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doi:10.2204/iodp.proc.336.106.2012

Inorganic geochemistry

Sediment pore water samples were collected from all four Holes, as well as from two sediment layers recovered during RCB coring in Hole U1382A (see the “Site U1382” chapter [Expedition 336 Scientists, 2012b]), using two different methods: extraction with Rhizon samplers and extraction by squeezing (see the “Methods” chapter [Expedition 336 Scientists, 2012a]). Eighty-nine pore water samples collected using Rhizon samplers were analyzed for dissolved nitrate, phosphate, ammonium, and chlorinity (Fig. F7; Tables T3, T4). Nitrate concentrations are greater than the bottom water value reported by Ziebis et al. (2012). In general, concentrations increase in the shallowest 5 m of the sediment column and remain uniform with depth. Within 5–10 m of the estimated sediment/​basement interface, concentrations generally decrease toward the seawater value. This trend is not observed in Hole U1382B, possibly because the hydrologic basement is at 98.8 meters below sea level (mbsl), 8.8 m deeper than the depth of the initial encounter with a hard layer. After initial contact with this hard layer, drilling progress was fast, similar to that of sediment, to 98.8 mbsf, when drilling progress slowed.

Phosphate values increase above the bottom seawater value in the shallowest few meters of the sediment and then generally decrease downcore. Values decrease to about 1 µM at the sediment/​basement interface. Ammonium values should be viewed with caution considering high dilutions necessary to analyze the samples, which results in high limits of detection. Samples from Holes U1383D and U1383E had a detection limit of 6 µM for ammonium because of the dilution used. Given the nature of potential contamination for ammonium, recorded values are likely high but suggest higher values near the sediment/​basement interface. The second batch (Holes U1382B and 1384A), with a much lower sample dilution before analysis, resulted in much lower concentrations and a detection limit of 1.5 µM.

Chlorinity was analyzed for each of these Rhizon samples but is not reported here. This is because values were sporadic with less discernible trends relative to those determined from measurements based on pore water extracted by squeezing sediment. The sporadic nature of these results probably stems from the samples beings stored frozen, introducing an artifact during the thawing phase.

One hundred and twenty-six samples collected from squeezing sediment were analyzed for chlorinity, pH, alkalinity, and the major, minor, and some trace elements in pore water. In general, our results are consistent with concentrations and trends observed from Ocean Drilling Program Hole 1074A (Shipboard Scientific Party, 1998). For example, chlorinities (and calculated Na) values increase to a maximum and then decrease to bottom seawater values at two sites but remain high at Site 1383, consistent with the diffusion of the glacial–interglacial signal through the sediment column (McDuff, 1985). Similarly, Sr concentrations increase to a maximum; however, values decrease to bottom seawater values in only one of the four boreholes.

Trends for the other major ions in seawater (Mg, Ca, K, S [as sulfate], and alkalinity) are consistent with shallow sediment and cool basement temperature. Aside from the alkalinity, concentrations are little changed from bottom seawater values with potential basement fluid compositions that are several percent different than bottom seawater values. Measured alkalinity, however, increases from the bottom seawater value to a maximum then decreases with depth to values below that in bottom seawater. Shipboard sulfate data are not reported because of analytical issues, and the resulting scatter in the data greatly exceeded that of the shore-based data.

The minor elements Li and B are generally uniform with depth. The other minor elements (Mn, Fe, and Si) are generally reactive. Hole U1382B has a clearly defined Mn maximum in the middle of the sediment column. Mn concentrations are about 1 µM in all of the other samples. The Fe values are generally below detection with several “spikes” that are generally <0.5 µM; only one datum exceeds 1 µM Fe. Dissolved silica concentrations are generally uniform with depth but trend toward higher values near the sediment/​basement interface.

Concentrations of the trace elements V and Rb do not vary downcore from seawater values; however, those of Mo, Cs, Ba, and U do show consistent trends with depth that deviate from the seawater value in at least several of the holes. Concentrations for some of these trace metals are different from seawater values at the sediment/​basement interface.

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