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doi:10.2204/iodp.pr.325.2010 Principal site resultsCores were recovered from 34 holes across 17 sites (M0030–M0058) (Table T1), with a conventionally calculated recovery of 26.63%. Hole depths ranged from 47.27 to 167.14 mbsl (LAT taken from corrected EM300 data), and cores were recovered from 42.27 to 208.5 mbsl. Four transects located within three geographical areas (Fig. F2) were drilled during Expedition 325: Hydrographer's Passage (2 transects; north and south), Noggin Pass (1 transect), and Ribbon Reef (1 transect). All holes were drilled within 125 m radius areas around the sites approved by the EPSP and were located within the transect areas defined by the GBRMPA. Borehole geophysical wireline logging was conducted at four holes. Because of space limitations on the Greatship Maya, only limited analysis of the cores was possible offshore. The bulk of the description and measurements on the whole and split cores was conducted during the Onshore Science Party (OSP) at the IODP Bremen Core Repository (Germany). Table T2 shows which measurements were conducted offshore and which ones at the OSP. Sedimentology and biological assemblagesThe core material shows that fossil reefs on the shelf edge of the GBR are composed of nine major lithologic types, defined as follows:
It is important to note that lithologic types of the same designation at different holes are not necessarily correlative in time. Comprehensive definition of distinct lithologic units and their correlation between sites will only be possible after a detailed analysis of the sedimentary facies and chronology data obtained during postcruise research. HYD-01C transect: Holes M0030A–M0039AThe northern Hydrographer's Passage transect, HYD-01C, includes (landward to seaward) Holes M0034A, M0030A and M0030B, M0031A, M0032A, M0033A, M0035A, M0036A, M0038A, M0039A, and M0037A at depths between 51.0 and 122.3 mbsl. Numerous holes are closely spaced (i.e., <20 m apart) and could be considered as a composite hole through distinct reef targets: Holes M0030A and M0030B in the 80 mbsl reef target, Holes M0031A–M0033A in the 90 mbsl reef target, and Holes M0038A, and M0039A in the 110 mbsl reef target. Figure F15 summarizes the major lithologic types and their distribution and recovery for the HYD-01C transect. Sedimentology and biological assemblagesCoralgal and coralgal-microbialite boundstones are the dominant lithologies recovered along the HYD-01C transect. Just below the modern seafloor, between one and three coralgal boundstone and coralgal-microbialite boundstone intervals occur in all the HYD-01C holes, except for Holes M0030A and M0030B, in which the recovery was low, and Hole M0037A, the most distal and deepest site on the transect (at 122 mbsl). Coralgal intervals (one or two, depending on the hole) contain little or no microbialite. Their thicknesses range from <1 m in Hole M0032A to 8 m in Hole M0031A. Coralgal intervals systematically overlie coralgal-microbialite intervals in Holes M0031A, M0032A, M0033A, M0035A, M0036A, M0038A, and M0039A. In Hole M0034A, a 2 m thick coral boundstone underlies an 18 m thick coralgal-microbialite interval, whereas in Hole M0036A, the coralgal boundstone is interbedded with 6 m of unconsolidated sediments. The main corals in the coralgal intervals are massive Isopora with smaller amounts of massive Porites, submassive to massive Montipora, and branching Acropora. Coralgal-microbialite intervals are dominated volumetrically by microbialites, and these boundstones are the thickest lithologies in every hole except Hole M0037A. They range in thickness from 10 m in Hole M0031A to ~30 m in Hole M0033A. Coral assemblages in the coralgal-microbial intervals are diverse. Although dominated by massive Isopora, branching Acropora, and Seriatopora, other corals such as massive Porites and Faviidae are the principal corals in localized intervals. In six of the eleven holes along the HYD-01C transect, an unconsolidated sediment interval ranging from <1 to 19 m thick underlies the upper coralgal-microbialite framework interval. In Hole M0034A, the unconsolidated interval is overlain by a coralgal level, whereas in Hole M0036A, the unconsolidated interval is included within the coralgal interval. This unconsolidated sediment interval was probably partly disturbed by coring operations and is composed of bioclastic lime sand to pebbles containing mollusks, larger benthic foraminifers, Halimeda, fragments of corals and red algae, bryozoans, echinoderms, and sea urchin spines. A thin (<3 m) skeletal packstone to grainstone interval rich in large benthic foraminifers, calcareous algae, and/or a dark coralgal–worm tube boundstone is interbedded with, or underlies, the unconsolidated sediment interval in Holes M0031A, M0032A, M0033A, M0035A, M0036A, M0038A, and M0039A. A second unconsolidated interval, similar to the first, forms the base of the recovered sequences in Holes M0031A and M0036A. As the most distal and deepest site (at 122 mbsl) along