Records Of Environmental Magnetism And Paleomagnetic Secular Variation Since The Last Deglaciation From Huguangyan Maar Lake Sediments In Zhanjiang City,guangdong Province

Characterizing spatiotemporal variability of the Asian summer monsoon（ASM）is critical for full understanding of its behavior,dynamics,and future impacts.The present knowledge about ASM variations since the last deglaciation largely relies on reocrds from central and northern China,while records from south China are scarce.In addition,notwithstanding a notable increase in reliable paleomagnetic secular variation（PSV）records from the northern hemisphere,the distribution of datasets is still distinctly uneven,with a high concentration in Europe,North America and Canada but a much lower concentration in East Asia,hampering a better understanding of the global PSV mechanisms.The sediments of Huguangyan Maar Lake（HML）,with high sedimentation rate,high-resolution and strong magnetism,are ideal archives to document the environmental and climatic changes and PSV in tropical–subtropical South and East Asia since the last deglaciation.Here we performed systematic analyses of rock magnetism,environmental magnetism,pollen,total organic matter（TOM）and microscopy,supplemented by geochemical and X-ray diffraction（XRD）analyses of the HML sediments from a new suite of overlapping piston cores covered the past 22 ky,which was constrained by AMS ¹⁴C and ¹³⁷Cs age determinations.Comparisons of both magnetic susceptibility（χ）and rare earth element（REE）distribution patterns for representative samples from different depths of the HML sediments,surrounding volcanic rocks,and typical loess–paleosol samples from the Chinese Loess Plateau show that the averageχvalue of the lake sediments is significantly greater than that of typical loess from the Chinese Loess Plateau but close to surrounding volcanic rocks.In addition,samples from local pyroclastic rocks and core sediments exhibit remarkably similar REE distributions,but they are significantly different from loess–paleosol samples,which consistently exhibit negative Eu anomalies,with enriched light REEs and relatively flat heavy REEs profiles.The above evidences unambiguously confirm that the HML sediments primarily originate from local pyroclastic rocks rather than from wind-blown material from arid North China.Detailed results of rock magnetism experiments including magnetization versus temperature curves,hysteresis loops,Day diagram,first-order-reversal curves and low-temperature rock magnetism,scanning electron microscope,transmission electron microscope and XRD demonstrate the predominance of titano-magnetite and siderite（mainly appear in magnetically strong intervals）,and a small amount of pyrrhotite and pyrite may also present.Results of the environmental magnetism,pollen and TOM demonstrate that:1)two magnetically depleted intervals with high percentages of tropical and subtropical plants,TOM and sulphur（S）content and low titanium（Ti）correspond to the early Holocene climatic optimum（ca.11500–6500 yr BP）and the B?lling–Aller?d（BA）warming,reflecting intense dissolution of titano-magnetite due to significantly enhanced diagenetic alteration.In contrast,two magnetically strong intervals with relatively low percentages of tropical and subtropical plants,TOM and S content and high Ti correspond to mid-late Holocene and the Younger Dryas（YD）cooling event,reflecting a weakly reductive environment.Comparisons among HML paleoenvironmental record,Chinese speleothemδ¹⁸O signals,the Chinese loess-based monsoon precipitation proxy indexes and the northern hemisphere summer insolation at 30^oN over the past 16 kyr BP show that HML records reveal a pattern of high-amplitude regional climatic fluctuations,including fine-scale oscillations during the BA warming,YD cooling event and eight bond-like cold/dry events（9.2,8.2,6.5,5.3,4.2,2.8,1.4 and 0.4 kyr BP）,which indicate a distinct influence of the North Atlantic circulation on low-latitude monsoon changes.The broad comparability between the HML paleo-proxies,Chinese speleothemδ¹⁸O records,and the northern hemisphere summer insolation since the last deglaciation suggests that solar insolation exerts a profound influence on ASM changes.Additionally,from 7 kyr BP,the subsequent brief intervals of high herbaceous pollen percentages but low percentages of tropical taxa correspond to high El Ni?o/Southern Oscillation（ENSO）activity,supporting a link between the weakening of solar isolation-driven ASM strength and a gradual intensification of ENSO.Alternating field demagnetization of natural remanent magnetization of u-channel samples suggests that HML record a reliable geomagnetic direction during the Holocene,however,the magnetic signal is largely affected by the redox processes in the lake resulting in the unsatisfactory relative paleointensity.The similarity between the HML PSV record and other independently-dated records from East Asia and geomagnetic field models corroborates the robustness of our age model and Holocene PSV record.Strikingly,centennial-to millennial-scale PSV features of the HML are correlatable,within age uncertainties,with other Holocene records from Europe,North America and Canada,suggesting that such directional patterns are likely to be hemispheric in scale.