Origin and content of alkenone of the Early Miocene marine sediments from IODP U1501 in the South China Sea
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摘要: 海洋沉积物中的长链烯酮由海洋单细胞钙化藻类颗石藻生产，是一种被广泛应用于古气候研究领域中的分子标记物。长链烯酮碳同位素是重建地质历史时期海水、大气CO2浓度的可靠方法之一。在此方法中，需要利用颗石大小对颗石藻生理参数b值进行修正，因此需要厘清哪类颗石藻对烯酮的贡献是一个重要的科学问题。目前认为新生代海洋沉积物中主要的长链烯酮生产者为Noelaerhabdaceae科的颗石藻，包含Emiliania huxleyi, Gephyrocapsa spp., Reticulofenestra spp., Cyclicargolithus spp.，但对它们具体的贡献程度仍然未知。因此，本文以南海国际大洋发现计划IODP U1501站早中新世海洋沉积物为研究材料，对比了沉积物中颗石与烯酮的绝对含量，发现Cyclicargolithus属的颗石丰度与烯酮含量具有显著的相关性（r=0.44，p＜0.01），而Reticulofenestra spp.的相关性较弱（r=0.09，p=0.5）。研究认为早中新世长链烯酮的主要生产者为Cyclicargolithus属，Reticulofenestra属次之。此外，研究还测量了各颗石属种的长度，并估算了颗石藻细胞的直径。研究认为Cyclicargolithus属对烯酮的高贡献源于其较大的细胞直径。从长链烯酮生产者的种类、细胞体积及其生命过程处着眼，有利于评估应用碳同位素分馏重建海洋中CO2浓度的可行性，对准确地还原地质历史时期大气CO2浓度具有重要作用。
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关键词:
- 长链烯酮 /
- 颗石藻 /
- 早中新世 /
- IODP U1501站 /
- 南海
Abstract: Alkenones are ubiquitous in ocean sediments and have been widely used in paleoceanography and paleoclimatology. Alkenones can be used to reconstruct the CO2 concentration of seawater in geological history, which is key to understanding the mechanism of climate evolution and predicting future climate change. In modern open ocean environments, the main producers of alkenones are Noelaerhabdaceae, including the calcifying haptophytes (coccolithophores) Emiliania huxleyi and Gephyrocapsa spp. It is generally believed that the most reliable producers of alkenones in the Cenozoic marine sediments belong to genus Reticulofenestra of Family Noelaerhabdaceae. By comparing the absolute abundance of coccoliths and alkenones in the Early Miocene sediments at IODP Site U1501 in the South China Sea, it is found that the abundance of Cyclicargolithus spp. coccoliths and alkenones are well correlated (r=0.44, p＜0.01). Therefore, we believe that Cyclicargolithus spp. is the main producer of alkenones in the Early Miocene, followed by Reticulofenestra spp. In addition, we measured the coccolith length of each species and estimate the diameter of coccolithophores cells. We inferred that the high contribution of Cyclicargolithus spp. to alkenones is due to its larger cell diameter. Our finding is helpful to evaluate the feasibility of using carbon isotope fractionation to reconstruct the atmospheric CO2 concentration in geological history.-
Keywords:
- alkenones /
- coccolithophore /
- Early Miocene /
- IODP U1501 /
- South China Sea
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图 1 国际大洋发现航次南海IODP U1501钻孔站位（a）以及年龄模式（b）
年龄模式根据Jian等[25]改绘；U1501站年龄模式通过钙质超化石事件、浮游有孔虫事件、古地磁及Sr同位素年龄，建立深度、年龄回归公式确定。
Figure 1. IODP Site U1501 in the South China Sea (a) and the age model (b)
The age model is modified according to Jian et al. [25] and is constructed based on the linear regression between depth and calcareous nannofossil, planktonic, paleo-magnetostratigraphy, and Sr isotope age.
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图 2 U1501C沉积物柱状样典型样品中的烯酮
样品深度为205.36～205.41 m。
Figure 2. U1501C alkenone in sediment columnar samples
The depth of the sample is 205.36～205.41 m beneath seafloor.
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图 3 南海IODP站位U1501C早中新世18～22 Ma的C37长链烯酮含量（a），Cyclicargolithus属颗石丰度（b），Reticulofenestra属颗石丰度（c）以及总Noelaerhabdaceae颗石丰度（d）
灰色阴影部分代表颗石丰度统计的95%的置信区间（±1.96倍的标准误差）。
Figure 3. Abundances of C37 long-chain alkenone (a), Cyclicargolithus spp. (b), Reticulofenestra spp. (c), and total Noelaerhabdaceae coccoliths (d) in the marine sediments in at IODP Site U1501 in the South China Sea from 18 to 22 Ma in the Early Miocene
The grey shading indicates the 95% confidence interval of the coccolith counting (standard error of ±1.96 times).
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图 4 南海IODP U1501站早中新世颗石藻Reticulofenestra与Cyclicargolithus细胞直径变化
Figure 4. Cell diameter of coccolithophore Reticulofenestra spp. and Cyclicargolithus spp. at IODP Site U1501C in the South China Sea in the Early Miocene
表 1 颗石绝对丰度与烯酮浓度之间Pearson相关性分析结果
Table 1 Pearson correlation analysis between the coccolith abundance and the alkenone concentration
相关系数r p值 Noelaerhabdaceae科颗石的总丰度 0.005 315 0.969 87 Cyclicargolithus属颗石丰度 0.443 09 0.000 892 Reticulofenestra颗石丰度 0.093 14 0.507 13 
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