The ST22 chronology for the Skytrain Ice Rise ice core – Part 1: A stratigraphic chronology of the last 2000 years

Abstract

Abstract. A new ice core was drilled in West Antarctica on Skytrain Ice Rise in field season 2018/2019. This 651 m ice core is one of the main targets of the WACSWAIN (WArm Climate Stability of the West Antarctic ice sheet in the last INterglacial) project. A present-day accumulation rate of 13.5 cm w.e. yr−1 was derived. Although the project mainly aims to investigate the last interglacial (115–130 ka), a robust chronology period covering the recent past is needed to constrain the age models for the deepest ice. Additionally, this time period is important for understanding current climatic changes in the West Antarctic region. Here, we present a stratigraphic chronology for the top 184.14 m of the Skytrain ice core based on absolute age tie points interpolated using annual layer counting encompassing the last 2000 years of climate history. Together with a model-based depth–age relationship of the deeper part of the ice core, this will form the ST22 chronology. The chemical composition, dust content, liquid conductivity, water isotope concentration and methane content of the whole core was analysed via continuous flow analysis (CFA) at the British Antarctic Survey. Annual layer counting was performed by manual counting of seasonal variations in mainly the sodium and calcium records. This counted chronology was informed and anchored by absolute age tie points, namely, the tritium peak (1965 CE) and six volcanic eruptions. Methane concentration variations were used to further constrain the counting error. A minimal error of ±1 year at the tie points was derived, accumulating to ± 5 %–10 % of the age in the unconstrained sections between tie points. This level of accuracy enables data interpretation on at least decadal timescales and provides a solid base for the dating of deeper ice, which is the second part of the chronology.

The ST22 chronology for the Skytrain Ice Rise ice core – part 2: an age model to the last interglacial and disturbed deep stratigraphy

“…This offset arises partly from dissolution of a small percentage of gas into the meltwater stream, as has been observed previously using CFA to measure methane (Rhodes et al, 2015). Continuous analyses started at 244 m depth and continued in all sections where the ice was of suitable quality to 649.4 m. A short section from 144.0-161.3 m was also analysed continuously for methane with an improved measurement setup which is discussed in the companion paper (Hoffmann et al, 2022).…”

“…We include a number of stratigraphic alignments to AICC2012, based on the data in the companion paper for the uppermost 2000 years, and using CH4, δ 18 Oair, 10 Be, and ice chemistry markers in deeper ice. Paleochrono was started with a prior for the accumulation rate (based on a simple relationship with water isotope ratios), air lock-in depth (prior set at a constant 58 m (Hoffmann et al, 2022)) and a simple ice thinning function. Paleochrono minimises a cost function that measures the misfit of the model with respect to the prior and the observations (tie points).…”

“…3. A number of discrete measurements were also made between 84 and 144 m at Oregon State University which are described in the companion paper (Hoffmann et al, 2022).…”

“…This combination ensures that Skytrain Ice Rise will almost certainly have remained as a separate ice dome, and would never have been overridden by inland ice, whatever the size of the WAIS. In a companion paper to this one (Hoffmann et al, 2022) we have used a variety of age markers, interpolated through counting of annual layers in chemistry, to derive an age scale for the last 2000 years (~200 m). In this paper we use a range of evidence to derive an age model for the rest of the core.…”

An age scale for new climate records from Sherman Island, West Antarctica

“…The WACSWAIN project (WArm Climate Stability of the West Antarctic ice sheet in the last INterglacial) aims to use ice core records to investigate the WAIS during the LIG. An ice core on Skytrain Ice Rise (Figure 1) was successfully drilled to the bed at 651 m, results from which are now being published (Mulvaney et al, 2021;Hoffmann et al, 2022). Drilling was carried out at a second site, on Sherman Island, located in the Abbott Ice Shelf in between continental Antarctica and Thurston Island, close to TG and PIG (Figure 1 and Table 1).…”