Talk by Corentin Reutenauer

Quantifying δ¹⁸Oatm variations during Heinrich Stadials under glacial boundary conditions

Abstract
Isotopic composition of atmospheric oxygen (δ¹⁸Oatm) undergoes millennial scale variations during the last glacial period, and systematically increases during Heinrich Stadials (HSs). Here, we quantify the response of δ¹⁸Oatm to such events with a freshwater hosing simulation performed under glacial boundary conditions, using an oxygen mass balance model incorporating outputs from general circulation model (IPSL-CM4) implemented with water isotopes (LMDZ-iso) and dynamic vegetation model (ORCHIDEE), and latest isotope fractionation factor estimations involved in respiratory and photosynthetic processes. Our global analysis enables us to decipher the processes behind the observed changes and highlights the dominant role of hydrology on δ¹⁸Oatm. While the model can simulate the observed δ¹⁸Oatm increase on a global scale, the climatic response during a HS is not spatially homogeneous, as shown by various paleoclimatic archives. We thus perform a model–data comparison to test the model performance on a regional scale. The model is able to reproduce the main observed features in terms of climatic conditions, vegetation distribution and δ¹⁸O of precipitation.

Our study suggests a coherent variability of regional monsoon activity during HSs, driven by a common global mechanism. In the simulation, the cooling of the NA followed by an AMOC reduction leads to a southward shift of the tropical rain belt, with a drying (wetting) north (south) of it. Comparison of CH4 mixing ratio and δ¹⁸Oatm variations during HSs seems to indicate that the Southern Hemisphere climate and vegetation distribution do not need to be significantly modified to trigger a δ¹⁸Oatm increase.