Microparticle Analysis of the Ross Ice Shelf Q - 13 Core and Preliminary Results from the J - 9 Core

The concentration and size distribution of microparticles are measured on the Ross Ice Shelf at three sites : Q-13, base camp, and J-9. Results from the analysis of 2 611 samples representing the 100 m core from site Q-13 are presented. Increasing particle concentrations since 1800 are interpreted as a reflection of the gradual movement of the Q-13 drill site northward toward the Ross Sea under greater influence of the dissipating cyclonic storms in the Ross embayment. A substantial increase in particle concentrations found between 1920-40 may reflect an increase in the frequency and/or intensity of cyclonic storms dissipating in the Ross Sea. This is under investigation. Preliminary microparticle analyses of a small portion of each of 39 sections from the 416 m J-9 core indicate a substantial increase in particle concentrations between 250 and 350 m, possibly signaling the presence of late glacial, or Wisconsin, ice. INTRODUCTION The Ross Ice Shelf Project (RISP) began during the 1973-74 austral summer with the establishment of a base camp (-82°30'S, 166°W) on 14 December 1973. RISP is an interdisciplinary program composed of glaciological, geophysical, and oceanographic investigations . The primary glaciological objectives are to measure snow accumulation, ice strain-rates, 10 m temperatures, ice thicknesses, and ice velocities. This information is necessary to interpret data from the bore holes and ice cores drilled in the Ross Ice Shelf. Microparticle concentration and size distribution have been measured in sample~ from three sites on the Ross Ice Shelf. The analytical aspects of the Coulter counter technique are described by Thompson (1977) . The physical dissimilarity of firn and ice demands separate methods for sample cutting and cleaning as firn is more easily contaminated and requires special handling (Mosley-Thompson 1980). One objective of these analyses is to examine the temporal variability in particulate deposition on the Ross Ice Shelf over two time scales : the last 400 a contained in the Q-13 core, and over many millennia contained in the J-9 core. A second objective is to determine if a seasonal cycle in microparticle deposition exists, and to test the use of microparticle concentration variations for dating ice stratigraphy on the Ross Ice Shelf. Clausen and others (1979) suggest that conditions on the Ross Ice Shelf are less favorable for a seasonal pattern of dust deposition and that the fallout from nearby volcanoes may mask the possible seasonal pattern in the sparse deposition. Scanning electron-microscope examination of the material in the Q-13 core does not indicate a great abundance of fresh volcanic fragments and the predominance of clay particles suggests that airfall from Mt Erebus is not a major contributor to the insoluble particulates. 100 m CORE FROM SITE Q-13 A 100 m core was drilled during the 1977-78 austral summer on the Ross Ice Shelf at site Q-13 (78°57 'S , 179°55'E)(for map see figure 1 in Thompson and Mosley-Thompson 1982). 2 611 samples representing the entire 100 m length (77 m water equivalent) were analyzed for micro-particle concentration and size distribution. The sample size at the top was 55 nrn firn (21 mm water: p = 388 kg m· 3)decreasing to 30 mm firn (27 mm water: p = 920 kg m-3) at the bottom. These sample sizes, coupled with the estimated annual accumulation of 160-190 nrn water, provide a resolution of between 7 and 9 samples per average annual accumulation increment. The Ross Ice Shelf is ~330 m thick at Q-13 and the ice moves approximately 900 m a-1 from the south-east corner of the ice shelf (Jezek unpublished). Flow-line maps based upon intensity variations of the bottom echo (Bentley and others 1979) indicate that ice at Q-13 has come from the vicinity *Contribution No.414 of the Institute of Polar Studies, Ohio State University, Columbus,