General laboratory procedures and equipment cleaning

All ice-preparation was carried out in a cold room (−20 °C) at the British Antarctic Survey, Cambridge, and all sample processing and analysis were carried out in clean laboratories at the University of Cambridge. Seawater analysis was carried out at Old Dominion University, USA. All clean work was carried out within a Class 1,000 HEPA-filtered clean laboratory and/or under Class 100 laminar flow air.

All water used was ultrapure (18.2 MOhm cm) and acids were either quartz-distilled (QD) to high purity from reagent grade or purchased as ultrapure (Fisher Scientific). Strict trace metal protocols were used, including wearing polyethylene (PE) gloves to minimize metal contamination during cleaning, processing and sampling. All plastic equipment was rigorously acid-cleaned following the procedures outlined below, which were based on previously published work45 or adapted from routine cleaning procedures in the Department of Earth Sciences at Cambridge.

New perfluroalkoxy (PFA) or fluorinated ethylene propylene (FEP) Teflon filter rigs, vials, jars, bottles and micro centrifuge tubes were cleaned by sequential immersion in weak (∼5%) Decon detergent (24 h), warm 50% (v/v) reagent-grade HNO 3 (24–48 h) and warm 50% (v/v) reagent-grade HCl (24–48 h). Teflon equipment was re-cleaned by immersion in either warm 50% (v/v) HNO 3 or HCl (24 h). All Teflon equipment was thoroughly rinsed with ultrapure water following each step and then left to dry on a clean drying rack under laminar flow. For any Teflon equipment that was to be in contact with ice or samples at near-neutral pH, the plastic was filled with ultrapure water to condition the surface. Nalgene low-density PE and polycarbonate (PC) bottles were cleaned following previously published cleaning methods for preparation of equipment for seawater sampling45: bottles were sequentially immersed in weak (∼5%) Decon detergent (48 h), 1 M reagent-grade HCl (48 h), 6 M reagent-grade HCl (48 h) and ∼1% (v/v) QD-HCl (>3 months), with extensive rinsing with ultrapure water between steps. Bottles were conditioned to neutral pH by filling with ultrapure water for 1 week before use. Whatman Nucleopore PC membranes for filtration (0.2 μm) were cleaned by immersion in 6 M QD-HCl (48 h) and then rinsed extensively with ultrapure water. Membranes were always handled with clean PFA tweezers to minimize contamination. Membranes were stored in and flushed with ultrapure water before use to condition the plastic to near-neutral pH for contact with ice samples. Pipette tips were either acid-cleaned45 with 50% (v/v) reagent-grade HCl (24 h) and rinsed with ultrapure water (seawater samples), or rinsed twice with 10% (v/v) QD-HNO 3 and twice with ultrapure water before use (non-seawater samples).

For handling and decontamination of ice samples, Kyocera ceramic knives and polytetrafluroethylene (PTFE) Teflon boards were used. Knives were cleaned by sequential immersion in 6 M reagent-grade HCl (24 h) and 1 M reagent-grade HCl (24 h), followed by extensive rinsing with ultrapure water. PTFE boards were cleaned by immersion in 6 M reagent-grade HCl and then rinsed extensively with ultrapure water. Arm-length PE gloves for handling ice were cleaned by immersion in weak (∼0.01% v/v) QD-HCl (24 h) and rinsed thoroughly with ultrapure water to ensure no transfer of acid to samples.

