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doi:10.2204/iodp.proc.325.101.2011 Principal site resultsCores were recovered from 34 holes across 17 sites (M0030–M0058) (Table T1) along the four transects, with a conventionally calculated recovery of 26.6%. Hole depths ranged from 47.27 to 167.11 mbsl (lowest astronomical tide 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). Borehole geophysical wireline logging was conducted at four holes. Because of space limitations on the Greatship Maya, only limited analysis of the cores was performed 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 assemblagesNine major lithologic types were recovered from fossil reefs on the shelf edge of the GBR. They are defined as follows:
To aid core description, these lithologic types were organized into distinct lithostratigraphic units numbered from the top of each hole (e.g., Units 1 and 2). Note that lithostratigraphic units of the same designation from different holes are not correlative in time. Comprehensive definition of distinct chronostratigraphic units and their correlation between sites will only be possible after detailed analyses of the sedimentary facies and chronology data during postcruise research. Transect HYD-01C: Holes M0030A–M0039AThe northern Hydrographer’s Passage transect, HYD-01C, consists of 11 holes. From landward to seaward, they are Holes M0034A, M0030A and M0030B, M0031A, M0032A, M0033A, M0035A, M0036A, M0038A, M0039A, and M0037A, starting at depths between 51.0 and 122.3 mbsl. Several holes are closely spaced (i.e., <20 m apart) and could be treated as a composite hole (or site) through distinct reef targets: Site 5 = Holes M0030A and M0030B in the 80 mbsl reef target, Site 6 = Holes M0031A, M0032A, and M0033A in the 90 mbsl reef target, and Site 8 = Holes M0038A and M0039A in the 110 mbsl reef target. Figure F125 in the “Transect HYD-01C” chapter summarizes the major lithostratigraphic units for transect HYD-01C, including their distribution and recovery. Sedimentology and biological assemblagesCoralgal and coralgal-microbialite boundstones are the dominant lithologies recovered along transect HYD-01C (see Fig. F125 in the “Transect HYD-01C” chapter). Just below the modern seafloor, between one and three coralgal boundstone and coralgal-microbialite boundstone units occur in most transect HYD-01C holes, except for Holes M0030A and M0030B, in which recovery was extremely low, and Hole M0037A, the most distal and deepest site on the transect (at 122 mbsl). The coralgal lithologies, spanning one or two sections depending on the hole, contain little or no microbialite and range from <1 m thickness in Hole M0032A to 8 m in Hole M0031A. These coralgal lithologies consistently overlie coralgal-microbialite units in Holes M0031A–M0033A, M0035A, M0036A, M0038A, and M0039A. In Hole M0034A, a 2 m thick coral boundstone underlies an 18 m thick coralgal-microbialite unit, whereas in Hole M0036A, the coralgal boundstone is interbedded with a 6 m of unconsolidated sediment unit. The main corals in the coralgal units are massive Isopora with lesser amounts of massive Porites, submassive to massive Montipora, and branching Acropora. The coralgal-microbialite units are dominated volumetrically by microbialites, and these boundstones are the thickest lithologies in every hole except Hole M0037A. They range from 10 m thick in Hole M0031A to ~30 m thick in Hole M0033A. They contain diverse coral assemblages dominated by massive Isopora, branching Acropora, and Seriatopora, but also locally abundant massive Porites and Faviidae. In six of the nine holes along transect HYD-01C, unconsolidated sediment from <1 m to 19 m thick underlies the upper coralgal-microbialite boundstone units and is composed of bioclastic lime sand to pebbles containing mollusks, larger foraminifera, Halimeda, fragments of corals and red algae, bryozoans, echinoderms, and sea urchin spines. In Hole M0034A, the unconsolidated unit is overlain by a coralgal lithology, whereas in Hole M0036A, the unconsolidated unit is bracketed by coralgal units. These unconsolidated sediments were probably partly disturbed by coring operations. A thin (<3 m) skeletal packstone to grainstone unit rich in larger foraminifera, calcareous algae, and/or a dark coralgal-worm tube boundstone is interbedded with, or underlies, the unconsolidated sediment unit in Holes M0031A–M0033A, M0035A, M0036A, M0038A, and M0039A. A similar unconsolidated unit also forms the base of the recovered sequences in Holes M0031A and M0036A. Hole M0037A, the most distal and deepest site at 122 mbsl along transect HYD-01C, has a different lithologic composition and succession, with almost uninterrupted unconsolidated sediments extending from the seafloor to the base of the hole. The uppermost 12 m of unconsolidated lime sands to pebbles overlie a thin (10 cm) interval of grainstone rich in foraminifera, coralline algae, and coral fragments that in turn overlies 8 m of lime sand rich in larger foraminifera and mollusks. Although there is clear evidence of downhole contamination in the upper part of each section, 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 transect HYD-01C. The cores were only partially saturated and often underfilled, thus impacting the data coverage and quality for the physical property measurements. Water depths and borehole depths are as follows:
Density and porosityDensity and porosity vary similarly in all of the boreholes drilled across transect HYD-01C. Discrete sample porosity ranges from 20% to 50% due to significant variability in the pore systems (e.g., moldic, vuggy, growth framework, and intergranular) (see Fig. F126 in the “Transect HYD-01C” chapter). 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 most likely due to the partial saturation of the cores but also a function of 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 (see Fig. F127 in the “Transect HYD-01C” chapter): where
This correlation demonstrates that the average grain density along transect HYD-01C is 2.77 g/cm3. Grain density varies between 2.7 and 2.85 g/cm3 and may represent a density value between the density of calcite (2.71 g/cm3) and aragonite (2.93 g/cm3). P-wave velocityA cross-plot of acoustic velocity (P-wave velocity, VP) versus porosity (both from discrete samples) for all sites shows an inverse relationship (see Fig. F128 in the “Transect HYD-01C” chapter). MSCL data, which were acquired cross core (over ~6.5 cm), range from 1500.34 to 1937.94 m/s, much lower values than those obtained from discrete measurements 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 those obtained from 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 coralgal and microbialite boundstone 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 × 10–5 and 5 × 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. Low resistivity values were given by unconsolidated or sandy sediments, (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 reflectanceColor reflectance spectrophotometry values 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 present. In most of the boreholes, slightly higher values of reflectance occur just below the seafloor where modern reef sediment was recovered. Along transect HYD-01C, Holes M0031A–M0033A are located in similar water depths and can be correlated. No significant trends were found in these cores, but the reflectance values for these boreholes are similar. This is also true for Holes M0035A and M0036A. Discrete measurements of reflectance values for all boreholes along transect HYD-01C are represented in Figure F129 in the “Transect HYD-01C” chapter. Boreholes are represented in landward to seaward order. All cores are presented in core depth below seafloor in meters (m CSF). 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 suite of downhole logging tools deployed at transect HYD-01C comprised the following:
Wireline logging operations on transect HYD-01C provided two sets of comparable through-pipe gamma ray data. Very few open-hole data were acquired in Hole M0036A because of hole instability. However, it was possible to discern three major logging units at these two sites based on the gamma ray data collected through American Petroleum Institute (API) pipe (see Fig. F130 in the “Transect HYD-01C” chapter):
Both total gamma ray (TGR) curves obtained at Holes M0031A and M0036A had a similar trend; therefore, it was expected that similar formations might be present. However, there are some significant differences in the deposits recovered by the drill holes (see Fig. F130 in the “Transect HYD-01C” chapter). 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. Transect HYD-02A: Sites M0040A–M0048AThe southern Hydrographer’s Passage transect, HYD-02A, includes nine holes. From landward to seaward they are Holes M0042A, M0048A, M0047A, M0043A, M0045A, M0046A, M0044A, M0040A, and M0041A, starting at depths between 50.8 and 126.1 mbsl. Some holes are closely spaced (i.e., <5 m apart) and these can be treated as a composite hole (or site) through the same reef target: Site 8 = Holes M0045A and M0046A in the 110 mbsl reef target; Site 10 = Holes M0040A and M0041A in the 120 mbsl reef target. Figure F88 in the “Transect HYD-02A” chapter summarizes the major lithostratigraphic units for transect HYD-02A, including their distribution and recovery. Sedimentology and biological assemblagesFew common patterns link lithologic successions in the eight holes along transect HYD-02A (see Fig. F88 in the “Transect HYD-02A” chapter). The following highlights describe some features along transect HYD-02A, focusing on shared features rather than on exceptions.