the HYD-01C transect, Hole M0037A also has a different sedimentary composition and lithologic succession, with a 12 m thick interval of unconsolidated lime sands to pebbles from the seafloor to the base of the hole. There is clear evidence of downhole contamination in the upper part of each section. This interval overlies very thin foraminifers, coralline algae, and coral fragment–rich grainstone. The base of the hole consists of an 8 m thick lime sand rich in large benthic foraminifers and mollusks. These deposits appear to be undisturbed and therefore are probably in situ with minimal disturbance from downhole contamination. Physical propertiesPartial recovery was achieved in holes drilled on the HYD-01C transect. With regard to physical property measurements, cores were only partially saturated and often underfilled, impacting the different types of data coverage and quality. Borehole depths are as follows:
In general, recovery was low, and the intervals recovered were often disturbed by drilling or partially unsaturated because of the unlithified to semilithified nature of the cored formations. Density and porosityDensity and porosity vary similarly in all of the boreholes drilled across the HYD-01C transect. Discrete sample porosity ranges from 20% to 50%. This is due to significant variability in the pore systems (e.g., moldic, vuggy, growth framework, and intergranular) (Fig. F16). Bulk densities of discrete samples vary between 1.7 and 2.4 g/cm3. Densities measured on whole cores with the multisensor core logger (MSCL) are <2 g/cm3. This is likely due in part to the partial saturation of the cores but also due to the majority of the core comprising unconsolidated fragments. There is a classic linear correlation between the porosity (φ) and the bulk density (ρ) of the discrete samples (Fig. F17):
where
This correlation demonstrates that the average grain density along the HYD-01C transect is 2.77 g/cm3. Grain density varies between 2.7 and 2.85 g/cm3 and may correspond to a value between the density of calcite (2.71 g/cm3) and aragonite (2.93 g/cm3). P-wave velocityA crossplot of velocity versus porosity (both from discrete samples) for all sites shows an inverse relationship (Fig. F18) between acoustic velocity (VP) and porosity. MSCL data, which were acquired cross core (over ~6.5 cm), range from 1500.34 to 1937.94 m/s, much lower values than discrete measurements acquired on core plugs. The scale dependency of petrophysical measurements, along with the (inevitable) difference in "selective" sampling of core as opposed to bulk MSCL measurements is evident: for a given porosity value, discrete measurements have higher VP values than MSCL measurements. 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, such as pore shape and connectivity, and textural properties of the coral and microbialite units. The differences on the low end of the range in velocity for a given porosity may originate from lack of burial compaction and/or pronounced diagenesis. Magnetic susceptibilityMSCL magnetic susceptibility data collected at this transect are difficult to interpret as a result of gaps in the data due to limited core recovery. However, it is clear that the majority of data falls between –5 x 10–5 and 5 x 10–5 SI across all the holes with occasional clear magnetic susceptibility highs defined by smooth curves. Electrical resistivityThe electrical conductivity of rock depends linearly on the electrical conductivity of the saturating fluid. In the presence of clays, an additional surface conductivity may be added to the previous volume conductivity. The volume conductivity of the rock is sensitive to the microgeometrical properties of the rock, such as porosity and tortuosity. Reliable resistivity measurements were difficult to obtain using the MSCL because of the presence of loose sediments or partially saturated rocks. When resistivity was measured on unconsolidated or sandy sediments, low resistivity values were found (e.g., Hole M0037A, 1–2 m CSF, where resistivity is between 1 and 2 Ωm). Relatively higher resistivities were found when measuring more consolidated sediments (e.g., Hole M0034A, 12–14 m CSF, where resistivity is between 10 and 30 Ωm [coral framework and microbialite]). A more detailed study of electrical properties of the sediments would require measurements with fully saturated discrete samples. Color reflectanceThe values of color reflectance spectrophotometry were calculated for each of the boreholes as discrete measurements. The main parameters measured are total reflectance (L*) and the color indexes a* (green to red, green being negative and red positive) and b* (blue to yellow, blue being negative and yellow positive). The ratio a*/b* was also calculated for all boreholes, as it can be used as a better proxy to identify changes in sediment characteristics than the independent values of a* and b*. Measurements were taken in the most uniform color zones in a unit. This is shown by the data in the sense that massive corals sampled in several points present a