Abstract
Abstract. The West Antarctic Ice Sheet Divide (WAIS Divide, WD) ice core is a newly drilled, high-accumulation deep ice core that provides Antarctic climate records of the past ∼68 ka at unprecedented temporal resolution. The upper 2850 m (back to 31.2 ka BP) have been dated using annual-layer counting. Here we present a chronology for the deep part of the core (67.8–31.2 ka BP), which is based on stratigraphic matching to annual-layer-counted Greenland ice cores using globally well-mixed atmospheric methane. We calculate the WD gas age–ice age difference (Δage) using a combination of firn densification modeling, ice-flow modeling, and a data set of δ15N-N2, a proxy for past firn column thickness. The largest Δage at WD occurs during the Last Glacial Maximum, and is 525 ± 120 years. Internally consistent solutions can be found only when assuming little to no influence of impurity content on densification rates, contrary to a recently proposed hypothesis. We synchronize the WD chronology to a linearly scaled version of the layer-counted Greenland Ice Core Chronology (GICC05), which brings the age of Dansgaard–Oeschger (DO) events into agreement with the U/Th absolutely dated Hulu Cave speleothem record. The small Δage at WD provides valuable opportunities to investigate the timing of atmospheric greenhouse gas variations relative to Antarctic climate, as well as the interhemispheric phasing of the "bipolar seesaw".
Highlights
Deep ice cores from the polar regions provide highresolution climate records of past atmospheric composition, aerosol loading and polar temperatures (e.g., NGRIP community members, 2004; EPICA Community Members, 2006; Wolff et al, 2006; Ahn and Brook, 2008)
Due to high accumulation rates of 22 cm ice a−1 at present and ∼ 10 cm ice a−1 during the Last Glacial Maximum (LGM), the WD core delivers climate records of unprecedented temporal resolution (Steig et al, 2013; Sigl et al, 2013) as well as gas records that are only minimally affected by diffusive smoothing in the firn column (Mischler et al, 2009; Mitchell et al, 2011, 2013; Marcott et al, 2014)
We present several improvements over previous work that reduce and quantify these uncertainties: (i) we combine the layercounted Greenland Ice Core Chronology (GICC05) and a recently refined version of the U / Th-dated Hulu speleothem record (Edwards et al, 2015; Reimer et al, 2013; Southon et al, 2012) to obtain a more accurate estimate of the ages of abrupt Dansgaard–Oeschger (DO) events (Sect. 4.4); (ii) we combine firn densification modeling, iceflow modeling, a new WD δ15N. Atmospheric N2 isotopic composition (δ15N)-N2 data set that spans the entire core, and a Monte Carlo sensitivity study to obtain a reliable age estimate (Sect. 3); and (iii) we compare four different interpolation schemes to obtain an objective estimate of the interpolation uncertainty (Sect. 4.5)
Summary
Deep ice cores from the polar regions provide highresolution climate records of past atmospheric composition, aerosol loading and polar temperatures (e.g., NGRIP community members, 2004; EPICA Community Members, 2006; Wolff et al, 2006; Ahn and Brook, 2008). Having a reliable ice core chronology (i.e., an age–depth relationship) is paramount for the interpretation of the climate records and comparison to marine and terrestrial paleoclimate archives. The West Antarctic Ice Sheet Divide (WAIS Divide, WD) ice core (79.48◦ S, 112.11◦ W; 1766 m above sea level; −30 ◦C present-day mean annual temperature) was drilled and recovered to 3404 m depth (WAIS Divide Project Members, 2013). The combination of high accumulation rates and basal melting at the WD site results in ice near the bed that is relatively young (∼ 68 ka) compared to cores drilled in central East Antarctica
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