[M-IS01_29PM1] Land-ocean linkages in East Asian marginal seas

★ [MIS01-03] The missing volcanic record captured by dispersed ash in sediment of the Japan Sea/East Sea and NW Pacific Ocean

*MURRAY Richard¹、SCUDDER Rachel P.¹、DUNLEA Ann G.¹、IKEHARA Ken²、IRINO Tomohisa³、TADA Ryuji⁴、ALVAREZ-ZARIKIAN Carlos A.⁵、KUTTEROLF Steffen⁶、SCHINDLBECK Julie⁶、SCIENTIFIC PARTY Expedition 346⁵ (1.Boston University、2.Geological Survey of Japan、3.Hokkaido University、4.University of Tokyo、5.IODP-TAMU、6.GEOMAR, Kiel)

キーワード：volcanic ash, sediment chemistry, Japan Sea, East Sea, volcanism

Volcanic ash in marine sediment provides a wealth of information not only about volcanism and arc evolution, but also potentially regarding climate change, geochemical mass balances, hydration of marine sediment during alteration, the geodynamics of subduction zones, and other key components of the earth-ocean-atmosphere system. Ash occurs both as discrete layers as well as isolated grains and shards dispersed throughout the bulk sediment, and with highly variable grain sizes. The study of this dispersed component has lagged behind the sedimentologic and chemical assessment of the ash layer record. For example, while decades of smear-slide studies of bulk sediment in volcanic-rich regimes have presented visual estimations of the abundance of volcanic glass, shards, and other components, the quantitative importance of the dispersed ash or cryptotephra remains largely unconstrained on local, regional, and global scales. Also, compared to the often visually stunning ash layer records, which in certain settings can leave single layers with thicknesses of 10s of cm, the dispersed ash component and cryptotephra are unable to be visually differentiated from detrital clay.We summarize here preliminary results regarding the distribution, composition, and accumulation of dispersed ash in sediment from the Japan Sea/East Sea (gathered during IODP Expedition 346, Asian Monsoon, and ODP Legs 127/128), and compare it to the record provided by discrete ash layers. We will interpret our work in the context of our ongoing studies of dispersed ash throughout the northwest Pacific, Nankai, and Izu-Bonin regions, which is based on sediment from DSDP/ODP/IODP Sites 52, 444, 579/581, and 1149, as well as from Sites C0011 and C0012.Multivariate statistical treatments are an integral part of our approach, as the bulk determination of the major, trace, and REEs provides the chemical context for our determination of provenance, and the statistical models allow distinctive resolution of the different aluminosilicate components based on their individual geochemical signature(s). A corollary benefit of our approach is an improved determination of the eolian component, as we are able to discern how contributions of dispersed ash have been inadvertently attributed to the eolian aluminosilicate inventory. Q-mode Factor Analysis can help determine the number, and composition of, potential end member contributions. Applying these results in conjunction with Total Inversion, a linear regression technique, allows determination of the compositional variation of these end members. Consistent with the qualitative smear-slide estimates, in these ash rich regions we find that the dispersed component can account for up to 40% of the total sediment. We are able to document abundances to a relatively high degree of precision (+/- 3-5%) on a sample-by-sample basis, and are further able to distinguish between different chemistries of the dispersed component, and document sources that change through time and space. In addition to providing an overview of “the missing volcanic record”, we will discuss some ongoing challenges, including how to best examine the relationship between the composition of the discrete ash layers compared to the discrete component, and what information can be gained from examining similarities and differences between their respective sources.