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doi:10.2204/iodp.proc.347.106.2015

Geochemistry

The geochemical concentration profiles at Site M0062, which is located in the Ångermanälven River inner estuary, are very similar to those of Site M0061 and reflect the strong temporal variations in depositional conditions (see “Lithostratigraphy” and “Biostratigraphy”).

Interstitial water

Salinity variations: chloride, salinity, and alkalinity

Chloride (Cl^–) concentrations increase from ~90 to 150 mM from the top to the bottom of the 35.9 m drilled sequence (Fig. F7A). Calculated salinities based on Cl^– concentrations (“Cl^– based salinity”) are consistent with the shipboard salinity, and both range ~6–10 (Fig. F7B–F7C; Table T4). The higher salinities at depth suggest that bottom water salinity was higher in the past than it is at present, corroborating observations at Sites M0059, M0061, and M0063. Alkalinity initially increases with depth and reaches a maximum of 22 meq/L at ~5 mbsf before gradually decreasing downhole to ~6 meq/L (Fig. F7D). The salinity and alkalinity profiles are very similar in magnitude and trend to those observed at Site M0061.

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

Sulfate (SO₄^2–) concentrations are typically <0.1 mM to ~16 mbsf (Fig. F8A). Deeper SO₄^2– concentrations gradually increase to ~5 mM at the bottom of the profile. Sulfide (H₂S) concentrations are below detection throughout the sediment column (Fig. F8B).

Ammonium (NH₄⁺) and phosphate (PO₄^3–) concentrations both reveal peaks near 7 mbsf, slightly deeper that of alkalinity, with peak values of ~0.9 and 0.3 mM, respectively (Fig. F8C–F8D). However, PO₄^3– concentrations sharply decrease to 0.02 mM to 13 mbsf, whereas NH₄⁺ gradually declines to 0.17 mM at the base of the profile. Together, the elevated values for alkalinity, NH₄⁺, and PO₄^3– provide evidence for microbial remineralization of organic matter as a more active process in the upper ~10 m of the profile.

Dissolved iron (Fe²⁺) concentrations sharply decrease from ~1100 µM near the surface to 10 µM by 7.5 mbsf (Fig. F8E). Deeper concentrations again rise to a peak of 400 µM centered at ~13 mbsf before declining to ~34 µM by the bottom of the section. Manganese (Mn²⁺) concentrations decrease from ~800 µM to near 90 µM by 10 mbsf, have a broad maximum around 15–20 mbsf (~220 µM), and further decline to near 50 µM by the bottom of the hole (Fig. F8F). Trends in pore water Fe²⁺ and Mn²⁺ are similar to those observed at Site M0061, although absolute concentrations at Site M0062 are higher. pH shows an increase in the upper 7.5 mbsf to a maximum of ~8.4, followed by a decline to 7.9 at ~15 mbsf, and then again a gradual rise deeper than this depth (Fig. F8G).

The profile of bromide (Br^–) shows a similar trend to that of salinity (Fig. F9A). As for Site M0061, a slight increase in the Br/Cl ratio relative to that of seawater is observed in the upper ~5 m of the sediment (Fig. F9B). Boron (B) concentrations show a maximum of ~140 µM at 7.5 mbsf and then decrease downhole to values <3 µM at ~20 mbsf before they slightly increase again in the lowermost part of the profile (Fig. F9C). Similar to Site M0061, B/Cl ratios show a maximum in the upper 8 mbsf of the profile with values clearly above the ratio of seawater, suggesting release from the solid phase (Fig. F9D).

