中更新世以来热带西太平洋颗石藻钙化作用研究

The response of coccolithophore calcification to ocean acidification has been studied in culture experiments as well as in present and past oceans. The response, however, is different between species and strains, and for the relatively small environment changes observed in natural environments, a uniform response of the entire coccolithophore community has not been documented so far. Previous palaeo-studies basically focus on changes in coccolith weight due to increasing CO2 and the resulting changes in the carbonate system, and focus on the influence of other environmental factors. Most of the previous studies are focused on the glaical terminations which are characterised by an increase in atmoshperic CO2 concentration. In fact, glacial to interglacial environmental changes have a strong impact on coccolithophore assemblage composition. However, only few studies of coccolithophore calcification focus on the glacial-interglacial timescale and the mid-brunches coccolithophore blooms event. In order to determine how these three process influence the calcite production of coccolithophores, and established the glacial-interglacial cyles and long term timescale relationship between coccolithophore calcification and seawater carbonate system parameters and environmental factors such as temperature and/or productivity, we here present a study on coccoith calcification from the mid-Pleistocene in the western tropical Pacific. The mid-Pleistocene, with is predominantly glacial-interglacial cycle atmospheric CO2, provides the conditions for such a comprehensive analysis. For an analysis on changes in major components since mid-Pleistocene coccolithophores under natural conditions, the family Noelaerhabdaceae was selected, which constitutes the main part of the assemblage in the western Pacific other than Florisphaera profunda. Coccolith weight estimates obtained with the automated coccolith recognition system SYRACO and the cross-polarizing microscope assemblage counts covering the last 800 ka from three sediments cores in the western Pacific will provide a new sight to the mechanism of coccolithophore calcification and the role of it plays in the global carbon cycle.

颗石藻作为一类重要的浮游植物，是海洋碳酸钙的主要贡献者，在全球碳循环过程中扮演了重要的角色。近年来，人类活动导致的大洋酸化对颗石藻钙化作用产生重要影响。室内培养实验和野外调查均显示出，颗石藻群落对大洋酸化存在着复杂的甚至相互矛盾的响应机制和适应策略。目前为止，颗石藻钙化作用对海洋酸化的响应机制并未得到解决。地质沉积物中颗石的保存，为研究长时间尺度颗石藻对自然环境变化的响应提供了新的视角。本项目选择位于热带西太平洋的3个柱状沉积物岩心，以颗石藻钙化作用为切入点，重建该区0.8Ma以来颗石藻群落组成和关键科优势种属颗石平均重量演变历史，以获得颗石藻钙化作用的演化特征。利用测试数据及前人已发表的海水碳酸盐系统参数和温度、盐度、生产力、水体结构等环境参数，通过对比分析，提取不同时间尺度下影响颗石钙化作用的主控因素，并探讨颗石藻在长时间尺度上对大洋酸化等环境变化的适应和演化机制。

研究中晚第四纪热带西太平洋颗石藻群落演替以及钙化作用演化历史，可以有效的恢复热带西太平洋生产力以及水文环境演化历史。 对于评估颗石藻在冰期旋回尺度和轨道时间尺度上全球碳循环中的重要作用，并理解颗石藻对环境变化的适应和演化机制方面具有重要的意义。本研究基于国际综合大洋钻探计划(IODP)363航次在西太平洋暖池核心区钻取的U1490站位岩芯，中法合作西太平洋MACRO-POLO2航次在西太平洋暖池核心区钻取的MD06-3050岩芯，以及深海大洋钻探计划(DSDP)31航次296站位的岩芯，通过颗石藻定量沉降方法，通过人工鉴定和自动鉴定和测量相结合的手段，重建了热带西太平洋暖池核心区，暖池边缘以及黑潮续流流域在中更新世以来颗石藻群落组成、颗石藻堆积速率、特征种属丰度变化特征以及平均重量和厚度等形态参数的演化历史，并讨论上新世以来颗石藻群落变化对暖池和黑潮变动的响应。发现研究区颗石藻主要以F. profunda、Gephyrocapsa Spp和Reticulofenestra spp为主，尤其是在中布容时期（MIS8-MIS12），G.oceanica和G.caribbeanica占统治地位，Reticulofenestra spp也贡献了较高的丰度。根据该岩芯的沉积速率和干密度等数据进行计算，获得了颗石藻Noelaerhabdaceae科颗石的绝对丰度、累积速率以及贡献的碳酸钙含量（图4）。以MD06-3050岩芯为例，研究发现，该科颗石的丰度在中布容时期呈现高值，尤其是MIS8和MIS10期，Noelaerhabdaceae科颗石快速堆积，贡献了大量的碳酸钙，根据该科颗石的平均重量以及碳酸钙的密度计算得到其贡献的碳酸钙量最高可以占沉积物样品总量的20%。颗石MDI指标，是指直径大于3um和直径小于3um的Noelaerhabdaceae颗石重量的差值，可以代表颗石群落演化过程中整体的钙化作用水平。对比MD06-3050，U1490和DSDP296三个岩芯获得的MDI数据，可以看出1Ma以来，暖池核心区、暖池边缘以及与暖池具有连通效应的黑潮续流区域均显示了变化趋势相似的MDI，指示了颗石的钙化作用在高纬度和低纬度的调控机制可能是统一的，支持了前人提出的天文轨道参数通过调过调控季节性变化和日照辐射变化来驱动颗石藻地质历史尺度上的演化