WACSWAIN ice core site characteristics

The WACSWAIN ice core was drilled at Skytrain Ice Rise (79°44.46′ S, 78°32.69′ W) at an altitude of 784 m above sea level (a.s.l.) ~50 km north of the closest outcrop of the Ellsworth Mountains near the Ronne Ice Shelf during the 2018–19 Antarctic field season (Fig. 1). The basal topography of the site is 133 m a.s.l. with a mean annual surface temperature of −26°C. The ice core was drilled to the bed at a depth of 651 m (Mulvaney and others, 2021). Ice core samples analyzed in this study were from 295 to 305 m depth below the surface. This depth interval was mainly chosen because (1) it is well below the firn–ice transition, (2) the data produced by all of the methods at this interval was of high enough quality for statistical comparison and (3) the condition of the ice at this interval did not affect the resulting data.

Fig. 1. Map of Skytrain Ice Rise ice core drilling site (79°44.46′ S, 78°32.69′ W).

WACSWAIN ice core drilling, transport and storage

The ice core was drilled using the British Antarctic Survey (BAS) deep drill (Mulvaney and others, Reference Mulvaney, Alemany and Possenti2007, Reference Mulvaney2021). After being extracted from the drill barrel, each ice core section was measured lengthwise and cut into 80 cm sections, using an electric cross-cut bench saw. For the first 291 m, the weight and dimensions of each section were recorded to enable ice density to be calculated. Each 80 cm section was then stored in layflat polythene tubing. During the field season, sections of the ice core were stored in insulated boxes in a 3 m deep covered trench that was excavated in the snow. The ice core storage trench maintained a temperature generally below −12°C, with briefly warmer conditions (which would have been buffered within the insulated boxes) on occasions when the store entrance was open. The core was flown out of the site by a Twin Otter, typically within 1 or 2 weeks of the drilling. Most cores spent a period of a few days stored in an undersnow storage cave at the Sky Blu field station before being flown for storage to a cold store either at the Rothera Research Station or to the RRS Ernest Shackleton that was moored at the English Coast. They were loaded into a commercial reefer container at −21°C and transported to the UK on the RRS Ernest Shackleton from January to April 2019. After reaching the UK, the ice core was stored in a commercial freezer storage facility at −22°C until sample preparation.

Ice core sample preparation

Prior to cutting, the length of each 98 mm diameter section of the Skytrain Ice Rise ice core was remeasured. Breaks, wind crusts and other features apparent in the ice core sections were recorded. The solid conductivity of each section was calculated based on the dielectric properties of the ice measured using a custom-built dielectric profiling instrument (Wilhelms and others, Reference Wilhelms, Kipfstuhl, Miller, Heinloth and Firestone1998). The Skytrain Ice Rise ice core was then cut using a band saw into six lengthwise sections in the −23°C cold room at BAS (Fig. 2). The core was first cut in half. One half of the core was preserved for future discrete total air content (TAC) and potentially for discrete trace gas measurements. The top of the other half of the core was then cut and stored for future laser ablation ICP-MS and potential physical properties measurements. Two sections at the outer part of this half core along its flat edge were cut and stored for discrete isotope measurements that could be used to supplement CFA isotope measurements in depth sections with poorer core quality. The inner portion of this half of the ice core was cut into two equally sized 3.2 cm×3.2 cm×80 cm sections. One of these sections was used for CFA. The sections of the ice core used for CFA were taken from the inner cross section of the cylindrical ice core to reduce contamination. All of the cut ice core sections were stored in a commercial freezer at −20°C in plastic layflat bags in insulated boxes. Prior to analysis, the ends of each ice core section were shaved using a metal file to reduce contamination and then remeasured lengthwise in the cold room at BAS. Whenever the core material had to be touched directly, nitrile gloves were worn.

Fig. 2. Schematic of WACSWAIN ice core cross-sectional cutting plan. Dashed lines show lengthwise cuts.

