposition of rock types within a complex detachment fault zone. The cores revealed a high degree of serpentinization and metasomatic alteration dominated by talc-amphibole-chlorite overprinting. Metasomatism is most prevalent at contacts between ultramafic and mafic domains (gabbroic and/or doleritic intrusions) and points to channeled fluid flow and silica mobility during exhumation along the detachment fault. The presence of the mafic lenses within the serpentinites and their alteration to mechanically weak talc, serpentine and chlorite may also be critical in the development of the detachment fault zone and may aid in continued unroofing of the upper mantle peridotite/gabbro sequences. New technologies were also developed for the seabed drills to enable biogeochemical and microbiological characterization of the environment. An in situ sensor package and water sampling system recorded real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential (Eh), and temperature and during drilling and sampled bottom water after drilling. Systematic excursions in these parameters together with elevated hydrogen and methane concentrations in post-drilling fluids provide evidence for active serpentinization at all sites. In addition, chemical tracers were delivered into the drilling fluids for contamination testing, and a borehole plug system was successfully deployed at some sites for future fluid sampling. A major achievement of IODP Expedition 357 was to obtain microbiological samples along a west–east profile, which will provide a better understanding of how microbial communities evolve as ultramafic and mafic rocks are altered and emplaced on the seafloor. Strict sampling handling protocols allowed for very low limits of microbial cell detection, and our results show that the Atlantis Massif subsurface contains a relatively low density of microbial life

Abiotic Hydrogen and Methane: Fuels for Life

International audienc

CO2 geological storage: Integrating geochemical, hydrodynamical, mechanical and biological processes from the pore to the reservoir scale

International audienc

Fast Determination of the Main Reduced Sulfur Species in Aquatic Systems by a Direct and Second-Derivative Spectrophotometric Method

International audienceThe determination of reduced sulfur species in aquatic systems is not an easy and fast task to accomplish regarding the numerous possible interferences and risks of oxidation that occur with the usual methods of quantification. e method presented here is a direct spectrophotometric method that can be used to quantify sulfides, sulfites, and thiosulfates in a simple and rapid way. e principle is based on the comparison of second-derivative absorbance spectra of the same sample at different pH (9.2, 4.7, and 1.0) and selected absorption wavelengths (250 and 278 nm). is method has been successfully tested and has demonstrated liability to (i) avoid the biases due to absorbance overlaps between the different major chemical species and (ii) keep, as a direct method, the advantages over indirect methods on interferences reduction. e limits of detections (LOD) reached for total sulfide, sulfite, and thiosulfate are 1.37, 7.32, and 1.92 µM, respectively. e method displays low accuracy mean and low relative standard deviation ( 0.999). Accordingly, this method represents a very robust alternative in terms of cost and rapidity for the quantification of reduced sulfur species in different aquatic environments, from freshwaters to saline and polluted systems

Tracking hidden organic carbon in rocks using chemometrics and hyperspectral imaging

Abstract Finding traces of life or organic components of prebiotic interest in the rock record is an appealing goal for numerous fields in Earth and space sciences. However, this is often hampered by the scarceness and highly heterogeneous distribution of organic compounds within rocks. We assess here an innovative analytical strategy combining Synchrotron radiation-based Fourier-Transform Infrared microspectroscopy (S-FTIR) and multivariate analysis techniques to track and characterize organic compounds at the pore level in complex oceanic rocks. S-FTIR hyperspectral images are analysed individually or as multiple image combinations (multiset analysis) using Principal Component Analyses (PCA) and Multivariate Curve Resolution – Alternating Least Squares (MCR-ALS). This approach allows extracting simultaneously pure organic and mineral spectral signatures and determining their spatial distributions and relationships. MCR-ALS analysis provides resolved S-FTIR signatures of 8 pure mineral and organic components showing the close association at a micrometric scale of organic compounds and secondary clays formed during rock alteration and known to catalyse organic synthesis. These results highlights the potential of the serpentinizing oceanic lithosphere to generate and preserve organic compounds of abiotic origin, in favour of the hydrothermal theory for the origin of life

Grating-Based X-Ray Computed Tomography for Improved Contrast on a Heterogeneous Geomaterial

International audiencePhase-contrast X-ray imaging has shown its potential to capture at micrometric scale and in three dimensions (3D), the structure and deformation of poorly-absorbing soft materials such as polymers and biological structures. When composed of constituents that attenuate X-rays differently, heterogeneous hard materials are often well resolved in 3D using absorption-based X-ray computed tomography (CT) techniques. As a result, phase-contrast techniques have been less frequently used to image such materials. However, many geomaterials contain similarly X-ray attenuating constituents, thereby complicating the use of absorption-based X-ray CT methods. Here we present the innovative use of grating-based synchrotron radiation computed tomography, a phase-sensitive technique, in order to better identify the distribution of constituents within geomaterials. We show that this approach enhances the contrast between similarly X-ray absorbing constituents, and can be used to identify spatially small structures such as pores or mineral grains that are typically poorly resolved with conventional absorption-based X-ray CT

Comparing biosignatures in aged basalt glass from North Pond, Mid-Atlantic Ridge and the Louisville Seamount Trail, off New Zealand - Fig 9

<p>top: SEM image of Louisville sample U1376A-15R-3W-37/41 in BSE mode showing organic matter microaggregates (1 & 2) within a palagonite rim (3 & 4) on top of fresh glass with several etch pits and below an SiO₂ filled fracture (5). bottom: sketch as side view (bottom left) and top view (bottom right), illustrating the spatial distribution of the tubular biosignatures and the organic matter remnants. The tubes originate from the fracture/fresh glass interface and propagate into the glass, are then overgrown by palagonite and sealed by precipitating SiO₂. The organic matter was only found within palagonites, suggesting that these putative remnants of microbial life were not associated with formation of tubular alteration textures.</p

Some of the remarkable tubular alteration textures in the Louisville samples in volcanic glass under plane polarized light.

<p>A (U1376A-16R-6W-65/71): Long, thin, and branched tubular alteration textures preserved in palagonite and indicated by white arrows. B (U1376A-16R-6W-65/71): A single long, wide, and branched tubular alteration texture preserved in glass. C (U1376A-16R-6W-65/71): Several very large tubular alteration textures preserved in palagonite and indicated by white arrows. D (U1372A-19R-1W-79/81): Remnants of tubules preserved (or formed) in palagonite (white arrows), red arrows show them protruding into fresh glass. E (U1372A-29R-3W-77/80): Long and wide tubule, which shows significant variation in width at the terminus along with simple branching F (U1376A-16R-6W-65/71): Significantly large textures preserved in palagonite, similar to those shown in D. The length of the scale bars is 50 μm in A, 20 μm in B, 200 μm in C, 100 μm in D, 20 μm in E, and 100 μm in F.</p

R_3/2 values as deducted from FTIR analysis of the organic matter found in the North Pond and Louisville samples.

<p>The values mostly plot above the values of whole bacterial and eukaryotic cells in both cases [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0190053#pone.0190053.ref043" target="_blank">43</a>;<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0190053#pone.0190053.ref044" target="_blank">44</a>]. Note that, individual cell constituents like membranes and lipids have lower R_3/2 values than whole cells, whereas proteins plot higher.</p

Typical FTIR spectrum of the organic matter found in the Louisville sample U1376A-16R-6W-65/71.

<p>The absorption bands indicate the presence of organics. 1: is the CH₃ asymmetrical stretch, 2: is the CH₂ asymmetrical stretch, and 3: the CH₂ symmetrical stretch. The broad main around 1250 cm^-1 absorption band corresponds to the Si-O bond. A.U. stands for arbitrary unit.</p