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Cores were recovered from 37 holes across 22 sites (M0005 to M0026) (Table T1), with a conventionally calculated recovery of 57.47%. Water depths at the sites ranged from 41.6 to 117.5 m, and cores were recovered from 41.6 to 161.8 meters below sea level (mbsl). Because of difficulties locating and operating at some the proposed drill sites (see Operations), the original strategy of coring along profiles was abandoned and new sites were chosen on the basis of water depth. Thus six drilling "areas" were targeted: Faaa, Tiarei inner ridge, Tiarei outer ridge, Tiarei marginal sites, Maraa western transect, and Maraa eastern transect. All new sites were drilled within areas approved by EPSP. Borehole geophysical wireline logging was conducted at seven sites (in ten holes).
Because of space limitations on the DP Hunter, only limited analysis of cores was conducted offshore, with the bulk of description and measurement conducted at the Onshore Science Party at the Bremen Core Repository. Table T2 shows which measurements were conducted offshore and at the Onshore Science Party.Sedimentology and Biological Assemblages
The cored material shows that the reefs around Tahiti are composed of two major lithological units: a postglacial carbonate sequence (Unit I) and an older Pleistocene sequence (Unit II). Modern sediments were often recovered above Unit I. Within Units I and II, the lithostratigraphy can be divided into subunits (e.g., Subunit IA, IB, IC, and so on), based on coral assemblages and internal structure. It is important to note that subunits of the same designation at separate sites are not correlative. Correlation of subunits between sites will not be possible until ages are obtained from postcruise research.Maraa Western Transect: Sites M0005, M0006, and M0007
The western transect drilled in the Maraa area (southwest Tahiti) includes Sites M0007, M0005, and M0006 (landward to oceanward) in water depths between 43 and 82 m. Figure F6 summarizes the major lithologic units, lithology, and recovery for all holes on the Maraa western transect.Modern Sediments
Modern sediments were recovered at all sites along this transect. They consist of a few decimeter-thick beds comprising rhodoliths, skeletal sands, and gravels rich in Halimeda segments, mollusk fragments, benthic foraminifers, and nongeniculate coralline algae along with clasts of Halimeda packstone and coral clasts.Lithologic Unit I (Postglacial Carbonate Sequence)
The top of Unit I is characterized by an abundance of thin crusts of nongeniculate coralline algae and extensive bioerosion. This sequence is primarily composed of coralgal-microbialite frameworks commonly interlayered with loose skeletal sediments including coral and algal rubble and skeletal sand and with skeletal limestone.
The coralgal-microbialite frameworks that form the bulk of this unit are characterized by widespread development of microbialites which locally represent the major structural and volumetric component of the reef rock. They develop within the primary cavities of the reef framework, where they generally overlie crusts of noneniculate coralline algae. The microbialites generally comprise a suite of fabrics including two end-members represented by laminated fabrics and thrombolitic accretions; the laminated fabrics generally correspond to the most abundant fabric.
The reef sequence is characterized by a general succession of distinctive coral assemblages, although many of them are intergradational, both laterally and vertically. Two successive subunits (Subunits IA and IB) displaying distinctive coral assemblages and internal structure were recognized:
At Site M0007, the base of Subunit IB corresponds to poorly lithified skeletal grainstone that contains fine sand-sized volcanic grains and a 30 cm thick interval composed of branching coralline algae in Core 310-M0007B-34R.Lithologic Unit II (Older Pleistocene Sequence)
The eastern transect drilled in the region of Maraa (southwest Tahiti) includes Sites M0017, M0015, M0018, and M0016 (landward to oceanward) in water depths from 56.45 to 97.35 mbsl. Figure F7 summarizes the major lithologic units, lithology, and recovery for all holes on the Maraa eastern transect.Lithologic Unit I (Postglacial Sequence)
The thickness of Unit I ranges from 24 m in the deepest hole (Hole M0016B at 97.35 mbsl) to 40 m in Hole M0018A. The base the unit has been recovered from 94 mbsl at the inner sites to 121 mbsl at the outer sites.
