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doi:10.2204/iodp.proc.325.106.2011 Transect NOG-01B summarySedimentology 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 (Fig. F155). Coralgal and coralgal-microbial boundstone units occur below the muds in these holes, and also below(?) the top of the recovered succession in the two holes in shallower water (Holes M0056A and M0057A). Coralgal boundstones are as thick as 4–15 m and are dominated by corals encrusted by coralline algae. Algal crusts often contain vermetid gastropods and the encrusting foraminifer Homotrema rubrum. Coralgal-microbial boundstone units are as thick as 10–16 m and contain thick crusts (as thick as 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 4–13 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 lithologies 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 units (Fig. F147). 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 show 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. Table T3 documents all the larger foraminifera described in this transect in association with hole, run, and depth (below seafloor). Physical propertiesRecovery at transect NOG-01B holes was much greater compared to the other transects visited during Expedition 325, with an average recovery of ~40%. Water depths and borehole depths reached in each hole from this transect are as follows:
Density and porosityBulk density was measured at transect NOG-01B using two methods: (1) gamma ray attenuation on a multisensor core logger (MSCL), which provides 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, which provide grain density and porosity data. Because of the higher levels of recovery and core quality in this transect, one can have more confidence in the MSCL data. Bulk density values measured on whole cores range from 1 to 2.52 g/cm3. Bulk densities measured on discrete samples vary between 1.57 and 2.62 g/cm3. Plug porosity varies between 6% and 68% (Fig. F156), whereas grain densities varied between 2.66 and 3.73 g/cm3. Some grain density values are <2.71 g/cm3, the density for calcite. This could be due to an anomalous measurement and/or the presence of clays in the plugs. Across the transect, bulk density increases as porosity decreases (Fig. F157). P-wave velocityA cross-plot of velocity (from discrete samples) versus porosity (from discrete samples) for all sites shows primarily an inverse relationship (Fig. F158) between acoustic velocity (P-wave) and porosity. However, there is a secondary group of data with extremely high porosity and relatively low P-wave velocity related to the lime mud units recovered in Hole M0058A. MSCL values acquired range from 1502m/s (Hole M0054A) to 1830 m/s (Hole M0053A). 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 shelf edge fossil reef holes, much lower values have been recorded for coralgal and coralgal-microbialite boundstone units compared with discrete measurements on core plugs. Discrete P-wave velocity measurements range from 1508 m/s (Hole M0058A) to 5322 m/s (Hole M0057A). Magnetic susceptibilityMagnetic susceptibility data from 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 395.32 × 10–5 SI (Hole M0055A). Small variations and trends are clearly visible in Hole M0054B between ~15 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. As with all other measurements, Hole M0058A exhibits the most continuous and convincing record obtained with the MSCL during Expedition 325. This transect represents the location from where the best resistivity measurements were taken on cores. Over the entire transect, resistivity is variable, with both the lowest (0.33 Ωm) and the highest (77.41 Ωm) values recorded in Hole M0055A. Trends in the data are much more visible at this transect—with some small fluctuations in Hole M0055A. Color reflectanceIn transect NOG-01B, Holes M0052A–M0052C and M0053A were located in similar water depths and can be correlated. The same also applies for Holes M0054A and M0054B. Hole M0058A was located in the fore reef slope area. Boreholes M0052A and M0052B had low recovery, and reflectance measurements were consistent for both of them, exhibiting a similar range of reflectance values. Hole M0052C also had low recovery, and only two measurements (not plotted) of color reflectance were taken for this borehole. Recovery in Hole M0053A was higher (~33%), and the color reflectance values taken in the top few meters are consistent with other boreholes in transect NOG-01B at the same depth (Holes M0052A–M0052C). Recovery for Hole M0054A was low but relates well with values obtained for Hole M0054B, which is the neighboring borehole. Discrete measurements of reflectance values for all transect NOG-01B boreholes are represented in Figure F159. Boreholes have been plotted from shallower to deeper water (left to right) using the same depth scale. Reflectance shows consistent trends for holes located at the same water depth, indicating a possible correlation between them. PaleomagnetismTransect NOG-01B comprises 10 holes at 5 sites, and materials recovered were predominantly corals and calcareous sediments (with the exception of the mud-dominated fore reef slope Hole