Geochemistry of Cryoconite Holes, Troll Blue Ice Area, Antarctica

Introduction

Cryoconite holes, snow and glacier ice were sampled in eight localities of blue ice in the vicinity of Troll Station, Antarctica during December 2019 and January 2020. The samples were collected as part of the Research Council of Norway-funded BIOICE Project (Grant No. 288402) by Prof. Andy Hodson and Dr Aga Nowak.

Data are available as a single Excel spreadsheet with two tabs: one containing the chemistry data and the other containing the location and dimension of the holes, where available.

Contact: Andrew.Hodson@unis.no

Methods

Cryoconite holes in the Troll blue ice area are typically covered by an ice lid, even during summer, when subsurface melting is caused by light penetration through the ice and its absorption by underlying debris particles. Access to the underlying water was therefore achieved first by drilling through the ice lid with a 5 cm Kovacs ice auger. The auger was cleaned at each site before use and the ice chips from the lid were sampled, taking care to ensure a depth-integrated sample was collected in each case. Once the lid was penetrated, a syringe was used to extract water from the hole beneath the lid. Samples of glacier ice were taken by drilling the ice auger into the glacier beside the holes.

Samples for major ion analysis (here Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^-, SO₄^2-) were syringe-filtered through 0.45 µm Whatman Puradisc Aqua 30 filters and stored in 50 mL Corning centrifuge tubes after being rinsed with filtrate. The analysis was conducted on Dionex DX90 Ion Chromatographs (University of Sheffield, UK) calibrated in the range 0.01-2 mg L^-1 for cations and in the range 0.25-2 mg L^-1 for anions. Precision errors for these ions were all <2% for mid-range standards, while the detection limit was ≤ 0.01 mg L^-1 for cations and 0.05 mg L^-1 for anions (calculated as three times the standard deviation of ten blanks). All ions described above are reported in mg L^-1.

Quantification of NH₄⁺, PO₄^3-, NO₃^- and Si in the above samples was conducted using a Skalar San++ Continuous Flow Analyser Autoanalyser (University of Sheffield, UK), calibrated in the range 0-3 mg L^-1. The limit of detection for these ions was ≤ 0.05 mg L^-1 (calculated as three times the standard deviation of ten blanks). These analyses employed standard colorimetric methods (based on The European Standard EN ISO, 1996, 2002, 2004 and 2005). Samples are reported as mgN L^-1, mgP L^-1 and mgSi L^-1, accordingly

For both dissolved organic and inorganic carbon (DOC and DIC), a 40mL aliquot was filtered through a Whatman Puradisc Aqua 30 0.45 um pore size filter and stored in Sievers-certified sterilised glass vials. DOC and DIC analyses used the membrane conductometric method of the Sievers 5310 Analyser with UV and persulphate digestion (University Centre In Svalbard, Norway) with a detection limit 0.01 mg L^-1 and < 5% precision errors according to repeat analysis of mid-range (0.4 mg L^-1) Sievers-certified calibration solutions. All samples are reported at mgC L^-1.

Samples for water isotope analysis were collected as unfiltered 20 mL aliquots in a screw-top HDPE bottle. The bottles were subsampled into 1.5 mL vials with septa closures and loaded into the auto-sampler tray of a CDRS (cavity ringdown laser spectroscopy) instrument (L1102-i Picarro water isotope analyzer and A0211 high-precision vaporiser, University of Cambridge). Each sample was injected nine times and the first three injections were rejected to reduce memory effects from the previous sample. Average values from the remaining six injections were averaged when in-run precision was less than ±0.1 for δ¹⁸O. Internal standards were run after eight samples and the external reproducibility of these standards was <1‰ 2σ. All results are reported in parts per thousand (‰) relative to V-SMOW.