consistent pattern of color. In these situations, the data obtained show a main value with a small deviation for the three parameters (L*, a*, b*). In the locations where Tubipora sp. was found, a strong signal in the red spectrum (a*) was found. In most of the boreholes, slightly higher values of reflectance are present just below the seafloor where modern reef sediment was recovered. Along the HYD-01C transect, Holes M0031A, M0032A, and M0033A are located in similar water depths and can be correlated. No significant trends were found in these cores, but the reflectance for all of these boreholes shows similar values. This is also true for Holes M0035A and M0036A. Discrete measurements of reflectance values for all boreholes along the HYD-01C transect are represented in Figure F19. Boreholes are represented in landward to seaward order. All cores are presented in core depth below seafloor in meters. Downhole measurementsDownhole 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 cannot be obtained from core, as downhole geophysical logging allows precise depth positioning of core pieces by visual (borehole images) or petrophysical correlation. The slimline suite deployed at transect HYD-01C comprised the following tools:
Wireline logging operations on the HYD-01C transect provided two sets of comparable through-pipe gamma ray data. Very few open-hole data were acquired in Hole M0036A because of the hole instability. However, it is possible to discern three major logging units at these two sites solely considering the gamma ray data collected through American Petroleum Institute (API) pipe (Fig. F20):
Both total gamma ray (TGR) curves obtained at Holes M0031A and M0036A show a similar trend; therefore, it could expected that similar formations might be observed. However, there are some significant differences (see Figure F20). It is not certain whether the large amount of unconsolidated material cored at Hole M0031A is truly representative of the in situ formation or whether the differences between the two holes, which are only ~800 m apart (Hole M0031A at ~90 m water depth and Hole M0036A at ~103 m water depth), are related to low recovery and core quality. HYD-02A transect: Sites M0040A–M0048AThe southern Hydrographer's Passage transect, HYD-02A, includes (landward to seaward) Holes M0042A, M0048A, M0047A, M0043A, M0045A, M0046A, M0044A, M0040A, and M0041A at depths between 50.8 and 126.1 mbsl. Closely spaced (i.e., <5 m apart) Holes M0045A and M0046A can be seen as a composite hole in the 110 mbsl reef target and Holes M0040A and M0041A form another in the 120 mbsl reef target. Figure F21 summarizes the major lithologic types and their distribution and recovery on the HYD-02A transect. Sedimentology and biological assemblagesNo common pattern appears to link the lithologic successions in the holes along the HYD-02A transect, as is the case along the HYD-01C transect. The following highlights describe shared features observed in the eight holes along the HYD-02A transect rather than exceptions:
The boundstone lithologies contain variable proportions of coral, coralline algae, and microbialite that define several coralgal, coralgal-microbialite, and microbialite boundstones that are also observed in the other GBR transects. The major corals in the boundstones are submassive to massive Porites, Montipora, branching Pocilloporidae, branching Acropora, massive Isopora, and submassive to massive Faviidae. The common patterns of boundstone distribution spanning most of the holes are
Physical propertiesRecovery for the HYD-02A transect sites averaged ~21%. However, recovery in Holes M0040A and M0041A reached ~50%. Cores were partially saturated and often disturbed, fractured, or contaminated, which affects the quality of physical property data that can be collected. Borehole depths are as follows:
Plugs and samples taken for discrete P-wave and moisture and density (MAD) measurements were obtained from both consolidated and unconsolidated core material. Density and porosityBulk density was measured for the HYD-02A transect using the gamma ray attenuation (GRA) densitometer on the MSCL, providing an estimate of bulk density from whole cores. Discrete MAD measurements were also taken with a pentapycnometer on plugs and/or rock fragments, providing grain density, bulk density (in the case of plug samples), and porosity data. As in the previous transect (HYD-01C), a classical linear correlation was observed between the porosity (φ) and the bulk density (ρ) of the discrete samples measured in all boreholes along the HYD-02A transect (Fig. F22). This correlation demonstrates that the average grain density along the HYD-02A transect is 2.77 g/cm3. Grain density varies between 2.7 and 2.85 g/cm3 and may correspond to a value between the grain density of calcite (2.71 g/cm3) and aragonite (2.93 g/cm3). Porosity values for all boreholes in this transect are shown in Figure F23. Similar trends in porosity can be picked out in Holes M0047A and M0043A with a zig-zag step decrease in porosity