One of the central assumptions of paleoenvironmental research using lipid biomarker isotope compositions in sedimentary archives is the isotopic stability of these compounds after deposition. If this indeed applies for open marine sediments with low sedimentation rates and strong biodegradation is, however, not well constrained. Here we study lipid biomarker isotope compositions from two sediment cores from the brine-filled Shaban Deep in the northern Red Sea that receive comparable water column flux due to close geographic proximity but are deposited under opposite anoxic (inside the brine) and oxic (outside the brine) sedimentary conditions. Despite almost complete degradation of some compounds in the oxic sediments, we show that the stable carbon isotope compositions of long-chain n-alkanes, alkenones and bulk organic matter (OM) remain stable among the two cores. The relative distribution of long-chain n-alkanes and alkenones is also preserved. Conversely, the stable hydrogen isotope composition of long-chain n-alkanes and bulk OM increases consistently during degradation (10‰ vs VSMOW for n-alkanes and 40‰ for of bulk OM) indicating the preferential degradation of labile bulk OM and n-alkane fractions that are isotopically lighter. For long-chain fatty acids we find shifts in relative distribution and variability of stable hydrogen and carbon isotope compositions indicating an overall marine sourcing of these compounds that contrasts their usual interpretation as plant derived markers. We conclude that isotope compositions of source-specific refractory molecules show only minor variations under strong degradation, while compounds with multiple sources can show strong variability potentially impeding their use as paleoenvironmental proxies.

在沉积档案中使用脂质生物标志物同位素组成的古环境研究的主要假设之一是这些化合物沉积后的同位素稳定性。如果这确实适用于沉积速率低且生物降解能力强的开放式海洋沉积物，则不会受到很好的限制。在这里，我们研究了来自红海北部充满盐水的沙班深部两个沉积物岩心的脂质生物标志物同位素组成，由于地理上的接近性，它们获得了相当的水柱通量，但沉积在相反的缺氧（在盐水中）和有氧（在卤水外）下。盐水）的沉积条件。尽管在含氧沉积物中某些化合物几乎完全降解，但我们证明了长链n-碳的稳定碳同位素组成烷烃，烯酮和大块有机物（OM）在两个核心之间保持稳定。长链正构烷烃和烯酮的相对分布也得以保留。相反，长链正构烷烃和本体OM的稳定氢同位素组成在降解过程中持续增加（正构烷烃的VSMOW为10‰ ，本体OM为40‰），表明不稳定的本体OM和n-的优先降解同位素较轻的烷烃馏分。对于长链脂肪酸，我们发现稳定氢和碳同位素组成的相对分布和变异性发生了变化，表明这些化合物的整体海洋来源与它们通常作为植物来源标记的解释形成了鲜明对比。我们得出的结论是，特定来源的难熔分子的同位素组成在强降解下仅表现出很小的变化，而具有多种来源的化合物则可能表现出强烈的变异性，从而有可能阻碍其用作古环境的代理。