[M-IS16] 古気候・古海洋変動

[MIS16-15] Reconstruction of Early Pleistocene sea-surface temperature in the Northwestern Pacific Ocean based on calcareous microfossils

*桑野 太輔¹、土屋 祐貴²、大浦 佑馬¹、林 広樹³、久保田 好美⁴、万徳 佳菜子⁵、亀尾 浩司¹ (1.千葉大学、2.名古屋大学、3.島根大学、4.国立科学博物館、5.国立環境研究所)

キーワード：石灰質ナノ化石、浮遊性有孔虫化石、モダンアナログ、古海洋学、更新世、上総層群

Planktonic foraminifera and calcareous nannofossils have been used to clarify past marine environments based on relationships between distributions of living taxa and sea-water conditions (e.g., Takemoto and Oda, 1997; Aizawa et al., 2004). In this study, we reconstruct the sea surface temperature (SST) based on the Modern Analog Technique (MAT) of calcareous microfossil assemblages. The MAT is a useful method for reconstructing SST by comparing the similarity between fossil and modern assemblages (e.g., Pflaumann et al., 1996). Around the Pacific coast of Japanese islands, many surface sediments have been obtained by grab sampler and piston corer, and the modern microfossil assemblages of those have been studied (e.g., Tanaka, 1991). We apply the MAT to 27 samples of the upper part of the Kiwada Formation, Kazusa Group, obtained by Kuwano et al. (in press). We estimated the SST by calculating the squared chord distance (SCD) between fossil and modern assemblages, subsequently taking the weighted temperature average of the top five samples with the highest similarity. The modern foraminiferal and nannofossil assemblage dataset were obtained from Takemoto and Oda (1997) and Tanaka (1991), respectively. The modern SST dataset was used in July from the Japan Oceanographic Data Center (2004).
The reconstructed SST tends to be high during interglacial, and the summer SST ranged between 23.4 and 27.7 ℃. On the other hand, during the glacial Marine Isotope Stage (MIS) 38, the SSTs temporarily dropped by 1–2 ℃. This sea-surface variation is similar to that of planktonic foraminiferal δ¹⁸O change although the nannofossil SSTs were sharp declines rather than planktonic δ¹⁸O changes in some horizons.

[Reference]
Aizawa et al., 2004, Marine Micropaleontology, 52, 277–297.
Japan Oceanographic Data Center, 2004, https://jdoss1.jodc.go.jp/vpage/bts_j.html
Kuwano et al., in press, Stratigraphy.
Pflaumann et al., 1996, Paleoceanography, 11, 15–35.
Takemoto and Oda, 1997, Paleontological research, 1, 291–310.
Tanaka, 1991, Sci. Rep., Tohoku Univ., 2nd ser. (Geol.), 61, 127-198.