Oceanographic and physiographic controls on modern sedimentation within Antarctic

EUGENE W. DOMACK Geology Department, Hamilton College, Clinton, New York 13323 SCOTT ISHMAN U.S. Geological Survey, Reston, Virginia 22092

ABSTRACT INTRODUCTION biological facies distributions to observed oceanic and glaciologic regimes within bays Physical oceanographic data and modern When compared to investigations of other and fjords, and present a quantified treatment surface sediments were collected from eleven depositional systems, the study of glacial ma- of modern glacial marine sediments that were Qords along the western side of the Antarctic rine deposits and processes is still in its in- collected from the western side of the Ant- Peninsula and South Shetland . Surface fancy (Reading, 1986, p. 522; Dowdeswell arctic Peninsula and South Shetland Islands sediment samples (62) were analyzed for tex- and Scourse, 1990; Anderson and Ashley, (Fig. 1). The data presented within this paper ture and total organic carbon content. The dis- 1991). This is partly due to the difficulty in- will also serve as a guide to interpreting the tribution of biogenic and terrigenous facies volved in conducting field studies of modern depositional record of Pleistocene/Holocene within the Qords is controlled by geometry environments but is also due to the real com- sequences along the Peninsula. and oceanographic regime. Climate plays a sec- plexity of glacial marine deposystems. This ondary role but, along with ice drainage basin complexity arises because of the great diver- SETTING size, controls the rate of terrigenous supply to sity of sediment sources and processes that the glacial marine environment. Specifically, introduce and redistribute sediment compo- The western margin of the Antarctic Pe- Qords along the Danco and Palmer Ar- nents in the glacial marine realm (Syvitski, ninsula represents an area where climatic chipelago with a high length to width ratio tend 1989). For instance, at least nine distinct conditions are transitional between polar and to have bottom sediments that are arenaceous mechanisms can be recognized for the re- subpolar. Mean summer temperatures are where ice-rafted sediment is released preferen- lease of terrigenous debris into the ocean just at, or slightly above, 0 °C (Fig. 2), result- tially at the head of the Qord. Biogenic facies (Anderson and others, 1983; Drewry, 1986; ing in limited surface melting and runoff. Late are favored where the bay geometry is com- Drewry and Cooper, 1981; and Kellogg and summer snow lines vary in elevation from plex. Where such complexity exists, separate Kellogg, 1988). Because of this complexity, —150 m (South Shetland Islands) to tens of oceanographic regimes develop that lead to few guiding principles have been established meters (Danco Coast and Palmer Archipela- separation of terrigenous and biogenic sedi- that can be used for the interpretation of strat- go). The adjacent land masses and islands are ments. Processes of interflow (mid- and deep- igraphic sequences, and hence our ability to all heavily glaciated. Seasonal sea ice cover water turbid cold tongues) and Coriolis deflec- link the depositional record of a glacimarine dominates most of the area while more per- tion produce terrigenous facies along the inner unit to climate variability is limited to the sim- sistent sea ice (land-fast ice) is found along Qord and western edges of a Qord system. ple inference of glaciation. Yet more precise the southern part of the study area. Hence, Warm outer bay waters tend to develop a sta- paleoclimatic inferences should be possible the area represents an ideal situation in which ble eddy circulation pattern that favors the because glaciation at sea level can occur un- to establish links between modern sediment productivity of phytoplankton in the surface der a variety of climatic settings from tem- facies and processes governed by climate, layers. Outer bays are therefore floored with perate oceanic, to subpolar, to polar (Ander- sea ice, glacial character, and oceanographic organic-rich siliceous muds and ice-rafted ma- son and Domack, 1991). One guiding regime. terial. Only in the South Shetland Islands is principle that has recently been put forward melt-water input significant enough to gener- by a number of authors is the role of melt- Ocean Circulation ate estuarine circulation within the Qord, but water sedimentation, as it may reflect con- here strong bottom currents result in arena- trasts between polar, subpolar, and temper- Circulation patterns along the western side ceous bottom sediments with no biogenic fa- ate climatic regimes (Eyles and others, 1985; of the Antarctic Peninsula are not well cies. Ice-rafted diamictons are produced prox- Domack, 1988a; Griffith and Anderson, known. So far, studies have concentrated in imal to the edges of small tide-water glaciers in 1989). In reality, the application of the prin- the Bransfield and adjacent regions of the South Shetlands. The facies relationships ciple of varying melt-water sedimentation is the Orleans Strait (Gordon and Nowlin, 1978; established in this study provide a strong ref- dependent upon a thorough understanding of Tokarc2yk, 1987; Stein, 1982; Stein and Ra- erence for paleoclimatic studies that utilize oceanic circulation patterns and basin geom- kusa-Suszcswewski, 1982; Sarukhanyan and downcore measurements of texture and or- etry, which act together to redistribute and/or Tokarczyk, 1988; Niiler and others, 1991). ganic carbon. enhance sedimentation. The results dis- Waters from the Bellingshausen Sea flow to cussed in this paper are an attempt to test the northeast and penetrate southern regions further the hypothesis of Griffith and Ander- of the Bransfield Strait, just north of Trinity son (1989). We will relate the sediment and . These waters continue to flow along

Geological Society of America Bulletin, v. 105, p. 1175-1189, 13 figs., September 1993.

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Bellingshausen Sea

Adelaide Island

Figure 1. Location of Antarctic Peninsula and regions mentioned in text. Locations of Figures 3, 4, and 12 are also shown.

