An Arctic Ocean ice shelf during MIS 6 constrained by new geophysical and geological data

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Abstract

The hypothesis of floating ice shelves covering the Arctic Ocean during glacial periods was developed in the 1970s. In its most extreme form, this theory involved a 1000 m thick continuous ice shelf covering the Arctic Ocean during Quaternary glacial maxima including the Last Glacial Maximum (LGM). While recent observations clearly demonstrate deep ice grounding events in the central Arctic Ocean, the ice shelf hypothesis has been difficult to evaluate due to a lack of information from key areas with severe sea ice conditions. Here we present new data from previously inaccessible, unmapped areas that constrain the spatial extent and timing of marine ice sheets during past glacials. These data include multibeam swath bathymetry and subbottom profiles portraying glaciogenic features on the Chukchi Borderland, southern Lomonosov Ridge north of Greenland, Morris Jesup Rise, and Yermak Plateau. Sediment cores from the mapped areas provide age constraints on the glaciogenic features. Combining these new geophysical and geological data with earlier results suggests that an especially extensive marine ice sheet complex, including an ice shelf, existed in the Amerasian Arctic Ocean during Marine Isotope Stage (MIS) 6. From a conceptual oceanographic model we speculate that the cold halocline of the Polar Surface Water may have extended to deeper water depths during MIS 6 inhibiting the warm Atlantic water from reaching the Amerasian Arctic Ocean and, thus, creating favorable conditions for ice shelf development. The hypothesis of a continuous 1000 m thick ice shelf is rejected because our mapping results show that several areas in the central Arctic Ocean substantially shallower than 1000 m water depth are free from glacial influence on the seafloor.

Introduction

The idea of an Arctic Ocean ice shelf grew from Mercer’s (1970) geographic comparison between the Eurasian Arctic Ocean and West Antarctica. Located close to the Earth’s north and south poles, both regions host large continental shelf areas with water depths of some hundreds of meters. Portions of the West Antarctic continental shelf are today occupied by the West Antarctic Ice Sheet (WAIS), while there is no equivalent ice sheet in today’s Eurasian Arctic (i.e. in the Barents, Kara and Laptev seas). The WAIS is marine-based, implying that its base is grounded well below sea level and that it flows, through systems of large ice streams, into floating ice shelves (Bentley, 1987, Anderson, 1999). The Ross Ice Shelf covers 511,680 km2 and the Filchner-Ronne Ice Shelf extends over 439,920 km2 (British Antarctic Survey, 2005). Together these two large ice shelves occupy an area corresponding to ca 21% of the deep central Arctic Ocean area beyond the shallow fringing shelves.

Mercer’s hypothesis was further developed by Hughes et al. (1977) to involve a continuous 1000 m thick ice shelf covering the Arctic Ocean that, together with surrounding grounded marine and terrestrial ice sheets, behaved as a single dynamic Arctic Ice Sheet during the LGM. In comparison, the mean thickness of the Antarctic ice shelves is estimated to be 440 m, although thicknesses >1000 m can be found near the grounding lines (Lythe et al., 2001). Grosswald and Hughes (2008) further refined the Arctic Ocean ice shelf theory and concluded that continuous 1000 m thick ice shelves were re-current elements during Pleistocene glacial maxima, including the LGM.

The present Arctic Ocean circulation involves net inflow of Atlantic water with about 1–1.5 Sv (1 SV = 106 m3 s−1) through the Fram Strait and >2 Sv via the Barents Sea (Maslowski et al., 2004, Rudels, 1995). These two flow paths merge off the St Anna Trough to form a cyclonic circulation along the slopes of the Arctic continental margins (Fig. 1). Transporting heat and salt into both the Eurasian and Amerasian Basins, the Atlantic water occupies water depths below the cold halocline between about 200 and 600 m and has temperatures from 0 to 2 °C. Therefore, the present Arctic Ocean circulation regime efficiently inhibits the development of major ice shelves. Existing Arctic Ocean ice shelves are tiny in comparison to the Antarctic ones and are located, for example, on the shallow continental margin along northern Ellesmere Island and in some deep Russian Arctic fjords, out of reach of warm Atlantic water and supported by the prevailing onshore wind direction (Jeffries, 1986, Williams and Dowdeswell, 2001, Dowdeswell and Jeffries, in press).

