The high-resolution IW sampling program at Site 1109 consisted of a total of 64 IW samples. Eight were collected from the first two cores, one was taken from each subsequent core down to 480 mbsf, and one from every other core until hard rock was encountered, unless core recovery or the composition of the material did not allow it. Except for Core 180-1109D-43R, in which a smaller whole round (7 cm) was taken because of the paucity of material available, the IW yield from whole rounds was always sufficient to permit shipboard analysis of the full suite of constituents and the acquisition of splits for all shore-based scientific interests.
The IW was analyzed for salinity, pH, alkalinity, major cations (Na+, K+, Ca2+, and Mg2+) and anions (Cl- and SO42-), SiO2, NH4+, Sr2+, and Li+. Results of shipboard inorganic chemical analyses are presented in Figure F70. The pH measurements made during the alkalinity determination in IW samples are considered to be generally reliable. We believe this to be the case because no significant degassing was observed, as evidenced by a rapid acquisition of a stable initial pH reading during the alkalinity titration (see "Inorganic Geochemistry" in the "Explanatory Notes" chapter).
The titration alkalinity profile shows four broadly defined zones. In the first zone, alkalinity rises sharply from ~3.5 mM in the shallowest cores to a maximum of 13.5 mM at 107 mbsf (Table T10; Fig. F70A) and decreases abruptly over the next 60 mbsf. A gentler negative concentration gradient between 160 and 330 mbsf gives way to another rise to a broad alkalinity submaximum between 375 and 535 mbsf, below which alkalinities are less than 1.5 mM to the bottom of Hole 1109D. The NH4+ profile shown in Figure F70A crudely parallels that of alkalinity with maxima and minima observed in the same depth ranges. However, the deep-seated NH4+ maximum exhibits proportionally much greater concentrations than its alkalinity counterpart. Dissolved SO42- is strongly depleted with increasing depth over the first 100 mbsf and mirrors the alkalinity profile down to ~160 mbsf. From 200 to 500 mbsf, SO42- remains almost fully depleted except for a few minor excursions to 3.0-4.5 mM. Sulfate appears to be fully depleted in IW recovered from the bottom 300 m of the hole, except possibly in the deepest sample collected immediately above hard rock.
The salinity exhibits a narrow range of 31-35. A systematic and gentle decrease from seawater values occurs down to 480 mbsf. Although a return to a salinity of 34 occurs by 717 mbsf, the deepest IW sample recovered immediately above hard rock displays a significantly lower salinity of 31 (Table T10).
The dissolved Na+ and Cl- profiles at Site 1109 (Fig. F70B) do not covary. Although Na+ concentrations generally decrease downhole throughout the entire sedimentary column, dissolved Cl- remains in a tight range of 550-560 mM down to ~550 mbsf. Below this depth, Cl- increases to 579 mM at 612 mbsf, followed by a decrease to the lowest value (522 mM) in the deepest (745 mbsf) IW sample collected. Dissolved Cl- measurements made by AgNO3 titration are better constrained than those by ion chromatography (IC) and are deemed more reliable. Excursions occur in Cl- data from IC on a run-by-run basis, with notable divergence of the titration and IC data observed in the first 150 mbsf (Fig. F70A). Dissolved K+ exhibits large fluctuations downhole between near-seawater values immediately below the mudline to near complete depletion at the bottom of the sedimentary sequence (Fig. F70B). A slight enhancement over seawater is observed in the shallowest sediments, whereas ~40% depletion relative to seawater concentration occurs by 310 mbsf. This trend reverses between 310 and 430 mbsf, below which a strong removal of K+ from pore fluids is once again observed throughout the deeper sediments.
Dissolved Li+ (Fig. F70B) exhibits a rather complex profile at Site 1109 and varies over the range of 10-86 µM. An initial minimum coincides with the alkalinity maximum and sulfate minimum. The fine structure of the Li+ profile appears to correlate with fluctuations of these two parameters down to 150 mbsf. Below this depth however, dissolved Li+ increases downhole through a series of concentration gradients of different slopes to a maximum at 568 mbsf, the bottom of lithostratigraphic Unit VI (see "Lithostratigraphic Unit VI"). The Li+ profile does not appear coupled strongly to that of any other IW constituents in this depth range. A submaximum in Li+ occurs near 680 mbsf, but concentrations drop dramatically in the two deepest IW samples.
