The SP19 chronology for the South Pole Ice Core – Part 2: gas chronology, Δage, and smoothing of atmospheric records

Abstract

Abstract. A new ice core drilled at the South Pole provides a 54 000-year paleoenvironmental record including the composition of the past atmosphere. This paper describes the SP19 chronology for the South Pole atmospheric gas record and complements a previous paper (Winski et al., 2019) describing the SP19 ice chronology. The gas chronology is based on a discrete methane (CH4) record with 20- to 190-year resolution. To construct the gas timescale, abrupt changes in atmospheric CH4 during the glacial period and centennial CH4 variability during the Holocene were used to synchronize the South Pole gas record with analogous data from the West Antarctic Ice Sheet Divide ice core. Stratigraphic matching based on visual optimization was verified using an automated matching algorithm. The South Pole ice core recovers all expected changes in CH4 based on previous records. Gas transport in the firn results in smoothing of the atmospheric gas record with a smoothing function spectral width that ranges from 30 to 78 years, equal to 3 % of the gas-age–ice-age difference, or Δage. The new gas chronology, in combination with the existing ice age scale from Winski et al. (2019), allows a model-independent reconstruction of the gas-age–ice-age difference through the whole record, which will be useful for testing firn densification models.

Gas isotope thermometry in the South Pole and Dome Fuji Ice Cores provides evidence for seasonal rectification of ice core gas records

“…We measured 14 depths in duplicate, giving us an estimate of analytical reproducibility. Our samples cover the time period from approximately 5,000 to 30,000 yr BP at an average resolution of 150 yr on the SP19 gas chronology (Epifanio et al, 2020). The measurements were made in two periods, between January and April 2018 and between October and December 2018.…”

“…The South Pole Ice Core SPC14 (hereafter SPICEcore) was drilled between 2014 and 2016 at a site close to the Amundsen-Scott South Pole Station (Casey et al, 2014;Winski et al, 2019;Epifanio et al, 2020;Souney et al, 2021). Ice cores were transported to the National Science Foundation's Ice Core Facility (NSF-ICF) in Denver, Colorado where 200g samples were cut and shipped to Scripps Institution of Oceanography in La Jolla, California.…”

“…Ice thickness and geothermal heat flux are held constant at 2600 m and 56 Wm -2 respectively-these values are not very well known as SPICEcore was not drilled to bedrock. The model can be run in an inverse mode, in which an automated algorithm is used to estimate the temperature and accumulation histories that best fit the observational δ 15 N data and the empirically reconstructed estimates of the gas age-ice age difference (Epifanio et al, 2020). We will refer to the optimized inverse scenario as the reference (REF)…”

Continuous-Flow Analysis of δ17O, δ18O, and δD of H2O on an Ice Core from the South Pole

“…A depth-age relationship for the core, tied directly to the timescale for the WAIS Divide ice core (WAIS Divide Project Members, 2013; WAIS Divide Project Members, 2015), was developed by Winski et al (2019) and is termed the "SP19" chronology. A timescale for the gas within the core is also available (Epifanio et al, 2020).…”

CH<sub>4</sub> and N<sub>2</sub>O fluctuations during the penultimate deglaciation

“…The impact of these trace gases on the Earth's radia-tive balance in the future depends on the sensitivity of natural sources to anthropogenic warming. Time periods of climate change in the past provide natural templates to study this coupling (Fischer et al, 2018). Reconstructions of greenhouse gas concentrations before the instrumental era are only enabled by analysing the composition of air trapped in tiny bubbles in polar ice cores, reflecting the atmospheric composition at the time the bubbles were formed.…”