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doi:10.2204/iodp.proc.320321.106.2010

Geochemistry

Sediment gases sampling and analysis

Headspace gas samples were taken at a frequency of one sample per core in Hole U1334A 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

Thirty-four interstitial water samples were collected using the whole-round squeezing approach (Table T23; Fig. F23). In addition, 61 samples were taken using Rhizon samplers from Sections 320-U1334B-13H-5 and 13H-6 and 320-U1334C-13H-5 through 23X-3 with a sampling frequency of one sample per section, resulting in a stratigraphic resolution of ~1.5 m (Table T24; Fig. F24). This depth interval was selected for Rhizon sampling to study the profiles of dissolved Mn and Fe of the interstitial water geochemistry revealed by the whole-round samples (Fig. F23) in more detail. Chemical constituents were determined according to the procedures outlined in "Geochemistry" in the "Methods" chapter. In this section, we first describe the overall site geochemistry based on the whole-round samples and then present a more detailed comparison of elements analyzed by squeezed and Rhizon samples in the depth interval of their overlap.

Chlorinity shows relatively little variability with depth, with values ranging mainly from 553 to 566 mM (Fig. F23; Table T23). However, chlorinity values reveal a distinct increase from 553 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 shows little variability with values ranging from 2.7 to 4.0 mM. Sulfate concentrations vary between 24 and 29 mM, with decreasing values in the upper 60 m CSF and higher values below 250 m CSF. Dissolved phosphate concentrations are ~2 µM in the shallowest sample, decreasing to ~0.5 µM in the uppermost ~15 m CSF. Dissolved manganese peaks with concentrations of up to 6 µM between ~50 and 150 m CSF, with peak manganese values (at ~110 m CSF) shallower than the peak dissolved iron value of 6 µM between 150 and 180 m CSF. Because of the relatively high sulfate concentrations, dissolved Ba concentrations are low and relatively homogeneous, with values between 0.8 and 1.5 µM. Concentrations of dissolved silicate increase with depth from ~400 to ~850 µM.

Calcium and magnesium concentrations are relatively uniform, with values ranging from 10.2 to 11.5 and from 50 to 53 mM, respectively (Fig. F23).

Lithium concentrations decrease from ~26 µM at the surface to 15 µM at ~100 m CSF, with the strongest decrease apparent between 10 and 20 m CSF. Lithium strongly increases below 220 m CSF toward basement. Strontium concentrations range between 78 and 107 µM. Values show an increase from the top toward 110 m CSF, followed by a decrease toward basement. Boron concentrations range between 400 and 500 µM, showing a relatively constant decrease from top to basement.

Interstitial water samples derived from Rhizon and whole-round squeezing show good agreement for some elements (Fig. F25). Because these two data sets were collected in different holes, data are plotted in CCSF-A depths to facilitate comparison. In the depth range of overlap, the more frequently sampled Rhizon profiles and the squeezed profiles give comparable absolute values and profile shapes for some elements (Fig. F25), especially when considering the analytical reproducibility of shipboard techniques (see the "Methods" chapter). This includes elements with relatively constant depth profiles (e.g., sulfate and silicate) and those with relatively large concentration changes (e.g., manganese).

The deepest three Rhizon samples were taken in the first three sections of the first core at this hole to be cored with the XCB. The more fragmented nature of the recovered sediments led to Rhizon samples that very rapidly filled with water and to results that appear more contaminated with seawater drilling fluid. Rhizon profiles are noisier, partially because of the greater depth resolution of sampling and the limits of analytical reproducibility. However, some of this variability appears related to actual sampling variability between Rhizons in a single depth profile and between holes regardless of sampling technique. For example, one Rhizon sample shows clear signs of drill fluid contamination as excursions toward seawater values are observed for several elements (alkalinity, silicate, lithium, and strontium in Sample 320-U1334C-15H-1, 75 cm; 161.98 m CCSF-A) (Fig. F25), but this sample was taken in an area of clear drilling disturbance.

We were particularly interested in the iron and manganese profiles, indicative of suboxic oxidation of organic carbon by manganese oxide and iron oxide reduction. The depth zone with high dissolved iron concentrations corresponds to the depth zone of low magnetic susceptibility (Fig. F25) and the tail of the dissolved manganese peak (Figs. F23, F25). We reran the squeezed samples with the Rhizon samples for dissolved manganese and iron, finding generally excellent analytical reproducibility on the replicate runs. The Rhizon and squeezed profiles agree well for dissolved manganese, with some occasional excursions in the Rhizon samples to higher manganese concentrations. The iron profiles also generally agree well. The exceptions are two substantially higher iron values in the squeezed samples in the 160–180 m CCSF-A range, the depth interval of the color change from yellowish gray to greenish gray. This may represent true interhole variability or a sampling artifact.

Bulk sediment geochemistry: major and minor elements

At Site U1334, bulk sediment samples for minor and major element analyses were distributed over the core depth to characterize the major lithologic units (0–280 m CSF; Hole U1334A). 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 sediments by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) (Table T25).

SiO₂ ranges between 7 and 54 wt%, with values decreasing from 50 wt% at the surface to values <10 wt% between 50 and 220 m CSF. Below 220 m CSF, SiO₂ concentrations vary between 7 and 54 wt%, with concentrations below 10 wt% near the basement. Concentrations of Al₂O₃ range from 0.2 to 12 wt%, with values decreasing in the upper 50 m CSF from 12 to <1 wt%. Between 50 and 250 m CSF, Al₂O₃ concentrations are mainly below 1 wt%. Around 250–260 m CSF, Al₂O₃ concentrations slightly increase to 2 wt%. A distribution with depth similar to that of Al is shown by TiO₂ (0.006–0.6 wt%), K₂O (0.1–2.2 wt%), Zr (18–240 ppm), and Sc (0.5–40 ppm).

Concentrations of Fe₂O₃ vary between 0.5 and 10 wt%, following the general pattern of SiO₂. Similar trends are also shown by MnO (0.07 to >0.2 wt%), MgO (0.4–4 wt%), copper (45 to >140 ppm), and vanadium (up to 115 ppm). The peak concentrations of Mn and Cu could not be quantified because they exceeded the calibrated range (Table T24).

Calcium (CaO) ranges from 1 to 42 wt%, with high values corresponding to minima in SiO₂ and Al₂O₃. Strontium concentrations range from 345 to >700 ppm, showing a similar pattern to CaO. Barium and P₂O₅ values range from below detection limit to >566 ppm and 1 wt%, respectively, showing minima at high CaO concentrations.

Bulk sediment geochemistry: sedimentary inorganic and organic carbon

CaCO₃, inorganic carbon (IC), and total carbon (TC) concentrations were determined on sediment samples from Hole U1334A (Table T26; Fig. F6). CaCO₃ concentrations ranged between <1 and 95 wt%. In the uppermost ~16 m CSF, CaCO₃ concentrations are very low (<1 wt%) and then, from 16 to 46 m CSF, vary greatly between <1 and 74 wt%. Carbonate concentrations are consistently high (74–95 wt%), from 46 to 247 m CSF, with a few relatively low concentrations at 57.9, 87.9, 103, and 116.9 m CSF. From 247 to 260 m CSF, CaCO₃ concentrations are low (<1–56 wt%). Below 260 m CSF, CaCO₃ concentrations are variable, ranging between 37 and 86 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 T26; Fig. F6) and are very low throughout the sediment column, with a range from below the detection limit to 0.15 wt% (Fig. F24).

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