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首页> 外文期刊>Organic Geochemistry: A Publication of the International Association of Geochemistry and Cosmochemistry >Carbon isotope composition of long chain leaf wax n-alkanes in lake sediments: A dual indicator of paleoenvironment in the Qinghai-Tibet Plateau
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Carbon isotope composition of long chain leaf wax n-alkanes in lake sediments: A dual indicator of paleoenvironment in the Qinghai-Tibet Plateau

机译：湖泊沉积物中长链叶蜡正构烷烃的碳同位素组成：青藏高原古环境的双重指标

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The carbon isotope composition (delta C-13 values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C-3 and C-4 photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting delta C-13 values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C-27 and C-29) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. -26.7% to -15.7% for C-29) similar to that of terrestrial C-4 plants. Thus, the use of delta C-13 values of sedimentary C-27 and C-29 n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C-4 plants. For sedimentary C-31, however, the contribution from submerged plants was minor, so that the delta C-13 values for C-31 n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the delta C-13 values of sedimentary C-27 and C-29 n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C-4 vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the delta C-13 values of sedimentary long chain n-alkanes (C-27, C-29 and C-31) may carry different environmental signals. While the delta C-13 values of C-31 were a reliable proxy for C-4/C-3 terrestrial vegetation composition, the delta C-13 values of C-27 and C-29 n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth. (C) 2015 Elsevier Ltd. All rights reserved.
机译：湖泊沉积物中长链正构烷烃的碳同位素组成（δC-13值）被认为是追踪具有C-3和C-4光合途径的植物陆生贡献变化的可靠方法。一个关键前提是用于同位素分析的长链叶蜡成分主要来源于陆生高等植物。然而，水生植物在影响湖泊沉积物中长链正构烷烃的δC-13值中的作用可能长期以来被低估了。在这项研究中，我们发现青海湖流域近岸沉积物中的大部分长链正构烷烃（C-27和C-29）是由淹没的水生植物引起的，这些植物表现出相对正的碳同位素组成（例如： C-29的-26.7％至-15.7％）与陆地C-4植物的相似。因此，使用沉积C-27和C-29正构烷烃的δC-13值追踪陆生植被组成可能会导致对陆地C-4植物比例的明显高估/低估。然而，对于C-31沉积物，淹没植物的贡献很小，因此地表沉积物中C-31正构烷烃的δC-13值与青海湖地区现代陆地植被的δ-13值一致。此外，我们发现沉积C-27和C-29正构烷烃的δC-13值变化与水深变化密切相关。下游分析进一步证明了湖泊沉积物中内源性脂质对于解释陆地C-4植被和相关环境/气候重建的重大影响。总之，我们的结果表明，沉积长链正构烷烃（C-27，C-29和C-31）的C-13增量值可能携带不同的环境信号。尽管C-31的C-13增量值是C-4 / C-3陆地植被组成的可靠代表，但C-27和C-29正构烷烃的C-13增量值可能已记录了湖泊的生态状况以及可能受湖泊水深影响的有机碳源。 （C）2015 Elsevier Ltd.保留所有权利。

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