Nature © Macmillan Publishers Ltd 1997

In the small Ikka Fjord in southwesternGreenland, we have studied remarkable

submarine tufa columns forming over alka-line springs by abiotic precipitation of themetastable, cold-water mineral ikaite(CaCO3.6H2O)1–3. The columns, which sup-port a rich marine life, are up to 20 m high,with annual growth of more than 50 cm.We wish to nominate the locality, whichseems to be unique, for classification as ageological World Heritage Site.

Ikka Fjord is a glacial valley, flanked bysteep, 500-m-high, flat-topped mountainsof Precambrian gneiss4. Syenitic and car-bonatitic rocks of the 1,300-million-year-oldGrønnedal–Ika igneous complex cut thefjord in a 3-km-wide belt with a north-westerly trend5. The fjord water is marineexcept for the uppermost 1–2 m, wherefreshwater runoff lowers the salinity to*20‰. The ikaite columns are in the shal-low, innermost part of the fjord (Fig. 1a).They are visible from the surface duringperiods of calm weather and at low tide asgreenish-white, pointed or mushroom-shaped towers.

We observed the columns in the fjord bysub-aqua diving, side-scan sonar andacoustic profiling. Columns are restricted toa 0.75 km2 ‘garden’ area, where there aremore than 500 individual columns of1–20 m in height. They form upright, trunk-like structures with diameters from a fewcentimetres to several metres (Fig. 1b). Thearea corresponds to shoreline outcrops of theGrønnedal–Ika complex. Columns are root-ed in bottom sediments, at the sides of rockyexposures or in massive, dome-shaped tufabuild-ups.

Often the columns occur as clustersgrowing from a common base. Branching israre except at the top of the highestcolumns, where metre-wide, saucer-shapedcrowns may form close to the surface.Growth zones consist of white, fine-crystalline ikaite without algal overgrowth,which contrasts with the greyish-green,algal-covered older parts of the columns.Growth is generally confined to the top ofthe columns, but is also evident as finger-shaped extrusions elsewhere on thecolumns. Cross-sections and radiogramsshow that the inner structure is porous andsometimes contains an irregular conduit.Coralline red algae (Lithothamnion andClathromorphum) encrust the lower part ofthe columns and thereby stabilize the deli-cate column structures.

Pauly6 has suggested that the columnsform by seepage of fresh water from thebottom of the fjord. To test whether seepagetakes place we cut eight columns one metrefrom their bases and fastened watertight

sample bags to the stumps. The bags werefilled within days by water that was lessdense than the surrounding sea water. Inmost cases, a precipitate of millimetre-sizedikaite crystals could be seen on the stumps.One stump, which we revisited after oneyear, showed vertical growth of more than50 cm (Fig. 1b).

We took pore-water samples directlyfrom columns with underwater syringes.The seep water is a sodium bicarbonate andsodium carbonate brine with high pH, highalkalinity and high phosphate content. Thecalcium concentration is significantly lowerthan in sea water (Table 1). The hydrogenand oxygen isotope composition of thewater is close to that of lake and streamwater sampled on the top of the intrusivecomplex. Cold homothermic springs(3.0–3.5 °C) surrounded by luxuriant plantcommunities, including three species oforchids, are found on the fjordward side ofthe intrusion. The springs have raised alka-linities (pH 8–9), in contrast to the nearlydistilled waters of springs and brooks out-side the intrusion area, and contain remark-able marine fauna elements such as saltmites (Halacarida).

The juxtaposition of ikaite columns,springs and carbonatites, and the total lackof columns outside the intrusive area, indi-cate a link. The highly fractured intrusivecomplex probably acts as an aquifer forgroundwater originating from precipitationon top of the intrusion. Dissolution of sodi-um-rich carbonate minerals in the intrusionwould increase the alkalinity and sodiumcontent of the water. If such alkaline ground-waters mix with sea water that is high in cal-cium at seeps on the bottom of the fjord,

immediate supersaturation would lead torapid precipitation of calcium carbonate.

