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doi:10.2204/iodp.proc.347.108.2015

Geochemistry

Interstitial water

Pore water samples from Site M0064 were recovered from the upper 16 m of the cored interval. For Holes M0064A, M0064C, and M0064D, this interval is composed of diamicton overlain by 7–9 m of glacial lake clays (see “Lithostratigraphy” and “Biostratigraphy”). For Hole M0063B, the lake clays transition into a sand layer at 4.6 mbsf. Low organic matter concentrations in these sediments and the present-day brackish conditions at the site govern the pore water profiles.

Salinity variations: chloride, salinity, and alkalinity

Pore water chloride (Cl^–) and salinity profiles (Fig. F5A–F5B; Tables T3, T4) illustrate good agreement between holes and between salinity measurements made shipboard with a refractometer and derived from Cl^– (Cl^– based salinity; Fig. F5C). Salinity is 13–14 in the shallowest sample at 1.35 mbsf, increases to ~15 at 6–8 mbsf, and then drops again to 11–12 toward the bottom of the holes. Alkalinity values are very low at Site M0064 (2–5 meq/L), with minor differences between holes; in Holes M0064A and M0064D, alkalinity decreases from ~6 meq/L at the surface to 3 meq/L at depth, whereas in Holes M0064B and M0064C alkalinity is relatively constant between 3 and 4 meq/L (Fig. F5D).

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

No methane or hydrogen sulfide measurements were conducted at this site. Pore waters record evidence of only minor organic matter degradation likely due to the overall low concentrations of organic matter in the sediments (see below). Sulfate (SO₄^2–) decreases from 10 mM in the shallowest samples to 5 mM at depth (Fig. F6A). A plot of SO₄/Cl (Fig. F6B) suggests that the SO₄^2– depletion represents sulfate reduction rather than dilution by low-salinity pore waters.

Pore water ammonium (NH₄⁺; Fig. F6C) indicates minor release from organic matter degradation, with values of 0.1–0.3 mM in the shallowest samples increasing to 0.4 mM at depth (Table T3). Pore water phosphate (PO₄^3–) concentrations are low (~0.01 mM) throughout most of the sampled interval, with a peak of 0.05 mM in the uppermost 3 mbsf (Fig. F6D). Both dissolved iron (Fe²⁺) and manganese (Mn²⁺) concentrations are scattered with broad peaks at 5–10 mbsf (Fig. F6E–F6F). Dissolved Fe²⁺ concentrations peak at ~200 µM, whereas in Hole M0064D the deepest sample returns to a higher value of 230 µM. Dissolved Mn²⁺ concentrations peak at 60–70 µM, with values of 20–30 µM in the shallowest samples and 10–25 µM in the deepest samples. pH has a slight and broad minimum of ~7.5 at 5–10 mbsf, with values of ~7.7 at the top and bottom of the sampled interval (Fig. F6G).

Pore water bromide (Br^–) concentrations (Fig. F7A) vary over a narrow range of 0.27–0.35 mM and follow the pattern observed for salinity and Cl^– such that the Br/Cl ratio is almost constant with depth (Fig. F7B), reflecting the seawater ratio. All holes have a similar pattern for boron (B), with surface concentrations of 150–190 µM that decrease to ~60 µM by 6 mbsf and maintain similar concentrations deeper than that depth (Fig. F7C). The shallowest B/Cl ratios are similar to the seawater ratio (Fig. F7D), but underlying ratios indicate uptake of B, possibly by weathering reactions or ion exchange.

Mineral reactions

Sodium, potassium, magnesium, and calcium

Concentrations of Na⁺ are relatively constant with depth and between holes, with a range of 130–240 mM (Fig. F8A; Table T3). Unlike other sites, K⁺ profiles at Site M0064 do not follow the same pattern as Na⁺. Instead, the K⁺ profile is similar to boron, with the highest concentrations of ~4 mM in the shallowest sample and values that decrease to 1.5–2 mM before leveling off deeper than ~6 mbsf (Fig. F8B). Pore water Mg²⁺ profiles are similar to Na⁺, with a narrow range of concentrations spanning 16–23 mM with depth (Fig. F8C). Pore water Ca²⁺ concentrations increase gradually from ~10 mM in the shallowest samples to a broad peak of ~25 mM between 6 and 10 mbsf, depending on the hole (Fig. F8D). Element to Cl^– ratios for the major cation concentrations highlight differences from seawater ratios (Fig. F8E–F8H). Both Na/Cl and Mg/Cl exhibit little variation from seawater values. Like B/Cl, the K/Cl ratios decrease to values below the seawater ratio, again suggesting weathering reactions or ion exchange as a sink. All Ca/Cl ratios plot well above the seawater values, indicating a solid phase source of Ca²⁺ at depth.

Strontium, barium, lithium, and silica

Strontium (Sr²⁺) concentrations are similar to Ca²⁺ profiles, with a gradual increase from 40 to 55 µM in the shallowest samples and a broad peak of ~125 µM at 8–10 mbsf (Fig. F9A). Concentration profiles of lithium (Li⁺) are similar to both Ca²⁺ and Sr²⁺. Li concentrations increase gradually from ~10 µM in the uppermost sample to a broad peak of ~15 µM at 8–10 mbsf (Fig. F9B). Dissolved silica (H₄SiO₄) concentrations increase subtly from ~200 µM in the shallowest sample to ~325 µM at 10 mbsf (Fig. F9C). Pore water barium (Ba²⁺) concentrations are very low at Site M0064, with values of 0.1–0.3 µM (Fig. F9D).

Sediment

Carbon content

The total carbon (TC) content at Site M0064 varies from 0.3 to 3.5 wt% across holes (Table T5; Fig. F10A). The highest total organic carbon (TOC) values (~2.3 wt%) occur in the uppermost ~0.70 m of the investigated profile, suggesting higher organic matter input during the more recent deposition of brackish-marine sediments, whereas the underlying freshwater and glaciolacustrine deposits are characterized by low TOC values of <0.4 wt% (Table T5; Fig. F10B).

The total inorganic carbon (TIC) content at Site M0064 increases with depth in the upper ~12 mbsf, reaching values of ~3.2 wt% (Table T5; Fig. F10C). The TIC content stays slightly elevated in the diamicton (lithostratigraphic Subunit IVa) and drops to an average value of 1.8 wt% deeper than ~20 mbsf.

Sulfur content

The total sulfur (TS) content is generally <0.5 wt% in sediments from the Hanö Bay (Table T5; Fig. F10D), with high values of 0.8–1.5 wt% being present only in the uppermost sediments (<1 mbsf). It is interesting to note that the lowest values of TS occur in the upper 1–7 mbsf (lithostratigraphic Subunit IIIa), where TOC values are slightly higher compared to the deeper sediment intervals.

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