Molecular fossil and paleovegetation records of paleosol S4 and adjacent loess layers in the Luochuan loess section, NW China

Abstract

Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identified, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and adjacent L5, L4 of the Luochuan loess section, Northwestern China. Based on these data, especially n-alkanes and high-resolution magnetic susceptibility and grain size data, the paleoenvironment and paleovegetation history during S4 was reconstructed. The CPI (Carbon Predominance Index) and correlation between n-alkanes and magnetic susceptibility and grain size data demonstrated that the molecular fossils in paleosol and loess layers can reflect the vegetation condition during the loess-paleosol formation, if the allochthonous organic inputs could be excluded reasonably. The ACL (average chain length) index is correlated well with paleomagnetic susceptibility and grain size variations, displaying their good synchrony with warm and humid climate. However, it relatively lagged behind the paleomagnetic susceptibility and the grain size variations when the climate began to deteriorate. During the formation period of paleosol, the n-alkanes was dominated by C 31 homologue, indicating that the primary organic input originated from herbs. Our study also demonstrated that the herbs were more flourish than wood plants in Loess Plateau, especially in the Luochuan area during the warm and humid phase, and there was no typical forest vegetation developed in the studied period. paleoenvironment Paleosol S4, Luochuan loess section, biomarker, paleovegetationBiomarker is a compound that could indicate the biologic origin of organic matters. Once the molecular structure and isotopic composition are identified, the source of the biomarkers and their composition character and distribution pattern could be figured out even to species level. Therefore, molecular fossil has been widely applied in the studies on paleoenvironmental and paleoclimatic changes [1,2] , such as aerosol [3] , marine sediment [4][5][6] , lacustrine and peat sediment [7][8][9][10] , stalagmite [11] , snow and ice [12] , loess-paleosol [13][14][15] and paleovegetation changes [16] . These works demonstrated the reliability of the biomarker research in the paleoclimate, paleoenvironment and paleovegetation and it could play an important role in tracking the environmental variation at molecular level.The loess is widely distributed in the Northwestern China and has served as one of the best archives for paleoenvironment reconstruction. Together with deep sea sediments and ice core studies, they were considered mainstays of the global changes [17] and great achievements have been documented in the recent decades [18][19][20][21][22] .

Diverse Soil Carbon Dynamics Expressed at the Molecular Level

“…The ACL suggests that the temperate site (C 27 -C 29 dominated) is influenced by deciduous tree species (Zhang et al, 2008), while the subalpine Podzol with pine trees which are known to contribute few alkanes to soils (Schäfer et al, 2016) may be potentially more strongly impacted by forb-derived (C 29 dominated) n-alkanes (Bush & McInerney, 2013). The δ…”

Northern Tibetan Plateau cooling and aridification linked to Cenozoic global cooling: Evidence from n-alkane distributions of Paleogene sedimentary sequences in the Xining Basin

“…For further comparisons, we use Q grass/plant = C 31 / (C 27 +C 29 +C 31 ) and Q wood/plant = (C 27 +C 29 )/(C 27 +C 29 +C 31 ), to represent the relative n-alkane abundance of grass and woody plants in the samples respectively, because the Q grass/plant and Q wood/plant indices have been widely believed also to represent the relative abundance of grass and woody vegetation [39,50]. n-alkanes, and might reflect local temperature variation [51,52], as well as different vegetation types [32,41,42].…”

“…The strongly hydrophobic nature of the n-alkanes in sediment and their affinity for clays makes them immobile. Molecular stratigraphic approaches using plant n-alkanes have been employed widely for marine sediments [29][30][31], lacustrine and peat sediments [32][33][34][35][36] and loess-paleosol [28,[37][38][39]. However, plant n-alkanes preserved in the Xining Basin have not been used to deduce paleoenvironmental information.…”

Climatic and environmental implications from n-alkanes in glacially eroded lake sediments in Tibetan Plateau: An example from Ximen Co

“…The leaf n-alkane distribution in an individual plant species varies with the seasons, mainly responding to temperature change [34]. This phenomenon is also observed in n-alkane studies in loess [35] and peat [36]. However, it should be noted that if climate variation causes the plant species to vary or results in sedimentary source changes, then the ACL value cannot effectively reflect the temperature changes and more accurately reflects changes in the vegetation [12].…”

“…The n-alkane distribution of cow manure in Ximen Co lake region ranges from n-C 25 to n-C 35 , with a maximum at C 31 , and shows a distinct odd-carbon preference which is indicative of an alpine meadow herb contribution ( Figure 4(c)). This distribution pattern is very similar to that for n-alkanes in the modern soil.…”