Because of time constraints at the end of the leg, samples were gathered only from Hole 1087A between 1.4 and 201.1 mbsf and from Hole 1087C between 224.5 mbsf to total depth. Whole-round samples were sampled at a frequency of one sample per core to 98.1 mbsf and every third core thereafter to 477.45 mbsf (Table 10). Only the samples from Hole 1087A were analyzed for properties other than salinity, pH, and alkalinity. Shore-based efforts will complete these traditionally shipboard analysis for Hole 1087C. Compared with the other Leg 175 Cape Basin drill sites, the chemical gradients observed at Site 1087 are intermediate between those at Sites 1085 and 1086, yet more similar to those at Site 1085.
Downcore profiles of alkalinity, sulfate, and ammonium (Fig. 12) reflect the degradation of organic matter. Alkalinity reaches a maximum value of 31 mM at 79 mbsf and subsequently decreases to a minimum value of 2.134 mM to the bottom of Hole 1087C. The deeper decrease likely is largely caused by alkalinity consumption during clay mineral formation ("reverse weathering"; see Mg2+ below). Sulfate is completely consumed by 79 mbsf, which is intermediate in depth between the depth of sulfate consumption observed at Sites 1085 and 1086. Ammonium reaches a maximum value of only ~4900 µM at 155 mbsf before starting to decrease with depth.
From the seafloor to 89 mbsf, the concentrations of dissolved Ca2+ and Mg2+ decrease sharply. The decrease in Ca2+ through this depth range (6 mM) is significantly less than the decrease in dissolved Mg2+ (13 mM). This lack of correspondence in both extent and pattern of these respective decreases suggests that dolomitization is not an important process at this site. Rather, we attribute the decrease in Ca2+ mainly to phosphate precipitation and the decrease in Mg2+ mainly to clay mineral uptake.
From 89 mbsf to the bottom of Hole 1087A, concentrations of dissolved Ca2+ increase smoothly, with the possible exception of the interval from ~98 to 212 mbsf, where there appears to be a positive excursion above the general trend of the increase (see shaded portion of Fig. 13). Through this same interval, dissolved Mg2+ concentrations may also be slightly above the overall decreasing trend. Regardless of these excursions, the broad increase in dissolved Ca2+ most likely reflects dissolution of biogenic calcite, whereas the decrease in Mg2+ reflects uptake by clay minerals.
Dissolved silica is present in interstitial waters from Site 1087 at concentrations greater than representative bottom-water values (Fig. 14), indicating dissolution of biogenic opal. There appear to be three depth domains of dissolved silica distribution. The first, from the seafloor to 22 mbsf, describes the greatest increase in dissolved silica. From this depth to 60 mbsf, dissolved silica concentrations remain essentially constant at ~615 µM. Following a rapid rise to a maximum of 714 µM at 79 mbsf, dissolved silica concentrations decrease with depth to the bottom of Hole 1087A. The decrease most likely records clay mineral uptake.
Dissolved phosphate concentrations increase with depth to reach a maximum of 50–70 µM from 60 to 70 mbsf (Fig. 14). Excluding the one point off the trend, the maximum itself is very broad and diffuse, and toward the deeper portion of the sequence dissolved phosphate concentrations decrease to values of ~10 µM, thereby recording uptake of dissolved phosphate into authigenic phases.
Dissolved Na+ increases rapidly through the uppermost 88 mbsf and decreases to 155 mbsf before increasing to the bottom of Hole 1087A (Fig. 15). Concentrations of dissolved K+ decrease to the bottom of the hole. There is a one-point maximum in dissolved K+ at 31.6 mbsf that corresponds with a one-point minimum in dissolved Na+; as discussed below, this may also be recorded in the salinity profile.
Salinity shows a general decrease from 35.0 to 32.5 through the sequence (Fig. 16) but records a strong increase to a maximum value of 37.5 at 50.6 mbsf. This maximum is not defined by a single point; rather, there is a marked increase in salinity from 22 mbsf to this maximum at 50.6 mbsf. This depth range includes the depth at which the one-point maximum in dissolved K+ is observed, suggesting that some—at this point unknown—reactions are occurring to simultaneously affect these chemical profiles. Concentrations of dissolved (Cl–) record an initial increase to a maximum of ~560 mM at 31.60 mbsf before decreasing to 547 mM at 126 mbsf. Below this depth, dissolved Cl– remains essentially constant to the bottom of Hole 1087A. The initial increase in dissolved Cl– may reflect changes in bottom-water chemistry associated with ice-volume variations through glacial periods. With the exception of the salinity variation in the uppermost sediment, both the salinity and Cl– profiles are very similar to those observed at Sites 1085 and 1086.