海洋地质与环境重点实验室知识库

太平洋位于东亚冬季风和西风的下风向，是亚洲内陆风尘重要的“汇”。太平洋风尘沉积记录了亚洲内陆古气候、古环境的演化历史。2014年夏季、2015年冬季以及2017年春季分别在热带西太平洋和中纬度西北太平洋收集了现代风尘沉积物，为开展海洋风尘沉积的研究提供了良好的材料。通过对西北太平洋夏季、冬季和春季风尘沉积物的沉积通量、矿物组成、常微量元素组成以及采样期间气团移动轨迹的对比分析，结合本哈姆高原T1站位500 m和2800 m水深沉积物的沉积通量、碎屑态的Sr-Nd同位素组成，以及有孔虫Nd同位素组成，本论文确定了中、低纬度西北太平洋现代风尘的物源及搬运动力，并讨论了陆源碎屑输入对海水Nd同位素组成的影响。

西北太平洋不同海区、不同季节风尘的矿物组成没有系统性的差别，因此矿物组成及其形貌特征仅能作为物源的参考指标。研究区的主要矿物为石英、斜长石和伊利石，而次要矿物主要有石膏、蒙脱石、高岭石和绿泥石等，碎屑矿物粒径大多集中在2~15 μm范围内，很少见大于20 μm的颗粒。研究区内矿物颗粒大多呈次棱角状-次圆状，这是矿物在风力作用下长距离搬运的典型特征，这些干冷气候条件下的指示矿物表明，西北太平洋的现代风尘中的陆源碎屑主要来自亚洲内陆干旱-半干旱地区，采样区附近火山岛弧的贡献可以忽略；沉积物中一些自生组分可能由海气边界层的大气过程产生。

2017年春季中纬度西北太平洋风尘主要由盛行西风从塔克拉玛干沙漠搬运而来，春季强盛的东亚冬季风也能携带少量亚洲东部沙漠（鄂尔多斯沙漠，巴丹吉林沙漠）的细粒沙尘至该地区；而低纬度西太平洋现代风尘的源区主要是鄂尔多斯沙漠，塔克拉玛干沙漠和巴丹吉林沙漠的贡献相对较小，主要由东亚冬季风输运至此。根据西北太平洋现代风尘碎屑态组分微量元素Zr-Th-Sc研究所获得的风尘源区与两地区柱状沉积物在地质历史时期的物源示踪结果一致，从而证明沉积物Zr-Th-Sc含量是风尘物源识别的可靠指标。

现代风尘的沉积通量受采样地点和气象条件的控制，与源区距离越远则风尘通量越低；降水和风速对风尘通量的影响较为复杂，沉降区降雨期间，湿沉降过程使长期悬浮的风尘颗粒快速沉降到海洋中，持续性降雨对空气中风尘颗粒的清除作用，使降雨后风尘通量急剧降低，而且沉降区风尘通量与源区降雨间存在延迟效应，延迟时间即为风尘物质从源区搬运到沉降区的时间。风尘通量与风速存在正消长关系，二者之间也存在一定的延迟效应。

2015年T1站位的现代沉积物表现为吕宋岛火山风化剥蚀产物和亚洲沙漠风尘的两端元混合，风尘组分主要来源于鄂尔多斯沙漠（>80%），仅有很少量来源于塔克拉玛干沙漠、巴丹吉林沙漠和腾格里沙漠（<20%）。研究区沉积通量具有季节性变化，主要受季风降水和东亚冬季风强度控制。2015年西太平洋500 m水深处浮游有孔虫ε_Nd值没有明显的季节性变化，表明陆源碎屑输入的季节性变化对表层和中层水的影响不大。2800 m水深处的浮游有孔虫ε_Nd值显著增大，且表现出明显的季节性变化特征。浮游有孔虫壳体在沉降过程中Mn覆层的吸附作用导致了深水中浮游有孔虫ε_Nd值增大。沉积物-海水的边界交换过程是影响研究区深层水Nd同位素变化的主要原因，冬季吕宋岛边缘出现低温近底流和海底地下水排放的增多，促进了火山物质-海水的边界交换过程，从而导致研究区冬季2800 m水深处的浮游有孔虫ε_Nd值明显增大。

