研究生知识库

当前全球年均温相对于前工业化时代已经升高了约1.0℃，如果全球气温按照当前的速度继续上升，那么在2030年–2052年之间，全球年均温极有可能会升高1.5℃。因此全面了解全球碳循环收支对全面了解地球系统及遏制全球气温的持续升高极其重要。然而当前碳循环收支并不平衡，岩石风化碳汇，特别是千年尺度内活跃的碳酸盐岩风化碳汇可能是碳失衡的一个来源，而且风化碳汇对气候变化和生态系统模式的改变非常敏感。当前在大尺度上风化碳汇的量级、空间分布模式、时空演变特征及其对全球变化及生态修复的响应关系还缺乏统一的认知。而中国是全世界碳酸盐岩分布最广泛、面积最大的国家之一，因此，充分了解我国以及全球尺度碳酸盐岩化学风化碳汇的量级、空间分布、时空演变特征以及典型喀斯特区域风化碳汇对全球变化及生态修复的响应，对全面系统地解读地球系统的化学过程，平衡碳循环收支、探明遗失碳汇可能的来源等问题具有相当重要的意义。针对上述问题，本文首先以中国典型碳酸盐岩类型即石灰岩区域为研究对象，利用融合机器学习的离子活度系数计算模型与碳酸盐岩热力学溶蚀过程模型，对2000年至2014年中国石灰岩区域的化学风化碳汇进行估算，并系统探讨其空间分布特征、时空演变特征，再对多个类型的碳酸盐岩风化碳汇效应进行评估；其次，针对我国西南典型的喀斯特槽谷，利用Lindeman-Merenda-Gold模型定量评估气候变化及生态修复因子对槽谷CSF的具体贡献，并将槽谷与黄土高原及珠江流域进行对比以辨析不同气候、水文及生态背景下的区域碳汇演变驱动机制；最后基于全球257个流域地球化学监测数据以及全球尺度气候水文生态数据，估算全球碳酸盐岩风化碳汇量，并探讨其在全球碳循环中的具体贡献。主要的研究结果如下：（1）2000~2014年我国石灰岩风化碳汇年均通量为4.28 t C km-2 yr-1，呈现出由西北向东南区域逐渐增加的态势；在纬度上，中国南方28.14°N以下是通量波动最大的区域，但整体上通量随着纬度的降低而呈现出增加的趋势；在气候带上，亚热带与热带区域是通量最大的区域，对于寒带、中温带、暖温带以及温带区域，荒漠气候带是这些气候类型中通量最小的区域，而草原气候带及阔叶林气候带是通量最大的区域。利用基于像元的趋势分析法对我国石灰岩风化碳汇通量的演变情况进行分析，发现我国轻微增加和基本稳定的风化碳汇通量的总面积（103.64万km2）占到石灰岩面积的62.71%，整体而言，中国石灰岩风化处于轻微增加的状态，其通量增长速率约为0.036 t C km-2 yr-1。（2）全国石灰岩风化碳汇总量约为7.07 TgC yr-1，中国石灰岩风化碳汇总量在研究期间处于波动的状态，在2002、2008及2010年为总量最大的三个年份，在2004、2009及2011年其量级处于较低的水平，且在2011年总量最低，整体而言处于轻微增加的趋势，总量增加的速率约为0.06 TgC yr-1。其中，西藏（1.20 TgC yr-1）是我国石灰岩风化碳汇最大的行政区，南方岩溶区为我国石灰岩风化碳汇最大的岩溶分区（4.95 TgC yr-1），其碳汇占到中国石灰岩风化总碳汇的70.01%。基于不同类型碳酸盐岩风化碳循环差异及其关系计算得到中国226.59万km2碳酸盐岩的风化碳汇总量可达11.37 TgC yr-1，其通量约为5.02 t C km-2 yr-1，中国碳酸盐岩风化碳汇相当于中国生物量碳汇的16.20%，这说明碳酸盐岩风化碳汇是我国陆地碳汇系统中的重要组成部分。此外，排除径流深为负值区域的干扰，中国碳酸盐岩风化碳汇通量可达6.54 t C km-2 yr-1。（3）槽谷的年均CSF约为9.42 t C km-2 yr-1，研究期间内处于增加的状态，其年均增长速率约为0.2 t C km-2 yr-1，CSF增加区域的面积占比约为89.28%，槽谷CSF受到气候变化因素（降雨、蒸散发、温度）及生态修复2方面的影响，其中降雨、温度及生态修复反馈因子FVC对CSF呈正面影响，ET对CSF呈负面影响，降雨对于研究区CSF的贡献率最大，达到了70.36%，其次为蒸散发，其贡献率约为11.72%，FVC及温度对于CSF的贡献率分别为10.63%和7.29%。珠江流域年均CSF约为10.34 t C km-2 yr-1，黄土高原CSF相对较低，约为1.44 t C km-2 yr-1，研究期间均表现为增加趋势，珠江流域与黄土高原CSF的增速分别为0.07 t C km-2 yr-1和0.05 t C km-2 yr-1。无论是干旱区还是湿润区域，降雨都是影响碳酸盐岩风化碳汇最主要的影响因素，但是降雨在湿润区域的贡献率更显著；在干旱区蒸散发的影响要更加明显，越是湿润的区域，蒸散发的贡献越低；温度对碳酸盐岩风化过程的影响相对较为复杂，过高的温度和过低的温度对 CSF的贡献均较低；植被恢复对岩石风化过程具有促进作用，在较干旱和植被覆盖度较差的北方，FVC对CSF的贡献相较于湿润和植被覆盖情况较好的南方更显著。（4）对全球2000年至2014年的碳酸盐岩风化碳汇的估算，模型估算风化碳汇通量与全球56个流域实测结果相关系数达0.92，P<0.001，拟合结果与实测结果的偏差仅为-0.08 t C km-2 yr-1，RMSE仅为1.17 t C km-2 yr-1，通过与2000年至2014年0.05°中国石灰岩地区的风化碳汇通量的对比结果表明，在中国碳酸盐岩区域，全球尺度模型计算结果与对比研究数据相关系数R达到了0.90，P<0.001，偏差约为2.38 t C km-2 yr-1，RMSE为4.71 t C km-2 yr-1，模型计算结果整体精度较高。（5）2000年至2014年全球喀斯特区域年均CSF约为3.08 t C km-2 yr-1，全球范围内碳酸盐岩露头区CSF高值主要分布在会促进溶蚀过程发生的温度和降雨都较丰富的赤道附近地区。低值主要分布在水文条件较差的亚洲中部及非洲北部的高原及沙漠地带，以及北半球的寒冷地带。其中CSF在2 t C km-2 yr-1以内的区域面积占比最大，超过了60%，总体而言，全球碳酸盐岩风化碳汇以通量在10 t C km-2 yr-1以内的区域占主导，其总面积比约为92%。（6）赤道以南0°–10°S范围内拥有全球最大的碳酸盐岩化学风化碳汇通量，该纬度带范围内CSF均值约为16.56 t C km-2 yr-1。在北半球，10°N–30°N之间的碳酸盐岩风化碳汇通量相较于其他纬度带而言，整体上处于较高的水平，中国南方岩溶区是北半球10°N–30°N内碳酸盐岩风化碳汇通量的主要贡献者。北半球20°N以北的区域是全球喀斯特分布面积最多的区域，该区域内全球碳酸盐岩分布面积达到了1734.21万km2，而20°N以南全球碳酸盐岩分布总面积仅为20°N以北区域碳酸盐岩面积的16.81%。在全球尺度的碳酸盐岩风化碳汇方面，北半球中高纬区域（20°N以北）是主要的贡献区。（7）全球碳酸盐岩风化碳汇通量最大值出现在赤道附近的热带雨林气候带，最低值出现在干旱气候区域。全球碳酸盐岩溶蚀碳汇通量15年均值大于8 t C km-2 yr-1的气候带主要为热带雨林气候（28.46 t C km-2 yr-1）、热带季风气候（13.15 t C km-2 yr-1）以及亚热带湿润气候（9.16 t C km-2 yr-1）。