【作者】 熊志方；

【导师】 李铁刚；

【作者基本信息】 中国科学院研究生院（海洋研究所） ， 海洋地质学， 2010， 博士

【摘要】 硅藻席将巨量的有机碳和生物硅输出到海底,提高海洋生物泵效率,引起大气CO2分压(PCO2)的变化,最终调节全球气候,其在全球碳、硅循环中的作用最近才予以重视。本论文利用从东菲律宾海帕里西维拉海盆获取的两个典型的含Ethmodiscus rex硅藻席岩心WPD-03和WPD-12,通过粘土矿物、生源组分、主、微量和稀土元素、总有机碳同位素和Ethmodiscus rex碳、硅同位素的地球化学系统研究,确定了Ethmodiscus rex勃发所需营养物硅的来源及利用程度,评估了硅藻席沉积期的生产力状况,查明了硅藻席沉积期的氧化还原环境,阐述了硅藻席在热带碳循环中的驱动作用,并探讨了由古海洋响应制约的热带西太平洋硅藻席的形成机制与沉积模式。粘土矿物和Ethmodiscus rex硅同位素组成表明硅藻质粘土沉积期发生过显著的风尘增强过程,风尘携带的丰富硅和铁促进了Ethmodiscus rex的勃发,从而沉积硅藻席。Ethmodiscus rex特殊的生态学特征(生长于次表层水体)以及对海洋环境的特殊需求(适宜于成层化水体)可解释其勃发对风尘输入的滞后响应。Ethmodiscus rex硅同位素组成并结合硅藻利用可溶硅的同位素分馏模型表明硅藻席沉积的末次盛冰期(LGM),Ethmodiscus rex勃发已完全利用尽了(次)表层水中的可溶硅,即对营养物硅而言,东菲律宾海在LGM与现代一样,也处于贫营养状态。生源组分、总有机碳同位素和Ethmodiscus rex碳同位素组成显示古生产力从远洋粘土沉积期→硅藻质粘土沉积期→硅藻席底部沉积期明显增加,在随后的硅藻席中、上部沉积期处于高稳定状态。硅藻席沉积期估计的初级生产力、有机碳雨率(Rain rates)和埋藏生产力平均分别为248.42 g/m2 yr、61.93 g/m2 yr和5.27 g/m2 yr,其初级生产力与现代高生产力的上涌海区可比。多种生产力的定量估算表明在巨型或“树荫种”硅藻生产力评估中要格外谨慎,应区分其代表的“深部”生产力和透光带普通浮游植物代表的“表层”生产力模式。主、微量和稀土元素组成表明远洋粘土、硅藻质粘土和硅藻席分别沉积于氧化、亚氧化和硫化缺氧的环境,但硅藻席沉积期的底层水为亚氧化条件。研究区LGM的深部大洋环流模式以及生产力状况联合表明硅藻席沉积的硫化缺氧环境并不是大洋侧向环流减弱引起的水体滞流造成,而主要是Ethmodiscus rex勃发导致的大规模有机质输出到海底而耗尽溶解O2所致。Ethmodiscus rex碳同位素组成表明LGM时,Ethmodiscus rex勃发消耗了(次)表层水中大量的溶解CO2(CO2(aq)),使硅藻质粘土-硅藻席过渡处CO2(aq)从15.4μmol/L锐减到6.5μmol/L,PCO2也相应地从545 ppmv急减到231 ppmv。硅藻席沉积期PCO2平均为220 ppmv,明显低于硅藻质粘土沉积期的PCO2(平均为440 ppmv),且PCO2总体上逐渐减小,在LDM上部(0～125 cm)沉积期明显低于LGM时全球PCO2(180～200 ppmv),综合表明硅藻席确实扮演“碳汇”角色,最终使研究区(东菲律宾海帕里西维拉海盆)逐渐演化成CO2的汇。综合研究表明东菲律宾海的硅藻席由成席的巨型“树荫种”硅藻Ethmodiscus rex吸收风尘硅,并由风尘铁刺激,在大洋成层化条件下勃发,随后“秋季倾泻”而沉积。现有资料似乎不支持“锋面作用”在研究区硅藻席的形成中起作用,也不支持南大洋“硅溢漏”作用带入可溶硅供Ethmodiscus rex勃发。更多还原

