AesraAcr: Small rafts of floating sediment, predominantly of granule and small pebble size, were observed along a tidewater coast on James Ross. Island ...
G R A V E L A N D S A N D F L O T A T I O N : A S E D I M E N T D I S P E R S A L P R O C E S S I M P O R T A N T IN C E R T A I N
NEARSHORE MARINE ENVIRONMENTS PER MOLLER ~ o OLAFUR INGOLFSSON Department of Quaternary Geology. Lund UntversiO,.Sflvegatan 13. S-223 62 Lurid, Sweden AesraAcr: Small rafts of floating sediment, predominantly of granule and Weddell Sea is noted for heavy ice conditions and strong tidal currents small pebble size, were observed along a tidewater coast on James Ross (The Antarctic Pilot 1974, p. 213-215), with a tidal range of about 3 m. Island, Antarctica. The sediment was Hfted from the beach by the ad- At the time of one of our observations the tide was rising, the air temvancing tidewater and kept allunt by surface tension. The current trans- peraature was 0*C, sea temperature +3 _+ 0.5"C, and there was no wind ported the rafts at least 100-150 m off the beach, where the rafts broke to generate waves in the lee of the coastal bluff. About 100-150 m offthe up dee to wind agitation of the sea surface and the gravel sank. This process coast a slight breeze caused small waves to form on the sea surface. can transport considerable volumes of sediment from the beach: it is esAs the tidewater inundated the gravelly beach, patches of gravel, one timated that flotation at the Naze launch ca. 500 kg per kilometer of grain thick, were lifted up and gathered in small rafts (Fig. 2A-C). Indishoreline per tidal cycle during calm and dry weather conditions, which vidual rafts were irregular in shape, from a few to about 600 cm2 (apfor one summer season might end up in a total of 0.022 tons of sediment proximately 20 cm × 30 cm). Figure 2C shows how the water surface is per meter of shoreline. The resulting sediments and sedimentary structures bent down beneath the rafts, and Figure 2B shows that the lift force is are concluded to be similar to those resulting from iceberg-, sea ice-, and strong enough even to support a camera lens cap put on one raft. After algae rafting, and sediment gravity flows. Floating can he an important the initial flotation the rafts were then moved outward and along the beach way of rafting gravels into fine-grained sublittoral or shallow-marine facies, to the northeast. This flotation occurred along at least a 2 km stretch of and should be included when dealing with processes related to sedimen- the beach. The rafts floated outward until they reached the wind-agitated tation in the nearshore marine environment. region offshore, where they broke up and sank. The process was observed on several occasions under similar physical conditions. The driving force for the flotation process thus is a rising tide under INTIRODUCTION very calm conditions. The shorefaee is quite steep (> 100), whereas the During a lithostratigraphic study of Late Quaternary sediments along a gravelly beach in front of the shore bluffhas a low gradient (ca. 5°). These coastal section on the Naze, .lames Ross Island, Antarctica (Fig. I) in circumstances make the tidewater encroach upon the beach very slowly, January 1993, we noticed that small rafts of predominantly granules and while the sea water penetrates the porous beach subaerially, forming small pebbles were lifted up from the beach along the advancing waterfront groundwater above the shallow permafrost. Rising tide causes the groundof a rising tide. These rafts were advected seaward and along the coast. water table to rise, causing a laminar, upward-directed flow through the We were struck by the effectivenessof the flotation process and the coarse- sediment. The uppermost beach-surface layer, one particle thick (and dry; ness of floated clasts. Floating coarse sediment was first described by this is a prerequisite), is thus wetted from below and lifted gently and held Hennessy (1871), and his study was followed by many observations near afloat on the water surface by the strength of surface tension. The incoming the beginning of this century (see reviews by McKelvey 1941 and Syvitski tide causes an inward-directed laminar underflow and an upward/outwardand van Everdingen 1981). These studies showed that the process is not directed overflow. The latter was shown by the oblique outward transport exclusive to cold environments. More recently, flotation of sand and coars- of the gravel rafts. When the mils reach turbulent water the surface-tension er sediment has been studied by Hume (1964) and Syvitski and van strength is lost and as a result the rafts are broken up and the floated Everdingen (1981). Hume (1964) and Gilbert (1990) believed that the particles sink. Stranded sea ice and small berg blocks were also lifted off the beach process is quantitatively unimportant or minor in many dynamic coastal environments, but Syvitski and van Everdingen (1981) showed that very during rising tide. These camed sand and gravel clasts adhered to their large volumes of sand were transported by flotation in a sheltered tidal- base, but by what could be visually judged, in very small volumes comflat environment. They also stated that "future sediment transport studies pared to the flotation