The high phosphate content of seepwater and low temperatures at seepage sitesfavour precipitation of ikaite rather thancalcite or aragonite7. The elongated shape ofthe monoclinic ikaite crystals causes theformation of the permeable framework ofthe columns and thereby ensures their ver-tical growth. In this model, seeps literally

NATURE | VOL 390 | 13 NOVEMBER 1997 129

Submarine columns of ikaite tufascientific correspondence

Table 1 Composition of seep water comparedwith local sea water

Component Seep water Sea water

Conductivity (mS cm11) 18.2 42.5

Salinity (‰ ) 9.3 31.1

Temperature (°C) 4.0 3.6

pH 10.4 8.1

Na& (mmol l11) 198 413

K& (mmol l11) 1.9 9.5

Ca2& (mmol l11) 0.17 8.9

Mg2& (mmol l11) 1.7 45.7

Cl1 (mmol l11) 21.2 506

SO421 (mmol l11) 2.8 28.6

PO431 (mmol l11) 0.26 below detection

Alkalinity (mmol l11) 153 *0.5

d2Hwater ‰ SMOW 195.7 110.4

d18Owater‰ SMOW 113.4 10.9

Seep water was obtained by underwater pumping ofpore water from an active column at 10 m water depth.Seawater for comparison was sampled 1 m from thecolumn. Conductivity, pH and alkalinity were measuredin the field. Alkalinity is expressed as total alkalinity.Isotopic values are given as ‰ deviations fromstandard mean ocean water (SMOW).

Figure 1 Distribution and typical shape of the ikaitetufa columns in Ikka Fjord. a, Bathymetric map ofthe inner part of the fjord, where locations of thelargest columns are shown in red. Columns developimmediately beyond the shallow waters of the deltaat the top of the fjord and occur continuously up tothe regional fault line bisecting the fjord. Syenitesand carbonatites belong to the Grønnedal–Ikaigneous complex. b, Ikaite column at 9 m waterdepth. Red and white subdivisions on the pole are20 cm each. Newly formed ikaite above the obliquecutting surface represents 13 months of growth.Older column below the cut surface is covered withLithothamnion and other algae.

Nature © Macmillan Publishers Ltd 1997

reading have a saccadic character and showmany of the features that characterize eyemovements.

Our subject, AI, is a 21-year-old femaleuniversity undergraduate. As a result of an,apparently congenital, extraocular muscu-lar fibrosis resulting in ophthalmoplegia, AIhas had no eye movements since birth.However, she reports no major visual prob-lem associated with her deficit and receivesno extra assistance, either with reading orwriting, in her studies.

The presence of an optokinetic nystag-mus (OKN) response is often taken as anindication of the presence of any residualeye-movement function2. We used a sensi-tive eye-movement recorder3 to track hereyes during fixation and when presentedwith a large-field sinusoidal grating. Theonly eye movements that we recorded werevery restricted drift movements (±0.5 deg atmost) which were not linked to the stimu-lus motion. Normal subjects showed a stan-dard OKN response to this stimulus whichcould not be suppressed.

Reading provides one of the clearest, and

When reading text, human subjects use apattern of eye movements consisting of fastsaccadic movements and fixations1. Wehave found a subject who cannot make eyemovements. Her visual perception is sur-prisingly normal and she is able to read athigh speeds. She uses movements of thehead to compensate for the absence of eyemovements. Her head movements during

create their own conduits in the form ofvertical, chimney-like columns. Only theformation of sea-ice during the winterwould prevent further upwards growth ofthe columns.

