[M-IS16] Paleoclimatology and paleoceanography

[MIS16-P03] Variations in shell morphology of the planktonic foraminifer Globigerina bulloides in the sediment off Kashima

*Yuya Hashimoto¹, Takuya Sagawa² (1.Kanazawa University College of Science and Engineering, 2.Institute of Science and Engineering, Kanazawa University)

Keywords:Planktonic foraminifera, morphology, Kashima

Planktonic foraminifer Globigerina bulloides inhabits a wide range of latitudes, especially in the mid-latitudes. Domitsu and Oda. (2005) focused on such shell morphology and reported that it can be divided into two major types, corresponding to warm and cold water masses in Japan Sea. However, there is no such comprehensive study on water masses and shell morphology in the Pacific Ocean. In this study, we focused on the waters off Kashima, where the temperature gradient in the latitudinal direction increases due to the mixing of the warm and cold water masses and examined the diversity of shell morphology of G. bulloides in response to such water mass mixing by observing and quantitatively measuring the shells. We used G. bulloides which is 180-250 μm size from 120 individuals each from Holocene, glacial, and last glacial samples. Measurements were made on images taken with a digital microscope. The following parameters were measured: short diameter [maximum distance between the first chamber from the last (F-1) and the third chamber from the last (F-3)], long diameter [maximum distance between the second chamber from the last (F-2) and the last chamber (F)], foraminiferal area (FA), the area of each foraminiferal chamber and the coiling direction of the shell. Cross-sectional observation by SEM was also conducted to see the difference in shell thickness. The ratio of short to long diameters (short diameter/long diameter) and the ratio of the area of F to the area of F-3 (F/F-3) were taken as quantitative indices to reflect the differences in shell morphology using the data obtained from the measurements. Histograms were made for each sample, and the values and shell morphology were compared among samples of different ages. As for F/F-3, there was an inverse correlation between the two indices, reflecting the difference in shell morphology. Although there was no significant difference in the shell morphology of the representative individuals of the populations located at both ends of the value range, observations showed that the shell thickness of the individuals from the warm period samples was thinner than that of the individuals from the cold period samples. This is consistent with Domitsu and Oda. (2005), who reported that thin-shelled individuals predominate in warm water masses and thick-shelled individuals predominate in cold water masses. It could not be adapted to the classification of Kashima. The mean values of short/long diameters were compared for each coiling direction, but no significant differences were found. So, we conclude quantitative data on thickness are also needed to classify G. bulloides with different water masses, and it is necessary to consider those with intermediate morphology off Kashima.