process. should not ignore the process of flotation" (Syvitski and van Everdingen 1981, p. 1321). The purpose of our study is to evaluate the importance of flotation in certain nearshore marine environments and its signature in the resulting sediments. rmAT~SgGRAVELIN ~rsacncA Field ObstrFatiom The Naze is a peninsula extending into Herbert Sound between James Ross Island and Vega Island in the westernmost Weddell Sea (Fig. 1). The coast where flotation was observed is bordered by a bluff 10-15 m high, eroded into a complex of middle Holocene till and glaciomarine sediments on top of Cretaceous clay shale (Ing61fssonet aL 1992). The bluffis probably formed by a combination of wave action during occasional strong summer storms at high tide and thermokarst erosion. This produces a gravelly beach below the bluff as line sediment is transported offshore during the storms. Most of the year the coast is sheltered by an extensive cover of sea ice and stranded icebergs, and open water usually persists only during the summer, between December and March. This part of the
Laboratoo Analysis We sampled floated sediment from the sea surface, and grain-size analyses of two samples (Fig. 3) show well sorted, slightly sandy gravel, with maximum diameter (b-axis) of - 3¢~(8 mm). Granules make up 60% and small pebbles 29% of the floated sediment. Axis measurements on the -2.5 to - 3 ¢ pebble mode show a mean a-axis length of 8.8 mm and that the largest sampled particle measures 15 m m x 7.9 m m x 4.5 ram. A surface sample of beach gravel just above the waterline, taken from a 10 cm x 10 cm area (Sample 3, Fig. 3), shows less soaing and indicates that clasts larger than - 3~ are not floated. The grain shape of the smallpebble population of Sample 2 is shown in Figure 4, split into two size fractions. From visual estimation and measurements of grain a-axes the largest particles are blade- to disc-shaped, whereas the smaller grains in one sieve population are more spheroidal. Pebble-size clasts are predominantly angular to subangular and none show any surface striations. The clasts are predominantly alkaline basalts; sandstone and limestone are minor constituents. The mean density p, of the sampled gravels is 2.25
JOUI~SALOFSEDIMEiqTARY R~EARCH,VOL.A64, No. 4, OCTOBER,1994,P. 894-898 Copyright© 1994,SEPM(SocietyforSedimentaryGeology) I073-130X/94/OA64.894/$03.00
GRAVEL AND SAND FLOTATION
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Ft6. l.-Map ofJames Ross Island, Antarctica.Ice-freeareas on northern James Ross Island are shaded. Insert map showsthe locationof James Ross Island (box at black arrow), located east of the Antarcticpeninsula. Punctuated line on insert map is the extension of ice shells. g/cm 3, thus indicating that individual clasts have some voids. Under the microscope many particles showed surfleial pitting.
Quutit=tive Estimates It is difficult to estimate the quantity of sand and gravel transported by flotation, but we made the following attempt for one 90 minute period of observations at the end of the rising tide cycle. As a minimum estimate for that period of observations, one 200 cm z raft departed from a 5 m stretch of waterline per minute. A raft of that size contains approximately 24 cm 3of sand and gravel, weighing ca. 28 g (dry bulk density 1.17 g/cm3). That gives a minimum estimate of 0.864 m 3, or approximately 1000 lg during the observation period for 2 km of shoreline (500 kg/km shoreline per tidal cycle). Calc~ated another way, if the rafts were broken up over a 50 m interval, let us assume 150-200 m offthe shore, and settled evenly over the sea floor, approximately 130 particles would be dropped per m 2 for one tidal cycle, which is equal to an input of 10 t/Inn2. Under periods of steady, calm, and dry conditions this would happen at each flood stage during the summer months with open water. It is reasonable to estimate that out of approximately lg0 tidal cycles with open-water conditions, flotation could be active during one day out of four. Forty-five tidal cycles could then float approximately 0.022 t/m shoreline. This estimated sediment input can be compared to those of different kinds of sea-ice rafting. Barnes et al. 0993) calculated the incorporation of beach sediment into an annually formed nearshore-ice complex (NIC) at Lake Michigan, U.S.A., as 0.23 tons per meter of shoreline, which is an order of magnitude higher than our estimate for flotation at the Naze. Reimnitz et al. (1992) calculated the sediment load entrained into sea ice by suspension freezing to be 810 t/kin 2 from the Northwest Passage of the Arctic Ocean. Sea-ice rafting operates on a seasonal scale and releases
FiG.2.-A) Overviewof the Naze coast during firing tide. All dark patches are gravelrafts floatingon the water surface. B) Verticalclose-upof gravelrafts above a totallysubmergedboulder. Note the agglomerationof granulesand smallpebbles into rafts. One raft supports a camera lens cap put onto it for scale. C) Close-up of gravel rafs that havejust been liRedabove the beach surface. Note the downbending of the water surfacebeneath the rafts and along their edges.Water depth is about 5 cm. debds over a large area upon ice melting, which results in a substantial reduction in sediment input per km 2 of sea floor.