The boundstone lithologies contain variable proportions of coral, coralline algae, and microbialite that define several coralgal, coralgal-microbialite, and microbialite boundstones similar to those in the other GBR transects. Major corals in the boundstones are submassive to massive Porites, Montipora, branching Pocilloporidae, branching Acropora, massive Isopora, and submassive to massive Faviidae. Common patterns of boundstone distribution in most of the holes are as follows:
The presence/absence of coralgal and coralgal-microbialite boundstones has no simple relationship to the geographic location and/or water depth of holes along transect HYD-02A. Physical propertiesRecovery for transect HYD-02A sites averaged ~21%. However, recovery in Holes M0040A and M0041A reached ~50%. Cores were partially saturated and often disturbed, fractured, or contaminated, thus affecting the quality of the physical property data. Water depths and borehole depths are as follows:
Density and porosityBulk density was measured for transect HYD-02A using the gamma ray attenuation (GRA) sensor on the MSCL, providing an estimate of bulk density from whole cores. Discrete moisture and density 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 bulk density (ρ) of the discrete samples measured in all boreholes along transect HYD-02A (see Fig. F89 in the “Transect HYD-02A” chapter). This correlation demonstrates that the average grain density along transect HYD-02A was 2.77 g/cm3. Grain density varied 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 F90 in the “Transect HYD-02A” chapter. 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 with ~30% porosity (~0–10 m CSF) increasing to ~50% toward the bottom of the drill holes. P-wave velocityA cross-plot of P-wave velocity versus porosity (both from discrete samples) for all sites show an inverse relationship (see Fig. F91 in the “Transect HYD-02A” chapter). Whole-core MSCL data (across ~6.6 cm) ranged from 1509 to 1896 m/s. As expected, because of the bias toward good quality material in the sampling of cores for discrete measurement compared to the more indiscriminate nature of the MSCL measurement, discrete P-wave data is generally higher and more reliable than the corresponding MSCL P-wave data. Magnetic susceptibilityOffshore MSCL magnetic susceptibility data were difficult to interpret for this transect because of limited core recovery in all holes. Magnetic susceptibility values were generally similar across the holes, with the majority of readings in the –1 × 10–5 to 1 × 10–5 SI range, delineated by short intervals of higher magnetic susceptibility. Electrical resistivityResistivity is variable over the entire transect, with the lowest values (0.56 Ωm) measured in Hole M0040A and the highest values (44.84 Ωm) recorded in Hole M0044A. Because of 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 exhibited 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 had a trend similar to that in the shallower holes; however, the reflectance measurements had a smoother distribution. Holes M0048A and M0044A had similar values. However, because of the lack of measurements with depth in Hole M0048A, there are no trends to compare. 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 exhibited less scatter in color reflectance measurements than other boreholes in this transect. Reflectance 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-A, and a decrease downhole to ~50% at 21 m CSF-A. Color reflectance measurements for all of the boreholes in transect HYD-02C are represented in Figure F92 in the “Transect HYD-02A” chapter; boreholes have been plotted from landward to seaward (left to right) at the same depth scale. Downhole measurementsWireline logging operations were performed at one API hole (M0042A) along transect HYD-02A. The priority imaging tools (acoustic borehole image [ABI40] and optical borehole image [OBI40]) were also run to see if 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. Unfortunately, the test run of the imaging tools in the API hole proved unsuccessful. Downhole logging in transect HYD-02A was conducted with the following set of wireline sondes:
Four main logging units were identified from the downhole data from Hole M0042A:
