海洋地质与环境重点实验室知识库

       冲绳海槽南部的高沉积速率为东亚边缘海地区的陆-海相互作用和构造活跃地区山溪性河流（台湾河流）流域长期以来风化和剥蚀历史的恢复及其古环境响应提供了宝贵的沉积档案。本学位论文以冲绳海槽南部获取的沉积物岩心S3为研究对象，测定了浮游有孔虫放射性碳年龄，沉积物的粒度、常微量元素、Sr-Nd同位素、质量堆积速率和重矿物组成，碎屑磷灰石的常微量元素和形貌学组成。探讨了晚全新世以来冲绳海槽南部陆源沉积物的来源、源区岩石的类型；重建了物源区晚全新世以来的河流排泄和硅酸盐风化历史；揭示了气候变化、突发性事件（台风和地震）和人类活动等因素对区域物理剥蚀和化学风化强度的潜在影响。
硅酸盐组分Sr-Nd同位素组成物源示踪结果表明，约3000 cal yr BP以来冲绳海槽南部的陆源沉积物主要来源于台湾东北部河流（兰阳溪）和台湾西部河流。中国大陆的大型河流（如，长江和黄河）和台湾东部河流搬运物、火山物质和风尘物质对海槽南部过去约3000年来的沉积作用没有起到显著的贡献。
       碎屑磷灰石地球化学组成物源判别结果显示，冲绳海槽南部的碎屑磷灰石具有混合来源的特征，沉积年龄较老的层位1中的磷灰石颗粒主要来自镁铁质/中性岩石，其他较年轻层位中的磷灰石颗粒主要来自镁铁质/中性岩石和高度演化型岩石。此外，各层位中还有少部分碎屑磷灰石可能来自碱性岩和变质岩。重矿物组合证据表明陆源物质是短距离搬运至沉积位置的。碎屑磷灰石主成分分析结果和区域地层学证据表明，冲绳海槽南部的陆源沉积物是台湾东北部和西部河流流域沉积岩和变质沉积岩风化剥蚀的产物，其终极来源包含多种岩石类型。台湾东部河流沉积物对沉积位置的陆源物质沉积没有明显的贡献。层位1中的碎屑磷灰石与层位2、层位3和层位4中的磷灰石在地球化学组成上表现出一定的差异性，推测可能是不同时期流域内风化剥蚀的沉积岩类型存在轻微差异造成的。
      根据沉积物的常量元素组成建立了化学蚀变指数（CIA）、α^Al_K、K/Al、Ti/Na和K/Na等化学风化替代指标，还根据碎屑磷灰石形貌学特征计算了平均蚀坑指数（EPI）作为化学风化替代指标，各指标记录显示过去约3000年来台湾河流流域的基岩经历了相对恒定且中等强度的化学蚀变。根据沉积物质量堆积速率（MAR）建立了MAR_{siliciclastic}和MAR_Ti值作为河流排泄强度替代指标，还根据碎屑磷灰的纵横比和晶面完整程度建立了平均纵横比（AR）和完整晶面百分比（PCF）作为河流排泄强度替代指标，各指标记录显示在3000-2100 cal yr BP阶段台湾地区的河流排泄强度相对较弱，从2100 cal yr BP开始增强并具有稳步递增的变化趋势。
       通过将本文的沉积记录与前人报道的台湾地区气候和人类活动记录进行比较，发现冲绳海槽南部海域陆源物质输入量的增加与过去约3000年来台湾地区降水量增大密切相关。在3000-2100 cal yr BP期间降水量相对较小，相应地河流排泄强度较弱；2100 cal yr BP开始降水量增大且维持在相对稳定的水平，相应地河流排泄强度开始显著增强；从约1500 cal yr BP开始陆源物质输入表现出稳步递增的特征，是恒定且较高水平的降水和逐渐加剧的人类活动共同作用的结果。
由于晚全新世以来台湾地区的物理剥蚀强度维持在一个较高的水平且后期稳步增强，强烈的物理侵蚀作用、陡峭的山势和湍急的水流很大程度上限制了台湾山区流域的硅酸盐风化，使得台湾近海沉积物一致表现出中等强度的化学蚀变。未来将构造背景活跃地区山溪性小河流近岸的沉积物化学记录用于解释/反映古气候变化时可能需要更加谨慎。从全球角度来看，与台湾类似的山溪性流域，虽然有极高的陆源物质通量，但由于经历了较弱的化学风化作用，通过硅酸盐（岩）的化学风化作用使大气中的CO₂向水圈转化/消耗的效率和通过碎屑磷灰石的化学风化作用将生物可利用P从岩石圈向水圈转化的效率均较构造稳定的大陆河流流域（如，长江）低。
       本文恢复了较长时期以来构造背景活跃的山溪性流域的风化和剥蚀历史，为构造活跃型地区河流流域的景观塑造和海-陆交互作用提供新的见解和认识，强调了降水和人类活动对河流物质输入的控制，这种类型流域虽然有更有利的化学风化条件（更快的化学风化速率），但并不会导致更高的沉积物化学蚀变程度。首次将碎屑磷灰石的地球化学组成和形貌学特征用于冲绳海槽沉积物物源的判别和源区风化和剥蚀历史的恢复，发现碎屑磷灰石是非常有前景的副矿物，可以为边缘海地区源-汇过程和古环境演化研究开辟新的途径。
 

    The high sedimentation rate in the southern Okinawa Trough provides an excellent sedimentary archive for the study of continent-ocean interactions in the marginal sea of East Asia and the reconstruction of chemical weathering and physical erosion history in tectonically active mountainous rivers (i.e., Taiwan rivers) in response to paleoenvironmental changes. The sediment core S3 retrieved from the southern Okinawa Trough is taken as the main research object of this dissertation. The radiocarbon age of planktonic foraminifera, the grain size, major element, trace element, Sr-Nd isotope, mass accumulation rate and heavy mineral compositions of the sediments, and the major element, trace element and morphological compositions of detrital apatite were determined to discuss the source and source rock types of terrigenous sediments in the southern Okinawa Trough since the late Holocene, reconstruct the history of fluvial discharge and silicate weathering in the source area since the late Holocene, and reveal the potential impacts of climate change, episodic events (e.g., typhoons and earthquakes) and human activities on regional physical erosion and chemical weathering intensities.
    The sediment source analysis of Sr-Nd isotope data suggests that the terrigenous sediments in the southern Okinawa Trough since ca. 3000 cal yr BP were mainly sourced from the rivers in northeastern and western Taiwan. Large rivers in mainland China (e.g., the Yangtze River and the Yellow River) and rivers from eastern Taiwan, volcanic materials, and aeolian dust have not contributed significantly to the deposition of the southern Okinawa Trough over the past approximately 3000 years.
    The results for provenance discrimination on the basis of detrital apatite geochemical data indicate that detrital apatite in the southern Okinawa Trough were derived from a mixture source with a wide variety of rock types. The apatite grains in Layer 1 with older sedimentation age were mainly originated from mafic/intermediate rocks, whereas the apatite grains in the three younger layers were derived from mafic/intermediate rocks and highly evolved rocks. In addition, a small part of grains in each layer might have contributed from alkaline rocks and metamorphic rocks. The principal component analysis (PCA) results of detrital apatite and regional stratigraphic evidence suggest that the terrigenous sediments in the southern Okinawa Trough are the products of weathering and erosion of sedimentary rocks and metasedimentary rocks in the river basins of northeastern and western Taiwan, and their ultimate sources include various rock types. The fluvial sediments from eastern Taiwan did not significantly contribute to the core site. There are some differences in geochemical compositions between the apatites in Layers 2-4 and apatites in Layer 1, suggesting that the weathered/eroded sedimentary rock types in the weathering profile of Taiwan river basins might have changed since Layer 2 (2010–1940 cal yr BP).
    The chemical index of alteration (CIA), K/Al, α^Al_K, Ti/Na, and K/Na were established as the proxies of chemical weathering intensity according to the major element compositions of the sediment. In addition, the average etch pit index (EPI) was calculated to infer the variations in the regional chemical weathering intensity on the basis of the morphological characteristics of detrital apatite. The results suggest that the bedrocks in Taiwan river basins have undergone relatively constant and moderate strength of chemical alteration since ca. 3000 cal yr BP. According to the mass accumulate rate (MAR) of the sediment, MAR_{siliciclastic} and MAR_Ti were determined to reflect changes in the strength of fluvial discharge. The average aspect ratio (AR) and the percent of crystal surface (PCF) of detrital apatite were also constructed as fluvial discharge intensity proxies. The results indicate relatively weak terrigenous sediment supply during the interval of 3000-2100 cal yr BP, followed by a progressively increased fluvial discharge from 2100 cal yr BP to present.
  By comparing the sedimentary records in Core S3 with previously reported climate and human activity records in Taiwan, indicating that the increased terrigenous sediment input from Taiwan to the southern Okinawa Trough since ca. 3000 cal yr BP was closely related to the changes in precipitation and anthropogenic processes on Taiwan Island. The strength of fluvial discharge was relatively weak during the interval 3000-2100 cal yr BP, corresponding well with the small rainfall on Taiwan during this period; the precipitation has increased and maintained at a relatively constant level from 2100 cal yr BP to present, which was consistent with a significantly enhanced fluvial discharge; the progressively increased terrigenous sediment supply since ca. 1500 cal yr BP was likely to be caused by the joint impacts of heavy rainfall and enhanced human activity.
    The physical erosion intensity in the mountainous rivers of Taiwan has maintained at a high level and increased steadily in the later stage since ca. 3000 cal yr BP. Such strong physical erosion with steep mountains and fast water flow have exerted a great limitation on regional chemical weathering intensity, making the offshore sediments of Taiwan deposited during the geological past all display moderate chemical alteration. It should be cautious to use the sediment chemistry records offshore of tectonically active mountainous rivers for the interpretation of paleoclimate change in the future. From a global perspective, although mountainous rivers (e.g., Taiwan rivers) have extremely high sediment flux, due to the relatively weak chemical weathering strength, the efficiency of CO₂ consumption/transfer from the atmosphere to the hydrosphere through silicate (rock) chemical weathering and the efficiency of bioavailable P conversion from the lithosphere to the hydrosphere through the chemical weathering of detrital apatite are both lower than those in river basins from tectonically stable continent (e.g., the Yangtze River).
  In this dissertation, relatively long-term chemical weathering and physical erosion history in small mountainous river basins have been reconstructed, which may provide new insights into the landscape shaping and continent-ocean interactions for river basins in tectonically active mountains. The results highlight the controls of precipitation and human activities on fluvial sediment discharge, although this type of river basin has more favorable conditions for chemical weathering (faster chemical weathering rate), it does not necessarily lead the sediment to experience more intense chemical alteration in such river catchments. The geochemical and morphological characteristics of detrital apatite have been used to distinguish sediment provenance and recover regional weathering and erosion history in the study area for the first time. Detrital is a promising accessory mineral, which will open up new avenues in the study of source-sink process and paleoenvironmental evolution in marginal sea of East Asia.