RESULTS

Concentrations of dissolved Rb and Ba in IW from Sites 1109, 1115, and 1118 are presented in Table T1; isotopic compositions (¹⁸O and ⁸⁷Sr/⁸⁶Sr) are given in Table T2.

Substantial fluctuations in the concentrations of dissolved Ba (Fig. F1) occur downhole. Full depletion of SO₄^2- occurs between 100 and 200 mbsf at each site, although SO₄^2- reappears in IW deep within Site 1118 (Taylor, Huchon, Klaus, et al., 1999). The absence of SO₄^2- throughout a large part of the sedimentary column of each site allows dissolved Ba²⁺ to accumulate in the IW; yet, the shapes of the depth profiles as well as the range of Ba²⁺ concentrations vary between sites. Dissolved Ba²⁺ increases from <1 µM in the upper 50 mbsf (Site 1109) to 100 mbsf (Site 1115), to a maximum of 17.7 µM at 480 mbsf at Site 1109. Lesser fluctuations are observed at Sites 1115 and 1118, the latter displaying only about half the Ba²⁺ concentration range observed at Site 1109.

Dissolved Rb⁺ concentrations also vary widely throughout the sedimentary column of each site (Fig. F1). Concentrations reach two- to threefold enrichment over seawater in localized maxima within Pliocene sediments of Sites 1109 and 1115, although concentrations decrease sharply near the Miocene unconformity. Unlike what was observed for Ba²⁺, the highest dissolved concentrations of Rb⁺ are present in IW from Site 1118, whereas variations in this constituent are most subdued at Site 1115, where Rb⁺ remains in a narrow range of ~2-2.5 µM in the upper 400 mbsf. Dissolved Rb⁺ is strongly depleted relative to seawater at and below the Miocene unconformity of Sites 1115 and 1109. A near total removal of Rb⁺ from IW occurs near the bottom of Site 1109, whereas concentrations of Rb⁺ remain more than twice that of bottom seawater in the deepest (842 mbsf) IW sample collected from Site 1118.

Sr isotope ratios display large variations in IW from the Woodlark Rise sites (Fig. F2). A systematic and similar decrease in ⁸⁷Sr/⁸⁶Sr with increasing depth is observed in the upper 300 mbsf of all three sites. Below this depth, however, values diverge. The widest range in ⁸⁷Sr/⁸⁶Sr ratio in the IW is observed at Site 1115, where it decreases from near-seawater values (e.g., ⁸⁷Sr/⁸⁶Sr = 0.70916) just below the mudline to a minimum of 0.70714 at 601 mbsf. The ⁸⁷Sr/⁸⁶Sr ratio increases slightly below 600 mbsf at Site 1115, before settling down near 0.708 in the deepest sections of the hole. IW from Site 1109 displays the narrowest range of ⁸⁷Sr/⁸⁶Sr values of the three Woodlark Rise sites, as well as a profile that mirrors that of Site 1115 between ~550 and 750 mbsf. In the deepest portion of Site 1109 the ⁸⁷Sr/⁸⁶Sr ratio decreases sharply again, approaching a value comparable to that observed at the same depth deep in Site 1118.

Profiles of oxygen isotopes in IW from the Woodlark Rise (Fig. F3) exhibit a general decrease in ¹⁸O downhole at Sites 1109 and 1115 and in the upper half of Site 1118 (i.e., 258-544 mbsf). At Sites 1109 and 1115 a small increase in the ¹⁸O is also observed between 20 and 50 mbsf. Unfortunately, because the upper 200 mbsf of the Site 1118 was drilled but not cored, no data are available for this depth range. At Site 1115, the largest decrease in ¹⁸O occurs across the Miocene unconformity, below which a minimum of -2.84 is observed at 667 mbsf. The trend in ¹⁸O values at Site 1109 is similar, although a much more subdued range is observed. The lowest value of -1.27 is present in the deepest sample (746 mbsf), which was recovered just above the Miocene unconformity. At Site 1118, there is a relatively steep negative gradient in ¹⁸O down to 544 mbsf where a value of -1.63 is recorded. However, unlike observed at the two other Woodlark Rise sites, a reversal in the ¹⁸O profiles occurs below 571 mbsf at Site 1118 and values trend back toward the contemporaneous seawater value, reaching -0.86 at 758 mbsf. Below 758 mbsf and approaching the Miocene unconformity, the profile resumes the decreasing trend in ¹⁸O, as observed deep in the other Woodlark Rise sites. Overall, the range of ¹⁸O values at Site 1109 is small with the lowest value of -1.27 measured in the deepest sample (746 mbsf), which was recovered just above the Miocene unconformity.

The trace element composition of the clay fraction (<2 µm) of selected whole-round cores corresponding to the IW samples is shown in Table T3. The corresponding X-ray mineralogy is given in Table T4.

The trace element concentrations of the clay samples were normalized to the primitive mantle composition (Hofman, 1988) to help determined their provenance and elucidate the relationship of clays to the abundant volcanic matter dispersed throughout the sediments of Site 1109. The normalized data plotted as "spider diagrams" are shown in Figure F4. The clays generally exhibit similar patterns, with depletion of Nb and enrichment of Pb, Ti, and Hf noted for all samples. The slope of the patterns is steepest in the samples collected deep within Site 1109, except from Section 180-1109D-38R-4, whereas samples recovered from younger sediments display less fractionated patterns. The level of enrichment or depletion relative to the trend described by the other trace elements, however, is subject to substantial variability. Additionally, selected clays display enrichments in Sr and Zr (and to a lesser extent Gd). Some samples recovered deeper within the sedimentary column, especially between Section 180-1109C-39X-2 and Section 39R-2 show a greater Sr enrichment relative to other samples. In the case of Zr, however, the enrichment is more variable, with no uniform pattern observed with increasing depth downhole. In fact, some samples recovered from younger sediments display the highest Zr enrichment (e.g., Section 180-1109D-6H-3). Clay isolated from the conglomerate interval (Section 180-1109D-34R-4) exhibits the lowest trace element enrichment as well as less fractionation across the trace element series than clays isolated from younger sediments. The clay sample recovered from below the Miocene unconformity (e.g., Section 180-1109D-38R-4) exhibits little fractionation across the series of elements comprising the spider diagram, including lower normalized Rb and Ba abundances than most other clays, although it does not have the lowest overall trace element concentrations. The latter is particularly true toward the end of the trace element series (e.g., Ti, Eu, Gd, Y, and Yb), where this sample actually exhibits the highest enrichment of these elements.

XRD investigations were conducted to identify the mineral assemblages of the clay fraction and to determine the composition and distribution of the various minerals. Quartz, feldspar, and illite are considered of detrital origin in all holes, representing the background hemipelagic sedimentary supply (Taylor, Huchon, Klaus, et al. 1999). The presence of talc and serpentine minerals in Sections 180-1109C-10H-4 to 26X-1 indicates erosion of metamorphic rocks for which the D'Entrecasteaux Islands or Papuan Peninsula are the obvious source areas. The XRD data indicate the presence of a significant amount of mixed-layer chlorite/smectite clays, suggesting a distinctive provenance for these intervals. The increased abundance of smectite with increasing depth downhole is probably related to an increase of volcanic alteration supported by advanced devitrification of volcanic glass shards.