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Ice Cover

Individual ice drainage systems and their flow can be determined from LANDSAT O L- imagery, and these data are shown for two 3 portions of the Peninsula (Figs. 3 and 4). (0 Xm From these maps it is clear that there are sig- O a. —a— Arctowski nificant differences in the size of ice drainage E Primavera systems that enter into the bays and fjords a> Almirante Brown along the Peninsula. Such differences in ice

c Palmer drainage size are likely to exert control upon (0 the delivery of terrigenous sediment to the a) Faraday ljord or bay environment, similar to the in- fluence that fluvial drainage basin size exerts upon sediment supply. Most of the bays and fjords along the west- ern side of the Antarctic Peninsula ex- perience seasoned changes in sea ice cover 5 6 7 (NAVAIR, 1985). During extremely cold Months years, land-fast ice persists throughout the Figure 2. Temperature variations along the Peninsula and South Shetland Islands during the summer in the inner reaches of ljords south of period from 1977 to 1981 (Arctowski represents 1978 to 1987). Data are taken from the following the Bismark Strait (Figs. 1 and 4). This sources: Faraday (courtesy of British Antarctic Survey), Palmer (Antarctic Journal of the United change appears to correspond to regional dif- States), Almirante Brown and Primavera (National Meteorological Service, Climatology Divi- ferences in winter/spring climates, as shown sion, Buenos Aires, Argentina), Arctowski (courtesy of Rukusa-Suszcswewski). Note north to by a comparison of meteorologic data along south gradient in mean summer temperatures, which controls the amount of ablation. Also note the peninsula (Fig. 2). Sea ice cover is impor- the general similarity in summer and fall climates and greater divergence of the winter and spring tant because it controls the amount of pri- regimes. Locations of stations are given in Figures 3, 4, and 15. mary production in the surface layer of the ocean by limiting light penetration and pro- ducing surface layer stability (enhanced pro- the southern edge of the South Shetland Is- Deep Water that is limited to areas south of ductivity) during spring melting. Temporary lands, while a strong component of the Ant- the Bismark Strait and west of the Palmer fast ice also serves as a rafting mechanism for arctic Circumpolar flows off their (see Fig. 4 below). rockfall debris, which accumulates along the northern . Waters from the Weddell base of the steep valley walls. Sea flow into the Bransfield Strait, from the Physiography northeast, and constitute a southwesterly Sediment Patterns flowing current as observed off the Davis The fjords and bays along the Danco Coast Coast. Interaction between the two water are bordered by precipitous cliffs, whereas Prior studies on surficial sediments in the types occurs at the juncture of the Bransfield farther north along the Davis Coast less steep area are presented by Anderson and others and Orleans Strait. Flow of water from the topography prevails (Figs. 3 and 4). This re- (1983), Griffith and Anderson (1989), Gruber Gerlache Strait into the Orleans Strait is in- sults in glaciers of varying longitudinal gradi- and others (1989), and Harden (1989). These dicated by minima in concentrations of ni- ent, so that systems to the south are small in studies show that the sediments distributed trate and phosphate, whereas the Weddell areal extent, steep, and drop dramatically along the Antarctic Peninsula margin vary Sea Water is marked by maxima in these nu- from the high regions of the plateau (Fig. 4). from diatomaceous muds to gravels. The dis- trients (Silva, 1986). Recent investigations by Glaciers to the north have rather gentle pro- tribution of these sediments is strongly con- the Research on Antarctic Coastal Ecosys- files near their termini and cover a broader trolled by primary productivity, terrestrial tems and Rates (RACER) project suggest areal extent (Fig. 3). Hence, the area of sur- sediment input (through melt water and ice that waters of the Gerlache Strait may con- face melting is much larger for the northern rafting), and oceanography. The results of stitute a unique water type that is charac- systems, even though the climate may be Griffith and Anderson (1989) are significant to terized by very warm surface-layer tem- quite similar to those areas in the south (Wil- our own studies in that they suggest strong peratures (>2.0 °C; Amos, 1991, personal liams and others, 1989). links between regional climatic and precipi- commun.; Niiler and others, 1991) and a very The South Shetland Islands are principally tation gradients and the nature of ljord basin strong northeasterly flowing current along volcanic and volcaniclastic in nature and are sediments. They suggest that as annual mean the northern side of the Strait. Localized high covered by small ice caps. The relief, though temperatures and/or precipitation increases, productivity is associated with this warm wa- dramatic, is not nearly as precipitous as along the role of terrigenous sedimentation be- ter in the vicinity of Hughes Bay (Holm- the Danco Coast. Edges of the ice caps ter- comes more important relative to biogenic Hansen and Mitchell, 1991). Our observa- minate as tidewater ice cliffs with some dis- (diatomaceous) sedimentation. Hence, they tions along the entire length of the Gerlache tinct but small valley glaciers. Portions of the report on fine-grained diatomaceous muds, Strait confirm the pattern and also delineate coast are marked by glaciofluvial and small in rugged bays, along the southern Danco the extent of warm (>1.0 °C) Circum-Polar deltaic systems. Coast and terrigenous sandy muds in the sub-

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64°20'—

Figure 3. Ice drainage pattern for Davis Coasts, with the location of individual bays mentioned in the text. P = Primavera.

dued (constructional relief) bays along the 1988 United States Antarctic Program E. Domack and L. Burkley. Textural ratios Palmer Archipelago and South Shetland Is- (USAP) cruise III of the R.V. Polar Duke of gravel, , and mud were determined lands. It is important to note, however, that (Domack, 1988b). Bottom surface sediments using standard wet sieving techniques with a their study did not address the role of sum- were collected with a Smith-Mclntyre grab reproducibility of about ±1%. Grain-size dis- mer temperatures, circulation patterns, sampler, which obtains a large, undisturbed tributions of the sand fraction were obtained currents, biogenic productivity, or the glaci- sample with an intact sediment/water inter- from settling tube analyses using a system ologie variables of drainage size and longitu- face. Splits of the surface samples were taken similar to that described by Anderson and dinal gradient. onboard ship. Samples are curated at the Kurtz (1979). Total carbon and total organic Antarctic Research Facility of State carbon were determined using a combination METHODS University and are available to qualified in- of acid consumption of calcium carbonate vestigators upon request. Texture and or- and a LECO induction furnace. Resultant to- The primary bottom sediment and océan- ganic carbon contents were determined at tal organic carbon contents have an error of ographie data were collected during the 1987- Hamilton College under the supervision of about ±0.01%.

