Examples from the Antarctic Peninsula and South ...
Marine geology and geophysics Climatic and oceanographic controls upon antarctic fjord sedimentation: Examples from the Antarctic Peninsula and South Shetland Islands EUGENE W. DOMACK
Geology Department Hamilton College Clinton, New York 13323
In late December, 1987, and January, 1988, the RIV Polar Duke conducted oceanographic and marine geologic investigations within bays and fjords along the western side of the Antarctic Peninsula and South Shetland Islands. These investigations were undertaken to test hypotheses related to climate and glacial marine sedimentation (Griffith and Anderson 1989) and to establish modern parameters for biogenic and terrigenous facies changes. Initial results are presented in Domack and Williams (1990), Domack et al. (1989), and Williams (1989).
This article presents a summary of our completed studies on modern surface sediments and related facies. Though climate controls the rate and mechanism by which terrigenous sediment is supplied to the glacial marine environment, our results demonstrate that the oceanographic and physiographic constraints, which act to influence primary productivity and redistribute the sediment, may be more important to the resulting facies patterns as observed in the study area. For example, the oceanographic regime within Admiralty Bay (King George Island) is dominated by strong currents and rapid exchange of waters with the Bransfield Strait (Sarukhanyan and Tokarczyk 1988; Domack et al. 1990). Hence, both terrigenous and biogenic constituents are effectively reworked and/or transported out of the system by currents and estuarine overflows. The efficiency of the latter process is aided or hindered by the prevailing wind regime. Strong winds from the west-northwest restrain down-fjord movement of the overflows and lead to enhanced suspended sediment concentrations (Domack et al. 1989, 1990). Bottom sediments are wellsorted silty sands and sandy silts with a dominance of volcaniclastic detritus. Diamictons occur within ice front aprons and as residual (sandy) deposits in the outer bay (figure 1). Meltwater-derived sediment is supplied to Admiralty Bay at the rate of 200 tons per day (Pecherzewski 1980). Organic carbon contents are less than 0.80 percent and show little cor-
Equilibrium line - 150ma..---Saturated zone
Outer Bay
Inner Bay/Inlet Ice Terminus ''- Moulins
PhYtoPIankton
Estuarine overflow terrigenous Diamicton â Meltwater conduits
n E -c 0.
Li Macroalgae
J
50
C)
C)
300
Lateral transport of biogenic SPM
very strong currents
c'j
I 1L1â' Fj% I f-r . .-- .
Diamicton
Muddy sand - - : - Relict morainal bank Muds& residual G.M. sediment (diamicton)
Iceberg turbation & resuspension many hiatuses
I Glaciofluvial delta as lateral equivalent
4 10 km
Figure 1. Diagram illustrating physical setting and associated sediment tacies based upon studies in Admiralty Bay, King George Island. (m denotes meters. km denotes kilometers. m a.b.s.l. denotes meters above sea level. G.M. denote glacial marine.) 1990 REVIEW
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Inner Basin Ice Terminus
Outer Bay
Surface melt
Iceberg and phytoplankton zone Concentrated ice
E - 0
Phytoplankton
0
100 CZ
200
300
Late summer snowline - 50m - -
-
Occasional surface plume
Surface gyre -
-LPOSd
-E ice zone Cold ice, no .) englacial melt tunnels
Crevasse flushing
Maximum biogenic flux Cold tongue
Grounding line cavity
Infrequent resuspension
Diamicton Sediment gravity flow
400
t.. Terrigenous facies
tâ Biosiliceous pebbly muds
4 - 5 km
Coast parrallel sand belt as lateral equivalent (in shallow areas)
Figure 2. Diagram illustrating physical setting and associated sediment fades based upon studies of fjords along the Danco Coast and Palmer Archipelago. (m denotes meter. km denotes kilometer.) relation to ice front distance or texture. The carbon is dominated by fragments of macroalgae which are in hydrodynamic equilibrium with sand-sized particles. Fjords along the Danco Coast and Palmer Archipelago contain both terrigenous and biogenic facies (figure 2). Generally cold summer temperatures limit surface melting and consequent runoff. Terrigenous sediment is supplied to the marine environment at or near the front of fjord-head glaciers. Ice rafting is at a maximum near the ice front because of debris dumping, during calving, and because icebergs can become concentrated by winds along the ice barrier (figure 2). terngenous particles, as coarse as very fine-grained sand, are transported out into the fjord at deep and mid-water depths by the horizontal movement of cold tongues away from the ice front. Short-term sediment supply rates via this mechanism may approach 173 tons per day for a single ice front system (Domack and Williams 1990). Sediment gravity flows are also generated and, together with particle settling from the cold tongues, contribute to ponding of terrigenous sediments in ice-proximal basins. Lateral deflection of turbid plumes by coastal currents and Coriolis forces limits terrigenous sedimentation within the central portions of large, complex fjord systems. This results in a sharp facies change with biosiliceous pebbly muds of the outer bay (figure 2). These sediments reflect high productivity in the warm waters of the outer bay and ice rafting via icebergs and sea ice. This facies is characterized by total organic carbon contents in excess of 1.0 percent, opaline silica greater than 10 percent, and low sand contents (generally less than 5 percent). Sedimentation rates are relatively high, at around 0.28 to 0.48 centimeters per year. Linear fjords contain a more gradual terrigenous/biogenic facies transition that may extend for more than 10 kilometers (Domack et al. 1989). The facies variations between the subpolar and polar settings so far examined along the Antarctic Peninsula reflect, in part, differences in corresponding climates. However, comparisons between systems that are fundamentally similar in bay ge60
ometry and current energy but significantly different with respect to climate are needed. A comparison to true polar fjords found further south or along the eastern side of the Peninsula may prove enlightening. This study was supported by the National Science Foundation programs in Research in Undergraduate Institutions and Research Experience for Undergraduates, as administered by the Division of Polar Programs (DPP 86-13565).
References Domack, E.W. 1990. Laminated terrigenous sediments from the Antarctic Peninsula: The role of subglacial and marine processes. In J.
Dowdeswell and D. Scourse (Eds.), Geological Society of London Special Publication No. 52. Domack, E.W., L.A. Burkley, and C.R. Williams. 1989. Character of modern glacial marine sediments: Antarctic Peninsula and South Shetland Islands. Antarctic Journal of the U.S., 24(5), 113-115. Domack, E. W., and C. R. Williams. 1990. Fine structure and suspended sediment transport in three Antarctic fjords. In C.R. Bentley (Ed.), Antarctic research series, first annual volume. Washington, D.C.: American Geophysical Union. Domack, E.W., K. Beaumont, L. Burkley, C. Budka, J. Domurad, and
C. Boies. 1990. Character of modern and Holocene sedimentation along the Antarctic Peninsula. Geological Society of America, Northeast
meeting. (Abstract with programs.) Griffith, T.W., and J.B. Anderson. 1989. Climatic controls upon sedimentation in bays and fjords of the northern Antarctic Peninsula. Marine Geology, 85, 181-204. Pecherzewski, K. 1980. Distribution and quantity of suspended matter in Admiralty Bay. Polish Polar Research, 1, 75-83. Sarukhanyan, E.J., and R. Tokarczyk. 1988. Coarse-scale hydrological conditions in Admiralty Bay, King George Island, West Antarctica, summer 1982. Polish Polar Research, 9, 121-132. Williams, C . R. 1989. Temperature and sediment characteristics of a polar fjord: Cierva Cove, Antarctica. (B.A. thesis, Hamilton College, Clinton, New York.) ANTARCTIC JOURNAL
