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doi:10.2204/iodp.proc.320321.103.2010

Geochemistry

Sediment gases sampling and analysis

Headspace gas samples were taken at a frequency of one sample per core in Hole U1331A as part of the routine environmental protection and safety monitoring program. All headspace gas 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-one interstitial water samples were collected using the traditional whole-round squeezing approach (Table T31). In addition, 14 samples were collected using Rhizon samplers from Section 320-U1331B-1H-6 at 10 cm spacing (Table T32). This section, with a visible color change at ~95 cm, corresponding to a change in calcium carbonate content, was chosen as a test of the logistics of this sampling on the JOIDES Resolution. Rhizon samplers were inserted through the core liner at an angle of ~55° along the length of the section so that they would not contact the typically disturbed core material near the core liner on either side (Fig. F21A). The section was positioned so the Rhizon samplers were in the vertical plane, with the sampler tubes exiting toward the top of the section (Fig. F21A). The samplers yielded 12 mL of fluid in the first 30 min, and water flow rate and total yield did not vary with depth in the section (Fig. F21B). We observed cracking in the sediment core as fluid collection proceeded (Fig. F21A), and visual observation raised concern that water near the core liner may be drawn into the sediment with time.

Chemical constituents were determined according to the procedures outlined in "Geochemistry" in the "Methods" chapter. We treated the results from the squeezed samples (Table T31) and Rhizon samples (Table T32) as single profiles with depth. Chlorinity shows relatively little variability with depth, with values ranging mainly from 555 to 565 mM (Fig. F22). Alkalinity ranges from 2.5 to 3.1 mM. Slightly lower alkalinities were observed in the uppermost 30 m CSF and between 80 and 120 m CSF. Sulfate concentrations are relatively constant and near seawater values throughout. 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, below detection limit in all but the uppermost ~25 m CSF. Because of the high sulfate concentrations, dissolved Ba concentrations are low and relatively homogeneous, with values between 1.5 and 2.0 µM. Concentrations of dissolved silicate increase with depth from ~450 to ~800 µM.

Calcium and magnesium concentrations show similar patterns with depth, with values ranging from 9 to 11 mM and 46 to 53 mM, respectively (Fig. F22). Lithium, boron, and strontium concentrations vary between 21 and 28 µM, 410 and 490 µM, and 70 and 82 µM, respectively, have pronounced minima between 10 and 20 m CCSF-A, and subsequently decrease with depth (Fig. F23).

Bulk sediment geochemistry: major and minor elements

Selected bulk sediment samples at Site U1331 were analyzed for major and minor elements to target a distinct lithologic transition (180–190 m CSF in Hole U1331C) at relatively high resolution (~1 m). We analyzed concentrations of silicon, aluminum, iron, manganese, magnesium, calcium, sodium, potassium, titanium, phosphorus, copper, chromium, scandium, strontium, vanadium, yttrium, and zirconium (Table T33) by inductively coupled plasma–atomic emission spectroscopy (ICP-AES). The notable results are discussed below.

SiO₂ ranges between 7 and 80 wt%, showing a general decrease from 180 to 188 m CSF and a slight increase to 190 m CSF. Concentrations of Al₂O₃ range from 0.4 to 11 wt% with peaks around 187 and 189 m CSF, which are separated by a distinct minimum, also present in SiO₂. A similar pattern is revealed by TiO₂ (0.01–0.6 wt%), K₂O (0.25–2 wt%), Zr (18–186 ppm), and Sc (0.6–40 ppm).

Iron and manganese show high concentrations between 186 and 188 m CSF of up to 15 and >0.2 wt% for Fe₂O₃ and MnO, respectively. Similar trends are shown by copper (50 to >140 ppm) and vanadium (0–150 ppm). The peak concentrations of Mn and Cu could not be quantified because they exceeded the calibrated range (Table T33).

Calcium (CaO) ranges from 0.4 to 40 wt%, with the maximum value corresponding to the minimum in SiO₂ and Al₂O₃. Strontium concentrations range from 60 to >700 ppm, showing a similar pattern compared to CaO. Magnesium varies between 0.3 and 9 wt%, with peak values between 186 and 187 m CSF. In summary, the bulk geochemistry of the selected interval of Site U1331 reflects the varying lithology of the sediments.

Bulk sediment geochemistry: sedimentary inorganic and organic carbon

Calcium carbonate (CaCO₃), inorganic carbon (IC), and total carbon (TC) concentrations were determined on sediment samples from Hole U1331A (Table T34; Fig. F24). CaCO₃ concentrations ranged between <1 and 82 wt%. From ~6 to 24 m CSF, CaCO₃ concentrations are high (34–82 wt%), with values <1% in the uppermost ~6 m CSF. From 26 to 70 m CSF, CaCO₃ concentrations are low, with some spikes at 33.8 and ~50–53 m CSF. Higher CaCO₃ concentrations of 16 to 63 wt% are present between 70 and 93 m CSF. Below 94 m CSF, CaCO₃ concentrations are generally low, with higher concentrations of 18–26 wt% present at ~115–119 m CSF. Variations in CaCO₃ concentrations correspond to lithostratigraphic variations (see "Lithostratigraphy").

Total organic carbon (TOC) concentrations were determined by two methods, normal and acidification (see "Geochemistry" in the "Methods" chapter) (Table T34; Fig. F24). TOC concentrations determined by using the normal method range from <0.1 to 1.7 wt%. These values may be overestimates because they are determined as a small difference between two numbers comparable in magnitude. This results in poor precision because of error propagation in very low TOC and high-CaCO₃ sediments. For example, determining a TOC value of 0.2 wt% from a sample with 10 wt% TC, with the TC value having an error of 0.03 wt% (0.3% relative standard deviation [RSD]) and IC values each having a nominal error of 0.04 wt% (0.4% RSD), results in an absolute error for TOC of ±0.05 wt% or 25% RSD. Therefore, we analyzed TOC using carbonate-free sediments after treatment by acidification. TOC concentrations by this acidification method are very low throughout the sediment column, with values ranging from below the detection limit (<0.03 wt%) to 0.05 wt% (Fig. F24).

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