Ice samples and cutting and decontamination

Detailed information about the age of ice sampled in this study is shown in Supplementary Data. EDC samples from 21 to 26 kyr BP were chosen to cover the very high dust portion of the LGM (∼0.6–1.2 mg kg−1 dust; ref. 15) based on previous work15,46. ‘A cuts’ (or ∼¼ of the core) from 15 separate ‘bags’ of ice (55 cm vertical section; ∼50 years) from the EDC ice core were supplied by EPICA (see EPICA cutting plan in Supplementary Fig. 2a). For EDC sampling, ∼3 cm pieces were typically cut from either the bottom or top of each bag, using a steel bandsaw within the cold room (−20 °C) at the British Antarctic Survey. Two vertical cuts were then made to create three subsamples of identical age (Supplementary Fig. 2b), which were then taken for decontamination and processing to generate subsamples of identical age for total or soluble analysis. The sampling resolution was therefore typically 50 years, although each sample represents only ∼3 years. Four bags (959, 984, 986 and 987) were sampled at higher resolution (two to four 3 cm subsamples per 50 years) to investigate higher-resolution variability as well as reproducibility between ice subsamples of identical age. Precise age for each 3 cm sample was calculated from depth within each bag and the EDC3 age scale47. Berkner samples were chosen to cover MIS 2–3, with two samples within the LGM (23 and 26 kyr BP) and 10 samples across the ‘A’ events48 of MIS 3 (36–50 kyr BP) based on the Berkner δD record (Mulvaney, unpublished data). Ice was supplied as ¼ pieces of Berkner core (diameter 50 mm), with 3 cm vertical pieces cut and subsampled analogous to EDC samples.

The outside of ice core material is typically unavoidably contaminated with trace metals from drilling, cutting with bandsaws and handling with gloves49. Thus, the outside of the ice must be decontaminated to remove outer layers where contamination overwhelms the natural signal. Previously, this has been carried out using a melt-head50, polyethylene lathe51, stainless steel blade52, ceramic blade or chisels53. Decontamination may require removal of several cm of core22,51,52,54 during low-dust or less-compacted intervals, or when elements of interest are only present at pg g−1 levels. Preliminary testing in this study demonstrated that, as expected, ultrapure water ice was severely contaminated following cutting with a bandsaw (20 ng g−1 Ca, 4 ng g−1 Fe and 8 ng g−1 Al). The small size of ice samples used in this study prevented the use of techniques that remove several cm of core, and so we developed a technique using Kyocera ceramic blades that sequentially removed contaminated ice-core material. Each piece of ice was placed on a PTFE board and held with a clean PE glove, while the outside few mm were scraped off with a clean ceramic blade. This was repeated three times using separate clean equipment. To achieve the lowest contamination levels, all ice sampling equipment and ice were only handled with clean PE gloves, and all ice decontamination was carried out under Class 100 laminar flow air within the cold room (−20 °C) at the British Antarctic Survey.

Decontamination blanks were 47 pg g−1 for Fe and 116 pg g−1 for Al (<0.1% of natural conc.), and testing established that using acid-cleaned equipment had no effect on the natural pH of ice (pH ∼5.3). Our simpler technique did not achieve the full decontamination of more rigorous chiselling methods22 (46 pg g−1 c.f. 4 pg g−1 Fe). However, our method can be applied to smaller pieces of ice, and the procedure reduces Fe contamination by factor of ∼100 × , resulting in insignificant contamination that represents <0.1% of typical natural Fe concentrations within ice from the high dust portion of the LGM.

Total digestion and pH ∼ 5.3 rapid-filtration procedures

Following decontamination, ice samples for total digestion were allowed to melt within PFA vials and acidified with 50 μl of QD-HNO 3 (conc.) per 5 ml. A 5-ml aliquot was evaporated to dryness, refluxed overnight at ∼80 °C with 100 μl QD-HNO 3 (conc.) and 200 μl Teflon-distilled conc. HF, evaporated to dryness, redissolved in 100 μl of 6 mol l−1 QD-HCl, refluxed at ∼80 °C overnight and evaporated to dryness. Finally, samples were redissolved in 250–500 μl of 0.1 mol l−1 QD-HNO 3 for analysis by inductively coupled plasma optical emission spectrometry (ICP-OES). Procedural blanks (n=26) were 1.9 ng g−1 (Fe), 5.7 ng g−1 (Al) and 2.8 ng g−1 (Ca), all <0.5% of natural concentrations.