Transect RIB-02C: Sites M0049A–M0051ATransect RIB-02C 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 (<5 m apart) and form a composite hole in the 100 mbsl reef target. Figure F27 in the “Transect RIB-02A” chapter summarizes the major lithologic types and recovery for all holes in transect RIB-02C. Sedimentology and biological assemblagesOnly a rough summary of the lithostratigraphic distribution pattern can be proposed for transect RIB-02 because of poor recovery in the four holes M0049A, M0049B, M0050A, and M0051A (see Fig. F27 in the “Transect RIB-02A” chapter). A limited lithological succession is proposed for three holes (M0049A, M0049B, and M0050A) in which drilling penetrated below the modern and subrecent seafloor. In Hole M0050A, recovered material probably represents subrecent seafloor sediment mixed with fossil material. The following lithological succession is proposed:
Physical propertiesRecovery for transect RIB-02A sites averaged ~16%. However, recovery in Holes M0049A and M0049B reached ~20%. Cores were partially saturated and often disturbed, fractured, or contaminated. This can affect the quality of physical property data collected. Water depths and borehole depths for this transect are as follows:
Density and porosityBulk density was measured on transect RIB-02A samples using the GRA sensor on the MSCL, providing an estimate of bulk density from whole cores. Discrete moisture and density 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. A clear linear correlation is observed between porosity (ϕ) and bulk density (ρ) of the discrete samples measured in all boreholes along transect RIB-02A (see Fig. F28 in the “Transect RIB-02A” chapter). The average grain density along transect RIB-02A is 2.78 g/cm3. Grain density varies between 2.75 and 2.79 g/cm3 and may represent 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 F29 in the “Transect RIB-02A” chapter. Porosity ranges from 17% to 45%; however, the majority of porosity values are ~30%. P-wave velocityOnly two core plugs were collected from this transect, both from Hole M0049B. A cross-plot of acoustic velocity (VP) versus porosity (both from discrete samples) for all sites shows primarily an inverse relationship between VP and porosity. Whole-core MSCL data (over ~6.5 cm) ranges from 1505 to 1845 m/s. As expected because of the targeted nature of taking discrete samples, much lower P-wave velocity values were recorded by the MSCL (offshore) for coralgal and microbialite boundstone 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. Very few values were obtained on whole cores, but data ranges from –0.64 × 10–5 SI (Hole M0049B) to high values of 31.6 × 10–5 SI (Hole M0050A). The most data were collected for Hole M0049B; however, no obvious trends are visible. Electrical resistivityOver the entire transect, resistivity is highly 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 relatively poor core quality and undersaturated cores, data should be treated with caution. Color reflectanceAlong transect RIB-02A, recovery for Hole M0051A was very low (<10 cm), and only one value of color reflectance spectrophotometry was measured. Holes M0049A, M0049B, and M0050A are located at the same water depth and are therefore comparable. Discrete measurements of reflectance values for all boreholes in this transect are represented in Figure F30 in the “Transect RIB-02A” chapter. No trends are observed in these three boreholes, but reflectance values are consistent in all cores where units were recovered at similar depths downhole. Transect NOG-01B: Sites M0052A–M0058ATransect NOG-01B 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 (<5 m apart) holes M0053A and M0052A–M0052C form a composite hole in the –100 mbsl reef target. Figure F155 in the “Transect NOG-01B” chapter summarizes the major lithologic types and their distribution and recovery for all holes along transect NOG-01B. Sedimentology and biological assemblagesThe uppermost parts of the seven holes in deeper water (Holes M0052A–M0052C, M0053A, M0054A and M0054B, and M0055A) contain mud and muddy sand (see Fig. F155 in the “Transect NOG-01B” chapter). Coralgal and coralgal-microbial boundstone units occur below the muds in these holes and at the top of the recovered succession in the two holes in shallower water (Holes M0056A and M0057A). The coralgal boundstones reach thicknesses of 4–15 m and are dominated by corals encrusted by coralline algae. The algal crusts often contain vermetid gastropods and the encrusting foraminifer Homotrema