The dissolved Ca2+ and Mg2+ profiles display four principal zones (Fig. F70C). The first zone, between 0 and 100 mbsf, is characterized by removal of ~90% of the Ca2+ and 40% of the Mg2+ from the pore water. In the second zone, between ~100 and 300 mbsf, dissolved Ca2+ concentrations return to near seawater values, whereas those of Mg2+ increase gradually from ~34 to 40 mM. A third zone of relatively constant Ca2+ and Mg2+ concentrations occurs between ~300 and 470 mbsf. The fourth zone displays a sharp increase in dissolved Ca2+ with increasing depth, whereas Mg2+ concentrations initially remain little changed (35-40 mM down to ~600 mbsf), before decreasing sharply toward the bottom of the hole. The Sr2+ profile is also characterized by four zones that correspond broadly to those noted above for Ca2+ and Mg2+. However, the shape of the Sr2+ profile is quite different, and the range of concentrations is much wider than those of Ca2+ and Mg2+ (Fig. F70C). The Sr2+ maximum at 661 mbsf is more than 15 times the seawater concentration and occurs at the same depth as the crossover point for the Ca2+ and Mg2+ profiles.
Dissolved SiO2 also displays a complex profile (Fig. F70C). Concentrations fall into three broad ranges downhole. In the first 140 mbsf, dissolved SiO2 remains mostly between 400 and 540 µM. Greater fluctuations are then observed, with several local maxima (461 and 803 µM) and minima (268 and 145 µM) evident between 140 and 500 mbsf. A sharp decrease to ~100 µM SiO2 occurs over the next 50 m, below which concentrations remain under 250 µM throughout the remainder of the sedimentary column.
Changes in the composition of interstitial fluids are particularly useful in fingerprinting diagenetic reactions; even small changes in the composition of the solid phase lead to large changes in fluid compositions because of orders of magnitude higher concentrations of most constituents in the solid phases relative to seawater (Sayles and Manheim, 1975). At Site 1109, the composition of IW reflects the bacterial oxidation of organic matter, its effect on early diagenesis of biogenic carbonate and silica, and the alteration of volcanic matter and subsequent formation of authigenic clay minerals, as well as transformations of pre-existing detrital minerals.
The bacterially mediated oxidation of organic matter (Claypool and Kaplan, 1974) following the general reaction shown below, is clearly evident in the strong depletion of SO42- and concomitant increases in alkalinity and NH4+ concentrations (Fig. F70A):
Although a decrease in pH normally results from the decomposition of organic matter, in the presence of aluminosilicate sediments pH can be buffered at higher values than those observed in carbonate sediments (Ristvet, 1978; Mackenzie et al., 1981). This is consistent with the increase in pH we observed over this depth interval but cannot account for larger increases in pH observed below 500 mbsf in lithostratigraphic Units VI and VII. The latter increase in pH may be attributable to precipitation of CaCO3 cements observed between 580 and 670 mbsf (see "Lithostratigraphy" and "Organic Geochemistry").
The sharp increase in alkalinity with increasing depth in the shallower sediments (~100 mbsf) induces dissolution and recrystallization of biogenic carbonates (Morse and Mackenzie, 1990). This is reflected in the dissolved Ca2+, Mg 2+, and Sr2+ profiles and in the initial drop in the Ca/Mg ratio (Fig. F70C). The early diagenesis of carbonates in the upper 100 mbsf of the sediments, in addition to the initial compaction of sediments and concomitant fluid expulsion, likely contributes to the observed decrease in sediment porosity (see Fig. F71).
A second potential source of alkalinity, although generally less important than the oxidation of organic matter, is the alteration of volcanic matter. The following reaction, which describes the alteration of volcanic matter, has been invoked previously to explain alkalinity increases observed in sediments from Sites 998 and 999 in the absence of significant bacterially mediated oxidation of organic matter (Sigurdsson et al., 1997):
Potassium feldspar can be substituted for Na-feldspar in the above equation.
