The Ordovician Chiatsun Group, exposed at the Jiacun section, Nyalam, in southern Xizang (Tibet), China, is biostratigraphically one of the best constrained Ordovician sections in the Himalaya margin, and thus plays a key role in correlating the Ordovician strata of this region. The Chiatsun Group is subdivided into the Adang Formation, the Alai Formation, and the Jiaqu Formation, in stratigraphically ascending order. Based on limestone samples collected from the Alai and Jiaqu formations, the Darriwilian chemostratigraphy is established and confirmed by conodont biozonation data. The Middle Darriwilian delta C-13 excursion (MDICE) is recorded in the Himalaya margin for the first time, with an amplitude of about 2 parts per thousand. Its rising limb starts near the boundary of the Histiodella holodentata and Histiodella kristinae zones, and the excursion reaches its maximum values in the Pygodus serra Zone. The decreasing limb is not exposed. The carbon isotope curve with its pronounced onset of the MDICE documents a high potential for chemostratigraphic correlations helping to disentangle the Kurgiakh orogeny in the Himalaya margin. The MDICE recorded in southern Xizang correlates well with that in Baltoscandia, Laurentia, the Argentine Precordillera, Siberia, South China, North China, and Tarim, and extends the distribution of the MDICE to a new block, and thereby strengthens its global significance.