1178 Geological Society of America Bulletin, September 1993

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linear geometry (high L/W) restricts lateral transport of suspended sediment by limiting the influence of coastal currents and Coriolis forces (Syvitski, 1989). At the same time it enhances the down-fjord transport of sus- pended sediment from the fjord head. Sand content and total organic carbon for bottom sediments are illustrated in Fig- ures 5A and 5B. Total organic carbon con- tents (TOC; a measure of the biogenic con- tribution) increase with distance down-fjord, from a low of around 0.20% to a high of around 1.60% (Fig. 5A). The increase of TOC with distance is quite uniform with correla- tion coefficients for linear regression of >0.9 (R2) for several individual systems. When all Kilometers from Fjord Head the fjord systems are compared, two domi- nant trends result (Fig. 5A). Only a single trend is observed for the sand contents (Fig. 5B). This indicates that there are real differences between the ijords in terms of ei- ther terrigenous sediment supply or more likely the vertical flux of biogenic constitu- ents (productivity). For instance, bottom sediments from Lapeyrere Bay show a very well-defined trend of increasing TOC with distance from the fjord head, so that at around 10 km, TOC exceeds 0.50% (Fig. 5A). Calving Line Zone In comparison, similar values for TOC in bot- tom sediments of Hughes Bay (Brialmont a ) are found as close as 2-3 km from the i/> ice front, and TOC values greater than 1.00% are found in sediments that are only 6-7 km

Iceberg Zone distant (Fig. 5A). The correlations of decreasing sand con- tent with distance from the fjord head are • • -r "I- quite striking and are marked by two predom- 1 5 20 25 inant relationships (Fig. 5B). The innermost B Kilometers from Fjord Head regions of the fjords are characterized by a linear decrease in sand content away from the Figure 5. A. Down-ljord trends of total organic carbon percent in surface sediment samples fjord head glacier. This passes gradually versus distance from the Ijord head glacier. Two linear regression lines are given for Baffin Island down-fjord (at between 5-10 km) into a zone Qords after Syvitski and others (1990). B. Down-fjord trend of sand content in surface sediment where the sand contents continue to de- samples versus distance from the fjord head glacier. crease, but at a much-reduced rate, produc- ing an overall exponential decrease in sand Oceanographic data were collected using a BAY PROCESSES AND content down-fjord. SEABIRD model SBE-19 continuous pro- SEDIMENTATION Oceanographic data were collected from filer, which was modified with a SEATECH all of the fjords and are summarized in other model ST-010A transmissometer. Details Linear Bay Systems of the Danco Coast and reports. We observed little or no suspended concerning data processing are discussed in Palmer Archipelago terrigenous particulate matter within the ice- Domack and Williams (1990). Transmissom- proximal regions of Lapeyrere Bay (Dixon eter data were calibrated against sensor drift Along the Danco Coast and Palmer Archi- and Domack, 1991). We made two visits, one while water bottle samples were collected pelago, seven bay systems were examined in late December and one in late January, and in order to assess the relationship of beam (Figs. 3 and 4). These include Borgen, Mark- found little evidence for significant melt-wa- attenuation coefficients and suspended par- man, Deloncle, Girard, Brialmont, Cierva, ter input (Dixon, 1991). In contrast, waters ticulate matter. Suspended particulate mat- and Lapeyrere. These systems are character- within Brialmont Cove contained significant ter (SPM) from the water samples was exam- ized by a single bay and/or a typical fjord ge- amounts of terrigenous sediment that was be- ined for composition and texture using ometry with lengths ranging from a few kilo- ing introduced via deep- and midwater inter- the Hamilton College scanning electron meters to >20 km. Length to width ratios flows or cold tongues (Domack and Williams, microscope. vary from a high of 5.44 to a low of 1.0. A 1990). More-recent observations made dur-

