Fletcher's Ice Research Articles

John Thomas Crowell, Jr. (1898-1985)

Jack Crowell supported, led, and advised the work of innumerable people for many years in Greenland, northern Canada, the Arctic Ocean and Antarctica. He was a master mariner by training and he applied the discipline and skills of that calling throughout the transition of polar development from the age of sail to the jet age. ... every aspect of polar activity and development interested him, and he gave it his best support for more than forty years. ... [In 1953] Crowell joined the staff of the Northeast Air Command at Fort Pepperell in St. John's, Newfoundland, as a technical advisor in the Arctic Division. He was thus directly involved in planning and the initial site landings for DEW Line stations in Canada and HIRAN sites on the Greenland Ice Cap. He was advisor to the reactivation of T-3, Fletcher's Ice Island, in the Arctic Ocean for the International Geophysical Year, and he was subsequently advisor to the U.S. Air Force for the examination of unprepared emergency airstrips at Polaris Promontory, Hall Land, and Bronlund's Fiord, Peary Land, North Greenland. Crowell then joined the National Science Foundation as special projects officer. He was responsible for the conversion and commissioning of the Eltanin for Antarctic oceanographic research, for the planning and construction of the research vessel Hero, and for planning an Arctic Ocean drifting research barge - a concept that has not yet come to pass. Crowell made two trips for NSF on icebreakers to the Antarctic Peninsula to select a site for Palmer Station. ... An island in Frobisher Bay, Baffin Island, a harbor in Labrador, and a mountain in Antarctica are named for Crowell ....

On the transfer of stratospheric ozone into the troposphere near the north pole

A series of nearly daily ozone vertical profiles obtained at station T-3 on Fletcher's Ice Island (∼85°N, ∼90°W) during the period January-March 1971 shows several significant ozone intrusions into the troposphere. These intrusions are not only associated with enhanced ozone amounts in the stratosphere but also require tropopause folding events to transport ozone into the troposphere. These folds in the Arctic tropopause appear to be capable of contributing significantly to the ozone budget of the Arctic troposphere during the late winter and spring seasons. The importance of tropopause folding for bringing ozone into the troposphere seen in the daily ozone profiles confirms the results found in the Arctic Gas and Aerosol Sampling Program aircraft flights.

Thomas Saunders English, 1928-1985

After three years of illness, Tom English passed away in Seattle, Washington, on 5 January 1985. ... Tom was born and grew up in Washington, D.C. The undergraduate years, 1946 to 1950, were spent at Iowa State University in Ames, first as a journalism major, later as a zoology student. He continued at Ames with graduate work in zoology and statistics, receiving a master's degree in 1951 with a thesis on age, growth, and life history of carps in a local lake. ... Before receiving his doctorate at the University of Washington in 1961, Tom did graduate studies at the University of Oslo and in Bergen, Norway (1954-55); he also served in the U.S. Air Force at Fairbanks and near the North Pole (1956-58) and spent one year as instructor in fisheries biology at the University of Alaska (1958-59). His Ph.D. thesis treated the distribution and abundance of planktonic flatfish eggs in Puget Sound. Tom joined the teaching faculty of the then Department of Oceanography of the University of Washington in 1959. The next quarter of a century of his professional life was more than filled with teaching, research, participation in base-line studies for the state and federal governments and administrative work in the department. ... Tom's research interests revolved around problems of sampling design and the plankton of the Arctic Ocean. ... The common method of estimating stock size of fishes with pelagic eggs from integrating areas within contour lines of egg abundance was critically assessed and a better method was proposed. ... Tom chose to discharge his ROTC obligations of undergraduate days by entering the Air Force ... and, apparently, elected to be detailed to the Arctic Aeromedical Laboratory outside Fairbanks. ... he spent two summers and autumns on Drift Station Alpha, an ice floe near the North Pole .... The biological investigations were performed under contract with the Arctic Institute of North America. Besides being among the first to SCUBA-dive under the ice ..., Tom established in great detail what earlier polar investigators could only suggest: that the annual phytoplankton cycle under the ice is primarily driven by underwater light, which in turn depends more on snow melt than incident radiation. ... In the mid-sixties, Tom's continuing interest in the plankton of the Arctic Ocean was directed to Fletcher's Ice Island (also called T-3). Plankton collections were made in eight summer seasons between 1966 and 1973, and in all or most other months from 1968 through April 1974. The island drifted during this time from about 75° N north of Point Barrow, to beyond 85° N, to north of Ellesmere Island. In addition, summer collections were made in 1975 at the main camp of the Arctic Ice Dynamics Joint Experiment in the Beaufort Sea. Predominantly north of 80° N, Tom, his students, and his assistants collected the following (approximate numbers): 5500 samples each for temperature and salinity, 5000 for oxygen, 3300 each for phosphate, nitrate, and silicate, 6000 for chlorophyll, and 4000 measurements of photosynthesis (14C); further, 5800 phytoplankton and 11 000 zooplankton samples were gathered .... While the emphasis was on the upper layers, some sampling extended to the bottom. The enormous, truly unique body of analyses has only partly been evaluated .... Tom chose not to spend much of his energy on the dissemination of research results through publications but, like a real college teacher, to devote his time primarily to transmitting knowledge by classroom teaching, as well as guiding students doing research. ...