The Pacific Ocean is located in the downwind direction of the East Asian winter monsoon (EAWM), and is an important “sink” area of Asian dust. The eolian dust deposits in the Pacific record the evolution history of paleoclimate and paleoenvironment of the Asian continent. Modern dust samples were collected during the summer of 2014, the winter of 2015 and the spring of 2017 in the mid-latitude and low-latitude Northwest Pacific, respectively, which provide a good opportunity for the research of ocean eolian deposits. The mass fluxes and microscopic mineral characteristics of modern eolian dust particulates and the trace element compositions of the siliciclastic fractions of these samples collected from the Northwest Pacific, together with air mass backward trajectory analysis of dust particulates therein in different seasons, are compared to better constrain the provenances and transport dynamics of dust delivered to these regions. The Sr and Nd isotopic compositions of the siliciclastic fractions and the Nd isotopic compositions of planktonic foraminifera selected from sediment trap sediments collected in 2015 at water depths of 500 m and 2800 m, are also investigated to quantify the eolian dust source to the West Pacific and to assess the influences of terrestrial inputs on the Nd isotopic compositions throughout the water column.

There is no systematic difference in the mineral compositions between the eolian sediments from different regions and different seasons in the Northwest Pacific, thus the composition and morphology of minerals can only be used as a reference index of sediment sources. The mineral assemblages of the modern dust mainly consist of quartz, plagioclase and illite, with minor gypsum, smectite, kaolinite and chlorite. The grain size of the detrital minerals mostly varies from 2~15 μm, and few particles are larger than 20 μm. The mineral particles are mainly subangular/subrounded, which is the typical characteristics of long-range transport by wind. These indicative minerals of dry and cold conditions suggest that the modern dust in this study are mainly derived from inland Asia, and the contribution of the volcanic island arc nearby is insignificant. As for the authigenic components, they may be generated by atmospheric processes over the marine-gas boundary layer.

Eolian dust transported by the Westerlies from the Taklimakan Desert dominates the flux of dust in the mid-latitude Northwest Pacific during the spring of 2017, enhanced EAWM can also carry a small amount of dust from the East Asian deserts (EADs; e.g., the Ordos Desert and the Badain Jaran Desert) to the mid-latitude Northwest Pacific in this season. Whereas，eolian dust in the low-latitude West Pacific is predominantly originated from the Ordos Desert, the contributions of the Taklimakan Desert and the Badain Jaran Desert are small. The EAWM is the dominant transport agent of dust to the low-latitude West Pacific. The provenance analysis results of the modern dust based on the Zr-Th-Sc compositions are consistent with that of core sediments in the geological past in both study regions, thus proving the index to be a reliable indicator of sediment provenances.

The mass fluxes of modern dust are dominantly controlled by sampling locations and meteorological conditions. The influences of the precipitation and wind speed on the mass fluxes are relatively complex. During the precipitation period in the subsidence area, the settling process is promoted due to wet deposition, thus causing sharply reduced mass fluxes after the rainfall because of the effective wet deposition removal of fine particles. The dust fluxes show a delayed response to the wet deposition processes in the source areas as a result of long-time transport. Furthermore, there is a delay positive effect between the wind speed and the dust fluxes.

The siliciclastic fractions isolated from sediment trap samples appear as a binary mixture of chemical weathering of volcanic matters from Luzon and eolian dust from Asian deserts. The provenance analysis demonstrates that dust is predominantly originated from the Ordos Desert (>80%), while the contribution of sediments from the Taklimakan Desert, the Badain Jaran Desert and the Tengger Desert is generally less than 20%. The mass fluxes display seasonal variability, and are mainly controlled by seasonal precipitation and the EAWM intensity. No seasonal variability was observed among the ε_Nd values of the planktonic foraminifera collected in 2015 at a water depth of 500 m in the West Pacific, indicating insignificant influence of seasonal changes in the terrigenous detrital input to the surface/intermediate water therein. However, the results at a water depth of 2800 m are obviously higher than the data collected at a water depth of 500 m, and tend to show seasonal variability characteristics. The adsorption of Mn coatings on the surface of the planktonic foraminifera during the settling process is considered to be the dominant factor account for the increase of the ε_Nd values of the planktonic foraminifera collected in at a water depth of 2800 m. The enhanced boundary exchange process and submarine groundwater discharge have great potential to influence the Nd isotopic compositions of deepwater in the West Pacific.