Methodology:

General information:

For each snow pit, the following general information is documented: date (YYYY-MM-DD), latitude (degrees N) and longitude (degrees E), and the snow height within the snow pit (in centimetres). Snow height was measured using a foldable ruler, which was placed at the snow pit front face and touching the ground (or ground ice, if present).

Temperature:

The temperature record was obtained over the full vertical extent of the snow pit wall at approximately 5-cm resolution using digital thermometers, which were horizontally pushed into the snow pit front. The temperatures are recorded in degrees Celsius, and the measurement height above the ground (in centimetres) is read from the foldable ruler placed at the snow pit front.

Stratigraphy:

The snow stratigraphy profile was created by visually and tactilely identifying any layers within the snowpack. They were labelled using plastic and describing them in terms of grain size, grain shape, and layer hardness based on the International Classification for Seasonal Snow (Fierz et al., 2009). The grain type categorisation also follows the International Classification for Seasonal Snow. Grain size readings are made using a hand-held loupe (10x) and a crystal size measurement grid (1-mm and 2-mm grid). Snow particles from a given layer are transferred from the snow pit front onto the crystal card and visually assessed by the operator using the magnifying lens. If a range of grain sizes is observed, both the minimum and maximum of that range are noted, and the average grain size is derived from these values (in millimetres). No grain size readings were taken for impenetrable layers. The abbreviations for hardness follow the International Classification for Seasonal Snow: Fist, four fingers (4F), one finger (1F), pencil (P), knife (K), and impenetrable.

Density:

The vertical density record is based on stainless-steel box density cutter sampling at approximately 4.5-cm intervals in combination with a spring balance. The measurement height is reported using the bottom level of the density cutter and given in centimetres above the ground. This value is read from the foldable ruler that was placed at the snow pit front.

Stable water isotopes (SWI):

We assessed the two stable water isotope ratios d18O and d2H, which are abbreviated as d18O and d2H, respectively, for 7 profiles (23rd - 30th March). Deuterium excess is abbreviated as dxs. The vertical SWI profile has a resolution of approximately 5 cm (average = 4.5 +/- 2.3 cm). Sampling was done by pushing sterilised, dry 50-mL polypropylene centrifuge tubes 5 cm horizontally into the snow pit front, which was freshly cut using a stainless-steel hand saw (STANLEY). Thus, the chemical signal is generally integrated over the 2.5 x 5 cm sampling dimensions (width x depth). The SWI measurements were conducted using a Picarro L2130-i setup based on cavity ring-down spectroscopy located at BAS in June 2023. The Sample ID is a system-internal identifier, which is assigned to the samples from fieldwork until data processing. The Picarro measurements are primarily processed using the LIMS software. It should be noted that we obtained SWI signatures both before and after tracer percolation experiments. This is why we named the CSV files, which contain SWI signatures of the specified snow pit before the experiment, "SWI_Before". The corresponding "SWI_After_Experiments" data are published in the related dataset.

All measurements made in the field were immediately recorded in a field notebook and digitised upon return to the NERC Arctic Station the same day. All further data processing of the above records was conducted using Microsoft Office Excel 365.

Data collection:

General:

GPS

Foldable ruler

Temperature:

Digital Thermometers: Salter 544A HBBKCR and Salter 518 WHCR, 0.1 deg C precision

Stratigraphy:

Layering: plastic labels

Grain type and size: crystal card, 10x loupe

Hardness: Pencil, knife

Density:

Stainless-steel box cutter, volume = 99 cm3, dimensions = 5.5 x 6 x 3 cm

Spring balance with plastic sample holder

SWI

Polypropylene (BIOFIL) centrifuge tubes

Picarro L2130-i

Data quality:

General:

All snow pits were dug to the ground to cover the full annual snowpack, and we assessed a snow pit wall that was permanently in the shadow and downwind to prevent artefacts through incoming solar radiation or snow re-deposition.

Density:

To ensure accurate density calculations, the weight of the sample holder, in which the content from the density box cutter is placed, is measured for daily scale calibration. Subsequently, the total weight of the sample holder containing a snow sample is measured in the field. As part of the data processing, each day's sample holder weight is subtracted from the total weight to accurately calculate each snow sample density.

SWI samples:

The discrete snow samples were stored in a freezer at the NERC Arctic Station until shipment to the British Antarctic Survey on 4th - 5th April 2023, where they were kept at -25 deg C until analysis. To prevent cross-contamination between samples during transport, all screw tops were sealed using Parafilm, sample tubes double-bagged in batches of 10 and kept upright within the transport box. While leakage was observed in ~15% of the samples despite these measures, we expect the material remaining within the sample tubes to be unaffected.

For every sample, seven injections were measured by the Picarro device, of which the first three were removed for memory effects, and the average of the last four makes up the respective SWI signature. d18O comes at an instrument accuracy of 0.2 ppth and d2H at 0.6 ppth. The measurements are presented in relation to the Vienna Standard Mean Ocean Water (V-SMOW), and the average 1 sigma precision of the here conducted measurements lies at 0.1 ppth for d18O and 0.3 ppth for d2H.

Empty cells within the CSV tables or labelled "na" indicate that information could not be recorded, e.g. grain size in a solid ice layer, or has been excluded due to erroneous measurements.