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geometry of Brialmont Cove. An additional r ? factor in the case of Hughes Bay to the west is the lateral deflection of terrigenous sources * •» (cold tongues) along the coast (Domack and J»'' - Williams, 1990). This lateral deflection is not possible in the longer fjords such as Lapey- rere Bay and Flandres Bay. A linear bay sys- tem results in terrigenous dilution of the bio- genic fraction that is maintained at a greater % distance from the ice front. Lateral deflec- tion, at a much smaller scale, was observed in Andvord Bay (see following discussion). The * - .. fact that sediment facies within Lapeyrere j»> - #. Bay are dominantly terrigenous while those j ' *• in Hughes Bay (Brialmont Cove) are terrige- nous and biogenic cannot be explained by contrasts in climate. Rather, contrasts in ba- thymetry, bay geometry, and ice drainage ba- X 1 1 6 0 20 k V 173-1 «0064 sin size are the primary reasons for the facies changes. Figure 6. Scanning electron photomicrographs of SPM from Brailmont Cove. Note quartz silt The observed increase in the total organic grains with sharp and angular edges. carbon content of surface sediments away from the main source of terrigenous debris (the ice front) would at first appear to be a ing the 1990 field season demonstrate that ap- Therefore, in the absence of other sources, reflection of terrigenous dilution of the bio- preciable terrigenous SPM is also transported such as major tributary systems, the input of genic constituents. Although dilution may be near or along the bottom and within surface ice-rafted terrigenous sediment will be great- the primary control, a number of secondary overflows. est at the calving line of the major fjord-head effects, such as bathymetry, sea-ice distribu- glacier. A net fining of the ice-rafted fractions tion, and outer bay circulation, are probably Interpretation of Linear Bay Systems will also be expected to occur in a down-fjord also important in determining the sediment direction. Estuarine circulation within the composition. These influences acting in com- The well-ordered trends of bottom sedi- water column would enhance this process bination may explain the diversity in the ment texture and composition reflect down- and distribute the fine fractions farther down- slope of the correlation lines as observed be- fjord distribution of terrigenous sediment fjord than would normally be the case if es- tween a number of systems (Fig. 5A). Water- from the head of the fjord. A linear bay ge- tuarine conditions did not develop. depth contrasts between the bays can only ometry favors this pattern for a number of Mid- and deep-water cold tongues are de- partially explain these observations, because reasons. First, ice rafting of material is at a veloped adjacent to the Cayley Glacier (Brial- water depth is not the dominant factor in con- maximum at the calving line of the fjord head mont Cove) and Breguet Glacier (Cierva trolling the texture and organic carbon con- glaciers. This is because dumping and debris Cove). The development of these systems, in tent of the surface sediments (Domack and release due to fracturing of ice is greatest response to possible tidal action and basal others, 1989). Bathymetric contrasts, how- where the glacier calves. Second, the calving melting of the glaciers, is discussed by ever, may be important. Lapeyrere Bay is an front serves as an effective barrier to drifting Domack and Williams (1990). The cold open, unsilled system, as recently shown by ice. Favorable winds tend to pile drifting ice- tongues act to increase the supply of sorted revised bathymetric mapping (Dixon and bergs and sea ice into a tightly packed con- terrigenous sediment in proximity to the ice Domack, 1991), whereas Brialmont Cove has centration along the glacier terminus. This terminus. This is especially noticeable in an inner basin that is silled at a depth of was observed in the field (1987 and 1990) Brialmont Cove where local bathymetric -300 m (Domack and Williams, 1990). Their within several of the systems, including La- conditions have formed a natural trap in the respective length to width ratios are also quite peyrere Bay and Cierva Cove. LANDSAT ice-proximal portion of the cove. Such bath- distinct. It is important to note the relation- images over a number of years also indicate ymetry does not influence sediment intro- ship between TOC and distance with respect that this is a consistent phenomenon within duced and transported within surface layers to Arctic fjords (Syvitski and others, 1990) as Lapeyrere Bay. Because of this barrier ef- but does restrict the near-bottom transport of well. fect, more debris will be released at, or im- SPM. Water samples from beneath, and Of these secondary variables, sea ice is mediately along, the glacier terminus because within, the cold tongues in Brialmont Cove probably the most important. The temporal the rate of ice rafting is determined to a large indicate the basal origin of the fine-grained pattern of sea-ice distribution is such that the degree by the residence time of debris-laden terrigenous sediment (Fig. 6). Inner-bay sed- innermost reaches of the fjord systems are ice (Dowdeswell and Murray, 1990). In con- iments in Brialmont Cove consist of a mix- typically the last to become ice free as the trast, the distribution of icebergs through the ture of poorly sorted and well-sorted terrige- austral spring advances. Also, fjord systems rest of a linear fjord is a more or less random nous sediments. Surface layers or overflows found under colder winter/spring climate re- process that should not favor any one site would be influenced by Coriolis forces and/or gimes (Fig. 2) may remain locked in fast ice over another in terms of residence time. coastal currents, especially in the open bay throughout most of the summer (NAVAIR,

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Figure 7. Bathymétrie map of Andvord Bay showing locations of bottom samples (triangles and squares) and oceanographic stations (open circles and filled circles). Triangles represent locations of bottom grabs, and squares represent locations of piston cores. Filled circles represent ocean- ographic stations occupied during January 1988, and open circles represent stations occupied during December 1987.

1985). This extended period of ice cover tive regions in all of the Antarctic coastal the highest values reported from Hughes Bay would severely restrict phytoplankton pro- ocean (Holm-Hansen and Mitchell, 1991). (as much as 373 mg C/m2/day; Karl and oth- ductivity within the innermost reaches of The development of very warm, highly pro- ers, 1987). These observations were made these fjords and may result in real differences ductive waters within the Gerlache Strait is only a few kilometers from the most distal in biogenic sedimentation between the inner believed to be a response to the high stability bottom samples of Cierva and Brialmont and outer reaches of the fjord. This partially and residence time of the surface water col- Cove. Therefore, one explanation for the el- explains the facies variability between the in- umn. These conditions are maintained by iso- evated TOC contents for surface sediments ner and outer regions of the bays and fjords as lation of the strait from open ocean mixing of Hughes Bay is high productivity in the illustrated in Figures 5A and 5B. (the island effect). Strong spatial and tempo- overlying water column. If the bottom sedi- The northern Gerlache Strait has recently ral contrasts in vertical carbon flux through ments in Hughes Bay accurately reflect pro- been recognized as one of the more produc- the water column were noted, with some of ductivity within the surface waters, as sug-

