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RV METEOR Cruise M58/1

The continental margin off Mauritania is characterized by coastal upwelling and high sedimentation rates on the shelf and the upper slope. Such rapidly deposited sediments might get instable and thus transported into the deep sea by turbidity currents, debris flows, and/or sediment slides. During Meteor-Cruise M58/1 of the Research Center Ocean Margins of the University of Bremen a suit of new bathymetric multibeam, narrow beam echosounder, and high resolution multichannel seismic data were collected off Mauritania. Different types of sediment transport were observed.

Figure: Cruise track of Meteor-cruise M58/1 off Mauritania

RV Meteor

The area between 17°00'and 18°30'N is affected by large-scale mass wasting, the so-called Mauritania slide complex. The upper boundary of the Mauritania Slide Complex is marked by a series of stepped headwalls up to 100 m high located in ~800m water depth. Multichannel seismic images show a stacked pattern of mass wasting events indicating that this part of the continental margin has been unstable over a long period. Sediment cores recovered at the edge of the uppermost unit of the Mauritania Slide Complex display that the uppermost debrite sheet is overlain by a 50 cm thick normal siliciclastic turbidite that again is covered by hemipelagic deposits. Radiocarbon dating of the base hemipelagite unit revealed an age of 11 ka.

Figure: (left) Narrow beam echosounder profile of the headwall area of the Mauritania Slide Complex. (right) Core of debris flow overlain by a siliciclastic turbidite.

One of the most unexpected results of the cruise was the discovery of a spectacular meandering slope channel offshore of Cap Timiris at ~19°45'N. The canyon was surveyed by us for ~215km from the shelf edge to ~3000m water depth, but it extends for several hundreds of km further offshore. The canyon cuts back to the shelf break and filled incised valleys were found on the shelf in this area. Sediment cores taken at the thalweg of the canyon indicate active sediment transport through the canyon. Hence the canyon is a direct pathway for sediment transport from the shelf into the deep ocean.

Figure: Bathymetric map of the newly discovered Cap Timiris Canyon.

Based on the newly established concept of geochemical element stratigraphy supplemented by AMS radiocarbon dates a precise chronostratigraphical framework was developed for these sediments that allowed a detailed reconstruction of post-glacial sediment dynamics in the canyon. The turbidite frequency was highest during deglacial sea level rise (13-11 ka) with typical quartz-rich fining-upward Bouma sequences pointing to a predominant supply from shelf sources. Turbidite activity in the Cap Timiris Canyon changes with the beginning of the Holocene. Turbidity flows are less frequent but recur obviously every 900 ± 200 years throughout the Holocene. This cyclicity is also observed in various Holocene palaeoclimate studies. Moreover, two phases can be distinguished through the Holocene: fine-grained turbidites occur from Early- to Mid-Holocene and 2) carbonate-rich turbidites are deposited during the Late Holocene.

The area off Cap Blanc between 20°30'N and 21°30'N is mainly characterized by undisturbed sedimentation on the shelf and upper slope except for some indications for contour parallel sediment transport but deposits of major mass wasting events were identified on the seismic records in ~200m subbottom depth.

The new data clearly show that even at the arid, presumably sediment-starved, continental margin off Mauritania with the Sahara in its hinterland, sediment transport operates with different rates and styles including a significant transfer of land-derived terrigenous and hemipelagic sediments to the deep sea.

Figure: Scientific team of Meteor-cruise M58/1.