Seawater Nd isotope variation in the Western Pacific Ocean since 80Â Ma (ODP 807, Ontong Java Plateau)

This study reports neodymium isotopic (εNd) variability at a time resolution of 0.5 to 3 Ma since the Late Cretaceous as recorded in a marine sedimentary core from the Western Pacific (ODP 807; 3°36′N, 156°3′E; Ontong Java Plateau). Our core is mainly fine-grained and composed of continuous sequences of nannofossil oozes. The εNd measured in the carbonate fraction was used as a proxy of εNd of seawater of the Western Pacific. On a long term, our results indicate a general increase in εNd of seawater by 4.5 εNd units from the Late Cretaceous (εNd = − 6) to modern times (εNd = − 1.7). This pattern was related to the emergence of the West Pacific margin and the progressive isolation of the Pacific Ocean from the other oceanic basins, resulting in its progressive shift to more radiogenic values through the Cenozoic. This long-term pattern is in accordance with previously published FeMn crusts data from the same study area. Nonetheless, by being at higher time resolution, our data records additional sharp and pseudo-cyclic variations (~ 7-11 Ma periods) superimposed on this long-term pattern from ~ 40 Ma to modern times. These oscillations might reflect the alternating dominance of the two main deep water masses (NPDW and UCDW) bathing our study area. In the same core, we also measured the εNd in the detrital fraction in order to trace back the local terrigenous inputs. The terrigenous record shows a significant variability up to + 12 εNd units. This was linked to the emergence of the west Pacific subduction zone ~ 50 Ma ago causing a higher input of radiogenic isotopes. In conclusion, the large variability observed in both seawater and detrital εNd records most probably result from a major tectonic and oceanic circulation reorganization of the Pacific Ocean.