Following decontamination, ice samples for soluble analysis were processed by either ‘rapid-filtration’ (melting at natural pH, referred to as ‘pH ∼5.3) or ‘seawater-leaching’ methods (see later sections). Ice samples for rapid filtration were allowed to melt at room temperature (∼2 h) on a 0.2-μm Whatman polycarbonate membrane inside a 47-mm PFA Savillex filter rig, under vacuum. Rigs were covered to prevent contamination. Liquid that formed was quickly filtered and collected in a PFA reservoir containing an appropriate volume of conc. HNO 3 to acidify samples to pH 0. On completion of melting, dust was leached with three 25 ml aliquots of ultrapure water, similarly acidified. This procedure, analogous to instantaneous leaches of modern aerosols27, was designed to minimize any FeOH 3 precipitation or wall adsorption, which are concerns with flow-through and long leaching procedures (Supplementary Discussion). Acidified samples were evaporated to dryness and redissolved in 300 μl of 0.1 mol l−1 QD-HNO 3 for analysis by ICP-OES. Fe concentration from the meltwater and subsequent ultrapure leaches were combined in final calculations. Procedural blanks (n=78) were 17 pg g−1 (Fe), 244 pg g−1 (Ca) and 83 pg g−1 (Na), all corresponding to 0–2% of natural concentrations.

Elemental analysis

Samples from total digestion or rapid filtration were analysed for dissolved Al, Ca, Fe and other cations using a Varian Vista ICP-OES at the University of Cambridge. Each sample was analysed six times and a mean concentration was determined by reference to a quadratic calibration curve based on five standards. Single-session accuracy on a 250-ng per gram internal multi-element reference standard for Al, Ca and Fe was >99.6% with precision of 0.41% (1 s.d.; n=10). Long-term reproducibility of the standard was 100±1% for all elements (n=60). Detection limits for Fe were 0.1–0.4 ng g−1. We assign 5% uncertainty on data to account for weighing, pipetting and %RSD of six ICP-OES analyses (typically <5%).

Sublimation

Sublimation allows the extraction of gases or dust from ice-core material while preventing melting, which could compromise the sample fidelity, and is achieved by lowering the pressure and temperature of ice below the triple point (0.01 °C, 6.1173, mbar). Sublimation has been previously used to investigate concentrations and isotope ratios of gases (for example, CO 2 and CH 4 ) released from bubbles trapped in ice-core material55,56,57,58, as well as having been utilized to cross-check CO 2 measurements obtained using other techniques in Bern59. In the present study, we were motivated to develop a sublimation technique to extract dust from ice cores without exposing the dust to liquid water at a pH lower than that of seawater (pH ∼8), which may alter the speciation or solubility of Fe within dust before analysis (Supplementary Discussion). The major mode of deposition of dust at Dome C during the LGM is thought to be dry20, meaning that EDC dust extracted by sublimation should be representative of dry dust deposition at Dome C during the LGM.

We designed a sublimation apparatus based on previous studies on gases58, adapted to cleanly extract large masses of dust57 from ice without melting. The apparatus consists of a Schott borosilicate glass desiccator, sealed with Apiezon N vaccum grease, a vaccum pump with oil cleaner and pressure gauge and a −100 °C freeze trap for removal of water vapour (Supplementary Fig. 3). To allow flexibility depending on the mass of dust/ice required by experiments, two configurations of the system were designed (Supplementary Fig. 3). In the present study, ice samples were placed directly on membranes within filter rigs within a Savillex PFA Teflon Jar with top ports open (25–50 g; Supplementary Fig. 3a). A second configuration, not used in the present study, allows for the collection of large dust masses, with larger pieces of ice (>50 g) placed within 1 l bottles within a 4-l bottle with holes drilled in the lid (Supplementary Fig. 3b). Although ice sublimes under any pressures and temperatures below the triple point, previous studies have shown that quasi- and surface-melting can occur on ice or ice–glass surfaces at temperatures (−1 to −4 °C) and pressures below the triple point60,61,62. To avoid this, it is necessary to keep the ice cold until the pressure is sufficiently lowered, and to operate at pressures much lower than the triple point (<2 mbar). To achieve this, first the glass desiccator was taken into a Class 100 laminar flow bench within the cold room and allowed to cool before being cleaned with propanol. Jars or bottles were then placed within the cleaned desiccator and the desiccator was closed and sealed before being transferred to the warm laboratory and connected to the pumps. This ensured that samples remained clean and that the ice remained cold for ∼10 min while the pressure was reduced below the triple point to the typical running pressure of 0.8–1.6 mbar.

