Biomarker and compound-specific δ13C evidence for changing environmental conditions and carbon limitation at Lake Koucha, eastern Tibetan Plateau
Biomarker and compound-specific δ13C evidence for changing environmental conditions and carbon limitation at Lake Koucha, eastern Tibetan Plateau
复制标题
DOI：
10.1007/s10933-009-9375-y
复制
发表时间：
2010-04-01
影响因子：
2.1
通讯作者：
Zhang, Chengjun
中科院分区：
文献类型：
--
作者：
Aichner, Bernhard;Wilkes, Heinz;Zhang, Chengjun
利用有机生物标志物及其稳定碳同位素特征对青藏高原东部库车湖沉积物岩心进行了研究。TOC含量、水生植物正构烷烃总量（如nC(23)）与TOC和nC(23) δ C-13值的相关性表明，库车湖在8 cal ka BP之前以大型植物为主。在7.2 cal ka BP时，该湖由咸水系统转变为淡水系统后不久，出现了各种藻类和细菌标记物，如藿烷和类异戊二烯，其中植烷、五甲基戊二烯（PMI）、多烯烯和双烯烯尤其丰富。植烷和PMI表现出不同的同位素信号（分别为1千分之1欧度和1千分之1欧度），表明它们的来源不同。植烷可能来源于蓝藻，而产甲烷的古细菌可能是PMI的来源。二磷烯和钼烯的同位素耗竭（60欧度下千分之一）表明存在甲烷氧化菌。在6.1 cal ka BP后，饱和的C-20高支化类异戊二烯（HBI）成为脂肪烃馏分的主要成分。这种优势很少在湖泊环境中报道，并且表明藻类（最有可能是硅藻）或蓝藻的强烈存在。在4.7 cal ka BP，出现了不饱和C-25 HBI，这是硅藻的特定生物标志物。此外，在最上面的两个样品中，nC(17)-烷烃的含量显著增加。这些结果表明，该湖泊在最后6.1 ka以浮游植物为主。相对较低的生物标志物浓度和δ C-13值分别为6.0、3.1和1.8 cal ka BP，这与青藏高原其他地区的推断一致。nC(23) δ C-13值在-23.5 ~ -12.6℃范围内，在约11 cal ka BP时大型植物丰度高峰值较高，在约6.1 ~ 2.8 cal ka BP时浮游植物丰度高峰值较高。因此，水生植物衍生的中链正构烷烃已被发现是碳限制条件的良好指标，这导致了同位素富集的碳种的同化。碳源的限制可能是发生在密集植物林分（如在核心的老部分）的局部现象，也可能是由高初级生产力（如在年轻部分）引起的。由于无机碳源的δ C-13值可能会发生变化，因此nC（23）和TIC的δ C-13值之间的偏移量可以作为湖泊环境中碳限制条件的更精确代表，反过来可以解释湖泊古生产力。
A sediment core from Lake Koucha (eastern Tibetan Plateau) was investigated using organic biomarkers and their stable carbon isotope signatures. The correlation between TOC content, total amount of aquatic macrophyte-derived n-alkanes (e.g. nC(23)) and delta C-13 values of TOC and nC(23) indicates that Lake Koucha was macrophyte-dominated before 8 cal ka BP. Shortly after the lake turned from a saline to a freshwater system at 7.2 cal ka BP, a variety of algal and bacterial markers such as hopanoids and isoprenoids emerged, of which phytane, pentamethylicosene (PMI), moretene and diploptene are particularly abundant. Phytane and PMI show different isotopic signals (a parts per thousand a'18 and a parts per thousand a'28aEuro degrees, respectively), which indicates that they originated from different sources. Phytane may have been derived from cyanobacteria, while methanogenic archaea may be the source of PMI. The isotopic depletion of diploptene and moretene (a parts per thousand a'60aEuro degrees) indicates the presence of methanotrophs. After 6.1 cal ka BP, the saturated C-20 highly branched isoprenoid (HBI) became the dominant constituent of the aliphatic hydrocarbon fraction. Such dominance has rarely been reported in lacustrine environments, and indicates a strong presence of algae (most likely diatoms) or cyanobacteria. At 4.7 cal ka BP, the appearance of an unsaturated C-25 HBI, which is a specific biomarker for diatoms, was noted. Furthermore, the level of nC(17)-alkane was observed to increase in abundance in the uppermost two samples. These results suggest that the lake was phytoplankton-dominated during the last 6.1 ka. Relatively low biomarker concentrations and delta C-13 values at 6.0, 3.1 and 1.8 cal ka BP indicate the occurrence of cool periods, which is in agreement with inferences from other locations on the Tibetan Plateau. The delta C-13 values of nC(23) range from -23.5 to -12.6aEuro degrees, with high values at the peak of macrophyte abundance at ca. 11 cal ka BP and at the phytoplankton maximum between ca 6.1 and 2.8 cal ka BP. Thus, aquatic macrophyte-derived mid-chain n-alkanes have been found to be excellent indicators of carbon-limiting conditions, which lead to the assimilation of isotopically-enriched carbon species. The limitation of carbon sources could be a localized phenomenon occurring in dense plant stands (as in the older section of the core), or it may be induced by high primary productivity (as in the younger section). Since the delta C-13 value of the inorganic carbon source may vary, the offset between the delta C-13 values of nC(23) and TIC could serve as a more precise proxy for carbon-limiting conditions in lacustrine environments, which could in turn be interpreted with respect to lacustrine paleo-productivity.