M0058A). The materials show mainly positive values of low-field and mass-normalized magnetic susceptibility. The arithmetic mean of these values indicates the presence of paramagnetic and/or ferromagnetic materials. Most peaks in magnetic susceptibility are located at particular depth intervals. Peaks were observed between ~14 and 22 mbsf for Sites 6 and 7 (Holes M0052A–M0054B), associated with lithologic variations. At Sites 2 and 5 (Holes M0055A–M0057A), peaks were recorded at 2–3, 5, ~10, ~15, and ~18 mbsf. Most variations appear to be related to the occurrence of detrital magnetic materials from terrigenous or windblown volcanic sources. However, this cannot be fully quantified until further research is conducted (e.g., Curie experiments). Magnetic susceptibility measured at these sites is stronger than at transects HYD-02A and HYD-01C, located further south. It is believed that this is related to the closer proximity of the modern reef systems and therefore a potential source of detrital magnetic materials. However, the Ribbon Reef transect (RIB-02A) shows average magnetic susceptibility values that are even higher than those measured at Noggin Pass (NOG-01B). These values suggest that the source of the magnetic minerals may be located north of the Great Barrier Reef or that the volume of magnetic mineral is directly related to the proximity to the mainland. Further studies of ferromagnetic material will detect and define the magnetic properties and geomagnetic behavior. Further rock magnetic studies on these layers may provide information on the nature and processes that produced and drove these fluctuations in magnetic susceptibility. Environmental magnetic studies may reinforce the climatic origin of these layers and provide information on the volume, composition, and grain size of the magnetic component retained. GeochemistryInterstitial waterA total of 77 interstitial water samples from transect NOG-01B were obtained from Holes M0052A (4), M0053A (13), M0054A (4), and M0058A (56) and analyzed for cations and anions (Table T4). Parameters including pH, alkalinity, and concentrations of ammonium were measured during the offshore phase of the expedition, whereas the major cations and anions were measured during the Onshore Science Party. The geochemical constituents were all determined to be within the normal range for marine sediments. Hole M0058A consisted of fine to coarse sediments, unlike other holes, and therefore continuous interstitial water sampling was achieved. There was no systematic vertical variation in the pH, alkalinity, and concentrations of chloride and bromide in Hole M0058A (Fig. F160). In contrast, variations in the concentrations of ammonia, magnesium, calcium, strontium, and sulfate were observed. Ammonia and strontium concentrations increased with depth from 0.1 to 2.2 mM and 90 to 451 µM, respectively, whereas opposite trends were observed in the other three parameters; as depth increased, concentrations of magnesium, calcium, and sulfate decreased from 53 to 34, 10 to 7, and 29 to 18 mM, respectively (Fig. F160). The notable characteristic of interstitial water from Hole M0058A is that two large anomalies occur at 6–10 and 29–36 m CSF-A along the profiles of total iron and manganese concentration (Fig. F160). These anomalies may indicate discrete periods of terrestrial material input to the reef environment during the past. Further investigations are needed to fully understand the cause of these anomalies. XRD resultsThe results of the semiquantitative HighScore mineral identification are presented in Tables T5 and T6 and in Figures F161 and F162. The average mineralogy for Hole M0058A is 54% carbonate (28% aragonite, 13% calcite, and 13% Mg calcite, with <1% other carbonates), 20% quartz (with <1% tridymite and cristobalite), 8% feldspars (dominantly albite, but also some alkaline feldspars), 13% clay minerals (dominantly kaolinite, 5.5%, and muscovite, 5%), 2% amphiboles and pyroxenes, and ~1% evaporite minerals, which are dominantly halite. In the case of feldspar and clay mineral identification, where the major mineral of a group is absent, it is often the case that another mineral of that group is present. For example, albite is near ubiquitous but is not present in some samples. Where it is absent, another feldspar is often present in a similar amount to the nearby samples for albite (e.g., Sample 325-M0058A-2X-1, 8–9 cm) (Table T5). Similar apparent substitutions are observed for clay minerals; where muscovite is absent it is “replaced” by illite or another similar clay, and where kaolinite is absent it is “replaced” by nacrite or another similar clay. It is probable that these apparent substitutions have arisen from misidentification of mineral peaks by the HighScore analysis because of the varying chemical compositions of the minerals causing the position of the peaks to differ from those of the reference database and the “substituted” minerals having similar diffraction spectra. To simplify interpretation of the mineral data and to negate uncertainty arising from inaccurate identification of similar minerals, minerals have been grouped: all carbonates (aragonite, calcite, and Mg calcite are still considered separately), quartz (all SiO2 polymorphs), all feldspars, muscovite and illite group clays, kaolinite group clays, and amphibole and pyroxene minerals. Lithologically, Hole M0058A is composed of fine sand-silt units punctuated by two coarse-grained units at 8.7–9.9 mbsf (Sections 325-M0058A-4X-1 through 4X-CC) and 28.9–31.3 mbsf (Sections 11X-3 through 12X-CC; gray bands in Figures F161 and F162). Each of these coarse units lies above sections of no recovery. Here, the fine-grained units are referred to as the upper, middle, and lower fine-grained units and the coarse-grained units as the upper and lower coarse-grained units. Mineral abundancesMineral abundances for Hole M0058A are shown in Figure F161 and Tables T5 and T6. The abundance of carbonate in Hole M0058A varies from 28% to 76%. The upper fine unit has a carbonate percentage profile that changes from 65% to 31% carbonate from 0.04 to 8.68 mbsf, which is similar to the lower part of the middle fine-grained unit. The upper coarse unit has a carbonate percentage that is slightly higher than the lower coarse unit at 50%–60%. The carbonate percentage of the middle fine-grained unit increases rapidly from ~40% at 14.79 mbsf to a high of 60%–70% at 17.78 mbsf and then decreases from 76% at 24.38 mbsf to 28% at 28.18 mbsf. The lower coarse unit has a lower carbonate percentage that also decreases downcore from 48% to 30%. The lower part of the core has a high carbonate content, which decreases from 65%–75% to 50%–60% from 33.59 to 40.27 mbsf. The profile for the percentage of quartz (all polymorphs) shows opposite trends to that of percent carbonate. The covariation of quartz and carbonate abundance has a negative correlation (r = –0.86). The percentage of quartz in the upper fine-grained unit increases downward from 15% to 37% from 0.04 to 8.68 mbsf, and the upper coarse unit has a low quartz content of 13%–15%. The percentage of quartz in the middle fine-grained unit also decreases downhole from ~30% at 14.79 mbsf to 12% at 24.38 mbsf, and then increases to 34% at 28.18 mbsf. In contrast, the lower coarse unit has a higher quartz content that increases downcore from 17% to 38%. The lower part of the core has a relatively low quartz content that increases from 10% to 22% from 33.59 to 40.27 mbsf. The percent clay data are variable and do not show a clear trend, although the coarse-grained units may have a slightly higher average clay content than the finer grained units: 17.0% ± 2.4% versus 12.9% ± 1.1% (at the 95% confidence interval). The percent feldspar data are also variable but do show trends. The coarse-grained units have a higher feldspar content (12.2% ± 2.6%) versus 7.5% ± 1.0% for the fine-grained units (at the 95% confidence interval), and the fine-grained units show trends that closely approximate the trends in quartz content, especially in the upper and middle fine-grained units. The percent amphiboles and pyroxenes data are sparser than the other mineral groups, with many samples not having any amphibole or pyroxene. A clear trend exists in the data with the greatest abundance of amphibole and pyroxene (as much as 9%) in the upper fine unit, lower abundance in the middle unit, and still lower abundance in the lower fine-grained unit. Relative abundance ratiosRelative abundance ratios for Hole M0058A are shown in Figure F162 and Table T7. The Mg calcite/calcite ratio is near constant in the fine-grained units (mean of 1.0 ± 0.2) but increases to mean ratios of 2.6 ± 0.3 in the lower coarse unit and is as high as 4.2 in the upper coarse unit. The carbonate/SiO2 ratio varies downcore in parallel with carbonate content and inversely to quartz content. Where feldspars are also included with SiO2 for the ratio, the pattern is similar because of the co-variation of quartz and feldspar content downcore. There is a rapid fall to ~1.4 at the top of the middle fine-grained unit. In the upper and middle fine-grained units, carbonate/SiO2 decreases from 6.8 to ~0.85 (4.4 for the upper unit) immediately above the coarse-grained unit. The carbonate/SiO2 ratio decreases continuously from the top of the upper fine-grained unit, but in the middle fine-grained unit it increases from 3.2 at 17.28 mbsf gradually to a 6.8 peak at 24.38 mbsf. The lower fine-grained unit has a carbonate/SiO2 ratio of 6–8; the ratio decreases rapidly at ~38 mbsf to ~3 at the bottom of the recovered unit. The aragonite/calcite (both calcite and Mg calcite) ratio shows an inverse relationship to the Mg calcite/calcite ratio downcore. The coarse-grained units have lower aragonite/calcite ratios with a mean of 0.6 ± 0.2 versus the mean for the fine-grained units of 1.2 ± 0.1. Although there is little variation in the aragonite/calcite ratio within the fine-grained units, subtle upward trends toward lower ratios exist in each of the upper, middle, and lower units. The ratios of clay mineralogy ([illite, mica]/kaolinite) and siliciclastic composition (feldspars/SiO2) are highly variable and display no clear trends downcore. However, there is a subtle increase in the (illite, mica)/kaolinite ratio between the middle and upper fine-grained units, which have means of 0.9 ± 