This unit is primarily composed of coralgal-microbialite frameworks commonly interlayered with loose skeletal sediments including coral and algal rubble. Coral and algal rubble are mostly composed of accumulations of fragments of corals (mostly encrusting Acropora and Montipora and branching Pocillopora and Porites), microbialites and mollusks, Halimeda segments, and rounded lithoclasts (e.g., skeletal sandstone rich in volcanic grains). Benthic foraminifers are usually scarce. Pebbles of basalt and sand-sized volcanic grains occur locally. Skeletal sand corresponds to Halimeda sand rich in fragments of corals (e.g., branching Porites) echinoids and mollusks.
The frameworks that form the bulk of Unit I include three subunits (Subunits IAIC) displaying distinctive coral assemblages and internal structure. Corals are usually thinly encrusted by nongeniculate coralline algae, except in Subunit IC, where the crusts are significantly thicker (up to 2 cm thick) and include vermetid gastropods. The last stage of encrustation over coral colonies corresponds to thick microbialite crusts dominated by massive laminated fabrics.
Unit II is primarily composed of irregular alternations of yellow and gray to beige poorly sorted skeletal limestone (grainstone to rudstone-floatstone) and coralgal frameworks with local intercalations of coral and algal rubble that display conspicuous alteration.
Subaerial diagenetic processes are indicated by recrystallization of coral skeletons and the occurrence of large solution cavities that display yellow and brown to red-brown staining. Some solution cavities are partly filled with sediments.
The grainstones, rudstones, and floatstones are rich in fragments of corals (e.g., robust branching Pocillopora, branching and massive Porites, branching Acropora, and encrusting Montipora), coralline algae, echinoids and mollusks, and Halimeda segments. Sand-sized volcanic grains are commonly associated with the skeletal grains.
The coralgal frameworks comprise encrusting and branching colonies of Porites, tabular colonies of Acropora, encrusting and massive colonies of Montipora, and encrusting colonies of agaricids; massive colonies of Porites occur locally.Tiarei Inner Ridge: Site M0023
Figure F8 summarizes the major lithologic units, lithology, and recovery for all holes on the Tiarei inner ridge.Lithologic Unit I (Postglacial Sequence)
Unit I is primarily composed of coralgal-microbialite frameworks. Manganese(?) impregnation and dark staining of corals is reported at the top of the unit. Basalt pebbles and volcanic grains are abundant toward the base of the unit.
Unit II comprises brown algal bindstone, microbialites, and coralgal frameworks that exhibit evidence of diagenetic alteration including the neomorphic transformation of coral skeletons and the occurrence of solution cavities. Basalt pebbles and lithoclasts occur throughout this unit.
Solution cavities present throughout this interval are filled with multigenerational infillings including well-lithified pale brownish limestone and poorly lithified dark brown sandy sediments including skeletal grains.Tiarei Outer Ridge: Sites M0009, M0021, M0024, M0025, and M0026
Figure F9 summarizes the major lithologic units, lithology, and recovery for all holes on the Tiarei outer ridge.Lithologic Unit I (Postglacial Sequence)
Unit I mostly comprises coralgal-microbialite frameworks that are locally interlayered with volcaniclastic sediments including coarse sand displaying skeletal elements (foraminifers, Halimeda segments, fragments of mollusks, bryozoans, corals, and especially branching Porites), rubble and sand composed mostly of coral fragments (mostly branching colonies and, to a lesser extent, tabular colonies), and skeletal packstone to floatstone rich in Halimeda segments and coral and mollusk fragments.
The top of Unit I is characterized by widespread development of micritic microbial crusts that display laminar and knobby morphologies. Extensive bioerosion, dark to brown staining within the uppermost 23 m of the unit, and hardgrounds are features that have been locally observed.
The development and morphologies of the microbial crusts are closely related to the morphology and size of the cavities in which they developed. In bindstone formed by encrusting coral assemblages, the microbialites are dominated by thrombolitic fabrics, whereas in frameworks made of branching and massive coral colonies they display greater development and are characterized by the development of compound crusts, up to 15 cm thick, formed by a succession of laminated and thrombolitic fabrics, where the thrombolites usually represent the last stage of encrustation. The thrombolites consist of closely spaced and vertically and laterally intergradational micritic masses that range from narrow millimeter-sized upward-radiating shrubs to broader dendritic clusters as high as 1 cm. Multiple generations may be closely packed and merge into micritic crusts as thick as several centimeters.Lithologic Unit II (Older Pleistocene Sequence)
The contact between Units I and II is characterized by an irregular unconformity typified by an abundance of large solution cavities partly filled with unconsolidated skeletal and volcanic sand including coralline algal branches and Halimeda segments and coral gravels (Pocillopora branches and fragments of Montipora colonies). Some cavities are partly filled with skeletal and volcanic sands and gravels and stalagmite crusts. Several unconformities occur in the upper part of Unit II.
