CH4 and N2O fluctuations during the penultimate deglaciation

Deglaciations are characterized by the largest natural changes in methane (CH4) and nitrous oxide (N2O) concentrations of the past 800 thousand years. Reconstructions of millennial to centennial-scale variability within these periods are mostly restricted to the last deglaciation. In this study, we present composite records of CH4 and N2O concentrations from the EPICA Dome C ice core covering the penultimate deglaciation at temporal resolutions of ∼100 years. Our data permit the identification of centennial-scale fluctuations during the transition from glacial to interglacial levels. At ∼134 and ∼129 thousand 5 years before present (hereafter ka BP), both CH4 and N2O increased on centennial-timescales. These abrupt rises are similar to the fluctuations associated with the Dansgaard–Oeschger events identified in the last glacial period. In addition, gradually rising N2O levels at ∼130 ka BP resemble a pattern of increasing N2O concentrations on millennial-time scales characterizing the later part of Heinrich stadials. Overall, the events in CH4 and N2O during the penultimate deglaciation exhibit modes of variability that are also found during the last deglaciation and glacial cycle, suggesting that the processes leading to changes in 10 emission during the transitions were similar but their timing differed.

To increase the sample throughput, standard gases are injected directly into the GC system, bypassing the extraction line employed for ice core samples. We periodically inject standards over gas-free ice samples to account for contamination along the extraction line, determined as the mean offset between the two injection pathways (line offset). For ::::: using :::: three :::::::: standard :::: gases ::::::::: bracketing ::: the :::::::::::::::: glacial-interglacial ::::: range :: of : CH 4 , the line offset depends linearly on concentrations (R 2 = 0.99) leading 125 to a downward revision of our measured values by up to 5 ppb(for concentrations ranging between 350 and 700 pbb). For N 2 O, the line offset is constant and leads to a downward revision of the measured values by 4 ppb.
Overall, the improved resolution of our records allowed us to identify features hidden in the current ::: not ::::::: resolved ::: in ::: the :::::::: previously ::::::::: published : CH 4 and N 2 O EDC datasets. In particular, the 134-ka event and the :::::: 130-ka ::::: event :: in : N 2 O increase at 180 ∼130.5 ka BP are resolved for the first time. Retrieving CH 4 and N 2 O concentrations from the same samples enable us to study the relative phasing of both trace gases in the course of these events without age uncertainty. At the onset of the 134 and 128-ka :::::: 129-ka : events, the rise in both trace gases occur simultaneously. In contrast, the 130.5-ka :::::: 130-ka event in the N 2 O record is not accompanied by a concomitant fluctuation in CH 4 concentrations.
However, the concept of freshwater forcing as a trigger of AMOC shutdown (either as iceberg discharges during HS or MWPs during glacial terminations) has become a matter of intense debate. Firstly, the history of MWPs is decoupled from 370 that of the AMOC and Greenland temperature in the course of TI (most notably exemplified by the absence of any such pulses for the YD stadial) (Tarasov and Peltier, 2005;Stanford et al., 2006). Secondly, iceberg discharge events within HS are consistently lagging behind oceanic circulation and Greenland temperature changes (Barker et al., 2015;Henry et al., 2016) . The current paradigm rather considers freshwater forcing as resulting from AMOC declines. This view is supported by modeling and experimental studies demonstrating that the interior of the ocean accumulates heat at times the AMOC collapses 375 (Galbraith et al., 2016;Pedro et al., 2018;Bereiter et al., 2018;Baggenstos et al., 2019), constituting a potent forcing for destabilizing glacial ice sheets and producing bursts of meltwater Marcott et al., 2011;Clark et al., 2020;Galbraith et al., 2016) . In summary, we are currently unable to propose a mechanism accounting for the relative brevity of the 134-ka event.

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Author contributions. The present study was designed by T.F.S, H.F and L.S. L.S and J.H performed the methane and nitrous oxide measurements. J.S provided the isotopic data. L.S wrote the text with inputs from all authors.
Competing interests. The authors declare that they have no conflict of interest.
Acknowledgements. The authors would like to thank Barbara Seth for the measurements of the isotopic composition of N2O, Gregory Teste for assistance in cutting ice samples, as well as Michael Bock and Jan Strähl for the construction of the new CH4 and N2O mea- The main logistic support was provided by IPEV and PNRA. This is EPICA publication no. XX.  Composite CH4 record (this study, as in Fig.1). (B) : B: Composite N2O record (this study, as in Fig.1). (C) : C: Speleothems δ 18 O(CaCO3) records: Sanbao SB25 (light green) (Cheng et al., 2009), Hulu Cave MSX (khaki) (Cheng et al., 2006), Hulu cave MSP (dark green) (Cheng et al., 2006), and Sanbao-Dongge composite (pale yellow) (Cheng et al., 2016).