## 4 Discussion  
The TOC and TN change trends of the sediment cores in this study were highly similar, with obvious stage characteristics and a significant correlation (\(r\) = 0.996, \(p\) < 0.01), indicating that the sources of TOC and TN were consistent. The \(\delta^{13}\)C\({}_{\rm org}\) value of the sedimentary core sequence was generally within the range of the \(\delta^{13}\)C\({}_{\rm org}\) value of plankton (phytoplankton and algae), indicating that plankton may play a major role in the accumulation of organic matter in lake sediments. The C/N value of the sediment core sequence was generally at a low level, except for in the mid-1950s to the mid-1960s, where the C/N was less than 10, and the C/N value has, overall, been between 4 and 10 since the 1970s. Combined with the change in the \(\delta^{13}\)C\({}_{\rm org}\) value in the middle, this indicates that the organic carbon in Aral Sea sediments is mainly endogenous, and plankton is the main endogenous contributor. As lake aquatic organisms are the main source of organic matter in the sediments of the North Aral Sea, the role of temperature and water environment on sediment organic matter is crucial. An existing study on the organic carbon of lake sediments in arid regions showed that the increase in temperature in the past 100 years will lead to an increase in the content of organic matter in lake sediments [52]. Under warm conditions, the suitable growth period of water plants is prolonged with an increase in the photosynthesis rate, and the productivity of organic matter increases [52], which further affects the accumulation of organic carbon in sediments. From the OCBR variations (Figure 4), the sediment rate has been increasing over the past 70 years, which is consistent with the regional temperature. The overall climate in the basin tends to be warmer, thereby, accelerating the OCBR, which is conducive to the accumulation of organic carbon. At the same time, an increase in temperature is conducive to the mineralization and decomposition of organic carbon, thereby, inhibiting its accumulation in lakes and restricting the burial efficiency of organic carbon [53; 54]. The research results indirectly indicate that the promotion effect of temperature on the input of organic matter may be stronger than its mineralization effect. Since most of the degradation of organic matter is carried out by microbial communities and extracellular enzymes [55], the temperature can directly affect its activity [56; 57], which changes the decomposition of organic carbon in sediments by microorganisms, which, in turn, affects the rate of mineralization. However, it can be seen from Figure 4 that, since 2015, the organic carbon burial rate in the North Aral Sea has shown a decreasing trend, and the increase in microbial respiration and the mineralization rate of organic matter in water bodies and sediments following the warming climate may lead to a decline in the rate of organic carbon burial. The amount of precipitation has a certain effect on the development of terrestrial plants in the watershed. As the organic matter in the sediments of the North Aral Sea is mainly derived from endogenous sources, the change in exogenous organic matter should have a small impact on the OCBR. However, in the 1960s, the source of organic matter in the sediments of the North Aral Sea was significantly different, which was manifested in an increase in the content of the exogenous organic matter. From the 1950s to the 1970s, an increase in precipitation typically promoted the carbon storage of vegetation and soil in a watershed [60; 61], thereby, increasing the input of organic carbon in sediments. The local government built a sand dam in 1992, and human intervention reduced the amount of water flowing out of the North Aral Sea into the Large Aral Sea, which caused the level of the Small Aral Sea to rise [62]. Since the dam was completely destroyed in 1999 AD [63], the level of the water dropped sharply during this period.