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

Abstract

Characterizing the K-isotope compositions of marine sediments is a necessary step to using this novel isotope 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 δ41K at both sites is mostly subtle, reflecting a consistently strong contribution of K from sources that have δ41K similar to the upper continental crust (UCC). Yet, in detail, the mean δ41K at both sites is about 0.05–0.10 ‰ lower than a recently proposed best estimate for UCC (δ41K = −0.44 ‰). We infer that the slightly low mean δ41K values 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 δ41K value (−1.33 ± 0.04 ‰) for one discrete ash layer at Site C0011, so volcanoclastic materials may lower the δ41K values of the bulk sediments, despite only accounting for a small fraction of the K in the sediments. Correlations between δ41K, 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 δ41K about −0.75 ‰) and terrigenous materials (about −0.46 ‰) can account for the observed variation in δ41K 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 δ41K 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 δ41K silicic volcanic ash in the lowermost sediments at Site C0011, supporting the view that heterogeneous volcaniclastics can give rise to significant δ41K variations in marine sediments.