0.2 and 1.7 ± 0.3, respectively. Carbon contentThe results for total organic carbon (TOC), total carbon (TC), and total inorganic carbon (TIC) are presented in Table T8 and Figure F163. TC content ranges from 5.27 to 10.18 wt% (average = 8.75 wt%), TOC content ranges from 0.19 to 0.51 wt% (average = 0.29 wt%), and TIC content ranges from 4.89 to 9.96 wt% (average = 8.46 wt%). Within the upper fine-grained unit, TC content ranges from 7.08 to 9.97 wt% (average = 8.72 wt%), TOC content ranges from 0.25 to 0.39 wt% (average = 0.33 wt%), and TIC content ranges from 6.49 to 9.58 wt% (average = 8.40 wt%). Within the middle fine-grained unit, TC content ranges from 5.27 to 10.18 wt% (average = 8.77 wt%,) TOC content ranges from 0.19 to 0.51 wt% (average = 0.28 wt%), and TIC content ranges from 4.89 to 9.96 wt% (average = 8.49 wt%). Within the lower fine-grained unit, TC content ranges from 7.74 to 10.10 wt% (average = 9.95 wt%), TOC content ranges from 0.21 to 0.42 wt% (average = 0.33 wt%), and TIC content ranges from 7.34 to 9.88 wt% (average = 9.62 wt%). The two coarse-grained units contain low TOC content with an average value of 0.25 wt%. ChronologyThe shallowest hole drilled on transect NOG-01B (Fig. F155), Hole M0057A (Site 2), was drilled into a feature at a 40 m water depth (lowest astronomical tide [LAT]), which returned an age of 12 cal y BP near the core top (Core 325-M0057A-5R) at 52 mbsl. Therefore, this hole seems to have recovered material from the middle of the last deglaciation. One further U-Th age determination on this hole was attempted in Core 325-M0057A-15R. However, the sample did not yield an age interpretation because it was diagenetically altered. Nevertheless, it is likely that the lower cores of Hole M0057A do capture material from prior to the Last Glacial Maximum (LGM). Hole M0056A (Site 5) penetrated a feature at 82 mbsl and returned ages of 34 and 39 cal y BP from near the core top (Cores 325-M0056A-2R and 5R). These ages indicate that the hole recovered material that predates the LGM, probably from marine isotope Stage 3. The deeper portion of this hole yielded an older age of 84 cal y BP (Core 325-M0056A-13R) from 114 mbsl, indicating significant accumulation from 84 to 39 ka. Hole M0055A (Site 5) drilled into feature located slightly deeper, at 90 mbsl, than Hole M0056A. Hole M0055A returned an age of 14 cal y BP from the shallowest core (Core 325-M0055A-1R), suggesting accumulation during at least the early to middle portion of the last deglaciation. Two ages of 23 and 25 cal y BP were recovered deeper in this hole at 100–103 mbsl (Cores 325-M0055A-4R and 5R), indicating accumulation of LGM material at this site. A deeper feature in 97 m water depth (LAT) was drilled by four holes (M0052A, M0052B, M0052C, and M0053A) at Site 6. The upper cores (325-M0052B-1R, 325-M0052C-1R, and 325-M0053A-3R) yielded ages of 17–14 cal y BP, suggesting that they span the early to middle portion of the deglaciation. Deeper in Hole M0053A, two ages of 20 and 24 cal y BP, at 106 and 123 mbsl, respectively (Cores 325-M0053A-9R and 25R), indicate substantial accumulation of material at Site 6 during the LGM. The deepest hole drilled on the shelf edge on transect NOG-01B (Hole M0054B at Site 7) penetrated a seabed feature at 116 m water depth (LAT). The upper cores of this hole yielded ages of 13, 18, and 20 cal y BP from 113, 119, and 123 mbsl (Cores 325-M0054B-1W, 3R, and 4R), respectively, indicating accumulation of material during the early and mid-deglaciation, whereas the cores below may represent material from the LGM. The last hole drilled (Hole M0058A at Site 8) at transect NOG-01B was on the fore reef slope sediments at 167 m water depth (LAT). The only sample analyzed for radiocarbon (Core 325-M0058A-4R) was beyond the limit of the radiocarbon method (Fig. F154). This indicates that the majority of Hole M0058A probably spans a significant period older than 50 cal y BP. Downhole measurementsOne hole (M0054B) was logged in transect NOG-01B. Despite having good core recovery (in Expedition 325 terms) (29.63%), the data set for this hole will be vastly improved by the inclusion of logging data from the suite of wireline tools deployed in the hole. The logging data provides a continuous downhole data set as well as allowing more precise core positioning within the hole. Borehole geophysical instrumentsThe suite of downhole logging tools deployed at transect NOG-01B is as follows:
Preliminary resultsWireline logging operations at 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 in order to try to displace the guar gum drilling mud and condition the hole for open-hole logging. The API conductor pipe and HQ drill string were sitting at ~17 m WMSF, and with a total hole depth of ~33 m WMSF open, leaving little margin for borehole infilling or collapse. With the exception of the ASGR through-pipe log, 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. Two main logging units were identified in Hole M0054B (Fig. F164):
Further work will be needed to integrate the depth discrepancies shown between the logging and lithostratigraphic unit boundary depths. |
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