The WAIS Divide deep ice core WD2014 chronology – Part 1: Methane synchronization (68–31 ka BP) and the gas age–ice age difference

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".

Analytical constraints on layered gas trapping and smoothing of atmospheric variability in ice under low-accumulation conditions

“…5 by comparing the Vostok record with the WDC record. High-frequency atmospheric variability is much better preserved in the WAIS Divide ice core because the accumulation rate is more than an order of magnitude higher (in the range from 18 to 22 cm ice yr −1 for the studied period; Buizert et al, 2015), and thus the firn densification and gas trapping are faster. For instance, the methane variation spanning between 59 000 and 58 800 yr BP is damped by ∼ 50 % in the Vostok record compared to WDC.…”

“…The atmospheric methane scenario used is the high-resolution methane record from the WAIS Divide ice core . The WDC gas age chronology (WD2014) was scaled to the GICC05 chronology (with present defined as 1950) dividing by a factor of 1.0063 as in Buizert et al (2015). For the rest of the paper we use this scaled WD2014 chronology to express WDC gas ages.…”

A 156 kyr smoothed history of the atmospheric greenhouse gases CO<sub>2</sub>, CH<sub>4</sub>, and N<sub>2</sub>O and their radiative forcing

“…t: mean data spacing; v: mean 1σ error; exact time framing is given by the age of the first (t start ) and last (t stop ) data point of the interval (in years BP); N: number of data points within interval. In the last column the underlying data source is briefly mentioned; see Table 5 Marcott et al, 2014;Buizert et al, 2015;Mitchell et al, 2013Mitchell et al, , 2011Sigl et al, 2016). Starting with the year 9821 BP, continuous CH 4 data from WDC with higher temporal resolution are now available and are used to support our spline .…”

Icechrono1: a probabilistic model to compute a common and optimized chronology for several ice cores

“…Des calages inter-carottes, c'est-à-dire des relations entre les profondeurs de deux carottes différentes, sont déter-minés pour pouvoir transférer des informations absolues d'une carotte à l'autre. Ils sont généralement basés sur la ressemblance des enregistrements de la composition atmosphérique globale par les différentes carottes (concentration en CO 2 , CH 4 et δ O) et des datations U/Th précises sur les spéléothèmes ont été utilisés pour ajuster la chronologie des carottes de glace (Barker et al, 2011 ;Buizert et al, 2015 ; Parrenin et al, 2007b). Cependant, ces exercices reposent sur l'hypothèse que les changements abrupts enregistrés dans les carottes de glace et les spéléothèmes sont simultanés.…”

“…Dans certain cas, des enregistrements hauterésolution de la composition isotopique de la calcite (δ 18 O) et des datations U/Th précises sur les spéléothèmes ont été utilisés pour ajuster la chronologie des carottes de glace (Barker et al, 2011 ;Buizert et al, 2015 ;Parrenin et al, 2007b). Cependant, ces exercices reposent sur l'hypothèse que les changements abrupts enregistrés dans les carottes de glace et les spéléothèmes sont simultanés.…”