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doi:10.2204/iodp.proc.347.109.2015

Geochemistry

Interstitial water

At Site M0065, freshwater and glaciolacustrine deposits are overlain by ~9 m of brackish-marine sediment (see “Lithostratigraphy” and “Biostratigraphy”). Because of safety reasons, the upper ~2 m of the sediment sequence was not sampled (see “Operations”). The brackish-marine deposit in this area is relatively thin compared to those at Sites M0059 and M0063, where they extend to ~47 and 30 mbsf, respectively. The pore water composition reflects a rise in salinity due to the transition from freshwater to brackish-marine conditions (Table T6).

Salinity variations: chloride, salinity, and alkalinity

Concentrations of chloride (Cl^–) are highest near the surface at ~250 mM and then decline to ~20 mM at depth (Fig. F8A). Pore water salinities from shipboard measurements determined with a refractometer and calculated from Cl^– concentrations show good agreement, with salinities derived from both measurements declining from ~15 to 1 across the sampled interval (Table T7; Fig. F8B–F8C). Alkalinity shows a broad maximum of ~40 meq/L from 3 to 10 mbsf, followed by a decline with depth to values of <5 meq/L (Fig. F8D). These results are in line with a transition from freshwater to brackish-marine conditions.

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

Methane (CH₄) is present to ~36 mbsf in the sediment (Fig. F9A; Table T8). Similar to Sites M0059 and M0063, observed scatter in CH₄ concentrations with depth is probably due to degassing upon core recovery, in particular shallower than ~25 mbsf. Deeper CH₄ concentrations gradually decrease following a smooth profile, suggesting that in this part of the sediment the measurements may reflect actual methane concentrations. Sulfate (SO₄^2–) concentrations in the pore water are generally <0.5 mM (Fig. F9B). The presence of sulfide (H₂S; ~0.7 mM) at depths shallower than ~5 mbsf suggests active SO₄^2– reduction in the upper part of the sediment (Fig. F9C). Pore water profiles of ammonium (NH₄⁺) and phosphate (PO₄^3–) (Fig. F9D–F9E) follow the general trend of alkalinity (Fig. F8D), which is consistent with organic matter degradation as a dominant control. Dissolved iron (Fe²⁺) and manganese (Mn²⁺) show distinct maxima in the pore water (Fig. F9F–F9G). Dissolved Fe²⁺ is mostly present in the former lake sediments (with concentrations as high as ~1000 µM), whereas dissolved Mn²⁺ is largely restricted to the brackish-marine sediments and peaks at ~250 µM. Pore water pH decreases from a value of ~8.1 near the surface to a broad minimum of ~7.5 at 17–25 mbsf. Deeper than this depth, pore water pH increases to a value of ~8 and then decreases to ~7.5 again.

The depth profile of dissolved bromide (Br^–) is similar to the chloride (Cl^–) profile, although the profile of Br/Cl does reveal a slight enrichment in pore water Br^– relative to Cl^– with depth (Fig. F10A–F10B). This increase may be linked to release of Br^– during the mineralization of marine organic matter. Dissolved boron (B) decreases from ~375 to 35 µM with depth (Fig. F10C). The maximum in B/Cl in the upper 5–10 m of the sediment may be indicative of release of B from the brackish-marine sediments (Fig. F10D).

Mineral reactions

Sodium, potassium, magnesium, and calcium

Depth profiles of sodium (Na⁺), potassium (K⁺), and magnesium (Mg²⁺) resemble those of chloride (Cl^–), suggesting strong control by seawater (Fig. F11A–F11C). The depth profile of Ca²⁺ is distinctly different, however, with a maximum concentration at ~20 mbsf (Fig. F11D). In the upper 10–20 m of the sediment, ratios of Na/Cl, K/Cl, and Mg/Cl (Table T7) are elevated relative to seawater, suggesting release of these cations from the sediment (Fig. F11E–F11G). Low Na/Cl, K/Cl, and Mg/Cl ratios relative to seawater values deeper than 10–20 mbsf may suggest removal of cations to a solid phase. Deeper than ~30 mbsf, salinities are so low that comparison of the ratios is no longer meaningful. Ratios of Ca/Cl in the pore water are always higher than the seawater Ca/Cl ratio and increase strongly downcore. This profile suggests that Ca²⁺ is released from the sediment to the pore water (Fig. F11H), possibly through ion exchange and/or mineral dissolution.

Silica, lithium, barium, and strontium

Concentrations of dissolved silica (H₄SiO₄) and lithium (Li⁺) are elevated in the upper 10–15 m of the sediment (Fig. F12A–F12B). This is possibly the result of enhanced mineral weathering or, for H₄SiO₄, dissolution of diatoms in the brackish-marine sediments. A large peak in barium (Ba²⁺) concentrations is observed between 10 and 20 mbsf, followed by a decline with depth (Fig. F12C). This trend is similar to that observed at Site M0059 and may be due to release of Ba²⁺ from the sediment through ion exchange or mineral dissolution linked to the intrusion of seawater into former freshwater sediments. Pore water strontium (Sr²⁺) shows a maximum concentration at 20 mbsf, possibly also reflecting release from solid phases (Fig. F12D).

Sediment

Carbon content

The total carbon (TC) content at Site M0065 varies from ~0.4 to 4.4 wt%, with high values in the topmost part of the profile and a pronounced minimum at the transition from freshwater to brackish-marine conditions (Table T9; Fig. F13A). Highest total organic matter (TOC) values (~4 wt%) are observed in the uppermost ~10 m of the investigated profile, possibly suggesting enhanced primary productivity and preservation of organic matter during the deposition of the brackish-marine sediments. The underlying freshwater and glaciolacustrine deposits are characterized by low TOC values that typically do not exceed 0.5 wt% (Table T9; Fig. F13B).

The depth profile of the total inorganic carbon (TIC) content shows minimum values (<0.7 wt%) in the uppermost 15 m of the sediment profile (Table T9; Fig. F13C). Deeper than this depth, the TIC content steadily increases, resulting in values above 3 wt% at the base of the investigated sequence.

Sulfur content

The total sulfur (TS) content ranges from 0.1 to 1.7 wt% (Table T9; Fig. F13D). The generally high TS (>1%) values in the brackish-marine deposits at the core top might be a result of sulfate reduction and the subsequent formation of iron sulfides in the sediments. In contrast, the underlying freshwater and glaciolacustrine sediments, which are characterized by only low sulfate concentrations, generally have TS values varying between 0.10 and 0.14 wt%. Note that the TS content, similar to the TOC content, slightly increases deeper than ~40 mbsf.

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