Werner, Kirstin (2011) Holocene variability of surface and seep water advection to the Arctic Ocean - a multiproxy perspective from the aastern Fram Strait. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 150 pp.

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Abstract

Micropaleontological, geochemical, and sedimentological parameters of two sediment cores from the eastern Fram Strait have been studied to reconstruct the variability of surface and deep water advection and related fluctuations of the marginal ice zone during the past ca ∼9,000 years with multidecadal resolution. The Fram Strait between Greenland and Svalbard is the only deep connection between the Arctic and adjacent subpolar oceans and is often referred to as the ‘Arctic Gateway’. Fram Strait thus plays a crucial role for the energy budget and density pattern of the Arctic Ocean. Large amounts of warm and saline Atlantic Water derived from the North Atlantic Drift transport most of the heat through eastern Fram Strait to the Arctic basin, resulting in year-round ice-free conditions. Arctic sea ice and cold and fresh waters exit the western part of the strait southward along the Greenland shelf. Compared to the ice-covered Arctic Ocean, the strong east-west temperature gradient results in higher bioproductivity and sedimentation rates in the eastern Fram Strait which allows for suitably tracking Holocene variations of the heat flux to the Arctic Ocean in continuous high-resolution sediment sequences. The multiproxy results presented in this thesis suggest that the Holocene climate and oceanographic development in the Fram Strait and possibly the Arctic Ocean was much more variable than previously assumed. The variation and interaction between warm and saline advection of Atlantic Water at the surface to subsurface into the Arctic Ocean and a correspondingly fluctuating sea ice margin characterise the eastern Fram Strait throughout the Holocene. The data imply that the transition from deglacial/Early Holocene to modern-like conditions occurred stepwise. Inferred from the high relative abundance of the subpolar planktic foraminifer species Turborotalia quinqueloba, intense advection of warm Atlantic Water to the Arctic Ocean marks the Early and Mid-Holocene interval (~9,000 to 5,000 years before present), concurrent with high insolation at that time. Superimposed on optimum climate conditions, repeated cold events such as the well-known ‘8,200 year cold event’ are observed. These cold events are likely related to repeated advances of the sea ice margin and the Arctic freshwater layer. A roughly 550-year cyclicity of bottom water inflow, indicated by benthic carbon isotope data, coincide with North Atlantic bottom sediment proxy records and may suggest that deepwater variations in the Fram Strait were linked to changes in thermohaline convection processes in the Nordic Seas. Modern (pre-industrial) climate conditions evolved after 5,000 years before present, simultaneous to the decreasing insolation and postglacial sea level highstand which likely resulted in the onset of modern-like sea ice production on the shallow Siberian shelves. Dominance of the coldwater-indicating planktic foraminifer Neogloboquadrina pachyderma and a significantly increasing amount of ice rafted material point to a weaker and/or cool subsurface Atlantic Water inflow and advances of the Arctic Front during the Late Holocene Neoglacial phase. Strong southeastward advances of Arctic sea ice and polar water likely prevailed and caused heavy winter sea ice conditions and relatively short ice-free summer seasons in the eastern Fram Strait during this period. Distinct changes linked to the variable Atlantic Water inflow and fluctuations of the sea ice margin occurred during the past ~2,000 years. More stable conditions and reduced influence of the sea ice margin characterise the well-known Medieval Climate Anomaly. Subsequently, colder conditions mark the onset of the Little Ice Age period which occurred in two phases in the eastern Fram Strait. A first phase from ~1350 to 1750 AD was characterised by frequent shifts of the marginal ice zone, indicated by high amounts of ice rafted material and highly fluctuating planktic foraminifer fluxes. After ~1750 AD a second, very cold phase with heavy sea ice conditions established, concomitant with an increased abundance of icebergs from advancing Svalbard glaciers. Changes in all studied proxies in the uppermost sediment layer confirm a strong climate shift during the past few decades. Highest relative abundance of subpolar planktic foraminifer species in the uppermost sediment layer and the application of two independent temperature reconstruction methods reveal a temperature increase of ∼2°C within the past ∼120 years. Seawater-derived neodymium and lead isotope compositions stored in ferromanganese oxyhydroxide coatings of sediment particles were investigated to reconstruct Holocene variations of deep water exchanges between the Nordic Seas and the Arctic Ocean through Fram Strait. Inflow of deep waters from the Nordic Seas into the Arctic Ocean can clearly be deduced for the period between 9,000 and 3,000 years before present. Thereafter, coeval with the Neoglacial cooling trend in the northern North Atlantic region and the onset of modern Arctic sea ice production, significantly more radiogenic neodymium isotope compositions may be related to the enhanced release of ice rafted material in the eastern Fram Strait during the Late Holocene.

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