Estilos de aprendizaje

2.2.1

Ice core analysis

The Mill Island ice cores were processed and analysed in the glaciology labo- ratory at ACE CRC, Hobart, Australia. This section describes the analytical methods used, including the ice core processing, and measurements for hydro- gen peroxide, water stable isotopes, and trace ion chemistry analysis.

Ice core processing

The Mill Island ice cores were processed in a clean freezer (=18‰) laboratory using similar techniques to those described by Curran and Palmer (2001). Ice core density calculations were carried out using the width, length, diameter and weight measurements for each core segment (∼1 m long). Visual ob- servation was also completed for stratigraphy studies. The cores were then transversely divided into three sticks using a clean band-saw. The sticks were used for analyses, including hydrogen peroxide, stable water isotope, and trace ion chemistry measurements. The sticks for hydrogen peroxide and water sta- ble isotope measurements were then cut into 4 cm samples and bagged for analysis. The chemistry sticks were cleaned to avoid contamination and sam- pled every 4 cm (i.e., approx. 25 samples per ∼1 m core segment). Cleaning was achieved by removing ∼3 mm of each surface with a microtome under a laminar airflow hood. Chemistry samples were stored in a Coulter cup and

2.2. METHOD 28

melted in a refrigerator overnight to avoid MSA loss (Abram et al., 2008), then refrozen again. The refrozen samples were melted prior to analysis. All tools used for processing ice cores were carefully pre-cleaned with deionised ultra-clean Milli-Q water (resistivity > 18 MΩ–cm), and polyethylene gloves were worn during the ice core processing, to minimise contamination.

Hydrogen peroxide (H2O2) measurement

Hydrogen peroxide (H2O2) is influenced by photochemical processes. This provides a strong seasonal variation, with a peak during summer when short- wave radiation fluxes are higher. Thus, it is useful for dating ice cores (Sigg and Neftel, 1988). Hydrogen peroxide measurements were carried out at ACE CRC, using a fluorescence detector as detailed by van Ommen and Morgan (1996). A 4 cm sample was analysed every 8 cm from the surface to a depth of 25 m, then every 12 cm for the rest of the core due to time constraints.

Water stable isotope measurement

The water stable isotope measurements (δD and δ18_{O) were carried out at} the ACE CRC/AAD isotope laboratory, Hobart, Australia, using a high- temperature elemental analyser (EA). The Eurovector EuroPyrOH-HT sys- tem was interfaced in continuous flow mode to an Isoprime isotope ratio mass spectrometer. Samples at 4 cm sample resolution were melted in a refrigerated unit prior to analysis. Liquid samples were sampled by a Eurovector liquid

auto-sampler (LAS EuroAS300). Analytical precision for δD is <0.5 ‡ and for δ18_{O is < 0.1} ‡_{, and values are expressed relative to the Vienna Stan-} dard Mean Ocean Water 2 (VSMOW2). D-ex was then calculated from the measured δD andδ18O using the following equation

D-ex =δD−8×δ18O (2.1)

(Paterson, 1994).

Trace ion chemistry measurement

Trace ion chemical measurements were carried out using a suppressed ion chromatograph (IC) as detailed by Curran and Palmer (2001). Samples were melted overnight in a refrigerator prior to analysis. Due to the high sea salt concentration, the melted samples were diluted 50 times in autosampler polyvials using a micropipette within a laminar flow. Further dilutions (5 to 100 times) were completed according to the sea salt concentration from the result of the initial analysis.

Samples were then analysed using a Dionex AS18 ICS-3000 (2 mm) mi- crobore ion chromatograph. Anions (i.e., MSA, Cl−, SO2₄−, and NO−₃) were analysed using an IonPac AS18 separation column and AG18 guard column. Cation (i.e., Na+_{, K}+_{, Mg}2+_{, and Ca}2+_{) analysis was performed using CS12A} separation columns. The system performed anion and cation analysis simul- taneously using dual isocratic pumps. The major ion species measured in this

2.2. METHOD 30

study were CH3SO₃− (MSA), Cl−, NO−₃, SO₄2−, Na+, K+, Mg2+, and Ca2+. The nssSO2₄− record was then calculated using the formula

[nssSO2₄−] = [SO2₄−]−kN a×[Na+] (2.2)

wherekN a is sea salt ratio of SO24−to Na+, 0.120 (Mulvaney and Wolff, 1994). All trace ions were calibrated using diluted standards (Curran and Palmer, 2001) expressed in concentrations of microequivalents per litre (µEq/L).

2.2.2

Dating

Accurate ice core dating is crucial for calibration and interpretation of ice core records. MI0910 was dated by counting annual layers of H2O2, water isotopes (δ18_{O, δD), and deuterium excess (D-ex) according to the methods presented} in Plummer et al. (2012). The results of this dating method are presented in Section 2.3.2. The layer counting method using H2O2, δ18O, δD, and D-ex was subsequently confirmed by the non sea salt sulphate (nssSO2₄−) record, which matches the timing of volcanic eruptions at Law Dome (LD) (Plummer et al., 2012) and at other ice core sites (Cole-Dai et al., 1997, 2000). The shallow cores MIp0910 and MI0809 were also dated using the layer counting technique to supplement the top of the MI0910 core (Table 2.1), and to verify the MI0910 dating.

Ice core Lat Lon Depth (m) Drill date MI0910 65o _{33’ 10” S 100}o _{47’ 06” E 120 2010-01-18}

MIp0910 65o _{33’ 10” S 100}o _{47’ 06” E 10.57 2010-01-15}

MIp0809 65o _{33’ 25” S 100}o _{33’ 26” E 16.69 2009-01-22}

Table 2.1: Mill Island ice core information

2.2.3

Stratigraphy

Visual stratigraphy observation was achieved by counting and logging the crust layers. The crust layers were measured for thickness and counted manually during processing of the ice core. An example of the crust layers is given in Figure 2.13, and the results are presented in Figure 2.14.