Purpose The distribution and composition of carbon isotopes (e.g., δ13C) in sediments are commonly used to investigate historical changes in organic carbon (OC) sources and environmental conditions in a watershed. However, information recorded in sedimentary profiles may be confusing due to an incomplete understanding of the factors that control n-alkanes associated with their compound-specific δ13C (δ13Cn−alk). Materials and methods This study determined the abundances of n-alkanes and compound-specific δ13Cn−alk in sediment cores from the Poyang lakefront zone, combined with the PCA−MLR analysis for quantitatively tracing the sources of OC and exploring their links to environmental changes. Results The detected <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\sum\limits_{i=15}^{35}\;n_i-alk\)</EquationSource> </InlineEquation> contents ranged from 340.5 to 1566.0 µg g− 1 OC, with the long-chain n-alkanes of nC26–nC35 dominating from 40.5% to 64.4%, followed by the short-chain ones (nC15−nC20, 16.6%−40.5%). The n-alkane concentrations decreased significantly after the 2000s, diverging from the rising OC trend and reflecting intense microbial activity stimulated by eutrophication-induced organic matter loading. The middle-chain and long-chain δ13Cn−alk values (−27.0‰ to −36.9‰ and −27.2‰ to −36.0‰, respectively) became progressively more depleted over time, while the short-chain δ13Cn−alk values showed no significant temporal trend. PCA–MLR analysis indicated that the contributions from aquatic macrophytes, algae, and terrestrial plants were 43.5%, 33.8%, and 22.7%, respectively, before the 2000s. This was followed by an increase in the respective contributions of terrestrial plants and algae to 51.9% and 48.1%, marking a gradual shift in dominance of OC sources from autochthonous to allochthonous origins. Conclusions The alteration in the sedimentary n-alkane composition observed in the early 21st century was driven by human activities, i.e., the operation of the Three Gorges Dam and extensive sand mining. These activities lowered lake levels, suppressing aquatic plant growth while stimulating increased allochthonous OC inputs, which substantially restructured the lake’s carbon composition.

Compound-specific isotopic fingerprints of lacustrine n-alkanes in the middle reaches of the Yangtze river basin associated with their environmental significance

期刊论文
发表日期：

2026年2月7日

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Jiayi Guo,
Minhan Sun,
Letian Ning,
Hong Wang,
Pengyuan Fu,
Mingli Zhang,
Yanhua Wang

摘要

Purpose

The distribution and composition of carbon isotopes (e.g., δ¹³C) in sediments are commonly used to investigate historical changes in organic carbon (OC) sources and environmental conditions in a watershed. However, information recorded in sedimentary profiles may be confusing due to an incomplete understanding of the factors that control n-alkanes associated with their compound-specific δ¹³C (δ¹³C_n−alk).

Materials and methods

This study determined the abundances of n-alkanes and compound-specific δ¹³C_n−alk in sediment cores from the Poyang lakefront zone, combined with the PCA−MLR analysis for quantitatively tracing the sources of OC and exploring their links to environmental changes.

Results

The detected \(\sum\limits_{i=15}^{35}\;n_i-alk\) contents ranged from 340.5 to 1566.0 µg g^− 1 OC, with the long-chain n-alkanes of nC₂₆–nC₃₅ dominating from 40.5% to 64.4%, followed by the short-chain ones (nC₁₅−nC₂₀, 16.6%−40.5%). The n-alkane concentrations decreased significantly after the 2000s, diverging from the rising OC trend and reflecting intense microbial activity stimulated by eutrophication-induced organic matter loading. The middle-chain and long-chain δ¹³C_n_−alk values (−27.0‰ to −36.9‰ and −27.2‰ to −36.0‰, respectively) became progressively more depleted over time, while the short-chain δ¹³C_n_−alk values showed no significant temporal trend. PCA–MLR analysis indicated that the contributions from aquatic macrophytes, algae, and terrestrial plants were 43.5%, 33.8%, and 22.7%, respectively, before the 2000s. This was followed by an increase in the respective contributions of terrestrial plants and algae to 51.9% and 48.1%, marking a gradual shift in dominance of OC sources from autochthonous to allochthonous origins.

Conclusions

The alteration in the sedimentary n-alkane composition observed in the early 21st century was driven by human activities, i.e., the operation of the Three Gorges Dam and extensive sand mining. These activities lowered lake levels, suppressing aquatic plant growth while stimulating increased allochthonous OC inputs, which substantially restructured the lake’s carbon composition.