Stepwise accelerations in the rate of sea-level rise in the area north of Tokyo Bay during the Early Holocene

Abstract

Tokyo Bay is situated near the triple junction of the North America, Pacific, and Philippine Sea plates, where relative sea-level changes since the Last Glacial Maximum (LGM) have been strongly affected by subduction tectonics. In this study, 55 sea-level index points were extracted from the post-LGM incised valley fills beneath the lowlands north of Tokyo Bay and used to construct a detailed relative sea-level curve for the Early Holocene. Sea-level jumps were detected at 10.1–10.0, 9.2–9.1, and 8.9–8.7 ka, each of 2–7 m and each followed by an acceleration in the rate of sea-level rise. These paired phenomena are attributed to coseismic subsidence and an interseismic decrease in the rate of uplift, respectively. The Early Holocene sea-level changes in Tokyo Bay are characterized by remarkably short sea-level jumps of <100 years and accelerations in the rates of sea-level rise with relatively long durations of 200–800 years. After separating these tectonic effects from relative sea-level changes, sea-level jumps that have been hypothesized to be eustatic events, such as Meltwater Pulses 1C and 1D and the pre-8.2-ka event, were not identified based on a sea-level detection limit of 3 m, but a decrease in the rate of sea-level rise at 8.2 ka, which can be regarded as a global phenomenon, was verified in Tokyo Bay.