Thornalley, David J.R.; (2008) Palaeoceanography of the South Iceland Rise over the past 21,000 years. Doctoral thesis (Ph.D), University of Cambridge.

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Abstract

The northern North Atlantic is a critical location in the Atlantic Meridional Overturning Circulation (AMOC) where inflowing surface waters are converted into a deep water return flow. This thesis uses four sediment cores between 1237 m and 2303 m water depth, located on the South Iceland Rise, to reconstruct past changes in these flows over the past 21,000 years, including the abrupt climate shifts associated with the deglaciation. The cores have been processed and analysed for: bulk sediment properties; grain size distribution; faunal and detrital assemblage counts; tephra abundance and composition; multi-species planktonic and benthic foraminifera paired stable isotope and minor element composition. Combined radiocarbon, tephra and stratigraphic chronologies indicate that surface radiocarbon reservoir ages south of Iceland increased to ~2 ky and 1.75 ky during Heinrich Event 1 (14.7–16.8 ka) and the Younger Dryas (11.7–12.9 ka) respectively, returning to modern values of ~0.4 ky by 10 ka. Using paired δ18O–Mg/Ca measurements on G. bulloides, G. inflata and N. pachyderma (s) the temperature and salinity of the surface inflow over the past 21,000 years were reconstructed. Near-surface waters show millennial timescale salinity variations (~0.5 psu) caused by southward migrations of the subpolar front, superimposed upon longer timescale trends including early Holocene freshening. Below the near-surface layer, inflow waters have undergone millennial timescale variations in temperature and salinity (3.5 oC and 1.5 psu respectively), remaining warm and saline during periods of significant meltwater release. Subpolar gyre dynamics control the properties of this water mass and are critical in modulating the salinity flux from low to high latitudes. This negative feedback may stabilise AMOC during periods of enhanced high latitude freshwater flux. Holocene near-surface and sub-thermocline changes combine to produce quasi-periodic upper-water column stratification events, which coincide with global records of climate change and glacier advance. The gradual change in deep water properties during the deglaciation is punctuated by two periods with low benthic δ13C and δ18O values, which do not lie on glacial or Holocene mixing lines. These periods correlate to the Younger Dryas and Heinrich Event 1, during which time meltwater input to the Nordic Seas and sea-ice formation led to brine rejection and export as an overflow into the North Atlantic. Iceland Scotland Overflow Water did not attain its modern flow strength until ~8 ka, in conjunction with major changes in surface water hydrography.

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