Lost island found

An abandoned ll‐by‐5‐km kidney‐shaped chunk of freshwater ice, used as a research station for 25 years, was rediscovered after the National Oceanic and Atmospheric Administration (NOAA) lost track of the island for 6 months. The recent find may foreshadow another loss, however: The island is drifting through the Greenland Sea and into the North Atlantic where it should melt within several months and d u m p its cargo of oil drums, equipment, and a wrecked plane into the ocean.Known as Fletcher's Ice Island—after Joseph O. Fletcher, a member of the first team of researchers to inhabit the island and a recently retired NOAA climate researcher—the ice chunk has already melted to a third of its original 49 m thickness. A pilot flying over the area to measure annual pollution buildup in the Arctic located the drifting island 242 km from the North Pole near the International Date Line.

New measurements of sulfate and chlorinity in natural sea ice

Precise determinations of sulfate and chlorinity were made on SIPRE core samples of natural sea ice. One ice was a multiyear ice that had remained attached to T‐3 (Fletcher's Ice Island) for over 20 years and the other was a shore‐fast annual ice collected near Barrow, Alaska. Our results show sulfate: chlorinity ratios different from the seawater ratio. A mixing model shows that on warming and aging, sulfate and chlorinity are lost in a constant ratio, indicating conservative behavior. Fractionation in sea ice appears to be determined in the freezing process and further enrichment does not take place with aging. The effects of sulfate fractionation in sea ice are too small and the residence times of both sea ice and Arctic Ocean surface water are too short to permit detectable modification of seawater composition by freezing.

Sediment waves and other evidence of paleo-bottom currents at two locations in the deep Arctic ocean

Sediment waves and other evidence of strong paleo-bottom current activity were observed in the deep Arctic Ocean during the 1967 through 1970 drift of Fletcher's Ice Island (T-3). On the crest of the Alpha Cordillera, between depths of 1100 and 2500 m, a blanket of sediment waves approximately 100 m thick overlies the 200–800 m thick sequence of stratified sediments which cover the basement topography of the cordillera. Maximum wave amplitudes were 55 m, with 1000 m wavelength. These waves were observed over an area of approximately 30,000 km 2. In one instance a fortuitous loop in the drift track showed that at least two of these waves parallel the 150°W meridian. Many sediment cores and camera stations indicate that these sediment waves are themselves blanketed by at least several meters of pelagic lutite and ice-rafted detritus, and show that the wave-forming process has been inactive since at least mid to Upper Pliocene times (3.0–3.5 m.y. B.P.). However, nepheloid layer and current measurements suggest that the present bottom circulation, while too weak to produce such waves, might be capable of producing the drastic change in regimen represented by these waves if current speeds were increased considerably. Such a regimen probably existed for a certain period prior to the development of the present Arctic ice-cover. At a second location, buried waves appearing seismically as two sequences of hyperbolic echoes, were observed in a 300 m thick zone, 550 m beneath the 3260 m deep Mendeleyev Abyssal Plain. This zone in turn overlies at least another 1500 m of horizontally stratified sediments. Adjacent to this plain, by the Mendeleyev Fracture Zone, other evidence for strong current activity was found, in the form of a depositional scarp apparently maintained by current scour, a zone of present day bottom erosion, and a network of relict abyssal channels. These waves may mark various stages of ‘commotion in the ocean’ such as those suggested by Berggren and Hollister (1977) for the Pliocene, Cretaceous, or perhaps even earlier. Such drastic changes in the sedimentological regimen, inherent in the mechanics of plate tectonics, may provide additional clues to the origin of the Amerasia Basin in the Arctic Ocean.