【Abstract】 Diatom mats play an important role in changing partial pressure of atmospheric carbon dioxide (PCO2) and, consequently modulating global climate by massive flux of organic carbon and biogenic silicon to seafloor and improved efficiency of marine biological pump. However, their significances in global carbon and silicon cycles have not heretofore been sufficiently emphasized. Two sediment cores (WPD-03 and WPD-12) with laminated Ethmodiscus rex diatom mats (LDM), located in the Parece Vela Basin of the eastern Philippine Sea, were used to trace the source and relative utilization degree of nutrient Si for the blooming of Ethmodiscus rex, evaluate paleoproductivity levels and reconstruct paleoredox environments during the LDM deposition by measurements of clay minerals, biogenic components, major, trace and rare earth elements, total organic carbon isotope, and Ethmodiscus rex carbon and silicon isotopes. Furthermore, we elucidated the deriving force of LDM in tropical carbon cycle and, finally explored the paleoceanographic constraints on the formation and deposition of LDM from tropical West Pacific.Clay mineral and Ethmodiscus rex silicon isotope compositions show that, during the diatomaceous clay (DC) deposition, it recorded a remarkable strengthened eolian accumulation, which imported plentiful silicon and iron to promote the blooming of Ethmodiscus rex probably and consequently resulted in the formation of LDM. Special ecological characteristics (i.e. populating in subsurface seawater) and peculiar demands on marine environments (i.e. adapted to stratified seawater) of Ethmodiscus rex could result in the lag response of their blooming to dust inputs.With the isotope fractionation model of diatom for dissolved silicon utilization, Ethmodiscus rex silicon isotope composition suggests that Ethmodiscus rex completely consumed the silicic acid during the Last Glacial Maximum (LGM) of LDM deposition. As a result, for the nutrient silicic acid, the eastern Philippine Sea was characterized by oligotrophic conditions during the LGM, the same as at present.Biogenic component, total organic carbon isotope and Ethmodiscus rex carbon isotope compositions indicate that paleoproductivity increased through time during deposition of the pelagic clay (PC), DC, and basal LDM, followed by a stabilization during deposition of middle and upper LDM. Primary productivity, organic carbon rain rates, burial productivity during the LDM deposition were averagely estimated to 248.42 g/m2 yr, 61.93 g/m2 yr and 5.27 g/m2 yr, respectively. Thereinto, the primary productivity is comparable to those of some modern continent-margin upwelling zones. Moreover, the estimations for several kinds of productivity above suggest that a caution must be taken to assess the‘deep’productivity represented by giant and‘shade flora’diatoms, which should be distinguishable from classic‘surface’productivity characterized by common phytoplankton in the photic zone.Major, trace and rare earth element compositions infer that the LDM, DC and PC accumulated under sulfidic anoxic, suboxic and oxic conditions, respectively, while redox conditions in eastern Philippine Sea bottom waters during the LDM deposition were probably largely suboxic. Sulfidic anoxic conditions within the LDM appear to have coincided with marine productivity maxima, thus suggesting that oxygen depletion in the pore water and at the seawater-sediment interface may have been linked to an increased flux of organic matter to the seafloor rather than to restricted lateral circulation. Ethmodiscus rex carbon isotope compositions suggest that the blooming of Ethmodiscus rex during the LGM consumed plenty of dissolved CO2 in seawater (CO2(aq)). In the DC-LDM transition, the CO2(aq) content rapidly decreased from 15.4μmol/L to 6.5μmol/L; Accordingly, PCO2 also distinctly reduced from 545 ppmv to 231 ppmv. Average PCO2 of about 220 ppmv during the LDM deposition is markedly lower than that of about 440 ppmv during the DC deposition, and PCO2 gradually decreased to the level evident lower than LGM global PCO2 arranging from 180 to 200 ppmv during deposition of upper LDM, suggesting LDM played a substantial‘carbon sink’role, and finally developed the eastern Philippine Sea into an obvious sink for CO2.Integrated analysis suggest that mat-forming, giant and‘shade flora’diatom Ethmodiscus rex utilized dust silicon, bloomed in the stratified seawater with the stimulation of dust Fe and then deposited to LDM by the‘fall dump’in the eastern Philippine Sea. Available data suggest that the LDM appear not to have been associated with frontal zone, and no dissolved silicon from the Southern Ocean described as the silicic acid leakage hypothesis promoted the blooming of Ethmodiscus rex in the eastern Philippine Sea.更多还原