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100 2 for the upper 100 m are illustrated for these • j boundary of "cold tongue" stations (Figs. 10A, 10B, and 10C). Deep-wa- ter temperatures are colder than —0.5 °C, 80 with a pronounced tongue of cold turbid wa- ter originating at the head of Lester Cove, at 60 U the Bagshawe Glacier terminus. Salinities of Terrigenous Facies O H the deeper water indicate that the cold * tongues are not the result of upwelling of S Biogenic Facies deep, cold water at the head of Lester Cove, because the isohalines are slightly down- 20 warped at the head of Lester Cove. Surface-water characteristics also demon- -0 strate the sharp boundary between waters of 0 8 1 o 1 2 1 4 6 Andvord Bay proper and Lester Cove. Wa- ters in Andvord Bay are less saline, warmer % sand Distance (km) (>2.0 °C), and significantly more turbid than % TOC near surface waters of Lester Cove. Sus- Figure 8. Sand content and total organic carbon percent in surface sediment samples versus pended particulates within the surface layer distance from the head of Lester Cove in Andvord Bay. of Andvord Bay are dominated by diatom and other phytoplankton material, whereas turbid water at the head of Lester Cove is gested, then a useful record of past variations axis of Andvord Bay that is dominated by dominated by a mixture of terrigenous and in this productivity may be preserved within biosiliceous, pebbly muds with very poorly biogenic components at the surface and at sediment cores. sorted sand fractions, total organic carbon depth. contents in excess of 0.5% (Figs. 8 and 9) and Complex Bay Systems: Andvord Bay opaline silica contents of about 12%-14% (Harden, 1989). This facies becomes increas- Interpretation of Andvord Bay Characteristic of embayments along the ingly rich in total organic carbon toward the central Danco Coast is Andvord Bay (Figs. 4 Gerlache Strait. The sand fraction is domi- Circulation within Andvord Bay appears and 7). It is a large coastal embayment with a nated by an equal mixture of quartz, feldspar, to be dominated by the movement of mid- length to width ratio of 3.4. Two smaller em- and lithic fragments; the latter consisting pre- and deep-water cold tongues away from the bayments (Lester Cove and the northeastern dominantly of black slate or mica schist. This ice margin, intrusion of deep and saline wa- cove) compose the head of the system, each facies appears to be restricted to the basins ters from the Gerlache Strait into the main of these having major glaciers (Bagshaw and along the northeast side of Andvord Bay. basin, and the development of a warm sur- Moser Glaciers, respectively) that feed into Sample 163, however, is of similar character, face layer within the main portion of the Bay. them. The entrance to Andvord Bay is and its presence along the northern flank of The generative mechanism for the cold flanked by small glaciers and ice aprons with the midbay high indicates an extension of this tongues is discussed in detail by Domack and one prominent glacier (the Deville Glacier) facies in this direction (Fig. 9). Williams (1990). Their results implicate tidal that terminates along the northeast coast. Hydrographic stations were occupied circulation as a means by which deep water is The bathymetry is marked by a series of lin- twice during the austral summer of 1987 and entrained within subglacial marine cavities. ear depressions, which lie along the north- 1988. The first series of profiles was collected This water contributes to melting along the eastern side of the bay. Water depths within along the western side of Lester Cove and in base of the glacier with consequent sus- the bay extend to just over 500 m. Lester front of the Bagshawe Glacier (Fig. 9) and pended sediment entrainment. The model Cove contains a small but deep (>500 m) de- demonstrates the development of a series of predicts a reduction in suspended sediment pression, which is oriented at a right angle to mid- to deep-water cold tongues, which orig- transport away from the marine cavity as the the main basin within Andvord Bay proper inate at the ice front and attenuate down the cold tongue mixes with surrounding water. (Fig. 7). Separating these two regimes is an axis of the fjord (Domack and Williams, The cold tongue model is fundamentally dif- elongate and somewhat sinuous high, which 1990). Toward the outer part of Andvord ferent from estuarine circulation patterns as- reaches depths of just under 100 m (Fig. 7). Bay, cold, saline deep water originates from sociated with typical fjord settings in that ter- Surface sediment distributions are defined the Gerlache Strait. A warm (-2.0 °C) sur- rigenous sediment that is derived from the by 15 surface grab samples and three gravity face layer was observed to lie in the outer glacial source is transported at mid- and deep- core tops (located in Fig. 7). Sediments portion of Andvord Bay (CTDT station water depths, not within surface overflows of within Lester Cove and along most of the No. 2; Fig. 9). This basic pattern of cold in- low-salinity water. Sediment dispersal will be midbay high are predominantly terrigenous, ner-bay waters at depth and a warm surface influenced not only by Coriolis deflection of as shown by total organic carbon contents of layer in the outer fjord was also demonstrated the cold tongues, but also by the configura- <0.5% (Fig. 8). This terrigenous facies is by the data obtained in late January 1988. tion of fjord bathymetry, as it may restrict the composed of sandy muds to muddy These data defined more clearly the bound- movement of the cold tongues. Both of these dominated by quartz and feldspathic miner- ary between the outer waters of Andvord influences are reflected in the distribution and als. The terrigenous facies is clearly defined Bay and those of Lester Cove. Tempera- character of surface sediments within And- from the biosiliceous facies within the main tures, salinities, and suspended particulates vord Bay.

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Figure 9. Océano- graphie transect through Andvord Bay and Lester Cove showing location of bottom sediment samples (triangles) and contoured values of beam attenuation coefficient. Note turbidity in ice-proximal environ- ment and concentration of phytoplankton in the sur- face layer at station No. 2. Station locations corre- spond to December data as shown in Figure 7.

Sand and organic carbon contents of sur- southern side of the midbay high and those (Fig. 8). The sand fraction is dominated by face sediments change dramatically away along the far western side of Andvord Bay poorly sorted (ice-rafted) lithic grains of slate from the head of Lester Cove (Fig. 8). also contain appreciable amounts of quartz- or mica schist. At present, transport of sorted Quartz-rich sands with low organic carbon ose, very fine-grained sand. Because the sand into this part of the basin is not occur- contents, as found along the northwestern weight percentage of this component within ring. This is because midwater cold tongues side of Lester Cove, reflect the movement of an entire sample is greatest to the west, it are restricted by the midbay high, thereby en- cold tongues away from the Bagshaw and indicates a dominant westward deflection of hancing the transport of sorted sand to the Grubb Glaciers and their deflection to the cold tongues by Coriolis forces. Periodic west. Further, surface overflows are appar- west. The muddy character of most of these transport down the main axis of Lester Cove ently not significant mechanisms of sediment sediments attests to the dominance of depo- may be related to infrequent events that in- transport in Andvord Bay today, as such sition of the sand fraction from suspension, as volve greater horizontal velocity and hence flows should widely distribute sediments re- persistent bottom currents of sufficient veloc- minimal deflection under the Coriolis effect. gardless of underlying bathymetry. Sediment ity to transport the sands would have win- Sediments along the northern side of the gravity flows are also apparently generated nowed the mud from these deposits. It is also midbay high lack the sorted sand fraction and near the head of Lester Cove and move down important to note that sediments along the contain higher organic carbon contents along the deep axis of the system. Such flows