High Arctic Benthic Isopods From Fletcher's Ice Island, T-3, With a Description of One New Species, Mirabilicoxa Fletcheri N. Sp

Mirabilicoxa fletcheri n. sp. recueilli dans le Haut Arctique a des profondeurs de 1253 a 1740 metres est decrit. D'autres Isopodes de meme provenance sont identifies et signales.

Sonar mapping of the underside of pack ice

An underice survey was carried out in a pack ice field near Fletcher's Ice Island (T-3) by means of a 48-kHz Kelvin Hughes transit sonar. The transducer was lowered through a hole, and discrete underice scans were made through 360°. Five sets of data have been collected and transformed into polar coordinate mosaics. The echo length, the shadow zone, the arrival time of echoes, and the sounding depth provide the basic information for interpretation. A complete underice map has been constructed to show the distribution of the underice features. The keel depths of the ridges have been estimated. A surface survey that included a cross-ridge survey was carried out. The top and bottom features have been compared. The average ratio of the peak top elevation to the keel depths is 1:7.6.

Benthic ecology of the high arctic deep sea

An analysis is made of 75 quantitative benthic samples collected by Mini-LUBS, and 28 qualitative benthic samples collected with the “small biological trawl”, from Fletcher's Ice Island, T-3, while it was drifting over the Alpha Cordillera region of the High Arctic Ocean during October, 1969 through February, 1970 and in March, 1972. The depth range was 1000 to 2500 m. Benthic foraminiferans account for about 53%, bivalves for 27%, sponges for 7%, and polychaetes for 5% of the total biomass. Other groups make up the remaining 8%. The weight ratio of macro- to meiofauna is 1:1. Numerically, excluding Foraminifera, polychaetes comprise 42%, nematodes 16%, sponges 11%, and bivalves 8% of the total fauna. The remaining 23% is composed of 13 other taxa. Biomass in the Amerasian Basin at depths of 1000 to 2000 m is extremely low (0.04 g/m); it is comparable to depths of 5000 to 6000 m in the oligotrophic red-clay area of the mid-Pacific Ocean, and is 40 times less than biomass at comparable depths from Antarctica and off Peru. Diversity, as calculated by the Shannon-Weaver method, is low, suggesting that the Arctic ecosystem is young, as reported in earlier studies (Dunbar, 1968; Menzies et al., 1973). Although the H' values are low, no biocoenoses of oligomixity in the deep Arctic are revealed, contrary to previous statements and beliefs. There may be fewer major benthic groups in the Arctic Ocean than in other parts of the world oceans. Following the conventional terminology of Petersen (1913) and Thorson (1957), we have called the High Arctic biocoenoses of the Alpha Cordillera region a Thenea abyssorum-Spirorbis granulatus community.

Optical Properties of the Arctic Upper Water

Optical properties of the Arctic Upper Water have been measured from Fletcher's Ice Island, T-3, in the Arctic Ocean. Beam transmittance for various wave-lengths and the upwelling and downwelling irradiance have been measured to a depth of 120 metres. In the spectral region of maximum transmittance, the beam transmittance was found to be 93.1% per metre and the diffuse attenuation coefficient for irradiance was 0.0444 per metre. The data show that the Arctic Upper Water, in early May before the snow cover on the ice has cleared, is optically uniform and very clear.