at 0–12 m CSF-A followed by an increase at ~15 m CSF-A and a gradual decrease to ~25 m CSF-A. Holes M0040A and M0041A have almost identical trends in porosity with ~30% porosity (~0–10 m CSF) increasing to ~50% to the bottom of the drilled holes. P-wave velocityA crossplot of velocity versus porosity (both from discrete samples) for all sites shows primarily an inverse relationship (Fig. F24) between acoustic velocity (VP) and porosity. Whole-core MSCL data (across ~6.6 cm) range from 1508.59 to 1895.75 m/s. As expected, because of the targeted nature of taking discrete samples, much lower VP values were recorded by the MSCL offshore for coral and microbialite units compared to discrete measurements on core plugs measured during the OSP. Magnetic susceptibilityOffshore MSCL magnetic susceptibility data are very difficult to interpret for this transect because of limited core recovery in all holes along the transect. Values are generally similar across the holes, with the majority of readings in the –1 x 10–5 to 1 x 10–5 SI range, delineated by short intervals of magnetic susceptibility highs. Electrical resistivityOver the entire transect, resistivity is very variable, with the lowest values (0.56 Ωm) measured in Hole M0040A and the highest values (44.84 Ωm) recorded in Hole M0044A. Because of the relatively poor core quality and undersaturated cores, data should be treated with caution. Color reflectanceIn the HYD-02C transect, Holes M0048A, M0047A, M0043A, M0044A, and M0046A are located in similar water depths and can be correlated (with <5 m between the drilled holes). Holes M0047A and M0043A exhibit similar trends, but Hole M0047A presented less scatter in the values of reflectance per section, probably because of the presence of massive corals. Hole M0046A shows a trend in data points similar to the shallower distributed holes; however, there is a smoother distribution of the reflectance measurements. Holes M0048A and M0044A have similar values. However, because of the lack of measurements with depth in Hole M0048A no trend can be compared. Recovery in Hole M0045A was so low that color reflectance was not measured. Holes M0040A and M0041A are located very close to each other at the same water depth. Both boreholes exhibit less scatter in color reflectance measurements than other boreholes in this transect. Data from these boreholes show a consistent pattern of L* values of ~50% in the top 2.5 m of the hole, a slight increase at 6–8 m CSF, and a decrease in reflectance after again reaching ~50% at 21 m CSF-A. Color reflectance measurements for all the boreholes in the HYD-02C transect are represented in Figure F25; boreholes have been plotted from landward to seaward (left to right) at the same depth scale. For boreholes found at similar depths, similar trends are present in the color reflectance data, which suggests a possible correlation between them. Downhole measurementsWireline logging operations were performed at one API hole (M0042A) along the HYD-02A transect. The priority imaging tools (acoustic borehole image [ABI40] and optical borehole image [OBI40]) were also run to see if quality image data could be obtained in an API hole. However, the standard maximum hole diameter for successful image data acquisition is 15 cm, and API holes tend to have a minimum diameter ~20 cm. Because of this difference, images were not successfully collected. The slimline suite deployed at transect HYD-02A comprised the following tools:
Four main logging units were identified from the downhole data from Hole M0042A:
RIB-02C transect: Sites M0049A–M0051AThe RIB-02C transect includes (landward to seaward) Holes M0051A, M0050A, M0049A, and M0049B at depths between 78.1 and 97.6 mbsl. Holes M0050A, M0049A, and M0049B are closely spaced (i.e., <5 m apart) and form a composite hole in the 100 mbsl reef target. Figure F26 summarizes the major lithologic types, lithology, and recovery for all holes in the RIB-02C transect. Sedimentology and biological assemblagesOnly a very rough summary of the lithofacies distribution pattern can be proposed for the RIB-02C transect because of the overall poor recovery in Holes M0049A, M0049B, M0050A, and M0051A. A limited lithological succession can be established only in Holes M0049A, M0049B, and M0050A, in which drilling penetrated below modern and subrecent seafloor. In Hole M0050A, the recovered material likely corresponds to subrecent seafloor sediments mixed with fossil materials. The lithological succession observed is as follows:
Physical propertiesRecovery for the RIB-02A transect sites averaged ~16%. However, recovery in Holes M0049A and M0049B reached ~20%. Cores were partially saturated and often disturbed, fractured, or contaminated, which affects the quality of physical property data that can be collected. Borehole depths for the transect are as follows:
Plugs and samples taken for discrete P-wave and MAD measurements were obtained from both consolidated and unconsolidated material. Density and porosityBulk density was measured for RIB-02A transect samples using the GRA densitometer on the MSCL, providing an estimate of bulk density from whole cores. Discrete MAD measurements were also taken with a pentapycnometer on plugs and/or on rock fragments, providing grain density, bulk density (in the case of plug samples), and porosity data. Data present a classical linear correlation between the porosity (φ) and the bulk density (ρ) of the discrete samples measured in all boreholes along the RIB-02A transect (Fig. F27). The average grain density along the RIB-02A transect is 2.78 g/cm3. Grain density varies between 2.75 and 2.79 g/cm3 and may correspond to a value between the grain density of calcite (2.71 g/cm3) and aragonite (2.93 g/cm3). Porosity values for measured boreholes in this transect are shown in Figure F28. Porosity ranges from 17% to 45%; however, the majority of porosity measurements lie around 30%. P-wave velocityOnly two core plugs were collected from this transect, both from Hole M0049B. A crossplot of velocity versus porosity (both from discrete samples) for all sites shows primarily a negative? inverse relationship (Fig. F29) between acoustic velocity (VP) and porosity. Whole-core MSCL data (over ~6.5 cm) range from 1504.7 to 1845.36 m/s. As expected, because of the targeted nature of taking discrete samples, much lower VP values were recorded by the MSCL (offshore) for coral and microbialite units compared to discrete measurements taken on core plugs during the OSP. Magnetic susceptibilityMagnetic susceptibility (MSCL offshore) data are difficult to interpret for this transect because of low core recovery in all holes along the transect. Very few values were obtained on whole cores, but data range from –0.64 x 10–5 SI (Hole M0049B) to high values of 31.6 x 10–5 SI (Hole M0050A). The most data were collected for Hole M0049B; however, no obvious trends are visible in the data. Electrical resistivityOver the entire transect, resistivity is very variable, with the lowest values (0.56 Ωm) measured in Hole M0040A and the highest values (44.84 Ωm) recorded in Hole M0044A. Because of the relatively poor core quality and undersaturated cores, data should be treated with caution. Color reflectanceAlong the RIB-02A transect, recovery for Hole M0051A was very small (<10 cm), and only one value of color reflectance spectophotometry was taken. Holes M0049A, M0049B, and M0050A were located at the same water depth and are therefore comparable. Discrete measurements of reflectance values for all boreholes in this transect are represented in Fig. F30. No particular trends were observed in these three boreholes, but reflectance values were consistent in all cores where units were recovered at similar depths downhole. NOG-01B transect: Sites M0052A–M0058AThe NOG-01B transect includes (landward to seaward) Holes M0057A, M0056A, M0055A, M0053A, M0052A–M0052C, M0054A and M0054B, and M0058A (fore reef slope) at depths between 42.3 and 167.1 mbsl. Closely spaced (i.e., <5 m apart) holes M0053A and M0052A–M0052C form a composite hole in the –100 mbsl reef target. Figure F31 summarizes the major lithologic types and their distribution and recovery for all holes along the NOG-01B transect. Sedimentology and biological assemblagesMud and muddy sand are found at the top of Holes M0052A–M0052C, M0053A, M0054A, M0054B, and M0055A. Coralgal and coralgal-microbial boundstones occur below the mud interval or from the top of the recovered succession in Holes M0056A and M0057A. The coralgal boundstone reaches thicknesses of 4–15 m. It is dominated by corals encrusted by coralline algae. The algal crusts in many cases contain vermetid gastropods and the encrusting foraminifer Homotrema rubrum. The coralgal-microbial boundstone can be as thick as 10–16 m. In this lithology, thick crusts of microbialite, up to several centimeters, are found in addition to corals and coralline algae. The main corals in these boundstones are a diverse assemblage of branching Acropora, Seriatopora, massive Isopora, Porites, Montipora, Faviidae, and Tubipora. Unconsolidated sands as well as grainstone and rudstone lithologies are encountered below the two boundstone facies, except in Holes M0052A–M0052C, in which drilling did not penetrate below the boundstone. Grainstones/rudstones are composed of shell and skeleton fragments of corals, coralline algae, Halimeda segments, mollusks, and benthic foraminifers. The consolidated grainstones and rudstones are 4–13 m thick. In Holes M0053A, M0054A, and M0054B, there is a lime sand interval below the grainstone/rudstone horizon. The downcore succession of unconsolidated and/or modern reef sediment, boundstones, grainstones/rudstones, and lime sand in these two holes is reminiscent of the pattern observed along the HYD-01C transect. No material underlying the grainstone was recovered in Hole M0055A. In Holes M0056A and M0057A below the uppermost grainstone/rudstone interval, there is a long succession of limestone that includes boundstone, grainstone/rudstone, and packstone facies. Coralgal boundstones, 12 m thick, are