The boundary between the two units is marked by a change from silts/sands to siltstone/sandstone and a color change from gray to orange-brown. Coincidentally, in Hole M0021B, a similar color change coincides with the Unit II/I boundary. Figure F10 summarizes the major lithologic units, lithology, and recovery for all holes of the Tiarei marginal sites.Lithologic Unit I (Postglacial Sequence)
The boundary between Units I and II is sharp. Unit II is composed mainly of well-lithified gray to light brown coralgal boundstone, coral rudstone, and skeletal sandy limestone, interlayered locally with horizons of gravels and rubble made of that material. The coral assemblage is dominated by foliaceous colonies of Pachyseris, tabular and branching colonies of Acropora, robust branching Pocillopora, encrusting and branching colonies of Montipora, and branching and massive colonies of Porites. Microbialites are abundant and include laminated and thrombolithic fabrics. The matrix of the limestone is rich in Halimeda segments; volcanic grains are locally abundant. Subaerial diagenetic processes are indicated by the alteration of coral skeletons and the occurrence of large solution cavities that are filled with volcaniclastic and skeletal sandstone and by gravels and rubble including branches of Pocillopora and basalt pebbles at the top of the unit.Faaa: Sites M0019 and M0020
The following description concerns Sites M0019 and M0020, which were drilled at 59.9 and 83.7 mbsl, respectively. The location of the boundary between Units I and II in the two holes was defined on the basis of lithologic and diagenetic features; it occurs at 82 m in Hole M0019A and at 92 m in Hole M0020A. Figure F11 summarizes the major lithologic units, lithology, and recovery for all holes of the Faaa sites.Lithologic Unit I (Postglacial Sequence)
Unit I at Sites M0019 and M0020 is 21 and 8 m thick, respectively, and displays a very similar composition in the two holes. It primarily comprises loose coralgal-microbialite frameworks (bindstone) interlayered with beds of coral rubble. The beds of coral rubble are composed of reworked and rounded fragments of corals (branching agaricids and Porites), coralline algal crusts, and microbialites that are extensively bored and stained.
The coralgal-microbialite frameworks are dominated by encrusting colonies of Montipora, agaricids (Pavona?), Acropora, Psammopora, and Echinophyllia associated locally with massive colonies of Porites, Montastrea, Cyphastrea, and Leptastrea and encrusting colonies of Leptoseris and fragments of robust branching colonies of Pocillopora, tabular colonies of Acropora, and branching Porites in addition to the coral colonies listed before. Microbialites consist of dark gray laminated dense and thrombolitic fabrics; the latter are usually dominant. Large cavities, partly to fully filled with skeletal sand rich in Halimeda segments (Halimeda packstone), commonly occur. Reddish brown to dark staining on the surface of reef rocks is conspicuous in some cores.Lithologic Unit II (Older Pleistocene Sequence)
Unit II at Sites M0019 and M0020 is 43 and 33 m thick, respectively, and displays a distinctive composition in the two holes. The coralgal-microbialite frameworks in the bulk of this unit are characterized by the widespread development of microbialites.
In Hole M0019A, Unit II includes three subunits (Subunits IIAIIC), characterized by their distinctive lithology and composition and separated by unconformities at 106.2 and 121.12 mbsl, respectively:
In Hole M0020A, Unit II is primarily composed of coralgal frameworks locally interlayered with skeletal limestone and rubble beds. The coralgal frameworks are dominated by branching and encrusting Porites, locally associated with encrusting Montipora and Pavona, and robust branching colonies of Pocillopora. The interlayered skeletal limestone consists of Halimeda wackestone and poorly sorted coral rudstone including fragments of branching and encrusting Porites, robust branching Pocillopora and Acropora, and encrusting Montipora. Other skeletal grains include Halimeda segments and fragments of mollusks and echinoids; volcanic silt to sand grains are locally abundant.Physical Properties
Carbonates are major constituents of the sedimentary rock record. Because coralgal reefs develop in shallow water, they are interesting systems to study. These kinds of sedimentary systems are most sensitive to climate and sea level fluctuations but also have great aquifer and reservoir potential. From a rock physics point of view, Expedition 310 was particularly interesting because it provided the unique opportunity to (petro)physically quantify modern reef development before pronounced overprinting by diagenetic processes (including mechanical compaction), which usually results in irresolvable complexities as very often encountered in aquifer and reservoir characterization.
