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doi:10.2204/iodp.proc.337.201.2015

Results

Results of the samples collected by the conventional sediment squeezer and Quicksilver probe are in close agreement, suggesting the success of the later sampling technique (Fig. F1). Strontium concentration increases from 1278 to 1989 mbsf with two distinct peaks at 1765 and 1939 mbsf (see the “Site C0020” chapter [Expedition 337 Scientists, 2013c]) (Fig. F1A). Accordingly, the isotope ratios decrease from 0.7086 at 1278 mbsf to 0.7062 at 1958 mbsf (Fig. F1B) and are slightly elevated around 1944 mbsf, which corresponds to the concentration peak at the same depth. Although the lack of measurements between 1959 and 2406 mbsf limits our interpretation, similar Sr concentration and isotope ratios between these two depths suggest a similar fluid source below 1959 mbsf.

To further investigate the source of pore water Sr, we plot the isotope ratios against the reciprocal of concentration in Figure F2. Seawater younger than middle Miocene, the age of our first measurement at 1278.1 mbsf based on the onboard biostratigraphy reconstruction (see the “Site C0020” chapter [Expedition 337 Scientists, 2013c]), has Sr isotope ratios that range from ~0.7088 to 0.7092 (McArthur et al., 2012). The Sr isotope ratios for volcanic materials from northeast Japan range from ~0.7059 to 0.7029 (Oba et al., 1983; Kurasawa et al., 1986; Ohki et al., 1994), a range very close to our measurements at 1978 mbsf. The trend in Figure F2 suggests mixing between seawater younger than middle Miocene and fluid derived from volcanic material alteration. Volcanic glass was commonly observed in cuttings and sediment cores. Smear slide observations confirm the existence of volcanic material (Fig. F1C, F1D). At 1940.44 mbsf, 1 m below the highest Sr concentration with the slightly elevated isotopic signature, a smear slide observation confirms weathered volcanic material (Fig. F1E) (see the “Site C0020” chapter [Expedition 337 Scientists, 2013c]).

Alteration of volcanic material can release various cations, bicarbonate, and silicate into pore water (Wallmann et al., 2008; White et al., 2011). These excess cations and anions not only stimulate the precipitation of authigenic minerals but may also provide the necessary nutrient for microbial life (Torsvik et al., 1998; Izawa et al., 2010). Both the elevated cell count and the geochemical signatures infer the active methanogenesis around the ~1950 mbsf coalbed (see the “Site C0020” chapter [Expedition 337 Scientists, 2013c]), where the Sr concentration and isotope ratios suggest active volcanic material alteration. Solute released during volcanic material decomposition may also be an important controlling factor for the microbial life at such harsh environment.

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