| Summary: | Understanding carbon cycle and climate dynamics in the past is crucial to project climate and CO2 changes in the future. Numerous geological archives of carbon isotope changes during the last deglaciation and Holocene provide an additional constraint on carbon cycle dynamics. To quantify a role of terrestrial mechanisms in atmospheric d13CO2 changes in the past, a model of 13C discrimination during terrestrial biogeochemical processes is added to the land surface module JSBACH of the MPI Earth System Model (MPI-ESM). The parameteresation of fractionation processes for C3 and C4 plants is based on the theory by Lloyd and Farquhar (Oecologia, 1994). The 13C model component simulates land-atmosphere carbon isotope exchanges on sub-daily time scale. The updated JSBACH model comprises a module for dynamical vegetation and disturbances by wind and fire and can be driven by climate forcing out of observations, reconstructions or directly coupled to ESMs of full or intermediate complexity. We will present a JSBACH model study focusing on the Holocene (last 6000 yrs). The climate forcing is taken out of a transient MPI-ESM simulation throughout the Holocene by Fischer and Jungclaus (Climate of the Past, 2010) based on a constant atmospheric CO2 concentration and an orbital forcing following the PMIP-2 exercises. The variability of modeled d13C will be shown as well as an effect of shifts in vegetation cover (changes in C3/C4 grass cover, boreal tree line, 'green Sahara' etc.) on the carbon isotope budget. Simulated changes in carbon isotope budget will be compared with ice core reconstructions of atmospheric CO2 and d13CO2.
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