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doi:10.2204/iodp.proc.347.104.2015

Geochemistry

Interstitial water

Pore water data at Site M0060 largely reflect organic matter degradation that varies with lithology, related mineral reactions, and the presence of fresher water in two deeper sand layers at 80–95 mbsf (Unit IV) and 117–146 mbsf (Unit VI) and the return of more saline water in the deepest diamicton (Unit VII) (see “Lithostratigraphy”).

Salinity variations: salinity, chloride, and alkalinity

The shipboard pore water salinity profiles (Fig. F8A; Table T7) indicate differences between Holes M0060A and M0060B. Salinities in the shallowest sections in both holes are ~30, which is consistent with chloride (Cl^–) measurements and calculations of Cl^– based salinity (Fig. F8B–F8C; Table T8). Largely similar salinity values are found to 70 mbsf in Hole M0060B, followed by a slight decrease to 25 between 70 and 85 mbsf. In contrast, shipboard salinity measurements in Hole M0060A evidence a drop from ~30 to 20 in the uppermost 45 mbsf, followed by a general but less steep decrease in salinity from 20 to 10 to ~120 mbsf. Shore-based Cl^– measurements for this interval are too discontinuous to verify the presence of distinct salinities in the two holes. Sediments deeper than 80 mbsf were only recovered from Hole M0060A. The profile indicates a low-salinity interval that corresponds to the sand layers of Units IV and VI. Deeper than 145 mbsf, salinity increases to ~20 in the underlying diamicton, indicating influence of deep and more saline waters.

Alkalinity measurements again indicate some differences between Holes M0060A and M0060B. In general there is a series of peaks in alkalinity. The most pronounced maximum of ~25 meq/L occurs at 15 mbsf, with smaller peaks of 10–15 meq/L at ~65, 85, 140, and 185 mbsf.

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

Several of the chemical profiles from Site M0060 preserve a record of microbial oxidation of organic matter. No methane is detected shallower than 90 mbsf, but the presence of methane (up to 2.2 mM) at ~100 mbsf and in the diamicton of Unit VII (Fig. F9A; Table T9) points to a deep subsurface zone of methanogenesis. Minima in sulfate (SO₄^2–) in the overlying sediment at 15 mbsf (1.7 mM SO₄^2–) and 60 mbsf (1.2 mM SO₄^2–) suggest two intervals of SO₄^2– reduction (Fig. F9B), although no hydrogen sulfide (H₂S) was detected in either hole (Fig. F9C). As expected, the minima in SO₄^2– correlate to peaks in the products of SO₄^2– reduction, such as alkalinity (Fig. F8D), ammonium (NH₄⁺), and phosphate (PO₄^3–) (Fig. F9D–F9E). The amplitude of peaks in NH₄⁺ and PO₄^3– concentration at 15 mbsf are more pronounced than those at 60 mbsf. Low concentrations of SO₄^2–, alkalinity, NH₄⁺, and PO₄^3– occur from 120 to 140 mbsf in the sand layers of Unit VI, again suggesting a different water composition in this interval. Scattered and relatively low SO₄^2– concentrations ranging from 4 to 13 mM were observed below 150 mbsf (Table T7). Higher concentrations of alkalinity, NH₄⁺, and PO₄^3– in the diamicton are consistent with evidence of methanogenesis in this layer.

Pore water concentrations of dissolved iron (Fe²⁺) peak at 50 mbsf (300 µM), decrease to 45–110 µM in the sands of Unit VI, and are quite scattered (e.g., 0–150 µM) in the diamicton of Unit VII (Fig. F9F). Pore water dissolved manganese (Mn²⁺) concentrations vary generally from 2 to 5 µM with a surface maximum of 10 µM and one high value of 30 µM in the diamicton (Fig. F9G). Variations in pH can be useful for determining the diagenetic reactions leading to the observed variations in pore water concentrations. Shipboard pore water pH is 8 ± 0.25 throughout the cored intervals with a broad minimum of ~7.3 from 20 to 70 mbsf, yet this minimum is only observed in Hole M0060B (Fig. F9H).

Mineral reactions: bromide, boron, sodium, potassium, magnesium, calcium, strontium, lithium, silica, and barium

The most obvious feature in the pore water concentrations of major and minor elements of seawater is the presence of water with a chemical composition distinctly different from seawater in the sand of Unit VI at ~115–145 mbsf. Within this unit, low salinities are associated with lower concentrations of bromide (Br^–), boron (B), sodium (Na⁺), potassium (K⁺), magnesium (Mg²⁺), and lithium (Li⁺) and higher concentrations of calcium (Ca²⁺) and strontium (Sr²⁺) than seawater or the overlying and underlying pore waters (Figs. F10, F11, F12). These differences can also be seen to varying degrees in the element/Cl profiles (Fig. F11E–F11H). With the exception of the pore waters from the sand layers of Unit VI, most of the element/Cl ratios (Table T8) plot close to the seawater ratio. There appears to be evidence for a contribution of diagenetic reactions and/or ion exchange to an increase in pore water Na⁺ and decreases in pore water Mg²⁺ and Ca²⁺ in Unit II (6–24 mbsf). The pore water profile of the upper 30 mbsf is also characterized by a decrease in K⁺ (Fig. F11B) and Rb concentrations (Table T7), which might be related to uptake by clay minerals (Gieskes, 1983).

Concentrations of dissolved silica in pore water are generally determined by the amount and the solubility of sedimentary biogenic silica and silicate minerals. The maxima of dissolved silica at 10 and 60 mbsf (Fig. F12D) are likely associated with dissolution of biogenic silica (see “Biostratigraphy”). Dissolved silica values ranged from 200 to 700 µM in the diamicton. Peaks in the Ba²⁺ profile at 15 and 65 mbsf correlate well with minima in SO₄^2– (Figs. F9B, F12B), suggesting dissolution of barite. As noted for Site M0059, analyses of solid phases will be required to identify the reactions occurring.

Sediment

Carbon content

The total carbon (TC) content at Site M0060 ranges from 0.23 to 3.95 wt% (Table T10; Fig. F13A). Variation in total organic carbon (TOC) content is primarily related to changes in lithology and varies from 0.17 to 0.79 wt% in the greenish gray clays of Units II and III and from 0.01 to 0.21 wt% in the sandy intervals of Units V and VI. Comparatively high TOC values (up to 1.85 wt%) occur in the diamicton of Unit VII and may be caused by the presence of charcoal clasts interbedded in the sandy mineral matrix (see “Lithostratigraphy”).

The total inorganic carbon (TIC) ranges from 0.31 to 3.5 wt% in the uppermost 80 m of the profile, with values reaching a maximum between 63 and 78 mbsf (roughly corresponding to lithostratigraphic Subunit IIIc). Deeper than 120 mbsf, coinciding with the deposition of well-sorted sand (Unit VI) and the sandy diamicton (Unit VII), TIC values are generally low (0.5–1.2 wt%) and show only little variation compared to the overlying sections.

Sulfur content

Total sulfur (TS) values could only be determined for a limited number of samples from Site M0060, and the low resolution does not allow any apparent trends with depth to be recognized. In general, TS contents are low throughout the investigated profile, ranging from 0.14 to 0.25 wt% (Fig. F13D).

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