Fine-grained shallow-marine depocenters in the environmental context.Role of rapid sea-level changes for sedimentary budget and climate system.
Final Report Abstract
The formation processes of confined shallow and deep water depocenters (mudbelts, contourite drifts), which represent major sinks of continent-derived clastic material, and the various environmental mechanisms having control on their development is insufficiently known. For the NW-Iberian continental shelf, we showed that the mudbelt started to form at 5.3 cal ka BP, i.e., with sea level stabilization. It expanded afterwards not, as classically assumed, from an origin close to the fluvial material supplier. Rather, it formed local nuclei in far-distance from the material source and grew even against the general ocean current system afterwards. A detailed volume, mass and budget calculation revealed 65 % of these fluviogenic muds do not remain on the shelf but are exported to the open ocean. With the 95 % decrease of fluvial sediment discharge due to river damming, massive sediment depletion of these shallow water habitats must be expected. Whilst the storm wave base commonly controls the position of the inner boundary of such a mudbelt, as a simple simulation showed, the location of the outer boundary is more difficult to explain. A combination of geological observations, hydrodynamic monitoring data, and numerical modelling suggests that internal waves play a major role here. The same is the case for a deep-water contourite system at the toe of the continental slope. The general paradigms assume that the core of a water mass, or alternatively the boundary between two water masses, carry highest energy to the seabed, thus being responsible for the geometry of a contourite system. We, in contrast, showed in an innovative approach by linking detailed field data with numerical modelling, that internal waves/ocean density fronts migrate through the wide transitions zone of two water masses. Result is pulse-like mobilization and distribution of sediments rather than a continuous transport and deposition process. The human impact in the form of chronic bottom trawling disturbs these deposital pattern significantly. Our novel calculations suggest that the amount of sediments mobilized by extensive seabedploughing on global scale sums up half the volume of sediments supplied to the world’s shelves by the global river. These precise numbers are absolutely novel and point to the urgent demand that stakeholders may start to consider the continental-shelf seabed as an area that needs systematic and highpriority protection to preserve habitats. A unique approach was developed using terrigenous radiogenic isotopes (Nd, Sr, Pb) on a continental margin, i.e., a complex semi-open sedimentary system, to unravel sediment sources, mixing rates, distribution mechanisms and retention times. A further highlight was the demonstration that continental shelves under hyperarid conditions store much more dust and dune sediments over a whole sea-level cycle (125,000 years) than previously assumed. With appropriate technique, this record can be used to decipher the stratigraphic, thus, climatic controlled history in high resolution. Rapid sea-level changes have major impact on features that are in fragile balance with the sea level. The Mekong River delta expanded rapidly during the past 3,000 years as a study combining ecological (mangrove), archaeological (salt production), and geological data (architecture) showed. However, the delta started to form a true progradational body not earlier than 4.8 cal ka BP. Against existing deltagrowth concepts, our finding was that not only a nearly stable sea level is required for delta formation but also the underlying morphology allowing for the opening of a sufficiently wide accommodation space. Though denied by the national stakeholders and several scientists, the outer Ganges-Brahmaputra delta shows a succession of events that led to coastal subsidence of about 5.4 ± 1 mm per year in the historical past. In front of the future accelerating sea-level rise scenario, follow-up stories arose as News Feature in Nature and in Spektrum der Wissenschaft. Finally, the first ever deployment of the MARUM-MeBo in a polar region (W’ Barents Sea) found great interest in the community as a technical pioneer approach. Target was deglacial deposits inside and in front of a former ice-stream trough to unravel the step-wise ice-stream retreat as response to unsteady deglacial sea-level rise dynamics.
Publications
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(2011) Formation, fate, and implications of depocentres along the sedimentary pathway on the Sunda Shelf (Southeast Asia) over the past 140 ka. Earth-Science Reviews 104, 92-110
Hanebuth TJJ, Voris HK, Yokoyama Y, Okuno J, Saito Y
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(2012). Early growth stage of a large delta – transformation from estuarine-platform to deltaic-progradational conditions (the northeastern Mekong River Delta, Vietnam). Sedimentary Geology 261-262, 108- 119
Hanebuth TJJ, Proske U, Saito Y, Nguyen VL, Ta, TKO
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(2013) Aeolian to shallowmarine shelf architecture off a major desert since the Late Pleistocene (Northern Mauritania). Geomorphology 203, 132-147
Hanebuth TJJ, Mersmeyer H, Kudrass, HR, Westphal, H
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(2013) Rapid coastal subsidence in the central Ganges-Brahmaputra Delta (Bangladesh) since the 17th century deduced from submerged salt-producing kilns. Geology 41 (9), 987-990
Hanebuth TJJ, Kudrass HR, Linstädter J, Islam B, Zander AM
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(2014) Volumetric budget calculation of sediment and carbon storage and export for a late Holocene mid-shelf mudbelt system (NW Iberia). Continental Shelf Research 76, 12-24
Oberle FKJ, Hanebuth TJJ, Baasch B, Schwenk T
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(2014) Wave-driven sediment mobilization on a storm-controlled continental shelf (Northwest Iberia). Journal of Marine Sciences 139, 362-372
Oberle F, Storlazzi C, Hanebuth TJJ
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(2014). Drilling Glacial Deposits in Offshore Polar Regions. EOS 95 (31), 277-278
Hanebuth TJJ, Rebesco M, Urgeles U, Lucchi RG, Freudenthal T
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Oceanic density fronts steering bottom-current induced sedimentation deduced from a 50 ka contourite-drift record and numerical modelling (off NW Spain). Quaternary Science Reviews Volume 112, 15 March 2015, Pages 207-225
Hanebuth TJJ, Zhang W, Hofmann AL, Löwemark LA, Schwenk T
