INORGANIC GEOCHEMISTRY

Thirty-one interstitial water samples were gathered from Hole 1080A over a depth range from 1.4 to 49.64 mbsf (Table 7), at a typical high-resolution frequency of one sample per section of core. Termination of drilling was caused by the presence of a dolomite layer at ~50 mbsf, and the high-resolution interstitial water program fortuitously was able to focus in detail on dolomitization processes operating throughout the recovered sequence. The chemical processes involved in organic carbon degradation and carbonate dissolution and reprecipitation dominate the interstitial water chemical distributions at Site 1080.

Downcore profiles of alkalinity, sulfate, and ammonium at Site 1080 (Fig. 14) record the degradation of organic matter. Sulfate is essentially completely consumed within the upper 10 mbsf. Alkalinity values increase dramatically through this uppermost depth interval, and the concentration of ammonium also shows the greatest increase. From 10 mbsf to the dolomite layer at 50 mbsf, alkalinity and ammonium both increase monotonically.

Concentration profiles of Ca²⁺, Mg²⁺, and Sr²⁺ reflect processes of carbonate dissolution and precipitation (Fig. 15). The upper 10 mbsf at Site 1080 is characterized by sharply decreasing concentrations of sedimentary calcium carbonate, with values changing from ~26 to <5 wt% through this depth range (see "Organic Geochemistry" section, this chapter). Concentrations of dissolved Sr²⁺ increase through this interval, indicating dissolution of sedimentary calcite. In addition to the Sr²⁺ distributions, the one-point maximum in both Ca²⁺ and Mg²⁺ concentrations at 2.9 mbsf may also be recording this dissolution.

Both Ca²⁺ and Mg²⁺ concentrations systematically decrease with depth down to ~25 mbsf. We interpret these decreases as recording carbonate precipitation including dolomitization; in the upper 25 mbsf, the decrease in Ca²⁺ (7 mM) almost matches the associated decrease in Mg²⁺ (10 mM). An unknown portion of the Mg²⁺ depletion is accounted for by uptake by clay minerals. From 25 mbsf to the dol-omite layer at 50 mbsf, both Ca²⁺ and Mg²⁺ remain essentially constant. Through the entire 50-m depth cored here, the overall decrease in Mg²⁺ describes a gradient of 0.18 mM Mg²⁺ per meter of sediment, which is the largest gradient observed so far during Leg 175. Gradients of the decrease in Mg²⁺ concentration at Sites 1075, 1076, 1077, 1078, and 1079 range between 0.06 mM/m and 0.12 mM/m. Although the formation of disseminated dolomite is occurring at these other sites, the presence of the completely lithified dolomite layer at 50 mbsf at Site 1080 suggests that dolomitization is of greater importance at Site 1080 than has been observed at previous Leg 175 sites.

These preliminary data also suggest that the dolomite layer responsible for terminating drilling is, in fact, a relict dolomite layer. Were the layer actively growing at the present time, the depletion gradients of Ca²⁺ and Mg²⁺ would record continual depletion toward the layer itself, rather than recording no change in either Ca²⁺ or Mg²⁺ from 25 mbsf to that horizon.

Dissolved silica increases in concentration very rapidly from representative bottom-water values through the uppermost 4 m of sediment (Fig. 16), recording the dissolution of biogenic opal. Below ~10 mbsf, the concentration of dissolved silica remains essentially constant to the bottom of the hole.

Dissolved phosphate increases from values representative of bottom water to a maximum of ~180 µM within the uppermost 10 mbsf. Phosphate concentrations below this depth increase only slightly to a maximum of 190 µM at 32 mbsf before decreasing to ~130 µM at the dolomite layer. The decrease in dissolved phosphate most likely records uptake by diagenetic apatite.

Concentrations of dissolved Na⁺ and K⁺ decrease through the uppermost 4 to 8 mbsf, below which concentrations of Na⁺ gradually increase with depth, whereas K⁺ remains essentially constant (Fig. 17). These variations are most likely reflecting uptake and release by clay minerals.

Salinity decreases smoothly downcore (Fig. 18), most likely reflecting the decreases of Mg²⁺ and Ca²⁺ described previously. Dissolved Cl^– records an initial increase to 8 mbsf, as has been observed at previous Leg 175 sites (and which we attribute to variability in the glacial ocean), followed by a continual decrease to the dolomite layer at 50 mbsf.