[BPT03-P09] Temporal size change of the middle Miocene planktonic foraminiferal species Paragloborotalia siakensis (LeRoy)
Keywords:planktonic foraminifera, temporal size distribution, global correlation , Miocene, IODP
We conducted size measuring of this species at Site U1338 from approximately 16 to 11 Ma. At the same time, we performed X-ray microcomputed tomography (XMCT) analyses and thin section observations at selected horizons. Seventy-five samples at an interval of approximately 0.05 Ma from the site were used for this study. These samples had been already examined for planktonic foraminiferal assemblage (Hayashi et al. 2012). Then, total 6895 specimens of P. siakensis were measured in maximum diameter. The size distribution was discussed with respect to previous geochemical and paleontological data. In the next step, we are examining the size distribution at the Site U1337 near the Site U1338. And some three-dimensional images of specimens collected from characteristic horizons were acquired by XMCT. The CT images enable us to visualize the inner structure such as the form of chambers, ontogenetic growth pattern, and density distribution in each test. Based on CT images, we can estimate the three-dimensional morphologic comparison, degree of maturation and obesity of each test. For mineralogical approaches, thin sections of foraminiferal tests were observed by a polarization microscope.
In a result, we detected twice giantisms and twice dwarfings in the size distribution pattern. The cycle of size change was approximately 2 Ma. One of the dwarfing events could be correlated with Mi3 event (Miller et al., 1991). Therefore, this dwarfing event could be caused by cooling of sea surface water. According to 3D profiles of foraminiferal chambers, tests of relatively small specimens were composed of both high and low CT value layers. In contrast, the larger tests were generally composed of a pair of low CT value layers. Considering previous CT studies, we assume that the difference in the inner layers might reflect the growth the rate for each individual: pair of low CT value layers might mean relatively rapid growth rate.