Instrumentation

The CFA system at BAS consists of a melting unit, a water and gas flow system and ten different sets of instrumentation for analyses (Fig. 3). This system has been used for previous ice core analyses and was improved, e.g. in terms of flow efficiency, for the WACSWAIN project. The instrumentation connected to the flow system consists of an Amber Science, Inc. 3082 series conductivity meter, a Dionex conductivity detector (CDM-1), an Abakus (Klotz) particle counter, three FIALab Precision Measurement Technologies photomultiplier tube fluorometers (PMT-FL), a Picarro L2130-i cavity ring down spectrometer (CRDS), a Picarro G2301 CRDS, an Agilent 7700x inductively-coupled plasma mass spectrometer (ICP-MS), a Dionex ICS-6000 fast ion chromatography (FIC) system, a Dionex ICS-3000 FIC system and a bubble detector (Sonotec, Sonocheck ABD06.85). Each of these systems is used to analyze components that are relevant for understanding past atmospheric composition. Methods that employ the ICP-MS and FIC ICS-3000 systems to detect sodium, magnesium and calcium, along with the fluorometer for calcium detection, are the focus of this study.

Fig. 3. Schematic of CFA system showing the melt system, pumps (p), valves (v), standard introduction, ultrapure water introduction, reagent introduction, flow rates (mL min⁻¹) and analytical systems in melting setup.

The ice core melting unit consists of a Perspex^® stand that is removed to load each 3.2 cm×3.2 cm×80 cm ice core section. The stand is then secured directly above the melt head. The melt head was designed, following Bigler and others (Reference Bigler2011), with a copper base, due to its high thermal conductivity, on a 5 μm layer of nickel with a 2 μm gold surface coating. The interface has grooves that direct meltwater flow through a 1.6 mm diameter hole in its center. The melt head is heated using heating cartridges, which are controlled using a PID controller. The ice core melter and sample tubing were cleaned daily by wiping with lint-free tissues and sequentially flushing with isopropyl alcohol and ultrapure water.

The ice core melt rate is determined by the nominal temperature of the melt head, the density of the ice core and the gravitational load exerted on the core piece onto the melt head. The melt rate was kept at ~3.0 cm min⁻¹, mainly by adjusting the melt head temperature, to ensure a sufficient water supply for all instruments attached to the flow system. There are five meltwater streams. Meltwater from the inner section of the ice core is directed through a hole in the center of the melt head. This fraction of meltwater is the least contaminated of the ice core because it originates from the center of the ice core cross section. Only the meltwater created in that center section is then directed to all analytical instruments. The four other meltstreams originate from the outer sections of the ice core and are directed to waste or are collected for discrete analysis less prone to contamination.

Meltwater is directed through holes in the melt head into temperated PTFE (sample) and PFA (waste lines) tubing through the wall of the freezer. After exiting the freezer, meltwater from the inner section of the melt head flows through a non-invasive clip-on bubble detector (Sonotec, Sonocheck ABD06.85) used to measure the air to water ratio as a proxy for TAC (Kaufmann and others, Reference Kaufmann2008). All of the meltstreams then flow through an Ismatec ISM932D peristaltic pump (pump 1). The meltwater from the inner section of the ice core flows through the pump at ~22 mL min⁻¹. It is then pumped through a Vici EHMA valve followed by a custom-built gravity-controlled debubbler.

Meltwater containing air from the top of the debubbler flows through an Ismatec ISM930 peristaltic pump to the Picarro G2301 CRDS for CH₄ analyses. Bubble-free meltwater flowing from the bottom of the debubbler is directed through the Ismatec ISM932D peristaltic pump a second time at a rate of 8 mL min⁻¹. This meltwater is then distributed to all of the other instruments using an Ismatec ISM934C peristaltic pump (pump 2). The whole flow system from the melt head up to peristaltic pump 2 has an internal volume of ~3.85 mL. From there, the lines going to the FIC, the ICP-MS and the fluorometer have volumes of 1.26, 1.16 and 0.11 mL, respectively (Fig. 3). The spatial distances between the melt head and the instruments based on the length of tubing sum up to 9.1 m for the FIC, 7.1 m for the ICP-MS and 5.0 m for the fluorometer.

