Hydrogen Isotopic Geochemistry Of Leaf Waxes In Aquatic Plants From Chinese Lakes

Plant leaf wax is a layer of wax covering the surface of plant leaves,which is consisted by a series of long-chain organic compounds.Their molecular distributions and isotopic compositions are widely utilized in paleoenvironmental changes.Among them,n-alkanes and fatty acids?FA?are the most common organic biomarkers.Long-chain n-alkanes and long-chain FAs in lake sediments are generally believed to be derived entirely from terrestrial plants,and are used to reconstruct land paleohydrological conditions.Aquatic plants is a large ecological group in lakes,but their effect on sedimentary long-chain n-alkanes and long-chain FAs is not very clear at present.In addition,current studies on hydrogen isotopic compositions??D?of aquatic plant leaf wax are still limited,greatly hammering the explanations and applications of the ?D values of lake sediments into paleoclimatic reconstructions.Therefore,in this study,by systematically investigating the concentrations,distributions and ?D values of n-alkanes and FAs in aquatic plants from Chinese lakes?including those in Tibetan Plateau,Yunnan and Inner Mongolia?,together with the ?D values of lake water and a sediment core,we obtained the following results:?1?The n-alkanes of aquatic plants are generally dominated by C₂₃ or C₂₅,but C₂₇ and C₂₉ n-alkanes also show relatively high abundances.In these aquatic plants,submerged plants exhibit high concentrations of long chain n-alkanes,close to those of terrestrial plants,suggesting that the contributions of long chain n-alkanes originating from submerged plants to lacustrine sediments should not be neglected.Conversely,algae may minimally contribute n-alkanes to lake sediments because of their significantly low concentrations.?2?The ?D values of aquatic plant n-alkanes reflect the ?D variations of lake water.In arid and semi-arid area,the aquatic plants in closed lakes record the signals of D enrichment in lake water relative to precipitation because of evaporation,while the aquatic plants in open lakes directly response to precipitation ?D changes.For each aquatic plant sample,we find uniformed ?D values of individual n-alkanes,implying that,the offset between the ?D values of n-alkanes with different lengths in lacustrine sediments can help determine the sources of sedimentary n-alkanes as well as inferring the hydrological characteristics of an ancient lake basin?open vs.closed lake?.?3?C₁₄-C₃₂ FAs in aquatic and terrestrial plants are mainly dominated by C₁₆ while the C₂₀-C₃₂ FAs in aquatic plants are predominantly C₂₄,and they also contain relatively high abundances of C₂₆.The submerged plants have high C₂₆-C₃₂ concentrations,which are close to those of terrestrial plants,indicating that the contributions of submerged plant C₂₆-C₃₂ FAs to lake sediments should be considered.On the other hand,algae exhibit low C₂₆-C₃₂ FA concentrations,indicating that their influence on lake sediment C₂₆-C₃₂ FA pools is minor.Meanwhile,we find that ACL14-32 and ATR values can be used to distinguish algae FAs from those of other plants,and STR values can be applied to differentiate the FA sources originating from submerged and terrestrial plants.?4?C₂₀-C₃₀ FAs in each aquatic plant sample have similar variation patterns in ?D values.In C₄-like taxa?Potamogeton,Ruppia,and Chara?,the C₁₆ and C₂₆ FAs have minor differences in their ?D values,while in the C₃-like Cladophora,?D values of C₁₆ FAs are significantly more negative?avg.-48 ± 7‰?than that of the C₂₆ FAs.We propose that different biosynthesis mechanisms in these algae and submerged plants account for the different inter-molecular ?D variations between C₁₆ and C₂₆ FAs.?D values of C₂₆ FAs in the carbonate coated alga Chara are significantly correlated with the ?18O values of carbonate encrustations.Such a co-variance suggests that in addition to lake water,other factors may also influence both lipid synthesis and carbonate encrustation building up in Chara.?5?By an investigation of lipid distributions and concentrations of aquatic plants in Yunnan and Inner Mongolia lakes,we found that the long-chain n-alkanes(e.g.C₂₇ and C29)and long-chain FAs(e.g.C₂₆ and C28)generally show high abundances in aquatic plant samples.Submerged plants have high concentrations of long-chain n-alkanes and long-chain FAs,close to those of terrestrial plants,while algae have low concentrations.Therefore,we believe that it is a common phenomenon that aquatic plants?especially the submerged plants?contain high abundances and concentrations of long-chain n-alkanes and long-chain FAs,illustrating that the contributions of aquatic plant leaf waxes to lake sedimentary lipid pools should not be negligible.?6?The hydrogen isotope fractionation factors???of n-alkane in aquatic plants are stable,and the FA ? values of aquatic plants show small variations.Therefore,the average hydrogen isotopic biosynthetic fractionation value of plant leaf wax is-161‰ for n-alkanes and-145‰ for FAs,and the small ? differences between FAs and n-alkanes can be related to the different magnitudes of their FAs utilized for n-alkane synthesis.?7?The C₃₁ n-alkane ?D values in Qinghai Lake are quite negative in the early Holocene?8-12 ka?,indicating a strong monsoon and greater precipitation during that period.During 4-8 ka,C₃₁ ?D values gradually became more positive,suggesting that the monsoon gradually weakened and precipitation gradually decreased.The C₃₁ ?D values are more positive in the late Holocene,showing that the regional monsoon was weaken and the amount of precipitation was low.In addition,the C₂₉ and C₃₁ n-alkane ?D values have similar changing trends during the past 12 ka in Qinghai Lake,but the C₂₉ ?D values are more positive than C₃₁ ones,which is more significant in the early Holocene,indicating that the water level of the lake was low in this period and aquatic plants have a large contribution to lake sedimentary n-alkane pools.This could be related to strong evaporation of lake water because of the high temperature in the early Holocene.