Early Diagenesis Of Peat Lipids And Their Responses To The Climate Changes Over The Past 13 Ka: Evidence From The Dajiuhu Peat Deposit, South China

Due to the specific conditions favoring organic matter preservation (acidic,waterlogged,oxygen deficiency,nutrition restricted,etc),peat deposits alwayspossess abundant and diverse lipid biomarkers.Previous studies principally focus onhigher plants derived lipids,such as the long-chain n-alkanes,fatty acids,alcohols,and n-alkan-2-ones.In fact,microbially derived lipid biomarkers occur widely inpeat deposits,such as hopanoids,branched and isoprenoid glycerol dialkyl glyceroltetraethers.The microbial activities strongly mediate the nutrients and carbon cyclesin the peatlands.In addition,microbial lipids have been proved to be sensitive toenvironmental changes.Dajiuhu Basin,located at the western margin of Shennongjia Forestry Area,HubeiProvince,is formed by glaciation,karstification and fluviation since the latePleistocene.The peat has accumulated in this closed subalpine basin from the latedeglaciation to the present.A multi-proxy study was conducted on the DZK-03 peatcore collected from Dajiuhu Peatland in May,2005.The analyses included pollen,POC,PON,lignin phenols,lipid biomarkers and their compound-specific carbonisotopes.Modern representative plants were also collected from Dajiuhu Peatland forlipid biomarkers and carbon isotope analysis.AMS ¹⁴C dating reveals that the peataccumulated through the recent 13 ka in the DZK-03 core.The general aim of thisstudy is to investigate the lipid biomarkers originated directly or indirectly frommicrobial activities,to discuss their transformations during very early diagenesis,andto explore their availabilities in paleoecological and paleoenvironmentalreconstructions.The investigation of leaf lipids in the peat-forming plants from Dajiuhu Peatlandreveals that some lipids are specific to certain plants;for example,24-methylcholest-7-en-3β-ol is relatively enriched in two moss species (Sphagnumpalustre,Polytrichum commune),while 24-ethylcholest-7-en-3β-ol is abundant in thenon-moss plants.Diploptene and hop-21-ene are enriched in mosses especiallyAulacomnium palustre.n-C₂₅ alkane is abundant in Quercus aliena var.acutesernataespecially in the dead leaves.The enrichment of n-C₂₅ alkane in dead leaves relativeto raw leaves of Q.aliena is probably caused by the additional input of thiscompound from fungi or other soil microorganisms.The Dajiuhu peat samples contain diverse groups of lipid biomarkers,including n-alkanes,long-chain 2- and 3-methylalkanes,terrestrial triterpenoids (includingaliphatic des-A,aromatic and polar homologues),hopanoids (hopanes,hopenes,hopanols,hopanones,hopanoic acids),steroids (sterols,sterenes,steroidal ketones),n-alkan-2-ones,fatty alcohols,fatty acids and fernenes.Some are seldom detected inthe peat deposit;e.g.,long-chain 2- and 3-methylalkanes,and C-ring cleavedaromatic triterpenoids.These results confirm that the peat deposit is perfect forchemical molecule preservation.The dominance of long-chain n-alkanes,fattyalcohols and fatty acids (＞C₂₂),terrestrial triterpenoids,and C₂₉ steroids suggests thathigher plants are the major source of lipid biomarkers.In addition,the occurrences ofabundant hopanoids,C₂₇ steroids,and short-chain fatty alcohols and acids (＜C₂₀)indicate that microbes are also important contributor to lipid biomarkers in the peatdeposit.This interpretation is further supported by the appearances of abundantaliphatic and aromatic terrestrial triterpenoids,stanols,steroidal ketones,and perhapsn-alkan-2-ones,which are diagenetic products derived from microbial activities.Plentiful intermediate products of early diagenesis of lipids occur in the Dajiuhupeat samples.In this young peat,terrestrial triterpenoids have already gone throughseveral types of degradations,including (1) aromatization of the A-ring followed byprogressive aromatization of the B-D rings ultimately resulting in piecene-likecompounds,(2) cleavage of the A-ring followed by progressive aromatization fromthe B to E-ring ultimately resulting in chrysene-like compounds,(3) cleavage of theC-ring of intact triterpenoids resulting in aromatized naphthyl/naphthylethylene-linked molecules,or (4) cleavage of the C-ring of the A-ring degradedtriterpenoids resulting in aromatized phenyl/naphthyl ethylene-linked molecules.17α(H),21β(H)-22R-homohopane (C₃₁) is the dominant hopanoids in both thesurficial and deeper peat horizons.The changes of C₃₁αβ/ββ-hopane ratio follow thegeneral trend of the Asian summer monsoon intensity over the past 13 ka,indicatingthatαβ-22R-homologue is the digenetic product of the biologicalββprecursor.Thistransformation is accelerated in the acidic condition.These results support the formerproposal that the occurrence ofαβ-22R-homohopane in recent sediments especiallypeats resulted from the oxidative and subsequent decarboxylation reactions ofbacteriohopanepolys with microbially mediated epimerization at position C-17catalyzed by the acidic environment in the mire.The results from the peat-forming plants and peat samples from Dajiuhu confirmthe former deduction that long-chain n-alkan-2-ones are the oxidized products oflong-chain n-alkanes