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doi:10.2204/iodp.proc.338.203.2015

Results

Analytical results are listed in Table T1, and depth profiles of B concentration sand δ11B for Sites C0021 and C0022 are shown in Figures F2 and F3, respectively.

At Site C0021, only one sample just below the seafloor had a high B concentration and a low δ11B relative to that of seawater (420 µm; +39.5‰) (Fig. F2). At 80 mbsf, where drilling operations at the site restarted, B concentration matched that of seawater and gradually decreased with increasing depth. The δ11B value was lowest just below the seafloor and increased with increasing depth. No anomalous B distribution was found in mass transport deposits.

At Site C0022, likewise, only one sample just below the seafloor had a high B concentration and a low δ11B relative to seawater (Fig. F3). B concentrations gradually decreased and δ11B values increased with increasing depth. Around the megasplay at ~100 mbsf, both values showed some scatter, which might be caused by local mechanisms (e.g., a submarine microlandslide) which stirred and mixed the sediments to smooth out any variation. Between 100 and 350 mbsf, both values were almost constant.

The fact that at both sites only one sample just below the seafloor had a higher B concentration and lower δ11B than seawater indicates that 10B is preferentially distributed in interstitial waters. The decrease in B concentrations and rise in δ11B values with increasing depth indicate that 10B is preferentially removed from seawater. Similar trends have been observed in interstitial waters from the Costa Rica margin (Kopf et al., 2000) and the southern Okinawa Trough (Huang et al., 2005). At Site C0022, drilling penetrated the megasplay, but we did not find any signals of deep-sourced fluids. This result suggests that the megasplay fault may not currently be active.