The sensitivity of the carbon sink by coupled carbonate weathering to climate and land-use changes: Sediment records of the biological carbon pump effect in Fuxian Lake, Yunnan, China, during the past century.

Recent studies show that the carbon sink attributable to the weathering of carbonate rocks may have been greatly underestimated if the biological carbon pump (BCP) effect in transferring dissolved inorganic carbon (DIC) to organic carbon (autochthonous OC) by aquatic photoautotrophs is neglected. The uptake of DIC by aquatic photoautotrophs may reach 0.2 to 0.7 Pg C/a globally, indicating that the carbon sink by the coupled carbonate weathering with aquatic photosynthesis mechanism (CCW) may be an important control in climate change. In order to understand the sensitivity of the CCW carbon sink to changes of climate and land-use, a systematic study of modern trap and 100-year-long core sediments was conducted in Fuxian Lake, (Yunnan, SW China), the second-deepest plateau oligotrophic freshwater lake in China. It was found that (1) the autochthonous OC in the lake sediments was characterized by lower C/N ratios and higher δC. By means of an n-alkanes compound calculation, the proportions of autochthonous OC were determined to be in the range, 60-68% of all OC; (2) increase in the autochthonous OC accumulation rate (OCAR) was accompanied by an increase in the inorganic carbon accumulation rate (ICAR) in both the trap and core sediments. In particular, the post-1950 OCAR was estimated to be about 6.9 times that for the period, 1910-1950; (3) OCAR in core sediments increased significantly with global warming and land-use change, from 1.06 g C m yr in 1910 to 21.74 g C m yr in 2017. The increasing carbon sink may act as a negative feedback on global warming if the trend holds for all lakes globally. This study is the first to quantify the burial flux of organic carbon generated by the BCP effect in lakes and may contribute to solving the problem of the missing carbon sink in the global carbon cycle.