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doi:10.2204/iodp.proc.310.101.2007 Site resultsCores were recovered from 37 holes at 22 sites (M0005–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). The initial strategy of coring along profiles has been applied, although the locations of some proposed drill sites were slightly changed because of difficulties locating and operating at those sites (see the “Operations” sections in the individual site chapters). Three transects were drilled in two regions: Tiarei (transect between inner and outer ridge sites) and Maraa (eastern and western transects). Other drilling areas include marginal sites at Tiarei and the Faaa sites. All sites were drilled within areas approved by the 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 assemblagesCored material shows that the fossil reef systems around Tahiti are composed of two major lithological units: a last deglacial 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–M0007The western transect drilled in the Maraa area (southwest Tahiti) includes Sites M0007, M0005, and M0006 (landward to oceanward) in water depths between 41.65 and 81.58 m. Figure F6 summarizes the major lithologic units, lithology, and recovery for all holes on the Maraa western transect. Modern sedimentsModern sediments were recovered at all sites along this transect. They consist of a few decimeter-thick beds comprising rhodoliths, skeletal sand, 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 (last deglacial carbonate sequence)The thickness of Unit I ranges from 33.22 to 44.56 m at Sites M0005 and M0007, respectively. 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 skeletal limestone and loose skeletal sediments, including coral and algal rubble and skeletal sand. 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 nongeniculate coralline algae. Microbialites generally comprise a suite of fabrics including two end-members represented by laminated fabrics and thrombolitic accretions; 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 top of Unit II was recovered at 92.85 and 86.21 mbsl at Sites M0005 and M0007, respectively. This sequence was drilled to 161.8 mbsl in Hole M0005D. The older Pleistocene sequence is composed of eight distinctive lithologic subunits:
Maraa eastern transect: Sites M0015–M0018The 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 81.8 mbsl. Figure F7 summarizes the major lithologic units, lithology, and recovery for all holes on the Maraa eastern transect. Lithologic Unit I (last deglacial sequence)The thickness of Unit I ranges from 35.44 in Hole M0015A at 72.15 mbsl to 39.05 m in Hole M0018A at 81.8 mbsl. The base of the unit has been recovered from 94.67 mbsl at the inner sites to 122.64 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 IA–IC) displaying distinctive coral assemblages and internal structure. Corals are usually thinly encrusted by nongeniculate coralline algae, except in Subunit IC, where the algal 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.
Lithologic Unit II (older Pleistocene sequence)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 grainstone, rudstone, and floatstone are rich in fragments of corals (e.g., robust branching colonies of Pocillopora, branching and massive colonies of Porites, branching colonies of Acropora, and encrusting colonies of Montipora), coralline algae, echinoids and mollusks, and Halimeda segments. Sand-sized volcanic grains are commonly associated with skeletal grains. Coralgal frameworks comprise encrusting and branching colonies of Porites, tabular colonies of Acropora, encrusting and massive colonies of Montipora, and encrusting colonies of agariciids; massive colonies of Porites occur locally. Tiarei inner ridge: Site M0023Figure F8 summarizes the major lithologic units, lithology, and recovery for all holes on the Tiarei inner ridge. Lithologic Unit I (last deglacial sequence)Unit I is primarily composed of coralgal-microbialite frameworks. Manganese(?) impregnation and dark staining of corals occurs at the top of the unit. Basalt pebbles and volcanic grains are abundant toward the base of the unit. Unit I is characterized by three successive coral assemblages (Subunits IA–IC):
Lithologic Unit II (older Pleistocene sequence)Unit II comprises brown algal bindstone, microbialites, and coralgal frameworks that exhibit evidence of late diagenetic overprints, including the alteration 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 that include skeletal grains. The coral assemblage includes massive and encrusting colonies of faviids, robust branching and tabular colonies of Acropora, encrusting colonies of agariciids, and branching colonies of Porites. Tiarei outer ridge: Sites M0009, M0021, and M0024–M0026Figure F9 summarizes the major lithologic units, lithology, and recovery for all holes on the Tiarei outer ridge. Lithologic Unit I (last deglacial sequence)On the outer ridge of Tiarei, Unit I was recovered from 81.7 to 122.12 mbsl. The thickest continuous sequence was recovered from Holes M0021A and M0021B, in which its thickness reaches 29.98 m. 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 colonies of 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 as well as coral and mollusk fragments. The top of Unit I is characterized by the widespread development of micritic microbial crusts that display laminar and knobby morphologies. Extensive bioerosion, dark to brown staining within the uppermost 2–3 m of the unit, and hardgrounds are features that were locally observed. Coralgal-microbialite frameworks are characterized by distinctive internal structure and coral assemblages. Five lithologic and coral assemblages (Subunits IA–IE) display a general consistent trend:
