Decline of soil respiration in northeastern Tibet through the transition into the Oligocene icehouse

Soil respiration (R_s), the production of carbon dioxide in soils, increases dramatically from deserts to forested ecosystems. R_s values thus provide a potential tool to identify past ecosystems if recorded in sedimentary archives. Here, we propose a quantitative method to reconstruct past R_s values from paleosols. This method reverses the soil paleobarometer, a proxy that estimates past atmospheric CO₂ concentration values (CO_2atm) from paleosols while considering a narrow range of variation for R_s. We use past CO_2atm values from marine proxies to reconstruct soil respiration from a 20 million year-long isotopic record from northeastern Tibet covering the transition from the Eocene greenhouse to the Oligocene icehouse. We show that R_s dropped at least 4-fold through the transition into the Oligocene icehouse, marking the spread of boreal desert-steppes of Central Asia. We show that increasing aridity and the decline of monsoonal rainfall, in parallel with global cooling, caused the fall of soil respiration. These highly dynamic R_s emphasize the need for a systematic screening of paleosol isotopic data before using the soil paleobarometer to reconstruct CO_2atm