The δ13C composition and genetic origin of a series straight long-chain alkyl naphthalene isomer pairs in the Upper Cretaceous lacustrine sediments of Songliao Basin, China

Abstract

A series of straight long-chain alkyl naphthalene isomer pairs (LANs) have been detected in the Upper Cretaceous lacustrine sediments of Nenjiang Formation (K₂n¹⁺²) in the Songliao Basin, and their distribution and carbon isotopic composition were investigated. The results indicate that the composition and distribution of LANs exhibit significant variations. The relative abundance of methyl- and dimethyl-naphthalenes shows a reciprocal correlation with that of LANs, increasing and declining in tandem. The δ¹³C composition of LANs varies from −24.6 ‰ to –32.6 ‰, with an average of −28.5 ‰, which is notably higher than that of n-alkanes, ranging from −31.9 ‰ to −34.5 ‰. The δ¹³C composition of methylnaphthalenes (MNs) ranges from −26.9 ‰ to −27.1 ‰, which is generally more positive than the average δ¹³C composition of LANs. Analysis of the carbon isotopic composition of n-alkanes, methylnaphthalenes, and the series of LANs, along with their distribution patterns, suggests the presence of multiple formation mechanisms. Specifically, the observed declining variation pattern of δ¹³C values for LANs along with the increasing carbon number of the alkyl chain may indicate a novel formation pathway. This pathway likely involves a direct reaction between ¹³C-enriched naphthalene and ¹³C-depleted straight long-alkyl chains during diagenesis. The relative abundance ratio of total methyl-substituted naphthalenes (ΣMNs) to the total LANs (ΣLANs), expressed as the ΣMNs/ΣLANs ratio, exhibits a profile variation that correlates with the profile changes of the Pr/Ph ratio, TOC content, and HI index. This correlation indicates that specific environmental conditions or source inputs, such as reduced sedimentary conditions or specific algal contributions, may be conducive to the formation of LANs.