Oligocene deep syn-rift lakes in the Gulf of Tonkin, Vietnam, represent exceptional paleoenvironmental archives, but their use for unravelling the regional paleoclimatic development has been hindered by poor dating. Here we present a high-resolution carbon isotope record of wood particles (δ13Cwood) and bulk organic matter (δ13Corg) from a 500 m thick cored Oligocene sedimentary succession representing a deep syn-rift lake. The obtained data allows for estimation of the age of the succession and gives a rare insight into the in-lake fractionation processes.

Correlation of the carbon isotopic signature of δ13Cwood with the global marine record narrows the duration of the deposition of the succession to 1.2 Ma (25.7–26.9 Ma), thus significantly improving any previous age estimates for the core. The comparison of the δ13Cwood with δ13Corg pinpoints parts of the δ13Corg signal influenced by in-lake fractionation processes and indicates two dominant factors controlling them. The lake productivity induced fractionation is seen as more positive δ13C values than expected from wood δ13C. This prevailed during more oxygenated periods, while organic matter degradation related processes dominated the fractionation during the more anoxic periods. The oxygenation of the lake was mostly dependent on changes in tectonic setting that resulted in some variations in lake depth and was strengthened by climatic influence.