Mangrove sediments are valuable archives of relative sea-level change if they can be distinguished in the stratigraphic record from other organic-rich depositional environments (e.g., freshwater swamps). Proxies for establishing environment of deposition can be poorly preserved (e.g., foraminifera) in mangrove sediment. Consequently, differentiating mangrove and freshwater sediment in the stratigraphic record is often subjective. We explore if biomarkers can objectively identify mangrove sediment with emphasis on their utility for reconstructing relative sea level. Our approach is specific to identifying in situ sediment, which has received less attention than identifying allochthonous mangrove organic matter. To characterize mangrove and non-mangrove (freshwater) environments, we measured -alkane, sterol, and triterpenoid abundances in surface sediments at three sites in the Federated States of Micronesia. Elevated taraxerol abundance is diagnostic of sediment accumulating in mangroves and taraxerol is particularly abundant beneath monospecific stands of spp. Taraxerol was undetectable in freshwater sediment. Other triterpenoids are more abundant in mangrove sediment than in freshwater sediment. Using cores from Micronesian mangroves, we examine if biomarkers in sediments are indicative of in situ deposition in a mangrove, and have utility as a relative sea-level proxy. Taraxerol concentrations in cores are comparable to surface mangrove sediments, which indicates deposition in a mangrove. This interpretation is supported by pollen assemblages. Downcore taraxerol variability may reflect changing inputs from spp. rather than diagenesis. We propose that taraxerol is a proxy that differentiates between organic sediment that accumulated in mangrove vs. freshwater environments, lending it utility for reconstructing relative sea level.

中文翻译：

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蒲公英醇丰度作为原位红树林沉积物的代表

如果红树林沉积物能够在地层记录中与其他富含有机物的沉积环境（例如淡水沼泽）区分开来，那么它们就是相对海平面变化的宝贵档案。红树林沉积物中用于建立沉积环境的替代物（例如有孔虫）保存不良。因此，区分地层记录中的红树林和淡水沉积物通常是主观的。我们探索生物标志物是否可以客观地识别红树林沉积物，重点是它们在重建相对海平面方面的效用。我们的方法专门用于识别原位沉积物，与识别外来红树林有机物相比，它受到的关注较少。为了表征红树林和非红树林（淡水）环境，我们测量了密克罗尼西亚联邦三个地点表面沉积物中的α-烷烃、甾醇和三萜类化合物丰度。蒲公英醇丰度升高可以诊断红树林中沉积物的积累，并且蒲公英醇在单种物种下特别丰富。在淡水沉积物中检测不到蒲公英醇。其他三萜类化合物在红树林沉积物中比在淡水沉积物中更丰富。使用密克罗尼西亚红树林的岩心，我们检查沉积物中的生物标志物是否表明红树林中的原位沉积，并具有作为相对海平面代理的实用性。岩心中的蒲公英醇浓度与表面红树林沉积物相当，这表明红树林中的沉积。这种解释得到了花粉组合的支持。地下蒲公英醇的变异性可能反映了来自物种的输入的变化。而不是成岩作用。我们认为蒲公英醇是区分红树林和淡水环境中积累的有机沉积物的代理，使其可用于重建相对海平面。