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.
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