Paleosecular variations of geomagnetic field from the Last Glacial Maximum to the Holocene in the north of South China Sea

The high-resolution geomagnetic field records from the Last Glacial Maximum to the Holocene, which possessed of a notable climate change, were scarce in the global area. In this abstract, two gravity piston cores ZSQD2 (114.16oE, 19.58oN, ~190 cm in length, water depth 681m) and ZSQD34 (114.74oE, 19.05oN, ~184 cm in length, water depth 1820 m), situated in the north of South China Sea, were selected to study the secular variations of geomagnetic field in this area. Radiocarbon ages of G.sacculifer suggest that the deposition rate varied with 56.1 cm/kyr and 3.7 cm/kyr during the Last Glacial and the Holocene, respectively. Rock magnetic results indicate that the pseudo-single domain magnetite with low coercivity dominate the properties of sediments. The characteristic remanent magnetization (ChRM) values are evaluated using the 5-8 AF steps when MAD values are generally <5. Constrained by the radiocarbon chronology, the secular variation curves since ~18 cal. kyr can be constructed using the ChRM directions and NRM/ARM ratios (as a proxy of relative intensity). Comparing the Holocene SV with that from terrestrial lakes in Southern China, similar shape corroborates the reliability of records and uniform pattern of non-dipole magnetic field. Two significant features on SV curves present the geomagnetic field characteristics from ~17 cal. kyr to the early Holocene. One is that the direction variations lag behind the relative intensity on the millennium time scale. Such as a major direction shift occurred around 13 cal. kyr while the relative intensity low was about 14 cal. kyr. Another feature is the fast and frequent wiggles both in direction and intensity between ~17 to ~14.5 cal. kyr. During this period, two significant negative inclination anomalies occurred at ~16.4 and ~15.4 cal. kyr associated with low intensity, respectively. Nevertheless, the corresponding declinations did not show the reversed features although they had also some slight fluctuations. The general shape of the relative intensity is characterized by a broad high peak between ~17.2 to ~14.6 cal. kyr and a notable low between ~14.6 to ~9.6 cal. kyr. This pattern is similar to other curves constructed from the ocean. Especially, five prominent sub-lows occurred at ~16.8, ~16.4, ~16.0, ~15.4 and ~15.2 cal. kyr respectively can find the corresponding features on the GLOPIS-75 curve. The climate context is very significant when we compare the secular variations with some climate proxies, such as GISP-δ18O from the Greenland ice core and the Chinese stalagmite δ18O. The most cold climate and weak monsoon between about 17.2~14.6 cal. kyr relate to the high relative intensity with fast and frequent change. Then, gradually warming associate with an increasing relative intensity.; Inclinatin and Declination variations of cores ZSQD2 and ZSQD34 sicne ~18 cal. kyr.