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doi:10.2204/iodp.proc.347.107.2015

Geochemistry

Interstitial water

Variations in pore water geochemistry at Site M0063 reflect the transition from Holocene brackish marine deposits in the upper ~27 mbsf of the core (Unit I) to the underlying glaciolacustrine sediments (Units II–VII) (see “Lithostratigraphy” and “Biostratigraphy“), microbial degradation of organic matter concentrated in the marine sediments, and mineral reactions that may include dissolution and ion exchange.

Salinity variations: chloride, salinity, and alkalinity

Pore water chloride (Cl^–) concentrations are ~170 mM in the uppermost samples at 1.3 mbsf, increase to 190 mM at 9–14 mbsf, decrease to <20 mM at ~50 mbsf, and continue to decline for the remainder of the profile (Fig. F12A; Table T8). Similar trends are observed in salinity, which from a maximum of ~15 at ~8 mbsf decreases to <2 at ~50 mbsf (Fig. F12B). Peak values for Cl^– based salinity are slightly lower (by ~2) than shipboard salinity measured by refractometer (Fig. F12B; Tables T8, T9). As at Site M0059, this difference can be explained when considering that additional ions related to organic matter degradation may contribute to salinity. Although peak pore water alkalinity concentrations of ~55 meq/L at Site M0063 (Fig. F12C) are a factor of 4 lower than those observed at Site M0059, Cl^– based salinity can be corrected to match shipboard salinity by assuming that HCO₃^– is the primary anion contributing to salinity and calculating anion-based salinity (Fig. F12D). Results imply that HCO₃^– is amplifying pore water salinity in the upper ~35 m of the profile but with less influence than that observed at Site M0059. Thus, Cl^– based salinity is a better estimate of bottom water salinity at Site M0063. It suggests a past period with higher bottom water salinity than today. Scattered salinity and Cl^– values in Hole M0063E, particularly between 48 and 70 mbsf, may indicate contamination from seawater (Table T8). Therefore, salinity and Cl^– data from Hole M0063E are listed in Tables T8 and T9 but are not included in subsequent plots.

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

Measurements of methane (CH₄) concentrations were only done for Hole M0063A (Fig. F13A; Table T10). There was extensive degassing of CH₄ prior to collection, and therefore the CH₄ profile should be viewed as an indicator of presence or absence rather than interpreted as absolute values. Methane is present at all depths to ~74 mbsf. Deeper than this depth, there is a consistent and distinct increase in sulfate (SO₄^2–) concentrations to 1.8 mM at ~102 mbsf, coinciding with the absence of methane (Fig. F13B). Though the source of SO₄^2– is uncertain, this pattern may suggest that SO₄^2– is the dominant electron acceptor at depth (Lovley and Phillips, 1987). Sulfate concentrations in all holes are generally <0.01 mM in the upper 74 m, but there is scatter in some parts of the profile. This scatter is particularly but not exclusively noted in Hole M0063E, suggesting the potential for contamination by seawater. Pore water H₂S was analyzed throughout Hole M0063A and also in one sample from the uppermost part of Holes M0063B and M0063D, with all samples below or at the detection limit. Traces of H₂S with values of 0.001 mM were only detected at ~1.4 mbsf in Hole M0063B.

The profiles for ammonium (NH₄⁺) and phosphate (PO₄^3–) (Fig. F13D–F13E) are similar to the alkalinity profile, suggesting production of these solutes is due to organic matter mineralization. There is considerable scatter in the upper 30 mbsf of the PO₄^3– profile for all holes, but a clear trend is still apparent. As with alkalinity, the maxima in PO₄^3– (1.5 mM) and NH₄⁺ (10 mM) concentrations are centered in the upper 15–20 mbsf, but compared to Site M0059, they are lower by factors of 2 and 4, respectively.

Dissolved iron (Fe²⁺) shows peak concentrations of ~230 µM in the uppermost samples at around 1.5 mbsf and decrease sharply to ~2.5 µM by 5 mbsf (Fig. F13F). A minor peak of ~75 µM is observed by 16 mbsf above a decline back to values of ~10 µM by 24 mbsf. Further downcore, concentrations of dissolved iron increase substantially to result in a maximum of 560 µM at ~35 mbsf before a sharp decline to near 10 µM by ~50 mbsf. There is some variation in the magnitude of the latter dissolved Fe²⁺ peak between holes, but the maximum is distinct in all holes. Deeper than 50 mbsf, in the glaciolacustrine sediments, Fe²⁺ concentrations in all holes slowly decline and are below detection by 65 mbsf and through the rest of the profile.