半干旱气候（1.67 t C km-2 yr-1）、冻原（1.32 t C km-2 yr-1）和干旱气候带（0.15 t C km-2 yr-1）是碳酸盐岩风化碳汇通量最小的气候带。热带雨林气候CSF增加速率最大，约为0.73 t C km-2 yr-1。热带草原气候带中的碳酸盐岩化学风化碳汇通量处于持续的减少过程，从2000年至2014年，CSF减少了65.74%，其减少速率达到了0.25 t C km-2 yr-1。（8） 2000年至2014年，全球碳酸盐岩风化碳汇总量约为62.27 TgC yr-1。亚洲碳酸盐岩分布面积最大，达944.32万km2，每年贡献了全球碳酸盐岩风化碳汇总量的47.42%，约为29.53 TgC。中国碳酸盐岩面积仅占亚洲碳酸盐岩分布面积的31.15%，但是其年均风化碳汇（13.76 TgC yr-1）却是亚洲碳酸盐岩风化碳汇最大的贡献者，占到亚洲风化碳汇总量的46.6%。北美洲碳酸盐岩分布面积达497.17万km2，占全球喀斯特分布的24.63%，其年均碳酸盐岩风化碳汇总量约为19.13 TgC，贡献了全球碳酸盐岩风化碳汇总量的30.72%。欧洲碳酸盐岩分布面积约为245.57万km2，占到全球碳酸盐岩面积的12.16%，其年均风化碳汇总量约为7.47 TgC。非洲、南美洲及大洋洲等区域的碳酸盐岩面积达331.7万km2，产生的风化碳汇总量共计6.68 TgC，占全球风化碳汇的10.73%。碳酸盐岩风化碳汇约占全球碳收支不平衡量（0.6 PgC yr-1）的10.38%，可能这个量级对于全球碳循环系统来说并没有那么重要，但是对于喀斯特分布较为广泛的区域，如中国南方岩溶区等，其化学风化过程对当地的生态系统、水循环过程等具有极其重要的影响。

The global average annual temperature has risen by about 1.0℃ compared with the pre-industrial era. If the global temperature continues to rise at the current rate, the global average annual temperature will probably rise by 1.5℃ between 2030 and 2052. Hence, a comprehensive understanding of the global carbon cycle budget is essential to fully understand the Earth system and to contain the continuous rise of global temperature. However, the current carbon cycle budget is unbalanced with rock weathering related carbon sink (CS), especially those of active carbonate rock weathering (CCS) in the millennium scale, may be a source of carbon imbalance, which is also very sensitive to climate change and ecosystem patterns variaties. At present, the magnitude, spatial patterns, spatiotemporal dynamics of CCS and its response to global change and ecological restoration on a large scale are still lack of a unified understanding. China is one of the countries with the widest distribution and largest area of carbonate rocks in the world. Therefore, fully understanding of the magnitude, spatial distribution, spatiotemporal variation of chemical weathering carbon sinks of carbonate rocks in China and even in the global karst area, also the response of CCS in typical karst regions to global change and ecological restoration is vital to comprehensively and systematically interpret the chemical processes of the earth system, balance carbon cycle and further find out the potential sources of missing carbon sink and so on.Aiming at the above problems, this paper firstly took the typical carbonate rock type in China, i.e. limestone area, as the research object, and estimated the chemical weathering carbon sink in China's limestone area from 2000 to 2014, using the calculating model of ionic activity coefficients based on machine learning and thermodynamic dissolution process model for carbonate rock, and further systematically explored the spatial patterns and spatiotemporal dynamics of the weathering CS of limestone in China. This thesis extended the research by evaluating the CS of different carbonate rock types. Secondly, the Lindeman-Merenda-Gold model was used to quantitatively assess the contribution of climate change and ecological restoration factors to the CS flux (CSF) of the typical karst valley in