The thickness and morphology 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, microbialites are dominated by thrombolitic fabrics, whereas in frameworks made of branching and massive coral colonies, they are characterized by the development of compound crusts, up to 15 cm thick, formed by a succession of laminated and thrombolitic fabrics, where thrombolites usually represent the last stage of encrustation. 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. Unit II comprises four lithologic subunits (Subunits IIA–IID) that are locally interlayered:
Tiarei marginal sites: Sites M0008, M0010–M0014, and M0022Holes dominated by volcaniclastic sedimentsHole M0008A exhibits a 38.7 m thick sequence (64.15–102.85 mbsl) composed of two lithological units:
The boundary between the two units is marked by a change from silt/sand to siltstone/sandstone and a color change from gray to orange-brown. 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 at Tiarei marginal sites. Holes composed of volcaniclastic sediments and carbonatesLithologic Unit I (last deglacial sequence)In holes located close to the outer ridge (Holes M0010A–M0014A), Unit I comprises three successive subunits (Subunits IA–IC) that were recovered from 78.85 to 117.98 mbsl:
Lithologic Unit II (older Pleistocene 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 colonies of Pocillopora, encrusting and branching colonies of Montipora, and branching and massive colonies of Porites. Microbialites are abundant and include laminated and thrombolitic 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 M0020Sites M0019 and M0020 were drilled at 58.5 and 83.3 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 81.94 mbsl in Hole M0019A and at 92.05 mbsl in Hole M0020A. Figure F11 summarizes the major lithologic units, lithology, and recovery for all holes at Faaa sites. Lithologic Unit I (last deglacial sequence)Unit I at Sites M0019 and M0020 is 23.19 and 8.75 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. These beds are composed of reworked and rounded fragments of corals (branching colonies of agariciids and Porites), coralline algal crusts, and microbialites that are extensively bored and stained. The coralgal-microbialite frameworks are dominated by encrusting colonies of Montipora, agariciids (Pavona?), Acropora, Psammocora, and Echinophyllia associated locally with massive colonies of Porites, Montastrea, Cyphastrea, and Leptastrea, encrusting colonies of Leptoseris, and fragments of robust branching colonies of Pocillopora, tabular colonies of Acropora, and branching colonies of Porites. 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 black 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 IIA–IIC) 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. These frameworks are dominated by branching and encrusting Porites, locally associated with encrusting colonies of 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 colonies of Porites, robust branching colonies of Pocillopora and Acropora, and encrusting colonies of Montipora. Other skeletal grains include Halimeda segments and fragments of mollusks and echinoids; volcanic silt to sand grains are locally abundant. Physical propertiesCoral reefs present special challenges for geological and geophysical studies. Reef growth is highly variable over small spatial scales, and widely spaced cores may not accurately resolve patterns of coral accumulation (Jordan and Wilson, 1998). Generally, carbonate systems are difficult to image using reflection seismic technologies because of their spatially highly variable acoustic properties, which are a result of their intrinsic pore systems during evolution and diagenetic overprint. At the same time, carbonate reef deposits hold more than half of the world’s hydrocarbon accumulations and represent major water aquifers in many places around the globe. Expedition 310 presents a unique opportunity to petrophysically quantify modern reef development before pronounced overprinting by diagenetic processes and to advance the understanding of such acoustically complex coralgal reef systems. Three sequences were identified during Expedition 310, each with specific physical properties. As the physical properties in carbonates are highly variable, no particular values of individual parameters are given in the following section; instead, they are included in individual site chapters. The last deglacial sequence comprises coralgal frameworks heavily encrusted by microbialites locally interlayered with coarse skeletal sand and gravels rich in coral and algal fragments. Large primary cavities in coralgal frameworks are open or partially filled with skeletal sand and gravels, including coral and coralline algal fragments, and with Halimeda segments. Microbialites are very abundant and represent the major and structural and volumetric component of the reef rock. The open framework and centimeter- to decimeter-size pores result in a highly variable physical system. Physical properties change on a centimeter scale and may range from low porosity, high density and velocity to 100% open pore space (see Fig. F67 in the “Maraa eastern transect” chapter). In situ corals, microbialites, and skeletal sand all have a distinct petrophysical signature (acoustic velocity, porosity, grain density, and color reflectance L* values) but cannot be correlated easily from hole to hole. The last deglacial sequences at Maraa and Tiarei display a difference in level of volcaniclastic component, with the reef at Tiarei containing a greater volumetric component, which can be observed on digital image scans and quantified by both diffuse color reflectance spectrophotometry and magnetic susceptibility logs. Minor