Occurrence and distribution of biomarkers in loess-paleosol sequences of Tajikistan (Central Asia): Implication for archaeo-ecological studies (original) (raw)
Polycyclic aromatic hydrocarbons in ancient sediments and their relationships to palaeoclimate
Organic Geochemistry, 1998
ÐCombustion-derived and land-plant-derived polycyclic aromatic hydrocarbons (PAHs) have been investigated for an Upper Triassic to Middle Jurassic sedimentary sequence from the Northern Carnarvon Basin, Western Australia. The former included¯uoranthene, pyrene, benzo¯uoranthenes, benzo[e]pyrene and benzo[a]pyrene, and the latter included retene, cadalene and simonellite. Combustion-derived PAHs are most abundant in the Upper Triassic and late Lower Jurassic sediments. The abundances of land-plant-derived markers all maximise in the upper part of the Lower Jurassic section, but vary independently of one another through time. The changes in the relative abundances of these markers have been related to the variation of regional climatic conditions, especially the changes in humidity and seasonality during the Late Triassic to Middle Jurassic times.
Frontiers of Earth Science in China, 2009
Carbon Preference Index (CPI values) of higher plant-derived long-chain n-alkanes extracted from 62 surface soil samples in eastern China exhibited a specific pattern of variations, namely gradual increase with the increasing latitudes. Such regular variations existed in both forest soil and grassland soil. Our data implied that CPI values of higher plant-derived long-chain n-alkanes had a certain connection with climatic conditions, and such a connection was not influenced by vegetation types. Together with previous data from marine sediments, loess/paleosol sequences, tertiary red clay and modern plants, our observation made us conclude that CPI values of higher plant-derived long-chain n-alkanes may be used as an excellent proxy for paleoclimatic studies.
Quaternary International, 2014
This study examines n-alkane distributions from a 120 m long borehole extracted from Palaeolake Tianyang, South China, which covers the last five glacial cycles. We aim to determine the palaeoclimatic/ environmental fidelity of n-alkane distributions in bulk organic matter (OM) from this tropical sedimentary archive. The Tianyang lithostratigraphy reveals a deep-lake sequence (120e42 m) dominated by diatomite, overlain by shallow-lake and marsh sediments in the upper 42 m. The Carbon Preference Index (CPI, C 23 eC 31 ) suggests that there is weak odd/even predominance (OEP) in long chain n-alkane distributions (C 27 eC 31 ) in OM from deep lake sediments, and a strong OEP of long chain n-alkane distributions in OM from shallow lake sediments. The opposite characteristics are evident in short chain nalkane distributions (C 16 eC 20 ). These CPI values suggest different sources of organic carbon within the deep (dominantly diatom synthesis) and shallow lake (dominantly high plant input), but cannot help differentiate glacial from interglacial sediments. The Terrigenous-to-Aquatic Ratio (TAR) shows high values throughout interglacial sediments (>4) and low values in glacial sediments (<4), corresponding to a higher productivity of high plants during interglacials, and vice versa. The proportion of aquatic plant nalkanes (P aq ) also reveals a clear change between glacial (0.31e0.39) and interglacial periods (0.22e0.28) during shallow lake stage, indicating a reduction in terrestrial OM input during glacial periods relative to interglacial periods. There is a greater amount of Unresolved Complex Mixture (UCM) in glacial sediments than within interglacial sediments. We suggest that the above n-alkanes indexes are useful environmental/climatic indicators within Palaeolake Tianyang, despite the fact that some other indexes, like Average Chain Length (ACL) , provide less definitive interpretations.
Science China-earth Sciences
To investigate characteristics of H isotope variation in long-chain n-alkanes (δDn-alkanes) from higher plants in surface soils under a single ecosystem, 12 samples from a basalt regolith were randomly collected from Damaping in Wanquan County of Zhangjiakou in North China. Molecular distribution and C (δ 13Cn-alkanes) and H isotopes of long-chain n-alkanes, as well as C isotopes of TOC (δ 13CTOC), were analyzed. Both δ 13CTOC and δ 13Cn-alkanes values from four representative dominant long-chain n-alkanes (n-C27, n-C29, n-C31, n-C33) derived from terrestrial higher plants show minor variations among the 12 samples, indicating the major contributor is from local grasses with a uniform C3 photosynthetic pathway. In contrast, variations in δDn-alkanes values of the four long-chain n-alkanes are relatively large, with the more abundant homologs generally showing more negative δDn-alkanes values and less variation. However, variation of <30‰ among weighted averaged δDn-alkanes values of the four long-chain n-alkanes is not only less than that among δDn-alkanes values for different modern terrestrial C3 grasses from the specific locations, but also less than the literature values of δDn-alkanes of long-chain n-alkanes for single species over different seasons. Thus, because the sources of long-chain n-alkanes in surface soils and sediments are similarly from multiple individual plants, our results are significant in confirming that paleoclimatic, paleoenvironmental and paleohydrological information can be interpreted more accurately from δDn-alkanes values of long-chain n-alkanes from sediments, particularly terrestrial sediments with organic matter derived from in-situ plants.
Proceedings of the National Academy of Sciences, 2003
A pseudohomologous series of branched aliphatic alkanes with a quaternary substituted carbon atom (BAQCs, specifically 2,2-dimethylalkanes and 3,3-and 5,5-diethylalkanes) were identified in warm (65°C) deep-sea hydrothermal waters and Late Cretaceous black shales. 5,5-Diethylalkanes were also observed in modern and Holocene marine shelf sediments and in shales spanning the last 800 million years of the geological record. The carbon number distribution of BAQCs indicates a biological origin. These compounds were observed but not identified in previous studies of 2.0 billion-to 2.2 billion-year-old metasediments and were commonly misidentified in other sediment samples, indicating that BAQCs are widespread in the geological record. The source organisms of BAQCs are unknown, but their paleobiogeographic distribution suggests that they have an affinity for sulfides and might be nonphotosynthetic sulfide oxidizers.
Geochimica et Cosmochimica Acta, 1997
The heterogeneous isotopic composition of C 3 and C 4 plants can be used to to follow the fate of plant carbon into soil organic molecules. Thus, after 23 years of cropping of maize (C 4 ) on a soil which was previously under C 3 vegetation, C 25 -C 33 soil n-alkanes are 13 C-enriched up to 9‰ relatively to the initial C 3 soil, reflecting the input of 13 C-enriched n-alkanes from maize waxes. In sharp contrast, C 16 -C 18 soil n-alkanes do not show any significant 13 C/ 12 C variation over the same time interval. This absence of isotopic variation, along with consideration of their relative concentration, absolute concentration and biodegradability, demonstrate that these substances must represent a regular input from an external source. Evidence of a large contribution of an ancient source, amounting to more than 65% of the alkane fraction, is given by a 14 C-age of 8510 yrs BP. Moreover, short-chain n-alkanes from soils, diesel fuel, diesel automobile exhaust and petroleum products exhibit similar distributions and δ 13 C values. These findings suggests that C 16 -C 18 soil n-alkanes represent a non-point source pollution of ancient hydrocarbons either carried by aerosols or entering the soil via continuous hydrocarbon seepage from the deep sedimentary rocks of the Paris basin.