Thursday June 1, 2023, 3 pm CEST
A reconstruction of environmental changes before and after the Anthropocene boundary (1950s-1960s) - the case of the inner part of Beppu Bay over the past 150 years using aquatic palynomorphs
*C/O East China Sea Research Center, Nagasaki University
A wide variety of organic microfossils are preserved in marine sediments. These include pollen grains, fern spores, fungal spores, dinoflagellate cysts, foraminiferal linings, ciliate remains, dormant crustacean eggs, turbellarian egg capsules, and acritarchs. These organic microfossils, except for pollen grains, fern and fungal spores, are called aquatic palynomorphs. For the reconstruction of past marine environments, observations and counting of these palynomorphs have been employed for long time. In my talk, I would like to clarify the environmental changes before and after the Anthropocene boundary in the inner part of Beppu Bay, Kyushu, Japan.
Stratigraphic cluster analysis using aquatic palynomorphs preserved in the core sediments revealed a rapid eutrophication due to anthropogenic activities from the mid 1960s in Beppu Bay. The aquatic palynomorph assemblages were divided into three major units: BP-I, BP-II and BP-III, whilst dinoflagellate cyst assemblages were divided into four different units in Beppu Bay: BP-A, BP-B, BP-C, and BP-D. Unit boundaries based on aquatic palynomorphs and dinoflagellate cysts were different except for the upper part, BP-III and BP-D, both of which clearly indicated anthropogenic eutrophication in both seawater and bottom sediments. On the other hand, in dinoflagellate cyst assemblages, Unit BP-A was characterized by a stable occurrence of the gonyaulacoid Spiniferites bulloideus and Spiniferites hyperacanthus, Lingulodinium machaerophorum, and a reduction of the heterotrophic peridinioid Brigantedinium spp. In unit BP-C, there was a clear decrease of L. machaerophorum. Unit BP-B was characterized by decreases of S. bulloideus, S. hyperacanthus, and L. machaerophorum, and a small increase of Spiniferites bentorii. Unit BP-C was characterized by an increase in S. bulloideus and the heterotrophic peridinioid Echinidinium spp. Unit BP-D was subdivided into Subunit BP-D1 where dinoflagellate cysts showed a marked increase in S. bulloideus accompanied by the appearance of L. machaerophorum and Tuberculodinium vancampoae, and Subunit PB-D2 where there was a decrease of the total of dinoflagellate cysts. From the dinoflagellate cyst assemblages, the marine environment of the period of unit BP-A was suggested to be warm and stable. However, L. machaerophorum started to decrease in BP-B. The clear decrease of L. machaerophorum suggests that the marine environment became cooler than that of Unit BP-A. Significant increases of S. bulloideus, S. hyperacanthus, L. machaerophorum, T. vancampoae, Brigantedinium spp., and Polykrikos kofoidii were characteristic of Unit BP-D. The increase in total dinoflagellate cyst density and the increase of the ratio of heterotrophic dinoflagellate cysts in Subunit BP-D1 are manifestations of the Oslo fjord Signal and heterotroph Signal, respectively. In addition, the decrease in microforaminiferal linings that continued from Unit BP-C to Unit BP-D might indicate a deterioration of the bottom sedimentary environment.