INORGANIC GEOCHEMISTRY

Thirty interstitial water samples were gathered from Hole 1084A between 1.4 and 599.90 mbsf. Whole-round samples were sampled at a frequency of one sample per core to 101.1 mbsf, and every third core thereafter to total depth (Table 12). The interstitial water chemistry at this site is dominated by the extremely high organic carbon concentrations in the sediment, which results in the generation of unusual chemical profiles. At this site we recorded the second highest concentrations of alkalinity and ammonium ever measured in the history of ocean drilling. Only Site 688 (Leg 112, Peruvian margin) recorded more extreme interstitial water variations; the alkalinity and ammonium values found here at Site 1084 exceed those found in the Santa Barbara Basin, Saanich Inlet, Oman Margin, and any other high organic region cored by DSDP or ODP.

Downcore profiles of alkalinity, sulfate, and ammonium (Fig. 20) through the upper 100 mbsf reflect the degradation of organic matter. Because of the extremely high total organic carbon (TOC) values at this site (see "Organic Geochemistry" section, this chapter), the rate of increase is very high. Alkalinity increases to a maximum value of 172 mM at 93 mbsf (approximately the boundary between lithostratigraphic Subunits IA and IB; see "Lithostratigraphy" section, this chapter) and remains at values >150 mM to ~220 mbsf before decreasing to the bottom of the hole. Within the uppermost 5.9 mbsf, sulfate is completely consumed. This is the shallowest depth of complete sulfate removal observed so far on this leg. Ammonium reaches a maximum of ~50 mM through the depth range of 130 to 250 mbsf before decreasing to the bottom of the hole. At previous sites on this leg we noted a more or less linear increase of dissolved ammonium with depth, a trend we attributed to the combined processes of organic matter degradation and clay mineral exchange. At Site 1084, in contrast, a broad ammonium maximum at depths between 100 and 300 mbsf reflects the continuing vigorous degradation of organic matter down to ~150–200 mbsf.

The concentration of dissolved Sr²⁺ increases within the uppermost 10 mbsf from a value near that of average seawater to ~125 µM (Fig. 21). This slight and shallow increase suggests that biogenic calcite dissolution is occurring only to a minor extent. Through litho-stratigraphic Subunit IA (see "Lithostratigraphy" section, this chapter) from 10 to 131 mbsf, dissolved Sr²⁺ concentrations stay approximately constant, suggesting no further dissolution. Through the remainder of lithostratigraphic Unit I and continually through Units II, III, and IV from 131 mbsf to the bottom of the hole, dissolved Sr²⁺ increases to a maximum of 464 µM. This deep increase records the dissolution of biogenic calcite, which releases Sr²⁺ to the interstitial waters.

Thus, through Subunit IA there appears to be calcite dissolution only in the uppermost 10 mbsf. From the seafloor to 73 mbsf, the concentration of dissolved Ca²⁺ decreases sharply, whereas that of Mg²⁺ increases through the same region. This is unlike observations at the previous Leg 175 sites, where Ca²⁺ and Mg²⁺ decreased in approximately equal proportions because of dolomitization. We interpret the Ca²⁺ profile at Site 1084 as recording precipitation of calcium carbonate driven by the high alkalinity. Below this depth, from 100 mbsf to the bottom of the hole, the increase in dissolved Ca²⁺ is consistent with the similar increase in dissolved Sr²⁺, both of which are caused by deep dissolution of biogenic calcite. We are unsure about the cause of the initial increase in Mg²⁺. We hypothesize that the deeper decrease in Mg²⁺ reflects ion exchange by clay minerals. The shallow increase in dissolved Mg²⁺ may also record ion exchange, or perhaps complexation of the cations with bicarbonate (J. Gieskes, pers. comm., 1997).

Dissolved silica is present in interstitial waters from Site 1084 at concentrations greater than representative bottom-water values (Fig. 22), indicating dissolution of biogenic opal. The concentration of dissolved silica continues to increase slightly with depth. Most notably, a local increase through lithostratigraphic Subunit IC and Unit II corresponds well with the increase in diatoms through this depth interval (see "Biostratigraphy and Sedimentation Rates" section, this chapter), as shown in the shaded region of Fig. 22. Below these diatomaceous sequences (see "Lithostratigraphy" section, this chapter), although dissolved silica continues to increase, the rate of the increase is similar to the rate through the nondiatomaceous lithostratigraphic units.

Because of remineralization of organic matter, dissolved phosphate concentrations increase with depth through Subunit IA and reach a maximum of nearly 50 µM at several intervals in this unit (Fig. 22). Through the remainder of the sequence, dissolved phosphate decreases to extremely low concentrations (~10 µM) reflecting the strong uptake of dissolved phosphate into diagenetic phases.

Concentrations of dissolved Na⁺ and dissolved K⁺ both increase from seawater values to maximum values at depth in lithostratigraphic Subunit IB before decreasing with depth to the bottom of the hole (Fig. 23). These profiles are broadly similar to those of alkalinity, ammonium, and, to a lesser extent, phosphate. In this context, it is important to recall that the Na⁺ concentration is not directly measured but is determined by charge balance. Because ammonium has the same ionic charge as Na⁺ and K⁺, its role may prove particularly relevant, and these positively charged species may all be responding to ion exchange with clay minerals.

Because of the large increases in alkalinity, ammonium, Mg²⁺, phosphate, Na⁺, and K⁺ through the upper 100 mbsf, as described above, salinity values increase to a maximum of 42.5 from ~90 to 100 mbsf (Fig. 24). Concentrations of dissolved Cl^– show a downcore pattern that is different from the previous Leg 175 sites; they exhibit a strong decrease from the seafloor down to a minimum of 531 mM at ~200 mbsf. Below this depth, dissolved Cl^– increases to a maximum value at the bottom of the hole. We are unsure of the processes controlling this distribution, although water exclusion during clay min-eral formation may play a role. The decrease in dissolved Cl^– is not caused by clathrate dissolution during coring and recovery because such dissolution would also decrease the salinity, and there is no other independent evidence of clathrate presence (see "Organic Geochemistry" and "Lithostratigraphy" sections, this chapter).