The Paleotemperature And Enhanced Drought Recorded By Microbial Tetraethers In Weinan Loess-paleosl Squence

The loess-paleosol sequences(LPS)in the Chinese Loess Plateau(CLP)have been proven to be good records of paleoclimatic changes during the Quaternary period.The interplay between the Asian summer and winter monsoon system(ASM and AWM)has been well documented by using a variety of geochemical and geophysical proxies.The(semi)quantitative reconstruction of temperature and precipitation independently are required to better understand the interaction between the climatic factors and their driving forces.The glycerol dialkyl glycerol tetraethers(GDGTs)areincreasingly used tools for quantitative paleotemperature reconstruction in the LPS.They are membrane lipids of archaea and some unknown bacteria that occur ubiquitously in terrestrial and marine environments.Microbial cell membrane is sensitive to the external environment changes.By adjusting the structure and composition of GDGTs compounds to maintain their liquidity and stability,the microbial can survival in environmental changes.Thus,the GDGTs record the climate change and show potential in paleoclimate reconstruction.Recently,the original nine brGDGTs were found to harbor 1-3 isomers that exhibit different positions of methyl group.The 5-and 6-methyl brGDGTs,respresenting methyl at ?/? 5 and ?/? 6 position,respectively,show completely different response to pH and MAT.Until recently,almost all the paleo-reconstructions using brGDGTs in the LPS from the CLP have been performed based on the traditional nine brGDGTs,whose isomers generally co-eluted in the liquid chromatography.It has been widely demonstrated that separation of 5-and 6-methyl brGDGTs can markedly improve the accuracy of temperature and soil pH reconstruction in modern surface soils.Here,we used two different liquid chromatography methodology to analyze 198 loess-paleosol samples from the Weinan section in the southern CLP for the past 350 kyr.The traditional one is using-CN column and result in 9 brGDGTs.The new improved liquid chromatography methodology using two silica columns in series and result in 15 brGDGTs containg 5-methyl brGDGTs and its 6-methyl isomers.We compare the difference between the two methods and compared to determine the suitable proxies to reconstruct the paleoclimate change in Weinan area.(1)The development of methods using liquid chromatography coupled to mass spectrometry to analyze glycerol dialkyl glycerol tetraethers(GDGTs)has substantially expanded the biomarker tool box and led to the development of several new proxies.Recent studies have shown that new high performance liquid chromatography methods have substantially improved separation of GDGT isomers and detection of novel isomers.And with the new chromatography method,iGDGTs showed some “shoulders” different with before.The “shoulders” should be included in the calculation of proxies and the total iGDGTs content remained about the same.Our results show that the new method leads to a less difference on TEX86,but no difference on BIT.(2)The 6-methyl brGDGTs comprise a major proportion of total brGDGTs,pointing to alkaline conditions for most soil samples from the Weinan LPS.The 6-methyl brGDGTs exhibited a higher abundance than their 5-methyl isomers in both loess and paleosols,accounting for 34.6% of the total brGDGTs on average.GDGT-Ia,-Ib and-IIa' collectively comprise 70% of total brGDGTs on average.Among them,brGDGT-Ia accounted for 17.8% to 46.5% of total brGDGTs,being the most abundant brGDGT component in general.MBT'5ME and MBT'6ME showed similar variation tendency in most intervals of the past 350 kyr in the Weinan LPS.In the profile,MBT'5ME generally changes ahead of MBT'6ME,but during some periods the variation of MBT'5ME and MBT'6ME exhibited an opposite trend.The decoupled profile variation of MBT'6ME and MBT'5ME suggests their response to different enviromental factors,possibly soil pH(or soil moisture)and temperature,respectively.This discrimination further corraborates that temperature and monsoonal precipitation were not in phase during the last four deglaciations on the CLP.(3)Temperatures reconstructed from six existing calibrations show similar trends but remarkably differ in amplitudes.The soil moisture appears to affect the global MBT'/CBT calibration,the global MATmr calibration and the Chinese local SSM(Stepwise Selection Method)calibration,resulting in a significant underestimation of late Holocene temperature.In contrast,the dry climate has no effect on the global MBT'5ME calibration.Of the six calibrations,only the Chinese local SSM calibration and global MATmr calibration produce temperature variation amplitude over the past 350 ka that is consistent with other independent proxy data.However,in the investigations of modern soils,pH have strong impact on 6-methyl brGDGTs,so we prefer the seperation of 5-methyl and 6-methyl brGDGTs obviously increase the accuary of paleotemperature reconstraction.So we pursue the temperature yeilded by MATmr caliration as the paleotemperature change in Weinan.During the past 350 ka,the temperature ranged from 13.1 to 23.7°C.The recently proposed local calibrations based on 5-and 6-methyl brGDGTs for North China yield the amplitude of temperature changes that is much larger than the results determined by other approaches.All the six calibrations have their own weakness in the MAT reconstruction,due partly to the inconsistency between the brGDGT distribution in the Weinan LPS and the modern soils used to establish these calibrations.(4)The abundance of archaeal iGDGTsrelative to bacterial brGDGTs,measured by Ri/b and BIT indices,is diagnostic of intensified or enhanced drought events.The Ri/b(and BIT)indices are stable and low(high)throughout most of the loess section spanning the last 350 thousand years,but they exhibit sharp transient peaks(valleys)during the intervals corresponding to the four ice age terminations,and especially those of Termination II and IV.The Ri/b(and BIT)indices are stable and low(high)throughout most of the loess section spanning the last 350 thousand years,but they exhibit sharp transient peaks(valleys)during the intervals corresponding to the four ice age terminations,especially those of Termination II and IV.These enhanced drought events,occurring immediately before the interglacial warmings,are different from,but much more severe than,the dry conditions during glacial periods.These enhanced megadroughts appear to be controlled by changing glacial-interglacial boundary conditions(e.g.,ice sheets,land albedo,sea ice)affecting the position of westerlies,but also amplified by a reduction in northward heat/moisture transport from the Indo-Pacific warm pool because of cooler SSTs and weaker Walker circulation during glacial terminations.