Previous   |    Next

doi:10.2204/iodp.proc.347.110.2015

Geochemistry

Interstitial water

At Site M0066, freshwater and glaciolacustrine deposits are overlain by <2 m of brackish-marine sediment (see “Lithostratigraphy” and “Biostratigraphy”). Because of safety issues, the upper ~2 m of the sediment was not sampled (see “Operations”). The chemical composition of the pore water (Table T2) is consistent with more limited input of organic matter and lower rates of sediment accumulation when compared to the nearby Site M0065.

Salinity variations: chloride, salinity, and alkalinity

Concentrations of chloride (Cl^–) are highest near the surface at ~170 mM and then decline to ~25 mM deeper than 20 mbsf (Fig. F3A; Table T2). Pore water salinity from shipboard measurements with a refractometer and as calculated from Cl^– concentrations are in close agreement and decline with depth from ~11 to 2 (Table T2; Fig. F3B–F3C). Given the limited thickness of the brackish-marine deposit, the penetration of Cl^– into the sediment at this site largely relies on molecular diffusion. Alkalinity is relatively constant with depth at ~4 meq/L (Fig. F3D).

Organic matter degradation: sulfate, sulfide, ammonium, phosphate, and bromide

Sulfate (SO₄^2–) penetrates to ~10 mbsf (Fig. F4A). Methane and sulfide were not analyzed at this site. Pore water ammonium (NH₄⁺) and phosphate (PO₄^3–) concentrations are relatively low, ~0.15 and 0.01 mM, respectively, around 3 mbsf and show only a minor but consistent decline with depth (Fig. F4B–F4C; Table T2). Dissolved iron (Fe²⁺) and manganese (Mn²⁺) concentrations are highest in the upper 5–10 m of the sediment, with maxima of ~140 and 40 µM, respectively (Fig. F4D–F4E). Pore water pH decreases in the upper part of the sediment and then increases and/or stays relatively constant deeper than ~10 mbsf in the different holes (Fig. F4F). Concentrations of NH₄⁺, PO₄^3–, and alkalinity are more than an order of magnitude lower at Site M0066 compared to Site M0065. This difference can be explained by the thinner brackish-marine deposit and associated lower rate of organic matter input and degradation at Site M0066.

The depth profile of dissolved bromide (Br^–) is similar to the Cl^– profile (Fig. F5A). The profile of Br/Cl does reveal a slight enrichment in pore water Br^– relative to Cl^– with depth (Fig. F5B; Table T3). Dissolved boron (B) concentrations decrease from 80 to 40 µM between 3 and 8 mbsf and stay relatively constant with depth deeper than 8 mbsf (Fig. F5C). Concentrations of boron at Site M0066 are distinctly lower than those measured at nearby Site M0065. Ratios of B/Cl increase with depth (Fig. F5D). However, given the low salinities at depth, this trend should be interpreted with caution.

Mineral reactions

Sodium, potassium, magnesium, and calcium

Profiles of sodium (Na⁺), potassium (K⁺), and magnesium (Mg²⁺) are similar to those of Site M0065, but concentrations are lower and maxima are limited to a more narrow depth zone near the sediment surface (Fig. F6A–F6C). Slightly elevated concentrations of calcium (Ca²⁺) are observed in the upper ~3–8 m of the sediment column when compared to deeper layers (Fig. F6D). Ratios of Na/Cl, K/Cl, and Mg/Cl are either at or below those for seawater, whereas those for Ca/Cl are consistently above the seawater ratio (Fig. F6E–F6H; Table T3). Given the low salinity of the pore water at this site, care should be taken in interpreting these ratios within the context of reactions.

Silica, lithium, barium, and strontium

Concentrations of dissolved silica (H₄SiO₄) and lithium (Li⁺) in the pore water are largely constant with depth and vary around 400 and 10 µM, respectively (Fig. F7A–F7B). Concentrations of barium (Ba²⁺) are generally low (<2 µM) but show a gradual increase with depth (Fig. F7C). Pore water strontium (Sr²⁺) concentrations slightly decrease with depth from ~40 to 27 µM (Fig. F7D). The contrasting results for Site M0066 compared to Site M0065 indicate differences in mineral reactions linked to differences in salinity and organic matter input.

Sediment

Carbon content

The total carbon (TC) content at Site M0066 varies between 0.6 and 3.2 wt% with maxima around 6 mbsf and between 17 and 20 mbsf (Fig. F8A; Table T4). The total organic carbon (TOC) content is generally low, with values ranging from 0.01 to 0.54 wt% and highest values observed at the core top (Fig. F8B). A second maximum with TOC values of up to 0.3 wt% is present in an interval from 16 to 22 mbsf. Note that any brackish-marine sediment that may have been present in the uppermost 2 m of the investigated profile was not sampled.

The total inorganic carbon content (TIC) varies from 0.5 to 3.0 wt% (Fig. F8C; Table T4), indicating that the major part of the carbon at Site M0066 is present in inorganic form. Similar to the TC profile, two maxima in TIC are observed at 6 mbsf and between 16 and 22 mbsf. Deeper than 22 mbsf, the TIC content gradually declines to values around 1 wt% close to the base of the investigated profile.

Sulfur content

The total sulfur (TS) content ranges from 0.1 to 0.7 wt% and shows a trend of increasing values with depth. Between 18 and 21 mbsf, TS values reach a maximum of 0.7 wt% (Fig. F8D; Table T4). Note that this maximum in sulfur content roughly correlates with that in TOC content, suggesting that sulfur in the deeper sediment may be largely incorporated in organic matter.

Top of page   |    Previous   |    Next