with n-alkan-2-ols as the intermediate.This diagenetic processprobably happens in a quite early stage such as leaf decomposition before leaves are transported into the sediments.Comparing to the raw leaves,dead leaves containhigher amount of long-chain n-alkanes,n-alk-l-enes,and n-alkan-2-ones,whichreveals that the decay process of raw leaves can release much more n-alkyl lipids.The above observations highlight the important role of decomposition on the qualityand quantity of geological lipids.The changes of higher plants,algae and bacterial activities are reconstructed overthe past 13 ka based on lipid biomarkers,Sphagnum spores,algal fossils,ligninphenols and the C/N ratios.The Paq and C/N ratios reveal that the proportion ofnonvascular aquatic plants gradually increases until reaching the maxima value at 0.6ka and then gradually decreases to the present.Comparing to pollen,lignin phenolsand the C/N ratios,lipid biomarkers possess the priority to reflect the microbialactivities in the past.Our lipid results show that bacterial activities are strong duringthe peat accumulation process and these fluctuations of bacterial activities probablycorrelate with environmental conditions such as temperature and humidity.Notably,the lipid and pollen records do not correlate in details in some case.Thisinconsistency is interpreted as the result of different modes of incorporation of thesignals.The lipids are largely derived from organisms in situ,while theautochthonous pollen can be diluted by large amount of allochthonous pollen.Inother words,the pollen records regional vegetation cover,while lipid data reflectlocal plant input.Both records are complementary in the reconstruction ofpaleoenvironmental conditions.The lipid-derived record of Asian summer monsoon intensity is consistent with thecarbonate oxygen isotopic records in Dongge Cave.This consistency between twosites with the distance more than 900 km indicates that these two sites are in thesame moisture passage and the moisture passage is constant over the past 13 ka.Bothour lipid-derived record and the Dongge records generally follow the changes ofsolar insolation in north hemisphere,revealing that the changes of Asian summermonsoon intensity are controlled by solar insolation at the millennial timescales.Thelipid-derived Dajiuhu record shows several centennial to millennial timescale weakmonsoon events,centering at 11.3,10.1,8.4,7.0,5.2,3.1 and 2.1 ka BP.The lipid and pollen derived temperature proxies record a cold YD Epoch,followed by a general warm trend during 11.2-9.3 ka BP.The relatively high andconstant temperature characterizes the Holocene Optimum at 9.3-7.0 ka BP.Thetemperature generally decreases through the recent 3.5 ka.At centennial to millennialtimescales,the general trend of temperature in Dajiuhu Basin is superimposed byseven centennial to millennial timescale cold events,centered at 11.0,10.0,7.0,5.5, 3.3,2.3 and 1.5 ka BP.These events are,within dating error,in phase with coldintervals recorded in South China.Some of them are possibly linked to Holoceneice-rafting events in the North Atlantic.The wet intervals characterized by Paq,average chain length of long-chainn-alkanes (ACL),and triaromatic triterpenoid content (TAT) occur at 12.9-11.7 ka BP(YD),8.8-4.8 ka BP,and 2.9-1.4 ka BP.In the early Holocene,the general trend iswet,which is consistent with the abnormal accumulation of olefinic compounds atthis interval.However,this general wet trend is punctuated by centennial tomillennial timescale dry events centering at 11.6,10.5,10.0 and 9.4 ka BP.Duringthe transitional period from middle Holocene to late Holocene,notable dry periodshappens at 4.8-2.9 ka BP,indicated by quite low content of TAT,Paq and high ACL.Notably,the 8.2-ka event is obscure in the Dajiuhu record.A moderately weakmonsoon event centers at 8.4 ka BP,whereas temperature and hydrology proxies donot show obvious change around this interval.This finding suggests that the 8.2-kaevent is possible not popular in the middle latitude region of north hemisphere.Alternatively,the absence of 8.2-ka event in the peat lipid record might result fromthe maintain terrain in Dajiuhu Basin.The 10.0-ka event is remarkable for its duration and magnitude in the Dajiuhulipid record of Asian summer monsoon intensity.Meanwhile,the lipid derivedtemperature and hydrology records exhibit moderate shifts.High or even exclusiveaccumulations of some olefinic compounds (des-A-triterpenes,sterenes and fernenes)occur at 10.3-9.5 ka BP.In addition,the hydrological conditions vary greatly at thisinterval.All these data characterize the 10.0-ka event is very special in our Dajiuhurecord.Whether this event is local or regional scale need further proof.Generally,the changes of temperature and humidity follow the shift of Asiansummer monsoon strength.The fit between monsoon intensity and temperature iscloser than that between monsoon intensity and humidity.During the two transitionalperiods of Asian monsoon intensity,both temperature and humidity show greatvariations.In addition,the associations of temperature and humidity are changeablein different intervals.Warm wet,warm dry,cool wet and cool dry conditions occurover the past 13 ka in our records.