Preview

PDF, English Download (31MB) | Terms of use

Abstract

Paleoceanographic studies are crucial for understanding the role of the ocean and its circulation in the Earth’s climate system. The patterns of the circulation can vary widely between different climate states, particularly in the course of the glacial-interglacial cycles of the Quarternary. In deep sea sediments, the isotopic composition of authigenic neodymium (expressed as εNd) can be utilized to trace back the provenance of the bottom water mass and therefore to reconstruct past circulation schemes. In Study I of this thesis, a profile of Nd isotopes at Ocean Drilling Program (ODP) Site 1063 in the Northwestern Atlantic is presented spanning the time period of 150 to 1027 thousand years ago (ka), which corresponds to Marine Isotope Stages (MISs) 6 to 29. Thus, it extends and augments the published record of εNd at the Bermuda Rise with a temporal resolution that is unprecedented for that time period in the Atlantic. The overall structure is dominated by the glacial-interglacial cycles, with Northern-Sourced Water masses (NSW) prevailing at the core site most of the time. The circulation experienced a major reorganization at the 900 ka event associated with the Mid-Pleistocene Transition, when for the first time during the past one million years, a higher proportion of Southern-Sourced Water masses (SSW) flooded the deep Northwestern Atlantic. This advance of SSW is also seen during most of the following glacials, however only at glacial maxima. An exception is the mild glacial MIS 14, when NSW was present at the core site. The circulation pattern in the Atlantic during MISs 12-11 was further explored in Study II. In addition to site ODP 1063, profiles of Nd isotopes were generated for six sites. They range from the Feni and Gardar Drift in the northeastern Atlantic (ODP 980 and IODP U1304, respectively), to the Cape Verde Plateau (ODP 659) and Ceara Rise (ODP 929) in the tropical North Atlantic to the Walvis Ridge (ODP 1267) and Rio Grande Rise (DSDP 517) in the subtropical South Atlantic. It is thereby the first systematically analysis of Atlantic wide εNd patterns for this time period. While a circulation pattern similar to the LGM prevailed during MIS 12, in MIS 11 NSW advanced further to the south compared to the Holocene. Therefore a strong AMOC is attested with active deep water formation even under ongoing freshwater input through melting ice sheets.