Generally, the Mn²⁺ profile resembles that of Site M0059, with a trend of rapidly declining pore water concentrations in the upper few meters followed by a more gradual decline starting at 15 mbsf. However, near-surface concentrations of 1200 µM in the uppermost samples (1.5 mbsf) at Site M0063 are twice those observed at Site M0059 (Fig. F13G). Similar to Fe²⁺, Mn²⁺ shows a local minimum around 3–5 mbsf, followed by a broader peak of ~300 µM by ~12 mbsf, and then a decline to 3 µM by the bottom of the profile at 102 mbsf.

pH varies between holes by as much as 0.5 in some portions of the profile, but an overall trend is still apparent (Fig. F13H). In general, pH increases from 7.7 at the top to peak above 8 at 10 mbsf. Deeper than this depth, pH decreases, with a minimum of ~7.5 at 35 mbsf, and then gradually increases to yield a second peak of ~8.5 in the lower part of the profile.

Pore water bromide (Br^–) and boron (B) profiles resemble those of salinity (Fig. F14A, F14C). Ratios of these elements to Cl^– are similar to those for seawater to at least 50 mbsf (Fig. F14B, F14D). Deeper than this depth, increasing and scattered element to Cl^– ratios likely result from very low Cl^– concentrations measured in the deeper core sections.

Mineral reactions

Sodium, potassium, magnesium, calcium, silica, lithium, barium, and strontium

Pore water sodium (Na⁺), potassium (K⁺), and magnesium (Mg²⁺) display trends similar to salinity with a broad peak in the upper portion of the profile followed by a gradual decline in concentrations to very low values around 50 mbsf (Fig. F15A–F15C). Calcium concentrations display two distinct peaks in the upper 50 m: a muted peak of ~5 mM in the upper 20 m and a larger peak of 10 mM at ~40 mbsf (Fig. F15D). Deeper concentrations gradually decline to values <1 mM. When normalized to Cl^–, it appears that bottom water salinity is a major control on Na⁺, K⁺, Mg²⁺, and Ca²⁺ concentrations (Fig. F15E–F15H). Deviations in element to Cl^– ratios from the assumed ratio of the primary bottom water indicate removal or addition to the pore waters through reactions or exchange with the sediment. However, concentrations of the individual elements Na⁺, Mg²⁺, K⁺, Ca²⁺, and Cl^– each become particularly low deeper than 70 mbsf, meaning that subtle changes in the elemental values may be exaggerated in the ratios. However, the increases in Ca/Cl starting near 25 mbsf and in Mg/Cl starting near 35 mbsf are more gradual and well defined than the scattered ratios deeper.

The trend for strontium (Sr²⁺) and lithium (Li⁺) resembles that of alkalinity with a peak centered within the upper 15–20 mbsf followed by a gradual decrease in concentration toward the basal parts of the profile (Fig. F16A–F16B). There is scatter in the upper 40 m of the dissolved silica (H₄SiO₄) profile (Fig. F16C), but an overall trend of gradually decreasing values is observed for all holes. Silica input into pore fluids may be attributed to diatom dissolution or weathering of silicates. As at Site M0059, the shape of the barium (Ba²⁺) profile (Fig. F16D) most closely resembles that of Ca²⁺.

Molybdenum, vanadium, and zirconium

Pore water trace element concentrations of molybdenum (Mo), vanadium (V), and zirconium (Zr) (Table T8) reveal trends that are noteworthy yet less well defined than the elemental profiles discussed above. Mo data reveal scatter in all holes in the upper ~30 mbsf with values ranging from below detection to 5.7 µM but without any observable trend. Deeper than ~30 mbsf, there is a consistent trend between holes with Mo concentrations steadily increasing to values of ~0.43 µM (~4 times that of typical seawater). Vanadium concentrations display two peaks, the first centered between 2 and 5 mbsf and the second centered between 15 and 25 mbsf. Overall, there is a general trend of V concentrations decreasing downcore to values below detection at the bottom. Zirconium concentrations display a single spike centered between 15 and 25 mbsf, similar to the depths of the larger V peak, and a general trend to increasing concentrations of ~110 nM (3 orders of magnitude above typical seawater) at the bottom of the section.

Sediment

Carbon content

Sedimentary total carbon (TC) values varied from 2–10 wt% in the brackish marine deposit to ~30 mbsf and steeply dropped to <1 wt% in the glaciolacustrine sediments deeper (Fig. F17A; Table T11). Most of the TC was present in its organic form with total organic carbon (TOC) values maximizing at 5 mbsf (~7 wt%) and 20 mbsf (~9 wt%). The underlying clays were on the contrary lean in TOC, with values typically not exceeding 0.5 wt% (Fig. F17B). Total inorganic carbon (TIC) also peaked at 5 and 20 mbsf and was low (~0.1 wt%) throughout the rest of the sediment column (Fig. F17C).

Sulfur content

Total sulfur (TS) concentrations were generally high in the brackish marine deposit, with peaks of 3–4 wt% at 5 and 20 mbsf and was only low (<0.1 wt%) in the organic-poor clay starting around 27 mbsf (Fig. F17D). This trend reflects the different salinity stages transitioning from a freshwater to a marine environment with increasing sulfate concentrations.

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