Stable carbon (C) and nitrogen (N) isotope ratios of sedimentary organic matter (OM) can reflect the biogeochemical history of aquatic ecosystems. However, diagenetic processes in sediments may alter isotope records of OM via microbial activity and preferential degradation of isotopically distinct organic components. This study investigated the isotope alteration caused by preferential degradation in surface sediments sampled from a eutrophic reservoir in Germany. Sediments were treated sequentially with hot water extraction, hydrochloric acid hydrolysis, hydrogen peroxide oxidation and di-sodium peroxodisulfate oxidation to chemically simulate preferential degradation pathways of sedimentary OM. Residue and extracts from each extraction step were analyzed using elemental analyzer-isotope ratio mass spectrometry and solid-state 13 C nuclear magnetic resonance spectroscopy. Our results show that stable C and N isotope ratios reacted differently to changes in the biochemical composition of sedimentary OM. Preferential degradation of proteins and carbohydrates resulted in a 1.2‰ depletion of 13 C, while the isotope composition of 15 N remained nearly the same. Sedimentary δ 15 N values were notably altered when lignins and lipids were oxidized from residual sediments. Throughout the sequential fractionation procedure, δ 13 C was linearly correlated with the C:N of residual sediments. This finding demonstrates that changes in biochemical composition caused by preferential degradation altered δ 13 C values of sedimentary OM, while this trend was not observed for δ 15 N values. Our study identifies the influence of preferential degradation on stable C isotope ratios and provide additional insight into the isotope alteration caused by post-depositional processes.

中文翻译：

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沉积有机质生化成分变化引起的同位素变化

沉积有机质 (OM) 的稳定碳 (C) 和氮 (N) 同位素比值可以反映水生生态系统的生物地球化学历史。然而，沉积物中的成岩过程可能会通过微生物活动和同位素不同有机成分的优先降解来改变 OM 的同位素记录。本研究调查了从德国富营养化水库采样的表层沉积物中优先降解引起的同位素变化。沉积物依次用热水提取、盐酸水解、过氧化氢氧化和过二硫酸钠氧化处理，以化学模拟沉积物 OM 的优先降解途径。使用元素分析仪-同位素比质谱法和固态 13 C 核磁共振光谱法分析每个提取步骤的残留物和提取物。我们的结果表明，稳定的 C 和 N 同位素比率对沉积 OM 的生化组成变化的反应不同。蛋白质和碳水化合物的优先降解导致 13 C 消耗 1.2‰，而 15 N 的同位素组成几乎保持不变。当木质素和脂质从残留沉积物中氧化时，沉积物 δ 15 N 值显着改变。在整个连续分馏过程中，δ 13 C 与残留沉积物的 C:N 呈线性相关。这一发现表明，优先降解引起的生化成分变化改变了沉积 OM 的 δ 13 C 值，而对于 δ 15 N 值没有观察到这种趋势。我们的研究确定了优先降解对稳定 C 同位素比率的影响，并提供了对沉积后过程引起的同位素改变的更多了解。