The carbon isotope 13C is commonly used to attribute the rise of atmospheric CO2 during the last deglaciation to various processes. Here we discuss the role of the growth of the Great Barrier Reef (GBR), the world's largest reef system, in the global carbon cycle marked by a pronounced decrease in d13C of 1.25‰ in absolutely dated fossil coral skeletons between 12.8 and 11.7 ka, which coincides with a minimum in atmospheric d13CO2. The abrupt start of this event follows the flooding of a large platform along the shelf off northeastern Australia and initiation of an extensive barrier reef system at 13 ka. We show the d13C decrease recorded by the corals to be mainly caused by a combination of reef carbonate production rates and rapid oxidation of organic land carbon on the newly flooded shallow-water platform. Both processes contributed together less than 1 ppm to the ongoing deglacial rise of atmospheric CO2 and, rather counterintuitively, to a small rise in atmospheric d13CO2 of less than 0.001‰. We can thus exclude that the GBR was even partially responsible for the minimum in atmospheric d13CO2 centred at ~12.4 ka.The measured d13C from the GBR are already archived under https://doi.org/10.1594/PANGAEA.833408.The simulation results of a 3-box model performed with MATLAB, including the MATLAB code are archived. Additionally, simulation results for atmospheric CO2 and d13CO using the BICYCLE-SE model (and its forcing in terms of CaCO3 accumulation rate and flooded area) are included here.