Heinrich Events and the British-Irish Ice Sheet: evidences of palaeoceanographic changes in the Rockall Trough
Citation:
Renken, Sabrina Jasmin, Heinrich Events and the British-Irish Ice Sheet: evidences of palaeoceanographic changes in the Rockall Trough, Trinity College Dublin.School of Natural Sciences, 2023Download Item:
Thesis_SabrinaRenken_2023.pdf (PhD Thesis) 20.64Mb
Abstract:
The Porcupine Bank is an offshore plateau connected to the Irish Shelf west of Ireland. Its western flank is affected by several water masses associated with the Atlantic Meridional Overturning Circulation (AMOC) and other water masses such as the Mediterranean Overflow Water or modified Antarctic Bottom Water flowing through the adjacent Rockall Trough. The proximity to the former British-Irish Ice Sheet (BIIS) and outermost reach of the former Laurentide Ice Sheet (LIS) makes the Porcupine Bank slope an ideal site for studying abrupt climatic events associated with changes in the above-mentioned water masses and ice sheets. This thesis aims to investigate the potential presence of BIIS and LIS ice-rafting signals at the Porcupine Bank slope during abrupt climatic events, particular Heinrich Events, and how these are connected to oceanographic changes (e.g. temperatures, salinities) to gain a better understanding of ice rafting triggering mechanisms. The sediments of the lower Porcupine Bank slope comprise of a variety of ice rafted debris (IRD) signals which are strongly connected to the dynamic behaviour of the BIIS over the last 55 000 years. A stratigraphic event analysis, carried out on the basis of high-resolution total IRD counts, grain size analyses, planktonic foraminifera census counts (cluster analysis, principal component analysis and N. pachyderma (sin) index) and non-destructive analyses such as X-ray imaging and multi-sensor core logging (magnetic susceptibilities), allowed a distinction between five preserved Heinrich Events (H1 to H5) and independent ice rafting events of the BIIS. The collected data in this thesis allowed a subdivision of the Heinrich Events on the basis of total IRD signatures, sortable silt data (bottom current strength estimates) as well as planktonic foraminifera derived temperatures (census count transfer functions and Mg/Ca) and salinities (δ18O) into three-phased Heinrich Events (H2, H4 and H5), which are strongly linked to so-called grey bands identified among the sediment of the Porcupine Bank slope, and discrete two-phased Heinrich Events (H1 and H3). The three phases of the grey band associated Heinrich Events reflect (1) a BIIS growth phase under cold conditions, weak bottom currents and changes in surface flow intensities most likely due to AMOC configuration alterations (onset or pre-phase), (2) a stable/oscillating BIIS during the main-phase with an incorporated LIS signature and (3) a BIIS short readvance but primary demise/retreat phase under superimposed climatically cold conditions (transition or post-phase). The main phase consists of three consecutive oceanographic events which indicate ocean warming as a result of either subducted warm surface waters or sub-surface warming associated with the Mediterranean Overflow Water, followed by a rapid increase in bottom water activity highlighting strong perturbation changes in the AMOC most likely linked to the restoration of the deep-water formation and a final increase in temperatures due to rapid solar induced surface warming. Ocean warming seemed to have also triggered H1, which only reflects the demise phase of the BIIS and LIS (second half). H3 is also linked to the transition of the warmer Marine Isotope Stage 3 to the cold Marine Isotope Stage 2, reflecting an initial BIIS growth phase halted by a steady/oscillation state when entering the last glacial period. BIIS-IRD signatures independent from Heinrich Events follow the Dansgaard-Oeschger cyclicity with increased ice rafting during stadial phases. With the onset of the Last Glacial Maximum this millennial-scale cyclicity ceases and climate-independent factors start controlling the BIIS dynamics. Other interesting finding of this thesis involve an unusual large local reservoir effect around H4 potentially linked to the Laschamp event in combination with increased carbon dioxide intake into the ocean, increased bioturbation and potentially strongly reduced accumulations rates as a hiatus during the same timing was observed. Additionally, unusual high Mg/Ca ratios indicate potentially increased early diagenesis processes during Greenland Interstadials in connection to Heinrich Events.
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Grant Number
Irish Research Council (IRC)
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APPROVED
Author: Renken, Sabrina Jasmin
Advisor:
Edwards, RobinPublisher:
Trinity College Dublin. School of Natural Sciences. Discipline of GeologyType of material:
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