southwestern China. Then this paper compared the valley with the Loess Plateau and the Pearl River Basin in order to identify the driving mechanism of regional CSF evolution under different climatic, hydrological and ecological backgrounds. Finally, based on the geochemical monitoring data of 257 basins and the global scale climatic, hydrological and ecological data, the global carbonate rock weathering related CS was estimated and its specific contribution to the global carbon cycle was discussed. The main results are as follows: (1) The annual average CSF of limestone weathering in China from 2000 to 2014 was 4.28 t C km-2 yr-1, showing a trend of increasing gradually from northwest to southeast; in latitude, the region below 28.14°N in southern China was the region with the greatest flux fluctuation, but as a whole, flux showed an increasing trend with the decrease of latitude; in climatic zone, subtropical and tropical regions were the regions with the greatest flux. The desert climate zone owned the lowest flux in the cold zone, the middle temperature zone, the warm temperate zone and the temperate zone, while the grassland climate zone and the broad-leaved forest climate zone owned the greatest flux. The evolution of CSF of limestone weathering in China was assessed, using the trend analysis method based on pixels. It was found that the total area of slightly increased and basically stable CSF (103.64×104 km2) accounts for 62.71% of the total area of limestone in China. Overall, the CSF of limestone weathering in China was slightly increasing, and the growth rate was about 0.036 t C km-2 yr-1.(2) The magnitude of CS by limestone weathering in China was about 7.07 TgC yr-1 with a fluctuant state during the research period. High total CS levels appeared in 2002, 2008, and 2010, and the minimum appeared in 2011 with a slightly increasing trend of the total CS being observed with a rate of 0.06 TgC yr-1. Tibet Autonomous Region was the administrative division with the largest total CS of limestone weathering (1.20 TgC yr-1) in China, and karst zones in Southeastern China had the largest total CS (4.95 TgC yr-1) which accounts for 70.01% of that in the three divided karst regions. On the basis of the diversity of rock chemical weathering carbon cycle mechanisms of different carbonate rock types, this paper estimated that the total CS of carbonate weathering in China may reach 11.37 TgC yr-1 (the sink was approximately 5.02 t C km-2 yr-1), which amounts to 16.20% of the total biomass CS in China, furthermore, the CSF of carbonate weathering in China can reach 6.54 t C km-2 yr-1 if excluding the interference of the negative runoff.