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. The following meters until temperature. Thus, in glaciers not subjected to seasonal the depth of 92.26 m constitute a transition zone, alternating snow melting at the surface (the melting water would
0o 0 1000 km
Weddell Sea Larsen Queen Maud Land ice Shelf South
Shetlands Filchner-Ronne Antarctic Peninsula Ice Shelf Dome Fuji
East Antarctica
Bellingshaussen Vinson Dome A Sea South Pole Massif Transantartic o o 90 W Subglacial Lake 90 E Mount Johns Vostok
West Mountains Vostok Amundsen Sea Antarctica
Marie Byrd Dome C Land McMurdo (USA) Victoria Exposed Rock Ross Ice Shelf Land Ice Sheet Ice Shelf 180o
Figure 1. Map of Antarctica showing the collection site in Mount Johns (79°55’28”S, 94°23’18”W, arrow) and the main toponyms mentioned in this paper (Figure adapted from Simões 2011). The perspective view in the lower right corner shows the Mount Johns (marked area) in relation to the southern hemisphere.
2Ice crystals larger than 3.00 mm formed between snow layers with great temperature difference (Simões 2004).
31 Brazilian Journal of Geology, 46(1): 29-40, March 2016 Drilling and first results of an ice core
Density (g/cm5) Stratigraphy 0.3 0.4 0.5 0.7 0.8
0.6 0.9 1.0 9 cm Surface 0 0 0
2.1 5 5 Fresh Snow Snow 10 4.8 10 Snow with grain > 3 mm Fim 15 7.3 15 Fim with grain > 3 mm Ice 20 10.1 20 Milimeter lenses of ice Depth Hoar
25 13.1 25
30 16.2 30
35 19.5 35 Depth (m eq H
40 22.9 40
45 26.3 Depth (m) Depth (m)
2 45 O)
50 29.9 50
55 33.6 55
60 37.3 60
65 41.1 65
70 44.9 70
75 48.8 75
80 52.8 80
85 56.8 85
90 60.9 90 62.7 92,26 m 95 95
Figure 2. On the left, Mount Johns ice core density profile (in g -3cm ). Note the two depth scales (real depth on the left side and meter water equivalent on the right side). The dashed line marks the transition between firn and ice (i.e., density greater than 0.83 g cm-3). The right profile shows the stratigraphy over the 92.26 m of the Mount Johns ice core.
32 Brazilian Journal of Geology, 46(1): 29-40, March 2016 Franciele Schwanck et al.
liberate latent heat and increase the temperature of the CCI Thermo Electron Element2 ICP-SFMS coupled to snow), the temperature between 10 and 15 m is the aver- an ESI model SC-4 autosampler. Working conditions and age air temperature at the site. measurement parameters are described in Table 1. The ICP- Temperature at 12 m deep at MJ ice core was measured SFMS is calibrated daily with five standards that bracket the by a probe composed of a silver strength previously cali- expected sample concentration range. Although there is no brated and placed onto the tip of an electric cable. The probe certified reference material for trace elements in polar snow remained in the pit 24 hours for establishment of thermal and ice, the analyzed samples were certified with water refer- equilibrium with the walls of the well. The value of the tem- ence material (SLRS-4, National Research Council Canada, perature measured was -33°C. Ottawa, Canada) to ensure the concentrations within the certification range, confirming the accuracy of this method Storage and transport (details in Osterberg et al. 2006). Sections of 1 m in length were packed in tubular coils of poly- ethylene, then organized in polystyrene boxes and transported by air in a cargo plane to Punta Arenas (Chile), where they were RESULTS stored in cold room at -20°C. The ice cores were flown under refrigeration in 2009 of Punta Arenas to New York and after in a refrigerator truck to Bangor, Maine, where it was kept solid in an Ice core dating industrial refrigerator until the time of preparation for analysis. The MJ ice core record (upper 45 m) was dated by annual layer counting based on seasonal variations of Na Laboratory work and S concentrations (Fig. 3). Based on the elemental con- centrations determined by ICP-MS, the annual seasonal Decontamination and subsampling by peaks were identified and estimated, sequentially, the depo- Continuous Ice Core Melter System sition dates (Steig et al. 2005). Layer dating by this type of Sample preparation