Investigation of the North Atlantic Heinrich events using molecular approach
The aim of this thesis is to investigate the application of biomarkers to study the provenance of allochthonous organic matter deposited in the North Atlantic during the last glacial period mainly as ice rafted debris (IRD) in Heinrich Layers (HLs). Two novel approaches are used in this work: The biomarker composition of sedimentary organic matter is used to characterise and compare possible sources and sinks of IRD• Glacigenic debris flows (GDFs) are used as proxies for possible sources of IRD in the deep ocean. The distribution of photosynthetic pigments, n-alkanes, highly branched aliphatic hydrocarbons (unresolved complex mixture (UCM) in the gas chromatogram) and long- chain alkenones in the sediments from the possible sources and sinks of IRD IS analysed. Sources of IRD are represented by GDF deposits from the Nordic Seas and the western Atlantic margins. Sinks are represented by deep-sea cores containing HLs: cores รบ90-09 and ODP 609 are from the main area of IRD deposition, "IRD belt", and cores MD95-2024 and HU87-025֊07P are from Labrador Sea. GDF and HL sediments generally contain a very high proportion of biodegraded and thermally mature organic matter originating from ancient outcrops eroded by former ice streams. Most hemipelagic sediments overlying GDFs or HLs in the deep-sea cores contain signatures of less reworked organic matter. GDF sediments at the North Atlantic margins are largely homogenous in their biomarker composition; they contain biomarker distributions that are characteristic and unique to each GDF deposit and thus significantly different from those of the overlying hemipelagic sediments. It was concluded that the biomarker fingerprints of the organic matter in each GDF can be considered as combined signatures of a variety of organic-rich outcrops eroded by a particular ice stream and therefore can be used to constrain the sources of IRD in the North Atlantic. “Typical" HLs 1, 2, 4 & 5 can be identified in the sedimentary records on the basis of the biomarker composition of sedimentary organic matter. Biomarker compositions of sediments from "untypical" HLs 3 and 6 in the two cores from the "IRD belt" differ significantly from those of the "typical" HLs 1， 2, 4 & 5 are similar to the overlying ambient sediments. In contrast, all HL samples from the Labrador Sea core MD95-2024 are similar. A shift in oceanic Polar Front location is reflected in the biomarker composition of ambient sediments deposited after HL3 in core SU90-09. These contain highly degraded organic matter of ice rafted origin whose biomarker composition differs from that of the "typical" HLs but not from that of HL3.The biomarker composition of the HL sediments from core SU90-09 varies both between and within HLs. All HLs in core SU90-09 can be distinguished from one another based on their biomarker composition. For the most part, the biomarker compositions of the sediments in the older HLs 4 and 5 are more similar to one another than to those in the younger HLs 1 and 2. Precursor events are recognised in the sedimentary record of HLs 1 and 2 but not HLs 4 and 5. Three steps can be distinguished in the deposition of HLs 4 and 5， with the narrow middle step different from the periods above and below it. HLl and possibly HL2 contain two steps. With regard to source-sink correlation, IRD in the HLs in core MD95-2024 contains a combined signal from North American and West Greenland sources. Similar signatures are present in the "typical" HLs from cores SU90-09 and ODP 609. A contribution from an additional unidentified source or sources is present in both cores, substantial in the former and minor in the latter. However, no significant input from the European ice sheets was detected. The variability between and within HLs is probably a result of variability in the relative input from different IRD sources. The biomarker composition of the "untypical" HLs 3 and 6 from ODP 609 is more consistent with aeolian rather than IRD input. Biomarker analysis provides a potentially more detailed and more specific approach than analysis of bulk properties of sediments to characterize sources and sinks of IRD.