(3) The annual average CSF of the valley from 2000 to 2014 was about 9.42 t C km-2 yr-1. The annual average growth rate was about 0.2 t C km-2 yr-1. The area ratio of increasing CSF in the valley accounted for 89.28%. The CSF variation of the valley was affected by climate change factors (precipitation, evapotranspiration (ET), temperature) and ecological restoration. Precipitation, temperature and ecological restoration factor of FVC, had a positive effect on CSF. On the contrary, ET had a negative impact on CSF. Precipitation contributed the most to CSF in the study area, reaching a magnitude of 70.36%, followed by ET, accounting for 11.72%. The contribution of FVC and temperature to CSF was 10.63% and 7.29%, respectively. The average annual CSF of the Pearl River Basin was about 10.34 t C km-2 yr-1, while that of the Loess Plateau was relatively low, about 1.44 t C km-2 yr-1. The CSF of the Pearl River Basin and the Loess Plateau bioth showed an increasing trend during the study period. The CSF growth rates of the Pearl River Basin and the Loess Plateau were 0.07 t C km-2 yr-1 and 0.05 t C km-2 yr-1, respectively. Precipitation was the most important factor affecting CSF in both arid and wet regions, but the contribution rate of precipitation in wet regions was more significant, while the effect of evapotranspiration in arid regions was more obvious, the more wet regions, the lower the contribution of evapotranspiration. The influence of temperature on carbonate weathering process was relatively complex. Excessive temperature and low temperature both contributed less to CSF. Vegetation restoration promoted rock weathering process, and FVC contributes more significantly to CSF in the northern regions with arid and poorly vegetated situation than the wet and better vegetation-covered southern regions.(4) The estimation of global CSF of carbonate rock weathering from 2000 to 2014 showed that the correlation coefficient between the estimated CSF and the observed results of 56 basins in the world was 0.92 (P < 0.001). The bias between the estimation and the observation was only -0.08 t C km-2 yr-1, and the RMSE was 1.17 t C km-2 yr-1. Comparison with CSF of limestone weathering in China from 2000 to 2014 with a spatial resolution of 0.05° showed that in the carbonate regions of China, the correlation coefficient of global scale estimation and comparative data reached 0.90 (P < 0.001), the bias was about 2.38 t C km-2 yr-1 and RMSE was 4.71 t C km-2 yr-1, which indicated that the overall accuracy of model estiamation is reliable.(5) From 2000 to 2014, the annual average CSF in global carbonate regions was about 3.08 t C km-2 yr-1. High CSF levels were mainly located in equatorial regions, where high temperature and rich precipitation can highly promote the dissolution process. Low CSF were mainly located in the plateau and desert areas in central Asia and Northern Africa with poor hydrological conditions, as well as in the cold regions of the northern hemisphere. The area ratio of CSF within 2 t C km-2 yr-1 accounted for the largest proportion, which was over 60%. Overall, the global CSF of carbonate weathering was dominated by the area with flux less than 10 t C km-2 yr-1 whose area ratio was about 92%.