Impacts Of Hydrological Conditions And Temperature On Microbial Tetraether Lipids In Soils And Lakes

The extreme climate events have drawn more and more attention.One important way to study them is to understand how the climate changes in the past,and thus link with global changes and predict the variations in the future.Environmental proxies are useful tools for the understanding of past climatic changes.A large part of the proxies are established for marine system,the number of proxies that can estimate past continental air temperatures,however,is relatively limited.Due to the heterogeneity in continental environment,many proxies can only be applied to some specific archives,such as lake sediments or stalagmites.But not all lakes or caves have archived paleoclimatic changes,and large parts of land have no lakes or caves,leaving these blank areas without paleoclimatic records.Promising candidates to fill this gap are the recently developed continental paleoclimate proxies based on branched glycerol dialkyl glycerol tetraether membrane lipids?b GDGTs?,which occur widely in terrestrial systems and are sensitive to environmental variations.However,proxies based on GDGTs may have different behavior in different terrestrial environment,like soils and lakes,which will impact the universality of these proxies.Therefore,?1?it is critically important to identify the different response strategy of these widely dispersed proxies in different environment or under different hydrological conditions,which will lay the foundation for their applications.On the other hand,The Asian Monsoon is not only a fundamental component of the Chinese and even global climate system but also a key factor influencing the sustainable development of densely populated East Asia.Our current understanding of past changes in the East Asian Summer Monsoon is mainly based on oxygen isotope records of stalagmites and grain size or magnetic susceptibility of loess-paleosol sequences.However,more and more studies have suggested that loess-and speleothem-based East Asian Summer Monsoon reconstructions may not reflect a pure precipitation signal,but potentially also contain a temperature component.Hence,?2?how to separate the coupled temperature and precipitation signals as well as to determine terrestrial proxies for reconstructing pure paleotemperature and paleoprecipitation is another key issue.For above issues,we start at a soil-lake transitional zone,and then deep into lake sediments.With previously published database of global soil,we compare the distribution and environmental dependence of GDGTs in lakes with that in soils.Our results indicate the influence of different environments on GDGTs-based proxies.The main conclusions are as follows:?1?The b GDGT-based MBT?‘?/CBT?MBT-methylation of branched tetraethers, CBT-cyclization of branched tetraethers?proxy has been widely applied in terrestrial paleotemperature reconstructions.However,large scatters of estimated temperature in arid and semi-arid regions always exist.By investigating the variations in GDGT abundances and distributions in soils along a large soil water content?SWC?gradient,we find that SWC has a strong impact on the relative abundance of b GDGTs and i GDGTs.We also find,for the first time,that the degree of methylation of b GDGTs?MBT'?,except for CBT,is significantly influenced by SWC.Our results show that the original MBT'reflect not only temperature signal,but also hydrological information.?2?After separating the 5-and 6-methyl b GDGTs,we find that SWC exerts a greater impact on MBT‘_6ME compared to MBT‘_5ME,implying a greater SWC control on the degree of methylation of 6-methyl b GDGTs compared to that of 5-methyl b GDGTs. Furthermore,we show that the degree of methylation of 6-methyl b GDGTs is mainly affected by SWC,whereas 5-methyl b GDGTs are more impacted by temperature,resulting in a co-variation of temperature and hydrological conditions recorded in MBT‘.?3?Based on above findings and the results of re-analyzing the published global soil database,we successfully separate the coupled temperature and hydrological information.We propose a diagnostic parameter,the IR_6ME?relative abundance of 6-methyl b GDGTs?index,to evaluate the applicability of b GDGT-based paleotemperature reconstructions.By separating the global dataset into two groups with an IR_6ME cut-off of 0.5,we find that MBT‘shows a significant correlation with mean annual air temperature when IR_6ME<0.5,allowing for the use of MBT‘/CBT as temperature proxy.However,MBT‘correlates significantly with mean annual precipitation when IR_6ME>0.5,implying that MBT‘may respond to hydrological change in these regions and can be used as a proxy for paleoprecipitation.?4?We have investigated the environmental controls on b GDGTs isomers in lacustrine system for the first time.The 5-and 6-methyl b GDGTs in Chinese lakes exhibitdifferentrelationshipswithtemperature.The5-methyl b GDGT-producing bacteria in lakes respond to temperature solely by regulating the abundance of hexamethylated III_5ME series,whilst the responding strategy of 6-methyl b GDGTs to temperature has no selectivity of compound series,i.e.can adapt to temperature by regulating all series.Thus,new calibrations have been established based on the investigations on Chinese lakes.The new calibrations proposed in this study may be useful for paleotemperature reconstruction in Chinese lacustrine archives and potentially elsewhere.Cotemporary study of b GDGTs isomers in present lakes helps improve the accuracy of paleoclimate reconstructions from lacustrine archives.?5?After comparing the behavior of 5-and 6-methyl b GDGTs in soils with lakes,we find that the 5-and 6-methyl b GDGTs respond differently to temperature in soils and lakes.The abundance of each 6-methyl b GDGTs series and their ratios correlate well with temperature in lakes;however none of them show any strong correlation with temperature in soils.The 5-methyl b GDGTs may respond to temperature by changing the relative abundance of pentamethylated II_5ME to I series in soils rather than by regulating only the abundance of III_5ME series in lakes.Therefore,the MBT‘_5ME index,which mainly traces the variation of I series,is sensitive to temperature in soils but is not influenced by temperature in lacustrine environments.This suggests that both 5-and 6-methyl b GDGTs-producing communities may be different in lakes and soils.This investigation identifies the reason responsible for different behavior of b GDGTs in soils and lakes,which lays the foundation for the applications of GDGTs-based proxies in different terrestrial environment.