Following publications have been announced by our department Matter Transport and Ecosystem Dynamics. For further information please contact Dr. Wenyan Zhang, author resp. co-author of the publications:
Zhang, W., Didenkulova, I., Kurkina. O., Cui, Y., Haberkern, J., Aepfler, R., Santos, A.I., Zhang, H., & Hanebuth, T.J.J. (2019): Internal solitary waves control offshore extension of mud depocenters on the NW Iberian shelf. Marine Geology, Volume 409, Pages 15-30, doi:10.1016/j.margeo.2018.12.008
Abstract:
Hydrodynamic conditions and near-bottom sediment transport on the NW Iberian shelf associated with a 5-day storm in September 2014 were monitored. During the post-storm relaxation period, active bottom sediment transport by internal solitary waves (ISWs) on a mid-shelf mud depocenter, located in between 110 and 130 m water depth (WD), was observed. To explore the potential of internal waves in sediment transport and its link to development of mid-shelf mud depocenters, we apply a weakly nonlinear model based on the variable-coefficient Gardner equation to estimate the flow fields and bottom shear stress induced by shoaling of mode-1 long internal solitary waves. Shoreward propagation of the ISWs in three representative periods (pre-, intra- and post-storm) is simulated, respectively. Transformation of the internal wave, from a single sech2 shape characterized by negative polarity and small amplitude to a dispersive trailing wave packet with varying amplitude and inverse polarity, are satisfactorily reproduced. Model results indicate enhancement of the maximum orbital velocity of the ISWs during and after the storm on the outer shelf (130–220 m WD) including the seaward margin of the mud depocenter. Bottom shear stress consequently becomes strong enough (≥0.1 Pa) to winnow unconsolidated sediment and constrains the offshore extension of the depocenter. The enhanced bottom orbital velocity and the asymmetry in the excursion direction of mode-1 long ISWs in the post-storm period prove to be efficient in transporting fine-grained sediment across shelf. Our results suggest that mid-shelf mud depocenters are not necessarily areas under permanently calm conditions where fine-grained sediment can settle straightforwardly. They could also result from convergent sediment transport from both onshore and offshore directions, and sediment may go through numerous cycles of resuspension-transport-deposition before its ultimate lasting burial.
Chen, H., Zhang, W., Xie, X., & Ren, J. (2019): Sediment dynamics driven by contour currents and mesoscale eddies along continental slope: A case study of the northern South China Sea. Marine Geology, Volume 409, Pages 48-66, doi:10.1016/j.margeo.2018.12.012
Abstract:
Based upon 3-D seismic reflection data, published drill core studies, oceanographic observations and numerical modelling, this study identifies a small scale contourite depositional system that is locally confined by canyons, slope failures, erosional terraces and erosional slopes on the Jianfeng slope, northern South China Sea (~1500 to ~2300 m water depth). Possible oceanic driving mechanisms for the spatial heterogeneity in sedimentation and erosion within the Quaternary deposits in this complex slope system are investigated. Numerical simulations show that the quasi-steady South China Sea deep water circulation, which affects the study area, is characterised by low velocities (maximum 4 cm/s), with a slight modulation by tides (±1 cm/s). This quasi-steady deep-water thermohaline circulation is able to keep sediment particles in suspension, rather than to erode seafloor sediment. The bottom hydrodynamic regime becomes energetic when mesoscale eddies (horizontal scale of 10 to 100 km) approach.
Our modelling study indicates that both surface and bottom mesoscale eddies are essential to account for the spatial heterogeneity in sedimentation pattern along the northern South China Sea continental slope. According to simulation, the eddy front contains the highest flow velocity over a mesoscale eddy cycle (45 days), exceeding the threshold for resuspension of unconsolidated sediment (15 cm/s). As a consequence massive resuspension is produced at various sites, and redistributed by sub-mesoscale (horizontal scale of 1 to 10 km) circulations originated from an eddy-topography interaction. In contrast to the surrounding areas, which are subject to erosional forcing for a relatively long part (>7% of an eddy cycle), both the locally-confined drift and a further upstream large elongated-mounded drift experience little erosion (<2.5% of an eddy cycle), and serve as depositional centres for sediment from remote areas and erosion from adjacent areas. Our study demonstrates a promising new perspective for bridging the scales between short-term sediment dynamics and long-term sedimentation through a comparison of modelled scenarios between normal conditions and energetic events.