the dominant lithologies in Hole M0057A, which includes a thin interval of packstones at the lowest interval of the boundstones. In Hole M0056A, coralgal-microbial boundstones, 8 m thick, overlie a 13 m thick succession of grainstones/packstones. Currently no pattern can be extracted in the succession of facies in these holes. The major corals observed in these deeper, older boundstones are encrusting submassive to massive Montipora, massive Porites, Faviidae, and occasionally Galaxea or Agariciidae. In Holes M0055A, M0056A, and M0057A, at the top of the uppermost grainstone/rudstone interval and separating intervals within the underlying boundstone, packstone, and grainstone lithologies, calcrete features including brownish staining, undulated dissolution surfaces, and rhizoliths are observed. Clear dissolution of the constituents, especially aragonitic particles, leave moldic porosity, dissolution, and neomorphism of parts of coral skeletons. These features clearly indicate several phases of immersion and weathering, including paleosol formation, of the recovered deposits. Hole M0058A, at 167 mbsl (the deepest Expedition 325 hole), was drilled to 41.4 m CSF-A and was mainly composed of unconsolidated green mud with two intercalated sections of fine to medium sand in addition to a few grainstone levels. The three mud sections in Hole M0058A are characterized by a lack of bedding. Scattered within the mud, small fragments of mollusk shells and small benthic foraminifers can be found. Planktonic foraminifers were observed only in Section 325-M0058A-1X-1, Core 7X, Sections 8X-1 and 11X-3, and Core 12X. Fragments of bryozoan colonies and clypeasteroid burrowing echinoids rarely occur. Cores 325-M0058A-11X and 13X show clear signs of bioturbation. The upper sand/grainstone section is at least 2 m thick and consists of fine to medium sand with fragments of well-cemented grainstone and visible fragments of mollusks, bryozoa, coralline algae, echinoids, larger benthic foraminifers, and serpulids. The grainstone is composed of shells and fragments of calcareous algae, larger benthic foraminifers, and mollusks. The lower sand section, characterized by fine to medium sand, is ~7 m thick and is less distinct than the upper one. Physical propertiesRecovery at NOG-01B transect holes was much greater compared to the other transects visited during Expedition 325, with an average recovery of ~40%. Borehole depths reached at each hole in this transect are as follows:
Density and porosityBulk density was measured along the NOG-01B transect using two methods: (1) GRA density measured on the MSCL, providing an estimate of bulk density from whole cores, and (2) discrete MAD samples measured with a pentapycnometer using 20 mm diameter plugs drilled from the working half of core sections and/or rock fragments, providing grain density and porosity data. Because of the higher levels of recovery and core quality in this transect, we have more confidence in the MSCL data than for the previous transects. Bulk density values measured on whole cores range from 1 to 2.52 g/cm3. Bulk densities measured on discrete samples vary between 0.62 and 2.49 g/cm3. Plug porosity varies between 20% and 50% (Fig. F32). These values are to be expected in carbonates, as they are formations known for high heterogeneity. Some grain density values are <2.71 g/cm3; this value is less than the calcite density (2.71 g/cm3). These low values could be due to an anomalous measurement and/or the presence of clays in the plugs. Data across the transect demonstrate a linear relationship between porosity and bulk density: bulk density increases as porosity decreases (Fig. F33). P-wave velocityA crossplot of velocity versus porosity (both from discrete samples) for all sites shows primarily an inverse relationship (Fig. F34) between acoustic velocity (VP) and porosity. However, there is a secondary group of data with extremely high porosity and relatively low VP. This group of data relates to the lime mud units recovered in Hole M0058A, where the MSCL data acquired (over ~6.5 cm crosscore) range from 1502.31 to 1829.983 m/s. In Hole M0058A, because of the high recovery (~82%) and nature of the core, MSCL values and discrete measurements are in accord. For all other holes, much lower values have been recorded for coral and microbialite units compared to discrete measurements on core plugs. Magnetic susceptibilityMagnetic susceptibility data obtained from the MSCL offshore for the NOG-01A transect can be used with more confidence than at the previous transects. Over this transect, magnetic susceptibility ranges from –1.6353 x 10–5 SI (Hole M0053A) to 38.2049 x 10–5 SI (Hole M0056A). Small variations and trends are clearly visible in Hole M0054B between ~15 m CSF-A and 22 m CSF-A, and an almost continuous record is available for Hole M0058A. Electrical resistivityObtaining reliable resistivity measurements on whole cores was much easier at this transect with improved recovery. Hole M0058A exhibits the most continuous and convincing record