[B-PT03] Biomineralization and Geochemistry of Proxies

[BPT03-P07] Variation of test morphology of planktonic foraminifer Globigerina bulloides in the western North Pacific

Yuya Hashimoto², Katsunori Kimoto³, *Takuya Sagawa¹ (1.Institute of Science and Engineering, Kanazawa University, 2.Graduate School of Natural Science & Technology, Kanazawa University, 3.JAMSTEC)

Keywords:planktonic foraminifera, morphological analysis

Globigerina bulloides, a species of planktonic foraminifera, is known as one of the species with diverse test morphology. Several studies have reported that the test morphology of this species may be related to the hydrological condition. This hypothesis is supported by biogeographic distribution of genotypes, with different genotypes, Type I and II, found to be distributed in warmer and cooler waters, respectively. However, the test morphological variation of G. bulloides has not been quantitatively investigated. Here, we used a digital microscope and micro-X-ray CT scanning to evaluate the test morphology of fossil G. bulloides. First, we compared tests from three Holocene sediments with different oceanic regions, the subarctic, subtropical, and mixed water regions in the western North Pacific. The ratio of short and long diameters (S/L) and test wall thickness from the subtropical and mixed water regions are very similar (warm-type) and distinct from those from the subarctic sediment, which have greater S/L and thicker test wall (cold-type). This is consistent with the modern hydrological condition, where the site in the mixed water region, offshore Kashima, is under influence of the Kuroshio Current. Next, down-core variation in test morphology was examined in the subarctic and mixed water region sediment cores. While the morphological characteristic of the Holocene and LGM specimens were nearly identical in the subarctic site, the LGM specimens in the mixed water region had greater S/L values and were twice as thick as the Holocene specimens. The LGM specimens in the mixed water region were similar to those in the subarctic site rather than Holocene specimens in the same core. The results suggest that the biogeographical distribution of different types of G. bulloides has changed in response to water mass migration and that ecologically different morphospecies switch at sites located near the boundary of different water masses, which may bias paleoceanographic reconstruction using foraminiferal tests.