Abstract The early Toarcian oceanic anoxic event (OAE) was a significant palaeoenvironmental perturbation that led to marked changes in ocean chemistry and climate, and which also had a long-lasting impact on marine ecosystems. The global significance of the event has been recognised from the widespread occurrence of a ~ 3–7‰ negative excursion in the carbon-isotope (δ13C) composition of marine organic and inorganic matter and terrestrial plant material. This feature of the event is indicative of a pronounced perturbation to the global carbon cycle; an inference further supported by widespread evidence for seawater deoxygenation and elevated rates of organic carbon burial. Nevertheless, the precise palaeoenvironmental impacts of this event from sections outside of the Boreal and Tethyan realms are uncertain. Here, we present the results of a multiproxy geochemical study of an expanded record of the early Toarcian event from the northwest Panthalassa Ocean margin exposed in southwest Japan (Toyora area, Yamaguchi prefecture). Our results indicate that in the studied succession, organic matter enrichment persisted through the early Toarcian event, but elemental redox proxies do not support persistent seawater anoxia. Analyses of terrigenously derived major and trace element abundances coupled with sedimentological observations reveal an increase in coarse-grained sediment close to the onset of a ~− 4‰ excursion in δ13Corg, and coincident with an inferred increase in terrestrial organic matter flux. These observations are consistent with previously published evidence for a marked strengthening of hydrological cycling and increased runoff that occurred contemporaneously with abrupt warming at the onset of the carbon isotope event.