Ocean circulation and water temperature underneath ice shelves are critical for the ice mass balance; basal accretion amounting to as much as 190 m of ice from the grounding line to the calving front is observed underneath the Amery Ice Shelf in eastern Antarctica (Fricker et al., 2001). On the other hand, warm water has been shown to protrude onto the Amundsen Sea continental shelf in West Antarctica, causing thinning of the floating margin of Pine Island Glacier (Walker et al., 2007, Wåhlin et al., in press). It seems clear that, in order to grow large and thick Antarctic-style ice shelves in the Arctic Ocean, the influx of Atlantic water must be shut down or restricted to deeper depths to prevent basal melting and allow accretion. The importance of ocean temperature and circulation for the mass balance of floating ice was also discussed by Broecker (1975), who suggested that “Floating Ice Caps” in the Arctic Ocean could have developed if a critical steady-state sea ice thickness was reached. Broecker concluded that the limiting factor for the development of thick ice was the degree to which ice was exported from the Arctic through the Fram Strait.

The increased use of acoustic mapping tools from icebreakers and submarines over the past decades has allowed a systematic testing of the hypothesis of a 1000 m thick ice shelf in the Arctic Ocean, because the grounding of deep-drafted ice on bathymetric highs leaves diagnostic scars on the seafloor. Ice grounding is mapped down to 1000 m present water depth on the central Lomonosov Ridge as well as on the Chukchi Borderland and Yermak Plateau (Jakobsson, 1999, Polyak et al., 2001) (Fig. 1). However, the limited spatial distribution of previously collected seafloor morphological data did not provide conclusive evidence for or against a continuous floating ice shelf covering the entire Arctic Ocean (Jakobsson et al., 2008b).

Here, we present new multibeam bathymetry and subbottom profiles acquired during three expeditions from previously un-surveyed areas of the southern Lomonosov Ridge north of Greenland, the Morris Jesup Rise, the Yermak Plateau, and the Chukchi Borderland that, together with previously collected data, allow us to conclude that large ice shelves existed during some glaciations. These new data do not support the idea of a single large and coherent 1000 m thick ice shelf covering the entire Arctic Ocean. We propose that a large ice shelf complex was mainly restricted to the Amerasian part of the Arctic Ocean and that its existence depended on a different Atlantic water circulation. Dating of sediment cores from ice grounded areas suggests that the most prominent grounding events occurred during MIS 6.

Section snippets

Materials and methods

The geophysical data and sediment cores presented here were primarily collected during four expeditions: (1) the Lomonosov Ridge Off Greenland (LOMROG) 2007 when Swedish icebreaker Oden supported by the Russian nuclear icebreaker 50 Years of Victory were the first ever surface vessels operating on the Lomonosov Ridge north of Greenland (Jakobsson et al., 2008c); (2) the Healy-Oden Trans-Arctic Expedition (HOTRAX) 2005 when Oden and the US Coast Guard Cutter (USCGC) Healy together transected the

Geophysical mapping

Along with the geophysical mapping data from LOMROG 2007, HOTRAX 2005 and HLY0905, we also include previously published results in this section in order to provide the circum Arctic Ocean view of existing data with relevance for the glacial history, in particular concerning the existence of marine ice sheets. On the Morris Jesup Rise, which protrudes from the northern Greenland continental margin (Fig. 1), traces of ice grounding in <1000 m water depths were previously postulated from subbottom

An Arctic Ocean Ice Shelf in the Amerasian Basin

The present collection of geophysical data from the Arctic Ocean does not support the existence of a continuous 1000 m thick Arctic ice shelf because numerous bathymetric highs that would have been affected by such an ice shelf show no evidence of glacial overriding (Fig. 8). Previous studies of the Chukchi Borderland suggest the re-current influence of ice grounding attributed to ice shelves extending from the northern margin of the Laurentide Ice Sheet combined with an ice rise formed over the

Conclusions

Geophysical mapping of the southern Lomonosov Ridge off Greenland, Morris Jesup Rise, Chukchi Borderland and Yermak Plateau in the central Arctic Ocean portray MSGL and iceberg plowmarks that constrain the existence of thick ice shelves and deep drafting icebergs during previous glacial periods. The deepest iceberg plowmarks are mapped on the Morris Jesup Rise and extend to 1045 m below present sea level. Dating of sediment cores from the ice grounded areas suggest that the deepest iceberg

Acknowledgements

The LOMROG 2007 expedition was carried out by the Swedish Polar Research Secretariat (SPRS) and the HOTRAX expedition as a collaboration between SPRS and the US National Science Foundation (NSF). We thank the Captains and Crews on the Oden and Healy icebreakers. Financial support was received from the Swedish Research Council (VR), the Swedish Royal Academy of Sciences through a grant financed by the Knut and Alice Wallenberg Foundation and the Bert Bolin Centre for Climate Research at

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