Carbon And Hydrogen Isotope Ratios Of N-alkyl Lipids In Lacustrine And Ocean Sediments From Ross Sea,Antarctica And Paleoclimatic Implications

Antarctica is an ideal region for studying past and present climate change and ecological response on the Earth because of its extreme natural climate per se,fragile ecosystem,and high sensitivity to fluctuations in global climate change.Although organic biomarkers(e.g.,n-alkyl lipids and alkenones)are a small group of total organic matter,these compounds are widely used to investigate biogeochemical cycle and reconstruct paleo-environmental and paleo-climatological changes due to their ubiquitous occurrence and excellent preservation in soils and lacustrine sediments.The distributions of organic biomarkers and their compound-specific carbon and hydrogen isotopic ratios in the Antarctic lake and ocean sediments are employed to quantitatively track the sources of organic matter and sedimentary environment changes,enabling us to reconstruct the paleo-environmental and paleo-climate variations.Previous studies were mainly focused on the sources of n-alkyl lipids.However,the distribution of n-alkyl lipids and compound-specific carbon and hydrogen isotope ratios in modern environmental samples from the Antarctica are lack of understanding.Furthermore,the implications of different organic biomarkers to climate and environmental changes are not differentiated,and the application of their compound-specific carbon and hydrogen isotopes for paleo-environment and paleo-climatological reconstructing is still very limited in Antarctic samples.In this study,we collected a variety of environmental samples in Antarctica,lacustrine sediments in the Ross Sea region,and ocean sediment core in the Southern Ocean.We systematic investigated the characteristics of n-alkyl lipid distributions and their ?13C values in the Antarctic samples,and accurately identify the sources of organic matter in lacustrine sediments and elucidate its potential implications on climate.In addition,multiple environmental proxies are used to reconstruct the historical climatic changes in the Ross Sea and Southern Ocean.Study designs and major findings are listed as follows:1.The characteristics of n-alkyl lipid distributions and their ?13C values in Antarctic environmental samplesThe geochemical characteristic of n-alkyl lipids in Antarctic environmental samples were obtained through analysis of a variety of environmental samples(moss,lichen,microbial mats,soil and moss-covered soil,fresh penguin guano and seal excrement,modern and ancient ornithogenic soil).Here we show that long-chain n-alkyl lipids are predominant biomarkers in moss,lichen and moss-covered soil samples with relative lower ?13C values ranging from about-38 to-33‰,which are similar to the values in lower latitude moss and lichen samples.However,the long-chain n-alkyl lipids in Antarctic lichen and moss are 2 to 3 carbon numbers longer in chain length than those in temperate lichen and moss;such differences of chain-length may be ascribed to the dry and cold environment of Antarctica.Biomarkers in microbial mats and soils are dominated by short-chain n-alkyl lipids,but long-chain species are extremely rare.The average ?13C values of n-alkyl lipids in microbial mat and soil samples are around-21‰,significantly higher than the values in moss,lichen and soil under moss.Animal excrements and ornithogenic soils have highest levels of short-chain n-alkanoic acids,along with some mid-chain species.The average ?13C values of short-and mid-chain n-alkanoic acids in animal excrement and ornithogenic soil samples vary from-32 to-28‰,while the average?13C values of long-chain n-alkanoic acids in ornithgenic soils range from-34 to-30‰.This study results show that the primary sources of long-chain n-alkyl lipids in Antarctic environment are moss and lichen and animal excrements can transport abundant short-and mid-chain n-alkanoic acids to natural environment.Our study can provide a reference for accurately tracking the sources of organic matter in Antarctic soils and lacustrine sediments using organic biomarkers and their isotopic signatures.2.The characteristics of carbon isotope ratios of n-alkyl lipids in Antarctic lacustrine sediments and its environmental implicationsLong-chain n-alkyl lipids have long been considered the most important biomarkers in higher vascular plant leaf waxes and widely applied to paleo-climate and paleo-environmental reconstructions.However,a critical question that has never been conclusively addressed over the past 50 years is whether or not non-vascular plant sources such as microbes can contribute significantly long-chain n-alkyl lipids to soil and lacustrine sediment.To resolve this problem,we selected two pond surface sediments(?L3 and ?L9)collected at high latitudes of Ross Sea,East Antarctica and investigated the distributions of n-alkyl lipids and their carbon isotope ratios.By comparing with Antarctic environmental samples,we found abundant long-chain n-alkyl lipids with high ?13C values varying from about-20 to-12‰ in IIL3 and IIL9 sediments,much higher than the values in Antarctic moss and lichen(-38 to-32‰).The high ?13C values of these long-chain n-alkyl lipids in lacustrine sediments,however,are similar to the values in aquatic microbial mats with an average bulk 813C value of-14.2±1.7‰,indicating that heterotrophic microbes could be the major source of these long chain n-alkyl lipids.This finding is consistent with previous study in the surface sediment of Darning Lake,East Antarctica.A binary carbon isotope mass balance model suggests that more than 80%long-chain n-alkyl lipids could have derived from heterotrophic microbes.Therefore,caution must be taken in using long chain n-alkyl lipids for paleo-climatic and paleo-environmental reconstructions on sites with little