Sublimation was allowed to proceed until all ice was gone, which was assessed visually or when the pressure had dropped to the background of the empty chamber (0.26 mbar). Rates of sublimation were relatively slow, dependent on ice mass and volume, with a maximum rate of ∼250 g per 24 h. Small samples (5–10 g) sublimed in several hours, but were typically left overnight to ensure all ice was gone.

Once all ice had sublimed, the desiccator was sealed, disconnected and returned to the laminar flow bench to slowly allow clean air to enter the chamber; this prevented both dust being blown away by a sudden increase in pressure and the entry of dirty air from the pump. The bottles containing the dry dust samples were then carefully bagged and taken for ‘seawater-leaching’ procedures (see next section). Blank contamination for the whole procedure over 24 hours was found to be acceptably low (∼40 pg Fe per ice sample).

Seawater-leaching procedures and Fe analysis

Ice samples for seawater-soluble Fe analysis (‘seawater-leaching’) were placed on a membrane on a PFA rig within a 1 l PFA jar within a glass desiccator at −20 °C, and quickly placed cleanly under vacuum (before moisture could form) at ∼18 °C. Samples remained under vacuum until all the ice had sublimed, leaving dry dust on the membrane (see ‘Sublimation’ section). The dust on the membrane was then leached with four 60-ml aliquots of low-Fe Ross Sea seawater at 2 °C. The Ross Sea seawater used in leaching was previously collected63 from 10 m depth in the Southern Ross Sea (76° 02′ S, 169° 53′ E) under clean conditions in December 2003. The water was filtered with 0.2 μm polypropylene cartridges and stored in 125 l polyethylene barrels, before being subsampled for this work using clean procedures63. We determined the background dissolved Fe concentration in this seawater to be 4.7 pg g−1 (0.11±0.02 nmol l−1, 2 s.d.; n=4), consistent with previous determination63,64 of 0.07±0.02 nmol l−1. Each aliquot of seawater was allowed to leach the dust momentarily, vacuum was applied and then the liquid was pulled through and acidified to pH 1.6 with 120 μl of conc. ultrapure HCl. Samples were left acidified for >2 weeks before being analysed for dissolved Fe by FIA-8HQ, following published techniques45. This method has low detection limits (<5 pg g−1) and analytical precision is typically 5–10% during routine analysis; we therefore applied 10% uncertainty on seawater Fe concentration. Fe concentrations from four leaches were combined in final calculations.

Berkner Deuterium and parameters and age scales

The ice core at Berkner Island was drilled to bedrock, and completed in 2005 (ref. 65 and Fig. 2). Deuterium (δD) ratios (expressed relative to the international standard Vienna Standard Mean Ocean Water) were measured at the NERC Isotope Geosciences Laboratory using standard methods66 and have a typical precision of 1.0‰. In Fig. 2b, the Berkner δD record across A events A1 and A2 is shown overlain on the EPICA Dronning Maud Land (EDML) δ18O record67, which is shown on the EDML1 age model that has been synchronized to EDC3 (refs 47, 67). In the absence of an official age model for this part of the Berkner Island ice core, Berkner δD is approximately visually tuned to the EDML δ18O record so that EDML δ18O, Berkner δD and Berkner Fe data are all shown on approximately the EDC3 age scale. Dust and temperature in Fig. 2a are from the EDC core15,68, while atmospheric CO 2 concentration (pCO 2 ) in both Fig. 2a,b is taken from the composite CO 2 record of Schilt et al.69

Fe solubility and flux calculation

Elemental fluxes were calculated:

where [Fe] is the concentration of Fe (mg kg−1) in the ice, A R is the calculated ice-accumulation rate (kg m−2 per year; ref. 47, Mulvaney, unpublished data).

Fe solubility was defined:

where [pH ∼5.3 or seawater-soluble Fe] and [total Fe] are the concentration in ice meltwater (ng g−1), combined from multiple leaches.

Calculating nss-Ca

nss-Ca in each ice-core sample was calculated in this study from measured dissolved Ca and Na concentration data using paired simultaneous equations50: