Geochemistry, physical properties, diatom abundance and datums of ODP Site 186-1150 and Hole 186-1151A

Detection of climate response to orbital forcing during Cenozoic long-term global cooling is a key to understanding the behavior of Earth's icehouse climate. Sedimentary rhythm, which is a rhythmic or cyclic variation in the sequence of sediments and sedimentary rocks, is useful for quantitative reconstruction of Earth's evolution during geological time. In this study, we attempt to (1) identify sources of natural gamma ray (NGR) emissions of core recovered during Ocean Drilling Program (ODP) Leg 186 by analyses of physical properties, major element concentrations, diatom abundances, and total organic carbon contents, (2) integrate whole-core NGR intensity of recovered core with wireline logging NGR measurements in order to construct a continuous sedimentary sequence, and (3) discuss changes in the NGR signal in the time domain. This attempt gives us preliminary information to discuss climate stability in relation to orbital forcing thorough geologic time. NGR values are obtained mainly by indirectly measuring the amount of terrigenous minerals including potassium and related elements in the sediments. NGR intensity is also affected by high porosity, which in these sediments was related to the amount of diatom valves. NGR signals might be a proxy of the intensity of the East Asian monsoon off Sanriku. A continuous sedimentary record was constructed by integration of the whole-core NGR intensity measured in sediments obtained from the drilled holes with that measured directly in the borehole by wireline logging, then using a stratigraphic age model to convert to a time series covering 1.3-9.7 Ma with a short break at ~5 Ma. High sedimentation rate (H) stages were identified in the sequence, related to intervals of low-amplitude precession and eccentricity variations. The transition of the dominant periodicities through the four H stages may correlate to major shifts in the climate system, including the onset of major Northern Hemisphere glaciation, the initial stage of the East Asian monsoon intensification, and the onset of the East Asian monsoon with uplift of the Himalayas and the Tibetan Plateau.

新生代长期全球变冷期间气候对轨道强迫的响应检测，是理解地球冰室气候行为的关键。沉积韵律（指沉积物与沉积岩序列中的韵律性或周期性变化）有助于定量重建地质时期的地球演化过程。本研究尝试：（1）通过分析物理性质、主量元素浓度、硅藻丰度及总有机碳含量，识别大洋钻探计划（Ocean Drilling Program, ODP）第186航次所取岩芯的自然伽马射线（natural gamma ray, NGR）发射源；（2）将所取岩芯的全岩芯NGR强度与测井NGR测量数据整合，以构建连续的沉积序列；（3）探讨时域内NGR信号的变化。这一尝试为我们提供了初步信息，以探讨地质时期气候稳定性与轨道强迫的关系。NGR值主要通过间接测量沉积物中含钾及相关元素的陆源矿物含量获得。NGR强度还受高孔隙度影响，而该沉积物中的孔隙度与硅藻壳数量相关。NGR信号或可作为三陆外海东亚季风强度的代用指标。通过整合钻孔沉积物中测得的全岩芯NGR强度与测井直接在井内测得的NGR强度，构建了连续沉积记录；随后利用地层年代模型将其转换为覆盖1.3-9.7百万年（Ma）的时间序列，其中在约5 Ma处存在短暂中断。序列中识别出高沉积速率（H）阶段，这些阶段与低振幅岁差及偏心率变化的区间相关。四个H阶段主导周期的转变可能与气候系统的重大转变相关，包括北半球大冰期的开始、东亚季风增强的初始阶段，以及伴随喜马拉雅山脉和青藏高原隆升的东亚季风的形成。