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Drilling, Processing and First Results for Mount Johns Ice Core in West

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Drilling, Processing and First Results for Mount Johns Ice Core in West DOI: 10.1590/2317-4889201620150035 ARTICLE Drilling, processing and first results for Mount Johns ice core in West Antarctica Ice Sheet Perfuração, processamento e primeiros resultados de um testemunho do manto de gelo da Antártica Ocidental Franciele Schwanck1*, Jefferson Cardia Simões1, Michael Handley2, Paul Andrew Mayewski2, Ronaldo Torma Bernardo1, Francisco Eliseu Aquino1 ABSTRACT: An ice core, 92.26 m in length, was collected near RESUMO: Um testemunho de neve de 92,26 m foi coletado em um dos the ice divide of the West Antarctica ice sheet during the 2008/2009 divisores da bacia de drenagem glacial do manto de gelo da Antártica Ocidental austral summer. This paper described the fieldwork at the Mount no verão austral de 2008/2009. Este artigo descreveu os trabalhos de campo no Johns site (79º55’S; 94º23’W) and presented the first results of the sítio Mount Johns (79º55’S; 94º23’W) e apresentou os resultados das análises upper 45.00 m record covering approximately 125 years (1883 – dos 45,00 m do registro superiores do testemunho e que representam cerca de 2008), dated by annual layer counting and volcanic reference ho- 125 anos (1883 – 2008) de precipitação de neve, datados por meio da conta- rizons. Trace element concentrations in 2,137 samples were deter- gem anual das camadas e dos horizontes vulcânicos de referência. Os elemen- mined using inductively coupled plasma mass spectrometry. The tos-traço estudados neste trabalho foram determinados usando espectrometria concentrations obtained for Al, Ba, Ca, Fe, K, Mg, Mn, Na, Sr and de massas por plasma indutivamente acoplado em 2.137 amostras. As concen- Ti are controlled by climate variations, the transport distance, and trações obtidas para Al, Ba, Ca, Fe, K, Mg, Mn, Na, Sr e Ti são controladas the natural sources of these aerosols. Natural dust contributions, pelas variações climáticas, pela distância de transporte e pelas fontes naturais mainly derived from the arid areas of Patagonia and Australia, are desses aerossóis. Contribuições naturais de poeira, oriundas principalmente das important sources for aluminum, barium, iron, manganese and ti- áreas áridas da Patagônia e da Austrália, são importantes fontes para alumínio, tanium. Marine aerosols from sea ice and transported by air masses bário, ferro, manganês e titânio. Os aerossóis marinhos, provenientes do gelo are important sources of sodium and magnesium. Calcium, potas- marinho e transportados pelas massas de ar, são fontes importantes de sódio e sium and strontium showed considerable inputs of both continen- magnésio. Cálcio, potássio e estrôncio apresentam aportes consideráveis tanto de tal dust and marine aerosols. poeira continental como de aerossóis marinhos. KEYWORDS: Trace elements; Ice core; West Antarctica ice sheet; PALAVRAS-CHAVE: Elementos-traço; Testemunho de gelo; Manto ICP-SFMS. de gelo antártico; ICP-SFMS. 1Centro Polar e Climático, Instituto de Geociências, Universidade Federal do Rio Grande do Sul – UFRGS, Porto Alegre (RS), Brazil. E-mail: [email protected] [email protected], [email protected], [email protected] 2Climate Change Institute and Department of Earth Sciences, University of Maine, Orono, USA. E-mail: [email protected], [email protected] *Corresponding author. Manuscript ID: 20150035. Received in: 09/29/2015. Approved in: 01/13/2016. 29 Brazilian Journal of Geology, 46(1): 29-40, March 2016 Drilling and first results of an ice core INTRODUCTION Twin-Otter, to the drilling site at 79°55’28 “S, 94°23’18 “W, 2,100 m of altitude, here called Mount Johns (MJ), where Air temperature on the surface of the Antarctic Peninsula the ice thickness exceeds 2,400 m (project data BedMap and West Antarctica have increased dramatically in recent 2: Bed Topography of the Antarctic; Fretwell et al. 2013). decades with some reports suggesting that these are the This drilling site (Fig. 1) was selected considering the regions which are warming faster on Earth (Turner et al. following characteristics: 2006, Bromwich et al. 2013). Even though climate models 1. it has a relatively high accumulation rate, suggest large natural climate variability over West Antarctica 2. it is located on a drainage basin divide (separating the (Hawkins & Sutton 2012), instrumental records in this part Pine Island Glacier drainage basin from the one that of the continent are scarce, and there is a lack of long-term flows to the Filchner-Ronne Ice Shelf), records, which make difficult the assessment on a larger 3. it is located in an area of air mass confluence time scale. (air masses from the Weddell, Amundsen and Weather information obtained from ice cores in Antarctica Bellingshausen seas), and provide a longer and detailed historical record than the cur- 4. no ice cores have been drilled nearby. rently available