青海湖钻孔沉积物中的羟基甘油二烷基甘油四醚类化合物及其环境意义

HYDROXYLATED GLYCEROL DIALKYL GLYCEROL TETRAETHERS IN LAKE QINGHAI SEDIMENTS AND THEIR PALEOCLIMATE IMPLICATIONS

来源于古菌细胞膜脂的类异戊二烯甘油二烷基甘油四醚类化合物（isoGDGTs）是古环境重建的有力工具。最近的研究发现，自然环境中还存在着一类isoGDGTs烷基长链上含有羟基的OH—GDGTs，其相对含量与温度负相关，具有示踪古温度的潜力。然而，目前湖相环境中关于OH—GDGTs的研究还很少。我们通过高效液相色谱-大气压化学电离-质谱分析，在青海湖8ka以来沉积物中检测到了OH—GDGTs的存在，其含量（相对含量％OH—GDGTs及浓度）与奇古菌特征标志物crenarchaeol相对含量和奇古菌amoA基因丰度一致，表明青海湖中的OH—GDGTs可能主要来源于奇古菌。进一步与湖泊水位指标对比显示，青海湖中％OH—GDGTs的变化可能主要反映了湖水深度变化导致的古菌群落的变化。因此，认为湖泊沉积物中的OH—GDGTs可指示古水文条件变化。

Archaeal lipids are ubiquitous in natural environments and have been proven to be useful biomarkers for paleoclimate studies. Recently, a new group of archaeal lipids, i.e., hydroxylated glycerol dialkyl glycerol tetraethers （OH-GDGTs） with one or two hydroxyl groups in the biphytanyl moieties of isoprenoid GDGTs （isoGDGTs） , was identified from marine sediments. Subsequent studies found increasing OH-GDGT contributions towards higher latitudes and lower water temperatures in marine and lake sediments, and therefore, the relative abundances of OH-GDGTs are potential proxies for paleotemperature reconstruction. Until now, however, knowledge is still limited about their occurrence and distribution in lakes. Lake Qinghai, located at the transition from the arid to the semi-arid climate zones, is the largest interior plateau lake in Central Asia. Due to the sensitivity of its regional climate to monsoon variation and global change, paleoclimate reconstructions from Lake Qinghai are of wide interest for understanding how complex forcing mechanisms could affect the regional climate. Hence, the development and validation of novel proxies for paleoclimate reconstructions are of great importance for this lake. In this study, we investigated the occurrence and distribution of OH-GDGTs in 12 downcore sediment samples （collection at 0-337cm in depth） of core QH-2011 （36°39′34″N, 100°35′37″E） with a whole length of ca. 5.8 m. This core was recovered at a water depth of ca.24 m in the southeastern sub-basin of Lake Qinghai and the samples used in this study covered the Middle and Late Holocene periods （past ca. 8ka）. The total lipid was extracted ultrasonically by using MeOH ： Dichloromethane （9：1, v/v） for 3 times, and fractionated over an activated silica gel column. The polar fraction containing OH-GDGTs was measured using high-performance liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry. The results showed that OH-GDGTs were generally abundant in the sediments of Lake Qinghai during the past 8ka, with OH-GDGT-0 being the most abundant. The concentration of OH-GDGTs ranged from 0. 8ng/g to 53.6ng/g, while the relative abundance of OH-GDGTs （to the sum of isoGDGTs and OH-GDGTs） ranged from 0.56% to 10.28%, both of which were relatively low in the early Middle Holocene sediments and high in the Late Holocene sediments. The fractional abundance of OH-GDGTs was positively correlated with the fractional abundance of crenarchaeol, a unique archaeal GDGT specifically from Thaumarchaeota （R2= 0.93; p 〈 0.01 ）, while the variation in the concentration of OH-GDGTs resembled that for thaumarchaeotal amoA gene abundance, pointing to a Thaumarchaeotal origin of OH-GDGTs in Lake Qinghai. Since the producer of OH-GDGTs, i.e., Thaumarchaeota prefers living in stratified waters with a deep mixed layer, where ammonium availability near the oxycline is high and the competition with photoautotrophs is low, we speculate that variations in OH-GDGT concentration and %OH-GDGTs are possibly controlled by changes in lake water depth. Further comparison between OH-GDGTs （both concentration and relative abundance ） and lake level proxies （including shoreline evidences, the relative abundance of crenarchaeol, and total organic carbon isotopes） supports that lake water depth plays an important role in affecting archaeal community structure and the production of OH-GDGTs. Therefore, we propose that OH-GDGTs preserved in lacustrine sediments might help to reconstruct the paleohydrological history of inland lakes.