RESULTS

The concentrations of the principal pore-fluid constituents for Sites 1003-1007 are presented in Table 1. Depth profiles are shown for the four principal sites of the Leg 166 transect (Sites 1003, 1005, 1006, and 1007) (Fig. 4). A brief description of the principal trends is given below for each of the main constituents.

Contents of dissolved Cl^- and Na⁺ have vertical gradients in the shallow Recent-Pleistocene intervals (~20-40 mbsf) at each site. Below this depth, profiles display a sharp increase in concentration that continues to increase down to the base of all holes. The increase is highest at Site 1005 and lowest at Site 1006 (Fig 4). The maximum dissolved Cl^- concentration occurs near the base of Site 1003 (1190 mbsf) and has a value of 1069 mM, roughly two times that of seawater. The concentrations of dissolved Na⁺ and K⁺ follow that of dissolved Cl^- at all sites except Site 1006.

Total alkalinity profiles increase sharply below 20-40 mbsf, reaching broad maxima between 100 and 300 mbsf at all sites. The highest increases in alkalinity content (>60 mM) occur at Sites 1004 and 1005, whereas the smallest increase (7.9 mM) is observed at Site 1006. A second maximum in alkalinity content is reached below 300 mbsf at Sites 1006 and 1007.

Ammonium (NH₄⁺) profiles are similar to alkalinity profiles in the upper 200 mbsf, with a broad maximum between 4 and 15 mM obtained 20-50 m below the alkalinity peak. The occurrence of the NH₄⁺ peak at greater depths than the sulfate peak is probably caused by differences in microbial end products within the sulfate reduction zone and lower methanogenic zone. Ammonium content either continues to increase to the base of the hole (Sites 1006 and 1007), decreases (Site 1005), or displays a second maximum near the base of the hole (Site 1003). The content of dissolved phosphate was very low at all sites (<6 µM) and is limited by a strong interaction with carbonate (Kitano et al., 1978).

Sulfate (SO₄^2-) profiles decrease sharply below 30 mbsf and reach a broad minimum between 80 and 200 mbsf at all sites. The sharpest decrease occurs at Site 1005, where SO₄^2- becomes undetectable (<2 mM) by a depth of 80 mbsf. At Site 1003, the minimum SO₄^2- concentration is 15.5 mM at a depth of 181 mbsf. Interestingly, at both Sites 1003 and 1005 the concentration of SO₄^2- increases further downhole in excess of bottom-water concentrations (30 mM). At Site 1003, SO₄^2- concentrations as high as 38.7 mM occur at a depth of 687 mbsf. (Table 1). In contrast, Sites 1006 and 1007 display a more typical SO₄^2- profile of decreasing concentration with depth in the upper 200 mbsf (base of sulfate reduction zone), and low to zero concentration down to the base of the hole.

Calcium (Ca²⁺) and magnesium (Mg²⁺) profiles show significant differences between sites and considerable structure within each site (Fig 4). As with the other elements, the profiles are near vertical in the upper 20-30 m of each site, with concentrations similar to bottom-water values. Large changes occur below the base of this zone. Calcium concentrations range between 6 and 34 mM, with significant decreases from seawater concentrations within shallow (upper 200 mbsf) zones of active sulfate reduction, and significant increases further downhole. Magnesium concentrations range between 25 and 63 mM, and generally decrease with depth at all sites. Strontium (Sr²⁺) concentrations increase to very high values below 30 mbsf at all sites. The highest concentration (~7 mM) was recorded at a depth of 453 mbsf at Site 1006. At Sites 1003, 1005, and 1007, Sr²⁺ concentrations decrease further downhole in an inverse relationship with SO₄^2-. Dissolved barium (Ba²⁺) ranges between 0.1 and 227 µM, with higher concentrations measured at the more distal sites. Barium profiles similarly show an inverse relationship with respect to SO₄^2-.

The oxygen stable isotopic composition (¹⁸O) of interstitial waters ranged from +0.05 to +0.08 in the upper 30 mbsf at all sites, becoming heavier further downhole at all sites except Site 1006. The highest ¹⁸O values (+2.8) occurred at Site 1003 at a depth of 910 mbsf. The carbon stable isotopic composition (¹³C) ranged from -2.57 to +4.86, with a mean value of +0.17.

Minor and trace element composition of the acid soluble sediment samples at Sites 1007 and 1005 are shown in Table 2. Also included are sediment porosity data determined by pycnometer on the same samples (F. Anselmetti and J. Kenter, unpubl. data). The insoluble content at Site 1007 ranges between 0.5% and 36%, with significant cyclic variation within the Miocene sequences. At Site 1005, insoluble content ranges between 1.5% and 10.5%. Strontium concentrations vary from 500 to 12,000 ppm. Iron concentrations range from 50 to 1800 ppm, with the highest values associated with higher insoluble fractions. Manganese concentrations vary from 0 to 220 ppm, displaying clear cyclic intervals downhole. Barium concentrations vary from 4 to 274 ppm and show distinct patterns downhole that are probably related to the abundance of acid-soluble barite in the sediments.