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Microparticle Analysis of the Ross Ice Shelf Q-13 Core

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Microparticle Analysis of the Ross Ice Shelf Q-13 Core Annals of Glaciology 3 1 982 @ International Glaciological Society MICROPARTICLE ANALYSIS OF THE ROSS ICE SHELF Q-13 CORE AND PRELIMINARY RESULTS FROM THE J-9 CORE* by Ellen Mosley-Thompson (Institute of Polar Studies, Ohio State University, Columbus, Ohio 43210, U.S.A.) and Lonnie G. Thompson (Institute of Polar Studies and Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210, U.S.A.) ABSTRACT One objective of these analyses is to examine the The concentration and size distribution of micro­ temporal variability in particulate deposition on the particles are measured on the Ross Ice Shelf at Ross Ice Shelf over two time scales: the last 400 a three sites : Q-13, base camp, and J-9. Results from contained in the Q-13 core, and over many millennia the analysis of 2 611 samples representing the 100 m contained in the J-9 core. A second objective is to core from site Q-13 are presented. Increasing particle determine if a seasonal cycle in microparticle depo­ concentrations since 1800 are interpreted as a re­ sition exists, and to test the use of microparticle flection of the gradual movement of the Q-13 drill concentration variations for dating ice stratigraphy site northward toward the Ross Sea under greater on the Ross Ice Shelf. Clausen and others (1979) influence of the dissipating cyclonic storms in suggest that conditions on the Ross Ice Shelf are the Ross embayment. A substantial increase in part­ less favorable for a seasonal pattern of dust deposi­ icle concentrations found between 1920-40 may reflect tion and that the fallout from nearby volcanoes may an increase in the frequency and/or intensity of mask the possible seasonal pattern in the sparse cyclonic storms dissipating in the Ross Sea. This is deposition. Scanning electron-microscope examination under investigation. Preliminary microparticle an­ of the material in the Q-13 core does not indicate a alyses of a small portion of each of 39 sections great abundance of fresh volcanic fragments and the from the 416 m J-9 core indicate a substantial in­ predominance of clay particles suggests that airfall crease in particle concentrations between 250 and from Mt Erebus is not a major contributor to the 350 m, possibly signaling the presence of late insoluble particulates. glacial, or Wisconsin, ice. INTRODUCTION 100 m CORE FROM SITE Q-13 The Ross Ice Shelf Project (RISP) began during A 100 m core was drilled during the 1977-78 aus­ the 1973-74 austral summer with the establishment of tral summer on the Ross Ice Shelf at site Q-13 a base camp (-82°30'S, 166°W) on 14 December 1973. (78°57 'S , 179°55'E)(for map see figure 1 in Thompson RISP is an interdisciplinary program composed of and Mosley-Thompson 1982). 2 611 samples representing glaciological, geophysical, and oceanographic inves­ the entire 100 m length (77 m water equivalent) were tigations. The primary glaciological objectives are analyzed for micro-particle concentration and size to measure snow accumulation, ice strain-rates, distribution. The sample size at the top was 55 nrn 10 m temperatures, ice thicknesses, and ice velocities. firn (21 mm water: p = 388 kg m· 3)decreasing to This information is necessary to interpret data from 30 mm firn (27 mm water: p = 920 kg m-3) at the the bore holes and ice cores drilled in the Ross Ice bottom. These sample sizes, coupled with the estimated Shelf. annual accumulation of 160-190 nrn water, provide a Microparticle concentration and size distribution resolution of between 7 and 9 samples per average have been measured in sample~ from three sites on the annual accumulation increment. Ross Ice Shelf. The analytical aspects of the Coulter The Ross Ice Shelf is ~330 m thick at Q-13 and counter technique are described by Thompson (1977) . the ice moves approximately 900 m a-1 from the The physical dissimilarity of firn and ice demands south-east corner of the ice shelf (Jezek unpub­ separate methods for sample cutting and cleaning as lished). Flow-line maps based upon intensity vari­ firn is more easily contaminated and requires special ations of the bottom echo (Bentley and others 1979) handling (Mosley-Thompson 1980). indicate that ice at Q-13 has come from the vicinity *Contribution No.414 of the Institute of Polar Studies, Ohio State University, Columbus, Ohio 43210, U.S.A. 211 Mos iey-Thompson and Thompson: MicPopaJ"ticie anaiysis of Ross I ce Sheif copes Ross Ice S he lf (0 13) stratigraphic studies in the Q-13 area to provide To ro! Porlicles::?. 