Inductively-coupled plasma mass spectrometry

An Agilent 7700x ICP-MS in no gas mode with argon carrier gas (1.09 L min⁻¹) was used to measure elemental sodium (with a mass-to-charge ratio (m z ⁻¹) of 23), magnesium (24 m z ⁻¹) and calcium isotopes (43 and 44 m z ⁻¹). Meltwater was delivered to the system at a flow rate of 1.42 mL min⁻¹. A stream of 2% nitric acid at a flow rate of 0.1 mL min⁻¹ was added to the meltwater flow online to improve sensitivity. This addition lowered the meltwater stream pH finally reaching the ICP-MS to ~1.7. The water was introduced into the instrument using a quartz nebulizer (Micromist) at an aspiration rate of 0.62 mL min⁻¹ and a spray chamber that was water-cooled at 2°C. The plasma was set with an RF forward power of 1550 W. Measurements were taken with an integration time of 0.1 s. Tuning parameters, including gas flows and ion optics, were adjusted daily using an Agilent tuning solution (1 μg L⁻¹ Ce, Co, Li, Mg, Tl and Y in 1 wt% HNO₃(aq)) and a standard solution that was also used for calibration (500 μg L⁻¹ Na and 100 μg L⁻¹ Mg, Al, Ca, K, NH₄, respectively, in 1 wt% HNO₃(aq)). The mass resolution achieved for sodium, magnesium, calcium-43 and calcium-44 was on average 0.5 amu. Sensitivities of ~20 000 counts per second (cps) per μg L⁻¹ for sodium, 13 000 cps per μg L⁻¹ for magnesium and 30 cps per μg L⁻¹ for calcium-43 could be achieved.

Fast ion chromatography

A Dionex ICS-3000 FIC with conductivity detection was used to measure sodium, magnesium, calcium, ammonium and potassium ions. This experimental set-up consisted of two identical analytical systems. It therefore allowed for measurements at a better depth resolution relative to a traditional IC system because one sample was already being loaded into the injection loop of one system while another was injected onto the column of the other system. For each analytical system, meltwater flowing from the melt head was loaded into a 1.0 mL injection loop at a flow rate only slightly above 1.0 mL min⁻¹ to ensure that the loop was filled completely. An automation manager valve built into the system allowed for automatic loading. Every 1.35 min the valve switched to load system 1 or 2. Any excess meltwater was diverted to waste.

Each 1.0 mL sample was separated using a Dionex Ionpac CS12A 5 μm 150 mm column and a 30 mM methanesulfonic acid (MSA) eluent at a flow rate of 1.0 mL min⁻¹. The eluent was generated electrolytically by an eluent generator cartridge (Dionex EGC III MSA). The CS12A column is designed for analyte separation with short retention times. A Dionex CERS 500e electrolytically regenerated suppressor was used to remove anions from the column effluent. The total run time for each sample was 2.7 min. In order to achieve this short run time for higher resolution measurements, calcium and magnesium ions from one sample injection were retained on the column and eluted on the following chromatogram. The retention times of sodium, ammonium and potassium were on average 2.2, 2.4 and 2.5 min, respectively. The retention times for magnesium and calcium in the following run were 0.40 and 0.80 min, respectively (Fig. 4).

Fig. 4. Chromatograms produced using the FIC method: (a) ultrapure water blank, (b) standard (50 μg L⁻¹ Na⁺, 10 μg L⁻¹ Ca²⁺, Mg²⁺, NH$_4^ +$ and K⁺) and (c) a WACSWAIN ice core sample.

Fluorescence detection

Fluorometry was employed as an additional method for calcium detection, using an FIAlab photomultiplier tube-fluorescence detector (PMT-FL). Ice core meltwater at a flow rate of 1.14 mL min⁻¹ was combined online with a reagent at a flow rate of 0.2 mL min⁻¹. The reagent was 0.01 mM of HPLC-grade Quin-2 (≥99%, Sigma-Aldrich) in a 35 mM Suprapur sodium hydroxide solution (Merck) with a 20 mM PIPES buffer (≥95%, Sigma-Aldrich). The mixture flowed through 0.8 m of coiled reaction tubing and a degasser (Systec, Multi Channel Vacuum Degasser) prior to introduction into the fluorometer, where it passed through a 15 mm long, 3 mm diameter, 100 μL Suprasil flow cell with a 3 mm optical path length. An LED light with a band pass filter excites the fluorescent Ca²⁺ compound at 320 ± 10 nm. The PMT detects emission of the light after it passes through another band pass filter at 492 ±10 nm.