1184 Geological Society of America Bulletin, September 1993

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have deposited fine-grained terrigenous muds Temperature: Andvord Bay (1/27/88) 8 g with low organic carbon contents (that is, sample 20) in an area of ponded sediment be- low the 500-m bathymétrie contour. In contrast, sediments within the main axis of Andvord Bay reflect the characteristics of the overlying surface water. The stable and warm surface layer is an area of high phyto- plankton productivity as indicated by the character and distribution of suspended par- ticulates (Fig. 10C). The dominance of ice- rafted (poorly sorted) sand and gravel as op- posed to sorted sand fractions indicates that most of the current derived fraction, if present, is to be found in the silt and clay sizes. The waters within Andvord Bay ap- 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Horizontal Distance (km) pear to compose a stable eddy, whereby clockwise circulation patterns distribute ice- Salinity: Andvord Bay (1/27/88) bergs and pack ice more or less evenly within the system. Clasts within the biosiliceous muds lack striations and are quite angular. This may reflect a dominance of englacial de- -20.0 - bris transport or sea ice rafting of rockfall debris. Figure 11 illustrates the role that cold tongues and outer bay circulation patterns play in the distribution of surface sediments. The generally sluggish current regime and ir- regular geometry of Andvord Bay, with a number of distinct embayments, allow this pattern to develop. The resulting facies pat- tern contrasts with those observed in more typical fjord systems of linear geometry (see 2.0 4.0 6.0 8.0 10.0 12.0 14.0 prior results) and those fjords with higher cur- Horizontal Distance (km) rent regimes (see following).

Suspended Sediment: Andvord Bay (1 /27/88) Complex Bay Systems: Admiralty Bay

Admiralty Bay is a complex bay system that contains three distinct or embay- ments into King George Island (Fig. 12). Ap- proximately one quarter of the coastline is bordered by the tidewater edges of an ice cap or smaller individual valley glaciers, which are fed by the upland ice cap. Mean summer temperatures on King George Island (Arc- towski station) average 2.0 °C (Fig. 2). Thus, the climate is somewhat warmer and wetter than the regions of the Danco Coast. The gla- ciologie setting is characterized by elevated -100. equilibrium lines (—150 m above sea level) 4.0 6.0 8.0 10.0 12.0 14.0 and the presence of a water-saturated snow Horizontal Distance (km) layer throughout the accumulation zone dur- Figure 10. A. Near-surface (100 m) temperature distributions in Andvord Bay. Note warm ing the summer months (Orheim and Gov- (>2.0 °C) water in surface layer and cold (< —1.0 °C) water at middepth near the glacier front. orukha, 1982). The presence of a thick super- Station locations are given along the top of the diagram and correspond to January stations in imposed ice zone also indicates that much of Figure 7. Contour interval = 0.25 °C. the surface melt water refreezes upon perco- B. Near-surface (100 m) salinity distributions in Andvord Bay. Note distribution of lowest lation throughout the firno r snow layers. This salinity (surface layer) in outer portion of the bay. See Figure 10A for station locations. Contour process must also take place to a greater ex- interval = 0.1%c. tent in glaciers along the Antarctic Peninsula. C. Near-surface (100 m) beam attenuation coefficients in Andvord Bay. Note turbidity in proximity to the glacier front and concentration of phytoplankton in the surface layer. See Figure 10A for station locations. Contour interval = 0.4 °C. Geological Society of America Bulletin, September 1993 1185

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Late summer snowline ~ 50m; Outer Bay Inner Basin Ice Terminus Iceberg and phytoplankton zone

Q. a> Q a5 03 £

Terrigenous fades CoaSt parallel sand belt as lateral equivalent (in shallow areas)

Figure 11. Cartoon of depositional processes active in fjords along the Danco Coast as exemplified in Andvord Bay. Modified after Domack, 1990.

