Previous   |    Next

doi:10.2204/iodp.proc.347.105.2015

Geochemistry

At Site M0061, silty and sandy sediments are overlain by ~8 m of clays (see “Lithostratigraphy”). The geochemical concentration profiles at Site M0061 reflect strong variations in depositional conditions in the outer Ångermanälven River estuary.

Interstitial water

Salinity variations: chloride, salinity, and alkalinity

Chloride (Cl^–) concentrations are lowest at the top of the profile with values around 100 mM and increase to ~130 mM at 8 mbsf (Fig. F9A). Deeper concentrations are almost constant with a slight decrease of 3–4 mM between 15 and 20 mbsf. Shipboard salinity measured with a refractometer ranges ~6–10 and shows the same general depth trend as Cl^– based salinity (Fig. F9B–F9C). Shipboard salinity values for Hole M0061C are higher than those for Holes M0061A and M0061B; however, Cl^– based salinities for all holes are very similar (Fig. F9C; Table T7). The salinity concentration profiles show freshwater impact in the surface sediments, likely related to fluvial input.

Alkalinity increases from 13.5 meq/L in the uppermost sample to ~20 meq/L at 5.5 mbsf followed by a gradual decrease to ~6 meq/L at the core bottom (Fig. F9D).

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

Sulfate (SO₄^2–) concentrations from Hole M0061C remain <0.25 mM until ~16 mbsf, where there is a gradual increase to 2.8 mM at 21 mbsf (Fig. F10A). In Holes M0061A and M0061B, SO₄^2– concentrations remain below detection until ~21 mbsf and then gradually rise to 3.8 mM by 27 mbsf. The overall similarity of the SO₄^2– profiles of the three holes suggest that contamination by drilling fluid is probably negligible and the measured SO₄^2– in the deeper sediment intervals may be provided by a subsurface source or a lateral inflow of sulfate-rich waters into the sandy deposits (see “Lithostratigraphy”).

Pore water sulfide (H₂S) was detected only in the uppermost sediment sequence (<8 mbsf) with values not exceeding 5 µM (Fig. F10B). The downhole trend in ammonium (NH₄⁺) is similar to that of alkalinity (Fig. F10C). Values increase from 0.5 mM in the uppermost sample to a maximum value of 1.3 mM at ~6 mbsf before gradually decreasing downhole to 0.24 mM by 27 mbsf. The phosphate (PO₄^3–) concentration profile shows an increase from 0.4 mM in the uppermost sample to a peak of 0.6 mM by 2.5 mbsf (Fig. F10D). Deeper than this maximum, concentrations slowly decline to values of 1–2 mM by ~8–10 mbsf and below detection by 22 mbsf. These results indicate high rates of organic matter degradation in the upper ~8 m of the core.

Dissolved iron (Fe²⁺) and manganese (Mn²⁺) concentrations are elevated near the sediment surface (95 and 250 µM, respectively), followed by a zone of depletion with the lowest values around 5 mbsf for both cations (Fig. F10E–F10F). Iron concentrations then increase to ~350 µM by 12 mbsf, followed by a gradual decrease to <100 µM at the bottom of the profile. Manganese concentrations gradually rise to ~120 µM by 15 mbsf and then decline to ~60 µM at 27 mbsf.

pH is characterized by some variation in the uppermost sediment sequence (<8 mbsf) but generally shows similar trends between holes (Fig. F10G). There is an increase from 7.8 in the uppermost sample to a peak of ~8 between 6 and 8 mbsf. Deeper pH values decline to near 7.6 by 12 mbsf and remain fairly constant for the remainder of the sampled interval.

The profile for bromide (Br^–) largely follows the salinity profile with an increase from 0.16 to 0.21 mM in the upper 6–7 mbsf. Deeper Br^– concentrations show little variation downcore (Fig. F11A). There are only small variations in the Br/Cl ratio (Fig. F11B). The low Br^– concentrations together with low alkalinity values suggest low rates of organic matter degradation at this site. Boron (B) concentrations show a maximum of ~140 µM at ~4 mbsf, followed by a decrease downhole and the absence of B deeper than ~20 mbsf (Fig. F11C). The B/Cl ratio is elevated above the seawater ratio and peaks at 4 mbsf, and then the ratio drops gradually to approach zero deeper than ~20 mbsf (Fig. F11D).

