Biomarkers Significance Of Polycyclic Aromatic Hydrocarbons In Geological Processes

Peat is the accumulation of loose organic matter with different degrees of decomposition,accumulated by anaerobic microorganisms after slow decomposition.Peat accumulation has good continuity and high temporal resolution,and is a good paleo-environment and paleoclimate recorder.The high concentration of organic matter in peat provides favorable internal conditions for the adsorption and storage of PAHs,while the weak acidity and anaerobic properties make the microbial activities weak,which can reduce the possibility of transformation between organic chemicals.The detailed characterization of PAHs sedimentary sequences and sources is of great scientific significance to improve the method of PAHs source analysis and enrich the geochemical behavior of PAHs as well as the research methods of and"discussing the present to understand the past".Dajiuhu（DJH）Wetland is a unique sub-alpine marsh wetland in central China.Since the last deglaciation,the thick layer of peat has developed in Dajiuhu Basin.Here,we explore the composition and distribution characteristics of parent polycyclic aromatic hydrocarbons（P-PAHs）and alkylated polycyclic aromatic hydrocarbons（Alk-PAHs）based on the Accelerator Mass Spectrometry ¹⁴C(AMS ¹⁴C).It is verified that the ratio method is not suitable for the source analysis of PAHs in peat core,combined with the fingerprint of PAHs of rock,weathered crust,saprolite,and soil samples as well as the investigation of atmosphere and atmospheric deposition from plain to mountain section.Molecular diagnostic ratio（MDRs）was established by using the markers of diagenetic PAHs combined with absolute principal components-multiple linear regression（APCS-MLR）to strip the sources of PAHs.The main conclusions are as follows:1.The sedimentary records of P-PAHs and Alk-PAHs in peat core:The variation of∑₄₅PAHs was consistent with that of∑₂₅P-PAHs in DJH peat core since 21 cal ky BP.Per occupied the majority of PAHs.The variation pattern of∑₂₀Alk-PAHs was similar to that of∑₂₄P-PAHs and∑₄₄PAHs(∑₂₄P-PAHs and∑₄₄PAHs represent∑₂₅P-PAHs and∑₄₅PAHs remove Per).From 21 to 18 cal ky BP,the concentration of∑₄₅PAHs,∑₂₅P-PAHs and∑₂₀Alk-PAHs all showed an upward trend,but the concentrations were all in the low value.From 18 to 16.2 cal ky BP,∑₄₅PAHs and∑₂₅P-PAHs increased exponentially,while∑₂₀Alk-PAHs,∑₂₄P-PAHs and∑₄₄PAHs increased first and then decreased.From 16.2 to 10.0 cal ky BP,the variation trends of∑₄₅PAHs and∑₂₅P-PAHs increased first and then decreased,while the∑₂₀Alk-PAHs first kept stable and then showed the oscillating trend of"four valleys and two peaks".From 10.0 cal ky BP to 3.0cal ky BP,∑₄₅PAHs and∑₂₅P-PAHs decreased significantly,while∑₂₀Alk-PAHs,∑₄₄PAHs and∑₂₄P-PAHs increased oscillation.From 3.0 cal ky BP to the present,both∑₄₅PAHs and∑₂₅P-PAHs decreased first and then increased.The proportion of Per was higher than 50%before 3.0 cal ky BP,and the highest was 98.78%.It increased with the depth increasing after 13 cal ky BP.High Per concentration was derived from sphagnum moss and in situ anaerobic formation under the action of microorganisms.It is also found that the ratios of Ant/（Ant+Phe）,Ba A/（Ba A+Chr）,Fla/（Fla+Pyr）and Icd P/（Icd P+Bghi P）are difficult to characterize the sources of PAHs from petrogenic and pyrogenic in peat core.2.The markers of petrogenic PAHs:According to the distribution and composition characteristics of PAHs in rock,weathered crust,saprolite,soil samples and rock soaked by weak acid in the DJH basin,PAHs are not easy to migrate in the process of rock weathering into the soil,and tend to be stored in situ.The distribution characteristics of45 individual PAHs and the ratio of alkylation derivatives and parent materials combined with RDA found that Ace,Bi P,Tri P,2,3,5-TMNap,1-MFlu and 2-MFlu monomer PAHs can be used as the markers and tracers of diagenetic PAHs in the process of rock soil-formation.The fingerprint verification shows that the four commonly used MDRs,Ant/（Ant+Phe）,Ba A/（Ba A+Chr）,Fla/（Fla+Pyr）and Icd P/（Icd P+Bghi P）to distinguish petrogenic and pyrogenic were based on fossil fuels such as coal and petroleum,so they are not suitable for the division of sedimentary records.3.Study on atmosphere and atmospheric bulk deposition in the long-distance section from plain to mountain area:It was found that the gas solid distribution caused by vapor pressure and octanol-air distribution coefficient of PAHs was mainly occupied by low molecular weight（LMW）PAHs,Nap,Flu and Phe had the highest abundance,followed by Fla and Pyr.The concentration of high molecular weight（HMW）PAHs was almost less than 1%.In the atmospheric bulk deposition,the dry deposition was mainly dominant,Phe and Nap were predominant in dry deposition,followed by Flu and Pyr.In the process of atmospheric transport,the preferential deposition effect of HMW-PAHs will highlight the input of diagenesis.Therefore,the commonly used MDRs of Ant/（Ant+Phe）,Ba A/（Ba A+Chr）,Fla/（Fla+Pyr）and Icd P/（Icd P+Bghi P）were not suitable to distinguish the petrogenic and pyrogenic PAHs caused by sedimentation.4.Separation of anthropogenic and natural source contributions and establishment of new reliable MDRs:Based on petrogenic markers,the APCS-MLR model was used to analyze the contribution of PAHs from 45 individual PAHs in peat boreholes from three sources:biogenesis,pyrogenic,and petrogenic.From 3.0 cal ky BP to the present,the main contribution was from pyrogenic source（50.5%）,followed by petrogenic contribution（25.1%）.10.0～3.0 cal ky BP pyrogenic（33.9%）and diagenesis（34.3%）contributed equally.In the 16.2～10.0 cal ky BP and 18 BP～16.22 cal ky BP stages the main contribution was from petrogenic（39.1%,48.5%）,followed by pyrogenic（30.4%,25.3%）and biogenesis（23.3%,9.3%）At the same time,based on the source contribution stripping and pyrogenic fingerprint,it was found that the ratio of 1,8-DMPhe/（1,8-DMPhe+3,6-DMPhe）was used to distinguish the source effectively.The fingerprint of diagenetic PAHs was used to determine that the ratio of 1,8-DMPhe/（1,8-DMPhe+3,6-DMPhe）>0.45 was the source of the pyrogenic,and below 0.45 was the source of the petrogenic.In this study,the inapplicability of the ratios of Ant/（Ant+Phe）,Ba A/（Ba A+Chr）,Fla/（Fla+Pyr）and Icd P/（Icd P+Bghi P）was demonstrated and explained through the characterization of petrogenic PAHs by rock weathered into soil and anthropogenic PAHs by atmospheric bulk deposition.The petrogenic PAHs marker components in the process of rock weathered into soil were determined by natural environment of rock weathering into soil samples and simulated weathering experiments.Combined with PAHs source stripping in the peat core,a new MDRs which is suitable for distinguishing petrogenic from combustion source in natural sedimentary process was established.