letters to nature NATURE | VOL 410 | 22 MARCH 2001 | www.nature.com 453 Here, 453M(C) is a mobility, w is the so-called gradient energy coef®cient, and the gradients are taken with respect to arc length. The ®rst term on the right-hand side describes the chemical effect leading to phase separation within the spinodal; the second term describes the effect that damps short-wavelength ¯uctuations. The mobility is proportional to the surface diffusivity D S and is given by MC =(D S /k B T)c(1 - c). The mobility is peaked for c = 0.5, and is zero for c = 0 and c = 1 (atoms do not diffuse in pure phases because there are no vacancies in our model). The normal velocity is given by v n C VC1 2 g-=k B Tk, where g is the surface free energy and V(C) is called the interface response function, equal to the velocity of a ¯at surface covered with a concentration C of gold. We ®nd in both simulation and experiment that the interface response is ®tted well by the functional form V(C)= V 0 (f)exp(-C/C*), where f is the overpotential and C* is a constant. Experimentally, the gold accumulation can be inferred by integrating the dissolution current versus time at ®xed overpotential; it is necessary to use an overpotential that is low enough to ensure that the surface remains planar (that is, porosity does not form) and also to catch the short initial transient rise in current as silver atoms are pulled from the ®rst few monolayers. This particular form for the interface response function is quite curious. Naively, one might expect that the local interface velocity would be proportional to the local concentration of silver exposed to the electrolyte, that is, V(C) ~ (1 - c). However, the decaying exponential form suggests that there is an evolving distribution of holes opening and closing within the interfacial region, controlling the accumulation rate. Physically, the mass conservation condition (equation (1)) is the statement that the total number CbDs of gold atoms in a length Ds of interface with lateral width b can change as a result of three distinct effects that correspond to the three terms on the right-hand-side of equation (1): the accumulation of gold atoms into the interfacial layer from the solid being dissolved; the local stretching of the interface (]Ds/]t = v n k Ds), which can either increase or decrease C depending on whether the solid is concave (k . 0) or convex (k , 0); and the motion of atoms along the interface driven by the surface diffusion ¯ux J S . Received 14 November 2000; accepted 10 January 2001. 1. Pickering, H. W. Characteristic features of alloy polarization curves. Corros. Sci. 23, 1107±1120 (1983). 2. Forty, A. J. Corrosion micromorphology of noble metal alloys and depletion gilding. Nature 282, 597± 598 (1979). 3. Masing, G. Zur Theorie der Resistenzgrenzen in Mischkristallen. Z. Anorg. Allg. 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The research of A.K. also bene®ted from computer time allocation at NU-ASCC. K.S. thanks the AFOSR for support. Correspondence and requests for materials should be addressed to J.E (e-mail: Jonah.Erlebacher@jhu.edu). ................................................................. Ice shelves in the Pleistocene Arctic Ocean inferred from glaciogenic deep-sea bedforms Leonid Polyak*, Margo H. Edwards², Bernard J. Coakley³ & Martin Jakobsson§k * Byrd Polar Research Center, Ohio State University, Columbus, Ohio 43210, USA ² Hawaii Mapping Research Group, Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, Hawaii 96822, USA ³ Department of Geology, Tulane University, New Orleans, Louisiana 70118, USA § Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden .............................................................................................................................................. It has been proposed that during Pleistocene glaciations, an ice cap of 1 kilometre or greater thickness covered the Arctic Ocean 1±3 . This notion contrasts with the prevailing view that the Arctic Ocean was covered only by perennial sea ice with scattered icebergs 4±6 . Detailed mapping of the ocean ¯oor is the best means to resolve this issue. Although sea-¯oor imagery has been used to reconstruct the glacial history of the Antarctic shelf 7±9 , little data have been collected in the Arctic Ocean because of operational constraints 10,11 . The use of a geophysical mapping system during the submarine SCICEX expedition in 1999 12 provided the oppor- tunity to perform such an investigation over a large portion of the Arctic Ocean. Here we analyse backscatter images and sub-bottom pro®ler records obtained during this expedition from depths as great as 1 kilometre. These records show multiple bedforms indicative of glacial scouring and moulding of sea ¯oor, combined with large-scale erosion of submarine ridge crests. These distinct glaciogenic features demonstrate that immense, Antarctic-type ice shelves up to 1 kilometre thick and hundreds of kilometres long existed in the Arctic Ocean during Pleistocene glaciations. The central Arctic Ocean contains relatively shallow areas (water depths ,1,000 m; see Fig. 1) on Yermak plateau, Lomonosov ridge and Chukchi borderlandÐwhich includes Chukchi plateau, Chuk- chi rise and Northwind ridge. During the SCICEX-99 expedition, conducted on the nuclear-powered submarine USS Hawkbill, shallow sea-¯oor areas were targeted for mapping to detect glacio- genic bedforms. Sea-¯oor images (collected using a submarine- mounted 12-kHz swath bathymetry and sidescan sonar 12 ) from the Chukchi borderland and the Lomonosov ridge show a variety of bedforms, including random or subparallel scours, parallel linea- tions, and transverse ridges. On the records from the chirp sub- bottom pro®ler, these bedforms are associated with planed ridge crests with rough microrelief and obvious angular unconformities cut into the strati®ed sediments. Randomly oriented furrows, typically ,100-m wide and up to 30-m deep, densely cover the shallowest, ,400-m-deep portions of sea ¯oor on the Chukchi borderland and adjacent continental margin (Fig. 2a). Isolated larger scours up to 700-m wide and over 10-km long occur as deep as 500 m. Even greater depths, exceeding 900 m, are attained by closely spaced, subparallel scours on the Lomonosov ridge. Sea-¯oor scours are known to be formed by the drift of icebergs and pack-ice ridges 13 . At present, icebergs in the Arctic Ocean have at most 50-m draughts 14 , whereas icebergs off Antarctica and Greenland reach depths of 500±550 m (refs 15, 16). The largest depths of gouged sea ¯oor, extending to 850 m, have been reported from the Yermak plateau 10 , matching the depth of sours on the Lomonosov ridge. Below the depth range of dense scouring, the sea ¯oor exhibits k Present address: Center for Coastal Mapping, University of New Hampshire, Durham, New Hampshire 03824, USA. © 2001 Macmillan Magazines Ltd