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doi:10.2204/iodp.proc.320321.107.2010

Geochemistry

Sediment gas sampling and analysis

Headspace gas samples were taken at a frequency of one sample per core in Hole U1335A as part of the routine environmental protection and safety monitoring program. All headspace samples resulted in nondetectable levels of methane (C₁; <2 ppmv), with no higher hydrocarbons, consistent with the low organic carbon content of these sediments.

Interstitial water sampling and chemistry

A total of 48 interstitial water samples were collected using the whole-round squeezing method, 45 from Hole U1335A and 3 from Hole U1335B (Table T21). 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 554 to 565 mM (Fig. F19). However, chlorinity values reveal a distinct increase from 554 to 565 mM in the uppermost 40 m CSF, most clearly seen in previous IODP Sites U1332 and U1334, 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 2.5 to 4.3 mM. Sulfate concentrations vary between 23 and 28 mM, with slightly decreasing values with depth. Dissolved phosphate concentrations are ~2 µM in the shallowest sample, decreasing to values ~0.5 µM in the uppermost ~50 m CSF. Dissolved manganese shows three distinct peaks with concentrations of up to 44, 13, and 5 µM between ~0 and 40, 50 and 80, and 150 and 210 m CSF, respectively. Generally, Mn peaks are sharp in the shallower part and broad at greater depth. Dissolved iron spikes to 6 µM at 6 m CSF and peaks with concentrations ~6 µM between 90 and 170 m CSF and between 190 and 370 m CSF. As with manganese, iron peaks become broader with increasing depth. Minima in dissolved Fe correspond to elevated Mn concentrations. The alternating pattern of dissolved Mn and Fe corresponds well to apparent color changes of the sediment (see "Lithostratigraphy").

Because of the relatively high sulfate concentrations, dissolved Ba concentrations are low and relatively homogeneous, with values below 2.2 µM. Concentrations of dissolved silicate increase with depth from ~500 to ~820 µM.

Calcium and magnesium concentrations are relatively uniform, with values ranging from 9.5 to 12 mM and from 44 to 51 mM, respectively (Fig. F19).

Lithium concentrations decrease from ~26 µM at the surface to 5 µM at ~300 m CSF. Lithium strongly increases below 300 m CSF toward basement. Strontium concentrations range between 82 and 250 µM, and this profile mirrors the lithium profile. Values increase from the top toward 200 m CSF, followed by a decrease toward basement. Boron concentrations range between 440 and 490 µM, slightly decreasing from the top to the basement.

Bulk sediment geochemistry: major and minor elements

At Site U1335, bulk sediment samples for minor and major element analyses were distributed over the core depth to characterize the major lithologic units (0–400 m CSF; Hole U1335A). We analyzed concentrations of silicon, aluminum, iron, manganese, magnesium, calcium, sodium, potassium, titanium, phosphorus, barium, copper, chromium, scandium, strontium, vanadium, yttrium, and zirconium by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) (Table T22).

SiO₂ ranges between 6 and 34 wt%, with generally higher concentrations in the upper sediment. Similar patterns to SiO₂ are displayed by Al₂O₃, with concentrations ranging from below detection limit to 3.6 wt%, TiO₂ (0.002–0.2 wt%), K₂O (0.1–1 wt%), Zr (12–90 ppm), and Sc (up to 14 ppm).

Concentrations of Fe₂O₃ vary between 0.4 and 2.7 wt%, following the general pattern of SiO₂. Similar trends are also shown by MnO (0.04 to >0.2 wt%), MgO (0.3–1.5 wt%), copper (below detection limit to >140 ppm), and vanadium (up to 20 ppm). Peak concentrations of Mn and Cu could not be quantified because they exceeded the calibrated range (values in brackets in Table T22).

Calcium (CaO) ranges from 26 to 43 wt%, with high values corresponding to the minima in SiO₂ and Al₂O₃. Strontium concentrations are >700 ppm, showing a similar pattern to that of CaO. Barium concentrations are >566 ppm, and P₂O₅ values range from below detection limit to 0.5 wt%, showing minima at high CaO concentrations.

Bulk sediment geochemistry: sedimentary inorganic and organic carbon

CaCO₃, IC, and TC concentrations were determined on sediment samples from Hole U1335A (Table T23; Fig. F5). Calcium carbonate concentrations ranged between 13 and 96 wt%. In the uppermost ~67 m CSF, CaCO₃ concentrations vary greatly between 12 and 87 wt%. From 67 to 157 m CSF, calcium carbonate concentrations are consistently high (72–92 wt%), and then CaCO₃ concentrations show more variation, ranging between 37 and 89 wt% from 157 to 222 m CSF. Below 222 m CSF, CaCO₃ concentrations are consistently high again (75–96 wt%). Variations in CaCO₃ concentrations correspond to lithostratigraphic changes (see "Lithostratigraphy").

TOC concentrations were determined by acidification (see "Geochemistry" in the "Methods" chapter) (Table T23; Fig. F5) and are very low throughout the sediment column, from below the detection limit to 0.10 wt% (Fig. F5).

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