B and Li Isotopes data of Kumano mud volcanoes' (Offshore SE Japan) pore waters extracted from cores taken during the SO222 expedition in 2012. Aliquots of 14 untreated fluids were utilized for B and Li isotopic composition analysis in the isotope geochemistry laboratory at the MARUM - Center for Marine Environmental Sciences, (University of Bremen), using a ThermoScientific Neptune Plus multi collector (MC) ICP-MS, equipped with the stable introduction system (SIS) and an x-cone. The B purification technique, measurement routine and data collection method are detailed in Hüpers et al. (2016). B isotope values are given relative to the NIST SRM 951 boric acid reference material in the conventional δ11B (‰) notation. Accuracy and precision for the separation procedure and measurement was checked through NIST SRM 951 and the internal bottom seawater (BSW) standard Susu Knolls. Fully processed NIST SRM 951 yielded an average δ11B value of 0.05 ± 0.03‰ (2SD with n = 2); the long-term repeatability for δ11B of NIST SRM 951 was 0.0 ± 0.2‰ (2SD, n = 25). Fully processed BSW SuSu Knolls (39.5 ± 0.2‰ (2SD with n = 2) overlap within uncertainty with literature values on modern seawater (39.61 ± 0.2‰ (2SDmean), Foster et al., 2010).The separation and purification of lithium was modified after Moriguti and Nakamura (1998) and is described in detail in Hansen et al. (2017), together with the measurement routine and data collection method. Li isotope ratios were measured using the standard-sample-bracketing method in 25 ng/ml Li solutions using reference material NIST RM 8545. Li isotope ratios are given in the delta δ7Li (‰) notation relative to untreated NIST RM 8545. Procedural blanks were lower than 0.01% of the Li in the processed sample aliquots and hence do not affect the sample composition. NIST RM 8545 and an internal seawater standard (bottom seawater from SuSu Knolls) were processed together with each sample series. The reference material NIST 8545, processed through the whole separation procedure, yielded a δ7Li value of 0.08 ± 0.04‰ (2SD with n = 2). Fully processed seawater shows a value of 30.8 ± 0.4‰ (2SD with n = 2), which is in agreement with literature values for IAPSO seawater (e.g. 31.2 ± 0.9‰ (2SDmean, n = 44) Pogge von Strandmann et al., 2010). The instrumental precision and internal long-term repeatability for δ7Li was -0.01 ± 0.18‰ (2SDmean, n = 82, obtained by the repeated analysis of NIST RM 8545). The uncertainty of the individual δ11B and δ7Li values is given as two standard deviation.

Further funding:* FWF Lise Meitner, Award: M3146-N