研究生知识库

自18世纪后期工业革命开始以来，由化石燃料燃烧、工业化和土地利用变化等造成的人为CO2排放已使大气CO2浓度上升到2018年的409 ppm。这引出了全球碳循环领域一个重要的问题，即CO2 收支不平衡。因此，确定遗失的碳汇是全球气候变化科学研究的重中之重。最新的研究发现，基于H2O-CaCO3-CO2-水生光合生物相互作用的碳酸盐风化碳汇作用被严重低估。水生光合生物对溶解无机碳的利用及其形成的有机碳（内源）埋藏（即生物碳泵效应--BCP效应），模型计算达到0.7 Pg C/a，使得碳酸盐风化碳汇无论在任何时间尺度气候变化的控制上可能都是重要的。然而，BCP效应背后的机制，和它产生的内源有机碳有多少可以被有效地埋藏及对气候和土地利用变化的响应关系仍然知之甚少。为了更准确地评估内源有机碳埋藏量，以及理解碳酸盐风化碳汇对气候和土地利用变化的响应机制，我们在中国第二深水高原贫营养化湖泊-云南抚仙湖通过结合现代监测、沉积物捕获器和百余年来柱芯沉积物记录的系统性研究发现：（1）通过在中国云南抚仙湖开展为期一年的水化学现代监测（1/2017–10/2017），发现抚仙湖水温季节性变化较大，在2017年4-10月，垂直剖面存在明显的热分层现象，温跃层分布深度在20~50 m之间，且随着时间变化而变化。（2）抚仙湖水化学类型为舒卡列夫分类法中的HCO3-Ca-Mg型，是典型的岩溶水。其水化学特征受到碳酸盐矿物和硅酸盐矿物风化的影响，但来自碳酸盐矿物风化的贡献占据主导地位。（3）抚仙湖的T（水温）、pH、EC（电导率）、DO（溶解氧）、[Ca2+]、[HCO3-]、SIc（方解石饱和指数）、pCO2（水体二氧化碳分压）和HCO3-的碳同位素组成（δ13CDIC）体现出明显的深度和季节变化。夏秋季节T、pH、DO、SIc和δ13CDIC升高，而EC、[HCO3-]、[Ca2+]和pCO2均下降，冬季与之相反；但无论是分层还是混合期，表水层相较深水层其pH、DO、SIc和δ13CDIC均较高，EC、[HCO3-]、[Ca2+]和pCO2则相对较低；此外，pCO2和DO在不同水深和季节均表现出明显的反相关关系，这些均表明水生光合生物对抚仙湖水化学和同位素季节变化的显著影响。（4）不同季节抚仙湖湖北LWHK和湖南JL两个样点的T、pH、EC、DO、[Ca2+]、[HCO3-]、SIc和pCO2均表现出显著的昼夜变化动态。白天湖水水生生物光合作用强烈，pH、DO和SIc增加，而[Ca2+]、[HCO3-]和pCO2降低；晚上呼吸速率超过光合作用速率，[Ca2+]、[HCO3-]和pCO2增加，pH、DO和SIc降低；抚仙湖昼夜尺度的pCO2和DO表现出相反的趋势关系，这些特征表明水生生物光合和呼吸作用的影响非常显著。（5）在2017年1和4月昼夜监测期间，抚仙湖的δ13CDIC值呈现出白天高晚上低的特点。而在7和10月，却表现出截然不同的趋势，整体上呈现白天低晚上高的特点。前者主要归因于白天水生生物光合作用期间，其对12CO2的吸收快于13CO2导致的δ13CDIC值通常偏正。造成后者的因素可能与白天湖水微生物活动加剧、有机质降解强烈、大气CO2的入侵及其产生的动力学分馏有关。（6）捕获器沉积物中C/N值较低，而δ13Corg与周围流域有机物质的δ13Corg值难以区分。利用C/N端元法得知，内源有机碳占总有机碳的比例在30％~100％之间，表明抚仙湖有机碳以内源为主，也因此很大程度上受到BCP效应的影响。同时，计算得出抚仙湖内源有机碳沉积通量在2017年为20.43 t C km2 yr-1，其主要受营养元素输入和温度等协同效应的影响。（7）抚仙湖柱芯沉积物低的C/N值和δ13Corg表明了较多的内源有机碳贡献，通过生物标志物法（正构烷烃）计算显示出三根岩芯沉积物中内源有机碳占总有机碳比例达60~68%。（8）抚仙湖柱芯沉积物内源有机碳沉积通量（OCARauto）的增加伴随无机碳沉积通量（ICAR）增加。尤其在1950年之后，OCARauto是1910–1950这个时期的6.9倍。（9）抚仙湖柱芯沉积物中的OCARauto随全球变暖和土地利用变化显著增加，从1910年的1.06 g C m–2 yr–1上升到2017年的21.74 g C m–2 yr–1。如果全球湖泊都保持这种趋势，那么增加的碳汇可能对全球变暖起到重要的负反馈作用。总之，在耦合水生光合作用的碳酸盐风化作用下，湖泊内源有机碳埋藏可能是一个重要的碳汇。本研究首次定量计算出BCP成因的湖泊内源有机碳埋藏通量，这有助于全球碳循环中“迷失碳汇”问题的解决。

Anthropogenic emissions of CO2 from burning fossil fuels, industry, and land use change have increased atmospheric CO2 concentration to 409 ppm in 2018 since the beginning of the industrial period in the late 18th century. A critical issue in the study of global climate change is the imbalance in the calculation of atmospheric CO2. Identifying missing carbon sink is therefore a top priority in the science of global climate change. 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 0.7 Pg C yr-1 globally, indicating that the carbon sink by the coupled carbonate weathering with aquatic photosynthesis mechanism (CCW) may be an important control in climate change. However, the mechanisms underlying BCP, and how much of the autochthonous OC it generates can be deposited effectively and the response to climate and land-use changes are still poorly understood. In order to make a more accurate evaluation of the buried autochthonous organic carbon and understand the sensitivity of the CCW carbon sink to changes of climate and land-use, a systematic study of modern hydrochemical monitoring, a sediment trap experiment 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) Throughout the year (January 2017–October 2017) of modern hydrochemical monitoring in Fuxian Lake, the water temperature showed an obvious seasonal variation, the vertical profiles of lake water temperature displayed thermal stratification and the thermocline was located between 20-50 m from April to October 2017. (2) The hydrochemical type was of simple HCO3-Ca-Mg, indicating typical karst water. The hydrochemical