A crossplot of velocity versus porosity for all sites shows a negative inverse relationship (Fig. F12) between acoustic velocity (VP) and porosity. Instead of showing individual data points for the bulky multisensor core logger (MSCL) data (15,191 data points), data density was contoured. MSCL data were acquired cross core (over ~6.5 cm), whereas core plugs were never longer than 3 cm and could only be drilled in appropriate core sections. The scale dependency of petrophysical measurements, along with the (inevitable) difference in "selective" sampling of core as opposed to bulk MSCL measurements is beautifully illustrated in Fig. F12: for a given porosity value, discrete measurements have higher VP values than MSCL measurements. Furthermore, in the comparison of velocity modeled using widely used velocity models and the real data, the modified (to improve relationships observed in larger-scale investigation of downhole wireline logging data) Raymer time average equation (Raymer et al., 1980) gives the best fit (smallest error) with MSCL data, whereas the Wyllie equation (Wyllie et al., 1958) shows the best fit for discrete samples. However, the difference between both velocity transforms and the real data points is still high. On the high end of the range in velocity for a given porosity, these differences can be interpreted as the added effect of pore characteristics like pore shape and connectivity and textural properties of the (overall coralgal) framework. The differences on the low end of the range in velocity for a given porosity may very well originate from lack of burial compaction and/or pronounced diagenesis.Downhole Logging
Downhole geophysical logs provide continuous information on physical, chemical, textural, and structural properties of geological formations penetrated by a borehole. In intervals of low or disturbed core recovery, downhole geophysical logs provide the only way to characterize the borehole section. This is especially true when recovery is poor and when comparable measurements or observations are obtained from core, as downhole geophysical logging allows precise depth positioning of core pieces by visual (borehole images) or petrophysical correlation.Borehole Geophysical Instruments
The set of borehole geophysical instruments utilized during Expedition 310 was constrained by the scientific objectives and the geological setting of the expedition. A suite of downhole geophysical methods was chosen to obtain high-resolution images of the borehole wall, to characterize the fluid nature in the borehole, to measure borehole size, and to measure or derive petrophysical or geochemical properties of the formation such as porosity, electrical resistivity, acoustic velocities, and natural gamma radioactivity. Because of environmental constraints, no nuclear tools were deployed during Expedition 310.
A total of 10 boreholes were prepared for downhole geophysical measurements. All measurements were performed under open borehole conditions (no casing) with the exception of a few of spectral gamma ray logs. After completion of coring, the drill string was pulled and the coring bit was changed for an open shoe casing to provide borehole stability in unstable sections and a smooth exit and entry of logging tools. In addition, a wiper trip was performed with fresh seawater (no drilling mud was used). Borehole conditions were extremely hostile, and very often the boreholes had to be logged in intervals where the HQ drill string was used as temporary casing, resulting in a nominally 100 mm diameter borehole. In order to record ultra-high-resolution geophysical downhole logging data, the acquisition was done in the rooster box, which, in the used piggy-back drilling system, is heave-compensated. Because of these difficult borehole conditions and time constraints it was not possible to log all tools in every borehole.Preliminary Results
Wireline logging operations at the Tiarei sites produced nearly complete downhole coverage of lithologic Unit I from 72 to 122 m below present-day sea level (Fig. F13). Because of very hostile borehole conditions around the Unit II/I boundary, it was not possible to image this boundary properly, with the exception of Hole M0023B.
The measured geophysical parameters, including optical and acoustic borehole wall images, provided the only source of continuous information of the drilled sequences during Expedition 310. Furthermore, by "unrolling" the images of the borehole wall (360°), into a two-dimensional view, a cross section ~31 cm is obtained as compared to a 6.54 cm cross section of a split core obtained by HQ-diameter drill bit. It was therefore possible to identify a typical stacking of lithofacies from the continuous downhole geophysical logs. A typical stacking of facies is grouped into a subsequence.