Why Does tke GravelFloat? Sea-water salinity was measured to be 32.2%~, which at 3°C corresponds to a density pw of 1.03 g/cm3. The density ratio P,/pw between our solid particles (p,= 2.25 g/cm3) and sea water is thus 2.18. At a density ratio > 1 a solid panicle usually sinks if immersed in quiet water. The panicle
896
PER MOLLER AND OLAFUR INGOLFSSON
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must, however, first penetrate the air-water interface. This is resisted by the surface tension of the water. McKelvey (1941) showed that the maximum diameter of a spherical particle that can float on water is given by d = {6T - (p+ - p , ) - ' , g-l}b~2
of incoming water on the floatability of sediment on a low.gradient sandy tidal flat. The tide came in as a nearly horizontal flow with velocities above the threshold entrainment velocity for particles larger than medium sand. It was concluded that the largest floatable grain size decreases with increasing flow velocity, and that the upper size limit is medium sand under normal flow velocities of an incoming tide. This is not supported by our field observations, which show that much of the floated clasts are around and above the theoretical maximum size. This must be explained by differences in environmental setting. The field test area of Syvitski and van Everdingen (1981) was a tidal flat with a very low gradient where the tide comes in as a tidal wave with a horizontal flow velocity that is above the critical velocity for particles larger than medium sand. At the Naze, with a much steeper shore gradient, the tide does not come in as a tidal wave, but causes a vertically upward-directed flow through the beach gravel, which does not exceed the critical velocity for the floated sediment. The effectiveness of flotation thus seems to be governed mainly by the availability of floatable and dry sediment on a beach, a relatively steep beach face, and a tidewater coast that at periods is in a sheltered position. Sediment in the lower part of the tidal zone at the Naze did not foat, probably because its surface layer did not dry out sufficiently between tides. The geographical distribution of reported observations of flotation (see reviews by McKelvey 1941 and Syvitski and van Everdingen 1981) indicates that Arctic/Antarctic coasts are favorable environments due to wave-breaking sea ice, although as previously stated, the process has been reported from several other environments.
(1)
where d is the diameter of the particle, T is the surface tension, p+ is the density of particles, p+. is the density of water, and g is the acceleration due to gravity. The surface tension at 3°C is approximately 75 dynes/cm (cf. Hume 1964). When applying Eq I with the input values above, the theoretical maximum diameter of a float,ableparticle should be 6.1 mm. This is considerably smaller than the largest clasts in our sampled gravel rafts. The difference must be due to grain shape: the largest ones are far from perfect spheres but are disc- to blade-shaped and angular, which increases the floatability (Syvitski and van Everdingen 1981). A reduction in salinity to 0%0, all other values being unchanged, would reduce the diameter of a spherical particle able to be floated by less than 0.1 ram; an increase in temperature to 20°(7 would reduce the diameter of a particle able to be floated by only about 0.1 mm. Salinity and temperature thus seem not to be important in controlling the size of floated clasts. Syvitski and van Everdingen (1981) studied the effect of flow velocity
DISCUSSION The presence ofterrigenous particles coarser than sand dispersed in finegrained marine mud, often making the bulk sediment bimodal, is usually a criterion for interpreting the sediment as glaciomarine (Fig. 5A). The introduction of the coarse member is usually claimed to be by ice(berg) rafting (IRD), i.e., release of debris from passing icebergs. Rafting of debris into a marine environment may, however, be quite complex, as shown by, e.g., Gilbert (1990), who discussed the significanceof active and passive sea-ice rafting and rafting by aloe, as well as active and passive iceberg rafting. Thus, an interpretation of a coarse mode in marine sediments as glacial is not entirely straightforward. Independently of the transport processes, the ice-rafted debris is released and deposited as drop or dump sediment at the sea floor, and the coarse particles will then be almost randomly distributed in the marine mud. Depending on sedimentation variations in suspended-sediment fallout,
