1] The International Bathymetric Chart of the Arctic Ocean (IBCAO) released its first gridded bathymetric compilation in 1999. The IBCAO bathymetric portrayals have since supported a wide range of Arctic science activities, for example, by providing constraint for ocean circulation models and the means to define and formulate hypotheses about the geologic origin of Arctic undersea features. IBCAO Version 3.0 represents the largest improvement since 1999 taking advantage of new data sets collected by the circum-Arctic nations, opportunistic data collected from fishing vessels, data acquired from US Navy submarines and from research ships of various nations. Built using an improved gridding algorithm, this new grid is on a 500 meter spacing, revealing much greater details of the Arctic seafloor than IBCAO Version 1.0 (2.5 km) and Version 2.0 (2.0 km). The area covered by multibeam surveys has increased from $6% in Version 2.0 to $11% in Version 3.0.

a b s t r a c tThe 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.

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The late Palaeozoic-Mesozoic Jameson Land basin in East Greenland was situated during early Tertiary break-up 300–400 km north of a presumed plume centre and 100–200 km landward of the initial line of North East Atlantic opening. The basin continues southward below Scoresby Sund fiord and the Scoresby Sund plateau basalts south of the fiord. By combining surface mapping with marine reflection and refraction seismics from Scoresby Sund and deep (6–12s) reflection seismics from the exposed part of the basin, it is possible to construct a composite image of the entire crustal structure in this region from the details of basalt stratigraphy at the top, through the basin fill, the foundation of the basin, and into the uppermost mantle. At the basin centre there is a very thin crystalline crust (6–8 km?) below as much as 16–18 km of basin fill. Virtually all crustal extension took place prior to mid-Permian time, and the post-rift Upper Permian to Cretaceous section contributes to only one third or less of the total thickness of the succession. During Tertiary break-up the basin was intruded by basaltic sills and dykes, and basaltic flood basalts flowed over the basin with an apparently decreasing thickness to the north. The sills are mainly exposed along the basin margins as rather thin (10–15 m) layers, but apparently increase in intensity and thickness at depth and towards the basin centre. It seems that magmas were intruded as sills up to 300 m thick in the deep (10–15 km) central parts of the basin, from where they ascended towards the basin margins and to much younger stratigraphic levels. Their geometry and possible volume make them potential candidates as mid-crustal magma chambers and crustal magma pathways for the flood basalts which show low-pressure fractionation. However, extension surface geological data and deep crustal reflection seismic data show no faulting or crustal extension associated with this intense break-up volcanism. In Scoresby Sund there is a general rather conformable relationship between the basin stratigraphy and the gross stratigraphy of the flood basalts, suggesting limited or no initial uplift prior to flood basalt volcanism. The present-day high elevation of the basalts is considered part of large regional margin uplift post-dating N Atlantic break-up by 20 Ma or more. The apparent guidance exerted by the basin on the break-up magmatic activity without renewed rifting of the basin itself, the apparent lack of a broad initial uplift during break-up, and the late regional margin uplift, all seem at odds with several current plume models.

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