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doi:10.2204/iodp.proc.320321.104.2010

Geochemistry

Sediment gases sampling and analysis

Headspace gas samples were taken at a frequency of one sample per core in Hole U1332A as part of the routine environmental protection and safety monitoring program. All headspace samples had nondetectable levels of methane (C₁; <1 ppmv), with no higher hydrocarbons, consistent with the low organic carbon content of these sediments.

Interstitial water sampling and chemistry

Twenty interstitial water samples were collected using the whole-round squeezing approach (Table T26). Additionally, 43 samples were collected using Rhizon samplers from Sections 320-U1332B-4H-1 through 7H-6 with a sampling frequency of two samples per section (Table T27). These sections were selected for Rhizon sampling because of a decrease in alkalinity revealed by the whole-round samples (Fig. F23). Rhizons were applied as described in "Geochemistry" in the "Site U1331" chapter. Chemical constituents were determined according to the procedures outlined in "Geochemistry" in the "Methods" chapter. In the following, we first describe the overall site geochemistry, combining the squeezed and Rhizon samples into single profiles with depth, and then present a more detailed comparison of squeezed and Rhizon samples in the depth interval where they overlap.

Chlorinity varies relatively little with depth, with values ranging mainly from 556 to 570 mM (Fig. F23). However, chlorinity values reveal a distinct increase from 556 to 565 mM in the uppermost 30 m CSF, potentially reflecting the change from the more saline ocean at the Last Glacial Maximum to the present (Adkins and Schrag, 2003). Alkalinity ranges from 2.3 to 3.4 mM. Alkalinities increase in the uppermost 30 m CSF from 2.3 to 3.1 mM and then drop to a distinct minimum of ~2.3 around 40 m CSF. Below 50 m CSF alkalinities increase steadily toward a value of 3.4 mM in the deepest sample. Sulfate concentrations are relatively constant and near seawater values. Low alkalinities and high sulfate concentrations indicate that organic matter supply is not sufficient to drive redox conditions to sulfate reduction. Dissolved phosphate concentrations are ~2 µM in the shallowest sample, decreasing to below detection limit in the uppermost ~25 m CSF. Dissolved manganese is 8–9 µM from 7 to 11 m CSF, with peak manganese values shallower than the peak dissolved iron value of ~1.0 µM at ~11 m CSF. Because of the high sulfate concentrations, dissolved Ba concentrations are low and relatively homogeneous, with values between 2 and 3 µM.

Concentrations of dissolved silicate increase with depth from ~400 to ~1000 µM. Superimposed on the gradual increase in dissolved silicate with depth is a pronounced minimum at ~80 m CSF. This is slightly deeper than the color change from light to dark that occurs at the lithologic Unit III to Unit IV transition (see "Lithostratigraphy").

Calcium concentrations increase slightly with depth, with values ranging from 10 to 12 mM and a local minimum around 50 m CSF (Fig. F23). Magnesium concentrations are relatively constant, ranging from 50 to 53 mM (Figs. F23).

Lithium concentrations decrease from ~26 µM at the surface to 20 µM near basement, with the strongest decrease apparent between 40 and 50 m CSF. Strontium shows relatively little variation, with concentrations ranging from 77 to 93 µM. Boron concentrations range between 400 and 490 µM, slightly decreasing between 40 and 50 m CSF.

Interstitial water samples derived from Rhizon samplers (Fig. F24) and whole-round squeezing generally show good agreement (Fig. F25). Because these two data sets were collected in different holes (Hole U1332B is 20 m north of Hole U1332A), data are plotted in CCSF-A, to facilitate comparison. Elements that show good agreement with respect to absolute concentrations as well as observed trends include Li, K, and Sr. Magnesium and calcium show similar trends in both data sets but with constant offsets of 1.25 and 0.25 mM, respectively. These correspond to 2.5% of the measured values, and they might be related to day to day variability of the inductively coupled plasma–atomic emission spectroscopy (ICP-AES) analyses. However, Mg/Ca ratios show a good agreement between both sampling techniques, irrespective of the applied corrections for Mg and Ca. Boron concentrations are identical in the upper part of the investigated interval, but Rhizon samples are slightly enriched in boron in the lower part. However, it is unclear if this feature is an analytical or sampling artifact. The same holds true for alkalinity values. Between 28 and 30 m CCSF-A and 50 and 65 m CCSF-A both sampling techniques reveal results indistinguishable within typical analytical precision. Rhizon samples did not reproduce the distinct decrease in alkalinity centered at 40 m CCSF-A obtained from the whole-round samples. Several aspects of Rhizon sampling might be responsible for this. First, Rhizon sampling was only conducted after the core sections were processed through the fast track system. Second, Rhizon sampling pulls a vacuum on the sediment to withdraw fluid. Third, because of work flow imposed by the rapidly acquired samples, alkalinities of the Rhizon samples were not measured directly after retrieval. We expect this third factor to have only small effects in these samples with no/limited sulfate reduction and relatively low alkalinities.

