南海184航次第四纪深海沉积物的自生10Be记录

Over 5000m-long deep-sea sediments retrieved during the ODP Leg 184 in the South China Sea have served as exceptionally high quality, continuous, carbonate-rich materials for paleoceanography research for the last 18 years. One weakness of the current studies on these cores is the lack of robust paleomagnetic data at most of the sites that would provide magnetostratigraphic framework for global correlation. This was primarily caused by the use of rotary drilling technology that partly distorted the integrity of sediments. This situation has been compensated by the excellent preservation of the carbonaceous constituents, which yielded high-quality stable oxygen isotope records for all the sites. This project proposes to generate 10Be records in the ODP Leg 184 Quaternary deep-sea sediments of the South China Sea. Authigenic 10Be and 9Be will be extracted from the sediments using weak acids and measured with accelerator mass spectrometry (AMS) and atomic absorption spectroscopy (AAS) respectively. Two main time intervals will be targeted, i.e. around the Matuyama/Bruhnes and Gauss/Matuyama polarity reversals at 0.78 Ma and 2.58 Ma respectively. We seeks to demonstrate the variations of sedimentary authigenic 10Be/9Be as a proxy for the production rate of the cosmogenic 10Be and in turn as an alternative approach to reconstruct changes in the intensity of the geomagnetic field. The profiles of the obtained authigenic 10Be/9Be ratio will be compared with the biostratigraphic events and oxygen isotope records for the two intervals with an aim of establishing the stratigraphic positions of the Matuyama/Bruhnes and Gauss/Matuyama transition and revealing the behaviors of the geomagnetic field during the two polarity reversals. Together with Ar-Ar dating of the microtektites from Site 1144, it is hoped the authigenic 10Be/9Be records will provide new time constraints on the sedimentary sequences of the South China Sea.

ODP184航次在过去的18年为南海古海洋学研究提供了长达5000余米的沉积物。然而，已有的岩芯研究中一个薄弱的环节是多数站位缺乏坚实的古地磁数据，从而影响了建立可用于全球对比的磁性地层学框架，这归因于钻探时使用了旋转式取芯技术，致使沉积物部分变形，幸运的是由岩芯中保存极佳的钙质化石得到的高质量氧同位素记录部分地弥补了这一不足。本项目将通过弱酸提取和加速器质谱及原子吸收光谱测量获得南海184航次第四纪深海沉积物的自生10Be/9Be，探讨其作为指示大气宇宙成因核素10Be产率和地磁场变化代用指标在南海沉积记录中的应用，结合生物地层事件和氧同位素记录，以及微玻陨石的氩-氩测年，揭示高斯/松山地磁极性倒转事件时期地磁场的变化特征，以及松山/布容由负极性转向正极性时地磁场强度变化的精细过程和幅度，厘定两个极性倒转的精确年龄和历时长度，为南海深海沉积地层提供新的年代控制点。

通过南海第四纪深海沉积物自生10Be/9Be指示的大气宇宙成因核素10Be产率变化确定地磁极性变化的地层位置，对于优化南海深海沉积年代地层学具有重要意义。鉴于沉积物自生组分的10Be/9Be比值与其上覆的深层海水关系复杂，本项目在测量深海岩芯沉积物之前，聚焦在从海水到沉积物的铍同位素来源、搬运等过程，测定了南海北部海水、珠江河口水及颗粒物、南海表层沉积物自生组分的10Be和9Be。研究发现，表层海水的9Be浓度介于8.8与43.6 pmol/kg之间，呈现出向东南方逐渐降低的趋势；表层海水的10Be浓度变化范围为118-576 atoms/g，最高值出现在东部站位。10Be/9Be分布基本与10Be分布特征一致。珠江口至SEATS站断面全水柱上层水的9Be分布与表层水一致，各层位从珠江口向海盆内部递减的梯度明显；10Be浓度和10Be/9Be比值向南递增，垂直方向上向下增加。南海北部表层海水的9Be受珠江河口输入的控制，10Be主要来自西太平洋表层水，水平10Be和9Be分布还受到季风洋流的影响。箱式模型计算得到南海北部沉积物清除的10Be沉积通量为1.1×10^22 atoms/yr，表明南海是边界清除强度较高的10Be汇。珠江口内部的溶解态9Be在河口混合过程中被颗粒物迅速清除，但颗粒态铍（9Be）仍主要为天然来源，即受人类活动影响不大。南海表层沉积物自生组分9Be为0.021-0.958 μg/g，10Be浓度为（0.13-9.10）×10^8 at/g，10Be/9Be变化范围为（0.04-2.43）×10^-8。 10Be和10Be/9Be分布呈现陆架/陆坡低、中央海盆高的趋势，海盆南部高于北部。本项目对南海海水、颗粒物和沉积物铍同位素的系统研究为仍在进行的南海184航次钻孔沉积物10Be/9Be分析和解释打下了基础，同时也显示了铍同位素在南海水团示踪方面的潜力。