【作者】 胡镕；

【导师】 凌洪飞；

【作者基本信息】 南京大学 ， 矿产普查与勘探， 2012， 硕士

【摘要】 大洋铁锰结壳是生长于大洋高地硬质基岩上的“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,并且在形成以后,其中主要元素(Fe、Mn)和一些微量元素(如稀土元素等)的活动迁移程度很小。因此,铁锰结壳的元素(如Fe、Mn)分布反映了其生长沉积时的海水化学特征；同位素组成(如Nd、Pb、Hf)可以示踪海水环流和物质来源。所以对铁锰结壳剖面进行元素和同位素的分析,可以揭示和恢复古海洋环流、古风化剥蚀物源以及古气候条件等的演化历史。为了进一步了解中北太平洋洋流的变化情况和铁锰结壳化学成分与气候变化的关系,本研究对中北太平洋2块生长致密、纹层发育较好的样品(MP3D07、CXD55)的表层剖面,进行了高分辨率主成分电子探针分析,并对2块位于马里亚纳岛弧附近不同水深的两块结壳样品(MKD13：1530m,MDD53：2700m)剖面进行了主成分电子探针分析和Nd同位素组成分析,同时采用Co含量经验公式法计算结壳的年龄。高分辨率的电子探针分析结果表明,结壳的Fe/Mn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,且冰期结壳的Fe/Mn比间冰期高。铁锰结壳中所显示的元素浓度比值在冰期-间冰期的显著变化信号,反映了至少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。我们的研究认为冰期结壳的Fe/Mn较高,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,均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。根据Nd同位素的分析结果,获得了对中新世以来北太平洋西部古洋流演化的初步认识：在中新世,结壳MDD53的Nd同位素组成保持稳定(εNd：-4.0～-5.0),且比北太平洋同时期类似水深的结壳贫放射成因Nd,反映了该时间段北太平洋西部深水主要受控于来自南太平洋贫放射成因Nd的深部西边界流；而浅水结壳MKD13的εNd值在中新世持续升高,是由于印尼海道陆续关闭,贫放射成因Nd的印度洋水体通过该海道流入太平洋浅部的水量陆续减少所致。在上新世,结壳MKD13的εNd值保持不变,说明该时间段岛弧来源Nd的量没有发生变化,并且在早上新世时印尼海道对印度洋-太平洋间浅层海水的流通已经关闭；所以结壳MDD53的CNd值迅速升高,不是由水体从浅层到深层的垂直输入的变化造成的,而是贫放射成因Nd的南大洋深水流入研究区深度范围(～2700m)的流通量从早上新世开始减弱的结果。更多还原

【Abstract】 Ferromanganese crust is shell-like precipitates growing on hard substrate in the marine environment. Hydrogenous ferromanganese crusts are considered to incorporate elements from ambient seawater during their growth on seamounts. Besides, after the crusts’formation, the major elements (like Fe and Mn) and some trace elements (like REE) seldom migrate. Accordingly, the distribution of elements such as Fe and Mn can reflect the chemical characteristics of the ambient seawater during the formation of the crusts, while the compositions of isotopes such as Nd, Pb, and Hf can trace the ocean circulation and source provenances. Therefore, investigation on the elements and isotopes of the crust profiles can reveal and reconstruct the evolution history about the paleocirculation, weathering inputs into the ocean, and the paleoclimate changes.In order to further understand the ocean circulation pattern of the central North Pacific and the relationship between the crusts’elements and the climate changes, crust MP3D07and CXD55have been analyzed for high-time resolution element contents by electron probe, and the other two crusts located near Mariana arc but at different water depths (MKD13:1530m, MDD53:2700m) have been analyzed for both the element contents and Nd isotope compositions. Co contents in the crusts are used to estimate the ages of these crusts.The high-time resolution electron probe results show that the Fe/Mn ratios of both crusts in this study reveal rhythmic variations, which coincide with benthic oxygen isotope fluctuation in glacial-interglacial cycles since～1Ma and the Fe/Mn ratios 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 to 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 led to higher primary productivity in Pacific surface water, and thus increased iron flux to the deep water by the release of biological particles, which were responsible for the increase of the Fe/Mn ratio of ferromanganese crusts.Based on the Nd isotopic records of the two crusts (MKD13and MDD53), the evolution of western Pacific deep water circulation since Miocene is explored. From the early to late Miocene, Nd isotopic compositions of MDD53remained stable, and they were also characterized by the least radiogenic signatures (sNd:-4.0to-5.0) compared with crusts of similar water depths in the Miocene North Pacific. Afterward, an abrupt increase in its ENd value occurred in the Pliocene. In contrast, Nd isotopes of MKD13became more radiogenic with time in the Miocene and were almost invariable thereafter. The continual increase in8Nd of shallower crust MKD13is interpreted as reflecting progressive closure of Indonesian Seaway in the Miocene, while the deep western boundary current sourced from the Southern Pacific may have dominated Nd isotopes of deeper crust MDD53during the same time interval. The lack of Nd isotopic variation of MKD13in the Pliocene indicates that there was no change in Nd sources in shallower waters and the final restriction between the Indian and Pacific might have only occurred since then. Therefore the observed large shift to more radiogenic Nd isotopes of MDD53in the Pliocene should have not been resulted from changes in vertical input from shallower to deeper water. Instead, a decreased ventilation of deep southern component current along the studied water depth range (-2700m) might have continued in the Pliocene.更多还原