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doi:10.2204/iodp.proc.347.103.2015

Geochemistry

Interstitial water

At Site M0059, freshwater and glaciolacustrine deposits are overlain by ~47 m of brackish-marine sediment (see “Lithostratigraphy”). The pore water composition at this site reflects the associated strong variations in bottom water salinity during the deposition of individual sediment units. The pore water composition also records large changes in the input and degradation of organic matter in the sediment and reveals associated changes in the dissolution and precipitation of minerals.

Salinity variations: salinity, chloride, and alkalinity

Concentrations of chloride (Cl^–), a conservative element in seawater (Fig. F13A), are high and relatively constant at ~380 mM in the upper 15 m of the sediment. This suggests relatively minor change in bottom water salinity during deposition of the more recent sediments. Cl^– concentrations then decline with depth to a minimum of 115 mM at 70 mbsf. This trend with depth is the net result of the increase in bottom water salinity associated with the transition from the freshwater to brackish-marine system and downward diffusion of Cl^– into the freshwater sediment. Deeper than 70 m, Cl^– concentrations rise again slightly, possibly suggesting a source of seawater from below.

Pore water salinity can be calculated from Cl^– concentrations (Table T7) when assuming that seawater with a salinity of 35 has a Cl^– concentration of 558 mM. Comparison of Cl^– based salinity to shipboard measurements of salinity analyzed with a refractometer (see “Geochemistry” in the “Methods” chapter [Andrén et al., 2015]) illustrates that dissolved salts other than those typically present in seawater contribute to salinity in the upper 50 m of the sediment (Fig. F13B). Deeper than 50 mbsf, Cl^– based salinity and shipboard measurements of salinity are largely similar. These results can be explained when considering that organic matter degradation and associated processes lead to enrichment of cations and alkalinity in pore water (e.g., Wallmann et al., 2008). These ions contribute to the salinity of the pore water. At Site M0059, alkalinity is exceptionally high in the upper ~50 m of the sediment, with a broad maximum of 200 meq/L centered around 20–40 mbsf (Fig. F13C). Cl^– based salinity can be corrected for additional anions by assuming that HCO₃^– is the dominant anion contributing to alkalinity. The resulting “anion-based salinity” (Fig. F13D) is similar to shipboard measurements of salinity by refractometry (Fig. F13B). Thus, whereas Cl^– based salinity results from the bottom water salinity during deposition of the sediment and the subsequent diffusive modification, the actual salinity of the pore water at present is significantly higher because of sediment diagenesis.

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

Microbial mineralization of organic matter leads to distinct changes in the geochemistry of pore water, often reflecting a combination of primary water column geochemistry during deposition and both past and ongoing diagenetic sedimentary alterations. Because of the highly transient nature of diagenesis at this site, the sequence of appearance of solutes with sediment depth differs from that typically observed in marine surface sediments (Froelich et al., 1979). At present, methane occurs in the sediment from the uppermost depth of measurement at 1.5 mbsf to 84 mbsf (Fig. F14A; Table T8). The measured methane concentrations are, however, not representative of actual methane concentrations because of the extensive degassing during core collection. The effect of degassing is probably greatest for the upper 65 m of the profile, whereas the drop in concentration deeper than 70 mbsf is probably real. Most of the methane likely formed in the brackish-marine sediments and diffused downward into the glaciolacustrine sediments.

Sulfate concentrations decrease from near 1.5 mM in the uppermost section to below detection at 2.2 mbsf (Fig. F14B). In this uppermost section (<0.8 mbsf), hydrogen sulfide (H₂S) is also present in the pore water (up to 2.45 mM), indicating active sulfate reduction in the near-surface sediments (Fig. F14C). Deeper than this, H₂S was below detection. Sulfate concentrations sporadically increase to 1.71 mM until 80 mbsf and deeper than this depth are as high as 10 mM. The cause of these relatively high sulfate concentrations remains unclear, but they may indicate occasional contamination of the sediment by seawater and/or drill fluid penetration during drilling (Table T9).

Pore water profiles of ammonium (NH₄⁺) and phosphate (PO₄^3–) (Fig. F14D–F14E) follow the general trend in alkalinity (Fig. F13D) with higher concentrations in the upper 50–60 mbsf. Both NH₄⁺ and PO₄^3– are products of organic matter degradation. Similar to alkalinity, they were largely formed in the brackish-marine sediments and diffused down into the glaciolacustrine sediments.