from instrumental observations. The high resolution of ice records provide information about the composition of the atmosphere in the past, in particular, Fieldwork changes in regions of aerosols origin and transport path- ways (Legrand & Mayewski 1997, Schneider et al. 2006, Drilling Mayewski et al. 2009). However, most of these records are Drilling was performed with the use of an electrome- sites in East Antarctica. Often, climate reconstructions from chanical drill FELICS (Fast Electromechanical Lightweight West Antarctica are discarded for producing inconsistent Ice Coring System) that retrieve cores of 8.5 cm in diameter results due to discontinuous records and the lack of data and up to 80 cm long, reaching a maximum of 160 m in depth (Monaghan et al. 2008, Steig et al. 2009). in the ice (Ginot et al. 2002). The ice core MJ reached a West Antarctica has lower elevation compared to East depth of 92.26 m. Antarctica, making West Antarctica more subject to the influence of marine air masses (Noone & Simmonds 2004, Density Nicolas & Bromwich 2011). Places with such influence are Each core obtained was immediately weighed with an important because they reflect more directly the atmospheric electronic balance, Bioprecisa BS3000A model, with a pre- conditions resulting from changes in ocean circulation and cision of 0.1 g. From this measure and the linear dimen- sea ice extent. sions (length and diameter), we made the density profile by In this study, we presented field data and the first lab- the depth (Fig. 2). Determination of density allows correct- oratory results of an ice core obtained in West Antarctica, ing the profile in equivalent water and then calculating the around 75 km from Mount Johns nunatak1 (79°33’S, annual accumulation. MJ ice core represents 62.70 m of 91°14’W). The core is part of the ITASE (International water equivalent after correction for density. This depth is the Trans-Antarctic Scientific Expedition) collection, an inter- sum of the length of each section multiplied by its density. national initiative that has the emphasis to collect data from MJ ice core has an average density of 0.70 g cm-3, rang- the past 200 years (although some records reach 1,000 years) ing of 0.34 to 0.91 g cm-3. The density increases gradually by ice cores spaced 200 km (Mayewski & Goodwin 1997). (as expected at a site without evidence of surface melting of snow) and reaches the transition firn/ice (density 0.83 g cm-3) at depth 68.88 m (44.00 m in water equivalent). MATERIALS AND METHODS Stratigraphy Drilling site The transformation of snow to ice occurs for densifica- The drilling mission in the austral summer 2008/2009 was tion (with partial air expulsion) and recrystallization. Note developed from a base camp near the airstrip at Patriot Hills that in polar glaciers this whole process occurs without for- (80°18’S, 81°22’W) in the Ellsworth mountains, as part of mation of seasonal melt water. The classification used by gla- the multidisciplinary expedition called “Deserto de Cristal”. ciologists (e.g., Cuffey & Paterson 2010) has the following From that point, four researchers flew by a twin-engine plane, sequence with increasing depth: 1A rock, often the top of a mountain, surrounded by a glacier, ice cap, or mantle of ice (Simões 2004). 30 Brazilian Journal of Geology, 46(1): 29-40, March 2016 Franciele Schwanck et al. 1. snow is usually restricted to material that has not changed layers of firn and ice. Throughout the MJ ice core, it is pos- much since it deposition; sible to observe millimeter ice layers. Depth hoar2 layers are 2. firn are the crystals in an intermediate phase, indicat- observed on some levels, for example the 18 m depth. ing a continuous transformation of the physical prop- The recorded observations do not show evidence of per- erties of snow, increasing the diameter of the crystals colation of water or refreezing across the profile. In addition, and partial expulsion of air, while maintaining the per- layers containing microparticles have not been observed meability and communication with the atmosphere; (e.g., volcanic ash), which could be correlated to specific 3. ice is snow that suffered intense compaction and meta- volcanic events. morphism, whose pores are finally closed (i.e., loss of permeability), which occurs approximately when snow Borehole temperature reaches a density of 0.83 g cm-3. The temperature distribution in glaciers and ice sheets depends on several factors, e.g. geothermal heat MJ stratigraphy was described visually using the crite- flux and heat generated by frictional heating basal slip ria above and density measurements (Fig. 2). First 4 m are warm or melt the base (Cuffey & Patterson 2010). constituted principally of fresh snow and snow with smaller However, in the first 10 to 15 m, the temperature of grains alternating with larger grain (> 3 mm) followed by firn the snowpack is controlled by seasonal variations in air layers until a depth of 68.80 m.
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