Dam-triggered organic carbon sequestration makes the Changjiang (Yangtze) river basin (China) a significant carbon sink

Abstract

Worldwide dam building in large river basins has substantially altered the carbon cycle by trapping much of the riverine transported particulate organic carbon (POC) in terrestrial reservoirs. Here we take the Changjiang (Yangtze) River basin, in which ∼50,000 dams were built over the past 50years, as an example to evaluate the effect of dam building on POC sequestration. We report the characteristics (elemental composition, radiocarbon and stable carbon isotopic compositions, and Raman spectra) of bulk POC in the lower Changjiang from October 2007 to September 2008, and we estimate the POC sequestration induced by dam building since the 1950s for the Changjiang Basin. Using radiocarbon measurements, we quantify the fraction of biospheric POC (POCbio) and petrogenic POC (POCpetro) in Changjiang POC. Over the study period, around 25% of the Changjiang POC is radiocarbon-dead POCpetro; the remaining is POCbio with a mean radiocarbon age of ∼3.5 kyr. Studies on the East China Sea (ECS) shelf along with an oxidation experiment suggest that, prior to dam building, the Changjiang POCbio was significantly oxidized in the ECS margin. In contrast, high preservation of POC is observed in Changjiang reservoirs. Combining our POC data with hydrometric data sets, our study indicates that, over the past five decades, dam building may have largely shifted the Changjiang POC burial site from the ECS margin to terrestrial reservoirs. This shift in burial site preserved labile POCbio that would have been oxidized, suggesting a new temporary carbon sink. We estimate that dam building in the Changjiang has sequestered ∼4.9±1.9 megatons POCbio every year since 2003, approximately 10% of the global riverine POC burial flux to the oceans.