IODP Proceedings    Volume contents     Search

doi:10.2204/iodp.proc.346.203.2017

Results

Measured concentrations at Site U1426 ranged 0.09–28.57 mM (S), 0.11–110.9 µM (Ba), and 0.14–32.3 µM (Fe). At Site U1427, these concentrations ranged 0.12–28.53 mM, 0.06–9.24 µM, and 0.15–25.7 µM. The MDLs were 1.2 µM for S, 0.01 µM for Ba, and 0.09 µM for Fe. Blank concentrations were below MDLs for all analytes in every sample. Spiked sample percent error was 1.59%, 2.41%, and 4.11%. Duplicate percent difference ranged 0.01%–34.69%, 0.01%–14.90%, and 0.03–5.19% for S, Ba, and Fe with average percent difference for all duplicates of 5.22%, 5.69%, and 12.55%. Strictly speaking, Fe concentrations should be reported with two significant figures; however, they are listed with three in Table T1 for comparison with shipboard data.

Smooth concentration profiles occur for all three elements at both sites. The maximum percent difference of “depth adjacent” samples, defined as the nearest sample in depth, is <98.18%, 79.17%, and 146.15% for S, Ba, and Fe (averages = 20.84%, 12.56%, and 28.69%). Although this calculation has little quantitative significance, it indicates a high degree of depth continuity, as expected for “diffusion dominated” systems. In short, the concentration profiles exhibit little scatter.

At Site U1426, S concentrations from the mudline to 7 mbsf are similar to SO42– concentrations measured on board the ship as well at Site 798 and piston core KH-77-3-L4; decreasing from ~28 to ~5 mM. Below 7 mbsf, however, S concentrations diverge from those determined on ship but follow the limited values determined at Site 798. Shipboard concentrations approach ~2 mM and roughly stay at this concentration. By contrast, the shore-based concentrations reach submillimolar levels at 9.56 mbsf and decrease to 0.14 mM at 37.39 mbsf. Likewise, Ba concentrations are similar to Site 798; however, shipboard results are 10 times higher.

Site U1427 S concentrations from the mudline to 5 mbsf are similar to SO42– concentrations examined on board the R/V JOIDES Resolution (~28–2.09 mM). Below 5 mbsf, S concentrations decrease to submillimolar concentrations at 6.95 mbsf, but shipboard SO42– results do not decrease to less than 1.83 mM. Site U1427 Ba concentrations are consistently at shipboard concentrations. Fe concentrations at Sites U1426 and U1427 follow shipboard profiles but have higher depth resolution.

Expedition 346 had one of the fastest core flow rates in the history of scientific drilling (~6.3 km of sediment recovered over ~6 weeks; see the “Methods” chapter [Tada et al., 2015a]). Inserting a high-resolution IW sampling program into such a cruise almost predictably led to issues: some IW samples could not be analyzed, and some analyses (although wrong) could not be correct in real time. The shipboard SO42– and Ba data determined for pore water at Sites U1426 and U1427 did not make sense. The SO42– concentrations did not approach zero below the SMT, and the Ba concentrations were astonishingly high. Since the shipboard analyses gave “smooth” profiles, we assume the errors lie with calibration and the accuracy of measurements. We suggest that for S, Ba, and Fe the shipboard data should be replaced with this new data set.