Investigating Holocene Changes in Deep North Atlantic Circulation Using High Accumulation Rate Marine Sediment Cores

In this thesis, the variability of North Atlantic Deep Water (NADW) – an important component of global ocean circulation – is investigated throughout the Holocene (0 to 11.7 thousand years before present (ka BP)), using high accumulation rate marine sediment cores. Firstly, neodymium isotopes are used to reconstruct watermass structure at Cape Hatteras at depths of 1.5 – 3.9 km. While limited by endmember changes and the local influence of the Gulf Stream, the results suggest that the structure of the Deep Western Boundary Current (DWBC) was stable on millennial timescales over at least the last 6000 years. High resolution sortable silt records from the upper-DWBC at Cape Hatteras are generated and compiled alongside a record from 2.3 km water depth in the Iceland basin. These records are used to investigate the multi-decadal to millennial variability of flow strength within and upstream of the DWBC. Long-term trends in the upper-DWBC (i.e., Labrador Sea Water, LSW) followed inferred subpolar gyre (SPG) strength changes, including large-scale shifts in the early and mid-Holocene. The influence of multi-centennial to millennial scale solar variability is also apparent. At times, the position of the Gulf Stream may impact flow speed at some Cape Hatteras sites. Based on a new synthesis of deep North Atlantic proxy records and new benthic-planktic 14C measurements from the Iceland Basin, it is estimated that NADW weakened by no more than ~20% during the 8.2 ka BP cold event. The small magnitude of NADW change suggests additional climate mechanisms may have contributed to the cooling. It was also demonstrated that low benthic foraminiferal δ13C events in the subpolar North Atlantic may be attributable to changes in local productivity, rather than a shoaling of NADW. This finding compromises previous interpretations of low benthic δ13C events, especially those used to infer instability of NADW strength during previous interglacial periods.