The Escamilla Case in Court

The issue of U.S. Criminal jurisdiction to try Mario J. Escamilla for the alleged slaying of Bennie Lightsy, station manager on ice island T-3 (Fletcher's Ice Island) on 16 July 1970 was argued before U.S. Federal District Judge Oren R. Lewis in Alexandria, Virginia, on 5 May 1971. The judge found sufficient grounds for taking jurisdiction and proceeded with the trial. Escamilla was then convicted of involuntary manslaughter. He has appealed. Following are some observations on the jurisdictional aspect of the case. 1) It was evident throughout the hearings that the presiding judge wished, if possible, to decide the jurisdictional issue exclusively on the basis of U.S. domestic law, and that he was not disposed to address himself to abstruse questions of international law, once the Canadian waiver was on the record. At no point, therefore, did he express firm views on such matters as the international legal significance of T-3's origin, the possibility of permanent "possession" of such an island, the implications of a "ship" analogy as such for ice islands as a class, distinctions if any between ice islands and occupiable ice floes, and a host of other theoretical questions of universal, or at least arctic, applicability that might be considered of interest to an international lawyer. 2) In view of the very real, long continued, and unchallenged activities of U.S. Government agencies and Government grantees on T-3, the factual close connection between that particular ice platform and U.S. interests was so clear that the finding of proper criminal jurisdiction in the Escamilla case should probably not be given too broad a theoretical application. 3) One may argue that the case illustrated the extreme narrowness of the exclusively territorial basis for criminal jurisdiction (exceptions were noted) and that the judge was so troubled at the consequences of applying its pure logic in the Escamilla case that he chose instead to proceed on the basis of practical common sense and social responsibility, leaving it to others to split hairs over the meaning of commas in Section 7 of Title 18 of the U.S. Code or the analogies of ice islands with ships, guano islands, or commercial aircraft flying over the high seas.

Geophysical Evidence for Ancient Sea-Floor Spreading from Alpha Cordillera and Mendeleyev Ridge: ABSTRACT

This paper presents the geophysical findings from Fletcher's Ice Island (T-3) for the period 1962 to mid-1970. During this time the station traversed the Chukchi Rise, parts of the Alpha Cordillera and Mendeleyev Ridge, and the Chukchi, Mendeleyev, and Canada plains. The findings support the suggestion of earlier investigators that the Alpha Cordillera is a fossil center of sea-floor spreading. Five fractures were observed to cut the Mendeleyev Ridge and the Alpha Cordillera, and many other closely spaced fractures are suggested by topographic, magnetic, and gravity trends. Seismic reflection profiles show a buried topography similar to that of the Mid-Atlantic Ridge. Offsets in the apparent axial rift suggest that the fractures are transform faults. The angular relation etween the Mendeleyev Ridge and the Alpha Cordillera appears to result from a southerly displacement of the Alpha Cordillera crest along numerous en echelon transform faults. Magnetic anomalies are consistent with the sea-floor spreading hypothesis, but no spreading dates are available. A broad zone of low amplitude anomalies over the central and southern Canada Plain might represent the Kiaman Magnetic Interval in Permian time or similar quiet intervals in the early Mesozoic. A crustal gravity model based on a 600 km long gravity and bathymetric profile and an unreversed refraction measurement from Station Alpha shows the observed gravity to be consistent with a section of East Pacific Rise type with a 5 km thick oceanic layer overlying 27 km of anomalous (p = 3.15) mantle. The relation of this ridge to the surrounding continental geology is explored. It is suggested that this ridge generated a sea floor which was consumed by marginal trenches along the Lomonosov Ridge (then the northern margin of the European continental block), the northern Alaskan coast, and the Canadian Archipelago. Sea-floor generation and consumption apparently ended with the separation of Greenland from Labrador in late Mesozoic time. The age of the basin is reflected in the large sediment thickness observed. Seismic reflection profiles show more than 2 km of sediment beneath the Mendeleyev and Canada plains, with prominent reflectors suggesting major climatic or depositional changes. Sediment cover on the ridge varies from several hundred meters to more than a kilometer. Sedimentary ridges blank t the crestal plateau of the Alpha Cordillera, apparently the result of currents which transport sediment across the crest from northwest to southeast. This process is presently inactive, and may have terminated with the initiation of continental glaciation as far back as late Miocene time. Similar sedimentary structures 700 m beneath End_Page 2483------------------------------ the Mendeleyev plain suggest a strong bottom circulation in the past. A zone of bottom erosion along the Mendeleyev Ridge flank may reflect a circulation of water through the Cooperation Gap, a trough which appears to cross the ridge. Two buried channels extending to subbottom depths of 700 m were observed between the Mendeleyev fracture zone and the Mendeleyev plain. End_of_Article - Last_Page 2484------------