0.6J.11m pu ~>' I of 5<1mp11 hlOsl confirmation that the well-preserved microparticle '° "' concentration cycles are annual. One approach successfully employed in a previous study at the South Pole (Mosley-Thompson 1980) is to assume that the particle concentration peaks are annual, convert the vertical separation between successive peaks to water equivalent using the density profile, and construct an accumulation profile. A total of 454 peaks were counted and a corresponding £ ~~~~ accumulation record was constructed. A Fourier trans­ .i ''1= formation of the data sequence and removal of periods less than 10 a was performed but no consistent trend l ,.,g~=::=o;=o was evident. Annual variability in net water accumul­ ation (Anl was found to be quite high, ranging between a minimum of 100 11111 a- 1 and a maximum of 260 mm a- 1 • A for the entire core is 168.8 mm a- 1, intermediate ~etween the earlier estimates of 180 mm a- 1 by Crary and others (1962) and more recent estimates which fall between 120 and 160 mm a- 1 (Bentley and Jezek 1981). Figure 3 illustrates the concentration of total A ~ • 200mm a·• particles (diameters >0.63 µm) per 500 µt of sample for the entire 100 m core, plotted in water Fig.1. Total particle concentrations in two sections equivalent depth. The dates assigned at each 5 m of the Q-13 core. Dates were assigned by counting interval were obtained by counting apparent seasonal the cyclical variations. An is the average variations in particle concentration downward from annual accumulation rate for each section. the surface as discussed above. The Q- 13 micropart­ icle record for the last 450 a bears no resemblance Ross Ice Shelf, Q- 13 to the only other detailed Antarctic microparticle To 1a 1 Porl lC!H::?: 0.63.11m per ~.IJ I of Somple !ilOs) . " Average Particles~ 0.63µm Annual per 500µ1 of Sample (10 3) Accumulation 0 30 60 90 mma-1 0 1972 167 1948 178 10 1920 178 15 1896 148 20 1860 161 25 1830 185 ~ 30 Q;"' 1803 :::. 185 .!: .c: 3 5 1776 a. 172 Cl"' c 4 0 1747 0"' 147 > ·5 4 5 CT 1713 I.LI 161 0"' 50 1682 ;;:: 151 55 1649 Fig.2. Total particle concentration in three sections 200 of the Q-13 core. Dates were assigned by counting 6 0 1624 the cyclical variations in microparticle concentra­ 161 tion. An is the average annual accumulation rate 6 5 1593 rate for each section. 178 70 1565 of West Antarctic ice streams A and B passing west 172 of the Crary Ice Rise. 75 1536 ·The detail of the microparticle record is illus­ trated by five representative sections (Figs.1 and 60 2). A tentative time scale is constructed by counting the best defined microparticle concentration peaks Fig.3. The concentration of total pa rticles in 2 611 downward from the surface. A discussion of the prob­ samples, representing the entire 100 m Q-13 core. lems inherent in the interpretation of microparticle Depth is in water equivalent. The estimated annual stratigraphy is presented elsewhere (Mosley-Thompson accumulation rate and data are included for each and Thompson in press). There have been no surface . 5 m increment. 212 Mo s ley- Thompson and Thompson: Mic r> opaT't icle analysis of Ros s Ice Shelf cor>es record, that from the South Pole (Mosley-Thompson latitudes as well as local · Antarctic material en­ 1980). Two prominent features of the Q-13 record are trained from exposed areas along the eastern margin the gradual increase in concentrations since - 1800 of the Ross embayment. and the very pronounced concentrations between 6.5 The very substantial increase in particle con­ and 10 m (water equivalent) representing the period centrations between 1920 and 1940 is quite intrigu­ 1920-40 (Fig.3). ing. Size distribution data indicate that the mater­ A gradual increase in particulate deposition ial is locally deri ved and consists of great quanti­ begins around 1800, increasing by a factor of 4 by ties of large (>1.0 µm) fragments. Very pronounced 1900 (Table I). Particulate concentrations increase broad peaks are centered at 1927, 1934, 1938, and rapidly after 1900 and by 1950 the concentrations are 1940, and are associated with modest accumulatio~ almost one order of magnitude greater than in 1730 increases. Scanning electron-microscope examination (Table I). The ratios of concentrations in 1951 to of the particles in the 1927 and 1940 peaks reveals 1730 show a small particle (0.63-0.80 µm) ratio of many large (>50 µm) fragments and very few fresh 8 and a large particle (>5.0 µm) ratio of 24, volcanic glass shards, which might be expected if demonstrating that a much greater proportion of direct air fall from Mt Erebus were the source. The larger fragments are being deposited at the current ratio of particle concentrations in the 1940 and Q-13 site than 220 a ago when the drill site was 1927 peaks to that in 1730 (Table I) provide strong nearly 200 km further inland.
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