and decontamination were per- counting is reliable in polar ice cores, as the original depos- formed at the Climate Change Institute (CCI), University ited snow sequence is preserved because melting, percola- of Maine, USA. During all transport and storage, the MJ ice tion and refreezing processes are rare in the Antarctic and core remained frozen at temperatures below -20°C. Greenland ice sheets (Cuffey & Paterson 2010). Decontamination of the MJ ice core was conducted in a Most of the Na in Antarctic snow precipitation origi- certified cold room (-20°C) of class 100. The outer layer of the nates from open ocean sources. Peaks in Na concentrations firn and ice was scraped (between 2 and 4 mm) using a clean in continental Antarctica occur primarily in winter/spring ceramic knife to prevent contamination (procedure performed due to more intense atmospheric circulation and trans- according Tao et al. 2001). The ends of each section were port during this time of the year (Legrand & Mayewski removed because they are most susceptible to contamination. 1997, Korotkikh et al. 2014). Sulfur deposition in the West The core was melted using a Continuous Ice Core Melter Antarctica ice sheet reflects both marine and non-marine System (housed in clean room class 100) developed by CCI researchers from the model proposed by Röthlisberger et al. Table 1. Instrumental conditions and measurement (2000), which enables a discrete sampling and automatic parameters for the CCI Thermo Electron Element2 quartering section. According to Osterberg et al. (2006), ICP-SFMS instrument. this system can simultaneously and continuously sample Forward power 1280 W Coolant gas ~16 L min-1 the ice core for three different glaciochemical analyses: Auxiliary gas ~0.85 L min-1 major elements by Ion Chromatography (IC), trace ele- Sample gas ~0.8 L min-1 ments by Inductively Coupled Plasma Sector Field Mass Sample cone Ni; 1.1 mm Spectrometry (ICP-SFMS) and stable isotopes by Cavity Skimmer cone Ni; 0.8 mm Ring-Down Spectroscopy (CRDS). The samples were col- Sample inlet ESI ApexQ Nebulizer ESI PFA-ST lected into acid-cleaned (Optima HNO3) LDPE vials and Low: 400; medium: 4,000 and high: acidified to 1% with double-distilled HNO3 before analysis. Resolution (m/mm) 10,000 -1 Determination of trace elements by ICP-SFMS Sample uptake rate 100 μL min 800,000 cps (counts per second) for Sensitivity Trace element concentrations (Al, Ca, Fe, K, Mg, Mn, 100 ng L-1 115In Na, Sr, and Ti) in 2,137 samples (corresponding to the Total measurement 2 min 20 s upper 45 m of the MJ ice core) were determined using the time per sample
33 Brazilian Journal of Geology, 46(1): 29-40, March 2016 Drilling and first results of an ice core
sources, and, in some cases, volcanic contribution is very First results on concentration important to the levels of this element (Steig et al. 2005). A of trace elements regular pattern was observed in the concentrations obtained We measured the concentrations of 10 trace elements from both marine as continental origin, except anomalous (Al, Ba, Ca, Fe, K, Mg, Mn, Na, Sr and Ti) in 2,137 sam- peaks in the concentration profile that have been identified ples. These samples represent the upper 45 m of MJ ice core, as major explosive volcanic events around the globe. totaling about 125 years of continuous record. Table 2 shows Major historical volcanic eruptions, such as Pinatubo a statistical summary of the trace element concentrations (1991), Agung (1963), Santa Maria (1902) and Krakatoa measured at Mount Johns site. (1883), were identified by large sulfur concentration peaks Contributions from primary natural sources in each and were used as absolute time horizons during the times- sample were estimated using the following indicators: alu- cale elaboration. Based on these data, the upper 45 m of minum (Al) for rock, and soil dust and sea-salt-sodium (ss- the MJ ice core cover the period from 1883 – 2008 (± 2 Na) for sea-salt spray. Aluminum and sodium were chosen years) A.D. for being the major constituent of the earth’s crust (Planchon