Potassium-isotope variations of marine sediments adjacent to the Izu-Bonin Trench and Nankai Trough

Characterizing the K-isotope compositions of marine sediments is a necessary step to using this novel iso-tope system to investigate K cycling both in the ocean system and in subduction zones. Here we report data for 70+ samples in order to characterize the K-isotope compositions of deep-sea sediments for two drill sites in the western Pacific: (1) IODP Site C0011 adjacent to the Nankai Trough and Southwest Japan arc and (2) Site 1149 adjacent to the Izu-Bonin trench and arc. Variation in delta 41K at both sites is mostly subtle, reflecting a consistently strong contribution of K from sources that have delta K-41 similar to the upper continental crust (UCC). Yet, in detail, the mean delta K-41 at both sites is about 0.05-0.10 %. lower than a recently proposed best estimate for UCC (delta K-41 =-0.44 %.). We infer that the slightly low mean delta K-41 val-ues relative to UCC are likely caused by secondary processes acting on the volcanogenic component of the sediments, though chemical weathering of (non-volcanogenic) terrigenous materials may also play a role. We measured a very low delta K-41 value (-1.33 & PLUSMN; 0.04 %.) for one discrete ash layer at Site C0011, so volcan-oclastic materials may lower the delta K-41 values of the bulk sediments, despite only accounting for a small fraction of the K in the sediments. Correlations between delta K-41, Cr/Al, and K/Al in the upper sediments of Site 1149 were used to investigate mixing between volcanic ash and terrigenous materials. This approach indicates that mixing of isotopically light volcanic ash (with delta K-41 about-0.75 %.) and terrige-nous materials (about-0.46 %.) can account for the observed variation in delta K-41 values in these sediments. In addition, we observe that at Site C0011 K isotopes exhibit a shift (by-0.08 %.) at-250 m below the seafloor from UCC-like compositions in the upper sediments to slightly lighter compositions in the lower sediments. This shift in delta K-41 coincides with an abrupt change in certain physical properties (porosity and cementation) of the sediments that may relate to the extent of weathering/degradation of the dispersed volcaniclastics, with more extensively weathered materials present in the lower sediments. Finally, we find evidence for variable admixture of a high delta K-41 silicic volcanic ash in the lowermost sediments at Site C0011, supporting the view that heterogeneous volcaniclastics can give rise to significant delta K-41 vari-ations in marine sediments.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Characterizing the K-isotope compositions of marine sediments is crucial for studying K cycling in the ocean system and subduction zones. The study reveals subtle variations in K-isotope compositions in deep-sea sediments in the western Pacific, influenced by mixing of volcanic ash and terrigenous materials.