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Heavy Mineral Composition of Surface Sediments Around the Ross Sea, Antarctica Christopher M. Agyingi*, Shuichi Tokuhashi**

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Heavy Mineral Composition of Surface Sediments Around the Ross Sea, Antarctica Christopher M. Agyingi*, Shuichi Tokuhashi** 17 堆 積 学 研 究, 41号, 17-25, 1995 J.Sed. Soc. Japan, No.41, 17-25, 1995 Heavy mineral composition of surface sediments around the Ross Sea, Antarctica ChristopherM. Agyingi*, Shuichi Tokuhashi** and Akira Nishimura In this paper are shown the results of the heavy mineral analysis of ice-rafted sand fractions of undisturbed surface sediments collected from several gravity cores in and around the Ross Sea shelf area during the TH91 and TH92 Antarctic research cruises by Technology Research Center, Japan National Oil Corporation, using R/V Hakurei-maru. The areal distribution pattern of the fre- quency of each heavy mineral suggests the existence of at least two petrographic provinces, i.e. western and central-eastern areas. Especially, olivine and clinopyroxene with titanaugite, which were probably supplied from the late Quaternary McMurdo alkaline basaltic rocks fringing the eastern margin of the Victoria Land, are exclusively or dominantly distributed in the western area of the Ross Sea. This results support the northward movement of icebergs nearly parallel to the east coast of the Victoria Land at least in the western part of the Ross Sea shelf area. key words : Antarctica, Ross Sea, glacial marine sediments, heavy mineral analysis. depth (approximately 800m) and the greatest depths Introduction on the shelf occur in its landward rather than sea- Sediments and sedimentary process on the shelves ward (Chriss and Frakes ,1972) . According to around Antarctica are very different from those Anderson et al. (1984) , the deeper (greater than around other continents and are characterized more approximately 300m) portions or basin floors of the or less by the actions of ice sheets, glaciers and Ross Sea continental shelf are covered by muddy icebergs. Generalized sedimentatry models in those sediments (compound glacial marine sediments), shelves are summarized by Anderson and Moila whereas shallower (above approximately 300m) por- (1989) , Anderson (1991) and so on. Characteristics tions or bank areas of the shelf are floored by of sediments and sedimentation in the Ross Sea were coarser (sand and gravel) deposits (residual glacial studied and discussed by several authors based on marine sediments), reflecting effective sediment sort- the sedimentologic al, geographical, oceanographical ing by bottom currents to depth of up to nearly and micro-paleontological data (e . g. Stetson and 300m. The muddy sediments in the deeper portions Upson,1937 ; Jacobs et al., 1970 ; Chriss and Frakes, consist of terrigenous fine silt and clay, siliceous 1972; Kellogg et al., 1979 ; Anderson et al., 1980, biogenic material, and poorly sorted ice-rafted de- 1984; Edwards,1987). bris. Therefore, heavy mineral composition of sand The Ross Sea is bordered on the south by the fraction of these sediments is expected to reflect the largest ice shelf in the world, the Ross Ice Shelf. The origin and flow pattern of the icebergs in the Ross shelf break of the Ross Sea occurs at relatively great Sea shelf area. In this paper are first shown the results of heavy Received May 22, 1944 ; Accepted September 30, 1994 *Cameroon mineral analysis of undisturbed surface sediments of , Former Guest Researcher (Former STA Fellow) at GeologicalSurvey of Japan several gravity cores in and around the Ross Sea ** Geological Survey of Japan , 1-1-3 Higashi, Tsukuba, 305 shelf area, collected during the TH91 and TH92 Japan Antarctic surveys conducted by Technology Re- 18 Christopher M. Agyingi, Shuichi Tokuhashi and Akira Nishimura 1995 search Center, Japan National Oil Corporation, using Anderson et al. (1984) . Samples of GC1307 and R/V Hakurei-maru. Then, the origin of some charac- GC1308, collected from the bank slope at the shelf teristic heavy minerals and flowing patterns of break of the Ross Sea, are, on the contrary, com- icebergs which transported those minerals will be posed of sand, reflecting the existence of bottom cur- shortly discussed. rents; seaward-flowing Antarctic Bottom Water (Chriss and Frakes, 1972) or impinging geostrophic Sample and Method current onto the shelf (Anderson et al ,1984). Other Characteristics of Samples samples were collected from more deeper portions Localities of samples are shown in Fig. 1 and de- off the Ross Sea shelf, such as continental slope scriptions of samples are summerized in Table 1. (GC1306) , upper continental rise (GC1203) and Samples of GC1303, GC1304 and GC1305 in the east- lower continental rise (GC1208) . These sediments ern and central Ross Sea and samples of GC1204, are composed of muddy sediments as like as those GC1205 and GC1206 in the western Ross Sea were collected from basin floors of the Ross Sea shelf collected from basin floors of the Ross Sea shelf, and area. they all consist of muddy sediments as mentioned by Fig. 1 Index map and sampling points. 