INORGANIC GEOCHEMISTRY

Shipboard chemical analyses of the interstitial water samples from Site 1165 followed the procedures outlined in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. Sixty-nine 10- to 20-cm-long whole-round core samples from 0.45 to 995.30 mbsf were squeezed for interstitial water. Samples were taken at 50-cm intervals from Hole 1165A. Samples were taken from Hole 1165B at 1.5-m intervals to ~60 mbsf, every ~10 m to 100 mbsf, and then every ~30 m to 636 mbsf. Samples were taken from Hole 1165C every ~30 m from 636 to 995 mbsf. Results from all three holes are considered to represent a single continuous profile and are presented in Table T7 and Figures F42 and F43.

Chlorinity decreases slightly downhole, from 555 mM near the seafloor to 531 mM at 995.30 mbsf. A local maximum of ~573 mM, between 30 and 50 mbsf, represents an increase of ~3% over seafloor values (Fig. F42). This increase is significant relative to the measurement precision (<0.3%) and likely reflects the global increase in ocean salinity during the last glaciation. The increase is not observed in the salinity measurements (Fig. F43), probably because of the relatively low resolution of the optical refractometer. The irregular downhole trend toward lower chlorinity values (564.5-531.0 mM; ~6%) shows no clear evidence of simple linear mixing between two separate water masses. The profile likely reflects diagenetic reactions within the core such as increasing clay mineral dehydration with increasing depth.

The pore-water profiles at Site 1165 may be referenced to three dominant chemical reactions: sulfate reduction, carbon dioxide reduction (methanic), and inorganic diagenesis. Although the first two reactions are largely exclusive, inorganic diagenesis occurs throughout the core.

The uppermost sediments at Site 1165 are strongly reducing, reflecting the active diagenesis of buried organic matter (see "Organic Geochemistry"). Dissolved sulfate (SO₄^2-) decreases linearly (R² = 0.984) downhole from seafloor values of 29 to 2 mM at 150 mbsf (Fig. F43). Sulfate depletion is highly negatively correlated (R² = 0.964) with a corresponding linear downhole increase in dissolved ammonium (NH₄⁺) (Fig. F43). Other key byproducts of organic matter decay increase roughly threefold through the sulfate reduction zone (SRZ), namely alkalinity (3-8 mM; derived from the oxidation of carbon in organic matter) and phosphate (HPO₄^2- = 3-10 µM; derived from the remineralization of organic phosphorous) (Fig. F43). In theory, the oxidation of organic matter during sulfate reduction follows the simplified stoichiometric form:

2 CH₂O (organic matter) + SO₄^2- = H₂S + 2 HCO₃^- (alkalinity)

or the full redfield ratio sulfate reduction reaction:

53 SO₄^2- + C₁₀₆H₂₆₃O₁₁₀N₁₆P =
39 CO₂^- + 67 HCO₃^- + 16 NH₄⁺ + 53 HS^- + 39 H₂O + HPO₄^2-

or

53 SO₄^2- + (CH₂O)₁₀₆ (NH₃)₁₆ H₃PO₄ + 39 H₂O =
106 HCO₃^- + 16 NH₄⁺ + 53 HS^- + 39 H₂O + HPO₄^2-,

wherein 2 mM of alkalinity are produced for each millimole of sulfate reduced. At Site 1165, significantly less alkalinity (and phosphate) are produced for each millimole of sulfate reduced (see "Conclusions").

The profile of dissolved manganese (Mn²⁺) within the SRZ is characteristic of the reduction of manganese oxides during the oxidation of organic carbon. Manganese concentrations remain below analytical resolution from the seafloor to 10 mbsf (Fig. F43). From 10 to 32 mbsf, manganese increases rapidly to 168 µM before dropping to ~8 µM at the base of the SRZ (~150 mbsf).

The interstitial water profiles of dissolved calcium (Ca²⁺), magnesium (Mg²⁺), and potassium (K⁺) (Fig. F43) all display marked inflections at 150 mbsf. Calcium shows a small but measurable downhole increase over 15 samples between the seafloor (10.8 mM) and 15 mbsf (11.2 mM). Calcium values then decrease nonlinearly downhole to 8.7 mM at 150 mbsf (Fig. F42). Magnesium concentrations decrease linearly (R² = 0.956) downhole, from the modern seawater value (~54 mM) near the seafloor to 37 mM at 150 mbsf. Dissolved strontium (Sr²⁺) increases downhole, from 106 µM at the seafloor to ~150 µM at 150 mbsf. The detailed profile shows repeated high-frequency excursions of ~10%, which remain unexplained (Fig. F42). The variations are unlikely to represent analytical drift because each excursion is characterized by four or more discrete analyses run in random order.