higher plant coverage(e.g.,Antarctic cold deserts,or other arid regions).3.The characteristics of hydrogen isotope ratios of n-alkyl lipids in Antarctic lacustrine sediments and its environmental implicationsRecent experimental and lacustrine sediment studies suggested long-chain n-alkyl lipids(n-alkanes and n-alkanoic acids)can be produced by aerobic microbes.One important question remains:which metabolic pathways(i.e.,heterotrophic versus autotrophic)do the microbes utilize to produce these compounds?To answer this question,we analyzed the hydrogen isotope ratios of long-chain n-alkyl lipids and the diversity of heterotrophic bacteria in two pond surface sediments(IIL3 and IIL9)from Ross Sea region,computed the hydrogen isotopic fractionations,and compared the results with the values reported in cultural studies.We found that the 8D values of long-chain n-alkyl lipids in the lacustrine sediments are significantly higher than the values in the lake waters by up to 300‰,and hydrogen isotopic fractionations with regards to the lake water(?Lipids/water)vary from-66 to 349‰.It is the first time that such strongly inverse hydrogen isotopic fractionation of long-chain n-alkyl lipids is observed in natural sediments.Based on recent extensive experimental data on microbial hydrogen isotopic fractionations,our data can only be explained by the predominant production of long-chain n-alkyl lipids from heterotrophic microorganisms in the study ponds.Furthermore,the high ?FA/water values of long-chain n-alkyl lipids may be rooted in the families of heterotrophic bacteria including SB-1,Flavobacteriaceae,Marinilabiaceae,Desulfobulbaceae,and Trueperaceae.These results are supportive of our isotope inference that long-chain n-alkyl lipids primarily derive from heterotrophic microbes in our study lakes.4.Paleoclimatic reconstructions for the past 3200 years in the Victoria Land,Ross SeaLakes ecosystem in Antarctica is sensitivity to fluctuations in global climate change and lake sediments are ideal recorder of paleo-environment and paleo-climatological changes.In this study,we reconstructed lake primarily productivity using ?13C values of long-chain n-alkanoic acids and pigments in two selected sediment profiles(IIL3 and IIL9)without input from animal excrement.By comparing with the results with other records from Ross Sea,we revealed the climate change process in the Victoria Land region for the past 3200 years.Our results suggested that more than 80%of long-chain n-alkanoic acids could have originated from heterotrophic microbes in the two sediment profiles,with little contribution of terrestrial moss.In addition,our results inciated that long-chain n-alkanoic acids from moss are extremely rare(average<5%)in this region,implying that the climate may be always drought over the past 3200 years.The ?13C values of the long-chain n-alkanoic acids showed a strong positive correlation with the ?13Croc values and thus can be used as a new proxy for the reconstruction of lake productivity and climate changes.The relatively high productivity recorded in the bottom layers of sediment cores(? 3200 a BP)indicated a warmer climate than younger times.This recorded warmer period may reflect the age of ice-sheet retreat in the Inexpressible Island.Furthermore,our results suggested two significantly warm(-2300-1300 a BP and?1100-700 a BP)and two cold(?3100-2600 a BP and?500-200 a BP)periods,which are consistent with other sedimentary records in the West Ross Sea region.The climatic variations in the Victoria Land region may be mainly controlled by Southern Annular Mode(SAM)and El Nino-Southern Oscillation(ENSO)which are related to solar activity changes by nonlinear dynamics.5.The characteristics of organic biomarkers in the Southern Ocean sediments and the implications of climatic changeVacillation of westerly winds in the Southern Ocean plays an important role in global climate and carbon cycle changes.Here we analyzed speciation and distribution features of long-chain n-alkanes and alkenones and carbon isotope ratios of long-chain n-alkanes preserved in a newly collected sediment core(R23),located in the far reaches of the Pacific sector of the Southern Ocean.Our main objectives are:(a)to evaluate the sources of long-chain n-alkanes based on their chain length distributions and compound-specific carbon isotopes,(b)to assess the UK37 and Uk'37-derived SST records.Our results show that long-chain n-alkanes(C29-C35)originated primarily from terrestrial higher plant waxes via long-range transport of dust,possibly from Australia and New Zealand.Mid-chain n-alkanes(C23-C25)with small odd-over-even carbon preference suggest that they stemmed from marine non-diatom pelagic phytoplankton and terrestrial sources.Furthermore,the C26 and C28 alkanes with lower ?13C values(?-34‰)indicate an origin from marine chemoautotrophic bacteria.The 813C values of the long-chain n-alkanes range from-30.8 to-24.8‰:the higher ?13C values suggest more contribution from C4 plants waxes.We found that Uk 37-derived SST displays a notable warm period,and therefore suggest the feasibility of using Uk37 in further SST reconstructions in the high latitude Pacific sector of the Southern Ocean.Based on the above results,alkane proxies(CPI,ACL and C31/C29)and ?13C values of C31 n-alkane variations in the sediment core suggest an expansion of C4 plants in the southern Austria and New Zealand region during the glacial(cold)periods,while the drier climate reflected equator-ward shifts and stronger in westerly winds.On the contrary,lower relative abundance of C4 plants indicates that the position of westerly winds moved polar-ward and became weaker during the interglacial periods.Our results document substantial orbital scale moisture changes in the southeast Australia and New Zealand over the past 330 ka,primarily paced by obliquity.