Establishing an accurate chronology is crucial for interpretation of ice core-based climatic records. While high snow accumulation rates characterise coastal Antarctica, thus enabling recovery of highly resolved climatic records, summertime melting at such low-elevation sites offers challenges in establishing a reliable chronological framework through traditional approaches using the seasonality of stable water isotope and ionic proxy records. Here, we assess visual stratigraphy (VS) obtained from line-scan images as a proxy for annual layer counting in firn section (top 50 m) of the IND-36/B9 ice core (dated 1919–2016 CE) from the Djupranen Ice Rise in central Dronning Maud Land, East Antarctica. We also used these images to obtain melt history for the site and found that traditional thickness-based quantification of melt proportion results in significant overestimations. Since density has dominant control on VS profile over the firn section, we first used circulant single-spectrum analysis to remove the secular trend and then we extracted the seasonal VS signals attributed to dust and sea-salt inclusions. We find that melt layers do not significantly alter the VS records if masked during pre-processing. The age–depth model based on the reconstructed VS profile revealed an excellent match with identified time-markers within an uncertainty of ±2 years.

Constant-load creep tests were performed at −10°C at various compressive stresses from 0.05 to 0.75 MPa on specimens taken every 10 m along a firn core extracted at Summit, Greenland in June 2017. The microstructures before and after creep testing were examined using both X-ray microtomography (micro-CT) and optical images from thin sections. An Andrade-like equation was used to describe the primary creep behavior and yielded the time exponent k of 0.17–0.76. The onset of secondary creep occurred at strains of ~0.5–3% but was sometimes not observed at all in shallow firn specimens and at stresses ⩽0.43 MPa even for strain up to 32%. For the 50–80 m firn crept at stresses ⩾0.55 MPa, secondary creep occurred at strains of 2.6 ± 0.28%, and the stress exponent, n, in Glen's law, was found to range from 4.1 to 4.6, similar to those observed for fully dense ice. Micro-CT observations of crept specimens showed that in most cases, the specific surface area, the total porosity and the structure model index decreased, while the structure thickness increased with increasing density. These microstructural characteristics are consistent with the densification of the firn. Optical images from thin sections showed that recrystallization occurred in some specimens that had undergone secondary creep.

We present in situ firn temperatures from the extreme 2012 melt season in the southwestern lower accumulation area of the Greenland ice sheet. The upper 2.5 m of snow and firn was temperate during the melt season, when vertical meltwater percolation was inefficient due to a ~5.5 m thick ice layer underlying the temperate firn. Meltwater percolation and refreezing beneath 2.5 m depth only occurred after the melt season. Deviations from temperatures predicted by pure conductivity suggest that meltwater refroze in discrete bands at depths of 2.0–2.5, 5.0–6.0 and 8.0–9.0 m. While we find no indication of meltwater percolation below 9 m depth or complete filling of pore volume above, firn at 10 and 15 m depth was respectively 4.2–4.5°C and 1.7°C higher than in a conductivity-only simulation. Even though meltwater percolation in 2012 was inefficient, firn between 2 and 15 m depth the following winter was on average 4.7°C warmer due to meltwater refreezing. Our observations also suggest that the 2012 firn conditions were preconditioned by two warm summers and ice layer formation in 2010 and 2011. Overall, firn temperatures during the years 2009–13 increased by 0.6°C.

The French-Italian Concordia (CCD) seismological station at Dome C is one of two observatories setup on the ice cap in the interior of the Antarctic continent. We analysed the seismic signal due to ambient noise at this station and at three temporary stations 5 km away from Concordia, in order to specify the ice properties beneath them. A method based on the horizontal to vertical (H/V) spectral ratio, commonly used to analyse soil response in seismic regions, was applied to the Antarctic stations. The main peak in the spectral ratios is observed at frequencies 6.7–8 Hz at the Dome C stations, but it is not observed at another station on the ice cap, QSPA, where the sensor is buried at 275 m depth. This peak can be explained by a 23 m thick unconsolidated snow or firn layer with a low S-wave velocity of 0.7 km s-1, overlying a consolidated layer with S-wave velocity 1.8 km s-1. Despite the non-uniqueness of the solutions obtained by fitting the H/V spectra, this model is preferred because the depth of the velocity contrast coincides with the density at which ice particles arrange themselves in a continuous, dense lattice. A small variability of this structure is observed around Dome C.