In contrast to the glaciers found farther south, vailing winds, which usually blow toward the cold tongues with lower salinities are also ob- however, glaciers on King George Island northeast. At that time, a well-mixed surface served periodically along the deep water contain englacial and subglacial melt-water layer was found within the head regions of front of the icecap. In summary, the ocean- conduits that exit along the terminus as gla- Martel that contained SPM concentra- ographic regime within Admiralty Bay is ciofluvial systems. tions > 12 mg/1. The particulates consisted of highly variable, dominated by strong cur- Water depths in Admiralty Bay show a sin- fine-grained silt and clay particles with almost rents, and characterized by estuarine circu- gle deep , >500 m deep, which lies no biogenic constituents. A more regular pat- lation with periodic dispersal of a great deal of along the main entrance of the Bay and ex- tern of hydrographic stations was occupied in terrigenous sediment. This contrasts with the tends along the edge of the ice cap (Fig. 12). late January 1988 in order to define the over- oceanographic stability observed within The three inlets are marked by somewhat all pattern of circulation and exchange with fjords of the Danco Coast and Palmer more gentle bottom profiles, which slope waters of the Bransfield Strait. Again, these Archipelago. gradually up to the edge of several fjord head data illustrated the organization of the surface Surface sediments within Admiralty Bay glaciers, , or small glaciofluvial deltas layer within Martel Inlet and its subsequent are uniformly silty sands, sandy silts, or (Fig. 12). dispersal out along the eastern side of the diamictons. These sediments contain gener- As discussed by other authors and con- main entrance to Admiralty Bay (Fig. 12). ally low amounts of total organic carbon firmed by our own observations in late 1987 SPM concentrations, though high, were no- (dominated by macroalgal detritus) and ice- and early 1988, the waters within Admiralty where near the values recorded earlier, how- rafted debris. Volcanic lithic grains dominate Bay are highly variable both spatially and ever. Particles consisted of mixtures of ter- the particle compositions and appreciably temporally. Sea ice cover is limited in most rigenous and biogenic debris. Higher-salinity more ice-rafted debris is found in ice-proxi- years to a two-month period during the aus- and less-turbid waters that flow into the bay mal versus ice-distal environments. Diamic- tral winter, July and August, although fast ice from the Bransfield Strait display a well-de- tons are found at the head of the inlets adja- may persist well into the spring within the fined boundary. This inflow occurs along the cent to the ice front of the glaciers. inlets. Because of warm air temperatures southwestern edge of the main entrance. during the summer, a significant amount of Observations by Sarukhanyan and Interpretation of Admiralty Bay melt water is generated from the surrounding Tokarczyk (1988) indicate that the interaction glaciers. This can lead to a well-defined es- between these two oceanographic systems is The composition of surface sediments tuarine surface layer that is characterized by quite variable and involves exchange and within Admiralty Bay reflects (1) melt-water low salinity and high terrigenous suspended frontal movements on a time scale as short as processes, (2) an abundance of volcaniclastic sediment concentration. Terrigenous sus- 4 to 14 hr. They reported currents as strong as detritus, (3) efficient dispersal of these parti- pended sediment input from melt water may 60 to 70 cm/sec (at 50- to 100-m depth) within cles by estuarine circulation, and (4) period- approach 200 tons/day (Pecherzewski, 1980). the inner reaches of the main entrance, as ically strong bottom currents (Fig. 13). The As observed in late December 1987, the sur- confirmed by the high wire angles on our own variable oceanographic regime and multiple face layer is influenced strongly by the pre- oceanographic deployments. Subsurface point sources for terrigenous input mean that

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Figure 13. Cartoon of processes involved with modern sedimentation in Admiralty Bay, South Shetland Islands. Modified after Domack, 1990.

there is no consistent pattern to bottom sed- nous units rather than a change to biogenic comparison to true polar fjords found farther iment facies, such as distal fining of textures sedimentation. The deposition of biogenic south or along the eastern side of the penin- and increased organic carbon contents away muds in Admiralty Bay would have to in- sula may prove enlightening. from the ice fronts. The well-mixed nature of volve a modification of the strong northeast- Bay systems along the western side of the surface waters and strong currents also erly current coupled with a decrease in melt- Antarctic Peninsula and Palmer Archipelago means that the products of biogenic produc- water-derived sediment input. differ in sediment character despite similari- tivity within the Bay (organic carbon and di- ties in climate, including precipitation. Such atom frustules) are dispersed and swept out DISCUSSION AND CONCLUSIONS differences reflect either contrasts in bay into the Bransfield Basin. Biosiliceous muds physiography, bathymetry, surface water are not found within the deeper regions of the Though climate controls the rate and productivity, or the effective coast-parallel Bay as modern surface sediments. Instead, mechanism by which terrigenous sediment is transport of terrigenous sediment away from these biogenic muds accumulate within the supplied to the glacial marine environment, bays with a low length to width ratio. The bathyal depths of the Bransfield Strait. The this investigation demonstrates that the océan- sedimentation patterns observed within the only biogenic constituents present within Ad- ographie and physiographic constraints, Antarctic Peninsula fjords display striking miralty Bay consist of small fragments of which act to influence primary productivity similarities to patterns observed in Arctic macroalgae, which are in hydrodynamic and redistribute the sediment, may be more fjords (Syvitski and others, 1990). Positive equilibrium with sorted sands. important to the resulting facies patterns. The correlation between increasing organic car- This pattern is controlled basically by the facies variations between the subpolar and bon concentrations and distance in the Ant- strong northeasterly flowing current, which polar settings so far examined along the Ant- arctic linear bay systems closely approxi- sweeps along the southern edge of the South arctic Peninsula do not adequately reflect dif- mates the organic carbon distribution Shetland Islands and promotes rapid ex- ferences in corresponding climates. What is patterns from Itirbilung and Coronation change of waters within Admiralty Bay. needed are comparisons between systems Fjords, Baffin Island (Fig. 5A) (Syvitski and Hence, periods of decreased terrigenous sed- that are fundamentally similar in bay geom- others, 1990), which are described as simple iment input, as a result of colder climates, etry and marine current energy but signifi- polar fjords. Alternatively, the Antarctic Pe- may be marked by hiatuses between terrige- cantly different with respect to climate. A ninsula complex bay systems exhibit organic

