[B-PT02] 地球生命史

[BPT02-02] Depth habitat of the late Eocene planktic foraminifers

*守屋 和佳¹、大山 琢²、石村 豊穂³ (1.早稲田大学 教育・総合科学学術院 地球科学専修、2.早稲田大学大学院 創造理工学研究科 地球・環境理工学専攻、3.京都大学大学院 人間・環境学研究科 相関環境学専攻)

Planktic foraminifers (Rhizaria) are widely distributed in modern ocean from equatorial to sub-polar oceans. Especially for surface dwelling species, distribution of those species is related to sea surface temperatures. For example, Globigerinoides sacculifer and Neogloboquadrina pachyderma predominantly inhabit tropical and polar oceans, respectively. In addition to this geographic variation, depth of habitat is also segregated from species to species. Globigerinoides ruber is well known as the shallowest dwelling species, Globorotalia menardii is deeper, and Globorotalia truncatulinoides is the deepest dwelling species. When calcifying their tests, oxygen isotope ratio (δ¹⁸O) of their test carbonates records temperature of their habitat. Therefore, δ¹⁸O of each species occurred from a given sample shows variation from positive to negative, hence, cooler to warmer. Using this temperature segregation, we can identify the depth of habitat of each species. This temperature/isotope segregation can be found in fossils species as well. Here, we report isotope segregation of the late Eocene planktic foraminifers in north Atlantic Ocean. We have determined δ¹³C and δ¹⁸O of 15 species at most within one sediment sample. The difference in δ¹⁸O between species showing the most negative and positive δ¹⁸O is approximately 3‰, corresponding to 12˚C. Turborotalia ampliapertura show the most negative δ¹⁸O, indicating surface dwelling nature of this species. On the other hand, the most positive δ¹⁸O is recorded by Globorotaloides quadrocameratus.