rubrum. The coralgal-microbial boundstone units can reach thicknesses of 10–16 m and contain thick crusts (up to several centimeters) of microbialite, in addition to corals and coralline algae. The main corals in these boundstones are diverse assemblages of branching Acropora, Seriatopora, massive Isopora, Porites, Montipora, Faviidae, and Tubipora. Unconsolidated sands, as well as consolidated grainstone and rudstone units, lie below the two boundstone units in all but the three holes (M0052A–M0052C) in which drilling did not penetrate below the boundstone. The grainstones/rudstones are 4 to 13 m thick and composed of shell and skeletal fragments of coral, coralline algae, Halimeda, mollusks, and benthic foraminifera. Although no material was recovered from beneath the grainstone/rudstone unit in Hole M0055A, three Holes (M0053A, M0054A, and M0054B) have a lime sand interval below the lower grainstone/rudstone. In these holes, the downcore succession of unconsolidated and/or modern reef sediment, boundstones, grainstones/rudstones, and lime sand resembles the pattern observed along transect HYD-01C. In contrast, the uppermost grainstone/rudstone units in Holes M0056A and M0057A overlie a long succession that includes boundstone, grainstone/rudstone, and packstone units. Coralgal boundstones, 12 m thick, are the dominant lithology in Hole M0057A and include a thin interval of packstones in the lowest boundstone interval. In Hole M0056A, 8 m of coralgal-microbial boundstone overlies a 13 m thick succession of grainstones/packstones. No consistent pattern has yet been extracted in the succession of facies in these holes. The major corals observed in the deeper, older boundstones are encrusting submassive to massive Montipora, massive Porites and Faviidae, and occasional Galaxea or Agariciidae. Three holes (M0055A, M0056A, and M0057A) contain packstone and grainstone lithologies, with calcrete features including brownish staining, undulating dissolution surfaces, and rhizoliths at the top of the uppermost grainstone rudstone unit and as layers separating intervals within the underlying boundstone. Obvious dissolution of constituents, especially originally aragonitic coral particles, has left moldic porosity, neomorphisms, and other dissolution features. These features are interpreted as several phases of emersion and weathering, including paleosol formation, within the recovered deposits. Hole M0058A is the deepest hole of Expedition 325, beginning at 167 mbsl. Its 41.4 m length consists mainly of unconsolidated green mud with two intercalated units of fine to medium sand and a few grainstone intervals (Fig. F155 in the “Transect NOG-01B” chapter). The three mud units in Hole M0058A are characterized by a lack of bedding. Small fragments of mollusk shells and small benthic foraminifera are scattered through the mud. Planktonic foraminifera are present only in Sections 325-M0058A-1X-2, 1X-7, 1X-8, 1X-11, and 1X-12, and there are occasional fragments of bryozoan colonies and clypeasteroid burrowing echinoids. Cores 325-M0058A-11X and 13X have clear signs of bioturbation. The upper sand/grainstone unit is at least 2 m thick and consists of fine to medium sand with fragments of well-cemented grainstone, mollusks, bryozoa, coralline algae, echinoids, larger foraminifera, and serpulids. The grainstone consists of shells and fragments of calcareous algae, larger foraminifera, and mollusks. The lower sand unit is ~7 m thick, consists of fine to medium sand, and is less distinct than the upper sand unit. Physical propertiesRecovery at holes in the NOG-01B transect was much higher than at other transects visited during Expedition 325, with an average recovery of ~40%. Water depths and borehole depths for each hole in this transect are as follows:
Density and porosityTwo bulk density measurements were taken on cores from transect NOG-01B. The first of these is conducted using a nondestructive method, measuring gamma density (a proxy for bulk density) on whole cores on a MSCL. The second bulk density measurement is taken on discrete samples using a pentapycnometer, on which porosity and grain density data are also acquired. Because of the higher levels of recovery and core quality in this transect, more confidence can be placed in the MSCL data than for the other 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% (see Fig. F156 in the “Transect NOG-01B” chapter). These values are to be expected in reefal carbonates, as they are formations known for high heterogeneity. Some grain density values are <2.71 g/cm3; which is less than the density of both calcite (2.71 g/cm3) and aragonite (2.93 g/cm3). These low values could be due to an anomalous measurement and/or the presence of clay in the plugs. Data across this transect demonstrate a negative linear relationship between porosity and bulk density, with density increasing with decreasing porosity (see Fig. F157 in the “Transect NOG-01B” chapter). P-wave velocityA cross-plot of P-wave velocity (VP) versus porosity (both from discrete samples) for all sites indicates an inverse relationship (see Fig. F158 in the “Transect NOG-01B” chapter). However, there was a secondary group of data with extremely high porosity and relatively low P-wave velocity. This group of data relates to the lime mud units recovered in Hole M0058A, where MSCL data (over ~6.5 cm crosscore) range from 1502 to 1830 m/s. The MSCL values and corresponding discrete measurements are in accord for Hole M0058A because of the high recovery (~82%) and nature of the core. For all other holes, much lower values were recorded for coralgal and microbialite units compared to discrete measurements on core plugs. Magnetic susceptibilityMagnetic susceptibility data obtained from the MSCL offshore for transect NOG-01A can be used with more confidence than at the previous transects. Over this transect, magnetic susceptibility ranges from –1.64 × 10–5 SI (Hole M0053A) to 38.20 × 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 exhibited the most continuous and convincing record obtained with the MSCL during Expedition 325, with the transect having the best resistivity measurements on cores, mainly because of improved recovery and core quality. Over the entire transect, resistivity is highly 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 transect NOG-01B, Holes M0052A–M0052C and M0053A were located in similar water depths and can be correlated. The same applies for Holes M0054A and M0054B. Hole M0058A was 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 color reflectance values taken in the first few meters were consistent with other boreholes along transect NOG-01B at the same depth (Holes M0052A–M0052C). Recovery for Hole M0054A was low but related well to values obtained for Hole M0054B, the neighboring borehole. Discrete measurements of reflectance values for all boreholes along transect NOG-01B are represented in Figure F159 in the “Transect NOG-01B” chapter; boreholes are plotted from landward to seaward (left to right) at the same depth scale. Reflectance has consistent trends for holes located at the same water depth. Downhole measurementsWireline logging operations for transect NOG-01B 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 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, acquisition was done in the rooster box, which is heave compensated. The downhole logging tool suite used in transect NOG-01B was as follows:
Two main logging units were identified in Hole M0054B (see Fig. F164 in the “Transect NOG-01B” chapter):
Lithological changes and logging unit boundaries do not concur perfectly; however, coring and wireline operations used different methods to measure depth. Further work will therefore be needed to more fully integrate the log and core data. GeochemistryHole M0058A consists of fine to coarse sediments, unlike other holes, and therefore continuous interstitial water (IW) sampling was achieved (see Table T4 in the “Transect NOG-01B” chapter). Several mud–fine sand units are punctuated by two coarse-grained units at 8.7–9.9 and 28.9–31.3 mbsf. Although there was no systematic vertical variation in the pH, alkalinity, chloride, ammonia, and strontium concentrations increased with depth from 0.1 to 2.2 mM and 90 to 451 µM, respectively. The notable characteristic of IW from Hole M0058A is that two large anomalies occur at the discrete coarse-grained units along the profiles of total iron and manganese concentrations. To determine the mineral abundances and total organic carbon (TOC) contents of sediments from Hole M0058A, X-ray diffraction (XRD) and carbon-sulfur measurements were conducted in the laboratories of University of Bremen. The percent carbonate measured by XRD fluctuated downcore, ranging between 28% and 76%. Total inorganic carbon (TIC), calculated as the difference between total carbon (TC) and TOC, had a profile similar to percent carbonate. The percent quartz profile showed opposite trends to that of percent carbonate. The two coarse-grained units contain low TOC content, with an average value of 0.25%, compared to the rest of the core. These data suggest that two coarse-grained units may reflect input from terrestrial sources during their deposition. Further investigations are needed to fully understand the cause of lithologic changes found in Hole M0058A. All transectsThe following disciplines consider trends across all four transects. 