Mineral reactions

Sodium, potassium, magnesium, and calcium

Sodium (Na⁺), potassium (K⁺), and magnesium (Mg²⁺) concentrations all increase with depth in the upper 5–8 mbsf (Fig. F12A–F12C). Deeper concentrations of Na⁺ and K⁺ gradually decline to ~75 and 1.3 mM, respectively, toward the base of the profile. Magnesium concentrations increase from 9 mM at the top to ~13 mM at 5 mbsf, decrease to 10 mM at 15 mbsf, and then rise again to ~13 mM at the bottom of Holes M0061A and M0061B. Note that Mg²⁺ concentrations remain near constant in the lower part of Hole M0061C (Fig. F12C). Calcium (Ca²⁺) concentrations increase steadily with depth from ~2.3 in the upper section to ~24 mM at 27 mbsf (Fig. F12D). The ratios of Na/Cl and K/Cl indicate that both Na⁺ and K⁺ are depleted relative to seawater ratios in sediments deeper than ~10 mbsf (Fig. F12E–F12F). Mg/Cl ratios are near constant (Fig. F12G), whereas those of Ca/Cl increase with depth (Fig. F12H). The high Ca²⁺ and Mg²⁺ concentrations between 10 and 22 mbsf in Hole M0061C compared to Holes M0061A and M0061B are not balanced by an equivalent concentration of anions. This suggests that the data for Hole M0061C should be viewed with caution.

The Ca²⁺ concentration profiles from all three holes clearly show release of Ca²⁺ from sediments. In combination with the Ca/Cl ratios, which display a gradual increase downcore, the Ca²⁺ data indicate a source of calcium in deeper subsurface sediments, including potential Ca²⁺ displacement from solid phases at depth by Na⁺ and K⁺ through ion exchange and/or dissolution of Ca-bearing phases.

Strontium, lithium, silica, and barium

The strontium (Sr²⁺) concentration profile is similar to that of Ca²⁺. Concentrations increase from 16 µM in the topmost pore water samples to 68 µM at the bottom of the profile (Fig. F13A).

Lithium (Li⁺) concentrations increase from ~5 µM in the uppermost section to 8.6 µM near 5 mbsf (Fig. F13B). Deeper concentrations decrease to a value of ~3 µM at 9 mbsf and then increase again to an average value close to 10 µM at the base of the profile.

Dissolved silica (H₄SiO₄) concentrations are elevated in the upper 8 mbsf with a concentration near 700 µM (Fig. F13C). Deeper, there is an abrupt drop of H₄SiO₄ close to 340 µM. Concentrations remain constant for the remainder of the profile. This trend may relate to a dissolution of diatoms, which are present in the upper 8 mbsf (see “Biostratigraphy”).

Barium (Ba²⁺) concentrations increase with depth from 0.4 µM near the surface to ~15–20 µM around 15 mbsf (Fig. F13D). Deeper than this peak, values gradually decrease to the bottom of the profile. Hole M0061C again shows distinct variation from Holes M0061A and M0061B. The Ba²⁺ enrichment with depth may be explained by Ba²⁺ desorption, weathering reactions, or dissolution of barium-containing minerals, which is enhanced when interstitial water sulfate is low.

Molybdenum, vanadium, and titanium

Molybdenum (Mo) and vanadium (V) are highly variable in concentration with depth and between holes (Table T8). Mo concentrations are relatively constant (e.g., 0.04 µM) in the upper 10 mbsf, followed by a maximum value directly deeper (up to 0.15 µM) and a decline toward the base of the profile. V concentrations, in contrast, decrease from 0.18 µM near the surface to values <0.1 µM deeper than 10 mbsf. Dissolved titanium (Ti) concentrations show a small but distinct trend consistent between holes in the uppermost sediments (Table T8). Concentrations of ~0.23 µM are observed at the core top, followed by a constant decline to values <0.01 µM around 9 mbsf.

Sediment

Carbon content

Sedimentary total carbon (TC) values (Fig. F14A; Table T9) reach a maximum of ~2.6 wt% in the upper 4–5 mbsf and then decrease to values below 0.2 wt% at 10 mbsf. The total organic carbon (TOC) profile largely resembles the one of TC, indicating that the majority of carbon is organic (Fig. F14B). The content of total inorganic carbon (TIC) in these sediments is low throughout the sediment column, with only slightly elevated concentrations (up to 0.25 wt%) in the upper ~8 mbsf (Fig. F14C).

Sulfur content

Total sulfur (TS) concentrations (Fig. F14D) follow the trend of the TOC profile with a maximum value of 2.3 wt% at the depth of the TOC maximum (~4.5 mbsf). Contents of TS decrease to low values (<0.2 wt%) deeper than the maximum to ~8 mbsf and stay low downcore. The high TOC and TS concentrations coincide with black bands described in the clays of lithostratigraphic Subunit IIb (see “Lithostratigraphy”) and are likely related to the presence of metal sulfide minerals in these sediment layers.

Top of page   |    Previous   |    Next