Heinrich Events

Session 5: IRD and Provenance

Session Description

During this session we aim to address the question of which ice-sheets were acting and reacting, and when, and whether precursor ice collapses had a significant impact upon the AMOC. In the broader sense, this session will address the issue as to whether the location of freshwater forcing is more important than the magnitude of freshening.

Session Talks

Modelling abrupt glacial North Atlantic freshening: rates of change and their implications for Heinrich events

Grant Bigg

Prof. Grant Bigg

Department of Geography, University of Sheffield

The abrupt delivery of large amounts of freshwater to the North Atlantic in the form of water

or icebergs has been thought to lead to significant climate change, including abrupt slowing of the Atlantic Ocean meridional overturning circulation. Here I examine intermediate complexity coupled modelling evidence to estimate the rates of change, and recovery, in oceanic climate that would be expected for such events occurring during glacial times from sources around the North Atlantic and Arctic periphery. We show that rates of climate change at both onset and recovery of Heinrich events are slower for events with a European or Arctic origin. Palaeoceanographic data are presented to consider, through the model results, the origin and likely strength of the controversial Heinrich event H3 during the last glacial period. We suggest that the dominant iceberg release for H3 is from the northern Fennoscandian Ice Sheet, rather than Hudson Strait.

Pattern and style of British Ice Sheet retreat around northern Scotland and Heinrich Event 1

Tom Bradwell

Tom Bradwell1, Martyn Stoker1, Derek Fabel2, Alun Hubbard3

1 British Geological Survey, Murchison House, Edinburgh, UK

2 Department of Geographical and Earth Sciences, University of Glasgow, UK

3 Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, UK

The detailed record of glacial geomorphology on the seabed around the northern UK preserves the 'footprint' of a retreating ice sheet. Using high-resolution multibeam and singlebeam bathymetric imagery we have identified suites of ice sheet moraines and grounding-line features that capture the pattern and style of British-Irish Ice Sheet retreat in its northern (Atlantic) sector. Seismic reflection profiles elucidate the sedimentary architecture of these seabed features and add valuable lithostratigraphic context. There is good evidence that the majority of these seabed features relate to the last Glaciation (Marine Isotope Stage 2-3). We have mapped, in detail, the distribution and morphology of the moraines across the Continental Shelf - revealing unexpected results. Our glaciological reconstructions point towards dynamically oscillating and predominantly grounded ice sheet retreat, but with lightly grounded and floating sectors. We suggest that ice sheet retreat was most rapid between ~20-15 ka BP, with large volumes of meltwater released in to the North Atlantic at this time, as the British-Irish Ice Sheet evolved from a largely grounded, coherent, marine-based ice sheet into several separate terrestrial ice centres. This ice sheet re-organisation occurred within the time interval of the newly proposed 'wider Heinrich Event 1 sequence' and the accompanying Atlantic meridional overturning circulation collapse. A developing cosmogenic surface-exposure chronology is helping to refine the rate and timing of ice sheet retreat in northern Scotland and its links to freshwater pulses into the North Atlantic.

A Laurentian contribution to "British-Irish" ice rafted - implications for rates and controls of IRD dispersal.

David Small

David Small

School of Geography and Geosciences, University of St Andrews, St Andrews, Fife, KY16 9AL, Scotland, UK

An understanding of provenance of Ice Rafted Detritus (IRD) provides a means to investigate the links between climate, oceans and ice sheets. Sediment core MD95-2007 from the St Kilda Basin, northeast Atlantic provides a remarkably high-resolution record for the period 17-11 ka consisting of benthic δ18O and ice-rafted detritus (IRD) flux records that can be tuned to the NGRIP δ18O record on the GICC05 timescale. This reveals a distinct ice-rafting event that occurred at the time of Greenland Interstade 1d (GI-1d). A new approach to IRD provenance analysis using Uranium-Lead (U-Pb) geochronology of detrital minerals rule out Scotland as the sole source of IRD and suggest instead a distal source demonstrating the widespread and rapid dispersal of IRD across the sub-polar North Atlantic during GI-1d. The occurrence of a geographically widespread peak in IRD concentration in ice distal sites at a time when increased freshwater flux to the surface ocean is inferred to have caused rapid cooling suggests that there is a mechanistic link between the two processes. We suggest that the IRD flux peak during GI-1d represents a hyrdro-graphic control on IRD dispersal, namely a reduction in sea surface temperature (SST) driven by a slowdown in the meridional overturning forced by freshwater input. This hypothesis is supported by the absence of IRD at southern locations at times of higher SST when the flux of icebergs from the pan-North Atlantic ice sheets to northern locations was continuing.

Dr. Jennifer D. Stanford, Geography & Environment, University of Southampton