Pore water concentrations of dissolved iron (Fe²⁺) and manganese (Mn²⁺) vary strongly throughout the sediment column (Fig. F14F–F14G). Dissolved Fe²⁺ concentrations are elevated in the glaciolacustrine sediments (up to 1500 µM) deeper than 47 mbsf, whereas Mn²⁺ pore water concentrations are highest in the upper 47 m of the sediment, which represents the brackish-marine interval. Deeper than 47 mbsf, two distinct zones in the Mn²⁺ concentration profile are distinguished: (1) 50–60 mbsf, where pore water Mn²⁺ concentrations are very low (a few micromolar), and (2) deeper than 60 mbsf, where Mn²⁺ concentrations are again slightly elevated. The exact processes leading to the production of dissolved Fe²⁺ and Mn²⁺ in these sediments remain to be explored.

Pore water pH can be of value for the identification of the various diagenetic processes active in sediments (Soetaert et al., 2007). At this site, pore water pH varies greatly between holes. However, pore water pH is generally higher in the brackish-marine sediments than in the underlying freshwater and glaciolacustrine sediments (Fig. F14H).

Mineral reactions

Bromide, boron, sodium, potassium, magnesium, and calcium

Comparison of profiles of the major and minor ions to those for Cl^– can provide insight into the role of reactions in the sediment versus changes in contributions from seawater. Ratios of these ions to Cl^– are particularly useful because elevated (or depleted) ratios relative to those for seawater are directly indicative of production (or removal) of ions. The depth profile of dissolved bromide (Br^–) (Fig. F15A) does not follow the exact trend of Cl^– (Fig. F13A) but instead is rather similar to that of salinity (which includes the ionic products of organic matter degradation). The Br/Cl ratios indeed indicate excess Br^– in the upper 70 m of the sediment column relative to seawater (Fig. F15B). Bromide is known to be incorporated in marine organic matter, and thus the excess Br^– is likely released from organic matter during degradation. The boron (B) profile (Fig. F15C) largely resembles that of alkalinity (Fig. F13C). Ratios of B/Cl reveal that, similar to Br^–, there is evidence for release of B in the brackish-marine sediments (Fig. F15D).

Sodium (Na⁺), potassium (K⁺), magnesium (Mg²⁺), and calcium (Ca²⁺) are all major components of seawater. Their concentrations in pore water may be altered by processes such as ion exchange, mineral weathering, and formation of new minerals. Trends in Na⁺, K⁺, and Mg²⁺ with depth resemble those of Cl^–, indicating the influence of seawater on the pore water chemistry in the upper ~50 mbsf (Fig. F16A–F16C). The Ca²⁺ profile is distinctly different and shows a concentration maximum deeper than 50 mbsf where the other profiles are at minima (Fig. F16D).

Ratios of the four cations to Cl^– indicate that their concentrations are all impacted by reactions in the sediment. In the upper 50–60 mbsf of the profile, ratios of Na/Cl, K/Cl, and Mg/Cl are elevated relative to those of seawater (Fig. F16E–F16G). This indicates release of Na⁺, K⁺, and Mg²⁺ from the sediment, possibly through desorption and exchange with pore water NH₄⁺ and through weathering of silicate minerals (e.g., Wallmann et al., 2008). Deeper than 50–60 mbsf, ratios of Na/Cl, K/Cl, and Mg/Cl are lower than those in seawater, suggesting removal to solid phases. Possible reactions include ion exchange and, for K⁺ and Mg²⁺, removal through formation of new clay minerals. The profile of rubidium (Rb), a trace element in the pore water that may be released from sediment solids through ion exchange, is very similar to that of K⁺ (Table T9).

Ratios of Ca/Cl in the pore water are always equal to or higher than those measured in the seawater (Fig. F16H). Two distinct zones of net production of Ca²⁺ can be distinguished. The first, less prominent zone occurs in the upper 15 m of the sediment, whereas the second zone is found at 40–80 mbsf, where Na/Cl and Ca/Cl profiles show opposite trends. Concentrations of Ca²⁺ in the upper 50 m of the sediment may be controlled by a combination of ion exchange, mineral precipitation, and dissolution reactions. In sediment deeper than 47 mbsf, the release of Ca²⁺ is possibly due to cation exchange with seawater Na⁺. In freshwater sediments, Ca²⁺ is typically the dominant adsorbed ion on clay particles, and exposure to seawater is known to lead to exchange with Na⁺ (e.g., Burdige, 2006).