Standardization

Intermediate standards were prepared using sodium (as NaNO₃ in 2% HNO₃, 1.000 mg L⁻¹ ±0.005 mg L⁻¹, Sigma Aldrich), magnesium (as Mg(NO₃)₂ in 2% HNO₃, 1006 mg L⁻¹ ±4 mg L⁻¹, Sigma Aldrich) and calcium (as CaCO₃ in 2% HNO₃, 1000 mg L⁻¹ ±2 mg L⁻¹, Sigma Aldrich). The intermediate mixed standard was diluted to mixed solutions of 50 mg L⁻¹  of sodium, 10 mg L⁻¹ of magnesium and 10 mg L⁻¹ of calcium in ultrapure water. Working standards, ranging from 0 to 1000 μg L⁻¹ of sodium, 0 to 200 μg L⁻¹ of magnesium and 0 to 200 μg L⁻¹ of calcium were prepared by dilution of these standards in ultrapure water monthly. All standards were refrigerated and shaken before use.

Calibration curves were produced daily over the range of the working standards for each method using a linear least squares regression. These curves were used to determine ice core sample concentrations. For FIC, the peak height was used for calibration. For the ICP-MS and fluorescence detection, the average level of the signal plateau after ~5 minutes of standard measurement was used for calibration. Standards were analyzed prior to ice core samples each day and after the end of ice core analysis each day.

Precision and limits of detection

Given previously reported minimum Antarctic ice core concentrations (Röthlisberger and others, Reference Röthlisberger2002; Sigl and others, Reference Sigl2016; Winski and others, Reference Winski2019), we aimed for a precision of ±0.5 μg L⁻¹ for sodium and ±0.1 μg L⁻¹ for magnesium and calcium, or ±5% of the lowest level standard used, which consisted of 10 μg L⁻¹ of sodium and 2 μg L⁻¹ of magnesium and calcium. In this study, the relative standard deviation (RSD) of the lowest level standard is used as a measure of precision. The RSD is defined as the std dev. of concentrations of the lowest level standards divided by their known levels after removal of outlier concentrations. The concentrations of the standards were calculated using their respective daily calibration curves. Outliers of the ICP-MS and fluorescence data were removed using the ‘rmoutliers’ Matlab function, in which values outside of the scaled median absolute deviation are removed. The RSD values of the ICP-MS and fluorescence methods for all of the compounds are below the 5% goal and they are all lower than those of the FIC method (Table 1). This result suggests that, in terms of reproducibility of measurements at expected concentration levels, the ICP-MS and fluorescence methods are superior to the FIC method.

Table 1. ICP-MS, FIC and fluorescence method ice core concentration ranges and means, LODs, RSDs and percentages of measurements below LOD

The RSD is defined as the std dev. of concentrations of standards (10 μg L⁻¹ for sodium and 2 μg L⁻¹ for magnesium and calcium) divided by their known concentrations. The number of blank and standard measurements used for the RSD and LOD calculations are also listed (n). The best LOD and RSD values of the methods used are highlighted in bold.

The limit of detection (LOD) is the lowest concentration of an analyte that can be reasonably measured using each method. We aimed to develop methods that reliably detect levels below minimum expected Antarctic ice core levels of sodium (5 μg L⁻¹), magnesium (1 μg L⁻¹) and calcium (1 μg L⁻¹). In this study, the LOD is defined by

(1)$${\rm LOD} = \mu{\rm _{b}} + 1.645( \sigma{\rm _{b}}) + 1.645( \sigma{\rm _{s}}) ,\; $$

where μ_b is the mean of the blank concentrations, σ_b is the std dev. of the blank concentrations and σ_s is the std dev. of the lowest level standard concentrations (10 μg L⁻¹ sodium, 2 μg L⁻¹ magnesium and calcium) (Armbruster and Pry, Reference Armbruster and Pry2008). The std dev. are multiplied by 1.645 in order to include the highest 95% of the measurements (assuming a Gaussian distribution).