堆 積 学 研 究41号Heavy mineral analysis, Ross Sea Antarctica 19 Table. 1 Brief description of analyjed samples. Method of Heavy Mineral Analysis The pinch clips at the bottom of the separating fun- Samples were sieved by a spray of water (wet nels were released slowly allowing the heavy miner- sieving) to obtain the fraction smaller than 250ƒÊm al fraction to pour into crucibles and followed by fraction. This was followed by washing and decanta- washing with ethanol to remove the heavy liquid tion of the clay and silt fraction, leaving a grain-size from the samples. The washed samples were dried in range from which optimal results had been obtained the oven about at 40•Ž. in test runs. The heavy minerals were then weighed and re- Acid digestion was then applied to the extracted corded. The magnetic heavy mineral fraction of each grain size range in order to rid the grains of coat- sample was separated from the none magnetic frac- ings of carbonates and oxides of iron. For this pur- tion using a horse shoe magnet followed by further pose the samples were placed in dilute HCl and weighing to obtain the weight of the magnetic and warmed over a hot plate in a fume cupboard with in- none magnetic heavy mineral fractions. termittent stirring for about thirty minutes. The Grain mounts of the none magnetic heavy minerals samples were then allowed to cool followed by de- were then made using resin as the mounting medium. cantation of the acid and a thorough wash with wa- For each mount the grains were evenly distributed ter to remove all soluble substances and remaining on the glass slide to avoid overlapping and dense clays and silts. The washed samples were dried in a packing of grains. oven at 80•Ž. The heavy mineral content of each grain mount Heavy mineral separation was achieved by gravity was identified by observation under a petrological settling using the heavy liquid tetra-bromoethane, microscope designated NIKON LABOPHOT 2-POL. 2.96 in specific gravity. The heavy liquid was filled The relative abundance of heavy minerals in each into separating funnels and weighed dry samples sample was determined by point counting using the (maximum 15g) were then added to the liquid and ribbon counting technique. stirred to ensure that the grains were thoroughly Results and Discussion wetted. The contents of the separating funnels were stirred intermittently until maximum separation was Results of heavy mineral analysis are summerized obtained (normally after 4-6 hours separating time). in Table 2, and areal distributions of the frequency 20 Christopher M. Agyingi, Shuichi Tokuhashi and Akira Nishimura 1995 Table. 2 Results of heavy mineral analysis. of individual transparent heavy minerals are shown western part than in the central and eastern parts of in Figs. 2A and 2B. In the western part (westward the shelf area. Especially, clino-pyroxene dominates from approximately long. 180•‹) of the Ross Sea shelf in the western area and olivine occurrs exclusively area, hornblende, clino-pyroxene (Cpx) , tremolite- in the western area. Other minerals such as horn- actinolite and garnet yield abundant or common blende, zircon, epidote and tourmaline , on the other occurrences. Relatively rare occurrence of olivine hand ,have a reversal tendency that they occur more and very rare occurrences of zircon, tourmaline, abundantly in central and eastern part than in the kyanite and andalusite are observed. In the central the western parts of the shelf area (Fig. 2B) . Espe- and eastern parts (eastward from approximately cially, hornblende predominates in the central and long. 180•‹) of the Ross Sea shelf area , on the other eastern parts and epidote is observed nearly exclu- hand, hornblende predominates. Rare occurrences of sively in the central and eastern parts of the shelf clino-pyroxene, garnet, and tremolite-actinolite, and area. very rare occurrences of epidote, tourmalire, and According to Anderson et al .. (1984) , east of andalusite are observed. approximately 180°, glacial ice is derived from West Anderson et al. (1984) appointed the existence of Antarctica, most of which flow into the Ross Ice distinct petrologic province of the western Ross Sea Shelf. West of approximately 180°, the Ross Ice Shelf and broardly similar but still distinct petrologic pro- consists of ice that has flowed from East Antarctica. vince of the central and eastern Ross Sea based on Along the mountaineous north Victoria Land coast, the petrography of sand fractions of surface sedi- outlet glaciers flow directly into the sea. Therefore, ments collected from many piston and gravity cores the existence of western and central-eastern petrog- in the Ross Sea, though no data were shown there. raphic provinces in the Ross Sea must reflect the They attributed the existence of such petrologic pro- difference of provenance between East Antarctica vinces to the flow patterns of icebergs in the Ross and West Antarctica. Sea. The difference of composition and relative Simplified geologic map around the Ross Sea is abundance of heavy minerals between in the western shown in Fig.
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