ALBERT

Description: The occurrence of gas hydrates at submarine mud volcanoes (MVs) located within the gas hydrate stability zone (GHSZ) is controlled by upward fluid and heat flux associated with MV activity. Determining the spatial distribution of gas hydrates at MVs is crucial to evaluate their sensitivity to known episodic changes in volcanic activity. We determined the hydrocarbon inventory and spatial distribution of hydrates at an individual MV structure. The Hakon Mosby Mud Volcano (HMMV), located at 1,250 m water depth on the Barents Sea slope, was investigated by combined pressure core sampling, heat flow measurements, and pore water chemical analysis. Quantitative pressure core degassing revealed gas-sediment ratios between 3.1 and 25.7, corresponding to hydrate concentrations of up to 21.3% of the pore volume. Hydrocarbon compositions and physicochemical conditions imply that gas hydrates incipiently crystallize as structure I hydrate, with a dissociation temperature of around 13.8 degrees C at this water depth. Based on numerous in situ measurements of the geothermal gradient in the seabed, pore water sulfate profiles and microbathymetric data, we show that the thickness of the GHSZ increases from less than 1 m at the warm center to around 47 m in the outer parts of the HMMV. We estimate the total mass of hydrate-bound methane stored at the HMMV to be about 102.5 kt, of which 2.8 kt are located within the morphological Unit I around the center and thus are likely to be dissociated in the course of a large eruption.

Description: Assessing frequency and extent of mass movement at continental margins is crucial to evaluate risks for offshore constructions and coastal areas. A multidisciplinary approach including geophysical, sedimentological, geotechnical, and geochemical methods was applied to investigate multistage mass transport deposits (MTDs) off Uruguay, on top of which no surficial hemipelagic drape was detected based on echosounder data. Nonsteady state pore water conditions are evidenced by a distinct gradient change in the sulfate (SO42−) profile at 2.8 m depth. A sharp sedimentological contact at 2.43 m coincides with an abrupt downward increase in shear strength from ∼10 to 〉20 kPa. This boundary is interpreted as a paleosurface (and top of an older MTD) that has recently been covered by a sediment package during a younger landslide event. This youngest MTD supposedly originated from an upslope position and carried its initial pore water signature downward. The kink in the SO42− profile ∼35 cm below the sedimentological and geotechnical contact indicates that bioirrigation affected the paleosurface before deposition of the youngest MTD. Based on modeling of the diffusive re-equilibration of SO42− the age of the most recent MTD is estimated to be 〈30 years. The mass movement was possibly related to an earthquake in 1988 (∼70 km southwest of the core location). Probabilistic slope stability back analysis of general landslide structures in the study area reveals that slope failure initiation requires additional ground accelerations. Therefore, we consider the earthquake as a reasonable trigger if additional weakening processes (e.g., erosion by previous retrogressive failure events or excess pore pressures) preconditioned the slope for failure. Our study reveals the necessity of multidisciplinary approaches to accurately recognize and date recent slope failures in complex settings such as the investigated area.

Description: The development of stable barium (Ba) isotope measurements provides a novel tool to investigate the geochemical cycling of Ba in the ocean and its sediments. In sediment pore waters, gradients of dissolved Ba concentrations result from various diagenetic processes. The distribution and fractionation of Ba isotopes in the pore waters are expected to further improve our understanding of these early diagenetic control mechanisms. Here, we present pore water profiles of dissolved stable Ba isotopic signatures (δ138Bapw) from shallow water sediments covering the entire Pearl River Estuary (PRE) in Southern China. We find pronounced depth-dependent Ba isotope variations generally showing a shift from heavy to light δ138Bapw signatures from the sediment surface down to 15 cm depth. These gradients are well reproduced by a diffusion-reaction model, which generates an apparent fractionation factor (138ε) of −0.60 ± 0.10‰ pointing to preferential removal of low-mass Ba isotopes from the pore water during solution-solid phase interactions. Consequently, the combined diagenetic processes induce the highest δ138Bapw values of +0.5 to +0.7‰ in the pore waters of the topmost sediment layer. Although the detrital fraction dominates the Ba content in the PRE surface sediments, the determined gradients of pore water Ba isotopes, together with concentration variations of Ba and other redox-sensitive elements such as manganese (Mn), show that non-detrital excess Ba carriers including Mn oxides and authigenic barite clearly affect the post-depositional Ba dynamics. Stable Ba isotopes are thus a potentially powerful tracer of Ba geochemistry during early sediment diagenesis in estuarine depositional environments. Key Points We present a data set of dissolved stable Ba isotopic compositions in surface sediment pore waters of a large river estuary Pore water Ba isotope values generally decrease with increasing sediment depth, reflecting post-depositional Ba isotope fractionation A diffusion-reaction model predicts the distribution and fractionation of stable Ba isotopes in the sediment pore waters well