[B-PT02] Biomineralization and Geochemistry of Proxies

[BPT02-10] Palaeoenvironmental reconstruction using inner bay ostracods–morphological methods–

*Gengo Tanaka¹ (1.Kumamoto University)

Keywords:Phanelozoic, Ostracoda, Palaeoenvironment

Seed shrimp (Ostracoda) belongs to Crustacea with two strongly calcified valves. They have been reported from many sedimentary rocks around the world since the Ordovician onwards. Marine and brackish-water species are important taxonomic groups in the reconstruction of palaeoenvironments because they are chiefly benthos throughout their life. In this research, I focused on the ostracod assemblages and surrounding environmental factors that live in the innermost bay of the Ariake Sea which is a famous place in the highest tidal range in Japan. Surface sediments at 28 locations were investigated, and environmental parameters (sediment, water depth, water temperature, salinity, dissolved oxygen, pH, chlorophyll concentration, and turbidity) were measured at the same time. Q-mode cluster analysis was performed on 23 of the 26 sites where 50 or more individuals have occurred, and two biofacies and five sub-biofacies were recognised. These were divided into the eastern biota, which is close to the rivers, and the western biota, which is far from the rivers.
The modern analogue technique (MAT) is for reconstructing paleoenvironments using morphological-based ostracod assemblages. This is a method that quantitatively reconstructs past paleoenvironments using modern community data sets and environmental parameters as dependent variables, and fossil communities as independent variables. However, in modern data, it is necessary to remove multiple correlations between the species in the ostracod assemblages, which serve as dependent variables, as well as environmental parameters. The multicollinearity (VIF) of the ostracod assemblages in the Ariake Sea was checked among the 107 species, most of the species had VIF > 6, and only 11 species were selected. Regarding environmental parameters, strong multicollinearity was observed in the salinity. After checking multicollinearity, we investigated the relationship between ostracod assemblages in each site and environmental parameters using canonical correspondence analysis (CCA) and found that ostracods from eight sites were affected by water temperature, and the ostracods from other points were affected by dissolved oxygen concentration, water temperature, turbidity, and pH. It turns out that many sites are influenced by different environmental factors. Using one of the eight points affected by water depth to ‘estimate’ the water depth at the point using the MAT. As a result, the estimated value was approximately 1.2 m shallower than the actual value. Thus, to use the MAT in the inner bay tidal flats such as the Ariake Sea, where there are many multiple correlations among species and where specific environmental factors cannot be determined, it is necessary to collect a large number of modern samples. However, other morphological characters, the opening ratio of the sieve-type pores (SP) of Cytheromorpha acupunctata, which lives on the sandy mud bottom of an inner bay, is useful in this study. By using the Akaike Information Criterion (AIC) to investigate the relationship between the opening ratio of SP and environmental parameters, the opening ratio of SP was related to water temperature, dissolved oxygen, and pH. Furthermore, a strong linear correlation (r = 0.83) was recognized between water temperature and the opening ratio of SP. By focusing on the community composition of ostracods and the morphological character of their shells, it is possible to estimate the paleoenvironment with high accuracy even in inner bay areas where tidal flats have developed and not easy to determine palaeoenvironmental conditions.