广西北部山地沼泽20 ka以来的 碳同位素组成与气候变化

根据广西北部山地泥炭沼泽沉积的有机碳同位素组成、TOC和C/N等古气候指标对末次盛冰期以来该区植被与气候演变过程进行研究，结果发现：末次盛冰期（21―14 cal. ka B.P.）偏正的δ13Corg数据反映了山地植被面貌曾发生显著变化，指示冰期气候的冷干变化，但并未导致山地森林向地带性草地退化，C3植物仍占优势。沉积物岩性和TOC等数据表明：在14―10 cal. ka B.P.期间沼泽湿地逐渐形成，指示了东亚夏季风降雨的增强，与石笋指示的季风增强起始年代基本一致。冰消期碳同位素曲线负偏的变化要早于TOC指示的泥炭沼泽堆积，即10―9 cal. ka B.P.前后富有机质的泥炭堆积进入旺盛期。碳同位素组成在9―8 cal. ka B.P.存在一个正偏的变化波动，可能与8.2 ka降温事件或局地物源输入变化有关。7.0―2.8 cal. ka B.P.期间古田湿地的有机质输入较高，碳同位素偏负，指示气候暖湿。从1.7 cal. ka B.P.至今，δ13Corg值的再次正偏可能指示夏季风强度减弱，但考虑到该阶段为人类农业活动快速发展期，因此不排除人为因素的影响。此外，末次盛冰期广西山地与雷州半岛沿海低地的碳同位素值对比发现：沿海平原区与华南山地的沉积记录存在明显差别，其主要原因是山区因山地降雨效应使山区的湿度即便在盛冰期也能保持较高水平；而低地平原冰期阶段的降雨则显著减少，加上平原区较高的蒸发量使干旱程度增加，C4草本也相应增多。实际上，影响亚热带山地沼泽δ13Corg值变化的原因比较复杂，在利用δ13Corg值推论古气候时需要同时考虑气候因素和湿地沉积体的物源及其地貌环境等背景。

Carbon Isotope Compositions and Climate Changes of the Past 20,000 Years Inferred from a Mountainous Peat Bog of Northern Guangxi

This study concerns the organic carbon isotopes, TOC and C/N in a mountainous peat bog in northern Guangxi province. Result reveals the local vegetation variations and the climate changes since the LGM in subtropical China. Positive δ13Corg indicates vegetation types change due to the cold and dry climate during the LGM. However, cold and dry condition didn’t lead to vegetation degradation from forest to grassland even during the last glacial period, as δ13Corg shows the dominance of C3 plants around the mountains, which is also proved by pollen records. It can be inferred from the lithology and TOC that local peat-land began to accumulate during the period of 14-10 cal. ka B.P., which developed at fastest speed between 10 and 9 cal. ka B.P. Carbon isotope change towards negative values occurred slightly earlier than the formation of the swamp, indicating that strengthening of summer monsoon occurred at about 14 cal. ka B.P., which is basically concordant with the record from stalagmites. Obvious variation in carbon isotopes which occurred between 9-8 cal. ka B.P. may be relative to 8.2 ka cooling event or changes of organic matter sources. High input of organic matter and negative carbon isotopes in the period during 7.0-2.8 cal. ka B.P. corresponded to the wet and warm condition. Relatively positive change of δ13Corg at 1.7 cal. ka B.P. might be caused by prehistory human impact. The comparison between the δ13Corg data and those of the coastal lowland records from the Leizhou Peninsula shows that a big difference existed between plain and mountainous areas, probably because humidity didn’t reduce too much due to the effect of orographic precipitation in the mountains even during the LGM, while decrease in precipitation and high evaporation were apparently shown in the plains. It should be pointed out that the factors are quite complicated for the interpretation of δ13Corg values in the peat records. The changes in sediment environment, origin of the organic matter and geomorphologic conditions should be taken into consideration when using δ13Corg to reconstruct paleoclimate.