The variability of sea ice in the Fram Strait throughout glacial-interglacial transitions of the Late Quaternary (MIS 11 to MIS 1)

Ruediger.Stein [ at ]


Given the pivotal role of Arctic sea ice in the global climate system, its excessive loss over the last decades is highly concerning. According to common climate models, an ice-free Arctic Ocean during summer may be reached by the middle of the century. While these profound changes are clearly visibly in the Arctic realm, the far-reaching consequences remain a matter of speculation. As the reliability of model simulations depend on their data base, the acquisition of comprehensive knowledge about past variation of Arctic sea ice cover is absolutely crucial. An ideal study area in this regard is the Fram Strait reflecting changes in the highly dynamic marginal ice zone of the Arctic Ocean. Therefore, the primary objective of this thesis was the reconstruction of sea ice variability in the Fram Strait throughout glacial-interglacial transitions of the Late Quaternary. For this purpose, analyses of molecular biomarkers were carried out on eight sediment cores along the western and northern Svalbard Shelf (PS93/006-1, PS92/039-2), the East Greenland Shelf (PS93/016-6, PS93/031-4) as well as the central Arctic Ocean (PS87/023-1, PS87/030-1, PS87/070-1, PS87/079-1). These records provided valid insights into the Arctic paleoenvironment since Marine Isotope Stage 11. In order to reconstruct past sea ice cover, the sea ice proxy IP25 was applied in combination with the related phytoplankton-IP25 sea ice index PIP25. Further specific biomarkers were used to reflect the sea surface temperatures (glycerol dialkyl glycerol tetraethers), the marine production (brassicasterol, dinosterol, tri-unsaturated highly-branched isoprenoids) and the input of terrigenous material (campesterol, β-sitosterol). The first study focused on the variation of sea ice at the eastern Yermak Plateau north of Svalbard (Core PS92/039-2) since MIS 6 (~160 ka). Here, the activity of the Svalbard Barents Sea Ice Sheet had a major impact on the sea ice cover. During intervals of ice sheet advance onto the continental shelf (MIS 6, MIS 5d - 5b, MIS 4, MIS 2), the Yermak Plateau was predominated by marginal sea ice conditions. Under the influence of katabatic winds blowing from the extended ice sheet and the upwelling of relatively warm Atlantic Water along its shelf break, wide polynyas may have formed north of Svalbard that triggered the establishment of a stationary ice margin above the Yermak Plateau. The Penultimate Glacial Maximum (~140 ka) experienced the formation of a perennial sea ice cover or a short-term advance of the ice sheet onto the plateau. The transition to interglacial stages, including the Last Interglacial (MIS 5e, ~123 ka) and the Holocene (MIS 1, 11 ka - present), was associated with a shift to extended, however variable sea ice conditions at the Yermak Plateau. Most probably, the sea ice margin followed the southward migration of the Svalbard Barents Sea Ice Sheet as the coastal polynya in front of it formed back. The repeated waxing and waning of the ice sheet, especially during MIS 6, triggered an enhanced input of glacially eroded terrigenous material along the continental margin of northern Svalbard. The high suspension of fine-grained material facilitated the downdrawal of marine organic matter due to aggregate formation. In a second study, the observations north of Svalbard were supplemented by reconstructions of the sea ice variability at the western Svalbard margin (Core PS93/006-1, ~190 ka). The comparison of both core sites revealed an opposing trend of glacial and interglacial sea ice conditions that highlights the regional impact of various environmental forces in the eastern Fram Strait. Along western Svalbard, glacial intervals were characterised by an extended seasonal sea ice cover probably due to the persistent, yet reduced, inflow of Atlantic Water. Indications for perennial sea ice conditions were associated with absolute insolation minima during early MIS 6, the Penultimate Glacial Maximum, the interstadial MIS 5d, MIS 4 and the Last Glacial Maximum. During interglacials (MIS 5, MIS 3, MIS 1) the western continental margin of Svalbard experienced a reduced sea ice cover with more frequent summer melt. The comparatively extended sea ice conditions during the peak interglacial MIS 5e implied a strong dominance of polar water masses that possibly repressed the inflow of warm Atlantic Water. The third study comprises sea ice reconstructions from the central Arctic Ocean and the northeastern continental margin of Greenland. These outcomes suggested a prevalence ofperennial sea ice at the Siberian part of the Lomonosov Ridge (PS87/070-1, PS87/079-1)since MIS 6 and at the Greenland side (PS87/023-1, PS87/030-1) since MIS 12. Likewise extended sea ice conditions predominated the northeastern Greenland margin at 82°N (Core PS93/016-6). Underlying the constant outflow of cold polar freshwater, this region was subject to a permanent sea ice cover regardless of the prevailing climate stage. Further south along the East Greenland margin (Core PS93/031-4, 79°N), distinct sea ice variations could be tied to major glacial-interglacial transitions since MIS 11 (~400 ka). According to a preliminary stratigraphy, intervals of extended glaciation (MIS 12, MIS 10, MIS 8, MIS 5d – 5b, MIS 4) were characterised by the establishment of marginal sea ice conditions at the outer shelf. Comparable to the eastern Yermak Plateau, the formation of open-water areas inthe otherwise ice-infested ocean might have been triggered by the expansion of theGreenland Ice Sheet. The glacials MIS 6 and MIS 2 showed evidence for a permanent seaice cover or an advance of the ice sheet onto the outer continental shelf area. Interglacialstages experienced most severe sea ice conditions at the East Greenland margin, probablycaused by the enormous drainage of glacial meltwater from the interior Arctic. In accordanceto the observations in the eastern Fram Strait, even the peak interglacials MIS 11 and MIS 5e were predominated by severe sea ice conditions. The thick layer of polar freshwater might have hampered the northward penetration of warm Atlantic Water.

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Kremer, A. (2018): The variability of sea ice in the Fram Strait throughout glacial-interglacial transitions of the Late Quaternary (MIS 11 to MIS 1) , PhD thesis, University of Bremen.

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