The blank signal was determined by measuring ultrapure water (R = 18.2 MΩ cm) directly from the IC Pure system as sample. It therefore does not account for any contamination effects in the melt system before valve 2 (Fig. 3). Blank concentrations were determined using the instrument response of the blank and the daily calibration curves of the ICP-MS and fluorescence detection methods. For the FIC method, blank levels (n = 16) were determined using peak heights of ultrapure water and daily calibration curves from three days of analysis.

Differences in data collection and processing between the methods affect their respective LOD values. For sodium and magnesium, the LOD values of all three methods are well below minimum values observed between the Skytrain ice core depths of 295 and 305 m. In terms of detection of low level concentrations, these methods are therefore acceptable for analysis of the Skytrain ice core. In general, the LODs for calcium are all well below mean levels of calcium in this depth interval. However, in some cases, the ICP-MS method for detection of calcium-44 and the fluorescence method resulted in calcium values below their LODs in this section of the core (Table 1). The calcium methods are therefore generally sufficient for analysis of the Skytrain ice core, but it is worth noting that some observations were below detection.

Depth resolution

The depth resolution of each method is particularly important in ice core science because it determines the temporal resolution of the record produced. In this study, the aim was to develop methods that achieved depth resolutions at or below the modeled annual layer thickness of the Skytrain ice core at 2000 years before present (~5–9 cm). Depth resolution is consistent throughout a measurement campaign at constant instrumental settings, but the temporal resolution decreases with depth because annual layer thickness decreases with depth. Depth resolution and the resulting temporal resolution of an ice core measurement depends on the sample injection rate, melt rate, flow rate and potential dispersion effects in the water lines as well as in the instruments. Depth resolution is determined by the mean difference in depth between each measurement. For continuous methods such as ICP-MS and fluorescence detection with very high sampling rates, the melt rate and the dispersion of the signal are the dominating and limiting factors (Sigg and others, Reference Sigg, Fuhrer, Anklin, Staffelbach and Zurmuehle1994). To quantify this dispersion effect on depth resolution for the ICP-MS and the fluorescence detection systems, the response of the signal to a step change in concentration, namely a calibration standard, was evaluated. This was done following the procedure described in Gkinis and others (Reference Gkinis, Popp, Johnsen and Blunier2010) and Emanuelsson and others (Reference Emanuelsson, Baisden, Bertler, Keller and Gkinis2015). Cumulative distribution functions of the lognormal distribution were fitted to the signal steps according to

(2)$$\delta{\rm _{\,fit}}( t) = {a_1\over 2}\left[1 + {\rm erf}\left({{\rm ln}( t) - b \over c \sqrt{2}}\right)\right] + a_2,\; $$

where t denotes the time and δ_fit(t) denotes the resulting fit function. The constants, a ₁, a ₂, b and c, have been determined using a least squares optimization. Examples of the derived fit functions for all investigated analytes using a medium concentration standard (50 μg L⁻¹ Na, 10 μg L⁻¹ Mg, Al, Ca, K and NH₄, respectively) are shown in Figure 5. Subsequent to fitting, the time between the decrease from 90 to 10% of the average step height was evaluated in the fit function. This was done for ten different measurement days (30 samples) over the course of 3 months for the ICP-MS data and for 25 measurement days (70 samples) in the same time frame for the fluorescence detection method. Multiplication of the derived median decay times with the average melt rate (3 cm min⁻¹) yields the dispersion dominated depth resolution. An overview of the resulting values is given in Table 2.

Fig. 5. Sodium (a), magnesium (b) and calcium (c, d) dispersion calculation for ICP-MS and fluorescence detection methods. Blue line shows decay of a standard (50 μg L⁻¹ Na, 10 μg L⁻¹ Mg, Al, Ca, K and NH₄, respectively). Red line is the fit following Eqn (1) (Gkinis and others, Reference Gkinis, Popp, Johnsen and Blunier2010; Emanuelsson and others, Reference Emanuelsson, Baisden, Bertler, Keller and Gkinis2015).

