Radionuclides of sediment cores and a sediment trap in the Weddell Sea

The scavenging of 230Th and 231Pa was investigated in the central Weddell Sea by combining results from a sediment trap and three sediment cores. Scavenging of both radionuclides is closely coupled with the annual cycle of particle fluxes. For 230Th the mean radionuclide flux measured in the trap is only 40% of its expected flux from production in the water column. This value is in excellent agreement with the long-term record in the sediment cores (33-43%). Similar results were obtained for 231Pa, although burial fluxes are generally higher. The data suggest that during the last 130 ka the Weddell Sea has been a net source for both radionuclides, with more than half of the 230Th and about half of the 231Pa being exported. As a consequence, 230Th normalized rain rates (assuming a constant flux equal to the production rate) overestimate the true rain rate in the Weddell Sea by 150%. The laterally transported 230Th and 231Pa activity exits the Weddell Sea to the north, where it is incorporated into the eastward flowing Antarctic Circumpolar Current (ACC). There it is scavenged in addition to local production. During its residence time in the S-Atlantic sector of the ACC about 3/4 of the dissolved 230Th imported from the Weddell Sea is transferred onto particles. Whether this particulate 230Th is entirely deposited in the S-Atlantic or is distributed over a larger area extending into the Indian and Pacific sectors of the ACC remains an open question. In the ACC, Th-normalization therefore leads to an underestimation of fluxes, but the effect is probably less than 50%. Interglacial-glacial shifts in the position of the productive belt are believed to cause temporal and regional variations in the depositon rate for 230Th.

本研究通过整合沉积物捕集器（sediment trap）与三根沉积岩芯的观测数据，对威德尔海中部海域的²³⁰Th（230Th）与²³¹Pa（231Pa）清除过程进行了探究。两种放射性核素的清除过程均与颗粒物通量的年度周期紧密耦合。针对²³⁰Th，沉积物捕集器测得的平均放射性核素通量仅为其在水体中原位生成通量的40%。该数值与沉积岩芯的长期记录（33%~43%）高度吻合。尽管²³¹Pa的埋藏通量整体更高，但其观测结果与²³⁰Th的趋势一致。数据表明，在过去130 ka（千年）间，威德尔海始终是这两种放射性核素的净源区，超过50%的²³⁰Th和约50%的²³¹Pa被向外输送。因此，采用以原位生成通量为恒定基准的²³⁰Th归一化沉降速率（rain rate），会将威德尔海的实际沉降通量高估150%。经横向输送的²³⁰Th与²³¹Pa活度向北流出威德尔海，并汇入向东流动的南极绕极流（Antarctic Circumpolar Current, ACC）。在该区域中，除原位生成的核素外，还会发生额外的清除过程。在南极绕极流南大西洋片区的驻留期间，从威德尔海输入的溶解态²³⁰Th中约有3/4被吸附至颗粒物上。目前仍未明确，这些颗粒吸附态的²³⁰Th是完全沉积于南大西洋，还是会被扩散至覆盖南极绕极流印度洋与太平洋片区的更大范围区域内。因此在南极绕极流中，采用Th归一化方法会导致通量被低估，但该误差幅度可能小于50%。现有研究认为，间冰期-冰期旋回中生产力带位置的迁移，会导致²³⁰Th的沉积速率出现时间与空间上的差异。