Description: Continental shelves represent areas of highest economical and ecological importance. Nevertheless, these sedimentary systems remain poorly understood due to a complex interplay of various factors and processes which results in highly individual construction schemes. Previous studies of sedimentary shelf systems have mainly focused on a limited number of cores, retrieved from Holocene fine-grained depocentres. As such, the relation between shelf architecture and sedimentary history remains largely obscure. Here, we present new data from the NW Iberian shelf comprising shallow-seismic profiles, a large number of sediment cores, and an extended set of radiocarbon dates to reveal the Late Quaternary evolution of a low-accumulation shelf system in detail. On the NW Iberian shelf, three main seismic units are identified. These overly a prominent erosional unconformity on top of the basement. The lowermost Unit 1 is composed of maximal 75-m thick, Late Tertiary to Pleistocene deposits. The youngest sediments of this unit are related to the last glacial sea-level fall. Unit 2 was controlled by the deglacial sea-level rise and shows a maximum thickness of 15 m. Finally, Unit 3 comprises deposits related to the late stage of sea-level rise and the modern sea-level highstand with a thickness of 4 m in mid-shelf position. Two pronounced seismic reflectors separate these main units from each other. Their origin is related to (1) exposure and ravinement processes during lower sea level, and (2) to reworking and re-deposition of coarse sediments during subsequent sea-level rise. According to the sediment core ground-truthing, sediments of the Late Tertiary to Pleistocene unit predominantly display homogenous fine sands with exceptional occurrences of palaeosols that indicate an ancient exposure surface. Fine sands which were deposited in the run of the last sea-level rise show a time-transgressive retrogradational development. The seismic reflectors, bounding the individual units, appear in the cores as 0.1 to 1-m thick deposits consisting either of shell gravels or siliceous coarse sands with gravels. The modern sea-level highstand stage is characterised by zonal deposition of mud forming a mud belt in mid-shelf position, and sediment starvation on outer shelf zones. Radiocarbon ages indicate that this mud belt was the main depocentre for river-supplied fine material on the NW Iberian shelf at least over the past 5.32 ka BP. The initial onset of this depocentre is proposed to be related to a shift in the balance between rate of sea-level rise and amount of terrigenous sediment supply. Various other stratigraphical shelf reconstructions reveal analogies in architecture which indicate that timing and shaping of the individual units on low-accumulation shelves is fundamentally controlled by eustatic sea-level changes. Other factors of local importance such as differential elevation of the basement and the presence of morphological barriers formed by rocky outcrops on the seafloor have additionally modifying influence on the sedimentary processes.

Description: Three 2D reflection seismic profiles from three glacial cross-shelf troughs of the Labrador Shelf, namely the Okak, Makkovik and Cartwright troughs, are presented. The data were collected onboard the Maria S. Merian during cruise and MSM84 using two different multi-channel streamers; a Geometrics GeoEel solid state digital high-resolution streamer (active length 75 m, channel spacing 1.5625 m; hereafter short streamer) and a digital Sercel high-resolution streamer system (active length 600 m, channel spacing 3.125 m; hereafter long streamer). Details regarding the used seismic systems can be found in the cruise report (doi:10.2312/cr_msm84). Data were processed using the commercial VISTA Desktop Seismic Data Processing Software (Schlumberger, version 2018). The standard processing approach included demultiplexing, bulk shifting, frequency filtering (Ormsby as well as in the FK-domain), spiking deconvolution, compensation of spherical divergence and common-midpoint (CMP) binning. The CMP bin size was set to 3.125 m for both datasets. For the short streamer a normal-move-out correction with a constant sound velocity of 1500 ms-1 was performed as the limited offset range of short streamer did not allow for a precise velocity analysis. Afterwards, a static correction was applied as the short streamer was towed close to the water surface. Finally, a CMP stack was calculated followed by a finite differences time migration. The dataset of the long streamer shows a strong receiver ghost, which is related to the tow depth of approximately 10 m below the water surface. This receiver ghost was suppressed. Afterwards, a data-driven velocity analysis was carried out by picking seismic velocities for every 500 CMPs. Normal-move-out correction, CMP stacking and a finite differences time migration were carried out using the velocity field. Finally, a white noise removal was performed. The objective of data recording on the Labrador Shelf is to map glacial landforms and reconstruct the dynamics of the Laurentide Ice Sheet in detail.