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carbon distribution patterns controlled by croscope photographs. We finally want to imentation in bays and fjords of the northern Antarctic Pe- ninsula: Marine Geology, v. 85, p. 181-204. several external variables (highly irregular thank Gail Ashley, Paul Carson, Thomas Gruber, N.L.S., Ayup-Zouain, R. N., and Filho, O. H., 1989, Con- tribution to study of glacio-marine sedimentology of Admi- bottom topography, wind, ice-front and Co- Cronin, Harry Dowsett, and Ray Kostas- ralty Bay, King George Island, Antarctica: International Ge- chuk for their reviews of the manuscript. ological Congress, 28th, Washington, D.C., Abstracts, p. 591. riolis-driven circulation patterns, and lateral Harden, S. L., 1989, Establishing rates of sediment accumulation on discharge). Similarly, patterns of organic car- 100-year and 1000-year time scales for glacial-marine deposits of the of the western Antarctic Peninsula: A bon distributions from Arctic fjords (Baffin REFERENCES CITED radiochemical approach [M.S. thesis]: Raleigh, North Caro- lina, North Carolina State University, 87 p. Bay) are highly variable due to side-entiy in- Anderson, J. B., and Ashley, G. M., 1991, Glacial marine sedimen- Holm-Hansen, 0., and Mitchell, B. G., 199i, Spatial and temporal puts, wind-driven mixing, etc. (Syvitski and tation; Paleoclimatic significance: Geological Society of distribution of phytoplankton and primary production in the America Special Paper 261, 232 p. western Bransfield Strait region: Deep-Sea Research, v. 38, others, 1990). What is remarkable is the well- Anderson, J. B., and Domack, E. W., 1991, Foreword, in Ander- p. 961-980. son, J. B., and Ashley, G. M., eds., Glacial marine sedimen- Karl, D. M., Tien, G., Jones, D., Tilbrook, B., Bailiff, M. 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The utility of the relationship between Wright, R., 1983, Development of a polar glacial-marine sed- tology, and Palaeoecology, v. 67, p. 51-74. imentation model from Antarctic Quaternary deposits and NAVAIR, 1985, Sea ice climatic atlas, Volume I, Antarctic: Ashe- distance from the glacier and texture/compo- glaciological information, in Molnia, B. F., ed., Glacial ma- ville, North Carolina, Naval Oceanography Command De- rine sedimentation: New York, Plenum Press, p. 233-264. tachment, NAVAIR 50-1C-540, 132 p. sition (Fig. 5) suggests that a temporal record Dixon, J. E., 1991, Circulation processes in Lapeyrere Bay, Anvers Niiler, P. 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W., 1988a, Biogenic facies in the Antarctic glatimarine matter in Admiralty Bay: Polish Polar Research, v. 1, warm, outer-bay waters and the pattern of environment; Basis for a polar glatimarine summaiy: Palae- p. 75-83. ogeography, Palaeoclimatology, Palaeoecology, v. 63, Reading, H. G., 1986, Sedimentary environments and facies: Ox- seasonal ice cover. Hence, down-core anal- p. 357-372. ford, England, Blackwell Scientific Publications, 615 p. yses within sediment cores may reflect a Domack, E. W., 1988b, Depositional environments of the Antarctic Sarukhanyan, E. J., and Tokarczyk, R., 1988, Coarse-scale hydro- continental shelf: Fjord studies from the R/V Polar Duke: logic conditions in Admiralty Bay, King George Island, West combination of glacial and sea ice (produc- Antarctic Journal of the United States, v. 23, p. 96-102. Antarctica, summer 1988: PoUsh Polar Research, v. 9, Domack, E. 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R., 1989, Char- Stein, M., and Rakusa-Suszcswewski, S. R., 1982, Geostrophic cur- The National Science Foundation sup- acter of modem sediments: Antarctic Peninsula and South rents in the South Shetland Islands area during FIBEX, in Shetland Islands: Antarctic Journal of the United States, Nemoto, T., and Matsuda, T., eds., Proceedings of the ported this project through their programs in v. 24, p. 113-115. BIOMASS colloquium: Tokyo, Memoir Institute of Polar Research in Undergraduate Institutions and Dowdeswell, J. A., and Murray, T., 1990, Modelling rates of sedi- Research, Special Issue 27, p. 24-34. mentation from icebergs, in Dowdeswell, J. A., and Scourse, Syvitski, J.P.M., 1989, On the deposition of sediment within glacier- Research Experience for Undergraduates, as J. D., eds., Glatimarine environments: Processes and sedi- influenced fjords: Marine Geology, v. 85, p. 301-329. ments: Geological Society of London Special Publication 53, Syvitski, J.P.M., William, K., LeBlanc, G., and Cranston, R. E., administered by the Division of Polar Pro- p. 121-137. 1990, The flux and preservation of organic carbon in Baffin grams (DPP 86-13565). We would like to Dowdeswell, J. A., and Scourse, J. D., 1990, On the description and Island Fjords, in Dowdeswell, J. A., and Scourse, J. D., eds., modelling of glatimarine sediments and sedimentation, in Glacimarine environments: Processes and sediments: - thank Dennis Cassidy and his staff formerly Dowdeswell, J. A., and Scourse, J. D., eds., Glatimarine logical Society of London Special Paper 53, p. 177-199. environments: Processes and sediments: Geological Society Tokaczyk, R., 1987, Classification of water masses in the Bransfield at the Antarctic Research Facility for their of London Special Publication 53, p. 1-13. Strait and southern part of the Drake Passage using a method curatorial assistance. We owe a special Drewry, D., 1986, Glacial geologic processes: London, Edward Ar- of statistical multidimensional analysis: Polish Polar Re- nold, 276 p. search, v. 8, p. 333-366. thanks to the participants of cruise III (1987- Drewiy, D., and Cooper, A. P., 1981, Processes and models of Williams, C, Boies, C., and Domack, E. W., 1989, Glacial drainage Antarctic glaciomarine sedimentation: Annals of Glaciology, systems along the Antarctic Peninsula and Palmer Archipel- 1988) of the R.V. Polar Duke, especially the v. 2, p. 117-122. ago: Antarctic Journal of the United States, v. 24, p. 116-117. Hamilton College students, crew, and scien- Eyles, C. H., Eyles, N., and Miall, A. D., 1985, Models of glaci- omarine sedimentation and their application to the interpre- tists from Rice University, including John tation of ancient glacial sequences: Palaeogeography, Palae- oclimatology, Palaeoecology, v. 51, p. 15-84. Anderson. We thank Ken Bart of the Hamil- Gordon, A. L., and Nowlin, W. D., 1978, The basin waters of the ton College SEM Laboratory for his assist- Bransfield Strait: Journal of Physical Oceanography, v. 8, MANUSCRIPT RECEIVED BY THE SOCIETY JULY 9,1992 p. 258-264. REVISED MANUSCRIPT RECEIVED DECEMBER 14,1992 ance in obtaining the scanning electron mi- Griffith, T. W., and Anderson, J. B., 1989, Climatic control on sed- MANUSCRIPT ACCEPTED FEBRUARY 18,1993

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