Precise interpolar phasing of abrupt climate change during the last ice age

Abstract

The last glacial period exhibited abrupt Dansgaard-Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard-Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard-Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard-Oeschger dynamics.

Does δ<sup>18</sup>O of O<sub>2</sub> record meridional shifts in tropical rainfall?

“…Figure B4. CH 4 in SD (Ahn et al, 2012Brook et al, 2005;Ahn and Yang, unpublished) and WD (Rhodes et al, 2015;Sowers, 2012): discrete SD measurements on original gas timescale (blue markers; Brook et al, 2005) and on new gas timescale (green markers, this study) are superimposed on WD interpolated curve (blue line) on WD2014 gas timescale (Buizert et al, 2015b). Root mean squared deviations between WD fitted curve and SD discrete measurements are 33.7 ppb on the original timescale and 18.3 ppb on the new timescale.…”

“…Figure B2 illustrates this CH 4 tie-point value matching and uncertainty process. ) and on new gas timescale (green markers, this study) are superimposed on WD fitted curve (blue line) on WD2014 gas timescale (Buizert et al, 2015b). Root mean squared deviations between WD fitted curve and SD discrete measurements are 0.029 ‰ on the original timescale and 0.022 ‰ on the new timescale.…”

“…Present mean-annual air temperature and accumulation rate at the WD ice core site are −30 • C and 22 cm ice equivalent per year (Banta et al, 2008;Morse et al, 2002). The youngest 31.2 kyr (2850 m) was dated by annual-layer counting (Sigl et al, 2016), while the oldest portion of the core (up to ∼ 68 ka) was dated by CH 4 synchronization to the Greenland NGRIP ice core, scaled linearly to match the timing of abrupt events recorded in U/Th-dated speleothem records (Buizert et al, 2015b (Dole, 1935;Morita, 1935) and has a modern-day value of ∼ 23.88 ‰ . Most of the Dole effect is attributable to the isotopic discrimination of marine and terrestrial respiration, both of which preferentially utilize 16 O, thus enriching atmospheric O 2 in 18 O (Bender et al, 1994;Luz and Barkan, 2011).…”

“…Siple Dome gas ages between 50 ka and 1950 CE were synchronized with the WAIS Divide WD2014 timescale (Buizert et al, 2015b) using the smooth annual layer thickness (ALT) method of Fudge et al (2014). The ALT method, as applied in this study, is a standard regularized linear least-squares method (Aster et al, 2013) that estimates an annual layer thickness history constrained by depth-gas age tie points and smoothness of the ALT time series.…”

“…Motivated by the recent high-resolution West Antarctic Ice Sheet (WAIS) Divide (WD) ice core methane record (Rhodes et al, 2015), in this study we explore a new record of atmospheric oxygen isotopes from WD and revisit published measurements in the Siple Dome (SD) ice core. A composite WD-SD record of atmospheric oxygen isotopes allows us to investigate past changes in the isotopic fractionation of O 2 on a common timescale (Buizert et al, 2015b) with other recent records.…”