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GRAVEL AND SAND FLOTATION
the resulting deposit may then show up as a massive sediment or as a rhythmically interlaminated sediment (cyclopelsor cyclopsams; cf. Cowan and Powell 1990) as end members and with randomly distributed coarse particles as the result of rafting. Another way to introduce coarse particles into the marine environment is by sandy-gravelly gravity flows.These may emanate from the grounding line of a marine-based glacier (e.g., Domack and lshman 1993, fig. 11) or from failure of oversteepened delta/shoreface deposits (e.g., Nemec 1990) that may or may not be fed by glacial streams. If an area is dominated by suspended-sediment fallout but is subject to frequent passage of sandygravelly gravity flows that deposit their main load farther downslope, the resulting deposit may show up as massive clay/silt beds, interbedded with very thin and more or less continuous sand partings and/or pebble stringers (Mrller et al. 1992) deposited by fallout from the tail of the flow (Fig. 5B). We propose that settling of floated sand and gravel clasts might produce very similar sediments and sedimentary structures, as discussed above, in certain environments and under special conditions. In an area of low to moderate sedimentation rate of suspended sediment we expect that the resulting deposit will be a massive silt/clay with three-dimensionally dispersed and randomly distributed coarse sand, granules, and pebbles, identical to a marine sedimentary environmentwith iceberg and sea-ice rafting. However, iceberg rafting is most important in deep water, whereas our observed process probably is not. Thus the possibility of misinterpreting an iceberg IRD peak in deep-water, shore-distant marine sediment is not likely. A criterion that also distinguishes flotation from berg rafting is that it does not support clasts with a-axes larger than 10-15 mm, which berg rafting certainly does. We believe, however, that the coarse mode might be misinterpreted as being due to active rafting of shore ice (stranded sea ice and small bergs) with freeze-on of beach gravel at the soles of ice blocks and subsequent redistribution of this sand and gravel when the ice lifts. This process would operate over the same period of the year as flotation and, as well as the latter, would be governed strongly by the tidal cycle. As shown by our calculations above, in calm weather the transported volumes and effectiveness of flotation might strongly outstrip shore-ice rafting in producing marine mud strewn with coarse sand to pebbles, especially in the nearshore marine environment because of rapid settling over a limited area when the rafts meet rough water. Shore ice has the potential to move farther out with slower and more dispersed release of the load. The sediments produced will, however, be very similar in appearance except that shoreice rafting can distribute and release coarser particles (e.g., Dionne 1993) than flotation. In areas of high sedimentation rates (several millimeters to a few centimeters per day) of suspended sediment, e.g., distal to a glacial meltwater stream flowing into a marine basin, we might find another misinterpretation trap of the coarse mode in marine mud. We believe that release and settling of the floated sand and gravel load might produce a distinct stratum that quickly becomes buried by mud. Each calm-weather tidal cycle might then produce another distinct coarse sand- to pebble-strewn horizon, building up a stratified sequence. In exposures of limited lateral continuity and in sediment cores, this could easily be interpreted as due to the passage of sediment gravity flows (cf. Fig. 5B). CONCLUSIONS This study shows that flotation can, under favorable conditions, transport considerable quantifies of sand and gravel from the beach and deposit it in the nearshore environment. Small pebbles are the largest grains that can float. It is dif~cult to distinguish floated gravels from other rafted particles and gravel tails of sediment gravity flows. A clear preference for grain sizes of small pebbles and finer is the only criterion applicable, together with circumstantial evidence of a nearshore paleocnvironment in a sheltered tidewater basin. The geological significance of the process
