Description: Highlights • 3D seismic imaging of an entire landslide complex. • Shallow gas accumulation within and underneath Tuaheni Landslide Complex. • Imaging of a basal shear zone within a subaqueous landslide complex. Abstract The Hikurangi margin is an active continental margin east of New Zealand's North Island. It is well recognized as a seismically active zone and is known for the occurrence of free gas and gas hydrates within the shallow sediments. A variety of subaqueous landslides can be observed at the margin, including the Tuaheni Landslide Complex off Poverty Bay. This slide complex has been interpreted previously as a slowly creeping landform, as its morphology and internal deformation is comparable to terrestrial earthflows and rock glaciers. In 2014, we acquired a high-resolution 3D seismic volume covering major parts of the Tuaheni South landslide. The 3D data show a variety of fluid migration indicators, free gas accumulations and manifestations of the base of gas hydrate stability in the pre-slide sedimentary units and the lower unit of the landslide system. The data also show that the landslide system is composed of an upper and lower unit that are separated by an intra-debris negative-polarity reflection. Free gas accumulations directly beneath the landslide units suggest that the debris acts as a boundary for rising fluids and only few migration pathways to the intra-debris reflector are observed in the distal parts of the landslide. Deformation within the landslide's debris is focused in the upper landslide unit, and we interpret the intra-debris reflector as a basal shear zone or ‘glide plane’ upon which the debris has been remobilized. The origin of the intra-debris reflector is unclear, but we suggest it could be a relatively coarse-grained horizon that would be prone to fluid flow focusing and the development of excess fluid pressure. Our seismic study provides one of the most detailed examples of a subaqueous landslide system and reveals insights into the fluid flow system and potential basal shear zone development of the Tuaheni Landslide Complex.

Description: Newly acquired bathymetric and seismic reflection data have revealed mass-transport deposits (MTDs) on the northeastern Cretan margin in the active Hellenic subduction zone. These include a stack of two submarine landslides within the Malia Basin with a total volume of approximately 4.6 km(3) covering an area of about 135 km(2). These two MTDs have different geometry, internal deformations and transport structures. The older and stratigraphic lower MTD is interpreted as a debrite that fills a large part of the Malia Basin, while the second, younger MTD, with an age of at least 12.6 cal. ka B.P., indicate a thick, lens-shaped, partially translational landslide. This MTD comprises multiple slide masses with internal structure varying from highly deformed to nearly undeformed. The reconstructed source area of the older MTD is located in the westernmost Malia Basin. The source area of the younger MTD is identified in multiple headwalls at the slope-basin-transition in 450 m water depth. Numerous faults with an orientation almost parallel to the southwest-northeast-trending basin axis occur along the northern and southern boundaries of the Malia Basin and have caused a partial steepening of the slope-basin-transition. The possible triggers for slope failure and mass-wasting include (i) seismicity and (ii) movement of the uplifting island of Crete from neotectonics of the Hellenic subduction zone, and (iii) slip of clay-mineral-rich or ash-bearing layers during fluid involvement. (c) 2009 Elsevier B.V. All rights reserved.

Description: Highlights • Release of dissolved Sr2+ with low 87Sr/86Sr, as well as Ca2+ and Ba2+ suggests ongoing volcanic ash alteration. • A concurrent increase in Fe2+ and a depletion of CH4 with a decrease in C of CH4 and DIC suggest Fe-AOM. • We for the first time document the potential linkage between ash alteration and methane oxidation via Fe-AOM. • The rate of Fe-AOM is estimated to be ∼0.4 μmol cm−2 yr−1, equivalent to ∼12% of total CH4 removal. Abstract We present geochemical data collected from volcanic ash-bearing sediments on the upper slope of the northern Hikurangi margin during the RV SONNE SO247 expedition in 2016. Gravity coring and seafloor drilling with the MARUM-MeBo200 allowed for collection of sediments down to 105 meters below seafloor (mbsf). Release of dissolved Sr2+ with isotopic composition enriched in 86Sr (87Sr/86Sr minimum = 0.708461 at 83.5 mbsf) is indicative of ash alteration. This reaction releases other cations in the 30-70 mbsf depth interval as reflected by maxima in pore-water Ca2+ and Ba2+ concentrations. In addition, we posit that Fe(III) in volcanogenic glass serves as an electron acceptor for methane oxidation, a reaction that releases Fe2+ measured in the pore fluids to a maximum concentration of 184 μM. Several lines of evidence support our proposed coupling of ash alteration with Fe-mediated anaerobic oxidation of methane (Fe-AOM) beneath the sulfate-methane transition (SMT), which lies at ∼7 mbsf at this site. In the ∼30-70 mbsf interval, we observe a concurrent increase in Fe2+ and a depletion of CH4 with a well-defined decrease in C-CH4 values indicative of microbial fractionation of carbon. The negative excursions in C values of both DIC and CH4 are similar to that observed by sulfate-driven AOM at low SO concentrations, and can only be explained by the microbially-mediated carbon isotope equilibration between CH4 and DIC. Mass balance considerations reveal that the iron cycled through the coupled ash alteration and AOM reactions is consumed as authigenic Fe-bearing minerals. This iron sink term derived from the mass balance is consistent with the amount of iron present as carbonate minerals, as estimated from sequential extraction analyses. Using a numerical modeling approach we estimate the rate of Fe-AOM to be on the order of 0.4 μmol cm−2 yr−1, which accounts for ∼12% of total CH4 removal in the sediments. Although not without uncertainties, the results presented reveal that Fe-AOM in ash-bearing sediments is significantly lower than the sulfate-driven CH4 consumption, which at this site is 3.0 μmol cm−2 yr−1. We highlight that Fe(III) in ash can potentially serve as an electron acceptor for methane oxidation in sulfate-depleted settings. This is relevant to our understanding of C-Fe cycling in the methanic zone that typically underlies the SMT and could be important in supporting the deep biosphere.