amounts of noncarbonate material, clay, and/or quartz silt/sand can dramatically change acoustic velocities (Stafleu et al., 1994; Kenter et al., 1997; Anselmetti and Eberli, 2001). Magnetic susceptibility values are lower in the last deglacial sequence at Maraa than in that of Tiarei (cf. Fig. F69 in the “Maraa western transect” chapter and Fig. F11 in the “"Tiarei inner ridge” chapter). Also, clear subdivision of intervals with different inputs of magnetic minerals is possible within individual sites. For instance, in Hole M0025B (see Fig. F67 in the “Tiarei outer ridge” chapter), the interval from 0 to 8 meters below seafloor (mbsf) displays a different magnetic susceptibility response than the one from 8 mbsf downward. It is inferred that the course of the Tiarei River changed through time and delivered different amounts of volcaniclastic material to the reef. The contact between the last deglacial sequence and the older Pleistocene carbonate sequence is characterized by an irregular unconformity; abundant solution cavities at the top of the underlying limestone are partly filled with unconsolidated skeletal and volcanic sand, including coralline algal branches, coral gravels, and Halimeda segments. The change in physical properties at this contact is sharp and abrupt. Density, velocity, and magnetic susceptibility increase, whereas porosity decreases (e.g., Hole M0007A, Fig. F69 in the “Maraa western transect” chapter, and Hole M0015A, Fig. F66 in the “Maraa eastern transect” chapter). The older Pleistocene sequence is composed of (1) well-lithified skeletal packstone to floatstone/rudstone, (2) well-lithified coralgal frameworks, and (3) beds comprising rubble and gravels primarily composed of coral clasts, limestone clasts, basalt pebbles, and reworked coral colonies. All three units are generally well lithified and have undergone several phases of diagenetic alteration. This results in tight, low porosity, high velocity limestones with a much smaller range in variation of physical properties than observed for the younger sequences. Volcanic pebbles and sands are locally interlayered and/or mixed with carbonate and are associated with an increase in magnetic susceptibility values (e.g., Hole M0005D, Fig. F68 in the “Maraa western transect” chapter). The third sequence, occurring in Hole M0008A, consists of volcaniclastic sediments and rocks: clays, un- to semilithified sandstone, basalt gravels, pebbles, and cobbles. Sandstone is commonly moderately lithified and has high densities and relatively low velocities. Basalts are high in density and velocity. Magnetic susceptiblity in this unit is generally high (e.g., Hole M008A, Fig. F9 in the “Tiarei marginal sites” chapter). Existing studies show that the primary factors controlling acoustic properties in carbonates are the amount and type of porosity, diagenetic properties, and mineralogy. Carbonates do not have large compositional variations that are, as is the case in other sedimentary rocks, responsible for velocity contrasts (e.g., Braaksma et al., 2003). Pure carbonates are characterized by a lack of any clay or siliciclastic content but are mostly produced and deposited on top of or on the slope of isolated or detached carbonate platforms, which have no hinterland as a source of terrigenous material (Wilson, 1975; Eberli, 1991). They consist of >95% of the carbonate minerals calcite (low and high Mg), dolomite, and aragonite. These minerals have very similar physical properties, which excludes compositional variation as a major reason for the large variability in velocity of carbonates. In Tahiti, however, reefs are contaminated with volcaniclastic input from the island. These inputs are more abundant at Tiarei than at Maraa but cannot be neglected for having influence on acoustic properties. Anselmetti and Eberli (1993) show that departures from general trends of correlation can be as high as 2500 m/s, and attribute this to the occurrence of different pore types that form both as a result of depositional processes and as specific diagenetic stages. Most studies show at the same time the poorly defined porosity-velocity trend and large deviations from classic and widely used velocity transforms as the Wyllie “time-average” (WTA; Wyllie et al., 1956, 1958) equation and the Raymer-Hunt-Gardner (RHD; Raymer et al., 1980) equation. In addition, theories that describe sonic wave propagation in porous media (Gassmann, 1951; Biot, 1956a, 1956b) are hard to apply in the complex system of pure carbonates because they form a variety of unique diagenetic rock fabrics with specific elastic properties. These complexities make the relationships between petrophysical properties and acoustic properties of carbonates rather complicated. A cross plot 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 bulky 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 difference in selective sampling of core as opposed to bulk MSCL measurements is illustrated in Figure F12; for a given porosity value discrete measurements have higher VP values than MSCL measurements. The WTA equation gives the best fit—the smallest error—with the MSCL data, whereas the RHD equation shows the best fit for the discrete samples. The deviation between both velocity transforms and the Tahiti data points, however, 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 mineralogical and textural properties of the coralgal-microbial frameworks. Differences on the low end of the range in velocity for a given porosity may very well originate from the lack of burial compaction and/or pronounced diagenesis. Downhole loggingDownhole 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 logs allows precise depth positioning of core pieces by visual (borehole images) or petrophysical correlation. The set of borehole geophysical instruments 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 (see the “Downhole logging” sections in the individual site chapters). 