INTERSTITIAL WATER CHEMISTRY AND HEADSPACE GAS

The interstitial water program at Site 1149 consisted of the extrusion and analysis of 46 samples from Hole 1149A as well as 6 samples from Hole 1149B. Despite poor core recovery through the porcelanites and cherts of lithologic Unit III (see "Unit III"), enough samples were gathered to provide adequate coverage of the entire sedimentary sequence, with the shallowest sample acquired at 1.4 mbsf and the deepest 0.60 m above the sediment/basement contact in Section 185-1149B-29R-1. Interstitial water data are given in Table T11.

The chemical composition of the interstitial waters at this site reflect several processes, including organic matter degradation, diagenetic formation of chert and porcelanite, carbonate diagenesis, ash alteration, authigenic clay formation, and basement alteration. In the following sections we show how the interstitial water chemistry at this site illustrates each of these various processes. The cherts and porcelanites of Unit III inhibit but do not totally restrict diffusive communication between the upper and lower portions of the sediment section. The low average sedimentation rate at Site 1149 (see "Sedimentation Rates") results in the recording of long-term diagenetic reactions in a relatively thin sedimentary sequence.

The organic loading at this site is minimal, as reflected by the low alkalinity and the slight enrichments in dissolved NH₄⁺ to ~200 µM and PO₄^3- to ~35 µM (Fig. F50). Dissolved SO₄^2- decreases only slightly downhole, and Fe is below the analytical detection limit throughout the entire sequence (Fig. F51). Dissolved Mn is present only in the upper 150 mbsf. These distributions are consistent with the very low concentrations of CH₄ in the headspace gases (Table T12) and collectively indicate only a slightly suboxic diagenetic environment at Site 1149.

Dissolved PO₄^3- reaches a maximum within the upper 10 mbsf, whereas dissolved NH₄⁺ reaches a maximum through the upper 50 mbsf, reflecting the sequential release of these species during organic matter degradation. The decrease in dissolved NH₄⁺ below this interval reflects uptake during clay diagenesis. Concentrations of dissolved PO₄^3- decrease sharply below this shallow maximum and reach a strong local minimum at ~20 mbsf, indicating a local sink of dissolved PO₄^3- at this depth. Dissolved Mn (Fig. F51) also records a minimum at this horizon, suggesting that the precipitation of diagenetic Mn oxides also removes PO₄^3- from the interstitial waters. The increases in dissolved Mn from 20 to 100 mbsf suggests a redox change through this depth interval, which will be addressed in greater detail in postcruise studies. Further downhole, from 100 to 150 mbsf, the rapid decrease in dissolved Mn suggests the precipitation of Mn oxides through lithologic Unit II. This decrease is broadly coincident with the decrease in dissolved PO₄^3- to essentially zero, suggesting that these Mn oxides also scavenge dissolved PO₄^3- from the interstitial waters. Considering the varying redox behavior in this portion of the sedimentary section, some of the sedimentary features identified as "ash layers" may in fact be relict Mn oxide precipitation horizons.

Dissolved silica is well above the representative concentration in seawater in even the shallowest sample recovered at Site 1149 (Fig. F52). Concentrations increase to maximum values at 100 mbsf, before decreasing precipitously to low values of ~230 µM through lithologic Unit II. Concentrations increase at the bottom of Hole 1149A and into Hole 1149B. Below Unit III, concentrations decrease with increasing depth to low values of ~230 µM again, with a local maximum in Unit V.

This dissolved silica profile is influenced by the diagenesis of biogenic silica as well as by the formation of authigenic clay minerals (the latter will be discussed more fully below). The steady increase through Unit I reflects the continual dissolution of biogenic opal from the diatoms contained in this lithologic unit. Were it not for the strong sink of dissolved silica caused by the ash and clay alteration occurring in Unit II, we predict the concentration of dissolved silica would have continued to increase to the opal-A-opal-CT transition found at ~170 mbsf. The first occurrence of opal-CT, however, is observed in Core 185-1149A-21X (~179.7 mbsf) (see "Unit III"), and this diagenetic transformation is the likely cause of the somewhat lower concentrations of dissolved silica at the top of Hole 1149B. The last occurrence of opal-CT is found in Core 185-1149B-18R (~301.7 mbsf) (see "Unit IV"), and the transition to diagenetic quartz draws down dissolved silica concentrations in Unit IV.

Considering the Adara temperature measurements made in Cores 185-1149A-6H and 8H, which indicate temperatures of 5.687° and 6.390°C, respectively (see "Run 1"), we estimate that the opal-A-opal-CT transition occurs at 6°-7°C, and the opal-CT-quartz transition occurs at 11°-12°C. These values are consistent with previous results from open-ocean depositional regimes (e.g., Hein et al., 1978).

Because there is little sedimentary carbonate in Units I, II, or III (see "Abstract"), increases in dissolved Sr²⁺ are minimal throughout this portion of the sedimentary section (Fig. F52). Through the uppermost 50 mbsf there is an increase in dissolved Sr²⁺ to values slightly above that of average seawater (see inset in Fig. F52). Below the sharp decrease in Unit II (see inset of Fig. F52), concentrations in dissolved Sr²⁺ increase at least in part as a result of the recrystallization of sedimentary carbonate in Unit IV. This is not the sole mechanism influencing the distribution of dissolved Sr²⁺, however, as shown by the widely varying concentrations at the depth where the effect of local sources and sinks may be important.

