廣瀬 孝太郎

Human activities in coastal areas have intensified over the last 200 years, impacting also high-latitude regions such as the Baltic Sea. Benthic foraminifera, protists often with calcite shells (tests), are typically well preserved in marine sediments and known to record past bottom-water conditions. Morphological analyses of marine shells acquired by microcomputed tomography (µCT) have made significant progress toward a better understanding of recent environmental changes. However, limited access to data processing and a lack of guidelines persist when using open-source software adaptable to different microfossil shapes. This study provides a post-data routine to analyze the entire test parameters: average thickness, calcite volume, calcite surface area, number of pores, pore density, and calcite surface area/volume ratio. A case study was used to illustrate this method: 3D time series (i.e., 4D) of Elphidium clavatum specimens recording environmental conditions in the Baltic Sea entrance from the period early industrial (the 1800s) to present-day (the 2010 s). Long-term morphological trends in the foraminiferal record revealed that modern specimens have ∼28% thinner tests and ∼91% more pores than their historic counterparts. However, morphological variability between specimens and the BFAR (specimens cm⁻² yr⁻¹) in E. clavatum were not always synchronous. While the BFAR remained unchanged, morphological variability was linked to natural environmental fluctuations in the early industrial period and the consequences of anthropogenic climate change in the 21st century. During the period 1940–2000 s, the variations in BFAR were synchronous with morphological variability, revealing both the effects of the increase in human activities and major hydrographic changes. Finally, our interpretations, based on E. clavatum morphological variations, highlight environmental changes in the Baltic Sea area, supporting those documented by the foraminiferal assemblages.

For assessment of the potential of the Beppu Bay sediments as a Global Boundaries Stratotype Section and Point (GSSP) candidate for the Anthropocene, we have integrated datasets of 99 proxies. The datasets for the sequences date back 100 years for most proxy records and 1300 years for several records. The cumulative number of occurrences of the anthropogenic fingerprint reveal unprecedented increases above the base of the 1953 flood layer at 64.6 cm (1953 CE), which coincides with an initial increase in global fallout of Pu-239+Pu-240. The onset of the proliferation of anthropogenic fingerprints was followed by diverse human-associated events, including a rapid increase in percent modern C-14 in anchovy scales, changes in nitrogen and carbon cycling as recorded by anchovy delta N-15 and delta C-13, elevated pollution of heavy metals, increased depositions of novel materials (spheroidal carbonaceous particles, microplastics, polychlorinated biphenyls), the occurrence of hypoxia (Re/Mo ratio) and eutrophication (biogenic opal, TOC, TN, diatoms, chlorophyll a), unprecedented microplankton community changes (compositions of carotenoids, diatoms, dinoflagellates), abnormally high spring air temperatures as inferred from diatom fossils, and lithological changes. These lines of evidence indicate that the base of the 1953 layer is the best GSSP level candidate in the stratigraphy at this site.

The timing and magnitude of coastal marine degradation in East Asia during a possible transition from the Holocene to the proposed Anthropocene have still not been quantified with well-dated proxy records over centennial-to millennial timescales. This study uses multi-proxy sedimentary records to address this issue and to document biological and environmental changes in an anoxic coastal marine basin, Beppu Bay, Japan. Analysis of sedimentary diatom and pigment records revealed a notable change in diatom and phytoplankton communities and an abrupt increase in their productivity beginning 1960 s CE. Biogeochemical indices, including total organic carbon, total nitrogen, biogenic opal, bromine, and nickel as well as total sulfur, showed a notable increase in values, reflecting the enhanced primary productivity in the water column due to eutrophication, and reduced oxygen-levels in bottom conditions. Biological changes seen in diatom concentrations and communities were unprecedented over the last 1300 and-670 years, respectively. The eutrophication-associated proxy records demonstrate that anthropogenic degradation of the coastal marine environment occurred in the 1960 s, a time which was associated with the exponential, world-wide spread of coastal marine degradation during the Anthropocene, proposed to commence in the mid-20th century. Beppu Bay sediments have a wealth of excellent anthropogenic proxy records and, therefore, could be a representative archive of coastal Holocene-Anthropocene transitions in East Asia.