ALBERT

Description: Altered pyroclastic (tephra) deposits are highly susceptible to landsliding, leading to fatalities and property damage every year. Halloysite, a low-activity clay mineral, is commonly associated with landslide-prone layers within altered tephra successions, especially in deposits with high sensitivity, which describes the post-failure strength loss. However, the precise role of halloysite in the development of sensitivity, and thus in sudden and unpredictable landsliding, is unknown. Here we show that an abundance of mushroom cap?shaped (MCS) spheroidal halloysite governs the development of sensitivity, and hence proneness to landsliding, in altered rhyolitic tephras, North Island, New Zealand. We found that a highly sensitive layer, which was involved in a flow slide, has a remarkably high content of aggregated MCS spheroids with substantial openings on one side. We suggest that short-range electrostatic and van der Waals interactions enabled the MCS spheroids to form interconnected aggregates by attraction between the edges of numerous paired silanol and aluminol sheets that are exposed in the openings and the convex silanol faces on the exterior surfaces of adjacent MCS spheroids. If these weak attractions are overcome during slope failure, multiple, weakly attracted MCS spheroids can be separated from one another, and the prevailing repulsion between exterior MCS surfaces results in a low remolded shear strength, a high sensitivity, and a high propensity for flow sliding. The evidence indicates that the attraction-detachment model explains the high sensitivity and contributes to an improved understanding of the mechanisms of flow sliding in sensitive, altered tephras rich in spheroidal halloysite.

Description: The half sections for archive were used for core imaging and description on ship board (GeoB 17701-1, GeoB 17703-1, GeoB 17705-1, GeoB 17708-1, GeoB 17711-1, GeoB 17715-1) and onshore (all others). The images were made using a handheld Canon EOS 450D reflex camera under fluorescent light conditions. A true color chart was used for color control. The core description provides information about core section length and position, core section number, core image, color, graphic lithology, sedimentary structures, coring disturbance, grain size and description text. Sediment color was determined qualitatively for core intervals using Munsell soil color charts. Grain size of the sand fraction was determined using a sand ruler, ranging from 63-90 µm for very fine sand, 90-250 µm for fine sand, 250-710 µm for medium sand, and 710-2000 µm for coarse sand.

Description: In this study we present a late Miocene - early Pliocene record of sixty-four zones with prominent losses in the magnetic susceptibility signal, taken on a sediment drift (ODP Site 1095) on the Pacific continental rise of the West Antarctic Peninsula. The zones are comparable in shape and magnitude and occur commonly at glacial-to-interglacial transitions. High resolution records of organic matter, magnetic susceptibility and clay mineral composition from early Pliocene intervals demonstrate that neither dilution effects nor provenance changes of the sediments have caused the magnetic susceptibility losses. Instead, reductive dissolution of magnetite under suboxic conditions seems to be the most likely explanation. We propose that during the deglaciation exceptionally high organic fluxes in combination with weak bottom water currents and prominent sediment draping diatom ooze layers produced temporary suboxic conditions in the uppermost sediments. It is remarkable that synsedimentary suboxic conditions can be observed in one of the best ventilated open ocean regions of the World.