Bulk sediment geochemistry: major and minor elements

At Site U1332, bulk sediment samples for minor and major element analyses were distributed over the complete depth range to target all major lithologic units (0–150 m CSF; Hole U1332A). We analyzed concentrations of silicon, aluminum, iron, manganese, magnesium, calcium, sodium, potassium, titanium, phosphorus, barium, copper, chromium, scandium, strontium, vanadium, yttrium, and zirconium in the sediment by ICP-AES (Table T28).

Bulk sediment SiO₂ ranges between 12 and 75 wt%, with values around 50 wt% from 0 to 40 m CSF and low values (10–20 wt%) between 40 and 65 m CSF. Below 70 m CSF, SiO₂ concentrations vary between 40 and 75 wt%. Concentrations of Al₂O₃ range from 0.5 to 13 wt%, with values decreasing in the upper 60 m CSF from 13 to 0.5 wt%. Below 60 m CSF, Al₂O₃ concentrations are between 0.5 and 2 wt%, with two peaks at 80 and 145 m CSF with values of 6 and 4 wt%, respectively. A similar pattern is displayed by TiO₂ (0.01–0.6 wt%), K₂O (0.25–2.4 wt%), Zr (20–205 ppm), and Sc (1.4–30 ppm).

Iron decreases from 6 wt% Fe₂O₃ at the surface to 0.4 wt% at 55 m CSF. Between 60 and 130 m CSF, concentrations vary between 1 and 5 wt%. Below 140 m CSF, values increase up to >13 wt% (measured value exceeded the calibrated concentration range). Similar trends are shown by MnO (0.03 to >0.2 wt%), MgO (0.03–21 wt%), copper (53 to >140 ppm) and vanadium (60 to >330 ppm). The peak concentrations of Mn, Cu, and V could not be quantified because they exceeded the calibrated range (see Table T9 in the "Methods" chapter).

Calcium (CaO) ranges from 0.4 to 40 wt%, with high values corresponding to the minimum in SiO₂ and Al₂O₃ between 40 and 70 m CSF and at 80 m CSF. Strontium concentrations range from 60 to >700 ppm, showing a similar pattern to CaO.

Bulk sediment geochemistry: sedimentary inorganic and organic carbon

CaCO₃, IC, and TC concentrations were determined on sediment samples from Hole U1332A (Table T29; Fig. F26). CaCO₃ concentrations range between <1 and 90 wt%. In the uppermost ~17 m CSF, CaCO₃ concentrations are very low (<1 wt%), below which concentrations vary up to 70 wt% at depths from ~17 to 32.6 m CSF. From 32.6 to 75.5 m CSF, CaCO₃ concentrations are consistently very high (48–90 wt%), and between 75.5 and 110 m CSF, CaCO₃ concentrations are low, except for a high of ~40 wt% at 98 m CSF. Below 98 m CSF, CaCO₃ concentrations are variable, ranging from 1.2 to 60 wt%. Variations in CaCO₃ concentrations correspond to lithostratigraphic changes (see "Lithostratigraphy").

TOC concentrations were determined separately by a difference method and by an acidification method (see "Geochemistry" in the "Methods" chapter) (Table T29; Fig. F26). TOC concentrations determined by the normal difference method range from <0.1 to 1.6 wt% (Table T29). These values are probably overestimates because they are determined as a small difference between two numbers comparable in magnitude. Therefore, TOC analyses were performed only by the acidification method for the remaining PEAT sites. For Site U1332, we analyzed TOC on carbonate-free sediments after treatment by acidification. We calculated a detection limit of 0.03 wt% for the TC measurements using the acidification technique. TOC concentrations by this acidification method are very low throughout the sediment column, with a range from below the detection limit to 0.18 wt% (Fig. F26). In the uppermost ~2.5 m CSF, TOC concentrations are slightly elevated (0.16–0.18 wt%). TOC concentrations are very low (below detection limit) from 32.6 to 75.5 m CSF, corresponding to the depths where CaCO₃ concentrations are high. Below 95 m CSF, TOC concentrations are slightly higher (0.04–0.05 wt%).

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