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doi:10.2204/iodp.proc.320321.105.2010

Geochemistry

Sediment gases sampling and analysis

Headspace gas samples were taken at a frequency of one sample per core in Hole U1333A as part of the routine environmental protection and safety monitoring program. All headspace sample analyses resulted in 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-five interstitial water samples were collected using the whole-round squeezing approach (Table T25). Chemical constituents were determined according to the procedures outlined in "Geochemistry" in the "Methods" chapter. Chlorinity shows relatively little variability with depth, with values ranging from 557 to 566 mM (Figs. F18). Chlorinity values slightly increase in the upper 50 m CSF and stay relatively constant below. Alkalinity ranges from 1.7 to 4.5 mM. Alkalinities increase in the uppermost 10 m CSF from ~2.1 to 2.8 mM and are relatively uniform to 50 m CSF downcore. Between 55 and 90 m CSF, alkalinity shows first a large increase to 4.5 mM followed by a local minimum of 1.6 mM. Between 130 and 140 m CSF, alkalinities are also reduced. Sulfate concentrations are relatively constant and near seawater values, ranging from 25 to 28 mM. Low alkalinities and high sulfate concentrations indicate that organic matter supply is not sufficient to drive redox conditions to sulfate reduction. The relatively low regeneration of organic carbon is also indicated by low dissolved phosphate concentrations, typically <1 µM. Because of the high sulfate concentrations, dissolved Ba concentrations are low and relatively homogeneous, with values between 1.0 and 1.6 µM. Concentrations of dissolved silicate increase with depth from ~400 to a maximum of ~800 µM at 135 m CSF, with a subsequent decrease to 730 µM.

Calcium concentrations increase slightly with depth, with values from 10 to 12 mM (Fig. F18). Magnesium concentrations are relatively constant, ranging from 50 to 53 mM, with minima around 20 and 120–140 m CSF.

Lithium concentrations decrease from ~28 to 22 µM in the upper 80 m CSF. Below 130 m CSF, Li concentrations increase again toward basement, except for the deepest sample. Strontium concentrations vary between 83 and 105 µM, showing an overall increase with depth and distinctly reduced concentrations between 130 and 140 m CSF. Boron concentrations range between 422 and 485 µM, showing reduced values between 130 and 140 m CSF.

Bulk sediment geochemistry: major and minor elements

At Site U1333, bulk sediment samples for minor and major element analyses were distributed over the core depth to characterize the major lithologic units (0–180 m CSF; Hole U1333A). We analyzed concentrations of silicon, aluminum, iron, manganese, magnesium, calcium, sodium, potassium, titanium, phosphorus, barium, copper, chromium, scandium, strontium, vanadium, yttrium, and zirconium (Table T26) in the sediment by inductively coupled plasma–atomic emission spectroscopy (ICP-AES).

SiO₂ ranges between 6 and 75 wt%, with values around 45 wt% in the top few meters and lower values (5–20 wt%) between 5 and 105 m CSF. Below 105 m CSF, SiO₂ concentrations vary mainly between 20 and 75 wt%, with concentrations <10 wt% near the basement. Concentrations of Al₂O₃ range from 0.2 to 6 wt%, with values decreasing in the upper few meters from 6 to <1 wt%. Below 5 m CSF, Al₂O₃ concentrations vary between 0.2 and 3 wt%. A distribution with depth similar to that of Al is shown by TiO₂ (0.01–0.3 wt%), K₂O (0.1–1.2 wt%), Zr (16–126 ppm), and Sc (up to 19 ppm).

Concentrations of Fe₂O₃ vary between 0.3 and 5 wt%, following the general pattern of SiO₂. Similar trends are also shown by MnO (0.04 to >0.2 wt%), MgO (0.3–2 wt%), copper (44 to >140 ppm), and vanadium (130 to >330 ppm). Peak concentrations of Mn, Cu, and V could not be quantified because they exceeded the calibrated range (Table T26).

Calcium (CaO) ranges from 0.5 to 40 wt%, with high values corresponding to the minimum in SiO₂ and Al₂O₃. Strontium concentrations range from 130 to >700 ppm, showing a similar pattern to CaO.

Bulk sediment geochemistry: sedimentary inorganic and organic carbon

CaCO₃, inorganic carbon (IC), and total carbon (TC) concentrations were determined on sediment samples from Hole U1333A (Table T27; Fig. F19). CaCO₃ concentrations ranged between <1 to 96 wt%. In the uppermost ~4 m CSF, CaCO₃ concentrations are relatively low (26–69 wt%) and then, from 4 to 35 m CSF, vary between 58 and 93 wt%. CaCO₃ concentrations are consistently high (76–96 wt%) from 35 to 111 m CSF. From 111 to 171 m CSF, CaCO₃ concentrations exhibit large fluctuations ranging from <1 to 74 wt%. In the basal section (173–180 m CSF), CaCO₃ concentrations are high (76–90 wt%). Variations in CaCO₃ concentrations correspond to lithostratigraphic changes (see "Lithostratigraphy").

Total organic carbon (TOC) concentrations were determined by acidification (see "Geochemistry" in the "Methods" chapter) (Table T27; Fig. F19). TOC concentrations determined using the acidification method are very low throughout the sediment column, with a range from below the detection limit to 0.05 wt% (Fig. F19).

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