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 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 a temporary casing. To be able to record ultra high resolution geophysical downhole logging data, acquisition was done in the rooster box, which, in the piggyback drilling system used, is heave compensated. Because of difficult borehole conditions and time constraints, it was not possible to log all tools in every borehole. The measured geophysical parameters, including optical and acoustic borehole wall images, provided a source of continuous information of the drilled sequences. During Expedition 310, the televiewer tools (OBI40 and ABI40) had particular value. By unrolling the images of the borehole wall (360°) into a two-dimensional (2-D) view, a section of ~31 cm is obtained, compared to a 6.54 cm cross section of a split core obtained by HQ-diameter drill bit. These images allow not only the precise positioning of pieces of core in areas of low recovery but also permit direct determination of coral growth forms and verification if coral colonies are in life position. In addition, the images provide a superb macro- and mesoscale characterization of the pore system in reef rocks. Centimeter- to decimeter-size pores and cavities are common in reef frameworks. Core recovery was calculated at ~57%, but image logs show that by including the open pore space, true recovery was actually much higher, potentially >90%. This contributes to the conclusion that coring was highly successful in carbonate reef environments during Expedition 310. Wireline logging operations at Maraa sites produced nearly complete downhole coverage of the last deglacial sequence from 41.65 to 102 mbsl. Tools have different data coverage in various holes (see the “Downhole logging” sections in the individual site chapters). In Figure F75 in the “Maraa eastern transect” chapter, borehole televiewer images, natural radioactivity logs (total counts), and electrical resistivity logs are plotted in meters below sea level. In each of the logged boreholes, the boundary between the last deglacial and older Pleistocene sequences is indicated. The depth below present-day sea level of the top of the older Pleistocene sequence is highly variable, indicating a rugged morphology prior to the development of the last deglacial sequence. Resistivity does not show any direct signal, but spectral gamma ray logs indicate a significant increase of counts in the older Pleistocene sequence (Holes M0005D and M0007A). Caliper, borehole fluid characterization, and acoustic tools all contain particular information on parameters for specific intervals and lithologies. Wireline logging operations at Tiarei sites produced nearly complete downhole coverage of the last deglacial sequence from 72 to 122 mbsl. Tools have different data coverage in various holes (see the “Downhole logging” sections in the individual site chapters). It was not possible to image the boundary between Units I and II, except in Holes M0021B and M0023B. In Figure F79 in the “Tiarei outer ridge” chapter, borehole images and natural radioactivity logs (total counts) are plotted in meters below sea level. In each of the logged boreholes, the boundary between the last deglacial and older Pleistocene sequences is indicated. The basal part of Unit I, directly overlying this boundary, is often semiconsolidated and poorly recovered and shows elevated natural gamma radioactivity values in proximal locations (e.g., Hole M0023B). Distal locations do not show this higher gamma ray signature, with the exception of Hole M0021B. In Figure F80 in the “Tiarei outer ridge” chapter, borehole images and formation electrical resistivity (resistivity) are plotted in meters below sea level. The general resistivity pattern and absolute values in the last deglacial sequence along this transect are comparable across the holes. In each hole, the basal interval has the lowest resistivity values and then values increase gradually to a maximum value, after which a sharp negative excursion to lower values can be observed. However, the interval of increasing resistivity values is interrupted once by a subtle but clear decrease in the middle part of the last deglacial sequence. The absolute values of this decrease are higher at distal sites than at proximal ones. Caliper, borehole fluid characterization, and acoustic tools all contain particular information on parameters for specific intervals and lithologies. As in the Maraa transect, the depth of the top of the older Pleistocene sequence below modern day sea level in the Tiarei transect is highly variable and indicates a very uneven paleotopography before the last deglaciation. No logging operations were carried out at Faaa sites. Despite operational difficulties associated with logging recent coralgal reef systems, geophysical downhole data are of high quality. The continuous information quantifies a variety of physical properties of the Tahiti fossil reef systems. The results, and especially the imaging of the borehole wall using optical and acoustic geophysical methods, complement the sedimentological descriptions and allow an unambiguous correlation of cores (through coral assemblages and sedimentological features), core logs (MSCL), downhole logs, and drilling parameters. MicrobiologySamples from Tahiti drill cores were analyzed for evidence of microbial activity, possibly related to the formation of microbialites. To date, onboard measurements have shown a certain degree of microbial activity directly