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doi:10.2204/iodp.proc.325.104.2011 Transect HYD-02A summarySedimentology and biological assemblagesFew common patterns link lithologic successions in the eight holes along transect HYD-02A (Fig. F88). The following highlights describe some features along transect HYD-02A, focusing on shared features rather than on exceptions:
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 Great Barrier Reef 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 in most of the holes are
Table T3 documents all the larger foraminifera described in this transect in association with hole, run, and depth (below seafloor). Physical propertiesRecovery at transect HYD-02A sites was partial with an average of ~21%. However, recovery in Holes M0040A and M0041A reached ~50%. Cores were partially saturated and often disturbed, fractured, or contaminated. All of these factors act to degrade the quality of any physical property data collected. Borehole depths for this transect are as follows:
Plugs and samples taken for discrete P-wave and moisture and density measurements were obtained from both consolidated and unconsolidated core material. Density and porosityBulk density was measured at transect HYD-02A using gamma ray attenuation. Gamma ray attenuation on the multisensor core logger provided an estimate of bulk density (also referred to as gamma density) from whole cores. Discrete moisture and density measurements were also taken with a pycnometer on plugs and/or on rock fragments. This provides grain density, bulk density (in the case of plug samples), and porosity data. As in the previous transect, HYD-01C, one observes a classic linear relationship between the porosity (ϕ) and bulk density (ρ = ρs[1 – ϕ] + ρwϕ) of discrete samples measured for all boreholes at transect HYD-02A (Fig. F89). Average grain density (ρs) is 2.77 g/cm3. Grain density varies between 2.70 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 can be viewed in Figure F90. Porosity in the transect ranges between 16% and 72%. Similar trends in porosity can be picked out in Holes M0047A and M0043A with a stepwise decrease in porosity between 0 and 12 m CSF-A followed by an increase at ~15 m CSF-A and gradual decrease to ~25 m CSF-A. Holes M0040A and M0041A have almost identical trends in porosity, with 25% porosity (~0–10 m CSF-A) increasing to 53% at 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. F91) between P-wave velocity and porosity. Whole-core multisensor core logger data ranges from 1508.59 to 1895.75 m/s. As expected, much lower, possibly erroneous (see “Physical properties” in the “Methods” chapter) P-wave velocity values have been recorded by the multisensor core logger for coralgal boundstone units, unlike the discrete measurements on core plugs, which are probably more reliable. Magnetic susceptibilityMagnetic susceptibility data are very difficult to interpret for this transect because of limited core recovery in all holes across transect HYD-02A. However, values are generally similar across the holes, with the majority of readings being in the –1 × 10–5 to 1 × 10–5 SI range, which is delineated by short intervals of magnetic susceptibility highs. Electrical resistivityOver the entire transect, resistivity is highly variable, with the lowest values (0.56 Ωm) measured in Hole M0040A and highest values (44.84 Ωm) recorded in Hole M0044A. Because of the relatively poor core quality (as mentioned above) and undersaturated cores, very little confidence can be placed in these data. Color reflectanceIn transect HYD-02A, Holes M0048A, M0047A, M0043A, M0044A, and M0046A are located at similar water depths and can be correlated (with <5 m separation between the drilled holes). Holes M0047A and M0043A exhibit similar trends, but Hole M0047A presents smaller dispersion in the values of reflectance per section. This is probably due to the presence of massive corals in Hole M0047A. Hole M0046A was corrected for depth because of its location on a pinnacle. This was taken into account when plotting it against the other boreholes in order to make direct comparisons across the transect. Hole M0046A shows a similar trend in data points to the shallower 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 identified. Recovery in Hole M0045A was so low that color reflectance was not measured. Holes M0040A and M0041A are located very close to each other in the same water depth. Both boreholes exhibit less dispersion in color reflectance measurements than other boreholes from this transect. Data from these boreholes show a consistent pattern of ~50% L* at 2.5 m CSF-A, a slight increase at 6–8 m CSF-A, and a decrease in reflectance below that, retuning to ~50% at 21 m CSF-A. Color reflectance measurements for all transect HYD-02A boreholes are represented in Figure F92. Boreholes are plotted from shallower to deeper water (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. PaleomagnetismTransect HYD-02A comprises nine holes located at five sites in the southern part of Hydrographer’s Passage. Recovered materials were dominated by corals and calcareous sediments. The materials show mainly low and/or negative values of