(6) The largest CSF of carbonate rock weathering appeared in the range of 0–10°S. The average CSF in this latitude zone was about 16.56 t C km-2 yr-1. In the northern hemisphere, the CSF between 10°N and 30°N was higher than that of other latitudes. The karst area in southern China is the main contributor to the CSF within 10°N–30°N in the northern hemisphere. Regions north of 20°N in the northern hemisphere owned the largest carbonate outcrops area in the world. The area of global carbonate rock in this area reached 17.34 million km2, while the total area of global carbonate rock south of 20°N was only 16.81% of the that north of 20°N. In terms of CSF of carbonate weathering on a global scale, the middle and high latitudes of the Northern Hemisphere (north of 20°N) were the main contributor.(7) The maximum CSF of global carbonate weathering occured in the tropical rainforest climate zone near the equator and the lowest in the arid climate zone. The climate zones with the global average CSF greater than 8 t C km-2 yr-1 in 15 years were mainly tropical rainforest climate (28.46 t C km-2 yr-1), tropical monsoon climate (13.15 t C km-2 yr-1) and subtropical humid climate (9.16 t C km-2 yr-1). Semi-arid climate (1.67 t C km-2 yr-1), tundra (1.32 t C km-2 yr-1) and arid climate zone (0.15 t C km-2 yr-1) were the climate zones with the lowest CSF of carbonate weathering. The increase rate of CSF in tropical rainforest climate was the highest, which was about 0.73 t C km-2 yr-1. CSF of carbonate weathering in the tropical steppe climate zone was in a continuous decreasing process. From 2000 to 2014, CSF decreased by 65.74%, and its decreasing rate reached 0.25 t C km-2 yr-1.(8) From 2000 to 2014, the global total carbon sink of carbonate weathering was about 62.27 TgC yr-1. Asia owned the largest carbonate outcrop area, reaching a magnitude of 9.44 million km2, contributing 47.42% of the total carbon sink of global carbonate weathering annually, which was about 29.53 TgC. The area of carbonate outcrops in China accounted for only 31.15% of the total carbonate rock area in Asia, but its annual average total weathering carbon sink (13.76 TgC yr-1) was the most significant contributor to the CS in Asia, accounting for 46.6% of the total weathering CS in Asia. North America's carbonate outcrops covered an area of 4.97 million km2, accounting for 24.63% of the global karst distribution. Its annual average total carbon sink of carbonate weathering was about 19.13 TgC, which contributed 30.72% of the global total CS. The distribution area of carbonate rocks in Europe was about 2.46 million km2, accounting for 12.16% of the world's carbonate rocks area, and the total annual average weathered carbon sink was about 7.47 TgC. The area of carbonate rocks in Africa, South America and Oceania was 3.32 million km2 and the total CS is 6.68 TgC, accounting for 10.73% of global CS. The estimated carbonate rock weathering related carbon sink accounted for about 10.38% of the global carbon budget imbalance (0.6 PgC yr-1). Perhaps this magnitude was not so important for the global carbon cycle system, but for areas with wider karst distribution, such as karst areas in southern China, its chemical weathering process had a critical impact on the local ecosystem, water cycle process and so on.