The abundance of ash layers in lithostratigraphic Unit II (see "Lithostratigraphic Unit II"), especially between 100 and 150 mbsf, likely also produces alkalinity and may contribute to the fine structure of the profile evident in Figure F70A. A small upturn in dissolved SO42- between 115 and 152 mbsf, which suggests a reduced rate of microbial activity, is consistent with this interpretation. The second and less pronounced alkalinity maximum observed between ~375 and 480 mbsf exhibits fluctuations that broadly correspond to the abundance of ash layers (see Fig. F36). In this section of the sedimentary column, microbial activity likely remains significant and contributes to the second broad NH4+ maximum, although weathering of mica is also known to release NH4+ to solution. The downcore decrease in NH4+ below 580 mbsf in the calcite-cemented sandstone of lithostratigraphic Unit VII could reflect a reduced contribution from microbial activity in combination with enhanced alteration and authigenesis of clay minerals. Detrital clay minerals represent a potential NH4+ sink through incorporation as a trace cation or by exchange with monovalent alkali cations (Stevenson and Cheng, 1972).
Examination of variations in the Ca/Mg ratio shown in Figure F70C reveals evidence of various sedimentary reactions. The initial decrease over the first 100 mbsf is consistent with a dissolution and recrystallization of biogenic carbonates, accompanied by release of Sr2+ to solution throughout sediments of lithostratigraphic Unit I, similar to that observed, for example, at the Great Bahamas Platform sites (Eberli et al., 1997). Between 100 and 300 mbsf, Ca2+ and, to a much lesser extent below 180 mbsf, Mg2+ are released to solution, as evidenced by an increasing Ca/Mg ratio. Here the source of dissolved Ca2+ is likely the alteration of volcanic minerals, especially plagioclase, dispersed throughout the sediments (e.g., Gieskes, 1981, 1983; Gieskes et al., 1982). Microscopy studies show that volcanic glass, lithic fragments, and individual crystals are more abundant above 380 mbsf (see "Lithostratigraphy"). Formation of authigenic clays that would remove Mg2+ does not appear to be an important process in this zone. Rather the presence of common chlorite/smectite mixed-layer clays (see "Lithostratigraphy") between ~100 and 200 mbsf suggests diagenetic transformation of pre-existing detrital clay minerals (McDuff and Gieskes, 1976; McDuff, 1981).
The negative K+ gradient (Fig. F70B) within the 100-200 mbsf interval can be attributed to removal of this constituent from pore fluids by illite formation. A shallower K+ gradient observed between 200 and 260 mbsf is consistent with the absence of illite in XRD patterns of the bulk sediments (see Table T3). The reappearance of this mineral in several XRD samples taken near 300 mbsf coincides with further depletion of K+ from the pore water. Below 300 mbsf, increases in dissolved K+ may be attributed to alteration of K-feldspars found within lithostratigraphic Unit V and, to a lesser extent, in the increasingly calcite-rich sediments of lithostratigraphic Unit VI. Very low abundances of illite, which can act as a sink for this constituent (see "Lithostratigraphy"), also contribute to the maintenance of elevated dissolved K+ concentrations. Within this depth interval, little change is observed in the Ca/Mg ratio, which is also consistent with a lack of authigenic clay mineral formation.
There are major changes in the lithology below 570 mbsf. Lithostratigraphic Unit VII is a calcite-rich sandstone in which dissolution of abundant biogenic carbonate components acts as a source of dissolved Ca2+, as reflected in the increasing Ca/Mg ratio (Fig. F70C). However, no significant change in the dissolved Mg2+ concentration is observed. Substantially lower porosity and enhanced calcite cementation were observed between 570 and 670 mbsf, an interval in which only a small change in the Ca/Mg ratio and the dissolved Mg2+ concentration was noted. The large negative K+ gradient throughout this interval (Fig. F70B), as well as a sharp drop in dissolved SiO2 (Fig. F70C), is consistent with the reduced porosity associated with cementation of the sediments. Substantially reduced pore-water yields during squeezing of the whole rounds of sediments from this portion of Site 1109 also support this inference.
