New constraints on the marine reservoir effect in Holocene around Kuroshio current from multiproxy sediment core analysis

To investigate the relationship between paleoenvironmental changes and marine reservoir effects, radiocarbon dating was conducted on paired marine shells and terrestrial plant samples collected from identical stratigraphic horizons within Holocene sediments. Two sediment cores (designated OR and HN1) were retrieved from the western margin of the Nobi Plain, a fluvial–coastal lowland primarily formed by the Kiso, Nagara, and Ibi rivers and influenced by the Kuroshio warm current. The drilling sites are located in a subsiding region associated with the Yoro fault. Based on lithological characteristics, molluscan assemblages, and radiocarbon age data, six sedimentary facies were classified, in ascending stratigraphic order: braided river, floodplain, estuary, prodelta, delta front, and delta plain. These facies transitions reflect paleoenvironmental changes that are largely driven by sea-level fluctuations during the deglacial period. Additionally, the accumulation curve of the terrestrial samples corresponds well with the stratigraphic level of the Kikai-Akahoya tephra, providing a chronological benchmark. Reservoir ages were determined from 21 paired samples of terrestrial plants and marine shells collected from estuarine to delta front facies, yielding values ranging from 170 ± 70 to 450 ± 60 14C years, and spanning a time interval from approximately 9500 to 2700 cal BP. Notably, variations in reservoir ages around 8000 cal BP—exhibiting both increasing and decreasing trends—indicate shifts in marine influence likely associated with changes in sea level and deltaic progradation.