In Figure F13, borehole images and natural radioactivity logs (total counts only) are plotted in meters below present-day sea level. In each of the logged boreholes, the boundary between lithologic Unit II and Unit I is indicated. In the Tiarei transect, the basal unit directly overlying the above-mentioned boundary consists of a semiconsolidated rubble interval having elevated natural gamma radioactivity values in proximal locations. Distal locations do not show this higher gamma ray signature and, although poorly recovered in downhole logging data, it usually consists of very open framework branching corals that are heavily encrusted.
In Figure F14, borehole images and formation electrical resistivity (resistivity) are plotted in meters below present-day sea level. At this larger scale of observation and by correlating the boreholes from the outer ridge to the inner ridge, it becomes clear that, although evolved on a relatively steep and irregular paleomorphology, the general resistivity pattern and absolute values of lithologic Unit I along this transect are essentially the same and comparable. In each borehole the basal interval has lowest resistivity values, values increase gradually to a maximum value, after which a more sharp negative excursion to lower values can be observed. The interval of increasing resistivity values is interrupted by a subtle but clear decrease in the middle of lithologic Unit I. The absolute values of this decrease are higher in distal boreholes than in the proximal boreholes.
Wireline logging operations at the Maraa sites produced nearly complete downhole coverage of lithologic Unit I from 102 to 41.65 m below present-day sea level (Fig. F15). Very hostile borehole conditions are caused by open framework coral morphologies and relatively soft microbialite encrusting along and over coral colonies. Overall, acoustic reflectivity values in the ABI-40 image logs are lower than those at Tiarei. These conditions did not allow image "recovery" as high as that for the Tiarei transect and, similar to Tiarei, the boundary between lithologic Units I and II could not be imaged. Spectral gamma ray logs through the steel casing do, however, indicate significant count increases in lithologic Unit II. In Figure F15, borehole images, natural radioactivity logs (total counts only), and electrical resistivity logs are plotted in meters below present-day sea level. In each of the logged boreholes, the boundary between lithologic Units I and II is indicated. The depth below present-day sea level of the Pleistocene boundary depends on paleoseafloor morphology at the time of the LGM. Although the quality and meters covered in imaging lithologic Unit I is less at Maraa than at Tiarei, a similar stacking of lithofacies can be identified.Microbiology
Drill core samples from Tahiti reefal environments were analyzed for evidence of microbial activity, possibly related to formation of microbialites. To date, onboard measurements have shown a certain degree of microbial activity, directly attached to mineral surfaces, which could be involved in microbialite formation. According to the activity measurements along the drill cores, the uppermost part, 04 meters below seafloor (mbsf) of the Tahiti reef environment, is the most active zone. This is a common trend in reef environments due the closeness to the photic zone inhabited by primary producing eukaryotes, such as algae. Pure microbiological activity was only observed in reef cavities where prokaryotic biofilms have appropriate conditions to develop (Figs. F16, F17).
Preliminary results show that the biofilms are diverse in structure and color. Figure F16 shows an association between a brown iron/manganese crust and biofilm, whereas Figure F17 shows an unusual a blue biofilm, which exhibited the highest degree of adenosine triphosphate (ATP) activity (20'600 RLU [relative light units]). In this sample, it was possible to define spherical assemblies of carbonate minerals embedded in the microbial exopolymeric substances (EPS). Figure F18 shows 4,6-diamindino-2-phenylindole (DAPI)-stained cells in high densities in this biofilm in conjunction with mineral precipitation, which is most probably carbonate.
The biofilms appear to have high diversity in macroscale observations, and they are equally diverse and heterogenic in microscale resolution, as observed by scanning electron microscopy (SEM). Carbonate minerals appear in close relation with claylike minerals (Fig. F19; Hole M0007C), carbonaceous microfossils (Fig. F20; Hole M0015B), and oxidized and reduced Fe minerals such as pyrite (Fig. F21; Hole M0023A). The metabolic processes responsible for the precipitation of this variety of minerals could not, as yet, be defined because of the complex and diverse conditions of the microenvironments where these biofilm were found.
Some evidence for heterotrophic metabolic activities is shown by exoenzyme measurements, which vary in the different biofilm samples. For instance, the samples from Holes M0020A (4.51 mbsf) and M0009D (3.64 mbsf) showed high phosphotase activity, suggesting a heterotrophic community that preferentially degrades organic bound phosphate compounds such as phospholipids or nucleic acids. In contrast, Hole M0007B (6.28 mbsf) showed only glucosidase and aminopeptidase activity, which is evidence for degradation and metabolization of polysaccharides and proteins.