Dissolved lithium (Li⁺) approximates seawater concentrations immediately below the seafloor (29 µM at 0.45 mbsf) but increases rapidly to 43 µM at ~3 mbsf before dropping back to seawater values between 5 and ~150 mbsf. Lithium also shows the unexplained high-frequency excursions observed for strontium. Interstitial water potassium levels are enriched by ~3 mM over standard seawater potassium in the top 4.45 m of Hole 1165A (Fig. F42). These high pore-water potassium concentrations coincide with traces of reworked glauconite (see "Lithostratigraphy"), which suggests that active dissolution of the grains is taking place. Between 4.45 and 4.95 mbsf, interstitial potassium values drop sharply to 9.6 mM and increase again to 12.4 mM at 22 mbsf before beginning a gradual downhole decrease to 7 mM at the base of the SRZ. Dissolved sodium concentrations are within ~1% of the modern seawater value (~480 mM) to the base of the SRZ (Fig. F43).

Near the seafloor, the interstitial water silica (H₄SiO₄) concentration of 522 µM is significantly enriched relative to modern deep-ocean water (typically <200 µM). Continuing dissolution of abundant siliceous microplankton within the near-surface sediments produces an exponential increase in dissolved silica to ~850 µM at 100 mbsf (Fig. F43).

When sulfate is depleted, CO₂-utilizing methanogenic Archaea bacteria rapidly take over as the dominant life form within the sediment (Fig. F44). In the presence of excess organic matter, the theoretical reaction follows the form:

(CH₂O)₁₀₆ (NH₃)₁₆ (H₃PO₄) = 53 CO₂ + 53 CH₄ + 16 NH₃ + H₃PO₄.

At Site 1165, headspace methane concentrations increase rapidly below 150 mbsf, marking the top of the CO₂ reduction zone (CRZ) (see "Organic Geochemistry"). The base of the active reaction zone is more diffuse and is therefore identified at ~400 mbsf on the basis of changes in pore-water ammonium and phosphate profiles (Fig. F43). Dissolved ammonium increases linearly downhole, from 400 µM at 150 mbsf to 800 µM at 400 mbsf, approximating the ammonium gradient measured in the overlying SRZ. The alkalinity profile deflects sharply at 150 mbsf (Fig. F43) and steadily decreases to the base of the hole. The 30% decline in alkalinity observed between 150 and 400 mbsf signifies the large-scale removal of HCO₃^- from the system. Phosphate decreases sharply to <2 µM between 150-200 mbsf and finally disappears from the interstitial waters below 400 mbsf.

Dissolved manganese shows a small increase from 7 to 23 µM between 191 and 265 mbsf, before decreasing to ~14 µM at the base of the CRZ. The dissolved calcium concentration increases linearly (11-19 mM; R² = 0.997) through the CRZ, reversing the trend observed in the overlying SRZ. Magnesium decreases nonlinearly downhole, from 32 to 30 mM, through the same interval. Dissolved strontium increases nonlinearly, from 160 µM at 150 mbsf to ~247 µM at ~400 mbsf. There is a sublinear correlation (R² 0.8) between calcium/strontium gain and magnesium loss through the CRZ.

Interstitial water lithium concentration increases sublinearly (R² = 0.819) downhole, from 28 µM at 191 mbsf to 59 µM at 382 mbsf. Dissolved potassium decreases linearly (R² = 0.984) to 2.8 mM at 487 mbsf. The dissolved sodium concentration begins a slow linear (R² = 0.937) downhole decline, from 483 mM at 265 mbsf to 399 mM at 880 mbsf. The dissolved silica concentration remains constant at ~1000 µM, approaching the solubility limit of opal-A.

Below ~400 mbsf, the fermentation processes driving active methanogenesis appear to decrease and inorganic reactions become more significant. Chloride and salinity both record low values between 600 and 800 mbsf (543 mM and 30.5, respectively, at ~700 mbsf). Ammonium approaches steady state below 400 mbsf, averaging ~800 µM to the base of the hole. The small change in the ammonium profile between Holes 1165B and 1165C may be an artifact of different sample batches. The sulfate profile shows a small downhole increase from ~0 to 2.2 mM between ~400 and 995 mbsf (Fig. F42). Alkalinity continues to decrease linearly (R² = 0.980) downhole to ~800 mbsf (1.26 mM). The lowermost two samples analyzed (851 and 880 mbsf) show indications that alkalinity may be increasing again, but the sample resolution is too low to draw any firm conclusions.