Large changes in the pore-water constituent profiles occur within and below lithostratigraphic Unit VII. A large increase in dissolved Ca2+ is observed in the lower portion of this unit, whereas the sharp maximum in the dissolved Sr2+ (Fig. F70C) and a Cl- maximum (Fig. F70A) is near the boundary between lithostratigraphic Units VII and VIII. A Li+ submaximum (Fig. F70B) is immediately below this boundary. The variations in IW composition beginning in Unit VII and continuing in Unit VIII, interpreted to be lagoonal sediments because of an abundance of shell, plant, and wood fragments, are bounded within a shalelike morphology that apparently minimizes diffusion. The source of dissolved Ca2+ and Sr2+ here is presumably the dissolution of bioclasts (e.g., aragonitic shells of gastropods; see "Lithostratigraphy"). The increasing dissolved Ca2+ and Cl- and the sharp uptake of Mg 2+ below the boundary of lithostratigraphic Units VII and VIII likely reflect diffusive profiles resulting from the alteration of K-feldspars and plagioclase and concomitant formation of the quite pure authigenic smectites observed in XRD samples collected from lithostratigraphic Units VIII and IX (see "Lithostratigraphic Unit VIII" and "Lithostratigraphic Unit IX").
Dissolved Li+ and SiO2 (Fig. F70B, F70C) display variations throughout the sediments that are interpreted to reflect both lithologic and structural variations. In the uppermost sediments, early dissolution of biogenic silica leads to a substantial enrichment of this constituent over seawater. Between 80 and 170 mbsf, however, low-temperature alteration of abundant volcaniclastic turbidites contributes to the dissolved SiO2 content of the IW. Relatively wide fluctuations in dissolved SiO2 concentrations below 200 mbsf generally coincide with changes in the porosity of the sediments (Fig. F71) or in substantial mineralogical variations. A submaximum in dissolved SiO2 at 260 mbsf in the upper portion of lithostratigraphic Unit IV (see "Lithostratigraphic Unit IV") coincides with a local porosity maximum (Fig. F71). This is a fault zone containing a conjugate fracture system that likely enhances fluid flow (see "Structural Geology" and "Downhole Measurements") and where a change in the dissolved Li+ gradient suggests upward diffusion of Li-enriched pore water from below. The next SiO2 maximum at 374 mbsf is at the base of a volcaniclastic sand layer (lithostratigraphic Unit V). It is also near another porosity maximum (Fig. F71) and coincides with a change in lithology to more mixed terrigenous, bioclastic, and volcanic material, as well as the onset of a steeper dissolved Li+ gradient. The drop in dissolved SiO2 to very low concentrations is at ~550 mbsf near the boundary between lithostratigraphic Units VI and VII (see "Lithostratigraphic Unit VI" and "Lithostratigraphic Unit VII"). and a sharp decrease in porosity (Fig. F71; see also "Physical Properties"). The boundary between high and low dissolved SiO2 concentrations also coincides with the Li+ maximum. Although calcite cementation of sediments in this interval may contribute to a reduced diffusion of dissolved SiO2 from above and help maintain higher dissolved Li+ in the remaining pore fluids, it is
more likely that this chemical composition reflects the uptake of dissolved SiO2 and the exclusion of dissolved Li+ during the formation of other silicate minerals.
The lowest dissolved Na+, Cl-, and Li+ concentrations at Site 1109 were measured in IW from Sample 180-1109D-43R-2, 82-89 cm. It was recovered from lithostratigraphic Unit X, which has a clayey silty matrix and is located slightly above the dolerite of lithostratigraphic Unit XI (see "Lithostratigraphic Unit X"). These low concentrations may represent the remains of a paleofreshwater signal that is being slowly erased with time by diffusion from overlying saline waters.
A summary of sedimentary geochemical processes is given in Table T11. The chemical composition of the IW reflects strongly both the influence of ash layers and other volcanic minerals dispersed throughout the sediments and the carbonate dissolution and recrystallization reactions induced by fluid compositional changes derived from the microbial oxidation of organic matter. Other carbonate mineral reactions and those involving silica and transformations of pre-existing detrital clay minerals also contribute to variations observed in pore-water profiles at this site. The inferences regarding geochemical reactions based upon the chemical composition of the IW are substantiated by lithostratigraphic observations, mineralogical identifications (XRD), and changes in the physical properties of the sediment.