Till J J Hanebuth

On the significance of sea-level variations and shelf paleo-morphology in governing sedimentation in the southern South China Sea during the last deglaciation

Abstract Deglacial sedimentation on the outer Sunda Shelf, the shelf margin and slope shows a close correlation with the stages of flooding of the emerged shelf. It is mainly controlled by the relationship between shelf paleo-physiography and changes in sea level and sediment supply. Based on the sedimentological and geochemical variability at five sites along a transect across the outer shelf and the continental slope covering the last 20 000 years, four intervals of significant depositional changes are identified. The first reorganization in the sedimentation regime, which was established during sea-level lowstand, is associated with the drowning of the lower course of the North Sunda River between 16.5 and 14.5 ka. During and following the rapid rise in sea level at 14.5–14 ka, sedimentation on the slope is characterized by an accelerated decrease in the supply of terrigenous material, associated with the flooding of the middle part of the paleo-valley. This abrupt change in sedimentation is followed by a period of gradual decrease of terrigenous supply to the slope, when the surrounding plains of the river valley were flooded (14–8.5 ka). The final change occurred between ca. 8.5 and 6 ka, when the coastline reached its modern position, and modern sedimentary patterns are established. This change probably marks the end of reworking by transgression. Given the synchroneity of sedimentological changes along the transect across the Sunda Shelf and into the deep sea with the environmental shifts on the central shelf, sedimentation on the margin and slope appears to be mainly controlled by the interrelationship of sea level, shelf paleo-physiography and sediment supply. Moreover, the records display lateral changes through time associated with stages of the deglacial transgression, which mask any potential sedimentological variability due to changes in precipitation, vegetation or atmospheric circulation. This fact cautions the interpretation of near-shore sedimentary records as unequivocal recorders of local climate change (e.g. SE Asian monsoon), and highlights the need to include variations in shelf physiography and sea level in future paleoceanographic studies.

An interdisciplinary investigation of a recent submarine mass transport deposit at the continental margin off Uruguay

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.

CapTimiris Canyon: A newly discovered channel system offshore of Mauritania

Intensive research over the past decades has greatly improved our understanding of processes operating in the deep ocean. There has been a particular focus on continental margins, as sediments deposited in these areas can provide a high-resolution record of past climatic changes, as well as serve to host some of the worlds major hydrocarbon reservoirs. However, the exploration and understanding of the deep ocean remains one of the great challenges of the 21st century [Stow and Mayall, 2000], and many fascinating features still wait to be found. The potential for new deep-water discoveries was recently highlighted during Meteor cruise M58/1 (depart Dakar, Senegal, 21 April 2003, return Las Palmas, Spain, 12 May 2003) of the Research Center Ocean Margins at the Universitat Bremen in Germany. A spectacular 400-km-long submarine meandering channel system was discovered off Mauritania. In this article, the system is called the Cap Timiris Canyon (Figure 1). Although a series of incisional gullies at the shelf break and uppermost slope have been described before [e.g., Rust and Wienecke, 1973], the enormous size and complex morphology of this submarine channel system were previously unknown.

Rapid coastal subsidence in the central Ganges-Brahmaputra Delta (Bangladesh) since the 17th century deduced from submerged salt-producing kilns

The densely populated, low-lying Ganges-Brahmaputra Delta is highly vulnerable to global sea-level rise. In order to estimate the rate of subsidence of the delta, we examined submerged salt-producing kiln sites in the coastal Sundarbans (a huge UNESCO-protected mangrove forest). These kilns were built just above the winterly spring high-tide level of the time, but their bases are currently located ∼155 cm below the corresponding modern level. According to optically stimulated luminescence (OSL) dating, the kilns were last fired ∼300 yr ago, and salt production was terminated by a catastrophic event that affected the kiln sites at different levels and locations. 14 C ages of charcoal at the kilns’ bases and associated mangrove stump horizons support the OSL dates. Based on the elevations and ages, the 300 yr average rate of sinking of the outer delta is 5.2 ± 1.2 mm/yr, which includes 0.8 mm/yr of eustatic sea-level rise. With the expectation of further acceleration of sea-level rise, the already-present problematic situation will be aggravated, and only prudent control of sediment accretion will keep southern Bangladesh above sea level.

The palaeoenvironmental development of the northeastern Vietnamese Mekong River Delta since the mid Holocene

Three radiocarbon-dated sediment cores from the northeastern Vietnamese Mekong River Delta have been analysed with a multiproxy approach (grain size, pollen and spores, macro-charcoal, carbon content) to unravel the palaeoenvironmental history of the region since the mid Holocene. During the mid-Holocene sea-level highstand a diverse, zoned and widespread mangrove belt (dominated by Rhizophora) covered the extended tidal flats. The subsequent regression and coeval delta progradation led to the rapid development of a back-mangrove community dominated by Ceriops and Bruguiera but also represented locally by e.g. Kandelia, Excoecaria and Phoenix. Along rivers this community seems to have endured even when the adjoining floodplain had already shifted to freshwater vegetation. Generally this freshwater vegetation has a strong swamp signature but locally Arecaceae, Fabaceae, Moraceae/ Urticaceae and Myrsinaceae are important and mirror the geomorphological diversity of the delta plain. The macro-charcoal record implies that natural burning of vegetation occurred throughout the records, however, the occurrence of the highest amounts of macro-charcoal particles is linked with modern human activity.

