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摘要
This study reports neodymium isotopic (epsilon(Nd)) 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 degrees 36'N, 156 degrees 3'E; Ontong Java Plateau). Our core is mainly fine-grained and composed of continuous sequences of nannofossil oozes. The epsilon(Nd) measured in the carbonate fraction was used as a proxy of epsilon(Nd) of seawater of the Western Pacific. On a long term, our results indicate a general increase in epsilon(Nd) of seawater by 4.5 epsilon(Nd) units from the Late Cretaceous (epsilon(Nd) = -6) to modern times (epsilon(Nd) = -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 Fe-Mn crusts data from the same study area. Nonetheless, by being at higher time resolution, our data records additional sharp and pseudo-cyclic variations (similar to 7-11 Ma periods) superimposed on this long-term pattern from similar to 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 epsilon(Nd) in the detrital fraction in order to trace back the local terrigenous inputs. The terrigenous record shows a significant variability up to + 12 epsilon(Nd) units. This was linked to the emergence of the west Pacific subduction zone similar to 50 Ma ago causing a higher input of radiogenic isotopes. In conclusion, the large variability observed in both seawater and detrital epsilon(Nd) records most probably result from a major tectonic and oceanic circulation reorganization of the Pacific Ocean.
- 出版日期2016-10-1