Silica, lithium, barium, and strontium

Dissolved silica (H₄SiO₄) is typically released to pore water through dissolution of diatoms and silicate minerals. Lithium (Li⁺) is also released through mineral weathering and may be removed through formation of authigenic mineral phases and adsorption to sediment particles. Concentrations of both H₄SiO₄ and Li⁺ are elevated in the upper ~50 m of the sediment (Fig. F17A–F17B), but the shapes of the profiles are very different. Dissolved H₄SiO₄ is highest (~1500 µM) near the sediment surface and then is largely constant at ~1100 µM throughout the brackish-marine sediments, likely suggesting that H₄SiO₄ is released to the pore water through dissolution of diatoms present in the sediment (see “Biostratigraphy”). The depth trend in dissolved Li⁺ in the upper 50 mbsf is similar to what is observed for the major cations Na⁺, K⁺, and Mg²⁺ (Fig. F16A–F16C). Deeper than ~50 mbsf, concentrations of dissolved H₄SiO₄ and Li⁺ are typically lower, with distinct minima around 60 mbsf.

Dissolved barium (Ba²⁺) in pore water is often the result of the dissolution of barite (barium sulfate mineral). It can also be released from minerals through dissolution or ion exchange. Pore water concentrations of Ba²⁺ are relatively low in the upper 50 m of the sediment column, mainly varying from 5 to 10 µm (Fig. F17C). A sharp peak in Ba²⁺ concentrations is observed between 50 and 60 mbsf, followed by a decline at greater depths. This profile may indicate release of Ba²⁺ from minerals linked to the intrusion of seawater into the former freshwater sediments. Similarly, pore water concentrations of strontium (Sr²⁺) are relatively constant throughout the upper ~50 m with a first maximum between 50 and 60 mbsf (Fig. F17D), possibly also reflecting release from solid phases. However, in contrast to Ba²⁺, there is also a strong increase in dissolved Sr²⁺ deeper than 75 mbsf. Again, analyses of the solid phases are necessary to elucidate the processes responsible for these trends with depth.

Sediment

Carbon content

The amount of total carbon (TC) in the sediment at Site M0059 is high throughout the profile (Fig. F18A; Table T10). In the brackish-marine sediments, total organic carbon (TOC) is the dominant form of carbon, with values up to 8 wt% (Fig. F18B). These high TOC values suggest high primary productivity at the time of deposition, high sedimentation rates, and/or low oxygen concentrations in the bottom water, all of which promote organic matter burial (Canfield, 1994). At ~52 mbsf, coinciding with the first downcore occurrence of the greenish gray clays of Unit III (see “Lithostratigraphy”), TOC values significantly decrease, and they remain low (<0.5 wt%) throughout the remainder of the profile.

The amount of total inorganic carbon (TIC) averages 1.2 wt% in the organic-rich sediments deposited under brackish-marine conditions (Unit I) but significantly increases to a maximum of 4.6 wt% in the greenish gray clays deposited under freshwater conditions (Fig. F18C). At 203 mbsf in Hole M0059B, TIC values reach 11.8 wt%, equivalent to ~98 wt% of calcium carbonate (Table T10). This is in good agreement with the sedimentologic observation indicating the presence of chalk at ~200 m (see “Lithostratigraphy”).

Sulfur content

Similar to TOC, total sulfur (TS) contents are high in the uppermost part of the investigated sediment sequence, with values ranging between 1.1 and 1.9 wt% (Fig. F18D). At ~48 mbsf, TS sharply decreases to <0.4 wt% and remains low downcore. Such a change from low to high TS values is typical for a transition from a freshwater to a brackish-marine environment where, given sufficient TOC input, sulfate reduction will become quantitatively more important (Berner and Raiswell, 1983). Contents of TOC and TS in the upper 48 m of the sediment correlate positively, and TOC/TS weight ratios are in the typical range for marine sediments overlain by oxygenated bottom waters (Berner and Raiswell, 1983). Much of the sulfur is likely present in the form of Fe sulfide minerals (see “Lithostratigraphy”).

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