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 δ¹³C_org. 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_auto) 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_auto was estimated to be about 6.9 times that for the period, 1910-1950; (3) OCAR_auto in core sediments increased significantly with global warming and land-use change, from 1.06 g C m^-2 yr^-1 in 1910 to 21.74 g C m^-2 yr^-1 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.

Combined use of radiocarbon and stable carbon isotope to constrain the sources and cycling of particulate organic carbon in a large freshwater lake, China

The concentrations and isotopic compositions of dissolved inorganic carbon (DIC) and particulate organic carbon (POC) were measured in order to better constrain the sources and cycling of POC in Lake Fuxian, the largest deep freshwater lake in China. Model results based on the combined δ¹³C and Δ¹⁴C, showed that the average lake-wide contributions of autochthonous POC, terrestrial POC, and resuspended sediment POC to the bulk POC in Lake Fuxian were 61%, 22%, and 17%, respectively. This indicated autochthonous POC might play a dominant role in sustaining large oligotrophic lake ecosystem. A mean 17% contribution of resuspended sediment POC to the bulk POC implied that sediment might have more significant influence on aquatic environment and ecosystem than previously recognized in large deep lakes. The contributions of different sources POC to the water-column POC were a function of the initial composition of the source materials, photosynthesis, physical regime of the lake, sediment resuspension, respiration and degradation of organic matter, and were affected indirectly by environmental factors such as light, temperature, DO, wind speed, turbidity, and nutrient concentration. This study is not only the first systematic investigation on the radiocarbon and stable isotope compositions of POC in large deep freshwater lake in China, but also one of the most extensive radiocarbon studies on the ecosystem of any great lakes in the world. The unique data constrain relative influences of autochthonous POC, terrestrial POC, and resuspended sediment POC, and deepen the understanding of the POC cycling in large freshwater lakes. This study is far from comprehensive, but it serves to highlight the potential of combined radiocarbon and stable carbon isotope for constraining the sources and cycling of POC in large lake system. More radiocarbon investigations on the water-column POC and the aquatic food webs are necessary to illuminate further the fate of autochthonous POC, terrestrial POC, and resuspended sediment POC, and their eco-environmental effects.

Bias and association of sediment organic matter source apportionment indicators: A case study in a eutrophic Lake Chaohu, China

The sources of sediment organic matter (SOM) could be explained by various indicators. To test their biases and associations, the present study determined multiple indicators for SOM source apportionment, including elemental analysis (carbon and nitrogen, and their stable isotope δ¹³C and δ¹⁵N), n-alkanes compositions as well as derivative indicators (e.g., terrigenous to aquatic ratio), and carbon isotopes of n-alkane in Lake Chaohu, a eutrophic lake. The spatial variation of anthropogenic effects could be revealed by SOM elemental variations. The n-alkanes of all samples had a bimodal distribution with the 1st peak at n-alkane with 17 carbons (C17) and the 2nd predominant peak at C29. The parity advantage index of n-alkanes indicated that the sediments had mixed characteristics of both endogenous and terrigenous sources. Some n-alkanes indicators also revealed eutrophication characteristics of dominant algae in Lake Chaohu. SOM received a mixed contribution of plankton (I), low-latitude terrestrial high-grade plants (II) and microbial material (III) as indicated by isotopic compositions of long-chain n-alkane. Multiport element model (MEM) showed the contribution of self-generated sources of organic matter in Lake Chaohu is >50%, indicating the historic serious eutrophication in Lake Chaohu. The main sources of SOM in the eastern part of the lake were algae and terrestrial input, with little input from microbes, and the contribution from algae decreased from west to east. The multiple indicators' judgment by MEM and principle component analysis (PCA) was of ecological significance and proposed because they offered scientific tools for disclosing the historic variations of SOM as well as their sources.