Nepheloid layers and bottom currents in the Arctic Ocean

Between 1965 and 1969, fifty-one profiles of light scattering were made in the central Arctic Ocean from Fletcher's Ice Island (T-3). The profiles, taken with an in situ photographic nephelometer extend from just below the surface to the bottom. Two distinctly different types of profiles were observed. At all stations the strongest scattering occurs near the surface, decreasing with depth in the upper layers. Over the Canada abyssal plain, light scattering is almost constant below 2000 meters, decreasing slightly with depth all the way to the bottom so that the bottom water is the clearest. Over the ridges and rises surrounding the Canada basin, however, scattering increases with depth below an intermediate scattering minimum. The zone of deep light scattering on the ridges and rises is called the bottom nepheloid layer. The bottom nepheloid layer is evidently caused by fine material that is maintained in suspension by the turbulent flow. Four spot measurements of bottom currents on the Mendeleyev ridge gave speeds of 4 to 6 cm/sec. One spot measurement over the Canada abyssal plain gave a speed of less than 1 cm/sec. This indication of swifter current speeds over the ridges is supported by bottom photographs in which animal tracks are much less evident on the ridges than on the Canada abyssal plain in spite of a greater abundance of life on the ridges. This is attributed to the higher current speeds, which obliterate the tracks. The observations suggest a counterclockwise deep circulation in the Canada basin with the currents confined primarily to the sloping margins of the basin. This pattern of deep circulation is in agreement with ideas and experiments on deep circulation with a concentrated source and distributed surface sink. Deep water enters the basin over a sill and leaves by upward diffusion through the halocline into the surface water, which then flows out of the basin.

Physical Oceanography in the Arctic Ocean: 1968

The support of drifting stations and other scientific work on the pack ice by the Naval Arctic Research Laboratory is basic for the US studies of the central Arctic Ocean. A synoptic picture of temperature/salinity variables is needed, as is information on spatial variations of the currents. During the summer 1967, the satellite navigation system gave fixes of Fletcher's Ice Island T-3, from which water motion at depths of 150, 500 and 1300 m was recorded. Results of analyses are graphically shown and discussed; the technique used to smooth the track, noted. The flow is similar in the three water masses; a long-term period variation in the record probably represents the T-3 motion; there is a marked oscillation of ~semidiurnal period which undoubtedly represents an inertial or semidiurnal tidal oscillation. Tracks of drifting stations during 20 yr define the Beaufort gyre circulation, the center of which, 80 W, 140 W, coincides with that of a mean atmospheric pressure anticyclone. West of 140 W, the dominant W and N drifts are segments in the Transpolar Drift Stream; east of 140 W, where drifts are mainly E to W, they are in the eastern Beaufort gyre. Ice drifts faster in the Transpolar Stream than along the Canadian Islands by about 1/2 nautical mi/day; a marked increase in speed occurs in summer or fall; the atmospheric pressure gradient variations agree qualitatively with the drift speed variations. Three experiments needed to solve the principal remaining problems are explained.

Inertial oscillations of Fletcher's Ice Island (T-3)

Observations with improved Roberts current meters, tethered drogues, and fathogram highlights are used to show that Fletcher's Ice Island (T-3), in the Arctic Ocean, often moves in clockwise circles with a diameter of about 1 km and a period of about 12 hours. The motions are inertial oscillations which represent the transient response of a floating ice mass to changing wind stress. Since the winds are often fluctuating, T-3 responds often with inertial motion. The following arguments indicate that these motions are inertial oscillations: (1) the period of the motions is closer to the inertial period, at this latitude, of 12.05 hr than it is to the lunar semidiurnal tidal period of 12.42 hr; (2) the amplitude of the periodic motion and the local wind speed are closely correlated; (3) the phase of the motion changes irregularly with time; and (4) the motion is restricted to the ice and to the uppermost layers of water.

The Soviet Drifting Ice Station, NORTH-67

In April 1967, one of the Arctic Research Laboratory's aircraft made two landings at the Soviet Drifting Ice Station NORTH-67. The station was on a floe about 2 metres thick which was somewhat cracked around the perimeter. The runway was in excellent shape, about 5,000 feet long and 125 wide, slick and hard. It was on a refrozen lead attached to the camp floe. Snow was scraped off, rather than dragged, with the use of tractors similar to small U.S. farm tractors. The first landing was made on 15 April while the aircraft was en route from Point Barrow to Fletcher's Ice Island T-3. The Soviet Drifting Ice Station was then at 76°40'N., 164°40'W., almost on a line between Point Barrow and T-3. The pilots decided to land at the Soviet station in part to encourage friendly relations between two groups engaged in comparable scientific research and combating the same austere environment and in part to satisfy a very human curiosity. Pilots Clifford Alderfer and Richard Dickerson, with their passengers from the Arctic Research Laboratory, Frank Spik, Al Magnuson, Simeon Akpik, and Loeb Wood, were cordially welcomed, and stayed from 1650 to 1830. ...