Mud depocenters on continental shelves—appearance, initiation times, and growth dynamics

Mud accumulates on continental shelves under a variety of environmental conditions and results in a diverse formation of mud depocenters (MDCs). Their three-dimensional architectures have been in the focus of several recent studies. Due to some terminological confusion concerning MDCs, the present study sets out to define eight individual MDC types in terms of surface sediment distribution and internal geometry. Under conditions of substantial sediment supply, prodeltas (distal zones off river deltas; triangular sheets), subaqueous deltas (disconnected from deltas by strong normal-to-shore currents; wedge-like clinoforms), and mud patches (scattered distribution) and mud blankets (widespread covers) are formed. Forced by hydrodynamic conditions, mud belts in the strict sense (detached from source; elongated bodies), and shallow-water contourite drifts (detached from source; growing normal to prevailing current direction; triangular clinoforms) develop. Controlled by local morphology, mud entrapments (in depressions, behind morphological steps) and mud wedges (triangular clinoforms growing in flow direction) are deposited. Shelf mud deposition took place (1) during early outer-shelf drowning (~14 ka), (2) after inner-shelf inundation to maximum flooding (9.5–6.5 ka), and (3) in sub-recent times (<2 ka). Subsequent expansion may be (1) concentric, in cases where the depocenter formed near the fluvial source, (2) uni-directional, extending along advective current transport paths, and (3) progradational, forming clinoforms that grow either parallel or normal to the bottom current direction. Classical mud belts may be initiated around defined nuclei, the remote sites of which are determined by seafloor morphology rather than the location of the source. From a stratigraphic perspective, mud depocenters coincide with sea-level highstand-related, shelf-wide condensed sections. They often show a conformable succession from transgressive to highstand systems tract stages.

Drilling Glacial Deposits in Offshore Polar Regions

High latitudes are of fundamental importance in the Earths climate system—they house ice sheets that govern global sea level heights, influence how much solar energy is reflected back to space, and create deep and bottom waters that drive the oceans ability to circulate energy and nutrients across the globe.

New constraints on oceanographic vs. seismic control on submarine landslide initiation: a geotechnical approach off Uruguay and northern Argentina

Submarine landslides are common along the Uruguayan and Argentinean continental margin, but size, type and frequency of events differ significantly between distinct settings. Previous studies have proposed sedimentary and oceanographic processes as factors controlling slope instability, but also episodic earthquakes have been postulated as possible triggers. However, quantitative geotechnical slope stability evaluations for this region and, for that matter, elsewhere in the South Atlantic realm are lacking. This study quantitatively assesses continental slope stability for various scenarios including overpressure and earthquake activity, based on sedimentological and geotechnical analyses on three up to 36 m long cores collected on the Uruguayan slope, characterized by muddy contourite deposits and a locus of landslides (up to 2 km3), and in a canyon-dominated area on the northern Argentinean slope characterized by sandy contourite deposits. The results of shear and consolidation tests reveal that these distinct lithologies govern different stability conditions and failure modes. The slope sectors are stable under present-day conditions (factor of safety >5), implying that additional triggers would be required to initiate failure. In the canyon area, current-induced oversteepening of weaker sandy contourite deposits would account for frequent, small-scale slope instabilities. By contrast, static vs. seismic slope stability calculations reveal that a peak ground acceleration of at least 2 m/s2 would be required to cause failure of mechanically stronger muddy contourite deposits. This implies that, also along the western South Atlantic passive margin, submarine landslides on open gentle slopes require episodic large earthquakes as ultimate trigger, as previously postulated for other, northern hemisphere passive margins.

Mud depocentres on the continental shelf: a neglected sink for anthropogenic contaminants from the coastal zone

Abstract In this study, published and unpublished data from the Santos Estuarine Complex and Bay and the adjacent continental shelf (São Paulo State, Brazil) were gathered in order to evaluate the entrapment of anthropogenic chemical contaminants (hydrocarbons, heavy metals) in a mid-shelf mud depocentre. Results show that these contaminants, produced by industrial activities in the adjacent coastal zone and released into the bay waters, are distributed far over the shelf since they are found in the mid-shelf mudbelt in locally significant concentrations. Two main aspects are highlighted by this study. The first underlines the fact that the material stored in the mudbelt is not related to a specific fluvial source discharging to the shelf. Instead, the contaminants, used as tracers, stem from multiple injection sources along the heavily used coastline of the Santos industrial zone. The second finding suggests that the anthropogenic compounds are not only accumulating in the surface sediments of fine-grained shelf depocentres. Rather, these substances are also already found several centimetres below the modern seabed. They can, thus, be easily reinjected into the water column by storms, benthic activity, and human disturbances such as seabed dredging and bottom trawling.

Pore Water Geochemistry as a Tool for Identifying and Dating Recent Mass-Transport Deposits

Several previous studies have shown that submarine mass-movements can profoundly impact the shape of pore water profiles. Therefore, pore water geochemistry and diffusion models were proposed as tools for identifying and dating recent (max. several thousands of years old) mass-transport deposits (MTDs). In particular, sulfate (SO 4 2− ) profiles evidentially indicate transient pore water conditions generated by submarine landslides. After mass-movements that result in the deposition of sediment packages with distinct pore water signatures, the SO 4 2− profiles can be kink-shaped and evolve into the concave and linear shape with time due to molecular diffusion. Here we present data from the RV METEOR cruise M78/3 along the continental margin off Uruguay and Argentina. SO 4 2− profiles of 15 gravity cores are compared with the respective acoustic facies recorded by a sediment echosounder system. Our results show that in this very dynamic depositional setting, non-steady state profiles occur often, but are not exclusively associated with mass-movements. Three sites that show acoustic indications for recent MTDs are presented in detail. Where recent MTDs are identified, a geochemical transport/reaction model is used to estimate the time that has elapsed since the perturbation of the pore water system and, thus, the timing of the MTD emplacement. We conclude that geochemical analyses are a powerful complementary tool in the identification of recent MTDs and provide a simple and accurate way of dating such deposits.