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Coral reefs are well known for playing an important role in the global carbon cycle, however little research has been done into their potential for documenting carbon cycles of the past. Here we report the δ13C of coral reef carbonate from a 928.75-m-long core (CK2) from the northern South China Sea (SCS). The chronological profile of CK2 has been well established based on Strontium isotopes and magnetostratigraphy. Our aim is to explore the potential for δ13C in coral reef carbonate to act as a proxy for the seawater δ13C in the northern SCS and related regional carbon cycle in the geological time scale. Our results demonstrate that variations in CK2 δ13C values were generally consistent with changes in seawater δ13C levels in the northern SCS (as inferred from planktonic foraminifer in the northern SCS) over the past 1.8 Ma. This indicates that the δ13C in our CK2 coral reef carbonate was influenced by seawater δ13C levels. The δ13C values of CK2 also presented obvious periodic variations corresponding to glacial-interglacial cycles, with values being higher during glacial periods and lower during interglacial periods. We speculate that the higher δ13C values during glacial periods indicate higher seawater productivity, i.e., increased photosynthesis intensity in coral reefs. In addition, we also found a coherent relationship between changes in CK2 δ13C values and changes in the East Asian terrestrial ecosystem, and we suppose that the CK2 δ13C values maybe partially influenced by the East Asian terrestrial ecosystem through its regulation of SCS seawater δ13C levels. Therefore, we suggest that δ13C values of coral reef carbonate from the northern SCS represent a potential proxy for the seawater δ13C in the northern SCS and related carbon cycle changes in the East Asia. • The δ13C of coral reef carbonate from northern South China Sea can well document changes in seawater δ13C and the regional carbon cycle in the geological time scale. • The evolution of East Asian terrestrial ecosystem could partially influenced CK2 δ13C values through regulate the seawater δ13C levels. • Higher CK2 δ13C values during glacial periods but lower during inter-glacial periods, indicating that seawater productivity, i.e. photosynthesis intensity in coral reefs was higher during glacial periods than during inter-glacial periods. [ABSTRACT FROM AUTHOR]