Description: In this study we aim on a reconstruction of mechanisms and kinematics of slope-failure and mass-movement processes along the northeastern slope of Crete in the Hellenic forearc, eastern Mediterranean. Here, subsidence of the forearc basin and the uplift of the island of Crete cause ongoing steepening of the slope in-between. The high level of neotectonic activity in this region is expected to exert a key role in slope-failure development. Newly acquired reflection seismic data from the upper slope region reveal an intact sediment cover while the lower slope is devoid of both intact strata and mass-transport deposits (MTDs). In a mid-slope position, however, we found evidence for a ∼ 4-km³-sized landslide complex that comprises several MTDs from translational transport of coherent sediment bodies over short distances. Morphometric analysis of these MTDs and their source scars indicates that this part of the northeast Cretan slope can be characterized as a cohesive slope. Furthermore, we reconstruct retrogressive development for this complex and determine a critical slope angle for both pre-conditioning of failure and subsequent landslide deposition near source scars. Consequently, data imply that the investigated shallower slope is stable due to low angles in the order of 3°, whereas 5°-inclined mid-slope portions favour both slope destabilization and landslide deposition. The failed mid-slope parts are dominated by sediment truncations from faults almost correlating with the orientation of head- and sidewalls of scars. We suggest that cohesive landslides and MTDs are generated and preserved, respectively, in such critical slope regions. If once generated, cohesive landslides reach the lower slope further downslope that exceeds the threshold gradient for MTD deposition (∼ 5°), they are transported all the way down to the foot of the slope and disintegrate to mass flows. From these observations we suggest that the mass-wasting history of the investigated Cretan slope area over a longer period of time is characterized by repeated sediment erosion and transport into the deeper Cretan Sea basin. The relocation of the critical slope portion in upslope direction and therefore recurrence of mass-wasting events is thereby likely controlled by the progressive steepening of the slope. This mechanism and restriction of sediment failure to narrow, critically-inclined and relocating slope portions likely explains how such an active margin setting can exhibit only scarce findings of MTDs on the slope despite an expected, extensive and widespread mass wasting.

Description: We present geochemical data collected from volcanic ash-bearing sediments on the upper slope of the northern Hikurangi margin during the RV SONNE SO247 expedition in 2016. Gravity coring and seafloor drilling with the MARUM-MeBo200 allowed for collection of sediments down to 105 meters below seafloor (mbsf). Release of dissolved Sr2+with isotopic composition enriched in 86Sr (87Sr/86Sr minimum = 0.708461 at 83.5 mbsf) is indicative of ash alteration. This reaction releases other cations in the 30-70 mbsf depth interval as reflected by maxima in pore-water Ca2+and Ba2+concentrations. In addition, we posit that Fe(III) in volcanogenic glass serves as an electron acceptor for methane oxidation, a reaction that releases Fe2+measured in the pore fluids to a maximum concentration of 184 μM. Several lines of evidence support our proposed coupling of ash alteration with Fe-mediated anaerobic oxidation of methane (Fe-AOM) beneath the sulfate-methane transition (SMT), which lies at ∼7 mbsf at this site. In the ∼30-70 mbsf interval, we observe a concurrent increase in Fe2+and a depletion of CH4with a well-defined decrease in δ13C-CH4values indicative of microbial fractionation of carbon. The negative excursions in δ13C values of both DIC and CH4are similar to that observed by sulfate-driven AOM at low SO2−4concentrations, and can only be explained by the microbially-mediated carbon isotope equilibration between CH4and DIC. Mass balance considerations reveal that the iron cycled through the coupled ash alteration and AOM reactions is consumed as authigenic Fe-bearing minerals. This iron sink term derived from the mass balance is consistent with the amount of iron present as carbonate minerals, as estimated from sequential extraction analyses. Using a numerical modeling approach we estimate the rate of Fe-AOM to be on the order of 0.4μmol cm−2yr−1, which accounts for ∼12% of total CH4removal in the sediments. Although not without uncertainties, the results presented reveal that Fe-AOM in ash-bearing sediments is significantly lower than the sulfate-driven CH4consumption, which at this site is 3.0μmol cm−2yr−1. We highlight that Fe(III) in ash can potentially serve as an electron acceptor for methane oxidation in sulfate-depleted settings. This is relevant to our understanding of C-Fe cycling in the methanic zone that typically underlies the SMT and could be important in supporting the deep biosphere.