An Archaeal Tetraether Perspective On LGM Environmental Change In The Lake Fuxian

The integrated samples from the lake basin,including surface soils,river surface sediments,the monthly lake suspended particulate matter（SPM）and water parameters within a one-year cycle through 100 m water columns,the surface sediments from lakeshore to lake center,and the 900 cm downcore of Lake Fuxian in southwestern China,were selected as the subject of our research.We detected glycerol dialkyl glycerol tetraethers（GDGTs）,including isoprenoid GDGTs（isoGDGTs）and hydroxylated GDGTs（OH-GDGTs）,aiming to clarify the GDGTs source-sink process and the environmental implications of archaeal GDGT distributions.At this point,a sedimentary record since the last glacial maximum（LGM）in southwest China was reconstructed to explore the evolution and possible driving mechanism of the Indian summer monsoon（ISM）.The main conclusions were as follows.（1）All samples collected from Lake Fuxian Basin had high isoGDGT abundance.The Group I.1b thaumarchaeotal,anaerobic methanogenic archaea,and Group I.1a thaumarchaeotal community contributed mainly GDGTs in soils and rivers and lakes,respectively.The methanogenic archaea in rivers might produce a high abundance of isoGDGT-1 to distort the tetraether index of ₈₆ carbon atoms(TEX₈₆)signal,which leads to the abnormally low TEX₈₆-inferred temperature.But the influence could be only observed in surface water of the lake during the rainy season.The minor OH-GDGTs in the rivers and surface lake water suggested that OH-GDGTs almost originated from in-situ archaeal communities.（2）IsoGDGTs in water columns were mainly derived from Group I.1a Thaumarchaeota.IsoGDGTs concentrations of water columns spiked in June and November;which might be mainly related with the lake mixing-stratified process,as well as the phytoplankton bloom.About 95%isoGDGTs occurred in the hypolimnion of the whole water columns.Vertical distribution characteristics here were associated with the Thaumarchaeotal niche selections on low light,low nutrition,and low oxygen.The relative abundance of isoGDGT-2 was the most variable component among all for calculating the TEX₈₆ values.The content of dissolving oxygen（DO）in the hypolimnetic lake significantly affected the distributions of isoGDGT,especially isoGDGT-2,which led to negative correlation between TEX₈₆ and DO levels.However,the abnormally warm signal of TEX₈₆ caused by low-DO content could be identified through the threshold of isoGDGT-2 with%isoGDGT-2>45%.（3）Similar to isoGDGTs,OH-GDGTs derived from Group I.1a Thaumarchaeota bloomed in June and November.Their abundance were mainly distributed in the hypolimnetic water layer.%OH-GDGTs correlated positively with the level of DO,and followed negatively with p H in water colum.Instead,RI-OH（RI-OH’）correlated negatively with the level of DO,and followed positively with p H.The opposite correlations above were associated with the inverse physiological function of the hydroxyl and cyclopentane moieties for Thaumarchaeota.OH-GDGTs in lake surface sediments appeared homologous to that of hypolimnetic water columns,leading to%OH-GDGTs increased with depth and RI-OH decreased with water depth.Furthermore,this depth tendency showed RI-OH and%OH-GDGTs could be used as lake-level proxies in Lake Fuxian.（4）Given the relationships between TEX₈₆,%OH-GDGTs,RI-OH and environmental factors in water columns of Lake Fuxian,we concluded that the fewer hydroxyl moieties played a critical role in‘membrane lipid homeostasis’under the setting of low DO,high p H,as well as high temperature for Thaumarchaeota;Instead,the more cyclopentane moieties played a critical role in regulating GDGT distributions,under the setting of low DO,high p H,as well as high temperature for Thaumarchaeota.（5）The reconstructed geological period of downcore（FXH-1）based on AMS¹⁴C dating was 26.0 cal.ka.B P.The paleoenvironment could be divided into four stages based on multi-proxies.StageⅠwas 26.0～17.7 ka.The lake levels and temperatures of Lake Fuxian were the lowest in the whole 26 ka.The cold and dry climate led to the low watershed primary productivity,and the basin suffered strong physical weathering.ISM was undergoing its weakest period since 26 ka.Stage II was 17.7～11.8 ka.The lake levels and temperature rose slowly,and the warm and wet climate led to the slightly increasing watershed primary productivity.At this stage,the multi-indicator in Lake Fuxian also recorded a series of abrupt cold climate events,such as the Heinrich1（H1）events and the Younger Dryers event（YD）.The intensity of ISM was slightly increased.StageⅢwas 11.8～5.7 ka.The lake levels,temperatures,and watershed primary productivity reached the highest rising rate and of amplitude since 26 ka,and peaked at 5.7 ka,suggesting that the study area was undergoing the Mid-Holocene Optimum period with the most suitable configuration of water and temperature.The intensity of ISM was strongest.StageⅣwas from 5.7 ka to the present.The climate gradually turned cold and dry,leading the primary productivity rapidly decreased.But this tendency reached the minimum at 1.6 ka.Overall,the ISM intensity was gradually decreasing in this period.（6）From the perspective of the driving mechanism,the changes of the paleoenvironment of Lake Fuxian mainly responded to the dynamic of the intensity of the low-latitude solar radiation.However,the launch of the last deglacial and the terminal of the Mid-Holocene Optimum lags behind solar radiation by about 3～4 ka,respectively.It might be related to the increasing global CO₂ level,corresponding to the northern hemisphere continental ice sheet retreat and the variation of tropical Indian ocean-atmosphere interaction.