事件沉积及其周期性的氦同位素指示

物理、化学或生物事件所造成的事件沉积一般都会在沉积层中留下痕迹.事件沉积的突发性使得不同物源的物质比如稀有气体He在尚未达到平衡时就被保存在沉积地层中.海底沉积物中地球4He的丰度受古大陆尘所支配,海底沉积物中的He主要源于3He/4He比率不足0.1Ra的大陆物质(Ra = 1.40×10-6,是空气的3He/4He比率)和3He/4He比率大于100Ra的地球外星际尘粒.计算表明:亚洲风成尘粒总的质量累积速率(mass accumulation rate,MAR)与4He通量之间有一个显著的相关性.由氧同位素变化所记录的冰期-间冰期循环的100 kyr周期的起始点与地壳4He通量中100 kyr周期的起始点清晰地匹配.在大洋钻探计划中,用海底沉积物中地球外3He的通量来考查星际尘粒的全球生长速率发现:3He埋藏通量也与100 kyr冰期-间冰期循环周期相关.对白垩纪-第三纪界面(KTB)粘土中较高的地球外3He( 3He]Et)计算,发现该界面事件的短期性.来自中国梅山和日本西南部Sasayama的二叠纪-三叠纪界面(PTB)富勒烯的He同位素限定了这种富勒烯形成于地球外环境.在整个宇宙体系中,各物源之间的He及其同位素组成存在着很大差异,这为解析沉积事件提供了新的高灵敏度的手段.

Helium Isotope Tracing on Depositional Events

There are diffrent marks on depositional events which are caused by physical, chemical, or/and biological events in the sediment sequences. The suddenness on event deposits will contain various materials from different sources into sediment sequences. Terrestrial 4 He abundance in the marine sediments is dominated by old continental dusts. Helium in seafloor sediments is predominantly derived from old continetal material with lower 3 He / 4He ratio.It is shown by theoretical calculationg that there is a considerable correlation between the bulk mass accumulation rates (MAR) of Asian Aeolian dust and 4 He flux in the ocean basins. The onset of 100 kyr glacial interglacial cycling in the oxygen isotope record clearly matches with the onset of 100 kyr periodicity in the crustal 4 He flux. Testing the IDP accretion rate by using extraterrestrial 3 He flux in the samples from Ocean Drilling Program (ODP), It can be seen that the burial flux of extraterrestrial 3 He is also associated with glacial interglacial cycles. The fact that a high extraterrestrial 3 He ( 3He] Et ) found in the Cretaceous Tertiary boundary (KTB) clay implies a short duration of the KTB event. The high 3 He/ 4He ratios found in the fullerenes from Meishan (China) and Sasayama (Japan) should reveal that the fullerene carriers are formed in an extraterrestrial environment. In the cosmos, the difference of helium content and isotopic composition in various sources and the high helium preservation in the seafloor sediments may provide a high sensitivity means for exploring depositional events.