Interstitial water from 28 samples at Site 1260 were collected: 25 from Hole 1260A (0.58–490.50 mbsf) and 3 from Hole 1260B (451.36–508.58 mbsf). The samples from both holes were taken to constitute a single depth profile. However, slight differences in lithology may cause minor breaks in concentration-depth gradients of some chemical parameters. An incomplete data set exists for Sample 207-1260A-33R-1, 0–12 cm, because of low pore water yield (Table T19).
Alkalinity, chloride, ammonium, and silica were determined by the standard shipboard procedures (see "Inorganic Geochemistry" in the "Explanatory Notes" chapter). The major ions Na, K, Mg, and Ca were analyzed by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) after 50-fold sample dilution with deionized water. The minor components Li, B, Si, Fe, Mn, and Sr were determined by ICP-AES from 10-fold diluted interstitial water samples. From the minor component dilution, we determined sulfate as total sulfur by ICP-AES. Details of the methods, including the emission lines used for analysis, are given in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. Results of the chemical analyses are presented in Table T19 and in Figure F21.
Interstitial water chemistry at Site 1260 is dominated by the black shales and associated organic matter–rich sediments. These sediments are of Cenomanian–Coniacian age (lithostratigraphic Unit IV [~390–475 mbsf]). Sulfate concentrations approach zero at the top of lithostratigraphic Unit IV (see Fig. F21F), and the gradient from the top of Unit III to the sediment/water interface is almost linear. These observations suggest the following:
The modest departure from linearity in the sulfate concentration profile seen below ~250 mbsf may indicate sulfate reduction by the oxidation of methane. This interpretation is consistent with the sharp uphole decrease in methane concentrations recorded across the transition from lithostratigraphic Units IV–III. The two samples that we collected from the quartz siltstones of Albian age at the base of the site (lithostratigraphic Unit V) suggest that the black shale sequence may also be supplied by the diffusion of sulfate from below (Fig. F21F), as is the case at Site 1257.
At Site 1260, we detected no odor of hydrogen sulfide during core-splitting procedures on the catwalk, in contrast to Site 1257. Occasionally, however, an H2S smell was detected from interstitial water whole rounds taken from lithostratigraphic Unit III during the routine scraping that is undertaken to remove contaminated material prior to squeezing. It is possible that pyrite formation in Unit III sediments is triggered by hydrogen sulfide diffusion upward from the Unit IV sediments, which are most likely Fe limited. Shore-based sulfur isotopic studies will help to test this hypothesis.
The reducing character of the sedimentary column is also seen in elevated Mn and Fe concentrations (Fig. F21O, F21P). At Site 1260, our first sample (Sample 207-1260A-1R-1, 58–63 cm) was taken at a very shallow depth (0.58 mbsf) and captures the Fe and Mn concentration maxima (Fe = >50 然 and Mn = >10 然, respectively) associated with organic matter respiration in the thin veneer of Holocene sediments that drape the outer Demerara Rise. The Fe profile shows broad secondary peaks between ~50 and 200 mbsf, but an obvious relationship with sediment color is absent. Very low Fe concentrations are attained in lithostratigraphic Unit III (clayey chalks of Campanian–Paleocene age) ("Lithostratigraphy").
With the exception of one questionable data point in our Fe profile (Fig. F21O), only very low pore water concentrations of the redox-sensitive metals are attained from lithostratigraphic Unit IV (Mn = <1 and Fe = 5 然). The same is true of Unit V. A similar association between organic matter–rich sediments and low interstitial water concentrations of Mn and Fe was observed at the previous sites. Our favored working hypothesis for this observation is that these redox-sensitive metals were completely remobilized during or shortly after the host organic matter–rich units were deposited, implying conditions of severe synsedimentary oxygen depletion. Alternatively, the low pore water Mn concentrations observed in Units IV and V reflect the formation of Mn-rich carbonate phases (e.g., ankerite/rhodochrosite). Shore-based chemical analysis of the interstitial water "squeeze cakes" will provide a definitive test of these two competing hypotheses.
Sulfate depletion in pore water samples is accompanied by increases in ammonium (Fig. F21G), which is consistent with organic matter consumption. Ammonium concentrations decrease almost linearly from the deepest sample toward the sediment/seawater interface. The modest departure between this linear ammonium profile and the dissolved sulfate profile in lithostratigraphic Units III and IV is consistent with the hypothesis that sulfate reduction in Unit III proceeds by methane oxidation.
The complete absence of sulfate in Unit IV at Site 1260 most probably promotes the same two phenomena inferred from the data sets obtained at Sites 1257–1259: (1) mobilization of Ba and (2) formation of dolomite.
Background concentrations of Ba at Site 1260 are unrealistically high, implying an analytical baseline problem, but the form of the downhole pore water Ba profile, together with sedimentological evidence, indicates Ba mobilization from near the base of the black shale sequence. Authigenic barite crystals of millimeter to centimeter scale are frequently observed in the overlying Upper Cretaceous chalks (lithostratigraphic Unit III) (see "Lithostratigraphy"). Similar sedimentological and mineralogical relationships are reported elsewhere (Brumsack, 1986; Torres et al., 1996).
The downhole interstitial water concentration profile for Mg is nearly linear from the sediment/water interface to the base of the drill hole at Site 1260. In contrast, the downhole interstitial water profile for Ca is nonlinear. Ca concentrations peak (~1.6 times seawater values) at ~200 mbsf, fall to lower values (~1.3 times seawater) near the base of lithostratigraphic Unit III, and then increase slightly to the base of the black shales (~1.4 times seawater) (Fig. F21I). At Sites 1257 and 1258, we interpreted a downhole decrease in dissolved Mg concentration in terms of dolomite formation in the sulfate reduction zone in the black shale sequence (Unit IV). Unfortunately, the gap in our sampling between ~375 and 450 mbsf does not allow us to determine whether the same is true at Site 1260.
