日本海中部60ka以来的风尘沉积对西风环流演化的指示

Evolution of westerly jet during the last 60 ka: Evidence from core deposits in the central Japan(East) Sea

对位于西风环流下风区日本海中部LV53-23岩芯沉积物的粒度特征进行了研究,重建了日本海末次冰期(~60 ka)以来的西风环流演化历史和空间变化规律.结果显示,岩芯沉积物以粉砂为主且粒度频率分布曲线多呈单峰分布,与现代日本黄沙和前人报道的邻近区域的岩芯粒度特征一致,具有典型的风尘沉积特征.众数粒组-粉砂组分的中值粒径分析表明,粉砂组分中值粒径可以用作岩芯沉积反映西风环流的替代性指标.在亚轨道及千年尺度上,粉砂组分中值粒径在MIS 3期的冰阶-间冰阶千年尺度旋回与西风环流密切相关:冰阶时,粉砂组分中值粒径增大,这与西风急流轴较长时间位于青藏高原南部,西风环流影响范围变大,风速增强,能携带更多近源的蒙古戈壁-中国东北沙地风尘至日本海有关;间冰阶时,粉砂组分中值粒径减小,这是由于西风急流轴位于青藏高原北部,携带更多远源的塔克拉玛干沙漠风尘至日本海.而在轨道尺度上,末次盛冰期时粉砂组分中值粒径的稳定低值与北半球65°N日照辐射量良好对应,表明北半球日照辐射量和中高纬温度梯度差对西风环流远距离输运粉尘起到重要作用,盛冰期时西风急流轴受北半球冰盖扩张影响显著南移导致西风环流范围也随之南移,从而对日本海40°~50°N上空西风影响减弱.

Aeolian dust has a significant environmental impact on the ocean biogeochemical cycles in different timescales. However, spatial and temporal variations of dust transported on long distances in the upper troposphere by the westerly jet, in particular over orbital-millennial timescales, have not been resolved yet. In this study, we present the evidence for changes in the main axis and path of westerly jet over East Asia at millennial-orbital scale during the last glacial period（~60 ka）. We examine the grain size characteristics of sediment core LV53-23 collected from the central Japan（East） Sea, which lies the downwind of westerly jet to discuss the evolution of westerly jet since the last glacial period. Our results show that core sediments are composed mainly of silt characterized of eolian dust, which is consistent with those sediment cores discussed in previous studies from the Japan Sea. The median grain-size of silt fraction has similar pattern with environmentally sensitive grain size extracted by standard deviation/grain-size method. The median grain size in silt fraction increased in stadial periods and decreased in interstadial periods, which might be related to changes in the path of westerly jet. During the stadial, the westerly jet mostly located in the the south of the Tibetan Plateau. It can influence the northern part of the Tibetan Plateau extending to 50°N with high speed, which is prerequisite of dust raised to westerlies in northern China and thus carried the coarser dust from nearby Mongolian Gobi and northeastern China to the Japan Sea. During the interstadial, the provenance of the finer aeolian dust was from Taklimakan Desert when westerly jet migrated northward and concentrated within a narrow band at northern Tibetan Plateau with a longer duration. We found the multi-millennial time scales variations during DOI 8, 12, 14, 16 in cores LV53-23 and MD01-2407 located in the southwestern Japan Sea based on the results of NGRIP and GISP 2 Greenland ice cores. Contrasting with grain size variations of the loess-paleosol records（e.g., Gulang Loess sequence）, the lowest grain size during the Last Glacial Maximum confirm that the silt fraction of Japan Sea sediment was mostly transported by westerlies instead of East Asian Winter Monsoon. The mechanism driving the orbital scale variability in grain size of aeolian dust in the Japan Sea is mainly forced by variations of ice volume due to summer insolation in the Northern Hemisphere. A southward migration in the main axis of westerly jet and its influential area was affected by a decrease of summer insolation and increase of ice volume in Northern Hemisphere. During the Last Glacial Maximum, although the cold surges and dust storms were intensive, the southward path of westerly jet may decreased the dust entrainment ability transported by westerlies so that downwind received finer grain size eolian dust. This argument is consistent with findings showing that the location of subarctic front in North Pacific during the Last Glacial Maximum was caused by a southward shift of the path of Kuroshio Current which is controlled by westerly jet. During the glacial period, the decreasing grain size of dust in northern site is explained by weaker westerlies in higher latitudes（40°–50°N） caused by the southward shift of westerly jet main axis, while the increase grain size in the southern site is controlled by the increasing of dust transport path because of westerlies speed in lower latitudes.