attached to mineral surfaces. According to the activity measurements along the drill cores, the uppermost part, 0–4 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. F13, F14). Preliminary results show that biofilms are diverse in structure and color. Figure F13 shows an association between a brown iron/manganese crust and a biofilm, whereas Figure F14 shows an unusual blue biofilm, which exhibited the highest degree of adenosine triphosphate (ATP) activity (20,600 RLU). In this sample, it was possible to define spherical assemblages of carbonate minerals embedded in microbial exopolymeric substances (EPS). Figure F15 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. Biofilms appear to have high diversity in macroscale observations, and they are equally diverse and heterogeneous in microscale resolution, as observed by scanning electron microscopy (SEM). Carbonate minerals appear in close relation with claylike minerals (Fig. F16; Hole M0007C), carbonate microfossils (Fig. F17; Hole M0015B), and oxidized and reduced Fe minerals such as pyrite (Fig. F18; Hole M0023A). The metabolic processes responsible for the precipitation of this variety of minerals could not be defined as yet because of the complex and diverse conditions of the microenvironments where these biofilms were found. Some evidence for heterotrophic metabolic activities is shown by exoenzyme measurements, which vary in the different biofilm samples. For instance, samples from Holes M0020A (4.51 mbsf) and M0009D (3.64 mbsf) showed high phosphotase activity, suggesting the occurrence of 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 indicates 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 (see Fig. F5 in the “Microbiology” chapter). 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 pyrite well distributed in the sediment supports the occurrence of 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. F17, F19). 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 usually no living biofilm could be detected in cavities along the core. GeochemistrySite M0008 interstitial water dataHole M0008A was drilled more proximal to the island of Tahiti than any other sites and recovered volcaniclastic sediments. This made Hole M0008A unique 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 (see Fig. F5 in the “Site surveys related to IODP Expedition 310” chapter), 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 sulfateAbove ~18 mbsf, the pH of IW samples is similar to ambient seawater, but below the diffusion barrier, pH decreases sharply with depth until pore waters are slightly acidic (Fig. F20A). The IW alkalinity profile essentially traces that of pH (Fig. F20B). This increase in free H+ ions results in an undersaturation of aragonite and calcite as calculated using the PHREEQC computer program (Parkhurst and Appelo, 1999) and shown in Figure F20A. Just below ~18 mbsf, significant amounts of ammonia are detected (Fig. F20C), indicating microbial activity. The ammonia appears to diffuse downward from this source just below the diffusion barrier. Chloride in pore waters is essentially similar to 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 SrThere is no significant deviation of IW Mg concentrations from that of seawater, whereas K becomes depleted with depth below the diffusion barrier at ~18 mbsf (Fig. F20D). Both Ca and Sr concentrations become highly elevated with depth below the diffusion barrier (Fig. F20E, F20F), 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 BaLi is depleted with respect to 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. F20G). 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. F20H). This pattern is unexpected 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 oxide–rich sediments containing small root fragments indicative of a laterite soil were recovered in this interval of Fe-free pore waters. It is 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 fluorescenceFive 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 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 family (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 changed by crystal accumulation of the olivine and pyroxene. Despite their small number, the rock samples apparently show downhole compositional variation (see Fig. F15 in the “Tiarei marginal sites” chapter). 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. Volcaniclastic sand/silt samples are compositionally different from 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 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 abundant pore water. Nevertheless, the upper sand/silt samples show many compositional similarities to the basalts, but the lower samples do not. The upper and lower sand/silt samples show apparent compositional differences (see Fig. F19 in the “Tiarei marginal sites” chapter). Downhole compositional variations are observed in pore water chemistry, and thus the compositional differences between the upper and lower sand/silt samples are probably real. Unlike the downhole compositional variation of the basalts, the sand/silt compositional differences are most probably due to differences in depositional settings between the upper and lower volcanic sand/silt intervals. Both parts of the island of Tahiti, 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, is a direct reflection of the geology of the island. Compositionally, the rock samples and the two upper sand/silt intervals plot with the rest of the igneous samples from Tahiti (see Fig. F20 in the “Tiarei marginal sites” chapter). 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 because 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. |
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