Isolation of microorganisms from biofilm samples was performed on agar plates using a medium that is selective for heterotrophic bacteria. After 2 weeks incubation time, 10 different heterotrophic colonies could be isolated (Fig. F22). From anaerobic experiments, only one isolation was successful. Distinct groups of microorganisms are associated in the biofilm that could range from aerobic to anaerobic metabolism. SEM investigation of microbialite samples shows evidence that anaerobic conditions must have prevailed at times. The occurrence of framboidal pyrite well distributed in the sediment supports a certain degree of anoxia in the environment. Some sediment samples also show a close spatial association between the mineral phase and microbes (e.g., Figs. F20, F23).
As a biogeographical summary of microbial activity in the Tahiti reef, northwestern Faaa Hole M0020A and southwestern Maraa Holes M0005C, M0007B, M0007C, M0015B, and M0018A were more active than the northeastern Tiarei sites, where often no living biofilm could be detected in cavities along the core.Geochemistry
Most importantly, interstitial water (IW) data from Expedition 310 demonstrate the ability to obtain geochemical signals (significant deviations from seawater chemistry) from fossil reef material. Rhizon sampling enabled IW samples to be taken where traditional whole-round squeezing was not possible and undesirable. For these reasons alone, the IW results from Expedition 310 are successful. The IW data indicate that no significant contamination resulted from using seawater as the drilling fluid through the lack of ubiquitous metallic enrichments compared to Tahitian seawater. However, storage of samples in sealed glass vials following IODP tradition resulted in serious B and Si contamination of the samples upon opening of the vials. In the future, acid cleaned plastic bottles should be used to store IW samples.General Remarks about Site M0008 Interstitial Water Data
Hole M0008A was drilled much more proximal to the island of Tahiti than any other sites and recovered volcanoclastic sediments. This made Hole M0008A unique compared to the other holes and offered opportunities to sample and analyze pore waters. In all the IW profiles, a barrier to diffusion is evident at ~18 mbsf, which corresponds to the position of a large basalt boulder recovered in Section 310-M0008A-8R-1. The lack of a chilled margin at the top and bottom of this basalt suggests it is not a continuous layer of basalt providing an impervious layer. However, the seismic profile used for site selection indicates a strong continuous reflector at approximately the same depth, suggesting the basalt may represent a continuous layer. Ultimately, with no recovery of Core 310-M0008A-7R above the basalt, the nature of the diffusion barrier will remain uncertain.pH, Alkalinity, Ammonia, Chloride, and Sulfate
Above ~18 mbsf the pH of IW samples is similar to ambient seawater, but below the diffusion barrier pH decreases sharply with depth until the pore waters are slightly acidic (Fig. F24A). The IW alkalinity profile essentially traces that of pH (Fig. F24B). This increase in free H+ ions results in an undersaturation of aragonite and calcite as calculated using PHREEQC (Parkhurst and Appelo, 1999) and shown in Figure F24A. Just below ~18 mbsf, significant amounts of ammonia are detected (Fig. F24C) indicating microbial activity. The ammonia appears to diffuse downward from this source just below the diffusion barrier. Chloride in the pore waters is essentially similar seawater with a slight depletion at the bottom of the section, precluding the influence of significant amounts of freshwater in these sediments. Sulfate concentrations do not significantly deviate from seawater values at any depth.Mg, K, Ca, and Sr
There is no significant deviation of IW Mg concentration from that of seawater, whereas K becomes depleted with depth below the diffusion barrier at ~18 mbsf (Fig. F24D). Both Ca and Sr concentrations become highly elevated with depth below the diffusion barrier (Fig. F24E, F24F), indicating dissolution of carbonate debris and/or weathering of the silicate material. The calculated undersaturation of calcite and aragonite in these pore waters suggests that carbonate dissolution must be contributing to these enrichments.Li, P, Mn, Fe, and Ba