Table 2. Decay times and resulting depth resolution for dispersion calculations for Na, Ca and Mg in ICP-MS and fluorescence detection

A melt rate of 3 cm min⁻¹ was used. n denotes the number of evaluated samples. For the decay times, the median and STD was evaluated.

For the FIC method, this direct evaluation of a step in analyte concentration is not possible because each measurement is a discrete volume of meltwater injected from the sample loop. The depth interval in each injection volume can be calculated according to

(3)$$D{\rm _{sample}} = {V{\rm _{sample}}R{\rm _{melt}}\over R{\rm _{\,flow}}},\; $$

where D _sample is the depth resolution of each sample injection, V _sample is the sample loop volume (1 mL), R _melt is the melt rate (3 cm min⁻¹) and R _flow is the flow rate (1.14 mL min⁻¹) (Cole-Dai and others, Reference Cole-Dai, Budner and Ferris2006). Using this equation, the depth resolution of each FIC measurement is 2.63 cm. However, each time meltwater flows through the sample loop, the loop is ‘overfilled’, meaning that some of the meltwater goes to waste. This is done to ensure that the sample loop does not contain any air or eluent prior to injection onto the column, which would affect the measurement. Due to this overfilling of the sample loop, some meltwater is lost between measurements. The depth interval lost between each FIC measurement can be calculated by

(4)$$D{\rm _{loss}} = R{\rm _{melt}}T - D{\rm _{sample}},\; $$

where D _loss is the depth interval lost between FIC measurements and T is the time interval over which the sample loop is filled (1.35 min) (Cole-Dai and others, Reference Cole-Dai, Budner and Ferris2006). This value can be interpreted as the time elapsed before the valve switches from system 1 to system 2. This calculation results in a loss of 1.42 cm between FIC measurements (35% of the meltwater is lost). Therefore, there is one FIC measurement every 4.05 cm. Consequently, the FIC method is only adequate for layer counting near the surface where annual layers span tens of centimeters (accumulation rate: 18 cm w.e. a⁻¹) and for detection of peaks lasting several years (e.g. some volcanic eruptions). It is not suitable for annual layer counting at greater depths. If necessary for a future project, an improved depth resolution could be achieved by reducing the melt rate and flow rate to the FIC system within the limitations of the FIC system and parallel experimental systems.

An FIC depth resolution of 2.63 cm initially appears to be better than the ~4 cm depth resolution calculated for the ICP-MS method (Table 2). However, it is important to note that the ICP-MS method has a measurement frequency equivalent to a depth interval of 0.04 cm (i.e. one ~4-cm sample every 0.04 cm) compared to 4.05 cm for the FIC method (i.e. one 2.63-cm sample every 4.05 cm). There are therefore 101 ICP-MS measurements for every one FIC measurement. There are also 70 fluorescence measurements (i.e. one 1.4-cm sample every 0.06 cm) for every 1 FIC measurement. The ICP-MS method also samples meltwater more frequently than the fluorescence method for calcium, but the fluorescence method has ~3× the depth resolution. This difference in depth resolution is caused by a larger dispersion of the signal in the ICP-MS measurement. This signal dispersion could be due to the proximity of the PMT-FL to the melter (5.0 m) relative to the ICP-MS (7.1 m) or differences in depth resolution produced within the instruments.

In essence, the ICP-MS and the fluorescence methods produce running averages at ~4-cm and 1.4-cm depth resolutions, respectively, with no loss of sample between measurements. The advantage of these ‘moving average’ type data is that the ICP-MS and fluorescence methods are easily capable of resolving annual peaks that are ~4 cm apart. They can therefore be used to count annual layers that are 5 cm thick. In contrast, the FIC method results in ‘bin-averaged’ type data at 4.05-cm intervals that can only be used to identify annual signals where the accumulation rate is high and the ice core has not undergone thinning.

