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Fluids and sediments from Deep Sea Drilling Project/Ocean Drilling Program Legs (56, 57, 87 and 186) along a transect extending from the subducting plate, across the midslope and upper slope of the Japan Trench forearc were analyzed for B and B isotopes in order to assess their composition and fluid–sediment interaction. At the reference Site 436 on the subducting plate, changes in B contents and B isotopes are controlled by the lithology and diagenesis only. The midslope Sites 440 and 584 showed stronger variations in the B geochemistry, which can be related to diagenesis and tectonic dewatering along faults. The strongest changes in the B geochemistry were observed on the upper slope Sites 1150 and 1151, where profound down-hole freshening (chlorinities as low as ∼310 mmol) coincides with a B enrichment (up to 9.3 × seawater concentration). The B isotope pore fluid profile of Site 1150 displayed a bimodal variation with depth, first increasing to values more positive than seawater, then shifting to lower signatures typical for deep-seated fluids, whereas Site 1151 showed a constant B decrease with depth. Sites 1150 and 1151 sediments showed B increases with depth to values as high as ∼164 p.p.m. and isotopic compositions ranging from ∼+4 to −9‰. A linear decrease in Bsolid/Bfluid ratio, suggests that B geochemistry of the upper slope sites is controlled by fluid–rock interaction and deep-seated fluid flow, whereas constant Bsolid/Bfluid ratios were observed at the reference site on the incoming plate. This fluid overprint is probably caused by normal faults in the sediment cover which might be interconnected to deep thrusts in the underlying Cretaceous accreted wedge. This suggests that the erosive Japan Trench margin is characterized by back-flux of deep-seated, B-enriched fluids into the ocean, which is facilitated by extensional normal faulting as a result of tectonic erosion and subsidence.