Highly dynamic sediment deposition in modern times on the submarine Amazon Delta - First results from the AMADEUS project (Cruise MSM 20/3, February/March 2012)

The submarine Amazon Delta is the major sink for the enormous sediment load of the Amazon River during modern times. Supported by its large drainage area (7 M km2) and the huge total run-off of 6300 km3/yr, which is 20% of the global run-off into the oceans, the Amazon River discharges the worldwide highest total sediment load to the oceans, namely 1200 Mt/yr. Characterized by sedimentation rates of decimeters per year, the submarine Amazon Delta can act as an ultra-high resolution archive of the Anthropocene climatic history of the Amazon basin. However, it has to be kept in mind that the accumulation on the delta is controlled by an interaction of sediment supply and the shelf current pattern including strong tides. To study in general the response of Amazon sedimentation to deforestation, land use and climate variability, the Project AMADEUS has been established in cooperation between the MARUM, Bremen, Germany and the University of Sao Paulo, Brazil. Within this project a first cruise was carried out with the RV Maria S. Merian in February/March 2012 in order to collect high-resolution seismic data (Mini GI Gun, 50 m analogue streamer), sediment echosounder data (Parasound; 4 kHz, 18 kHz, 40 kHz), swath-sounder data (bathymetry and backscatter), sediment samples and water column samples in front of the Amazon River mouth. The presentation will focus mainly on the seismo-acoustic data sets. As a first step, the data were analyzed to understand the internal architecture of the recent clinoform build-up in time and space. The main emphasis were set on the foreset and bottomset, where most of the accretion occur, to allow volume and sediment budget calculations in relation to the denudation history of the Amazon basin. Especially the Parasound data reveal significant changes of the clinoform along the delta front in terms of thickness, slope angle, shape (oblique-sigmoidal-convex) and internal architecture (especially depths and character of erosional unconformities). A special outcome of the new data is the comparison with Parasound data collected in 1996 during RV Meteor Cruise M34/4. Due to several crossing points of both data sets and even a repetition of a profile shot in 1996 it is now possible to carry out direct measurements of the accumulation during the last 16 years. It turned out, that up to 6 m of sediments has been deposited at some locations since 1996, and that the progradation of the delta was around 50 m/yrs, pointing probably to a strong modern denudation in the Amazon Basin. Moreover, it is visible that the style of the clinoform changed slightly within the last 16 years indicating a change of deposition and transport processes during this time. Using the derived sedimentation rate it will be possible to date the identified erosional unonformities which represents obviously drastic changes in the transport and/or current regime. By utilizing the reflector representing the sefloor from 1996 and the unconformities as key horizons, the seismo-stratigraphic analysis in combination with coring results will be continued to enable finally a calculation of sediment budgets on yearly, decadal and centennial scales.