Diagenetic processes acting within the ash- and clay-rich sediments of Units I and II (see "Unit I" and "Unit II") dramatically affect several constituents of the interstitial waters. The concentration of dissolved silica exhibits a pronounced minimum in Unit II (see shaded area in Fig. F52), reaching values as low as those found in the diagenetic quartz portion of Unit IV much deeper in Hole 1149B. The dark brown pelagic clays of Unit II clearly define a unique diagenetic environment compared to the other units at this site. Concomitant depletions are observed in dissolved Sr²⁺ (see inset in Fig. F52) and dissolved K⁺ (Fig. F53), whereas enrichments are present in alkalinity and dissolved NH₄⁺ (Fig. F50) and dissolved Li⁺ (Fig. F53), reflecting the effects of cation exchange and authigenic clay formation. Additionally, there is a slight decrease in dissolved Cl^- as well, which may reflect dehydration during authigenesis.

The uppermost several tens of meters of the sedimentary section are also active in terms of clay-mineral diagenesis. Through the upper 10-20 mbsf, dissolved Na⁺ increases, whereas dissolved Li⁺ shows a pronounced depletion (Fig. F53). These changes may in part reflect ion exchange between the microbially produced NH₄⁺ (Fig. F50) and Li⁺. General changes with depth of these and other cations also most likely reflect a contribution by clay alteration.

Drilling at Site 1149 recovered the sediment/basement contact in Core 185-1149B-29R. Sediments in the overlying Units IV and V were unlithified enough to yield several interstitial water samples, allowing a terrific chance to document diffusional signals responding to basement alteration.

The distributions of dissolved Ca²⁺ and Mg²⁺ record the release of Ca²⁺ and uptake of Mg²⁺ during basalt alteration (Fig. F54). The interstitial water samples record an extreme enrichment in dissolved Ca²⁺, reaching 135 mM at 407 mbsf, along with a depletion of dissolved Mg²⁺ to a low value of 15 mM at the same depth. The greatest change in both concentration profiles occurs through Units IV and V, indicating that the decreased permeability of the cherts and porcelanites effectively inhibits diffusion through this portion of the sedimentary section.

The strongly linear relationship between dissolved Ca²⁺ and Mg²⁺ in the lower reaches of the sedimentary section (r² = -0.99; see upper left panel in Fig. F55), along with the shape of the paired concentration-depth profiles, argues persuasively for a dominantly diffusive control on the concentration of these species in Units IV and V. This is consistent with many previous observations of dissolved Ca²⁺ and Mg²⁺ in other sedimentary sequences (e.g., Gieskes, 1983). We can use the well-documented relationship between dissolved Ca²⁺ and Mg²⁺ to assess the effect of basement alteration on other dissolved species at Site 1149 (Fig. F55). Considering the five interstitial water samples recovered from Units IV and V, and assuming Mg²⁺ is quantitatively removed by basement alteration (and thus an end-member concentration of 0 mM), we can assess the effect of basement alteration on other dissolved constituents by comparing the linearity of their relationship to Mg²⁺. This indicates that the distributions of dissolved K⁺ and Na⁺ appear to be dominated by diffusion into basement alteration products, that the distribution of dissolved NH₄⁺ appears to be moderately affected, and that the distribution of dissolved Sr²⁺ appears to be unaffected by basement alteration (Fig. F55). Thus, basement alteration is sequestering Mg²⁺, K²⁺, and Na²⁺. The inverse relationship with dissolved NH₄⁺ may reflect cation exchange in the sediments involving other species. Finally, the slight net increase in dissolved silica in the deepest sample (Fig. F52) suggests the release of silica during basement weathering.

In Units III and IV, with the exception of the deepest sample, concentrations of dissolved Cl^- increase toward the basement (Fig. F56). These concentrations are moderately correlative to those of dissolved Mg²⁺ (r² = -0.64). Whereas the increase in dissolved Cl^- with depth over the uppermost 20 mbsf most likely reflects the freshening (with time) of ocean water since the last glaciation, the sharp increase in dissolved Cl^- with depth toward the bottom of the hole indicates hydration of the basaltic crust.

The relatively simple sedimentary sequence at Site 1149 provides an exceptional natural laboratory to examine diagenetic processes operating over a long time scale (~135 m.y.) in a sequence bounded by basaltic crust and the oceanic reservoir. At this site, the distribution of dissolved constituents in the interstitial waters is affected, in a broad burial sequence, by degradation of organic matter and associated changes in the redox environment, authigenic clay formation, diagenetic recrystallization of biogenic silica and carbonate, and basement alteration.

These processes cannot be considered in isolation from one another. For example, release of dissolved PO₄^3- is followed by sequestering in Mn oxides, whereas the release of dissolved silica by the dissolution of diatoms is followed by sequestering in the authigenic clays of Unit II, and cation exchange of species influenced by basaltic alteration (e.g., Na²⁺ and K⁺) affects other constituents (e.g., NH₄⁺) in the deepest recovered interstitial waters. Although further shore-based research will refine the preliminary observations discussed here and identify further problems to study, it is clear that the chemical environment at Site 1149 is responding to the combined effects of a variety of sources and sinks throughout the sedimentary and basaltic sequence.