Twenty-four interstitial water samples were gathered from Hole 1081A over a depth range from 2.4 to 441.2 mbsf, at a sampling frequency of one sample per core for the upper 100 mbsf and every third core to total depth (Table 10). Unlike the previous Leg 175 sites, the drilling at this hole targeted the deeper portions of the sequences, thereby enabling the interstitial water program to document an additional suite of chemical behaviors that was not evident at sites with less penetration. Throughout the sequence, interstitial water variations are observed that can be tied directly to the local stratigraphic lithologic unit, as well as the variations caused by the changes in organic matter concentration, as have been observed previously. In particular, the depositional and paleoceanographic conditions that result in the diatom-rich clays of lithostratigraphic Subunit IB appear to have a strong influence on chemical processes occurring at Site 1081.
Downcore profiles of alkalinity, sulfate, and ammonium (Fig. 30) through the upper ~100 mbsf record the degradation of organic matter found in these shallowly buried sediments. Sulfate is completely consumed within the upper 60 mbsf. Through this uppermost depth interval, alkalinity values increase sharply, and the concentration of ammonium also shows the greatest relative increase. These alkalinity values are lower than those measured at previous Leg 175 sites.
Additional chemical sources and sinks are active in the deeper portions of the hole. Below a minimum in alkalinity at 100 mbsf (see discussion of carbonate reactions below), values gently increase to a maximum of 30 mM at ~200 mbsf. Alkalinity then decreases steadily to a deep minimum value of 14.8 mM at the bottom of the hole. We interpret the maximum at 200 mbsf as reflecting a higher organic supply during the times in which lithostratigraphic Subunits IB and IC were deposited.
Through this same deeper interval, ammonium concentrations also reach maximum values before decreasing toward the bottom of the hole. Again, we interpret the maximum as reflecting a greater availability of organic matter through that portion of the sequence. The decrease toward the bottom of the hole may reflect cation exchange reactions during clay diagenesis.
Concentration profiles of Ca2+, Mg2+, and Sr2+ reflect processes of carbonate dissolution and precipitation through the various shallow and deep portions of the sedimentary sequence at Site 1081 (Fig. 31). First, from the seafloor to ~50 mbsf, the interstitial water chemical distribution of these elements records carbonate precipitation, including dolomitization, accompanied by calcite dissolution. The evidence for calcite dissolution is not as unambiguous as at the previous Leg 175 sites; however, the net increase from dissolved Sr2+ values near that of modern seawater at the shallowest sample to values approaching 150 µM at ~130 mbsf reflects the diagenetic release of Sr2+ by the dissolution of biogenic calcite. Superimposed on this increase is a localized minimum at ~50 mbsf, which indicates the local presence of a strong Sr2+ sink that is most likely diagenetic apatite.
Stoichiometrically, the decrease in Mg2+ concentrations through this portion of the stratigraphy (an 8-mM decrease from 2.4 to 43.3 mbsf) essentially exactly mimics the decrease in Ca2+ concentration, suggesting that the paired decrease in these elements represents their simultaneous incorporation into diagenetic dolomite. From 43 mbsf to the bottom of the hole, Ca2+ and Sr2+ concentrations increase, whereas those of Mg2+ decrease. We interpret these chemical gradients as recording the dissolution of biogenic calcite and the incorporation of Mg2+ into clay phases. The several local minima superimposed on the general increase with depth of dissolved calcite broadly correspond to the lithologic unit boundaries and reflect the contrasts between these units (see "Lithostratigraphy" section, this chapter). Additionally, although there are portions of the stratigraphic section in Subunits IB and IC that contain dolomite layers in them, the interstitial water chemistry clearly indicates that these are trapped layers not actively growing; the zone of currently active dolomitization appears to be confined to the upper ~100 mbsf. As sedimentation occurs, this zone of precipitation moves upward, leaving behind the relict layers.
Dissolved silica increases in concentration very rapidly from representative bottom-water values to a maximum of 1074 mM at 100 mbsf (Fig. 32), recording the dissolution of biogenic opal. This maximum occurs within the diatom-rich clays of lithostratigraphic Subunit IB (see "Lithostratigraphy" section, this chapter), but its stratigraphic position does not appear to correspond to the first-order abundance index of diatom distributions (see "Biostratigraphy and Sedimentation Rates" section, this chapter). Because the maximum in dissolved silica occurs near the top of the diatomaceous unit, we interpret this chemical profile as recording first the diagenetic dissolution of biogenic opal in the youngest sediments of the subunit, followed by the formation of authigenic clays in the older sediments, which subsequently removes dissolved silica from the interstitial waters. In lithostratigraphic Subunit IC and Unit II, the local maxima in dissolved silica correspond well with local maxima in the diatom abundance index centered on ~300 and 410 mbsf (see "Biostratigraphy and Sedimentation Rates" section, this chapter).
Dissolved phosphate concentrations increase with depth within lithostratigraphic Subunit IA, recording the remineralization of organic matter. There is a pronounced local maximum centered at 150 mbsf in the middle of Subunit IB. The diatoms in this subunit are characteristic of upwelling species (see "Biostratigraphy and Sedimentation Rates" section, this chapter). This is consistent with the high dissolved phosphate concentrations recording a greater supply of organic matter during the time in which the unit was deposited. Also, the highest TOC concentrations found at Site 1081 are distributed through this portion of the sequence. Dissolved phosphate concentrations decrease through Subunits IC and ID, recording the removal of phosphate most likely into either precipitating authigenic phases or onto iron oxides.
The concentration of dissolved Na+ increases from seawater values to maximum values at depth (Fig. 33). This increase may be recording the release of Na+ from clay minerals. The concentration of K+ (Fig. 33) initially increases from seawater values in the uppermost 10 mbsf and remains essentially constant to a depth of ~300 mbsf. From 300 mbsf to the bottom of the hole, the concentration of dissolved K+ decreases by almost one-half. This decrease does not correspond with any lithologic boundary. Broadly, it is through a similar depth range that ammonium concentrations also decrease. Collectively, we interpret these concentration profiles as reflecting deep clay exchange reactions, but we cannot distinguish between purely diagenetic causes and those causes that may reflect a mineralogic or source variation.
Salinity decreases smoothly downcore to a broad minimum from 90 to 178 mbsf before increasing with depth to the bottom of the hole (Fig. 34). The initial decrease most likely is recording the combined decreases of dissolved sulfate, Ca2+, and Mg2+, whereas the deep increase is responding to increases in Na+, Cl–, and Ca2+. Concentrations of dissolved Cl– record a relative maximum at 72 mbsf and a continual increase to the bottom of the hole.