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 were 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. However, it was observed that magnetic susceptibility for transects NOG-01B and RIB-02A was significantly stronger than for the southern Hydrographer’s Passage transects (HYD-01C and HYD-02A); the signal along transect RIB-02A was stronger than that observed along transect NOG-01B. 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 GBR. 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 (transect HYD-02A) were also discussed at the OSP. The “noisy” natural remnant magnetization (NRM) demagnetization paths are attributed to the relatively low intensity of magnetizations (1.08 × 10–9 to 2.19 × 10–7 A/m with a mean of 2.02 × 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 was 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 did not demagnetize at all whereas others had 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 suggested no overprinting, or for which much of the drilling overprint had been removed, could be used to conduct further studies, such as paleointensity experiments. The generally positive and high inclination values obtained for Expedition 325 samples are not expected in the low paleolatitude 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. GeochemistryA total of 115 IW samples acquired during Expedition 325 were obtained from transects HYD-01C (16), HYD-02A (20), RIB-02A (2), and NOG-01B (77) (numbers in parenthesis indicate the number of samples obtained from each transect). The majority of the IW samples were collected from the holes drilled into the shelf edge fossil coral reefs. The only exceptions were from Hole M0058A, which is located in the deep fore reef slope at Noggin Pass (transect NOG-01B). Recovered material from Hole M0058A is composed of fine to coarse sediments. Measurements of pH, alkalinity, and ammonium concentrations of all of IW samples were made during the offshore phase of the expedition, whereas concentrations of chloride, bromide, sulfate, and major/minor elements were determined by ion chromatography and inductively coupled plasma–optical emission spectrometry (ICP-OES) during the OSP. The pH, alkalinity, and ammonium concentrations of IW collected from the holes drilled into the shelf edge fossil coral reefs did not indicate any apparent depth-related, not transect-specific variation because of the scarcity of IW samples at each transect (see Tables T5 in the “Transect HYD-01C” chapter, T4 in the “Transect HYD-02A” chapter, T4 in the “Transect RIB-02A” chapter, and T4 in the “Transect NOG-01B” chapter). 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 show significant variation between sites and transects. 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 multicollector inductively coupled plasma–mass spectrometry at the University of Oxford (United Kingdom) and radiocarbon measurements were made by accelerator mass spectrometry 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 30 to 0 cal y BP, with eight older ages interpreted from the preliminary data (Fig. F11). The 60 ages from 30 to 0 cal y BP are from core catchers that were drilled from between 130 and 51 mbsl. Preliminary age interpretations therefore demonstrate that Expedition 325 has successfully recovered a complete sequence of material from the LGM 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 Heinrich Events 1 and 2, 19ka-MWP, the Bølling-Allerød, MWP1A (and MWP1B), and the YD. The distribution of coral ages recovered during Expedition 325 also fills a gap in the coral record from 16.8 to 14.7 ka (Fig. F11). Paired U-Th and 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. |
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