To our knowledge, the Ikka columnsrepresent a phenomenon unique in theworld. The now inactive tufa towers foundat the shores of Mono Lake8 and PyramidLake9,10, in the western United States, mayrepresent structures formed in a similar wayto the Ikka columns, but in non-marineenvironments. The high scientific and aes-thetic value of the Ikka columns make themappropriate for international protection.Bjørn Buchardt, Paul Seaman*Gabrielle Stockmann, Marie VousUffe WilkenGeological Institute, University of Copenhagen,Øster Voldgade 10, DK-1350 Copenhagen K,Denmark *and Department of Geology,Imperial College of Science, Technology and Medicine,London SW7 2AZ, UKe-mail: bjornb@geo.geol.ku.dkLene Düwel, Aase KristiansenBotanical Institute, University of Copenhagen,Ø. Farimagsgade 2, DK-1353 Copenhagen K,Denmark Christopher JennerETSU, Harwell Laboratories, Oxford OX11 ORA, UKMichael J. WhiticarSchool of Earth and Ocean Sciences,University of Victoria, PO Box 3050, Victoria,British Columbia, V8W 2Y2 CanadaReinhardt M. KristensenGodtfred H. Petersen, Lone ThorbjørnZoological Museum, Universitetsparken 15, DK-2100 Copenhagen K, Denmark

1. Pauly, H. Arctic 16, 263–264 (1963).

2. Marland, G. Geochim. Cosmochim. Acta 39, 83–91 (1975).

3. Suess, E. et al. Science 216, 1128–1131 (1982).

4. Berthelsen, A. & Henriksen, N. Ivigtut 61 V.1 Syd. Geological

Map of Greenland 1:100,000 (Geol. Surv. Greenland,

Copenhagen, 1975).

5. Emeleus, C.H. Medd. Grønland 172(3), 1–75 (1964).

6. Pauly, H. Naturens Verden Argang 1964, 168–192 (Copenhagen,

1963).

7. Johnston, J., Merwin, H. E. & Williamson, D. E. Am. J. Sci. 41,

473-493 (1916).

8. Bischoff, J. I. et al. Geochim. Cosmochim. Acta 57, 3855–3865

(1993).

9. Shearman, D. J., McGugan, A., Stein, C. & Smith, A. J. Geol. Soc.

Am. Bull. 101, 913–917 (1989).

10.Benson, L. Palaeogeogr. Palaeoclimatol., Palaeoecol. 109, 55–87

(1994).

scientific correspondence

130 NATURE | VOL 390 | 13 NOVEMBER 1997

best characterized1 examples of how visualprocesses and eye movements are coordi-nated to acquire information. Eye move-ments during reading have a number ofdefining features. The text is scanned in asaccadic manner, the eyes alternatingbetween short fast movements and fixa-tions, where the eye is stable. Most saccadesare to the right, although a small propor-tion are regressive. Rightwards saccades aretypically between seven and nine characterslong, although the length can vary consid-erably depending on the text and the indi-vidual reader. Steady fixation is maintainedbetween saccades, typically lasting between200 and 250 ms. It is during fixation thatinformation is gathered from the text. Atthe end of each line, subjects make a largeleftwards return saccade that orients theeyes to the beginning of the next line.

AI’s overall reading speed for standardpassages of text with a wide range of difficul-ty was 257 words min11 (range 183 to 435),which is consistent with slow but not abnor-mal reading1. This impressive reading speedis supported by movements of her head (Fig.

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Figure 1 Recordings of head and eye movements. a, Head movements from AI (left) and eye movementsfrom a control subject (right) during text reading recorded using a head-mounted search coil. Gaze positionwas sampled at 40 Hz. The horizontal eye or head displacement (left is down) is plotted against time. Theslight overshoot in the eye-movement records is due to lens distortion7. AI is considerably slower overalland her head stability is not as good as that of the control subject. There seems to be a small, high-frequency tremor, probably reflecting the inherent instability of the head over the eye. b, The distribution offixation durations and head movement sizes during single-sentence reading. Records were divided into fixa-tion and movement periods on the basis of a velocity exceeding 10 deg s11 over a 125-ms period. This elim-inates the small-tremor head movements during fixation but allows for the detection of the onset of largermovements. AI’s head movements are characteristic of normal eye-movement during reading. c, AI’s headmovements while viewing pictures, taken from the study of picture scanning by Yarbus4. The pictures wereviewed for 20 s and head position was sampled at 100 Hz .

Saccades without eye movements