The floor of the Arctic Ocean comes into sharper focus with the publication of an improved version of a bathymetric chart first released in provisional form in 1999, and as version 1 in 2001. Accurate mapping of the ocean bottom is essential for modelling deep ocean circulation, but also has a political angle in defining the extent of the continental shelf — a serious consideration in such a politically sensitive part of the world as the Arctic.

The story behind the improved bathymetric chart — IBCAO Version 2.0 — is told by Martin Jakobsson and colleagues in Geophysical Research Letters (M. Jakobsson et al. Geophys. Res. Lett. 35, L07602; 2008). Its production is an instructive case of new data being married to a reinterpretation of old.

Most of the new data come from mapping missions carried out since 2000 with multibeam sonar equipment aboard various vessels, including USCGC Healy, RV Polarstern and IB Oden. Multibeam sonar systems differ from the sidescan systems used, for example, to look at the shape of the sea floor or to detect wrecks, in providing information mainly about depth.

The more dramatic changes to version 2 over version 1 are that, as the authors laconically put it, the “deep abyssal plains are systematically ca. 50–60 m deeper ...”. The revision stems from a metadata analysis of records collected by US Navy submarines over several decades, which are a central source of bathymetric information at high northern latitudes in particular. Conversion of data for version 1 was based on an assumption that the figure for the speed of sound in water used for the original calculations was 1,500 m s−1. But in many cases the figure applied was 1,463 m s−1. Hence the change in estimated depth, which also helps to explain several anomalies evident in version 1.

The three-dimensional views shown here are depictions of the Alaskan Slope and Northwind Ridge before (upper image) and after Jakobsson and colleagues' exercise in producing version 2. The image is about 650 km across, and the black area at the upper left is Alaska; the Northwind Ridge is the 'peninsula' on the right. The improved definition is evident in the sharper depiction of the gullies, caused by erosion, that scar the Alaskan Slope.

The new map is far from the final word. The authors point out that a near-perfect bathymetric model will require comprehensive multibeam coverage, which won't be available anytime soon. Meanwhile, more details on version 2 and derivations of it are available from http://www.ibcao.org.