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doi:10.2204/iodp.proc.325.105.2011

Transect RIB-02A summary

Sedimentology and biological assemblages

Only 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) (Fig. F27). 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.

At the top of two of the three holes (M0049A and M0050A), the uppermost sediment consists of brown-stained fragments of coralgal boundstone in lime sand that is rich in Halimeda. The fragments appear to include both lithified and unconsolidated modern or subrecent seafloor sediment. In Holes M0049B and M0050A, coralgal-microbialite boundstones occur below the modern sediment. The recovered unit varies from 8 to 16 m thick. No underlying lithologic information was obtained.

Table T3 documents all the larger foraminifera described in this transect in association with hole, run, and depth (below seafloor).

Physical properties

Recovery at transect RIB-02A sites was partial with an average recovery of ~16%. However, recovery at Holes M0049A and M0049B reached ~20%. Cores were partially saturated and often disturbed, fractured, or contaminated, which affects the quality of physical property data collected. Borehole depths for this transect are as follows:

• Hole M0049A = 97.63 mbsl, 3.50 m DSF-A.

• Hole M0049B = 97.63 mbsl, 15.6 m DSF-A.

• Hole M0050A = 97.63 mbsl, 10.5 m DSF-A.

• Hole M0051A = 79.63 mbsl, 2.50 m DSF-A

Plugs and samples taken for discrete P-wave and moisture and density measurements were obtained from both consolidated and unconsolidated material.

Density and porosity

Bulk density was measured at transect RIB-02A using the gamma ray attenuation sensor on the multisensor core logger (MSCL). Gamma ray attenuation provides an estimate of bulk density (also referred to as gamma density) from whole cores. Discrete moisture and density measurements are also taken with a pentapycnometer on plugs and/or rock fragments. This provides grain density, bulk density (in the case of plug samples), and porosity data. One observes a classic linear relationship between the porosity (ϕ) and bulk density (ρ = ρ_s[1 – ϕ] + ρ_wϕ) of discrete samples measured for all boreholes at transect RIB-02A (Fig. F28). Average grain density (ρ_s) 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 both measured boreholes in this transect can be viewed in Figure F29. Porosity ranges from 17% to 45%; however, the majority of porosity measurements acquired hover around 30%.

P-wave velocity

Only two core plugs were collected from this transect, and both were from Hole M0049B. Whole-core MSCL data (over ~6.5 cm) ranges from 1504.7 to 1845.36 m/s. Values at the lower end of this range (1500 m/s) should be treated with a degree of caution because the proximity of the values to the velocity of seawater suggests that core quality issues (including inundation by drilling fluid) may have compromised the data validity. As expected, because of where discrete samples are taken from cores (biased toward well-consolidated, good-quality samples that should yield good data), much lower P-wave velocity values have been recorded by the MSCL for coralgal and coralgal-microbialite units compared to discrete measurements on core plugs.

Magnetic susceptibility

Magnetic susceptibility data are very difficult to interpret for this transect owing to low core recovery in all holes. Very few values were obtained on whole cores, but data range from –0.63 × 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 in the data.

Electrical resistivity

Over the entire transect resistivity is highly variable, with the lowest value (0.63 Ωm) measured in Hole M0050A and the highest value (78.34 Ωm) recorded in Hole M0049B. Because of the relatively poor core quality and undersaturated cores, very little confidence can be placed in these data.

Color reflectance

In transect RIB-02A, recovery for Hole M0051A was very low (<10 cm), and only one value of color reflectance spectrophotometry 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 the boreholes in this transect are represented in Figure F30. No particular trends were observed in these three boreholes, but reflectance values were consistent in all cores where units were recovered from similar depths downhole.

Paleomagnetism

Four holes at two sites were drilled in transect RIB-02A, with recovered materials mainly composed of corals and calcareous sediments. The recovered materials show mainly positive values of low-field and mass-normalized magnetic susceptibility. The arithmetic mean values of the sites indicate the presence of paramagnetic and/or ferromagnetic materials.

Most of the peaks in magnetic susceptibility occur at specific depth intervals of 2–3, ~5, 17–20, and 27–30 mbsf, associated with variations in lithology. The features at 17–20 and 27–30 mbsf are related to the occurrence of sandy layers or other terrigenous material.

Magnetic susceptibility for these sites is stronger than at the southern sites on transects HYD-01C, HYD-02A, and NOG-01B, possibly because the Ribbon Reef transect is located much closer to land and thus to a source of magnetic materials. Further studies of ferromagnetic material will facilitate detection and definition of important features of the magnetic record and geomagnetic behavior. Further rock magnetic studies on these layers may provide information on the nature and processes that have contributed to the presence of these features. Environmental magnetic studies may reinforce the climatic origin of these layers and provide information on the amount, composition, and grain size of the magnetic component retained. The magnetic susceptibility record will allow alternative hypotheses to be investigated other than a simple concentration of magnetic minerals due to proximity to the eroded continent.

Geochemistry

Only two interstitial water samples (one aliquot each from Holes M0049A and M0050A) were collected from transect RIB-02A. Samples were analyzed for cations and anions (Table T4). All measurement parameters including alkalinity, pH, and concentrations of ammonium, cations, and anions were 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.

Chronology

A short hole (M0051A at Site 3) (Fig. F27) drilled into an 80 m feature at 78 m water depth (lowest astronomical tide [LAT]) returned an age of 10 cal y BP (Core 325-M0051A-2R), indicating that this hole recovered material from the middle of the deglaciation. Three more holes (M0049A, M0049B, and M0050A; Site 4) were drilled at 97 m water depth (LAT) in transect RIB-02A. These holes were drilled into a 100 m feature, and the youngest age recovered was 4 cal y BP (Core 325-M0049B-2R), suggesting some accumulation of Holocene material. Other ages from these holes range from 16 to 11 cal y BP (Cores 325-M0050A-2R, 325-M0049B-4R, and 9R), indicating material was accumulating during the last deglaciation at this site.

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