The first millennial-resolution triple greenhouse gas record over the MPT using novel sublimation extraction/laser spectrometry

The Mid-Pleistocene Transition (MPT) is characterized by a shift from 40 kyr to 100 kyr glacial cycles and increasing glacial ice volume. The reason for this change is still a matter of debate, but a plausible explanation could be a long-term decline of greenhouse gas (GHG) radiative forcing during glacial times across the MPT, which would have allowed global ice sheets to grow over a longer time interval and to greater size.Although recent developments in marine CO2 proxies (for example Nuber et al., 2025) and CO2 measurements on blue ice samples (Marks Peterson et al., 2025) have led to first results to constrain the atmospheric CO2 over the MPT, an ultimate answer to the question how glacial/interglacial radiative forcing changed over the MPT is still missing. Marine CO2 proxies are limited in terms of precision and accuracy, making it difficult to reconstruct CO2 changes smaller than app. 20 ppm. Blue ice records show stable mean CO2 (and CH4) over the MPT with surprisingly little glacial/interglacial variations. The latter is likely due to glaciological reasons specific for blue that do not impact the continuous Beyond EPICA deep ice core.Using the novel Laser Sublimation Extraction and multi-beam Quantum Cascade Laser Absorption Spectrometer developed at the University of Bern (in particular for the Beyond EPICA ice core, where availability of ice as old as the MPT is extremely limited due to the glacial thinning) we are able to measure CO2 (and its carbon isotopic composition!), CH4, and N2O concentrations all on the same ice core sample of only 15 g with highest precision and accuracy. We applied this technique to discrete samples from the Beyond EPICA ice core to reconstruct the first multi-millennial record for all three GHG over the MPT, which allows us to quantify changes in the total GHG radiative forcing. The preliminary results confirm minimal secular changes across the MPT, but in contrast to the blue ice record reveals significant glacial/interglacial variations in all three GHG.This poster will introduce the analytical details of this unique analytical system, present the latest results for the Beyond EPICA greenhouse gas records and discuss the implications and limitations of these results for the interpretation of the MPT. ReferencesMarks Peterson, J. et al., Ice cores from the Allan Hills, Antarctica, show relatively stable atmospheric CO2 and CH4 levels over the last 3 million years, Research Square preprint under review, https://doi.org/10.21203/rs.3.rs-5610566/v1.Nuber, S. et al., Mid Pleistocene Transition caused by decline in atmospheric CO2, Research Square Preprint under review, DOI: https://doi.org/10.21203/rs.3.rs-6480074/v1.