Samples from Hole 1138A were prepared for diatom analysis using standard procedures. Two preparations were made for each sample, including an unprocessed or "raw" preparation and a processed preparation. Two subsamples (~0.25 g) for each interval were placed in 15-mL centrifuge tubes. One set of subsamples was disaggregated in deionized water, and the other was chemically treated. The treated samples were initially reacted with ~5 mL of 10% hydrochloric acid in order to remove the carbonate component. The centrifuge tubes were shaken on a vortex stirrer and then set aside to react for 1 hr. Following these steps, ~3 mL of 30% hydrogen peroxide was added to remove organic material and help disaggregate biosiliceous "clumps." The samples were soaked overnight in the HCl/H2O2 solution and then thoroughly washed by centrifuging three times at ~1500 rpm for 10 min. The samples were not heated during chemical treatment.
Strewn slides of all samples for diatom analysis were made on 20-mm x 40-mm coverslips. Each centrifuge tube was filled with ~8 mL of deionized water and then mixed on a vortex stirrer. After settling for 30 s to let the sand-sized fraction settle out, a small aliquot was removed from the center of the suspension with a pipette. Two or three drops of sample were then pipetted onto a coverslip containing a thin film of deionized water. All samples were mounted in Norland optical adhesive 61 (refractive index [RI] = 1.56). A few selected samples for photomicrography were sieved to obtain a >10-µm fraction (using nylon screens) and mounted in Naphrax (RI = 1.74).
Because of low-to-moderate core recovery in the Miocene section of Hole 1138A, only core catcher samples were examined for Cores 183-1138A-32R through 15R. Above this level, many discrete core samples were examined in addition to the core catcher sample. A full sample list is given in the left column of Table T1.
Relative diatom abundance was determined from strewn slides of the unsieved, chemically treated (HCl and H2O2) preparations. The total relative abundance of diatoms (as a group) was determined at 600x magnification and was based on the average number of specimens observed per field of view. Several traverses were made across the coverslips, and abundance estimates were recorded (Table T1; Fig. F2) as follows:
The qualitative abundance of individual diatom taxa in Hole 1138A (Table T1) is based on the approximate number of specimens observed per field of view at 1000x. Individual species abundance categories are listed below. Generally, one-quarter to one-half of the 20-mm x 40-mm coverslip was examined (40 mm = ~200 fields of view). After initial abundance determinations were made at 1000x, the slides were then routinely scanned at 600x to identify rare taxa.
2 valves per field of view).
1 valve or identifiable fragment per traverse of coverslip).
The degree of siliceous microfossil fragmentation often mirrors dissolution, but the two factors are not necessarily dependent (i.e., well-preserved samples can be highly fragmented). Preservation of diatoms, therefore, was qualitatively based on the degree of dissolution and was rated as follows:
In addition to diatoms, the relative abundance of a few ebridian and endoskeletal dinoflagellate taxa was also noted (Table T1). The stratigraphic distribution of silicoflagellates in the Neogene section of Hole 1138A is treated in a separate chapter in this volume (McCartney et al., this volume).
To compare with diatom abundance, the relative abundance of calcareous nannofossils was also noted in the "raw" sediment preparations (Table T1; Fig. F2). These estimates were made in cross-polarized light at 1000x using the qualitative abundance designations outlined by Wei and Wise (1992).
Four tephra horizons were sampled for this study (Fig. F2). These samples were heated in dilute acetic acid to remove calcareous microfossils and then wet sieved to remove the <32-µm component, which was dominated by siliceous microfossil material. Glass shards from the tephra samples were examined by scanning electron microscopy (SEM), and ~70 shards were analyzed for major elements by electron probe using techniques described by Wallace (2002).
Crystallization ages of glass shards and minerals in discrete ash layers were measured using the 40Ar-39Ar incremental heating method. The chief advantages of this technique over conventional K-Ar dating are in separating the contributions of primary igneous and secondary alteration phases to the total sample Ar composition and in identifying any initial, nonatmospheric Ar, if present. This is accomplished, after neutron irradiation, to produce 39Ar from 39K by heating the sample in increasing temperature steps and analyzing the composition of Ar released at each step (e.g., Dalrymple et al., 1981; McDougall and Harrison, 1999). Crystallization ages are then interpreted from convergence of step ages toward a mid- to high-temperature plateau age and independently from the slope of collinear step compositions in Ar isotope ratio plots (i.e., an isochron age).
Experiments were run using both glass shards and biotites separated from the ash layers described in Coffin, Frey, Wallace, et al. (2000). Samples were selected for dating on the basis of examination under binocular microscope to assess shard morphology (size and vesicularity) and state of alteration of either glass or biotite grains. After handpicking to obtain 20-30 mg of material, separates were cleaned in dilute nitric acid followed by ultrasonic washing in distilled water. The samples were then wrapped in Cu foil, labeled, loaded in quartz vials, and interspaced with 10-mg aliquots of biotite monitor FCT-3 (28.04 ± 0.12 Ma; calibrated against Mmhb-1 hornblende at 523.5 Ma [Renne et al., 1994]). Quartz vials were evacuated, sealed in standard Al tubes, and irradiated for 6 hr at 1 MW power in the center ring of the TRIGA reactor at Oregon State University, Corvallis, Oregon.
FCT-3 biotite was placed at multiple vertical positions along the 80-mm center vial, which provided neutron flux measurements (J values) that varied smoothly within a ~10% range. Horizontal gradients in J values are known from previous experience to be <1%. J values for the sample positions were interpolated from a second-order polynomial fit to the monitors. Errors in sample J values (0.5%) accumulated from the individual monitor measurements and gradient fitting.
Ar isotopic compositions of samples were measured with a MAP-215/50 mass spectrometer connected to an ultra-high vacuum resistance furnace and Zr-Al getters. Samples were heated from 400°C to fusion in 50°-100°C increments. The system was operated in the peak-hopping mode (for m/z = 35, 36, 37, 38, 39, and 40) by computer. Peak decay is typically <10% for the MAP system, which has a measured sensitivity of 4 x 10-14 mol/V, and regressed peak heights against time follow linear fits. Mass discrimination on the MAP system was measured using zero-age basalt disks run in the same way as the samples and was constant at 1.005 (for 2 amu). The background for the mass spectrometer is 1.5 x 10-18 mol at m/z = 36, 2 x 10-18 mol at m/z = 39, and 1.5 x 10-16 mol at m/z = 40. Procedure blanks range from 3.0 x 10-18 mol 36Ar and 5.4 x 10-16 mol 40Ar at 600°C to 6.4 x 10-18 mol 36Ar and 1.7 x 10-15 mol 40Ar at 1400°C.
