The data presented here comprise Ryazanian–Valanginian carbon isotope ratios analyzed from fossil wood and belemnites from the shallow marine Boyarka River succession in Siberia. Additional belemnite carbon isotope ratios from the Izhma River succession (also Ryazanian–Valanginian) in Russia are also presented. The wood-derived and belemnite-derived isotope ratios are considered to primarily reflect changes in the terrestrial and marine carbon isotope reservoirs respectively. The δ13Ccarb and δ13Cwood records reveal a distinct mid-Valanginian positive carbon isotope excursion, with the initiation occurring near the Boreal Russian michalskii–polyptychus zone boundary, which is broadly time-equivalent Tethyan campylotoxus–verrucosum boundary. The Ryazanian–Valanginian δ13Ccarb values fluctuate between c. −1 and +1.5‰ but reach a maximum of +4.1‰ in the Late Valanginian, whilst the δ13Cwood values fluctuate between c. −27 and −23.5‰ and reach a Late Valanginian maximum of −21.2‰. The excursion maximum in the Boreal Russian bidichotomus zones corresponds with the peak of the Tethyan marine carbonate excursion in the verrucosum–peregrinus zones, the peak of a marine carbonate excursion recorded in the Argentinean atherstoni Zone and also with the peak of a terrestrial organic carbon isotope excursion in the Crimean trinodosum–callidiscus ammonite zones. The synchroneity of the positive carbon isotope event between the marine and terrestrial records and between the northern and southern hemispheres and Tethys, clearly indicates a strong coupling of the ocean–atmosphere system at this time and also confirms that this was a global event, which would have affected the total exchangeable carbon reservoir.\ud \u
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