obtained with the MSCL during Expedition 325, with the transect representing the location from where the best resistivity measurements were taken on cores, mainly because of improved recovery and core quality. Over the entire transect, resistivity is very variable, with the lowest values (0.33 Ωm) measured in Hole M0058A and the highest values (38.20 Ωm) recorded in Hole M0056A. Trends in the data are much more visible at this transect, with some small fluctuations in Hole M0055A. Color reflectanceAlong the NOG-01B transect, Holes M0052A, M0052B, M0052C, and M0053A were located in similar water depths and can be correlated. The same applies for Holes M0054A and M0054B. Hole M0058A is located in the fore reef slope area and represents the longest continuous record obtained during Expedition 325. Holes M0052A and M0052B had low recovery, and reflectance measurements exhibited a similar range across both of them. Hole M0052C also had low recovery, and only two measurements of color reflectance were taken for this borehole. Recovery in Hole M0053A was higher (~33%), and the color reflectance values taken in the first few meters are consistent with other boreholes along the NOG-01B transect at the same depth (Holes M0052A, M0052B, and M0052C). Recovery for Hole M0054A was low but relates well to values obtained for Hole M0054B, the neighboring borehole. Discrete measurements of reflectance values for all boreholes along the NOG-01B transect are represented in Figure F35; boreholes are plotted from landward to seaward (left to right) at the same depth scale. Reflectance shows consistent trends for holes located at the same water depth, indicating a possible correlation between them. Downhole measurementsWireline logging operations for the NOG-01B transect were performed in one HQ hole (M0054B). This provided the only opportunity to run both of the high-priority imaging tools in a "logging" hole. After completion of coring, ASGR logging through-pipe was performed, and then the HQ drill string was pulled and the coring bit exchanged for an open shoe casing to provide borehole stability in unstable sections and a smooth exit and entry of logging tools. In addition, seawater was pumped into the hole in order to try and displace the guar gum drilling mud and condition the hole for open-hole logging. With the exception of the ASGR log through-pipe, logging was obtained over a maximum interval of ~8.5 m. Borehole conditions were relatively hostile, and the lower portion of the hole began to infill. In order to record ultra high–resolution geophysical downhole logging data, the acquisition was done in the rooster box, which is heave compensated. The slimline suite deployed at transect NOG-01B comprised the following tools:
Two main logging units were identified in Hole M0054B (Fig. F36):
Lithological changes and logging unit boundaries do not perfectly concur; however, coring and wireline operations use different methods to measure depth. Further work will therefore be needed to more fully integrate the log and core data. PaleomagnetismMaterials acquired during Expedition 325 generally yielded a low concentration of ferromagnetic materials, coupled with a strong drilling overprint. Therefore, it was very difficult to obtain an integration and assessment of the paleomagnetic results from Expedition 325 during the OSP. Several peaks have been detected across some of the holes. However, the nature of these is uncertain, and any possible correlations need to be further investigated through additional rock magnetic studies. Environmental magnetic studies may help refine the climatic origin of these magnetic susceptibility signals and provide information on the volume, composition, and grain size of the magnetic component retained. However, it was observed that the magnetic susceptibility for the NOG-01B and RIB-02A transects is significantly stronger than for the southern Hydrographer's Passage transects (HYD-01C and HYD-02A); the signal along the RIB-02A transect is stronger than that observed along the NOG-01B transect. One hypothesis is that this may be linked to proximity to a source of magnetic mineral input into the system, suggesting that such a source may exist in the northernmost/north of the Great Barrier Reef. The majority of results are derived from Holes M0040A, M0041A, and M0058A, which provide longer records because of improved recovery rates. In addition, preliminary results obtained from the paleomagnetic study of a U-channel taken from Section 325-M0041A-12R-1 (HYD-02A transect) are also discussed. The "noisy" natural remanent magnetization (NRM) demagnetization paths are attributed to the relatively low intensity of magnetizations (1.08 x 10–9 to 2.19 x 10–7 A/m with a mean of 2.02 x 10–8 A/m). Only a few samples are characterized by high NRMs, and these are associated with layers of high values of magnetic susceptibility. Consistency of the NRM inclinations of the discrete cubes measured can also be correlated to the intensity of magnetization results. The demagnetization method