From shelf dynamics to shelf export: Evidence from sedimentologic and palaeoceanographic slope records

Abstract

The continental shelf and slope are among the most dynamic sedimentary and oceanographic environments on Earth, strongly influenced by processes within the terrestrial and marine realm. The shelf break, geographically separating the two areas, marks one of the major physiographic transitions on Earth. The oceanographic and sedimentary processes that lead sediments across this boundary represent the dynamic link between both sedimentary systems. With a multi-proxy approach, this thesis tries to elucidate i) how strong sedimentary and (palae)oceanographic processes modify the primary shelf export signal, ii) if sedimentary records from the slope can be used as reliable archives for reconstructing sedimentary and paleoceanographic shelf dynamics, and iii) how important the local morphology is, in influencing shelf sediment export routes, transport process and/or the creation of depocentres on the slope. The key study areas of this thesis cover two different continental slope settings, the lower sloperegion (off Northwest Iberia) and the uppermost slope (off Uruguay). Off Northwest Iberia, based on high frequency sediment echo-sounding profiles and a large sediment core data set the late Pleistocene sedimentary history of the shelf system, as well as the detailed late Holocene evolution of fluviogenic depocentres on the middle shelf was revealed during the beginning of this thesis. Detailed knowledge of the late Pleistocene shelf sedimentary patterns enables the differentiation of local shelf export signals from over-regional palaeoceanographic signals within the sedimentary patterns at the Northwest Iberian slope. Applying this knowledge, multi-proxy investigation of three slope records off Northwest Iberia reveal the interplay of slope morphology as well as palaeoclimatic effects on shelf slope sediment dynamics and depositional patterns during the late Quaternary. At the uppermost slope off Uruguay a small morphologic terrace represents a further type of external archives for the effective and direct recording of shelf sediment export. Contrasting the regional morpho-sedimentary structure of the Southeast South American continental margin, the uppermost slope terrace off Uruguay is not influenced by contouritic sediment transport. Instead, sediment dynamics on the terrace are directly linked to the sedimentary shelf configuration, which is strongly controlled by sea-level rise during the Holocene. Very highsedimentation rates on the terrace (at least approx. 50 cm/kyr) enable the application of these sedimentary sequences for high-resolution palaeoceanographic reconstructions. Holocenelatitudinal shifts of the subtropical shelf front across the Southeast South American shelf and uppermost slope were primarily forced by palaeoclimatic variations (northward migration and extension of the Southern Westerlies; middle to late Holocene evolution of the El Niño Southern Oscillation). Moreover, for the most recent 200 years an anthropogenic forcing is suggested. Overall this thesis makes the attempt to study the complex interplay of palaeoceanographic and sedimentary processes within shelf sediment export and slope sedimentary patterns. It is shown that shelf sedimentary export signals can be recognized as far down-slope as the lower slope. Two different types of slope settings are presented as qualified external records of sedimentary and palaeoceanographic shelf dynamics and export archives. An uppermost slope terrace at the Southeast South American continental margin represents a reliable and direct archive for shelf dynamics, whereas off Northwest Iberia the linkage of middle to lower slope sedimentary patterns with the last deglacial to Holocene history of sediment dynamics on the shelf is more complex. Palaeoceanographic dynamics have little modifying effect on the Holocene shelf export patterns off Southeast South America shelf, off Northwest Iberia deglacial paleoceanographic changes on the outer shelf considerably foster export of shelf sands. Moreover, local margin morphology constitutes a crucial component for shelf slope linkage as well as in slope sedimentary patterns: At the uppermost slope, small morphologic steps might act as excellent off-shelf transport archives, whereas slope incisions might act as efficient sediment conduits. At the middle to lower slope, morphologic highs may deflect sediment transporting contour currents, which causes deposition on the stoss-side of these obstacles. The combined findings of this thesis demonstrate that slope records can serve as external archives for shelf sedimentary and palaeoceanographic dynamics, and doing so have great potential for contributing to our integral understanding of sedimentary shelf-slope linkage.