low-field and mass-normalized magnetic susceptibility. The arithmetic means of the measurements taken suggest that they are related mainly to diamagnetic materials. Further studies may detect minimal fractions of ferromagnetic material and further define the magnetic properties and geomagnetic behavior of the records obtained thus far. The lower magnetic susceptibility values related to diamagnetic material are difficult to correlate to a common geological or paleomagnetic feature. However, high magnetic susceptibility values for samples located in the uppermost 1–2 m or between 2 and 5 mbsf are common to the majority the sites in this transect, are most evident in the deep Site 10 location (Holes M0040A and M0041A), and are mainly associated with lithologic variations. Variations between 17 and 20 mbsf and 27 and 30 mbsf can be attributed to the occurrence of sandy layers, which may represent an important variation in the growth regime of the Great Barrier Reef, potentially marking significant paleoclimatic changes. At Site 10 (Holes M0040A and M0041A), strong magnetic susceptibility spikes at 14–15 and 16–17 mbsf were recorded. Magnetic susceptibility for these layers is strong enough to suggest alternative hypotheses beyond a simple variation in the concentration of magnetic minerals as a result of a paleoclimatic pulse. However, further rock magnetic studies are required to define the nature and processes that produced these susceptibility variations. Environmental magnetic studies will define the climatic origin of these layers and provide further information on the volume, composition, and grain size of the magnetic component retained in these layers. Drilling contamination appears to have occurred in the uppermost ~10–30 cm of some cores from Holes M0040A and M0041A. The strong signal may be caused by rust from the pipe. Alternatively, the high values of susceptibility may be due to sulfide pieces that were not washed out of the hole during drilling and accumulated at the top of each core. GeochemistryA total of 20 interstitial water samples from transect HYD-02A were obtained from Holes M0040A (4), M0041A (6), M0042A (2), M0043A (4), M0046A (2), and M0047A (2). Samples were analyzed for cation and anion concentrations (Table T4). Parameters including pH, alkalinity, and concentrations of ammonium were measured during the offshore phase of the expedition, whereas major cations and anions were measured during the Onshore Science Party. All geochemical constituents were determined to be within the normal ranges for marine sediments. Because of the scarcity of interstitial water samples in this transect, interpretations relating to vertical variations could not be made. ChronologyThe shallowest hole on transect HYD-02A (Fig. F88), Hole M0042A (Site 2), was drilled into a 50 m feature and returned ages of 13–10 cal y BP from 65 to 51 meters below seafloor (mbsl) (Cores 325-M0042A-1R and 10R), thus recording the middle of the last deglaciation. Deeper in Hole M0042A, an age of 169 cal y BP was recovered from 86 mbsl (Core 325-M0042A-24R), indicating the potential for recovering pre-Last Glacial Maximum (LGM) material in deeper cores from this hole. Sites 7–12 drilled a range of features from 98 to 108 mbsl. Where dated, the tops of holes at these sites return ages of 15–14 cal y BP (15 cal y BP, Core 325-M0043A-2R, 106 mbsl; 14 cal y BP, Core 325-M0044A-2R, 109 mbsl; 15 cal y BP, Core 325-M0047A-3R, 102 mbsl; 14 cal y BP, Core 325-M0048A-2R, 101 mbsl), and the lower portions of the two holes at Site 12 that have greater penetration (Holes M0043A and M0047A) returned ages of 20 and 21 cal y BP from 129 and 127 mbsl (Cores 325-M0043A-18R and 325-M0047A-11R), respectively. Therefore, at these sites, the early portion of the deglaciation was recovered, whereas the LGM has been captured in the deeper cores at Site 12. The deepest site on transect HYD-02A, Site 10 (Holes M0040A and M0041A), was drilled into a feature at 127 mbsl. Ages of 17–10 cal y BP have been returned from the tops of the two holes at this site (Cores 325-M0040A-2R, 4R, and 325-M0041A-2R), indicative of accumulation during the last deglaciation. However, the lower portion of Hole M0040A returned an age of 25 cal y BP (Core 325-M0049A-8R) from 138 mbsl, indicating that Site 10 also recovered material representative of the LGM. Downhole measurementsDownhole geophysical logs provide continuous information on a broad range of formation properties. In this transect, this continuous data set provides important data where core recovery is limited. Borehole geophysical instrumentsThe wireline sondes deployed at HYD-02A were as follows:
Preliminary resultsWireline logging operations were performed in one API hole (M0042A) on transect HYD-02A. The priority imaging tools (ABI40 and OBI40) were also run to see if good image data could be obtained in an API hole. The standard maximum hole diameter for successful image data acquisition is 15 cm, and API holes tend to have a minimum diameter of ~20 cm, so images were not successfully collected. Four main logging units were identified from the downhole data from Hole M0042A:
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