Li is depleted from seawater value (~174 µg/L) at all depths in Hole M0008A, suggesting Li uptake by clays in the siliclastic sediments (e.g., Zhang et al., 1998). P displays little variability with depth. Enrichment of the pore waters in Mn is observed in all samples from Hole M0008A, but there is an important source at ~20 mbsf below the diffusion barrier (Fig. F24G). Fe is greatly elevated in the IW samples from above the diffusion barrier at ~18 mbsf but was below detection in the samples from below the diffusion barrier (Fig. F24H). This pattern is interesting because conventional wisdom suggests that Mn oxide reduction occurs above the zone of Fe oxide reduction in marine sediments (e.g., Berner, 1980). One possible explanation for this pattern is that most Fe has already been lost from the older sediments below the diffusion barrier. However, iron oxiderich sediments containing small root fragments indicative of a laterite soil were recovered in this interval of Fe-free pore waters. It is very interesting that Mn can be mobile in these sediments below the diffusion barrier and Fe is not. Ba is highly enriched in the pore waters below the diffusion barrier at ~18 mbsf, leading to a calculated barite oversaturation.X-Ray Fluorescence
Five samples of volcanic sand/silt units and nine individual basalt samples were selected for bulk rock analysis by energy dispersive polarized X-ray fluorescence (EDP-XRF) analysis. All the samples analyzed by this technique were taken from Hole M0008A. Analytical results are shown in Table T3. The low SiO2 but fairly high K2O, P2O5, and TiO2 contents (Na2O was not analyzed) suggest that the volcanic rock samples belong to the alkalic basalt clan (i.e., alkalic basalt, basanite, tephrite, and nephelinite). The two texturally similar samples that apparently come from a single boulder show many compositional similarities, but they also have differences, especially with respect to MgO contents. The differences may be due to flow differentiation, which is known to create compositional variation within a lava flow, or to instrumental error. Two samples are fairly primitive (>14.0 wt% MgO and >350 ppm Ni), but one is highly olivine-pyroxene phyric, and thus its composition may have been compromised by crystal accumulation of the olivine and pyroxene. Despite their small number, the rock samples apparently show downhole compositional variation (Fig. F25). Samples from the upper part of the hole have slightly higher incompatible element contents (e.g., K and Rb) but lower SiO2 than those from the lower part of the hole, indicating that the shallow basalt samples are compositionally more alkalic than the deeper samples.
The volcaniclastic sand/silt samples are compositionally different from the whole rock basalt samples in that they have lower SiO2 contents, which translates to lower total weight percent of all major oxides, obviously due to higher volatile contents, mainly seawater, as evidenced by their high Cl contents. Sulfur is also high in the sand/silt samples, particularly in the upper samples. The only rock sample that is high in both S and Cl is a scoriaceous pebble that probably contains a lot of pore water. Nevertheless, the upper sand/silt samples show many compositional similarities with the basalts, but the lower samples do not. The upper and lower sand/silt samples show apparent compositional differences (Fig. F26). Downhole compositional variations are observed in pore water chemistry, and thus the compositional differences between the upper and lower sand/silt units are probably real. Unlike the downhole compositional variation of the basalts, the sand/silt compositional differences are most probably due to differences in the depositional environment between the upper and lower volcanic sand/silt units.
Both Tahiti-Nui and Tahiti-Iti are composed predominantly of lava flows of moderately alkalic to strongly alkalic basalts; plutonic rocks are subordinate in amount and only exposed near their eroded centers (e.g., McBirney and Aoki, 1968; Cheng et al., 1993; Duncan et al., 1994). Differentiated rocks occur in subordinate amounts and were erupted mostly in the waning stages of volcanic activity. Thus, the lithologic composition of the volcaniclastic sediments drilled during Expedition 310, particularly the preponderance of lithic basalt clasts, unsurprisingly, is a direct reflection of the geology of the island. Compositionally, the rock samples and two upper sand/silt units plot with the rest of the igneous samples from Tahiti (Fig. F27). The two primitive basalts plot with the relatively fewer high-MgO basalts from Tahiti, whereas the other basalt samples plot with the more abundant, lower-MgO basalts. Many of the lower-MgO basalts in Tahiti can be modeled as crystal fractionation daughters of the higher-MgO parents (e.g., Cheng et al., 1993; Duncan et al., 1994). The two upper volcaniclastic sand/silt samples plot between the two basalt groups, either because their lithic basalt components are less fractionated than the lower-MgO basalts or they are similar to the latter but contain additional pyroxene mineral components. The three lower sand/silt samples generally plot outside the Tahiti lava field, suggesting that their compositions have been modified or altered.