Marine Os isotopic fluctuations in the early Eocene greenhouse interval as recorded by metalliferous umbers from a Tertiary ophiolite in Japan

The Mineoka umbers occur within the Tertiary accretionary complex of central Japan, immediately overlying ~ 53.7 Ma tholeiitic basalts. The associated tholeiitic basalts exhibit typical normal mid-ocean ridge basalt (N-MORB)-type geochemical signatures and are interpreted to be a remnant of Tertiary oceanic crust. Centimeter-resolution bulk-rock chemical analyses were performed to cover the entire section of the ~ 35 cm thick umber deposit. There are striking similarities in major element, trace element, Re, and Os contents between the umbers and modern metalliferous sediments along the East Pacific Rise. These results demonstrate that the Mineoka umbers are ancient counterparts of MOR metalliferous sediments that are plume fall-out ferromanganese precipitates scavenging various seawater-derived elements, including P, V, Y, REEs, and Os, and confirm that the umbers subaerially exposed in the accretionary complex generally preserve their primary geochemical signatures at the time of deposition. The marine 187Os/188Os ratios inferred from the Mineoka umbers gradually increase up-section with moderate fluctuations. The 187Os/188Os composition, ranging from 0.47 to 0.54, is significantly higher than that of the prior period (post-Paleocene–Eocene thermal maximum (PETM); 187Os/188Os ~ 0.38). This was probably the result of an increase in the chemical weathering rate of continental crust caused by the long duration of noticeably warm climate during the early Eocene greenhouse interval. Several fluctuations observed in the marine 187Os/188Os record during this interval are most likely related to PETM-like, brief hyperthermals, supporting the suggestion that the intensity of continental crustal weathering controlled by climatic changes can fluctuate on timescales of ~ 105 yr.