The nonlinearity of the Ca profile from the base of Unit III to the sediment/water interface indicates carbonate diagenesis in sediments of Late Cretaceous and Paleogene age as deduced from associated trends in alkalinity (Fig. F21E). The alkalinity and Ca concentration depth profiles show a particularly pronounced structure in lithostratigraphic Unit III (clayey chalks of Campanian–Paleocene age). This general association between these interstitial water chemistry parameters and sediment CaCO3 content (see Fig. F4) is similar to that seen at Site 1258 and implies some local control by ongoing carbonate dissolution and reprecipitation reactions. On the other hand, the downhole dissolved Sr and ammonium profiles, which are widely thought to be sensitive proxies for recrystallization of biogenic carbonate and organogenic bicarbonate, show minimal local control. Instead, we see near-linear downhole increases in both parameters, indicating sources at depth and simple diffusion to the sediment/water interface (Fig. F21J). One interpretation of the decoupled behavior between these parameters (Ca and alkalinity vs. Sr and ammonium) is that carbonate diagenesis in the Upper Cretaceous chalks (Unit III) is dominated by calcite precipitation fed by chemical diffusion from below rather than dissolution or in situ recrystallization. In fact, the form of the Sr pore water depth profile (Fig. F21J) suggests that the main locus of carbonate recrystallization most likely lies below the strata that we drilled at Site 1260. We interpret the simple diffusion-dominated Sr and ammonium profiles to reflect minimal accumulation of sediments younger than middle Eocene age (see Fig. F10). Similar findings have been reported from Blake Nose, where Eocene-age sediments crop out at the seafloor (Rudnicki et al., 2001). Average linear pore water Sr depth gradients (~2 然/m) are lower than those seen at Sites 1257 and 1259 (~5 and ~3 然/m, respectively) and those at Site 1258 (~1 然/m) and in the range of many Deep Sea Drilling Project/ODP sites (Rudnicki et al., 2001).
Dissolved silica concentrations at Site 1260 are relatively high from ~40 to 300 mbsf and thus do not correspond to the abundance of biogenic Si as recorded in the lithostratigraphy at the site (Fig. F4). As is the case at other Leg 207 sites, the clay-rich Paleoecene and Albian sediments both above and below the black shale sequence appear to serve as a sink for K (Fig. F21D). In fact, significant parallels exist between the K and Mg profiles at Site 1260. This observation suggests that clays may act as an important sink for Mg at the site.
Two relatively low salinity and Cl concentration anomalies occur in lithostratigraphic Unit III at Site 1260. The first anomaly occurs at ~318 mbsf, and the second one occurs at ~375 mbsf (Fig. F21A, F21B). These anomalies are in contrast to our findings at Sites 1257 and 1259, where we infer the presence of a brine in the black shale sequence of lithostratigraphic Unit IV but in remarkable accordance with the Cl profile above the black shale sequence at Site 1258 (see Fig. F20B in the "Site 1258" chapter). Both of the low Cl concentration anomalies at Site 1260 are paralleled by Na, and the average Na/Cl ratio at the site is 0.84, which is very close to the International Association for the Physical Sciences of the Ocean (IAPSO) seawater value of 0.86. The two Cl minima (<560 mM) indicate near-seawater concentrations, which is a small (<4%) freshening relative to the highest Cl concentrations measured at the site. At Site 1258, the Cl anomalies in lithostratigraphic Unit III implied a similar degree of freshening but overall absolute Cl concentrations are significantly lower (less than seawater) than at Site 1260 (see Fig. F20B in the "Site 1258" chapter). Furthermore, at Site 1258, a much larger interstitial water Cl anomaly is seen below the black shale sequence (~17% freshening relative to seawater). Unfortunately, the two samples that we collected from the Albian quartz claystones at the base of the site (lithostratigraphic Unit V) do not lie far enough below Unit IV to determine whether the same is true at Site 1260 (Fig. F21B). Taken alone, the modest Cl anomalies seen in Figure F21 would not invite extensive comment, but the parallels between the pore water Cl profiles at Sites 1260 and 1258 are striking. Significant concentrations of CH4 in headspace gas analyses (>50,000 ppmv) (see "Organic Geochemistry") are consistent with the anomalies having been caused by dissociation of gas hydrates (see "Inorganic Geochemistry" in the "Site 1258" chapter). Alternative explanations for the Cl anomalies are clay dehydration reactions and dilution by meteoric water. The former possibility seems unlikely given the lithologies encountered, but the latter possibility cannot be excluded even though the nearest landmass is located >300 km away. Li pore water concentrations show significant elevation at Site 1260 (Fig. F21L), suggesting that the association between high Li concentrations and the brines of the black shale sequence at Site 1257 may be coincidental, but the cause of these significant Li anomalies remains enigmatic.
In summary, the interstitial water chemistry profiles from Site 1260 primarily reflect ongoing organic matter diagenesis in the black shales and carbonate diagenesis. In sharp contrast to our findings at Sites 1257, 1259, and 1261 where lithostratigraphic Unit IV appears to act as an aquifer for fluids of relatively high salinity, we observe relatively low salinity and chlorinity anomalies in lithostratigraphic Unit III at Site 1260 that are strikingly parallel in form to those observed at Site 1258. We hypothesize that these anomalies are caused by either gas hydrate dissociation or dilution by meteoric water.