东海北部外陆架EY02-1孔磁性地层研究

对长达70.20m的东海浅钻EY02-1进行了岩石磁学和古地磁分析,证明沉积物的载磁矿物主要为低矫顽力的磁铁矿,磁性地层揭示了发生于9.62~8.58m的磁极性事件,结合钻孔上部的AMS14C测年证明它为全新世初期的哥德堡磁极性漂移,线性外推的时间是距今12681~10206Ma,为全新世开始时地磁场是否发生过短期的磁极性漂移提供了新证据;与东海高分辨率的浅地层地震剖面以及典型钻孔(中法联合东海地震调查和DZQ4钻孔)对比还揭示,在中更新世地层中也出现过两次磁倾角变化。在钻孔中下部54.00~50.94m(2271—2151号样品)出现一段磁倾角变小甚至变成负值,但是由于该段沉积物以粗颗粒的砂为主并且负向样品并不连续,依据研究的标准不作为反磁极性事件。第二个比较连续的负向样品段出现在最底部70.20~64.31m。虽然研究区域内不乏揭示中更新统地层的地震剖面,但至今没有足够长的钻孔在时间上予以佐证。根据东海地震相对比和沉积物中海侵和海退旋回的不同特征以及布容期以来报道的反磁极性事件发生的时间来推测下部地层的时代归属。由于钻孔最底部的沉积主要是粗颗粒的粉砂质砂和细砂,同时钻孔也穿透了倒数第二冰期的杂乱地震相地层和其下的平行透明海相层,所以推测下部的倒转可能为发生在MIS8晚期的CR0反磁极性事件(距今265~255ka)。

Magnetostratigraphy of Borehole EY02-1 in the northern outer continental shelf of the East China Sea

Rock magnetic and paleo magnetic study is under taken for the 70.20 m borehole in the East China Sea(ECS),showing that the mainmagnetic carrier is low coercivity magnetite.Magnetostratigraphy in this borehole reveals excursion at 9.62~8.58 m that is proved to be Gothenburg excursion at the beginning of Holocene with the linearly extrapolated calendarage o f 12 681~10 206 a BP from AMS ¹⁴C dating,which provides a new evidence for the aut henticity of this excursion.The stratigraphic sequences are determined for the Late Pleisto cenebased on high-resolution subbottom seismic profiles and boreholes(typical seismic profiles and boreholes in the ECS)and two rever sals are indicated.One is from 54.00 to 50.94 m with a shallow ornegative inclination,which may be an Iceland Basin event,but this is not fully discussed because of coarse sediment(which is considered as an unreliable recorder of reversal)and discontinuo us negative sample inclinations.The second continuous negative section occurs at the bottom from 70.20 to 64.31 m. Althought here is seismic profile penetrating int othe Middle Pleist ocene sequence,real boreholes reaching such long time are absent.So the bottomage is deduced according to the seismic correlation,the cycles of transg ression and regression and the time span of rever sals occurring in the Brunhes chron.The sediments of the bottom reversal are mainly coarse silty sand and fine sand,and also the length of the borehole is calculated to cross the layers of the chaotic seismic phase of MIS6 and parallel stratigraphies of marine phase underneath,so it is concluded that the bottom reversal is CR0(265~255 ka BP)happened at late MIS8.