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doi:10.2204/iodp.proc.347.111.2015

Geochemistry

Interstitial water

At Site M0067, five samples were taken from a 4 m interval in Hole M0067A and three samples were collected from an 11 m interval in Hole M0067B. Interpretations of the geochemical data (Table T6) are limited because of the low sampling resolution. However, the observed geochemical variations can be linked to lithology represented by an organic-rich sapropel of marine-brackish origin overlying gravel and sand (see “Lithostratigraphy” and “Biostratigraphy”).

Salinity variations: chloride, salinity, and alkalinity

Chloride (Cl^–) and shipboard salinity measured with a refractometer have very similar patterns and limited variability with depth (Fig. F7A–F7B). Chloride concentrations range 330–350 mM with the exception of a lower value of ~300 mM in the deepest sample in Hole M0067A. Shipboard measured salinity and Cl^– based salinity have similar trends representing brackish values of 19–22 (Fig. F7B–F7C). Alkalinity at Site M0067 is roughly an order of magnitude lower than at nearby Site M0059 (Fig. F7D), with the suggestion of a maximum in the upper organic-rich layer of Holes M0067A and M0067B.

Organic matter degradation: sulfate, hydrogen sulfide, iron, manganese, ammonium, phosphate, pH, bromide, and boron

Pore water sulfate (SO₄^2–) concentrations scatter between 2 and 6 mM (Fig. F8A) and are depleted relative to the seawater ratio at this site (Fig. F8B), suggesting degradation of organic matter coupled to SO₄^2– reduction. Further evidence for this comes from sulfide (H₂S) concentrations measured for Hole M0067A, which were as high as 2.2 mM (Fig. F8C; Table T6). Extensive H₂S accumulation leading to formation of iron sulfides provides an explanation for low dissolved iron (Fe²⁺) concentrations that are near the detection limit in the upper part of the sediment. In the underlying sand layer, the only measured Fe²⁺ concentration was 238 µM (Fig. F8D). In contrast to Fe²⁺, Mn²⁺ was present in the upper 5 mbsf at concentrations of 5–16 µM (Fig. F8E). The moderately high alkalinity, which is linked to organic matter degradation in the upper 5 mbsf, is reflected in correspondingly high ammonium (NH₄⁺) and phosphate (PO₄^3–) concentrations averaging ~1.7 mM and 0.2 mM, respectively (Fig. F8F–F8G). pH was around 8 in the upper 5 mbsf and slightly dropped to 7.5 at 11 mbsf. (Fig. F8H).

Pore water bromide (Br^–) and boron (B) have concentrations of ~0.55 mM and 400 µM, respectively, in the upper meters of sediment (Fig. F9A, F9C). The Br/Cl and B/Cl ratios are slightly above the seawater ratio, particularly in the uppermost organic-rich clay (Fig. F9B, F9D).

Mineral reactions

Sodium, potassium, magnesium, and calcium

Pore water sodium (Na⁺), potassium (K⁺), and magnesium (Mg²⁺) are relatively constant with depth, whereas calcium (Ca²⁺) increases slightly from 8 to 12 mM (Fig. F10A–F10D). When normalized to Cl^–, Na/Cl and K/Cl ratios plot near the value for seawater, whereas Mg/Cl is generally above the seawater value and Ca/Cl is consistently above the seawater ratio (Fig. F10E–F10H; Table T7). Together, the element to Cl^– ratios indicate little diagenetic alteration of the bottom water Na⁺ and K⁺ and inputs of Ca²⁺ and Mg²⁺ to the pore water.

Strontium, lithium, dissolved silica, and barium

Strontium (Sr²⁺) and lithium (Li⁺) increase from 20 to 120 µM and from 17 to 21 µM with depth, respectively, whereas dissolved silica (H₄SiO₄) decreases from ~900 to 500 µM with depth (Fig. F11A–F11C). Barium (Ba²⁺) possibly displays a peak of ~5 µM in the organic-rich sediments similar to alkalinity and NH₄⁺ (Fig. F11D).

Sediment

Carbon content

The total carbon (TC) content at Site M0067 varies between 0.2 and 7.6 wt% (Table T8; Fig. F12A). The total organic carbon (TOC) content is comparatively high in the marine-brackish clay of Unit I (0–4.4 mbsf) (Fig. F12B). Deeper than this depth, TOC drops below 0.2 wt%, displaying typical depositional differences between the glaciofluvial to glaciodeltaic deposits and the overlying brackish marine sediments. The total inorganic carbon (TIC) content is low throughout the sediment with values below 1.2 wt% (Table T8; Fig. F12C).

Sulfur content

The total sulfur (TS) content ranges from 0.2 to 2.8 wt%, with low values restricted to the glaciofluvial deposits deeper than 4.4 mbsf (Table T8; Fig. F12D). Similar to the TOC concentrations, the TS values are highest in the marine-brackish deposit of Unit I (upper 4.4 mbsf), indicating increased sulfate reduction rates related to the elevated TOC content and subsequent formation and burial of iron sulfides in the sediments.

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