used is not able to remove the magnetization for all core sections. Other methods, such as thermal demagnetization experiments, could be used to remove the overprinting that may be related to the presence of high-coercivity magnetic minerals such as hematite and goethite, and thereby reduce the NRM intensity. However, overprinting cannot be erased with standard alternating-field (AF) demagnetization, and there are still uncertainties regarding how the secondary overprint has been acquired and why some samples do not demagnetize at all whereas others have the potential for demagnetization. The component of any drilling-related overprint that may remain will have a negative effect on both the inclination and declination results. However, samples for which the data analysis suggests no overprinting, or for which much of the drilling overprint has been removed, can used to conduct further studies, such as paleointensity experiments. The generally positive and high inclination values obtained for Expedition 325 samples are not what is expected for the low paleolatitude of the sampling sites (latitudes between 17° and 19°S) with the corresponding geomagnetic axial dipole (GAD) values of ~31° to –38°S. One interpretation of the results is that a significant portion of the drilling overprint remains on the majority of the samples studied. Alternatively, there may be a pervasive present-field overprint that was not possible to remove with AF demagnetization experiments. GeochemistryMeasurements made during the offshore phase of the expedition showed that pH, alkalinity, and ammonium concentrations did not indicate any apparent variation between transects (Tables T3, T4, T5, and T6). The pH values varied from 7.34 to 7.96, which is slightly lower than standard seawater values. Alkalinity ranged from 2.3 to 8.1 mM with an average of 4.1 mM. Ammonium concentrations varied from 0 to 2.2 mM with a mean of 0.3 mM. Inorganic carbon and inductively coupled plasma–optical emission spectrometry (ICP-OES) measurements conducted during the OSP showed that concentrations of chloride, bromide, sulfate, and most of the characterized major and trace elements vary within the normal range for marine sediments and did not vary significantly between sites and transects (Tables T3, T4, T5, and T6). ChronologyDuring the offshore phase of Expedition 325, 68 samples were subsampled from core catcher materials near the base, middle, and top of each hole for preliminary chronology measurements (20 for U-Th and 48 for radiocarbon). These measurements provided approximate age information for each hole before the OSP, thereby aiding the development of targeted sample requests and sampling strategies. U-Th measurements were performed by MC-ICP-MS at the University of Oxford (United Kingdom), and radiocarbon measurements were made by AMS at the Australian National University following sample preparation at the University of Tokyo (Japan). To ensure rapid sample throughput and presentation of data before the OSP, no sample screening for diagenesis or detrital contamination was performed for either U-Th or radiocarbon measurements. Therefore, age interpretations may be inaccurate and will need to be refined by further measurements after the OSP. Of the 33 holes drilled during Expedition 325, 26 had at least one preliminary dating measurement and 18 had at least three measurements. Of the holes that had more than one preliminary dating measurement, the age interpretations in all but one (Hole M0037A) were in stratigraphic order, adding confidence to the age interpretations and the notion that core catcher material is often broadly representative of in situ stratigraphy. A total of 60 age interpretations were from 0–30 ka, with eight older ages interpreted from the preliminary data (Fig. F37). The 60 ages from 0 to 30 ka are from core catchers that were drilled from between 51 and 130 mbsl. Preliminary age interpretations therefore demonstrate that Expedition 325 has successfully recovered a complete sequence of material from the LGM interval, through the first half of the last deglaciation up to 10 ka. Therefore, the material recovered will enable investigation of the magnitude and nature of sea level change around the LGM, as well as the rise of sea level out of the glacial period. The rise of sea level during the early stages of deglaciation will also be refined with further chronological and paleoenvironmental analysis. Thus Expedition 325 has recovered material from key periods of interest for sea level change and environmental reconstruction, including MWP1A (and MWP1B), 19ka-MWP, the YD, the Bølling-Allerød, and Heinrich Events 1 and 2. The distribution of ages of the corals recovered during Expedition 325 also fills a gap in the coral record from 14.7 to 16.8 ka (Fig. F37). Paired U-Th:radiocarbon measurements from corals within this gap will be crucial in providing data to refine the radiocarbon calibration, thereby enabling investigation into the carbon cycle during this period of environmental change. | ||||
