Nd and Sr isotopic signatures of fine-grained clastic sediments: A case study of western Pacific marginal basins

摘要

The Nd-Sr isotopic signature of fine-grained clastic strata from western Pacific marginal basins has been examined to test the applicability of isotopic fluctuations in basin characterization, stratigraphic correlation, and provenance determinations. Neodymium and strontium isotopic analyses were conducted on Plio-Pleistocene sediments (n =70) from DSDP/ODP drill cores in the Sea of Japan, Shikoku Basin, and the Sulu Sea. The isotopic values from these basins demonstrate the utility of Nd-Sr isotopic signatures for both interbasinal characterization and provenance evaluation, and intrabasinal stratigraphic correlations. The isotopic signature of the Sulu Sea and the Sea of Japan fluctuates within predictable limits defined by the geology of the source area, and the signature is sensitive to changes in sediment flux from isotopically homogeneous source regions. Isotopic values in the Shikoku Basin deviate strongly from predicted isotopic values with neodymium values significantly lower than expected (ε_(Nd) (- 7) to (- 9) vs. predicted ε_(Nd) (-2) to (+5)). This isotopically evolved signature could not have been derived from the crustal domains on the basin margins, and requires input from a previously unrecognized source. The isotopic signature of the Shikoku Basin is interpreted to represent aeolian sediment flux into the basin from eastern Eurasia coincident with paleoclimatic change in Late Pliocene to Early Pliestocene time. Stratigraphic isotopic fluctuations within the Shikoku Basin are sharp, spanning five ε_(Nd) units, and roughly synchronous across the topographically complex basin floor. Fluctuations in the isotopic signature are used to correlate drill holes across over 5400 km2 of basin floor. The isotopic signature of fine-grained sediments reflects the average isotopic composition of the entire source region, and stratigraphic fluctuations in the isotopic signature record variations in the relative importance of isotopically distinct crustal domains through time. Isotopic variations are synchronous across the Shikoku Basin, and are interpreted to reflect episodic basin margin volcanism superimposed on a background of hemipelagic sedimentation with a strong eolian component. Fluctuations in the isotopic signature of basinal strata ultimately result from geologic events on the basin margins and paleoclimatic influences, and may therefore be useful in reconstruction of basin evolution.