上新世亚北极太平洋硅藻种群特征及其环境控制因素

The Pliocene is a key period in studies of paleoclimate and paleoceanography, because the global climate changed gradually from warm to cold during this period. Diatoms, the most important primary productivity in the subarctic Pacific Ocean, have been widely applied to reconstruct paleoceanographical environments. However, most diatom species living in the Pliocene evolved or were extinct in the subarctic Pacific Ocean. Furthermore, there are rare detailed studies of influences of Asian aeolian dust iron input on diatom growth during the Pliocene, and diatom distribution and spatially difference of their environmental backgrounds. These facts lead to unclear environmental interpretation using diatom records during the Pliocene, and impede the application of diatoms in palaeoceanography studies in the subarctic Pacific Ocean. To make the environmental backgrounds of diatoms more clear, in this project, sediments in the Pliocene are chosen in two ODP sites, 882 and 885/886, which locate in the northwest and central-south of the subarctic Pacific Ocean, respectively. Diatom assemblage changes will be firstly studied, including diatom abundance, species composition and frustule size of key species. Then, I will study the three main environmental backgrounds controlling diatom growth, including iron mineral characteristics (e.g. the category, content, and grain size) delivered from Asian aeolian dust, main nutrients (e.g. nitrogen) input under control of upwelling strength and sea surface temperature (SST) changes, through detailed environmental magnetism investigations, stable nitrogen isotope measurements and collection of previously reported SST results, respectively. At last, correlations between diatom assemblage characteristics and palaeoceanographical environmental backgrounds, and their spatial differences will be analyzed. The results of this project will provide solid bases for diatom application in paleoceanography studies in the subarctic Pacific Ocean.

上新世全球气候逐渐由暖转冷，是古气候-古海洋研究的焦点。硅藻作为亚北极太平洋最主要的初级生产力，其沉积记录是恢复该区古海洋环境的关键手段。然而，上新世该区硅藻主要属种已进化或灭绝，并且前人对上新世亚洲风尘铁输入对硅藻的影响以及该区内部硅藻分布及其受控环境因素的空间差异的研究尚不深入，导致硅藻环境指示意义并不明确，影响了硅藻在古海洋环境研究中的应用。因此，本项目拟以分别位于亚北极太平洋南北部的ODP 885/886和882站位上新世沉积物为研究材料。首先研究硅藻丰度、属种组成、优势种壳体大小以及总生产力等种群特征；然后通过详细的磁学和稳定氮同位素分析，并结合前人已有海表温度等结果，探讨亚洲风尘输入含铁矿物特征（包括类型、含量以及颗粒大小等）、上升流强弱控制下主量营养盐输入、海表温度等环境因素对不同空间区域硅藻种群的影响。该项目将为硅藻记录在该区古海洋环境研究中的应用提供详实的理论基础。

硅藻在全球碳、硅等元素的生物地球化学循环过程中扮演着重要角色，也是亚北极太平洋最重要的初级生产力。亚北极太平洋是典型的高营养盐低叶绿素海域，硅藻种群受风尘等陆源Fe输入、混合作用控制下主量营养盐供应等多因素影响。目前，已有较多研究关注该区域冰期-间冰期等尺度上硅藻种群与风尘输入、通风作用、海表温度等背景的关系，但是，上新世硅藻种群与其环境背景之间的关系还缺乏深入认识。因此，该项目以ODP882、885两站位上新世沉积物为材料，通过环境磁学、主微量元素、氮同位素、硅藻等多指标分析，获得了以下几方面新认识：1）上新世北太平洋中层水，即通风作用，比现今强，且在3.5-2.73 Ma比上新世其他时段更强，可能已加强为深层水，这与3.5 Ma之后北半球冰川作用开始出现有关；2）硅藻属种对亚北极锋移动较敏感，结合亚洲风尘通量、粒度等，发现从~5.7到4.7 Ma，西风环流北移且减弱，~4.7 Ma以后，特别是3.7 Ma以后，西风环流南移并增强，且分别与东亚夏季风的增强和减弱对应；3）上新世亚洲风尘对硅藻种群的影响较小，而火山灰却能促进硅藻生长，主要是因为亚洲风尘中含铁矿物主要以难溶解的粗颗粒（钛）磁铁矿、赤铁矿、针铁矿等为主，不利于硅藻吸收利用，而火山灰中含较多易溶解的纳米级细颗粒含铁矿物，易被硅藻利用，进而促进硅藻勃发；4）在ODP882站位海表温度越高C. marginatus壳体越大，但在ODP885站位C. marginatus壳体大小与海表温度的关系较复杂；5）上新世亚北极太平洋磁性矿物受生物还原溶解作用影响显著，在硅藻生产力较高层段粗颗粒（钛）磁铁矿、赤铁矿等不易溶解矿物相对含量升高，其他未受还原溶解作用影响层段，磁学手段对陆源风尘、冰筏碎屑的输入具有重要的指示意义。该项目深入的揭示了上新世北太平洋中层水演化特征、风尘等陆源Fe输入对硅藻生产力的影响、北半球西风与东亚季风的关系，为进一步探讨上新世全球气候变化机制提供了重要基础，对今后更好的利用硅藻和环境磁学手段研究该区域古海洋环境演化也具有重要的指导意义。

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