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 50 years, 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 (POC bio ) and petrogenic POC (POC petro ) in Changjiang POC. Over the study period, around 25% of the Changjiang POC is radiocarbon-dead POC petro ; the remaining is POC bio 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 POC bio 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 POC bio 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 POC bio every year since 2003, approximately 10% of the global riverine POC burial flux to the oceans.

Global perturbation of organic carbon cycling by river damming

“…For the Yangtze River basin, the estimated 2030 reservoir OC burial and mineralization fluxes are 172 and 152 Gmol per year (2.1 and 1.8 Tg C per year). The corresponding total OC removal by reservoirs (172+152=324 Gmol per year) agrees well with the reduction in riverine OC flux since the 1950s of 408±158 Gmol per year, derived independently by Li et al 42. using sediment core data from the lower reaches of the Yangtze River.…”

An Analysis of Terrestrial and Aquatic Environmental Controls of Riverine Dissolved Organic Carbon in the Conterminous United States

“…This result is in accordance with previous findings that the primary production of aquatic ecosystems is more limited by phosphorus than by nitrogen [70]. The impact of dams should also be taken into consideration in explaining DOC spatial variability [71]. Positive correlations between dam density and DOC concentration may be attributed to the elevated algae growth following the construction of dams [72].…”

Fate of Organic Carbon Burial in Modern Sediment Within Yangtze River Estuary

“…Sources of C org buried in marine sediments are commonly inferred from the C/N ratios and isotopic compositions of δ 13 C org and δ 15 N. Previous studies have revealed that terrestrial organic carbon in suspended particle matter from the Yangtze River is characterized by values of C/N 14.6 ± 4.6 (on average) (Wu et al, ), δ 13 C org from −26.6‰ to −24.8‰ (Li et al, ; Yu et al, ), and δ 15 N between 1.18‰ and 4.14‰ (Deng et al, ). Marine‐derived organic matter is typically assumed to have C/N < 10 (Meyers et al, ), δ 13 C org between −22‰ and −18‰ (Ramaswamy et al, ), and δ 15 N with mean values of around 5‰ to 7‰ (Lamb et al, ).…”

“…Differences in the burial of C org in large river‐dominated continental margins are primarily controlled by changes in the terrestrial sediment supply (Syvitski et al, ), SARs (Allison et al, ), hydrodynamics (Tesi et al, ), and biological processes (Ramirez et al, ). There were dramatic alterations in the transfer of suspended particles from the land to the sea between the 1950s and 2015, associated with river damming and cultivated land changes in the drainage basin, and the particle loads and amounts of terrestrial C org to the YRE decreased by >60% and >86%, respectively (Figure S2; Li et al, ; Yang et al, ). Significant changes in sediment load will inevitably decrease SARs and enhance physical erosion and/or bioturbation, thereby influencing the burial and preservation of sedimentary C org in modern sediments (Ramirez et al, ).…”