characteristics in Fuxian Lake were influenced by carbonate weathering and silicate weathering, and the former being predominant. (3) Temperature, pH, EC (electric conductivity), DO (dissolved O2), DIC, [Ca2+], SIc, and partial pressures of CO2 (pCO2), as well as carbon isotopic compositions of HCO3– (δ13CDIC) in Fuxian Lake water all displayed distinct seasonal and vertical variations. There were higher values of T, pH, DO, SIc and δ13CDIC in the summer and fall, while lower EC, [HCO3-], [Ca2+] and pCO2, the opposite happened in winter and spring; the pH, DO, SIc and δ13CDIC in the epilimnion were higher and EC, [HCO3-], [Ca2+] and pCO2 were lower than those in deep water during both summer stratification and winter mixing. Especially there was an inverse correlation between pCO2 and DO. All indicate that the variations of hydrochemistry and δ13CDIC in lake waters were mainly controlled by the photosynthetic metabolism of aquatic phototrophs. (4) All parameters (T, pH, EC, DO, [Ca2+], [HCO3-], SIc and pCO2) at sites LWHK and JL in Fuxian Lake showed pronounced diurnal hydrochemical cycles. During the day, when aquatic photosynthesis was strong, pH, DO and SIc increased and [Ca2+], [HCO3-], and pCO2 decreased; During the night, the rate of respiration exceeded that of photosynthesis, [Ca2+], [HCO3-], and pCO2 increased and pH, DO and SIc decreased; pCO2 and DO showed inverse direction of change, suggesting a potential significant role of terrestrial aquatic photosynthesis. (5) There were higher values of δ13CDIC during the day and lower during the night in January and April 2017. However, in July and October 2017, an opposite trend appeared. For the former, during photosynthesis, 12CO2 is preferentially utilized, leading to an increase in δ13CDIC values. The reason for the later may be a combination of increased microbial activity, increased degradation of organic matter, the invasion of atmospheric CO2 and its kinetic fractionation. (6) The lower C/N ratios were recorded in the trap sediments, and δ13Corg were influenced by factors of signature overlapping of end-members. From observed C/N ratios, the proportions of autochthonous OC were estimated to be 30%–100% of all OC, indicating that autochthonous OC was an important contributor to sedimentary OC. It was calculated that autochthonous OC flux in the Fuxian Lake was 20.43 t C km2 y-1 in 2017, and this was affected by synergistic effects, including nutrient availability and climatic factors.(7) Autochthonous OC in the Fuxian Lake core sediments was characterized by lower C/N ratio and higher δ13Corg. By determining the n-alkane distributions, the proportion of autochthonous OC was found to be in the range of 60 - 68%.(8) The increase in the accumulation rate of autochthonous OC (OCARauto) was accompanied by an increase in the inorganic carbon accumulation rate (ICAR) in the trap and core sediment samples. Particularly important was the determination that the post-1950 OCARauto was about 6.9 times that for the previous period, 1910–1950.(9) OCARauto in the core sediments has increased significantly with global warming and land-use change, from 1.06 g C m–2yr–1 in 1910 to 21.74 g C m–2 yr–1 in 2017. This increasing carbon sink may act as an important negative feedback on global warming if the trend holds for all the lakes in the world. To sum up, lacustrine autochthonous OC by combining carbonate weathering with aquatic photosynthesis could potentially be a significant carbon sink. 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.