晚第四纪中北太平洋铁锰结壳Fe/Mn变化:对古气候变化的响应

Fe/Mn Variations of Late Quaternary Ferromanganese Crusts from the Central North Pacific: Implications for the Paleoenvironment Change

大洋铁锰结壳是沉积于海山硬质基岩上的“壳状”沉积物。水成型铁锰结壳被认为是从海水中直接沉淀出来的，并且，形成以后Fe，Mn等元素活化迁移程度很小。因此，铁锰结壳的Fe，Mn含量可以大致反映其生长沉积时的海水化学特征。由于结壳生长通常会受到许多因素（包括水动力环境）的影响而使其生长不够稳定，从而难以准确获取结壳元素在千年尺度上变化的规律。为了克服这一影响，本文选取了中北太平洋两块生长韵律较好，结构致密的结壳（MP3D07和CXD55）进行了高精度化学成分探针分析。结果表明，两块结壳的Fe／Mn比值都可以和～1Ma以来的深海底栖有孔虫氧同位素曲线很好地对应起来，冰期结壳的Fe／Mn比值比间冰期高。铁锰结壳的Fe／Mn比值在冰期一间冰期的显著变化，说明至少1Ma以来，铁锰结壳中Fe—Mn的含量变化与全球气候变化紧密相关。我们认为冰期结壳较高的Fe／Mn比值，是由于冰期风沙输入量增大带来大量的Fe和其它营养元素，加之亚北极水流的影响，使得太平洋表层的生物生产力增大，进而导致由生物颗粒向深水释放的Fe通量增高造成的。

Ferromanganese crust is shell-like precipitate growing on hard substrate of deep sea mountains. Hydrogenous ferromanganese crusts are considered to incorporate elements from ambient seawater during their growth on seamounts. After the formation of crusts, the mobility of their constituent elements is low. Accordingly, the contents of Fe and Mn can reflect the chemical characteristics of the seawater during the crust formation. Since the crusts can easily be influenced by many factors, including hydrodynamic environment, they usually cannot grow in a steady condition. Therefore, it is difficult to judge the chemical evolution process of the crusts＇ elements in millennium scales. To circumvent this problem, this study selected two densely structured crusts （MP3D07 and CXD55） with well rhythmic growth pattern and analyzed their spatially high-resolution element contents by an electron probe. The results show that the Fe/Mn ratios of both crusts in this study reveal rhythmic variations, which coincides with benthic oxygen isotope fluctuation in glacial-interglacial cycles since ～1 Ma and the Fe/Mnratios in glacial stages are higher than those in interglacial stages. Such distinct glacial and interglacial signal extracted from the ferromanganese crusts indicates that the Fe-Mn contents of crusts are related with global climate change. During the glacial stages, together with the influence of Subarctic water mass, the increased eolian input which brought large amount of iron and other nutritive elements lead to higher primary productivity in Pacific surface water, and thus increased iron flux to deep water by release of biological particles, which are responsible for the increase in the Fe/Mn ratios of ferromanganese crusts.