FiG.5.-A) Massiveclay with scattered pebble-sizeclasts, consistingpredominantly of chalk (smallwhite dots). It is interpretedas a glaciaquatiesediment, derived from suspended-sedimentdepositionand icebergrafting(Land Diamicton, southernSweden;MalmbergPerssonand Lagerlund1990).B) Massiveclayey silt, interlaminatedwith very thin sand partingsand/or distinct horizonsof dispersed granulesand pebbles.It is interpretedas a glaeiomarin¢sedimentderived from suspended-sedimentdepositionof the massiveclayeysilt beds and by deposition from sandy high-densityturbidity currents, bypassingthe area (late Weichselianlower-alluvial-fandelta sequenceon Edge~a, Svalbard;MiSIleret al. 1992). is dependent on the magnitude of other sedimentary inputs to the basin, but under favorable circumstances flotation probably can exceed rafting by sea ice and algae. Hotation should therefore be included in conceptual models for sources and processes related to sedimentation in the ncarshore (giacio)mafine environment. ACKNOWLEDGI~NTS Thisworkwasfinancedbythe SwedishNaturalSoenceResearchCouncil(NFR) and Lund University.Travel and logistics were funded by the Swedish Polar
PER MOLLER AND OLAFUR INGOLFSSON
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Research Secretariat and the Argentinean Antarctic Institute (lnstituto An~rtico Argentino). We thank J.C. Dionne, E. Reimnitz and J.B. Southard, who provided valuable comments and suggestions for improving the manuscript. REFERENCES BARN~, P.W., gJ~Un~MA,E.W., REIMN,~., E., McCoRulCK,M., WEaE~,W.S., ANDl-hvI~,, E.C., 1993: Beach profile modification and sediment transport by ice: an overlooked process on Lake Michigan: Journal of Coastal Research, v. 9, p. 65--86. CowA,, E.A., ~HDPOWELL,R.D., 1990, Suspended sediment transport and deposition of cyclically interlamioated sediment in a temperate glacial Oord, Alaska, U.S.A., in Dowdeswell, J.A., and Scourse J.D., eds., Glacimarine Environments: Processes and Sediments: Geological Society of London Special Publication 53, p. 75--89. DION~E, J.C., 1993, Sediment load of shore ice and ice raring potential, upper St. Lawrence Estuary, Qu6bec, Canada: Journal of Coastal Research, v. 9, p. 628....646. DOMA~, E.W., A,o IS, M~S, S., 1993, Oceanographic and physiographic controls on modern sedimentation within Antarctic fjords: Geological Society of America Bulletin, v. 105, p. 1175-1189. G,La~RT,R., 1990, Rafting in glacimarine environments, in DowdeswelL J.A., and S¢ourse LD., eds., Glacimarine Environments: Processes and Sediments: Geological Society of London Special Publication 53, p. 105--120. H ~ v , H., 1871, On the flotation of sand in a tidal estuary: Geological Magazine, v. 11, p. 316..-318. H~u~ J.O., 1964, Floating sand and pebbles near 8arrow, Alaska: Journal of Sedimentary Petrology, v. 23, p. 523-536.
]~G6LFSSON,O., ]'IJOl~T,C., BJORCI~,S., ANt)SMITH,R.I.L, 1992, Late Pleistocene and Holocene glacial history of James Ross Island, Antarctic Peninsula: Boreas, v. 21, p. 209.-222. MALMBERGPER.gSON,K., ANDLAGERLUND,E., 1990, Scdimentology and dcpositional environments for the Land Diamicton, southern Sweden: Boreas, v. 19, p. 181-199. McKztv~v, g£., 1941, The flotation of sand in nature: American Journal of Science, v. 239, p. 594-607. M/iLl,r, P., IOONBOrC,C., Joamsso,, K., AnDSTUS~UP, O.P., 1992, The Quaternary history of Visdalen, Edgeaya, eastern Svalbard, in Mbller, P., Hjort, C., aM Ing61fsson,O., ¢ds., Weichsclian and Holocen¢ Glacial and Marine History of East Svalbard. Preliminary Report on the PONAM Field Work in 1991: Land University, Department of Quaternary Geology, LUNDQUA Report, v. 35, p. $5-137. N~EC, W., 1990, Aspects of sediment movement on steep delta slopes, in Colella, A., and Prior, D.B., eds., Coarse-Grained Deltas: international Assooation of Sedimentologists Special Publication 10, p. 29-73. RE~Mr~,~E., MAglSCOVlCH,L., McCoaM,cK,M., ~D BRaC~S,W.M., 1992, Suspension freezing of bottom sediment and biota in the Northwest Passage and implications for Arctic Ocean sedimentation: Canadian Journal of Earth Sciences, v. 29, p. 693-703. Swtrsg,, J.P., ANOvAN EVERDINGES,D.A., 1981, A revaluation of the genlogic phenomenon of sand flotation: a field and experimental approach: Journal of Sedimentary Petrology, v. 51, p. 1315-1322. T,E AmARCr~CI~LOT,4th Edition, 1974, The Hydrographer of the Royal Navy, LondoiL 333 p. Received 17 February 1994; accepted 26 April t994.