Dissolved manganese shows no clear trend between 400 and 600 mbsf, with values ranging from 11 to 23 µM. Two minor downhole-decreasing trends are apparent between 677 and 765 mbsf (25-18 µM) and from 794 to 995 mbsf (26-5 µM). Dissolved calcium increases exponentially below the CRZ (R² = 0.986), reaching a maximum of 89 mM at 995 mbsf. The magnesium profile is significantly more complex. An upper reaction zone, from ~400 to ~600 mbsf, shows a continuation of the nonlinear gradient observed in the overlying CRZ. The lower two reaction zones, from ~600 to 800 mbsf and ~800 to 937 mbsf, show distinct nonlinear gradients. Magnesium concentrations reach a low of 6.6 mM near the base of the hole. Dissolved strontium increases exponentially downhole, from 217 µM at 421 mbsf to 764 µM at 995 mbsf. The 250% downhole increase in strontium compares to a 367% increase in calcium and a 78% decrease in magnesium concentrations over the same interval.

The dissolved lithium profile appears to show marked inflections at ~600 and 880 mbsf. Lithium shows significant scatter between 400 and 600 mbsf, but the dominant trend is toward higher values downhole (50 to ~80 µM). The lithium profile then decreases linearly (79-21 µM; R² = 0.964) to 880 mbsf before increasing linearly (21-41 µM; R² = 0.987) to the base of the hole. The dissolved potassium profile from 487 to 937 mbsf shows minor variations in concentration, although the overall downhole trend is toward lower values (1.3 mM at 937 mbsf). Sodium decreases sublinearly (R² = 0.924) downhole, from 467 mM at 420 mbsf to 399 mM at 880 mbsf. Dissolved silica declines steadily below 420 mbsf, decreasing from ~972 to ~130 µM at 938 mbsf.

Interstitial water chemistry at Site 1165 reveals three distinct reaction zones within the sedimentary section. The SRZ shows the expected depletion of dissolved sulfate and salinity, with associated increases in ammonium, phosphate, alkalinity, and silica. In theory, 2 mM of alkalinity are produced for each 1 mM of sulfate reduced. Within the SRZ, alkalinity increases by <0.2 mM/mM sulfate reduced (Fig. F45), indicating rapid removal of bicarbonate from the system. Likely mechanisms for this reaction are calcium carbonate precipitation or silicate reactions. If the alkalinity deficit is assumed to be caused solely by carbonate precipitation, preliminary saturation-state calculations predict a decrease of 5.5 mM calcium in the interstitial waters at the base of the Site 1165 SRZ. This compares to the observed calcium decrease of only 1.9 mM between the seafloor and ~150 mbsf (Fig. F46). Associated sediment calcium carbonate values are generally <1 wt% (see "Organic Geochemistry"), further confirming that calcite precipitation is not the principle mechanism involved in the alkalinity removal. It should be noted that this simplified mass-balance approach ignores the possible role of advective and diffuse transport in controlling element distribution.

Below 150 mbsf, the CRZ shows the expected increases in methane and ammonium. Removal of dissolved CO₂ by methano-bacteria lowers alkalinity, raises pH, and accelerates carbonate precipitation. The decrease in dissolved phosphate within the CRZ is likely due to the precipitation of authigenic phases such as Ca₃(PO₄)₂, Fe₃(PO₄)₃, MgNH₄PO₄, or Ca₅(PO₄)₃F.

Calcium and magnesium are inversely correlated through much of the sedimentary section (Fig. F47). The correlation is mostly nonlinear, suggesting independent diagenetic reactions are responsible for the profile variations. No firm conclusions can be drawn from the available data; however the following may be considered: although diagenetic rather than diffusive reactions appear to dominate throughout much of the core, calcite or dolomite dissolution/precipitation alone cannot explain the observations. Clay mineral reactions, such as smectite formation, could produce the observed downhole decrease in dissolved magnesium and alkalinity. Dissolution of calcium-rich plagioclase may account for part of the exponential downhole increase in dissolved calcium, although no clear correlation is evident from comparison with plagioclase estimates from XRD analyses (see "Lithostratigraphy"). The sharp downhole decline in dissolved potassium is closely matched by a relative increase in K-feldspar within the sediment (Fig. F48). It seems likely that much of the K-feldspar observed at Site 1165 is authigenic in origin.