Study On Lipid Biomarkers And Isotope Geochemistry In Lake Sediments From Typical Antarctic Regions

The Antarctic region is an important place for studying climate change and environmental ecological responses.Lake sediments are considered to be good carriers for reconstructing climate and ecological history because of their high resolution and relatively good age sequences compared with terrestrial sedimentary materials.Proxies,such as biomarker and carbon isotopic compositions,are commonly employed in research of Antarctic lacustrine sediments.These Proxies provide information on both local history and global environmental changes.The species,content,distribution characteristics and carbon isotopic composition of the biomarker compounds in the Antarctic lake sediments can indicate the changes in the organic matter and the depositional environment,and reconstrut the regional evolution history.In this study,we collected the lake sediments in the Ross Sea region,the surface sediments of the Fildes peninsula and the Larsemann Hills,biological remains and some environmental media samples.With the methods of lipid molecular markers and isotope geochemistry,we discussed the characteristic of the organic geochemical distribution and the response of paleo-ecology to historical climate and environmental changes in the typical Antarctic regions.Main study contents and results are given as follows:1.Predominance of sedimentary leaf waxes derived from fungi:evidence from Antarctic lakesLong chain(>C24)n-alkanes with a strong odd over even predominance and n-alkanoic acids with a strong even over odd predominance in aquatic sediments,or collectively,long chain n-alkyl lipids(LCNALs),are traditionally ascribed to a terrestrial vascular plant origin,because these compounds are the major components of higher plant leaf waxes.However,it is extremely difficult to detect the existence of long chain n-alkyl lipids derived from non-vascular plant sources in aquatic sediments,because of the prevalence of higher plants in most environments around the globe.Here we show that LCNALS can be derived from aquatic sources by studying glacial-fed Antarctic lakes,with no vascular plants present in the surrounding land mass.We found abundant LCNALs in both Long lake and Darning lake sediments.More importantly,we found a significant amount of lichen and moss surrounding Long Lake,but not Darning Lake.The carbon isotopic ratios demonstrated conclusively that these compounds were derived from aquatic sources in the lake sediment.This was most likely an aquatic fungal microorganism that decomposes lake algae in the lake.In comparison,we show that when vascular plants are present around a high latitude Antarctic lake,LCNALs are predominantly derived from higher plant sources,with a negligible contribution from aquatic sources.2.Characteristics of organic geochemistry in biological excrement sediment from Beaufort Island,East AntarcticaWe analyzed molecular geochemical compositions of biological excrement sediment from Beaufort Island,East Antarctica.The results showed that n-alkanoic acids were mainly composed of short chain(C16 and C18)acids,which were most likely derived from lake algae and biological excrement of penguins and seals.Fecal sterol was the dominant alcohols in the sediment samples.Cholesterol and epicoprostanol were used as proxies of the population of penguin and seal respectively.Results suggested that historic penguin and seal population increased from 1680 to 1770 AD and from 1860 AD to recent,and the tendency of historical seal population showed a more dramatic change.The change profiles of vegetable sterols,chlorophyll a and historical penguin/seal population proxies exhibited similar fluctuations against depth in the sediment core.The results suggested a close interaction between animals and aquatic vegetation around the study site,which means the input of biological feces provided nutrient for the growth of algae and thus promoted the growth of algae.3.Source of organic matter and paleo-environmental implications inferred from carbon isotope composition of lacustrine sediments at Inexpressible Island,Ross Sea,AntarcticaIn order to identify the source of organic matter,we determined the carbon isotopic composition of organic matter(?13Corg)in two sediment cores(IIL1 and IIL9)collected from Inexpressible Island,Ross Sea,Antarctica.The ?13Corg values of IIL9 sediments were found to vary between-13.06‰ and-11.06‰,with a mean of-12.34‰(n=48),significantly higher than those in IIL1 sediments.The ?13Corg values of IIL1 varied between-23.63‰ and-20.66‰,with a mean of-21.97‰(n=55).Different ?13Corg values in these two sediment cores indicate alternate sources of organic matter.The relatively high ?13Corg values in IIL9 is in accordance with a source from algae,while the low ?13Corg values in IIL1 evince significant influence from penguin guano with some mixing of guano and algae.Compared with the reference data from other high-latitude lake sediments and plants,the ?13Corg values in IIL9 were extremely high,a result likely related to intense competition for CO2 assimilation among algal species during the growing season in this relatively shallow pond.The fluctuation of ?13Corg with depth in the IIL9 sediment profile displayed significantly positive correlations with changes in TOC and TN,which are indicative of algal production.Thus ?13Corg in the sediments predominately composed of pond algae can be used as a reliable proxy for changes in primary production.4.Historical seal distribution and environmental change in the Cape Bird and Marble Point,Ross Sea,AntarcticaWe applied coprostanols and seal hair numbers as proxy indicators to reconstruct the history of seal inhabitation and climate changes at Cape Bird.The results indicated that seal occupied the study region during 1280-1660 AD,which means,it was suitable for seal living during this period.Through the analysis of the ancient seal hairs,the results suggested that seal species in Cape Bird was mainly elephant seals.The seal hairs collected from Marble Point recorded the presence of seals at around 2700 a BP,in accordance with the seal optimum.The profile of MB6 showed that n-alkanoic acids mainly consisted of C16 and C18.We infer that the short chain acids were likely derived from lake algae and excrement.The carbon isotope values of the molecular markers were between-34.7 and-28.5‰ and the short chain n-alkanoic acids indicated an interaction with biological excrement in the profile of MB6 sediments.Thus the carbon isotope can indicated the source of organic matter and reconstructed the paleoecology and climate changes in Antarctica regions.