The seasonal variation in the sound-scattering layer observed at Fletcher's Ice Island (T-3) with a 12-kc/s echo sounder : Hunkins Kenneth, 1965. Deep-Sea Res., 12 (6): 879–881.

The seasonal variation in the sound-scattering layer observed at Fletcher's Ice Island (T-3) with a 12-kc/s echo sounder : Hunkins Kenneth, 1965. Deep-Sea Res., 12 (6): 879–881.

The seasonal variation in the sound-scattering layer observed at Fletcher's ice island (T-3) with a 12-kc/s echo sounder

FOR NEARLY tWO years, from June 1963 to the present, an echo sounder has been operated continuously at Fletcher's Ice Island (T-3), a drifting research station in the central Arctic Ocean. Diffuse reverberations are frequently seen on the echograms during the summer months. These reverberations are similar in general appearance to the deep scattering layers of other oceans. In non-polar oceans, the deep scattering layer is usually found at depths between 200 and 600 m during the day. At night it rises almost to the surface and weakens or disappears. Two important features distinguish the Arctic Ocean scattering layers from these non-polar scattering layers : (1) the Arctic Ocean scattering layers are relatively shallow, 50-200 m, and (2) the Arctic scattering layers have a pronounced annual, rather than diurnal, cycle. These effects are probably related to light conditions peculiar to polar oceans. First, the light is relatively weak beneath the permanent ice cover. Second, the d a y becomes effectively one year long at these high latitudes so that the Arctic scattering layers are present at moderate depths during the summer light period and then disappear during the winter dark period. In addition to the scattering layers, discrete echoes from shallow depths are frequently recorded. These discrete echoes occur throughout the year but are particularly frequent during the winter. These echoes take a hyperbolic shape indicating relative movement between the ice station and the reflector. Presumably they are caused by nekton such as fish or seals. This is believed to be the first observation of sound scattering layers in the central Arctic Ocean. An examination by DIETZ and SHUMWAY (1961) of echograms taken on the nuclear submarines Nautilus and Skate during their Arctic Ocean cruises revealed no deep scattering layers or discrete echoes. However, the submarines probably cruised within or below the scattering layer. The echo sounder at the ice station was better placed for detecting a shallow scattering layer. T-3 drifted along the track shown in Fig. 1 during the period of echo sounding measurements. The Precision Depth Recorder system consisted of a standard Edo transducer, a spark-type drum recorder and a sonar transceiver manufactured by T. H. Gifft and Associates. The system operated with an electrical power output of 800W at a frequency of 12 kc/s. The outgoing ping was 80/zsec (l cycle) in length and was repeated once each second. The drum rotated once each second

Ice Drilling in Fletcher's Ice Island (T-3) with a Portable Mechanical Drill

Reports an Apr 1964 attempt by General Motors Corp engineers to assess the feasibility of using the Houston Model V-100 portable drilling rig (illus) for drilling through ice islands. When used in its "pressure mode", it proved satisfactory to a depth of 100 ft. A method by which the drill hole can be kept free of ice for subsequent investigations, using diesel oil and an electric heat drill is described.

Arctic ocean geophysical studies: Chukchi Cap and Chukchi abyssal plain

A bathymetric chart of the Chukchi Cap region was compiled with soundings obtained from Fletcher's Ice Island (T-3), as well as from other ice stations and from U.S. Navy icebreakers. New details of the Chukchi Cap are shown, including two submarine troughs on the southwest side. West of the Chukchi Cap, a small abyssal plain was found with a depth of 2230 m. This abyssal plain is connected through an abyssal gap with the deeper Canada Abyssal Plain.The prominent magnetic anomaly discovered during the drift of Station Charlie was crossed more recently by T-3 and by aeromagnetic flights. The continuity of the anomaly along the western and northern sides of the Chukchi Cap was further established by the new measurements. An interpretation was made of the anomaly as an expression of induced magnetization in basement rocks. The interpretation shows a basement ridge beneath the anomaly maximum at the edge of the Chukchi Cap. The Cap itself is interpreted as being underlaid by a 12 km thickness of sediments. Both magnetic and gravity data were used for an interpretation of total crustal thickness along the same section. Crustal thickness ranges from 18−12km beneath the Chukchi Cap to 32 km beneath the large basement ridge.