过去53万年来东亚冬季风与北半球冰量和大气二氧化碳浓度的关系

Relationship of East Asian winter monsoon with Northern Hemisphere ice sheets and atmospheric carbon dioxide concentrations over the last 530 ka

关键词:  黄土高原  粒度  东亚冬季风  北半球冰量  大气CO2浓度

Key words:  Chinese Loess Plateau  grain size  East Asian winter monsoon  Northern Hemisphere ice sheets  atmospheric CO2 concentrations

摘要:

亚洲季风作为全球气候系统的重要组成部分，是高、低纬气候变化相互联系的纽带，对亚洲区域气候环境演变具有重要影响。深入研究过去东亚季风演化规律及其驱动机制有助于加深对未来气候变化的认识，并为应对潜在风险提供科学依据。黄土古气候研究表明，东亚冬季风在轨道尺度上主要受北半球冰量和大气 CO₂浓度调控，表现出显著的冰期-间冰期旋回特征。然而，其对外部强迫的响应并非始终呈线性关系。文章基于洛川黄土-古土壤剖面上部约37 m、53万年以来的粒度记录发现，冬季风整体上以10万年周期变化为主，主要受北半球冰量和大气 CO₂浓度共同驱动，但在特定阶段冬季风与北半球冰量和大气 CO₂浓度存在明显的非线性响应。全球海平面、深海底栖有孔虫壳体氧同位素 （δ¹⁸O） 与大气 CO₂浓度表明，深海氧同位素阶段8 （MIS 8） 冰期具有较MIS 7间冰期中的亚冰阶 （MIS 7d） 更大的北半球冰量和更低的大气 CO₂浓度，然而MIS 8时期冬季风强度却弱于MIS 7d；同样，MIS 11时期相较于 MIS 13表现出显著减少的北半球冰量和显著升高的大气CO₂浓度，而MIS 11时期冬季风强度与MIS 13相当，甚至强于MIS 13。上述现象表明，在冰期-间冰期尺度上冬季风对北半球冰量和大气CO₂浓度的变化幅度存在差异性响应，反映了气候系统内部的非线性特征。

Abstract:

Background, aim, and scope The Asian monsoon is an important component of the global climate system and exerts profound influences on the environment of South and East Asia, where more than half of the world’s population resides. It consists of seasonally alternating cold/dry northerly Asian winter monsoon (AWM) and warm/moist southerly Asian summer monsoon (ASM). The eolian loess-paleosol sequence on the Chinese Loess Plateau (CLP) consists of alternating yellow loess and red paleosol layers. These dust sediments were transported primarily by Asian winter monsoon from inland arid regions to the CLP. The grain-size distribution of the loess-paleosol sequence is closely linked to variations in AWM intensity, because stronger winter monsoon enhances dust transport and promotes accumulation of coarser sediments on the CLP. The red paleosol development within the yellow loess sequence reflects increased summer monsoon precipitation on the CLP during warm interglacials, which drove intense pedogenesis and formation of abundant iron oxides. Here we investigate the detailed orbital-scale East Asian winter monsoon variability and its relationship with Northern Hemisphere ice sheets and atmospheric carbon dioxide concentrations over the last 530 ka using a grain-size record from the Luochuan loess-paleosol section on the central CLP. Materials and methods After cleaning and removing the surface outcrop, we collected 365 samples from the upper about 37 m of Luochuan loess-paleosol section (35°48′N, 109°24′E), spanning from the top of paleosol S5 (Late Pleistocene) to paleosol S₀ (Holocene). Samples was conducted at 10 cm intervals, yielding an average temporal resolution of about 1—4 ka for grainsize analyses. The grain-size distributions were measured with a Malvern 2000 Laser particle-size analyzer after the removal of organic matter and carbonate in the State Key Laboratory of Loess Science at the Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China. Results The grain-size record from the Luochuan loess-paleosol section exhibits a pronounced about 100-ka cyclicity that correlates glacial-interglacial variations in global sea-level and marine benthic foraminiferal oxygen isotope (δ¹⁸O) records. Generally, glacials have higher contents of ＞40 μm particles and stronger East Asian winter monsoon than interglacials. This is consistent with dominant controls of Northern Hemisphere ice sheets and atmospheric CO₂ concentrations on the East Asian winter monsoon over glacial-interglacial variability. However, our results indicate that the response of the East Asian winter monsoon to Northern Hemisphere ice sheets and atmospheric CO₂ concentrations was not always linear over the past 530 ka. Despite larger ice sheets and lower atmospheric CO₂ concentrations during Marine Isotope Stage (MIS) 8 compared with MIS 7d, the higher coarse-particle content in MIS 7d indicates a stronger East Asian winter monsoon during MIS 7d. Likewise, MIS 11 exhibits higher coarse-particle contents and stronger winter monsoon intensity than MIS 13, even though ice sheets were smaller and CO₂ concentrations were higher during MIS 11. Discussion Comparable discrepancies among coarse-particle content, ice sheets, and CO₂ during MIS 7d, MIS 8, MIS 11, and MIS 13 shown by the Luochuan loess-paleosol profile are also reflected in other central and western profiles across the CLP, showing the asymmetric responses of winter monsoon to ice sheets and CO_2. Glacial-interglacial stages are characterized by substantial variations in both the extent and spatial distribution of ice sheets, as well as in key climatic boundary conditions, including atmospheric CO₂ concentrations and orbital parameters. Such variations may contribute to asymmetric responses of the East Asian winter monsoon to ice sheets and CO₂ forcing. Conclusions East Asian winter monsoon responded dynamically to Northern Hemisphere ice sheets and atmospheric CO₂ concentrations, but not always linearly. Pronounced nonlinear and asymmetric responses are evident during MIS 7d, MIS 8, MIS 11, and MIS 13. Recommendations and perspectives The specific mechanisms responsible for these nonlinear responses remain unclear and require further investigation through the integration of paleoclimate proxy data with climate model simulations. It is necessary to further explore the driving mechanism of the East Asian winter monsoon beyond the CLP.