南海北部约17cal.ka以来地磁场长期变记录

Paleosecular variations of geomagnetic field since ca.17cal.ka in the northern South China Sea

近些年来地磁场长期变（PSV）研究取得了长足进展，不但为探索地球内部动力学过程提供了不可多得的材料，也为千年至万年时间尺度沉积地层的精细对比和定年提供了强有力的工具。但是在众多的资料当中，末次盛冰期至全新世早期这一段时期内可靠的高分辨率古地磁场记录相对较少。本文以南海北部钻孔ZSQD2沉积物为研究对象，重建古地磁场方向和相对强度的变化，补充该时期内地磁场变化的记录。浮游有孔虫的AMS 14C定年结果表明约190cm钻孔沉积物年龄在约4~17cal.ka之间，岩石磁学和环境磁学实验证明沉积物中主要控磁矿物为软磁组分，如磁铁矿类矿物，沉积物中磁性矿物的浓度、粒度和类型的变化较为单一，适合进行古地磁方向，尤其是相对强度的研究。该段时期内，地磁场的倾角和偏角均存在多次峰、谷变化，其中在约12.4~13.4cal.ka之间发生明显的地磁场异常，指示了“哥德堡地磁漂移（Gothenburg Excursion）”的存在，持续时间大约1ka。在该漂移发生之前，地磁场强度先开始衰减，相对强度的低值发生在约13.2~15.0cal.ka之间，早于古地磁场方向异常约1.6ka。

Few of high-resolution paleomagnetic secular variations（PSV）since ca.17ka were reported, which hinders our understanding of the geomagnetic field and its correlation with climate change during the period from the Last Glacial Maximum to the Holocene.In this study, paleosecular variations including relative paleointensity from the northern South China Sea were investigated in gravity piston core ZSQD2（19.58°N, 114.16°E）with 190cm core length and 681m water depth.Grey silty clay composed mainly of the sediments characterized by the variant sedimentary structure and the amounts of foraminifera.Foraminifera enriched in the top 8cm and assembled in the irregular conglomeration between 8cm and 65cm.Some irregular hard clay mass with light color（most of them displayed in narrow strip）dotted in the grey silty clay.In the interval of 84~87cm, a large amounts of foraminifera enriched in a thin layer of fine-silt clay.Total 186 cylinder samples were continuously selected using glass box in 1cm diameter and 1cm height for paleomagnetic measurement, and total 91 cubic samples using the plastic box in 2×2×2cm parallel to cylinder samples were selected for magnetic fabric experiments.The AMS radiocarbon ages for selected planktic foraminifera constrained the individual chronology of the core, show that the core ZSQD2 provides high-resolution sediments during about 4~17cal.ka.Mineral magnetic and paleomagnetic measurements were carried out using discrete sediment samples.Rock magnetic results indicate that the main magnetic mineral is magnetite with a very homogeneous grain-size distribution in the pseudo-single domain range.Changes in the amount of magnetite are relative homogeneous while a little fraction of maghemite play less contribution to the magnetic properties below 30cm.Slight changes in magnetic minerals concentration, grain size and coercivity also meet the criteria for reconstructing the relative paleointensity records.Then, secular variations（directions and relative intensity） between about 4~17cal.ka can be reconstructed from this core.Four peaks and three vales both in inclination （‘a^g’） and declination （‘A^G’） present the general pattern of the geomagnetic field in the interval of 7~17cal.ka was overlooked when discussing PSV owning to the deposition rate difference）.Comparison of our PSV features with other records from the western Pacific Ocean and stacked global curve displays the good consistent pattern.The significant direction anomaly between 12.4~13.4cal.ka both in inclination and declination can find counterparts in the paleomagnetic documents of MD81 core（6°30＇N, 125°49.2＇E, from the western Equatorial Pacific Ocean） and the Japan Sea cores KT89-18（32°09＇N, 133°54＇E） and GH98-1232 （44°48.09＇N, 139°41.97＇E）.The good consistent among these records suggest that this anomaly respond to the Gothenburg excursion, which was reported in Chinese and European lacustrine and marine sediments although it was still argued.However, the relative intensity low is between the interval of 13.2~15.0cal.ka, ahead of the direction anomaly about 1.6ka.