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Nicholas Owen, Trinity College Dublin (Ireland)
Lee Toms, University College Dublin (Ireland)
Peter Haughton, University College Dublin (Ireland)
Robin Edwards, Trinity College Dublin (Ireland)
Patrick Shannon, University College Dublin (Ireland)
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A smooth, stable section of the Porcupine Bank (NE Atlantic Ocean) has been studied in order to construct a downslope lithostratigraphy. This section was selected because it is relatively stable, is known to be / have been swept by northward flowing bottom-currents, and high resolution seismic profiling reveals that the section is plastered by thin contourite sheets. These factors make this site ideal for investigating controls on the fidelity of the offshore record in relation to climate change, and also how this record can be linked to terrestrial climate changes. Four cores from a depth transect on the western flank of the Porcupine Bank have been examined. This has resulted in the identification of twenty-four planktonic foraminiferal species and sixty-eight taxa of benthic foraminifera from two hundred and thirty samples.
Key findings include a consistent variation in sea surface temperatures between interglacial (average 18-19°C) and glacial (average 3-6°C) episodes (summer estimates). There is also a consistent offset between SST estimated from MAT transfer functions of planktonic foraminifera and δ18O palaeotemperature estimates derived from tests of G.bulloides. This implies glacial-interglacial and stadial-interstadial variation in the depth or strength of the seasonal thermocline (shallower during glacials) and also, probably, the timing of the seasonal bloom of G.bulloides. Spectral analysis of the δ18O data indicates that precession modulated eccentricity could be the main forcing mechanism in the variation of δ18O.
The variation between colder and warmer climate episodes is clearly displayed in the data for core 9-97-10 and closely follows what might be expected from a study of the sediments alone, but provides much greater detail regarding the leads and lags according to planktonic data and the response in the deep ocean as inferred from sediments and benthic foraminifera. An episode of enhanced productivity associated with Termination I is also present at a core depth of 0.15m in core 9-97-10. This is supported by a large peak in the abundance of T.quinqueloba and a strongly negative planktonic δ13C value. Data from a deep-water core (9-97-27-II, 2756m water depth) indicates a clear sensitivity to instabilities in climate during inferred Marine Isotope Stages 2-3, particularly Heinrich-like events and possible clathrate destabilisation episodes. There is some evidence to suggest that variations in planktonic and benthic foraminiferal (particularly evidence of sorting and size fraction analysis) assemblages at different depths are responding to variations in bottom-water current velocity, although further work is required to absolutely discriminate bottom-water current velocity from other causal mechanisms at this stage.
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