Comparison of elemental composition characteristics and 87 Sr/ 86 Sr-143 Nd/ 144 Nd between fossil and modern fish teeth and the significance of enrichment mechanism of REE in deep sea sediments

Xiaoyu Zhang ^1* Zhijie Xu ¹ Kongyang Zhu ¹ Mu Huang ² Miao Yu ² Zheng Li ¹ Zhongrong Qiu ¹ Yanfang Lu ¹

• ¹ Zhejiang University, Hangzhou, China
• ² First Institute of Oceanography, Ministry of Natural Resources, Qingdao, Shandong Province, China

Fossil fish teeth are important carriers of rare earth elements (REE) in deep sea sediments. Meanwhile, 87Sr/86Sr-143Nd/144Nd in these fossils have been widely used in paleoceanography. However, when and how REE enters the fish teeth keeps ambiguous, which hinders elucidating the enrichment mechanism of REE in deep sea sediments, and the effectiveness of 87Sr/86Sr-143Nd/144Nd in paleoceanography are doubted. This study checked the contents of REE, major and trace elements, and 87Sr/86Sr-143Nd/144Nd for modern fish teeth and for fossil fish teeth from deep sea sediments. The results indicated that elemental geochemical properties and redox environment are main factors controlling their enrichment procedures in fish teeth. At least three categories of trace elements are classified during two different stages (physiological process of living fish and post deposition of fossils): (1) elements of Type Ⅰ mainly belong to IA, ⅡA, ⅢA, ⅣA and transition group with active chemical properties, entering into the fish teeth during life; (2) Type Ⅱ represents elements from IA, ⅤA, transition elements, and Actinides group, are enriched uninterruptedly from the living fish teeth to the post-deposition fossils with variable valence under different redox conditions. (3) Type Ⅲ are elements accumulated only after deposition under oxidizing or sub oxidizing, are members of Lanthanides and rare dispersed elements group with large atomic radius and strong chemical activity. For the distinctly different enrichment processes of Sr and Nd, 87Sr/86Sr and 143Nd/144Nd in fossil fish teeth may represent different material sources and should be cautious when employed in paleoceanography. The continuous rehabilitation of 87Sr/86Sr in fossil fish teeth after deposition would change the original 87Sr/86Sr of sea water recorded by fish teeth but the content of Sr keeps unchanged. As for 143Nd/144Nd, when Nd entering the fossils after deposition, exchange of 143Nd/144Nd between particles and pore liquid gradually ceased for the saturation of isomorphism lattice. 143Nd/144Nd of fossil fish teeth may record more information of deep sea water at seawater– sediments interface. This study is beneficial to the enrichment mechanism of REE in the deep sea sediments, and the cautious utilization of 87Sr/86Sr and 143Nd/144Nd of fossils is prerequisite.