Analyte recovery

All of the methods in this study resulted in similar levels, ranges, and means from 295 to 305 m in the Skytrain ice core (Table 1). The variability of the levels of the compounds with depth was also similar for all of the methods (Figs 6, 7). A comparison between the concentrations detected using ICP-MS data from depths nearest to depths assigned to the FIC data resulted in slopes near 1 for all three elements with r ² values above 0.76 (n = 204; Fig. 8). The FIC method can only detect ions in solution. The similarity in analyte levels between the FIC and ICP-MS methods therefore suggests that the ICP-MS method also only detected soluble sodium, magnesium and calcium components. This result is in agreement with past studies that discuss the need to subject discrete samples to acid digestion in order to recover insoluble particles using ICP-MS (Knüsel and others, Reference Knüsel, Piguet, Schwikowski and Gäggeler2003; Ruth and others, Reference Ruth2008; Rhodes and others, Reference Rhodes, Baker, Millet and Bertler2011; Arienzo and others, Reference Arienzo, McConnell, Chellman and Kipfstuhl2019; Erhardt and others, Reference Erhardt, Jensen, Borovinskaya and Fischer2019). This result is especially relevant for calcium which has significant soluble and insoluble terrestrial dust components (Ruth and others, Reference Ruth2008; Rhodes and others, Reference Rhodes, Baker, Millet and Bertler2011; Arienzo and others, Reference Arienzo, McConnell, Chellman and Kipfstuhl2019).

Fig. 6. Skytrain ice core: (a) sodium and (b) magnesium from 295 to 305 m depth. Measurements were made using ICP-MS (blue) and FIC (orange) methods.

Fig. 7. Skytrain ice core calcium from 295 to 305 m depth: (a) calcium-43 (blue) and calcium-44 (orange) ICP-MS measurements; (b) calcium levels using the FIC method (violet) and fluorescence detection (gray).

Fig. 8. Skytrain ice core ICP-MS and FIC method comparison. Points are (a) sodium, (b) magnesium or (c) calcium (one example) levels using each method from 295 to 305 m (n = 204). Points for the ICP-MS method are ICP-MS data at depths nearest to depths assigned to the FIC data. Solid line is the linear fit (R ² = 0.76 for Na, R ² = 0.87 for Mg and R ² = 0.77 for Ca). Dashed line is the standard error of the fit.

Prior to laboratory analysis, we calculated an expected sodium concentration deposited at the Skytrain Ice Rise site of 60 μg L⁻¹. This value is based on the sea-water sodium/chloride ratio and the expected chloride concentration 550 km from the edge of the Ronne Ice Shelf, which is based on a 30% per 100 km reduction in concentration with distance from the coast reported in Minikin and others (Reference Minikin, Wagenbach, Graf and Kipfstuhl1994). This 60 μg L⁻¹ estimate is only slightly higher than mean sodium levels of 55 μg L⁻¹ (ICP-MS) and 58 μg L⁻¹ (FIC) in the Skytrain ice core from 295 to 305 m (Table 1). This analysis confirms that the Skytrain ice core sodium levels are consistent with the pattern observed in a previous study on the Ronne Ice Shelf, where the sodium concentration was indicative of distance from the ice edge (Miniken and others, 1994).

Preliminary interpretation of sources

A comprehensive climatic interpretation of the sodium, magnesium and calcium signals in the 10 m core section presented here is beyond the scope of this study, but we performed some initial analyses to explore possible interpretations of the records. Using the results of the ICP-MS method, the mean ratio of magnesium to sodium is 0.141 with a std dev. of 0.028 (19.6% of the mean). This ratio is only slightly higher than the sea-water magnesium to sodium ratio of 0.119, suggesting that most of the magnesium detected in this section of the ice core is likely of marine origin. The mean ratio of calcium (ICP-MS calcium-43) to sodium of 0.112 is substantially higher than the sea-water calcium to sodium ratio of 0.038. However, with a std dev. of 0.063 (56% of the mean), this ratio is three times as variable as the magnesium to sodium ratio and ratios of calcium to sodium range from 0.0165 to 0.562 along this depth interval. Variability of sodium levels and ratios of calcium to sodium are also strongly out of phase along the depth scale. These findings suggest a periodic shift in the sources of calcium to the ice core site, which needs to be investigated further.