Mineral reactions

Sodium, potassium, magnesium, and calcium

Sodium (Na⁺) concentrations increase with depth from ~70 to ~120 mM in the upper 10 m of the sediment and then remain relatively constant (Fig. F10A). Potassium (K⁺) and magnesium (Mg²⁺) concentrations both increase in the upper 7–10 mbsf and then decline to 17.5 mbsf before increasing again (Fig. F10B–F10C). Calcium (Ca²⁺) concentrations gradually increase from near 3 to ~22 mM by 20 mbsf and then remain high to the bottom of the profile (Fig. F10D). Concentrations of Na⁺ normalized to Cl^– are close to seawater values all the way down the profile, although with some scatter (Fig. F10E). Ratios of K/Cl suggest release of K⁺ in the upper ~8–10 m of the sediment and removal deeper than that interval (Fig. F10F). Ratios of Mg/Cl are nearly constant and near seawater throughout the profile (Fig. F10G), but those of Ca/Cl increase with depth and are consistently above seawater ratios, suggesting release of Ca²⁺ from solids at depth (Fig. F10H). The trends in K⁺, Mg²⁺, and Ca²⁺ at Site M0062 are very similar to those for Site M0061; however, the Na⁺ profile is distinctly different. Whereas Na⁺ concentrations decrease with depth at Site M0061, they are relatively constant deeper than ~10 mbsf at Site M0062.

Strontium, lithium, dissolved silica, and barium

Strontium (Sr²⁺), lithium (Li⁺), barium (Ba²⁺), and dissolved silica (H₄SiO₄) profiles are very similar to those for Site M0061. Strontium concentrations increase with depth from ~20 to ~50 µM (Fig. F11A). Lithium concentrations show a maximum of 6 µM around ~6 mbsf (Fig. F11B). Deeper concentrations decrease to ~3 µM around 10 mbsf and then increase again to ~8 µM by 20 mbsf. Barium concentrations are near 1 µM to 6 mbsf and then sharply rise to a peak of 18 µM around 15 mbsf (Fig. F11C). Deeper Ba²⁺ values drop back to near 1 µM at 20 mbsf. Dissolved silica concentrations increase from the surface to a peak at ~800 µM over the uppermost 8 m of the sediment (Fig. F11D) and then decline to 300–400 µM after passing through a minimum of ~250 µM at 12 mbsf. The higher H₄SiO₄ concentrations in the upper ~8 m correlate well with the occurrence of diatoms in these layers, suggesting potential dissolution of siliceous microfossils (see “Biostratigraphy”).

Molybdenum, vanadium, and titanium

Pore water molybdenum (Mo) concentrations show a maximum of ~0.6 µM between 10 and 15 mbsf (Table T5). The profile is very similar to that of Site M0061, but maximum concentrations are a factor of 3 higher. Vanadium (V) concentrations decline from ~0.2 µM at the surface to ~0.05 µM by 10 mbsf (Table T5). This same trend and similar concentrations are also observed at Site M0061. Concentrations of titanium (Ti) decrease from near 0.2–0.3 µM at the surface to below detection at 10 mbsf (Table T5) with a profile that is very similar to that of Site M0061.

Sediment

Carbon content

The amount of total carbon (TC) in sediments recovered from Site M0062 varies from 0.11 to 2.03 wt% (Fig. F12A; Table T6). The total organic carbon (TOC) content ranges from 0.06 to 1.86 wt% with elevated concentrations (average = 1.06 wt%) present in the uppermost 11 m of the cored sequence (Fig. F12B), roughly coinciding with the gray laminated silty clays of lithostratigraphic Subunit Ia. The pronounced maximum (1.86 wt%) in TOC at 7.4 mbsf seems to be related to the presence of a distinct black interval containing iron sulfide precipitates (see “Lithostratigraphy”). Deeper than ~17 mbsf and therefore associated with the first occurrence of the fine to medium sands of Unit II, TOC contents remain uniformly low with values <0.1 wt%.

The total inorganic carbon (TIC) content of sediments from Site M0062 stays low throughout the cored sequence and ranges from below detection to 0.17 wt% (Fig. F12C; Table T6). TIC thus constitutes only a minor component (on average 0.07 wt%) of the total carbon pool preserved in sediments of Site M0062. Similar trends in TIC are observed in Holes M0062A, M0062B, and M0062D, whereas notably lower values are found to occur in Hole M0062B.

Sulfur content

Total sulfur (TS) concentrations generally show little variation with depth and average 0.13 wt% (Fig. F12D; Table T6). An exception is found in the depth interval between 6 and 9 mbsf, which is characterized by two greenish black layers with frequent iron sulfide banding (see “Lithostratigraphy”), in which TS values increase to >1 wt% across all holes.

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